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Cyclic deformation of aluminium alloys after the preliminary combined loading
EFA-02939; No of Pages 11 Engineering Failure Analysis xxx (2016) xxx–xxx
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Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Cyclic deformation of aluminium alloys after the preliminary combined loading Volodymyr Hutsaylyuk a,⁎, Lucjan Snieżek a, Mykola Chausov b, Janusz Torzewski a, Andrii Pylypenko b, Marcin Wachowski a a b
Military University of Technology, Gen. S. Kaliskiego Str. 2, 00-908 Warsaw, Poland National University of Life and Environmental Sciences of Ukraine, Geroiiv Oborony 12, 03041 Kyiv, Ukraine
a r t i c l e
i n f o
Article history: Received 12 November 2015 Received in revised form 12 May 2016 Accepted 15 July 2016 Available online xxxx Keywords: Aluminium alloys 2024-T351 and D16CzATW Pre-combined loading Cyclic deformation
a b s t r a c t This work explores sensitivity aluminium alloys under the influence of preliminary combined loading. There were examined 2024-T351 and D16ChATV aluminium alloys. The research was carried out under the conditions of tensile static and constant-amplitude fatigue tests on samples made of virgin material and after pre-combined loading. As a result of the research, it was concluded that the pre-combined loading leads to the modification of the mechanical properties of the tested materials. During the study reached similar results in increased strength for two aluminium alloys after the pre-combined loading, despite some differences in the structure of the output. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction In real operating conditions, the majority of structural elements and machine parts made of aluminium alloys are usually subjected to various types of loads. Almost always, these loads are complex. Conditionally, we can distinguish two basic types of their application. One type is preliminary load and another is the basic load. Real operating conditions make it quite difficult to separate their application into different stages. For most structures made of aluminium alloys, the basic load assumed as cyclic load. Despite the fact that the basic load largely determines the lifetime of the material, preliminary load also has a significant input on the lifetime of the material. In recent years, much attention was paid to research on maintaining mechanical properties of structural materials at high strain rate of ε = 103 … 106 s−1. At the same time, experimental tests have been conducted of the structure of materials that have been subjected to a high strain rate as a result of which, in the course of implementing deformation mechanisms, a new groups of structure elements were created (e.g. macro and micro groups) [1–6]. In order to describe the behaviour of materials under complex loads associated with a shock change in the strain rate, a large number of phenomenological models have been developed, allowing to perform calculations on the strength of structural elements with the shock change of the strain rate [7–11]. There are a number of studies that show that the preliminary load significantly affects the mechanical properties of the material and its strength. Special attention should be paid to the preliminary load applied by monotonic loading with an additional impulse component [12–15]. ⁎ Corresponding author. E-mail addresses: [email protected] (V. Hutsaylyuk), [email protected] (L. Snieżek), [email protected] (M. Chausov), [email protected] (J. Torzewski), [email protected] (A. Pylypenko), [email protected] (M. Wachowski).
http://dx.doi.org/10.1016/j.engfailanