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Vision-based measurement of crack generation and evolution during static testing of concrete sleepers
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Engineering Fracture Mechanics journal homepage: www.elsevier.com/locate/engfracmech
Vision-based measurement of crack generation and evolution during static testing of concrete sleepers ⁎
Liu Rui , Emanuele Zappa, Andrea Collina Department of Mechanical Engineering, Politecnico di Milano, Milan, Italy
A R T IC LE I N F O
ABS TRA CT
Keywords: Crack propagation Concrete sleeper Digital image correlation Crack tracking technique
In this paper, vision-based measurement system is developed to monitor the crack generation during the static test of concrete sleepers. In particular three point bending test described in the European Standard, EN 13230-2 [1] is considered. This test is the acceptance tests for concrete sleepers, which are declared in the mentioned Standard as “safety critical components for railway applications”. In this ambit, the validation and the check of the reliability of crack opening technique is a crucial point for the safety of passengers and infrastructure. The mentioned Standard imposes to measure the width of the main crack appearing on the sleeper and track it during the test, but it does not prescribe the measuring technique to be used or the maximum uncertainty that can be accepted. Crack width is often estimated manually, with the help of a handheld microscope. The measuring uncertainty is strongly related to the experience of the operator. In this work, a vision-based measuring technique, based on digital image correlation (DIC) technique, is proposed and validated. In the proposed approach, the crack path is first detected automatically, relying on the strain map generated by DIC, and then the crack width is estimated, relying on the displacement map. Finally, the proposed measurement approach is compared with the state-of-the-art technique based on the manual analysis of images acquired with a handheld microscope. The results show that the crack width measured with the proposed approach is compatible with the data obtained by the manual technique. Thanks to the proposed full-field measurement, the crack width can be estimated not only in a few manually detected points along the crack, but for the full length of the crack with an automatic procedure. Moreover, the proposed technique can be applied in an unmanned way, since the camera does not need to be manually moved along the sleeper side to detect the crack: it is mounted in a fixed position and the image processing software automatically detects the crack path and width. The operator is therefore not requested to work close to the sleeper during the loading, with advantages also for the safety.
1. Introduction In ballasted railway tracks, the most widely adopted for railway lines, the sleepers have the main duty to transfer the axles loads from the rails to the ballast, keeping at the same time the correct gauge between the rails and their inclination. In order to assure their functionality sleepers must guarantee an adequate strength level, under quasi static, dynamic loads [1] as well as shock conditions [2,3]. To ensure the safety of passenger and infrastructures, the sleepers must guarantee a proper mechanical behavior, in particular
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Corresponding author. E-mail address: [email protected] (L. Rui).
https://doi.org/10.1016/j.engfracmech.2019.106715 Received 5 January 2019; Received in revised form 18 September 2019; Accepted 7 October 2019 0013-7944/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Liu Rui, Emanuele Zappa and Andrea Collina, Engineering Fracture Mechanics, https://doi.org/10.1016/j.engfracmech.2019.106715
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Fig. 1. Sleeper bending according to correct (a) and not correct (b and c) ballast packing under the sleeper (). adapted from [4]
in terms of crack generation and extreme loading before failure. The International Standard EN 13230-1:2009 ([1]) declares that the sleepers are “safety critical components for railway applications” and describes the acceptance tests for these components. The tests are based on three-points bending tests and crack detection and tracking. In this ambit, the validation and the check of the reliability of crack opening technique is a crucial point. Concrete sleepers are basically subjected to bending moment resulting from the vertical loads applied by the wheels and the reaction given by the continuous support provided by the ballast. Since ballast may be not uniformly distributed under the sleeper depending on its settlement in the long term, the following conditions can occur [4] [see Fig. 1]:
• In regular condition, the area under each rail seat is subjected to an almost constant distributed pressure, leading to a maximum •
bending moment in correspondence of the middle section of the rail seat and a relatively small bending moment in the central section of the sleeper (Fig. 1a) In the case that part of the pressure distribution is shifted outwards or inwards, an additional positive or a negative bending moment results in the central section of the sleeper (Figs. 1b and c).
The resistance of the sleeper is then defined in terms of bending resistance of the sections in correspondence of the rail seat and of the central section. In order to check the bending resistance of the section of the sleeper, the homologation procedure defined in the international standard [1] adopts a three-point bending test in order to create a bending moment in the considered section and check through static and fatigue testing the resistance of the sleeper. Considering that the sleeper is a composite structure (concrete and prestressed steel bars), three levels of bending moments are evaluated corresponding to three different status of stress in the section:
• first opening of a crack under applied load, corresponding to the end of elastic range • load corresponding to a prescribed residual opening of the crack, indicating the onset of plastic range • final failure, corresponding to the ultimate bending moment resistance. As for the first two levels, in the routine activity of homologation testing, optical comparator [5] or a portable microscope can be used, manually scanning the surface of the sleeper in the region where cracks are expected. Alternatively, a microscope connected to a camera enables to acquire an image of the crack, getting its opening from measurement performed on the acquired image. In scientific literature, more sophisticated means to measure displacement, deformation or crack opening on concrete beams and on sleepers are available. In particular, several measuring approaches are proposed in literature for these purposes. Gencturk developed a set of six LVDTs mounted on the surface of the specimen and measured the displacement of the point of the concrete specimen during the test and compared the results with DIC measured data [6]. Strain gauge is another conventional sensor to measure the strain of one point, especially at the critical region/discontinuities of engineering structures [7–9]. Purkiss used a strain gauge and a displacement gauge to compare the mechanical behavior of the fibre reinforced and unreinforced concrete beams [10]. Robins combined electrical strain gauges with semi-automated grid method to investigate the fracture of steel-fiber-reinforced sprayed concrete under bending test [11]. Obviously the point-measurement sensor does not allow to obtain the full-field measurement and it is difficult to obtain the crack propagation during the test, since the crack path is unpredictable. Recently the development of non-contact measurement techniques allowed a wide application of these devices for many fields of measurement, including the analysis of the mechanical behavior of concrete manufacts. In the early age, Haavik applied an optical comparator to measure crack width [12]. Dias-da-Costa developed a technique for the visualization of displacements, strains and cracks of the concrete by means of photogrammetry and image post-processing [13]. Anwander et al. used the laser speckle with a correlation data-processing procedure to successfully measure large mechanical strain of various materials at high temperature [14]. This technique is a potential candidate to estimate the evolution of the crack, by means of the analysis of the large deformation occurring during crack propagation. Bernstone developed a digital image analysis technique for the estimation of crack development in concrete dams and of the crack opening at the surface of the structure [15]. Rodríguez used distributed optical fiber to measure the crack width in the concrete, and the result was well exploited to calibrate FEM model [16]. Although the mentioned measurement techniques can provide reliable data on crack opening, a denser measuring technique would be desirable in order to monitor a large area of the sleeper surface and automatically detect the crack path and width. To this purpose, digital image correlation (DIC) is promising, since it provides a dense displacement and strain field. Choi successfully measured the non-uniform deformations of the surface of the concrete specimens by means of the full-field digital image correlation technique [17]. Biswal used DIC to measure the 2
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surface deformation, and further compute the crack reopening load accurately in concrete structures with prestressing force [18]. Fayyad applied DIC technique to monitor the crack propagation of reinforced concrete in the three-point bending test and quantified the fracture properties of the concrete by means of a high resolution camera [19]. In order to understand behavior of sand under complex loading condition, Salvatore combined DIC and X-ray microtomography to obtain the intensive strain localization near failure of specimens [20]. Cholostiakow applied DIC to study the development and degradation of shear resisting mechanisms in fiber reinforced polymer–reinforced concrete beams [21]. Kuntz realized an in-situ measurement of a shear crack in a reinforced concrete beam during a bridge static load test with DIC displacement measuring technique [22]. Based on DIC Hoult proposed a preliminary method to compute the crack width and slip in reinforced concrete beams [23]. He also suggested this technique can be used to measure the crack movement. Sabato built structural health monitoring system with 3D DIC measurement technique, and analyzed the railway structure. It can accurately monitor the condition of the structure to identify defects or damage [24]. In the current paper, an approach to run the static tests on sleepers described in [4] is proposed with the assistance of an automatic vision-based system that allows to estimate the crack position and opening with an automatic procedure, without the need to locate additional sensors displacement or strain sensors, still keeping the test set up arrangement requested by the standards. The goals of the proposed approach are: i. obtain the results with a deterministic procedure that does not depend on the operator’s skills and experience; ii. analyze the full area of cracking of the sleeper, and not only the region around the first crack. This allows to monitor the global phenomenon of crack generation and sleeper failure, providing more information on the response of these structures to mechanical loading; iii. track the growth and the opening of the cracks during the whole test, from crack generation to the final value of residual opening; iv. obtain a detailed representation of the phenomenon, allowing to deliver the data necessary to estimate also the energy associated with the sleeper failure, directly referring to the cracked area. The proposed measurement approach is based on image of acquisition and processing, using the Digital Image Correlation (DIC) technique [26] to estimate the full displacement and strain field of the area of interest. By means of processing the DIC data, the crack position and development is detected. The large amount of information that can be obtained with the proposed measuring approach is expected to be a useful support for the optimization of the design of both existing and innovative sleeper’s types. Moreover, the automatic and continuous monitoring of the cracks, can in perspective allow to run unmanned laboratory tests of the sleepers. The paper is organized through the following points:
• Recall of the steps of the static test for the rail seat section, to which the approach developed in this paper is applied • The set up for the DIC test, and all the steps carried out for its implementation • The image and data processing techniques and obtained results • Final remarks and lines of future work 2. Static tests on concrete sleeper in the international standard In this section a brief description of the standard procedure for sleeper testing will be provided, focusing on the static test under rail seat. Fig. 2 shows the equipment used in this work for standard three-point bending test of the sleeper. Different types of pretensioned sleepers can be tests with this set-up, with a number of reinforcement bars ranging from 2 to 8 and with some variability of the sizes. In Fig. 2 the main dimensions of the sleeper tested in this work are provided. Standards refer directly to the applied load and not the resulting applied bending moment, having prescribed the distance between the two supports. Although in this work the attention is mainly focused on the static tests prescribed for the rail seat section, the proposed measuring approach can be applied
Fig. 2. The three-point bending test of the sleeper at rail seat section (left: sketch of the test rig, right: example of set-up). 3
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Fig. 3. The static test procedure.
also for the tests at the centre section, recommended by the same standard. Fig. 3 shows loading procedure for the whole standard static test [1], which is composed of three stages. In the first stage the applied load is increased by steps of 10 kN, starting from an initial loading Fr0. The value of Fr0 depends on the class of the sleeper, defined by the standard. The prescribed loading rate is 120 kN/min. Every load value should be maintained for at least 10 s and no more than 5 min (time t in the figure). The load is increased until the first crack formation is visible in the bottom of the sleeper seat section. The load value that generates a visible crack is named Fr. When the first crack is detected, the load is removed and the first stage is complete. The condition corresponding to Fr defines the limit for the linear elastic range. The second stage is composed of a sequence of loading-unloading cycles, with increasing loading level (Fig. 3). The increment of the maximum loading between two subsequent cycles is 10 kN. After every loading-unloading cycle the crack width is measured in unloaded conditions: if the crack width is larger than 0.05 mm the test is completed, otherwise another loading cycle is applied. The load value that generates the residual crack width of 0.05 mm is named Fr0.05. Note that the duration of crack checking operation should not exceed 5 min, this is to be taken into account for real time processing of the data. The Fr0.05 load corresponds to the onset of plastic phase at a prescribed residual deformation. The third and final stage is aimed to estimate the final breaking value, reached in a single cycle test. Load is continuously increased until the final breaking value FrB reported in Fig. 3, is reached. The present paper deals with the first two stages, since stage 3 does not need any crack opening measurement. In the view of the standard, the key point of the measurement is to detect the crack position and measure its opening. The crack width is usually measured using an optical micrometer or a handheld digital microscope manually moved along the bottom of rail seat section, allowing the user to detect the crack. During the tests more than one crack might appear in different positions at the bottom of the rail seat section and the operator is responsible of detecting the main one. The main crack is defined in the European Standard EN13230-1 [25] as the first crack that propagates up to 15 mm from the bottom of the sleeper. 3. Experimental set-up and image processing technique 3.1. The digital image correlation technique Digital Image Correlation (DIC) technique is a contactless full-field measurement that enables to estimate the displacement and strain field of the specimen under test [26]. The possibility to obtain a full map of displacement and strain, instead of single point measurement along the whole crack, is one of the most relevant advantages of DIC with respect to most of the traditional techniques. Thanks to this characteristic, DIC is often applied in the measurements aimed at the qualification of the behavior of materials under stress. In DIC the displacement and strain fields of the region of interest are computed relying on an image correlation algorithm. The basic procedure is to acquire an image of the specimen before loading and an image of the loaded specimen; the first image is then divided into regions of interest (usually square), and each of them is correlated with the image of the specimen under loading to identify its displacement. As soon as the correlation is applied to all the regions of interest, the displacement map is estimated and the strain map can be computed by means of local numerical derivative of the displacement map [27]. To ensure a reliable image correlation, the specimen surface must have a random gray intensity distribution (speckle pattern). The speckle pattern can be the natural texture of the specimen surface or artificially made: in this application the speckle was generated using a stencil technique, as described in Section 3.3. The overall size of the field of view and the measurement resolution strongly depends on the hardware characteristics and on processing parameters: the camera resolution, the focal length of the optics, the size of the region of interests and so on. In this work the vision system and the specimen surface were optimized to guarantee a resolution of the measurement that allows to early detect the cracks and to measure the evolution of the width of the crack during the tests. DIC technique has a promising merit in the concrete fracture field. Due to the manufacture procedure of the concrete, stochastic voids and micro-cracks are always presented inside the concrete sleeper. This leads to the unpredictable crack growth and unpredictable crack path. Comparing with the conventional crack detection technique, DIC technique can capture the crack propagation more easily, since it allows to monitor the full seat section of the sleeper, therefore it allows to detect the crack wherever it appears. Because of these reasons, DIC technique is applied in this work to detect cracks and measure their width evolution for sleeper testing 4
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Fig. 4. (a) Vision-based measurement system and the sleeper under test; (b) The scheme of the measurement set-up; (c) The fastening system of the sleeper and the loading plate.
according to the standards [1]. 3.2. Experimental set-up A three-point bending test is conducted to analyze the crack propagation of the concrete specimen under static condition. Fig. 4(a) shows the real arrangement of the whole measurement set-up, and Fig. 4(b) shows the scheme of the measurement set-up. The whole set-up includes mainly loading and controller facility, concrete specimen and vision-based measurement system. The whole test procedure follows the international standard explained in Section 2. The sleeper under test is a commercially available RFI 260 type (2600 mm long), used for high speed network. During the tests, a rubber pad with characteristic prescribed by the standards is placed between the sleeper and each support, in order to equalize load distribution at the contact surfaces, as happens in the final application of the sleepers. The loading surface is fitted with the fastening system of the sleeper, connecting an articulated support that transfers the load from the actuator to the rail seat of the sleeper, instead of the rail (Fig. 4c). The standard of rail pad is included in the fastening device. The loading is imposed to the sleeper by means of a 1000 kN MTS hydraulic actuator. Cracks are supposed to originate from the bottom of the sleeper. As the loading increases, the crack propagates upward, progressively increasing its width at each loading step. The vision-based measurement system for the estimation of the crack width is composed of a camera, two lighting sources and a software for image acquisition and processing. As Fig. 4(a) shows, an IDS 5490SEM-GL camera is mounted in front of the concrete sleeper to acquire the image of the surface of the specimen. The adopted camera is equipped with a 3264x2448px CMOS sensor. The required size of the field of view is about 120x160mm, in order to cover the region of interests (ROI) marked with a red rectangle in Fig. 2. This ROI is the region where the main cracks are expected to appear during the tests. To obtain this field of view, it was necessary to select a wide angle optics (focal length 6 mm), since the maximum distance from the camera to the target is imposed by the main frame of the testing machine and is about 300 mm. With this hardware configuration the resolution of the image is around 0.05 mm/px. Speckle for DIC is produced on the sleeper surface as described in Section 3.3. The camera operation is fully controlled by software, developed in National Instruments Labview environment. During the test, the image acquisition frequency was set to 0.2 Hz. A set of LED lamps (see Fig. 4) were used and pointed to the target from two different directions to ensure a proper lighting of the specimen during the tests. 3.3. DIC speckle generation using the stencil technique The accuracy of the DIC results are strongly related to the characteristics of the speckle generated on the surface of the specimen [5]. Size and distribution of the speckles, as well as the contrast between the speckle and the background have to be optimized to improve the accuracy of the measurement. In particular, average speckle size should be around 3px (see [29]), corresponding, for this application, to about 0.15 mm. The conventional speckle producer based on spray technique is therefore not suitable in this work, because the spray technique produces speckles with average size well below 0.1 mm. To control the characteristics of the speckle, a stencil technique (see [30]) is applied here. In order to evaluate the quality of the speckle produced by means of the stencil technique, a widely accepted parameter is the Mean Intensity Gradient (MIG), proposed in [28]. MIG is estimated as the average value of the modulus of omnidirectional image gradient within the DIC pattern. The equation to compute the MIG value is: W
MIG =
H
∑ ∑ |∇f (xij)|/(W × H ) (1)
i=1 j=1
where W and H represents the image resolution, |∇f (x ij )| =
fx (x ij 5
)2
+ f y (x ij
)2
represents the modulus of local intensity gradient
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Fig. 5. One portion of the speckle of the specimen.
vector with the x- and y-directional intensity derivatives fx (x ij ) and f y (x ij ) at pixel ( x ij ). As can be seen in Eq. (1), the MIG parameter is proportional to the absolute value of the derivatives of image brightness, therefore it is function not only of the speckle size but also of the spackle spacing. Moreover, the use of the stencil technique, allows to keep the size of the speckles as uniform as possible and close to the ideal value of 3px. MIG has been computed for many speckle images, acquired during the tests on different beams, to obtain an indication of the speckle quality. The MIG values of these images is around 11, and it is reasonable for DIC analysis. Fig. 5 shows an example of a portion of the speckle generated on the specimen. 3.4. Optics aberration compensation The wide angle optics, needed to ensure the necessary field of view (as described in Section 3.2), generates a relevant image distortion, visible in Fig. 6a. This image distortion must be compensated for, prior to run the DIC analysis. To this purpose, a camera calibration procedure was applied, taking the image of a thin calibrator with a regular grid of circles mounted on the surface of the sleeper (Fig. 6a). The spacing between the circles was 20 mm, while their diameter was 10 mm. Based on the calibrator image, the parameters of the distortion model are computed and then compensated by applying a proper warping to the image. Before DIC analysis, all the images acquired during the test are compensated with the same warping. Fig. 6b and c show an example of the original image and the image after compensation respectively. In addition, the camera calibration allowed to estimate the scale factor of 0.05 mm/pixel, used to convert the DIC output from pixel to millimeters. 3.5. Crack opening measurement To run the DIC analysis, the commercial software VIC 2D was used. The subset size was set to 21 × 21 px, with overlap of 16px. The displacement and strain fields generated by DIC were further processed to obtain the information on crack path and opening. In particular, the strain map was used to estimate the path of the crack, since along the crack the strain assumes very large values. These large values are apparent (i.e. do not represent the actual strain of the sleeper); they are a consequence of the strain computation algorithm that does not account for the displacement discontinuity generated by the crack opening. As soon as the path of the crack is identified relying on the mentioned large strain values, the crack width is determined as the difference of the displacement of two
Fig. 6. Calibration and compensation of the image: (a) calibrator; (b) original image; (c) image after compensation. 6
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Fig. 7. The crack detection during the loading continuously stage.
points across the crack. As described in Section 2, the standard imposes two stages in the test: first stage involves a continuously increasing loading, while second stage is based on loading-unloading cycle. For the first stage, although several cracks generate in the field of view, the standard imposes to consider only the crack that firstly propagates vertically for 15 mm. The width of this crack must be monitored during the test. The procedure to measure the width of the crack relying on DIC data is (see also Fig. 7): 1. A reference line is defined, 15 mm from the bottom of the sleeper (thanks to the knowledge of the scaling factor, this line can be easily identified in the DIC data). 2. During the loading, the values of the horizontal strain along the reference line are extracted. If a crack propagates up to the reference line, a peak appears in the horizontal strain values (Fig. 8). However, due to the heterogeneity of the concrete specimen and the noise of the images, local high strain value can be found, leading to the risk of erroneously predicting the presence of a crack. In Fig. 8a a spurious crack is visible at the horizontal coordinate × around 4 mm, while the real crack is presented around x = 100 mm. Since spurious strain peaks are due to noise in the measurement, it was observed in different tests that, averaging the horizontal strain trends of different lines allow to remove spurious data. Therefore, to correctly detect the crack, we averaged the horizontal strain trends of all the lines in a stripe between the lines L1 and L2 (Fig. 7), which was heuristically defined as a proper size for averaging. 3. To distinguish the peaks of strain due to the presence of a crack from the ones due to measurement noise, a threshold value is needed. In this work, the threshold value was heuristically set to 0.003 m/m, analyzing the data at different loading steps. Since the detection is done on the strain profile estimated averaging on a 10 mm stripe (between L1 and L2) this threshold allowed to correctly detect the crack. The step 2 and 3 should be conducted until the crack is up to the reference line, and then the position of the crack is obtained. 4. The opening of the crack is estimated as the difference between the horizontal displacement uA and uB of two points (A and B in
(a)
(b)
Fig. 8. The distribution of the strain in the reference line in case of the force 208kN: (a) without average; (b) with average. 7
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Fig. 9. The crack detection during the loading-unloading stage.
Fig. 7) across the crack. The distance d between the crack and the points A and B cannot be too small to avoid that the DIC data are affected by the discontinuity induced by the presence of the crack. On the other hand, d should not be too large because displacement of points A and B have to be representative of the crack opening. It was experimentally estimated that a proper value of d in this application is 1.75 mm. To improve the signal to noise ratio in the estimation of uA and uB , we averaged the horizontal displacement values of all the points along two vertical lines: L3 and L4 respectively (see Fig. 7). The length of the vertical lines L3 and L4 is 2 mm since this was found to be a good tradeoff between the noise reduction and the need to ensure a local displacement measurement. In the second stage of the tests a sequence of loading-unloading cycles is applied until the width of the main crack in the unloading condition is beyond 0.05 mm. The European standard does not point out in which position of the main crack the width has to be checked, however in the first stage this crack is identified as the first one that reaches a length larger than 15 mm. Since in the second stage the crack is usually much longer than this limit, in our work we estimate the width of the crack as the mean opening in a stripe of 20 mm height: from a reference line to the bottom of the sleeper (Fig. 9). The procedure to measure the width of the main crack produced in the first stage includes the following steps: 1. A reference line is defined, 20 mm from the bottom of the sleeper (thanks to the knowledge of the scaling factor, this line can be easily identified in the DIC data). 2. In loading-unloading cycle, the opening of the crack must be measured along the full crack path between the reference line and the bottom of the sleeper. For every row, the cracked region is detected as the points where the horizontal strain is larger than a given threshold (in this application equal to 0.005 m/m). Horizontal position of the mid crack line is detected as the mid-point of the cracked region. 3. For each row, the opening of the crack is estimated as the difference between the horizontal displacement uC and uD of two points (C and D in Fig. 9) across the crack at a distance d1 from the mid-point of the cracked region. In this application d1 is set to 12.5 mm for the loaded condition, and to 1.25 mm for the unloaded one. The value of d1 is larger in the case of loaded sleeper since the width of the crack is much larger in this condition, therefore DIC shows a significant increase in the uncertainty of the data very close to the crack edges. 4. At the end of the previous steps, the horizontal displacement uC and uD is estimated for each point of the lines L5 and L6. The width of the crack can be considered to average the crack width of each point along the crack path by means of uC − uD . It should be noted that the EN13230 Standard ([1;25]) impose a strict procedure for the testing of concrete sleepers, and prescribes to tune the loading to obtain a main crack of given characteristics, in operating conditions the main crack is characterized by very stable characteristics (length and width). Therefore, the parameters for DIC data processing described in the points 3 and 4 of the procedure above, have a general validity in sleeper testing.
4. Test results and analysis This section shows some examples of data obtained with the measuring technique described in the previous section. The test was carried out following the routine manual procedure for crack width estimation and acquiring at the same time the images for DIC. The initial loading value Fr0 in the first stage prescribed by the standard was 168 kN for the sleeper under test. The Fr value corresponding to the appearance of the first eye-visible crack was obtained at a loading of 288 kN. In the second stage, the force value Fr0.05, corresponding to a residual crack opening of 0.05 mm, was 368 kN. For the manual estimation of the crack width, a Dyno Lite handheld microscope was used. The results of each part of the measurement are reported in the following sections.
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Fig. 10. Examples of exx strain maps at increasing loading levels.
4.1. Detection of the first crack and estimation of the crack width evolution 4.1.1. Results for strain The crack detection is based on the analysis of the strain map of the specimen, as described in Section 3.5. Fig. 10 shows some examples of the strain map in the horizontal direction (exx) at increasing loading levels. The crack path is visible as a region with high strain value. By means of the characteristic of the crack, the opening and the propagation of the crack can be captured in the strain map. In the test, there are more than one crack visible in the strain map. The crack that first crosses the reference line shown in Fig. 7 is considered as the main crack (see Section 3.5 for details). 4.1.2. Evolution of the width of the crack in the 1st stage The evolution of the crack width is estimated as described in Section 3.5, and it is shown together with the load in Fig. 11 (as a function of the time in the left, and width vs load in the right). The first level of the applied load is 168 kN and it is subsequently increased by 10 kN steps. Each step is retained for 20 s. In correspondence of an applied load of 228 kN, the rate of crack’s opening changes, indicating a modification of structural response at local level. At 288 kN, the crack becomes visible from visual inspection. At this load level, the width of the crack reaches its maximum value around 0.042 mm (see point A in Fig. 11). According to the European Standard [1], having reached the formation of a visible crack under loading condition, 1st stage is ended. During the unloading step from 288 kN to zero the width of the crack reduces to 0.009 mm (point B). Since the residual crack opening is lower than 0.05 mm, the second stage is activated. 4.2. Residual crack opening after each loading-unloading cycle 4.2.1. Evolution of the crack width in the 2nd stage In this stage, the crack detected in the 1st stage is focused. A series of loading-unloading cycles (as reported in Fig. 3) are applied
Fig. 11. Evolution of crack’s width and applied load during the 1st stage. 9
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Fig. 12. The exx map for the crack path detection at the third loading level (left) and after unloading (right).
until the residual opening of the crack at unloading condition reaches 0.05 mm. As described in Section 3.5, the opening of the crack is estimated the mean of the width in the stripe below a reference line (Fig. 9). With the manual measurement only a limited number of points along the crack can be measured. On the contrary with the DIC approach, a mean crack opening can be estimated, as described in the Section 3.5. Fig. 12 shows the path of the crack at the third loading-unloading cycle as an example of the application of the method to detect the crack path described in Section 3.5. Note that the map shown in Fig. 12 is only a portion of the full data map (see Fig. 10). The bottom of the figure corresponds to the bottom of the sleeper. The crack path can be clearly identified both in loaded and unloaded conditions, thanks to the high apparent strain values described in Section 3.5. Please, note that the color bar limits are different for the loaded and unloaded strain maps, since the apparent strain values are significantly larger in the loaded conditions. The black circles highlight the automatically estimated mid-crack path. Fig. 12b shows the crack path estimated in the case of unloaded sleeper. As soon as the crack path is identified, the width of the crack can be computed by means of the method presented in Section 3.5. The distance d1 from the points C and D the crack mid-line (Fig. 9) is set to 2 mm for the unloading conditions and is increased to 12.5 mm for the loading conditions, to avoid the low reliable DIC data measured in the vicinity of the crack, as explained in Section 3.5. The horizontal strain values are negligible close to the crack, because of the discontinuity due to the crack itself. It is therefore expected that the increase of the d1 value to 12.5 mm does not affect values of the estimated horizontal displacement significantly. This is confirmed by the data in Fig. 13b, where, for the largest loading values, the crack opening is estimated two times: with d1 = 6.25 mm and with d1 = 12.5 mm and the obtained results are compatible. Crack width is measured along the full crack path.
Fig. 13. The distribution of the width of the crack along the crack path: in the unloaded conditions (left) and in loaded conditions (right). The numbers in the legend correspond to the cycles of the 2nd stage. 10
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Fig. 14. Mean width of the crack and its standard deviation, for each loading cycle. Value of the load is indicated on the top of each step.
Fig. 14 shows the mean crack width and the corresponding standard deviation range for each loading/unloading cycle. The standard deviation values include two contributes: the trend of the crack opening along the crack path (opening is larger in the lower part of the sleeper, in particular in loading conditions) and the measuring uncertainty of DIC. The variability of the crack opening is clearly visible in Fig. 14, where the standard deviation of the estimated opening values is shown to be much larger in loading conditions. As the load of each step is increased, the mean width of the opening crack is progressively increased from around 0.05 mm to around 0.52 mm while the standard deviation of the estimated width is one order of magnitude lower of the width itself. As the cycle number progresses, the width of the opening crack after unloading increases from lower than 0.01 mm to around 0.05 mm due to the damage accumulation. As Fig. 14 shows, by means of DIC technique, in the unloading step of the 8th cycle, the average crack width is beyond the threshold value 0.05 mm. More details of the comparison between DIC technique and the microscope technique are presented in the following section. In order to have more insight into the DIC application to crack’s opening measurement, the width of the opening crack along the crack path is examined in both the loading and unloading condition (see Fig. 13). It is worth remarking that in each step, there are more than one images acquired and the full results are shown in Fig. 14. We point that the platform corresponding to each step means there is no obvious difference among the images. Therefore, only the result of one of them is displayed for clarity. Clearly with applied load the opening is larger and the uncertainties in crack’s profile detection are bigger. Moreover, it can be observed that, as expected, DIC technique allows to estimate the crack width all along the crack path. It should be noted that, during the test, more than one crack appears on the sleeper. However, as described in Section 2, the international Standard [1] impose to focus on the first crack that reaches a length of 15 mm (named main crack), since this is the crack that leads the failure of the sleeper in the vast majority of the tests. The long experience of one of the authors in sleepers testing confirms this evidence. Obviously the main crack can generate in any of the sides of the sleeper, therefore the crack monitoring, performed either with a manual approach or with the proposed automatic technique, has to be applied to both the sides. 4.2.2. Comparison between current practice and proposed techniques During the test, the usual microscope-based technique was applied together with DIC, in order to compare the results between the two approaches. For each unloading condition the handheld microscope is manually placed along the main crack. Fig. 15 shows examples of these images. Since it is not possible for the microscope to scan the whole crack in a unique image (the field of view is approximately 1.8 mm), only some positions chosen according to the experience of the operator are considered. Therefore, the positions where the crack opening is estimated are not necessarily the same for all the images. In addition, the crack width along the path is not perfectly uniform due to the anisotropic behavior of the concrete. Note that, to estimate the crack width, two points on the two edges of the crack are manually selected by the operator with mouse click, therefore the experience of the operator plays a crucial role in the measuring process. The operator was asked to measure the width in 20 different positions for each image acquired by the microscope in order to estimate the mean crack opening and the associated uncertainty, computed as the standard deviation of the 20 repeated measurements. Further, the mean crack width Wm and the standard deviation σm are expressed by the Eqs. (2) and (3):
Wm =
σm =
1 20
i = 20
1 19
∑
(x1i − x2i )2 + (y1i − y2i )2
(2)
i=1
i = 20
∑
( (x1i − x2i )2 + (y1i − y2i )2 − Wm)2
(3)
i=1
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Fig. 15. Portions of the main crack in unloading conditions after the following loading cycles: (a) 368 kN; (b) 378 kN; (c) 388 kN. Images acquired by means of the manual microscope.
where the coordinates (x1i , y1i ) and ( x2i , y2i ) represent the position of the point at the left edge of the crack and the right edge of the crack respectively. DIC technique obviously allows to obtain more dense information of the crack width, as shown in Fig. 13. DIC measurement results also demonstrate the non-uniform crack width along the crack path due to the property of the concrete. Therefore, the average crack width Wc along the path are considered as the crack width of the test, and the standard deviation σc of the measurement can be calculated as:
Wc =
σc =
1 n
i=n
∑ (uiC − uiD)
(4)
i=1
1 n−1
i = 20
∑
((uiC − uiD ) − Wc )2 (5)
i=1
where the symbols uC and uD have the same definition in Section 3.5. The symbol i represents the position of the crack along the path. Its size depends on the DIC measurement. The measures of the two techniques are shown in Fig. 16, where the mean crack width is shown together with error bars
Fig. 16. Comparison of the crack width in unloaded condition measured with the DIC technique and with the microscope. 12
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representing the standard deviation of repeated measurements. The results of the two techniques are compatible, even if for cycles from 5 to 7 the mean values estimated with the DIC are approximately 2 µm larger than the corresponding data from the microscope. This discrepancy may be due to the non-perfectly uniform crack opening along the crack path (as shown by DIC data in Fig. 13 left). While DIC averages the opening along the full crack path, microscope data is more local and could be more affected by the variability of the width. For both the techniques, the estimated average crack width is larger than 0.05 mm after the 8th loading/unloading cycle (i.e. after the loading cycle at 368 kN). The standard deviation of the crack opening along the crack path, estimated for the two techniques, assume values lower than 6 μm in the case of the experimental set-up proposed in this work. Note that, in loaded conditions, the standard deviation values represent the combination of the measuring uncertainty and the variability of the crack opening along the crack path, which is quite evident in the case of loaded sleeper (Fig. 13 right). In fact, in loading conditions, the width of the crack obviously increases from the reference line to the bottom of the sample, according to the mechanical behavior of the crack. On the contrary, in unloaded conditions the variability of the crack opening is limited (see Fig. 13 left), therefore in unloaded conditions the standard deviation provides a reasonable index of measuring uncertainty. 5. Concluding remarks This paper proposes the use of DIC technique to monitor the cracks propagation in concrete sleepers during laboratory qualification tests. The acceptance tests for concrete sleepers are crucial, since the sleepers are declared in the Standard [1] as “safety critical components for railway applications”. In this ambit, the validation and the check of the reliability of crack opening technique is a crucial point for the safety of passengers and infrastructure. The main goal of this work was the automatic crack detection and the measurement of crack opening as a function of the applied load. In the current practice crack opening is evaluated through an optical comparator or a handheld microscope, but these approaches need the manual detection of the first crack by means of an inspection of the operator and the manual positioning of the microscope every time the crack width has to be estimated (i.e. after each loading/ unloading cycle). The use of DIC instead of the manual technique gives several advantages. First of all, the camera is mounted before the beginning of the test, so the presence of the operator in the vicinity of the loaded sleeper is no longer required, avoiding any safety problem. Moreover, the crack position can be automatically detected during the first stage of the test and the crack width along the full crack path can be measured continuously and automatically during the whole test. Another advantage of the DIC-based measuring technique is that the measuring procedure is fully automatic, therefore the effect of operator’s skill is removed and the results are more objective. Declaration of Competing Interest Author declares that there is no conflicts of interest. References [1] Railway applications-track-concrete sleepers and bearers-Part 2: prestressed monoblock sleepers, European Standard, EN 13230-2. [2] Kaewunruen S, Remennikov AM. Progressive failure of prestressed concrete sleepers under multiple high-intensity impact loads. Eng Struct 2009;31:2460–73. [3] Kaewunruen S, Remennikov AM. Dynamic crack propagations in prestressed concrete sleepers in railway track systems subjected to severe impact loads. J Struct Eng 2010;136:749–54. [4] Esveld C. Modern railway track, II Ed., MRT Productions, Zaltbommel, ISBN: 978-1-326-05172-3, 2016; 241. [5] Mazzoleni P, Zappa E, Matta F, et al. Thermo-mechanical toner transfer for high-quality digital image correlation speckle patterns. Opt Laser Eng 2015;75:72–80. [6] Gencturk B, Hossain K, Kapadia A, et al. Use of digital image correlation technique in full-scale testing of prestressed concrete structures. Measurement 2014;47:505–15. [7] Liao D, Zhu SP, Correia JAFO, et al. Computational framework for multiaxial fatigue life prediction of compressor discs considering notch effects. Eng Fract Mech 2018;202:423–35. [8] Zhu SP, Liu Y, Liu Q, et al. Strain energy gradient-based LCF life prediction of turbine discs using critical distance concept. Int J Fatigue 2018;113:33–42. [9] Zhu SP, Liu Q, Peng W, et al. Computational-experimental approaches for fatigue reliability assessment of turbine bladed disks. Int J Mech Sci 2018;142–143:502–17. [10] Purkiss JA, Blagojevic P. Comparison between the short and long term behaviour of fibre reinforced and unreinforced concrete beams. Compos Struct 1993;25:45–9. [11] Robins P, Austin S, Chandler J, et al. Flexural strain and crack width measurement of steel-fibre-reinforced concrete by optical grid and electrical gauge methods. Cement Concrete Res 2001;31:719–29. [12] Haavik DJ. Evaluating concrete cracking by measuring crack width, Boston, MA: The Aberdeen Group, 1990 Publication #C900553. [13] Dias-da-Costa D, Valenca J, Julio ENBS. Laboratorial test monitoring applying photogrammetric post-processing procedures to surface displacements. Measurement 2011;44:527–38. [14] Anwander M, Zagar BG, Weiss B, et al. Noncontacting strain measurements at high temperatures by the digital laser speckle technique. Exp Mech 2000;40:98–105. [15] Bernstone C, Heyden A. Image analysis for monitoring of crack growth in hydropower concrete structures. Measurement 2009;42:878–93. [16] Rodríguez G, Casas JR, Villaba S. Cracking assessment in concrete structures by distributed optical fiber. Smart Mater Struct 2015;24:035005. [17] Choi S, Shah SP. Measurement of deformations on concrete subjected to compression using image correlation. Exp Mech 1997;37:307–13. [18] Biswal S, Ramaswamy A. Measurement of existing prestressing force in concrete structures through an embedded vibrating beam strain gauge. Measurement 2016;83:10–9. [19] Fayyad TM, Lees JM. Experimental investigation of crack propagation and crack branching in lightly reinforced concrete beams using digital image correlation. Eng Fract Mech 2017;182:487–505. [20] Salvatore E, Ando E, Viggiani G, et al. Compression and extension triaxial tests on sand, studied with X-ray microtomography and digital image correlation. 1st IMEKO TC-4 International Workshop on Metrology for Geotechnics, Benevento, Italy. 2016. p. 157–61. [21] Cholostiakow S, Benedetti MD, Guadagnini M, et al. Experimental and numerical study on the shear behaviour of geometrically similar FRP RC beams. 8th
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International Conference on Fiber Reinforced Polymer (FRP) Composites in Civil Engineering (CICE2016), Hong Kong, China. 2016. [22] Kuntz M, Jolin M, Bastien J, et al. Digital image correlation analysis of crack behavior in a reinforced concrete beam during a load test. Can J Civ Eng 2006;33:1418–25. [23] Hoult NA, Dutton M, Hoag A, et al. Measuring crack movement in reinforced concrete using digital image correlation: overview and application to shear slip measurements. Proc IEEE 2016;104:1561–74. [24] Sabato A, Niezrecki C. Feasibility of digital image correlation for railroad tie inspection and ballast support assessment. Measurement 2017;103:93–105. [25] Railway applications-track-concrete sleepers and bearers - Part 1: General requirements, European Standard, EN 13230-1:2009. [26] Sutton MA, Orteu JJ, Schreier HW. Image correlation for shape, motion and deformation measurements. New York: Springer; 2009. [27] Pan B, Qian K, Xie H, et al. Two-dimensional digital image correlation for in-plane displacement and strain measurement: a review. Meas Sci Technol 2009;20:062001. [28] Pan B, Lu Z, Xie H. Mean intensity gradient: an effective global parameter for quality assessment of the speckle patterns used in digital image correlation. Opt Laser Eng 2010;48:469–77. [29] Reu P. All about speckles: speckle size measurement. Exp Tech 2014;38:1–2. [30] Mazzoleni P, Matta F, Zappa E, et al. Gaussian pre-filtering for uncertainty minimization in digital image correlation using numerically-designed speckle patterns. Opt Laser Eng 2015;66:19–33.
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Contents lists available at ScienceDirect
Engineering Fracture Mechanics journal homepage: www.elsevier.com/locate/engfracmech
Vision-based measurement of crack generation and evolution during static testing of concrete sleepers ⁎
Liu Rui , Emanuele Zappa, Andrea Collina Department of Mechanical Engineering, Politecnico di Milano, Milan, Italy
A R T IC LE I N F O
ABS TRA CT
Keywords: Crack propagation Concrete sleeper Digital image correlation Crack tracking technique
In this paper, vision-based measurement system is developed to monitor the crack generation during the static test of concrete sleepers. In particular three point bending test described in the European Standard, EN 13230-2 [1] is considered. This test is the acceptance tests for concrete sleepers, which are declared in the mentioned Standard as “safety critical components for railway applications”. In this ambit, the validation and the check of the reliability of crack opening technique is a crucial point for the safety of passengers and infrastructure. The mentioned Standard imposes to measure the width of the main crack appearing on the sleeper and track it during the test, but it does not prescribe the measuring technique to be used or the maximum uncertainty that can be accepted. Crack width is often estimated manually, with the help of a handheld microscope. The measuring uncertainty is strongly related to the experience of the operator. In this work, a vision-based measuring technique, based on digital image correlation (DIC) technique, is proposed and validated. In the proposed approach, the crack path is first detected automatically, relying on the strain map generated by DIC, and then the crack width is estimated, relying on the displacement map. Finally, the proposed measurement approach is compared with the state-of-the-art technique based on the manual analysis of images acquired with a handheld microscope. The results show that the crack width measured with the proposed approach is compatible with the data obtained by the manual technique. Thanks to the proposed full-field measurement, the crack width can be estimated not only in a few manually detected points along the crack, but for the full length of the crack with an automatic procedure. Moreover, the proposed technique can be applied in an unmanned way, since the camera does not need to be manually moved along the sleeper side to detect the crack: it is mounted in a fixed position and the image processing software automatically detects the crack path and width. The operator is therefore not requested to work close to the sleeper during the loading, with advantages also for the safety.
1. Introduction In ballasted railway tracks, the most widely adopted for railway lines, the sleepers have the main duty to transfer the axles loads from the rails to the ballast, keeping at the same time the correct gauge between the rails and their inclination. In order to assure their functionality sleepers must guarantee an adequate strength level, under quasi static, dynamic loads [1] as well as shock conditions [2,3]. To ensure the safety of passenger and infrastructures, the sleepers must guarantee a proper mechanical behavior, in particular
⁎
Corresponding author. E-mail address: [email protected] (L. Rui).
https://doi.org/10.1016/j.engfracmech.2019.106715 Received 5 January 2019; Received in revised form 18 September 2019; Accepted 7 October 2019 0013-7944/ © 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Liu Rui, Emanuele Zappa and Andrea Collina, Engineering Fracture Mechanics, https://doi.org/10.1016/j.engfracmech.2019.106715
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Fig. 1. Sleeper bending according to correct (a) and not correct (b and c) ballast packing under the sleeper (). adapted from [4]
in terms of crack generation and extreme loading before failure. The International Standard EN 13230-1:2009 ([1]) declares that the sleepers are “safety critical components for railway applications” and describes the acceptance tests for these components. The tests are based on three-points bending tests and crack detection and tracking. In this ambit, the validation and the check of the reliability of crack opening technique is a crucial point. Concrete sleepers are basically subjected to bending moment resulting from the vertical loads applied by the wheels and the reaction given by the continuous support provided by the ballast. Since ballast may be not uniformly distributed under the sleeper depending on its settlement in the long term, the following conditions can occur [4] [see Fig. 1]:
• In regular condition, the area under each rail seat is subjected to an almost constant distributed pressure, leading to a maximum •
bending moment in correspondence of the middle section of the rail seat and a relatively small bending moment in the central section of the sleeper (Fig. 1a) In the case that part of the pressure distribution is shifted outwards or inwards, an additional positive or a negative bending moment results in the central section of the sleeper (Figs. 1b and c).
The resistance of the sleeper is then defined in terms of bending resistance of the sections in correspondence of the rail seat and of the central section. In order to check the bending resistance of the section of the sleeper, the homologation procedure defined in the international standard [1] adopts a three-point bending test in order to create a bending moment in the considered section and check through static and fatigue testing the resistance of the sleeper. Considering that the sleeper is a composite structure (concrete and prestressed steel bars), three levels of bending moments are evaluated corresponding to three different status of stress in the section:
• first opening of a crack under applied load, corresponding to the end of elastic range • load corresponding to a prescribed residual opening of the crack, indicating the onset of plastic range • final failure, corresponding to the ultimate bending moment resistance. As for the first two levels, in the routine activity of homologation testing, optical comparator [5] or a portable microscope can be used, manually scanning the surface of the sleeper in the region where cracks are expected. Alternatively, a microscope connected to a camera enables to acquire an image of the crack, getting its opening from measurement performed on the acquired image. In scientific literature, more sophisticated means to measure displacement, deformation or crack opening on concrete beams and on sleepers are available. In particular, several measuring approaches are proposed in literature for these purposes. Gencturk developed a set of six LVDTs mounted on the surface of the specimen and measured the displacement of the point of the concrete specimen during the test and compared the results with DIC measured data [6]. Strain gauge is another conventional sensor to measure the strain of one point, especially at the critical region/discontinuities of engineering structures [7–9]. Purkiss used a strain gauge and a displacement gauge to compare the mechanical behavior of the fibre reinforced and unreinforced concrete beams [10]. Robins combined electrical strain gauges with semi-automated grid method to investigate the fracture of steel-fiber-reinforced sprayed concrete under bending test [11]. Obviously the point-measurement sensor does not allow to obtain the full-field measurement and it is difficult to obtain the crack propagation during the test, since the crack path is unpredictable. Recently the development of non-contact measurement techniques allowed a wide application of these devices for many fields of measurement, including the analysis of the mechanical behavior of concrete manufacts. In the early age, Haavik applied an optical comparator to measure crack width [12]. Dias-da-Costa developed a technique for the visualization of displacements, strains and cracks of the concrete by means of photogrammetry and image post-processing [13]. Anwander et al. used the laser speckle with a correlation data-processing procedure to successfully measure large mechanical strain of various materials at high temperature [14]. This technique is a potential candidate to estimate the evolution of the crack, by means of the analysis of the large deformation occurring during crack propagation. Bernstone developed a digital image analysis technique for the estimation of crack development in concrete dams and of the crack opening at the surface of the structure [15]. Rodríguez used distributed optical fiber to measure the crack width in the concrete, and the result was well exploited to calibrate FEM model [16]. Although the mentioned measurement techniques can provide reliable data on crack opening, a denser measuring technique would be desirable in order to monitor a large area of the sleeper surface and automatically detect the crack path and width. To this purpose, digital image correlation (DIC) is promising, since it provides a dense displacement and strain field. Choi successfully measured the non-uniform deformations of the surface of the concrete specimens by means of the full-field digital image correlation technique [17]. Biswal used DIC to measure the 2
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surface deformation, and further compute the crack reopening load accurately in concrete structures with prestressing force [18]. Fayyad applied DIC technique to monitor the crack propagation of reinforced concrete in the three-point bending test and quantified the fracture properties of the concrete by means of a high resolution camera [19]. In order to understand behavior of sand under complex loading condition, Salvatore combined DIC and X-ray microtomography to obtain the intensive strain localization near failure of specimens [20]. Cholostiakow applied DIC to study the development and degradation of shear resisting mechanisms in fiber reinforced polymer–reinforced concrete beams [21]. Kuntz realized an in-situ measurement of a shear crack in a reinforced concrete beam during a bridge static load test with DIC displacement measuring technique [22]. Based on DIC Hoult proposed a preliminary method to compute the crack width and slip in reinforced concrete beams [23]. He also suggested this technique can be used to measure the crack movement. Sabato built structural health monitoring system with 3D DIC measurement technique, and analyzed the railway structure. It can accurately monitor the condition of the structure to identify defects or damage [24]. In the current paper, an approach to run the static tests on sleepers described in [4] is proposed with the assistance of an automatic vision-based system that allows to estimate the crack position and opening with an automatic procedure, without the need to locate additional sensors displacement or strain sensors, still keeping the test set up arrangement requested by the standards. The goals of the proposed approach are: i. obtain the results with a deterministic procedure that does not depend on the operator’s skills and experience; ii. analyze the full area of cracking of the sleeper, and not only the region around the first crack. This allows to monitor the global phenomenon of crack generation and sleeper failure, providing more information on the response of these structures to mechanical loading; iii. track the growth and the opening of the cracks during the whole test, from crack generation to the final value of residual opening; iv. obtain a detailed representation of the phenomenon, allowing to deliver the data necessary to estimate also the energy associated with the sleeper failure, directly referring to the cracked area. The proposed measurement approach is based on image of acquisition and processing, using the Digital Image Correlation (DIC) technique [26] to estimate the full displacement and strain field of the area of interest. By means of processing the DIC data, the crack position and development is detected. The large amount of information that can be obtained with the proposed measuring approach is expected to be a useful support for the optimization of the design of both existing and innovative sleeper’s types. Moreover, the automatic and continuous monitoring of the cracks, can in perspective allow to run unmanned laboratory tests of the sleepers. The paper is organized through the following points:
• Recall of the steps of the static test for the rail seat section, to which the approach developed in this paper is applied • The set up for the DIC test, and all the steps carried out for its implementation • The image and data processing techniques and obtained results • Final remarks and lines of future work 2. Static tests on concrete sleeper in the international standard In this section a brief description of the standard procedure for sleeper testing will be provided, focusing on the static test under rail seat. Fig. 2 shows the equipment used in this work for standard three-point bending test of the sleeper. Different types of pretensioned sleepers can be tests with this set-up, with a number of reinforcement bars ranging from 2 to 8 and with some variability of the sizes. In Fig. 2 the main dimensions of the sleeper tested in this work are provided. Standards refer directly to the applied load and not the resulting applied bending moment, having prescribed the distance between the two supports. Although in this work the attention is mainly focused on the static tests prescribed for the rail seat section, the proposed measuring approach can be applied
Fig. 2. The three-point bending test of the sleeper at rail seat section (left: sketch of the test rig, right: example of set-up). 3
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Fig. 3. The static test procedure.
also for the tests at the centre section, recommended by the same standard. Fig. 3 shows loading procedure for the whole standard static test [1], which is composed of three stages. In the first stage the applied load is increased by steps of 10 kN, starting from an initial loading Fr0. The value of Fr0 depends on the class of the sleeper, defined by the standard. The prescribed loading rate is 120 kN/min. Every load value should be maintained for at least 10 s and no more than 5 min (time t in the figure). The load is increased until the first crack formation is visible in the bottom of the sleeper seat section. The load value that generates a visible crack is named Fr. When the first crack is detected, the load is removed and the first stage is complete. The condition corresponding to Fr defines the limit for the linear elastic range. The second stage is composed of a sequence of loading-unloading cycles, with increasing loading level (Fig. 3). The increment of the maximum loading between two subsequent cycles is 10 kN. After every loading-unloading cycle the crack width is measured in unloaded conditions: if the crack width is larger than 0.05 mm the test is completed, otherwise another loading cycle is applied. The load value that generates the residual crack width of 0.05 mm is named Fr0.05. Note that the duration of crack checking operation should not exceed 5 min, this is to be taken into account for real time processing of the data. The Fr0.05 load corresponds to the onset of plastic phase at a prescribed residual deformation. The third and final stage is aimed to estimate the final breaking value, reached in a single cycle test. Load is continuously increased until the final breaking value FrB reported in Fig. 3, is reached. The present paper deals with the first two stages, since stage 3 does not need any crack opening measurement. In the view of the standard, the key point of the measurement is to detect the crack position and measure its opening. The crack width is usually measured using an optical micrometer or a handheld digital microscope manually moved along the bottom of rail seat section, allowing the user to detect the crack. During the tests more than one crack might appear in different positions at the bottom of the rail seat section and the operator is responsible of detecting the main one. The main crack is defined in the European Standard EN13230-1 [25] as the first crack that propagates up to 15 mm from the bottom of the sleeper. 3. Experimental set-up and image processing technique 3.1. The digital image correlation technique Digital Image Correlation (DIC) technique is a contactless full-field measurement that enables to estimate the displacement and strain field of the specimen under test [26]. The possibility to obtain a full map of displacement and strain, instead of single point measurement along the whole crack, is one of the most relevant advantages of DIC with respect to most of the traditional techniques. Thanks to this characteristic, DIC is often applied in the measurements aimed at the qualification of the behavior of materials under stress. In DIC the displacement and strain fields of the region of interest are computed relying on an image correlation algorithm. The basic procedure is to acquire an image of the specimen before loading and an image of the loaded specimen; the first image is then divided into regions of interest (usually square), and each of them is correlated with the image of the specimen under loading to identify its displacement. As soon as the correlation is applied to all the regions of interest, the displacement map is estimated and the strain map can be computed by means of local numerical derivative of the displacement map [27]. To ensure a reliable image correlation, the specimen surface must have a random gray intensity distribution (speckle pattern). The speckle pattern can be the natural texture of the specimen surface or artificially made: in this application the speckle was generated using a stencil technique, as described in Section 3.3. The overall size of the field of view and the measurement resolution strongly depends on the hardware characteristics and on processing parameters: the camera resolution, the focal length of the optics, the size of the region of interests and so on. In this work the vision system and the specimen surface were optimized to guarantee a resolution of the measurement that allows to early detect the cracks and to measure the evolution of the width of the crack during the tests. DIC technique has a promising merit in the concrete fracture field. Due to the manufacture procedure of the concrete, stochastic voids and micro-cracks are always presented inside the concrete sleeper. This leads to the unpredictable crack growth and unpredictable crack path. Comparing with the conventional crack detection technique, DIC technique can capture the crack propagation more easily, since it allows to monitor the full seat section of the sleeper, therefore it allows to detect the crack wherever it appears. Because of these reasons, DIC technique is applied in this work to detect cracks and measure their width evolution for sleeper testing 4
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Fig. 4. (a) Vision-based measurement system and the sleeper under test; (b) The scheme of the measurement set-up; (c) The fastening system of the sleeper and the loading plate.
according to the standards [1]. 3.2. Experimental set-up A three-point bending test is conducted to analyze the crack propagation of the concrete specimen under static condition. Fig. 4(a) shows the real arrangement of the whole measurement set-up, and Fig. 4(b) shows the scheme of the measurement set-up. The whole set-up includes mainly loading and controller facility, concrete specimen and vision-based measurement system. The whole test procedure follows the international standard explained in Section 2. The sleeper under test is a commercially available RFI 260 type (2600 mm long), used for high speed network. During the tests, a rubber pad with characteristic prescribed by the standards is placed between the sleeper and each support, in order to equalize load distribution at the contact surfaces, as happens in the final application of the sleepers. The loading surface is fitted with the fastening system of the sleeper, connecting an articulated support that transfers the load from the actuator to the rail seat of the sleeper, instead of the rail (Fig. 4c). The standard of rail pad is included in the fastening device. The loading is imposed to the sleeper by means of a 1000 kN MTS hydraulic actuator. Cracks are supposed to originate from the bottom of the sleeper. As the loading increases, the crack propagates upward, progressively increasing its width at each loading step. The vision-based measurement system for the estimation of the crack width is composed of a camera, two lighting sources and a software for image acquisition and processing. As Fig. 4(a) shows, an IDS 5490SEM-GL camera is mounted in front of the concrete sleeper to acquire the image of the surface of the specimen. The adopted camera is equipped with a 3264x2448px CMOS sensor. The required size of the field of view is about 120x160mm, in order to cover the region of interests (ROI) marked with a red rectangle in Fig. 2. This ROI is the region where the main cracks are expected to appear during the tests. To obtain this field of view, it was necessary to select a wide angle optics (focal length 6 mm), since the maximum distance from the camera to the target is imposed by the main frame of the testing machine and is about 300 mm. With this hardware configuration the resolution of the image is around 0.05 mm/px. Speckle for DIC is produced on the sleeper surface as described in Section 3.3. The camera operation is fully controlled by software, developed in National Instruments Labview environment. During the test, the image acquisition frequency was set to 0.2 Hz. A set of LED lamps (see Fig. 4) were used and pointed to the target from two different directions to ensure a proper lighting of the specimen during the tests. 3.3. DIC speckle generation using the stencil technique The accuracy of the DIC results are strongly related to the characteristics of the speckle generated on the surface of the specimen [5]. Size and distribution of the speckles, as well as the contrast between the speckle and the background have to be optimized to improve the accuracy of the measurement. In particular, average speckle size should be around 3px (see [29]), corresponding, for this application, to about 0.15 mm. The conventional speckle producer based on spray technique is therefore not suitable in this work, because the spray technique produces speckles with average size well below 0.1 mm. To control the characteristics of the speckle, a stencil technique (see [30]) is applied here. In order to evaluate the quality of the speckle produced by means of the stencil technique, a widely accepted parameter is the Mean Intensity Gradient (MIG), proposed in [28]. MIG is estimated as the average value of the modulus of omnidirectional image gradient within the DIC pattern. The equation to compute the MIG value is: W
MIG =
H
∑ ∑ |∇f (xij)|/(W × H ) (1)
i=1 j=1
where W and H represents the image resolution, |∇f (x ij )| =
fx (x ij 5
)2
+ f y (x ij
)2
represents the modulus of local intensity gradient
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Fig. 5. One portion of the speckle of the specimen.
vector with the x- and y-directional intensity derivatives fx (x ij ) and f y (x ij ) at pixel ( x ij ). As can be seen in Eq. (1), the MIG parameter is proportional to the absolute value of the derivatives of image brightness, therefore it is function not only of the speckle size but also of the spackle spacing. Moreover, the use of the stencil technique, allows to keep the size of the speckles as uniform as possible and close to the ideal value of 3px. MIG has been computed for many speckle images, acquired during the tests on different beams, to obtain an indication of the speckle quality. The MIG values of these images is around 11, and it is reasonable for DIC analysis. Fig. 5 shows an example of a portion of the speckle generated on the specimen. 3.4. Optics aberration compensation The wide angle optics, needed to ensure the necessary field of view (as described in Section 3.2), generates a relevant image distortion, visible in Fig. 6a. This image distortion must be compensated for, prior to run the DIC analysis. To this purpose, a camera calibration procedure was applied, taking the image of a thin calibrator with a regular grid of circles mounted on the surface of the sleeper (Fig. 6a). The spacing between the circles was 20 mm, while their diameter was 10 mm. Based on the calibrator image, the parameters of the distortion model are computed and then compensated by applying a proper warping to the image. Before DIC analysis, all the images acquired during the test are compensated with the same warping. Fig. 6b and c show an example of the original image and the image after compensation respectively. In addition, the camera calibration allowed to estimate the scale factor of 0.05 mm/pixel, used to convert the DIC output from pixel to millimeters. 3.5. Crack opening measurement To run the DIC analysis, the commercial software VIC 2D was used. The subset size was set to 21 × 21 px, with overlap of 16px. The displacement and strain fields generated by DIC were further processed to obtain the information on crack path and opening. In particular, the strain map was used to estimate the path of the crack, since along the crack the strain assumes very large values. These large values are apparent (i.e. do not represent the actual strain of the sleeper); they are a consequence of the strain computation algorithm that does not account for the displacement discontinuity generated by the crack opening. As soon as the path of the crack is identified relying on the mentioned large strain values, the crack width is determined as the difference of the displacement of two
Fig. 6. Calibration and compensation of the image: (a) calibrator; (b) original image; (c) image after compensation. 6
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Fig. 7. The crack detection during the loading continuously stage.
points across the crack. As described in Section 2, the standard imposes two stages in the test: first stage involves a continuously increasing loading, while second stage is based on loading-unloading cycle. For the first stage, although several cracks generate in the field of view, the standard imposes to consider only the crack that firstly propagates vertically for 15 mm. The width of this crack must be monitored during the test. The procedure to measure the width of the crack relying on DIC data is (see also Fig. 7): 1. A reference line is defined, 15 mm from the bottom of the sleeper (thanks to the knowledge of the scaling factor, this line can be easily identified in the DIC data). 2. During the loading, the values of the horizontal strain along the reference line are extracted. If a crack propagates up to the reference line, a peak appears in the horizontal strain values (Fig. 8). However, due to the heterogeneity of the concrete specimen and the noise of the images, local high strain value can be found, leading to the risk of erroneously predicting the presence of a crack. In Fig. 8a a spurious crack is visible at the horizontal coordinate × around 4 mm, while the real crack is presented around x = 100 mm. Since spurious strain peaks are due to noise in the measurement, it was observed in different tests that, averaging the horizontal strain trends of different lines allow to remove spurious data. Therefore, to correctly detect the crack, we averaged the horizontal strain trends of all the lines in a stripe between the lines L1 and L2 (Fig. 7), which was heuristically defined as a proper size for averaging. 3. To distinguish the peaks of strain due to the presence of a crack from the ones due to measurement noise, a threshold value is needed. In this work, the threshold value was heuristically set to 0.003 m/m, analyzing the data at different loading steps. Since the detection is done on the strain profile estimated averaging on a 10 mm stripe (between L1 and L2) this threshold allowed to correctly detect the crack. The step 2 and 3 should be conducted until the crack is up to the reference line, and then the position of the crack is obtained. 4. The opening of the crack is estimated as the difference between the horizontal displacement uA and uB of two points (A and B in
(a)
(b)
Fig. 8. The distribution of the strain in the reference line in case of the force 208kN: (a) without average; (b) with average. 7
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Fig. 9. The crack detection during the loading-unloading stage.
Fig. 7) across the crack. The distance d between the crack and the points A and B cannot be too small to avoid that the DIC data are affected by the discontinuity induced by the presence of the crack. On the other hand, d should not be too large because displacement of points A and B have to be representative of the crack opening. It was experimentally estimated that a proper value of d in this application is 1.75 mm. To improve the signal to noise ratio in the estimation of uA and uB , we averaged the horizontal displacement values of all the points along two vertical lines: L3 and L4 respectively (see Fig. 7). The length of the vertical lines L3 and L4 is 2 mm since this was found to be a good tradeoff between the noise reduction and the need to ensure a local displacement measurement. In the second stage of the tests a sequence of loading-unloading cycles is applied until the width of the main crack in the unloading condition is beyond 0.05 mm. The European standard does not point out in which position of the main crack the width has to be checked, however in the first stage this crack is identified as the first one that reaches a length larger than 15 mm. Since in the second stage the crack is usually much longer than this limit, in our work we estimate the width of the crack as the mean opening in a stripe of 20 mm height: from a reference line to the bottom of the sleeper (Fig. 9). The procedure to measure the width of the main crack produced in the first stage includes the following steps: 1. A reference line is defined, 20 mm from the bottom of the sleeper (thanks to the knowledge of the scaling factor, this line can be easily identified in the DIC data). 2. In loading-unloading cycle, the opening of the crack must be measured along the full crack path between the reference line and the bottom of the sleeper. For every row, the cracked region is detected as the points where the horizontal strain is larger than a given threshold (in this application equal to 0.005 m/m). Horizontal position of the mid crack line is detected as the mid-point of the cracked region. 3. For each row, the opening of the crack is estimated as the difference between the horizontal displacement uC and uD of two points (C and D in Fig. 9) across the crack at a distance d1 from the mid-point of the cracked region. In this application d1 is set to 12.5 mm for the loaded condition, and to 1.25 mm for the unloaded one. The value of d1 is larger in the case of loaded sleeper since the width of the crack is much larger in this condition, therefore DIC shows a significant increase in the uncertainty of the data very close to the crack edges. 4. At the end of the previous steps, the horizontal displacement uC and uD is estimated for each point of the lines L5 and L6. The width of the crack can be considered to average the crack width of each point along the crack path by means of uC − uD . It should be noted that the EN13230 Standard ([1;25]) impose a strict procedure for the testing of concrete sleepers, and prescribes to tune the loading to obtain a main crack of given characteristics, in operating conditions the main crack is characterized by very stable characteristics (length and width). Therefore, the parameters for DIC data processing described in the points 3 and 4 of the procedure above, have a general validity in sleeper testing.
4. Test results and analysis This section shows some examples of data obtained with the measuring technique described in the previous section. The test was carried out following the routine manual procedure for crack width estimation and acquiring at the same time the images for DIC. The initial loading value Fr0 in the first stage prescribed by the standard was 168 kN for the sleeper under test. The Fr value corresponding to the appearance of the first eye-visible crack was obtained at a loading of 288 kN. In the second stage, the force value Fr0.05, corresponding to a residual crack opening of 0.05 mm, was 368 kN. For the manual estimation of the crack width, a Dyno Lite handheld microscope was used. The results of each part of the measurement are reported in the following sections.
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Fig. 10. Examples of exx strain maps at increasing loading levels.
4.1. Detection of the first crack and estimation of the crack width evolution 4.1.1. Results for strain The crack detection is based on the analysis of the strain map of the specimen, as described in Section 3.5. Fig. 10 shows some examples of the strain map in the horizontal direction (exx) at increasing loading levels. The crack path is visible as a region with high strain value. By means of the characteristic of the crack, the opening and the propagation of the crack can be captured in the strain map. In the test, there are more than one crack visible in the strain map. The crack that first crosses the reference line shown in Fig. 7 is considered as the main crack (see Section 3.5 for details). 4.1.2. Evolution of the width of the crack in the 1st stage The evolution of the crack width is estimated as described in Section 3.5, and it is shown together with the load in Fig. 11 (as a function of the time in the left, and width vs load in the right). The first level of the applied load is 168 kN and it is subsequently increased by 10 kN steps. Each step is retained for 20 s. In correspondence of an applied load of 228 kN, the rate of crack’s opening changes, indicating a modification of structural response at local level. At 288 kN, the crack becomes visible from visual inspection. At this load level, the width of the crack reaches its maximum value around 0.042 mm (see point A in Fig. 11). According to the European Standard [1], having reached the formation of a visible crack under loading condition, 1st stage is ended. During the unloading step from 288 kN to zero the width of the crack reduces to 0.009 mm (point B). Since the residual crack opening is lower than 0.05 mm, the second stage is activated. 4.2. Residual crack opening after each loading-unloading cycle 4.2.1. Evolution of the crack width in the 2nd stage In this stage, the crack detected in the 1st stage is focused. A series of loading-unloading cycles (as reported in Fig. 3) are applied
Fig. 11. Evolution of crack’s width and applied load during the 1st stage. 9
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Fig. 12. The exx map for the crack path detection at the third loading level (left) and after unloading (right).
until the residual opening of the crack at unloading condition reaches 0.05 mm. As described in Section 3.5, the opening of the crack is estimated the mean of the width in the stripe below a reference line (Fig. 9). With the manual measurement only a limited number of points along the crack can be measured. On the contrary with the DIC approach, a mean crack opening can be estimated, as described in the Section 3.5. Fig. 12 shows the path of the crack at the third loading-unloading cycle as an example of the application of the method to detect the crack path described in Section 3.5. Note that the map shown in Fig. 12 is only a portion of the full data map (see Fig. 10). The bottom of the figure corresponds to the bottom of the sleeper. The crack path can be clearly identified both in loaded and unloaded conditions, thanks to the high apparent strain values described in Section 3.5. Please, note that the color bar limits are different for the loaded and unloaded strain maps, since the apparent strain values are significantly larger in the loaded conditions. The black circles highlight the automatically estimated mid-crack path. Fig. 12b shows the crack path estimated in the case of unloaded sleeper. As soon as the crack path is identified, the width of the crack can be computed by means of the method presented in Section 3.5. The distance d1 from the points C and D the crack mid-line (Fig. 9) is set to 2 mm for the unloading conditions and is increased to 12.5 mm for the loading conditions, to avoid the low reliable DIC data measured in the vicinity of the crack, as explained in Section 3.5. The horizontal strain values are negligible close to the crack, because of the discontinuity due to the crack itself. It is therefore expected that the increase of the d1 value to 12.5 mm does not affect values of the estimated horizontal displacement significantly. This is confirmed by the data in Fig. 13b, where, for the largest loading values, the crack opening is estimated two times: with d1 = 6.25 mm and with d1 = 12.5 mm and the obtained results are compatible. Crack width is measured along the full crack path.
Fig. 13. The distribution of the width of the crack along the crack path: in the unloaded conditions (left) and in loaded conditions (right). The numbers in the legend correspond to the cycles of the 2nd stage. 10
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Fig. 14. Mean width of the crack and its standard deviation, for each loading cycle. Value of the load is indicated on the top of each step.
Fig. 14 shows the mean crack width and the corresponding standard deviation range for each loading/unloading cycle. The standard deviation values include two contributes: the trend of the crack opening along the crack path (opening is larger in the lower part of the sleeper, in particular in loading conditions) and the measuring uncertainty of DIC. The variability of the crack opening is clearly visible in Fig. 14, where the standard deviation of the estimated opening values is shown to be much larger in loading conditions. As the load of each step is increased, the mean width of the opening crack is progressively increased from around 0.05 mm to around 0.52 mm while the standard deviation of the estimated width is one order of magnitude lower of the width itself. As the cycle number progresses, the width of the opening crack after unloading increases from lower than 0.01 mm to around 0.05 mm due to the damage accumulation. As Fig. 14 shows, by means of DIC technique, in the unloading step of the 8th cycle, the average crack width is beyond the threshold value 0.05 mm. More details of the comparison between DIC technique and the microscope technique are presented in the following section. In order to have more insight into the DIC application to crack’s opening measurement, the width of the opening crack along the crack path is examined in both the loading and unloading condition (see Fig. 13). It is worth remarking that in each step, there are more than one images acquired and the full results are shown in Fig. 14. We point that the platform corresponding to each step means there is no obvious difference among the images. Therefore, only the result of one of them is displayed for clarity. Clearly with applied load the opening is larger and the uncertainties in crack’s profile detection are bigger. Moreover, it can be observed that, as expected, DIC technique allows to estimate the crack width all along the crack path. It should be noted that, during the test, more than one crack appears on the sleeper. However, as described in Section 2, the international Standard [1] impose to focus on the first crack that reaches a length of 15 mm (named main crack), since this is the crack that leads the failure of the sleeper in the vast majority of the tests. The long experience of one of the authors in sleepers testing confirms this evidence. Obviously the main crack can generate in any of the sides of the sleeper, therefore the crack monitoring, performed either with a manual approach or with the proposed automatic technique, has to be applied to both the sides. 4.2.2. Comparison between current practice and proposed techniques During the test, the usual microscope-based technique was applied together with DIC, in order to compare the results between the two approaches. For each unloading condition the handheld microscope is manually placed along the main crack. Fig. 15 shows examples of these images. Since it is not possible for the microscope to scan the whole crack in a unique image (the field of view is approximately 1.8 mm), only some positions chosen according to the experience of the operator are considered. Therefore, the positions where the crack opening is estimated are not necessarily the same for all the images. In addition, the crack width along the path is not perfectly uniform due to the anisotropic behavior of the concrete. Note that, to estimate the crack width, two points on the two edges of the crack are manually selected by the operator with mouse click, therefore the experience of the operator plays a crucial role in the measuring process. The operator was asked to measure the width in 20 different positions for each image acquired by the microscope in order to estimate the mean crack opening and the associated uncertainty, computed as the standard deviation of the 20 repeated measurements. Further, the mean crack width Wm and the standard deviation σm are expressed by the Eqs. (2) and (3):
Wm =
σm =
1 20
i = 20
1 19
∑
(x1i − x2i )2 + (y1i − y2i )2
(2)
i=1
i = 20
∑
( (x1i − x2i )2 + (y1i − y2i )2 − Wm)2
(3)
i=1
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Fig. 15. Portions of the main crack in unloading conditions after the following loading cycles: (a) 368 kN; (b) 378 kN; (c) 388 kN. Images acquired by means of the manual microscope.
where the coordinates (x1i , y1i ) and ( x2i , y2i ) represent the position of the point at the left edge of the crack and the right edge of the crack respectively. DIC technique obviously allows to obtain more dense information of the crack width, as shown in Fig. 13. DIC measurement results also demonstrate the non-uniform crack width along the crack path due to the property of the concrete. Therefore, the average crack width Wc along the path are considered as the crack width of the test, and the standard deviation σc of the measurement can be calculated as:
Wc =
σc =
1 n
i=n
∑ (uiC − uiD)
(4)
i=1
1 n−1
i = 20
∑
((uiC − uiD ) − Wc )2 (5)
i=1
where the symbols uC and uD have the same definition in Section 3.5. The symbol i represents the position of the crack along the path. Its size depends on the DIC measurement. The measures of the two techniques are shown in Fig. 16, where the mean crack width is shown together with error bars
Fig. 16. Comparison of the crack width in unloaded condition measured with the DIC technique and with the microscope. 12
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representing the standard deviation of repeated measurements. The results of the two techniques are compatible, even if for cycles from 5 to 7 the mean values estimated with the DIC are approximately 2 µm larger than the corresponding data from the microscope. This discrepancy may be due to the non-perfectly uniform crack opening along the crack path (as shown by DIC data in Fig. 13 left). While DIC averages the opening along the full crack path, microscope data is more local and could be more affected by the variability of the width. For both the techniques, the estimated average crack width is larger than 0.05 mm after the 8th loading/unloading cycle (i.e. after the loading cycle at 368 kN). The standard deviation of the crack opening along the crack path, estimated for the two techniques, assume values lower than 6 μm in the case of the experimental set-up proposed in this work. Note that, in loaded conditions, the standard deviation values represent the combination of the measuring uncertainty and the variability of the crack opening along the crack path, which is quite evident in the case of loaded sleeper (Fig. 13 right). In fact, in loading conditions, the width of the crack obviously increases from the reference line to the bottom of the sample, according to the mechanical behavior of the crack. On the contrary, in unloaded conditions the variability of the crack opening is limited (see Fig. 13 left), therefore in unloaded conditions the standard deviation provides a reasonable index of measuring uncertainty. 5. Concluding remarks This paper proposes the use of DIC technique to monitor the cracks propagation in concrete sleepers during laboratory qualification tests. The acceptance tests for concrete sleepers are crucial, since the sleepers are declared in the Standard [1] as “safety critical components for railway applications”. In this ambit, the validation and the check of the reliability of crack opening technique is a crucial point for the safety of passengers and infrastructure. The main goal of this work was the automatic crack detection and the measurement of crack opening as a function of the applied load. In the current practice crack opening is evaluated through an optical comparator or a handheld microscope, but these approaches need the manual detection of the first crack by means of an inspection of the operator and the manual positioning of the microscope every time the crack width has to be estimated (i.e. after each loading/ unloading cycle). The use of DIC instead of the manual technique gives several advantages. First of all, the camera is mounted before the beginning of the test, so the presence of the operator in the vicinity of the loaded sleeper is no longer required, avoiding any safety problem. Moreover, the crack position can be automatically detected during the first stage of the test and the crack width along the full crack path can be measured continuously and automatically during the whole test. Another advantage of the DIC-based measuring technique is that the measuring procedure is fully automatic, therefore the effect of operator’s skill is removed and the results are more objective. Declaration of Competing Interest Author declares that there is no conflicts of interest. References [1] Railway applications-track-concrete sleepers and bearers-Part 2: prestressed monoblock sleepers, European Standard, EN 13230-2. [2] Kaewunruen S, Remennikov AM. Progressive failure of prestressed concrete sleepers under multiple high-intensity impact loads. Eng Struct 2009;31:2460–73. [3] Kaewunruen S, Remennikov AM. Dynamic crack propagations in prestressed concrete sleepers in railway track systems subjected to severe impact loads. J Struct Eng 2010;136:749–54. [4] Esveld C. 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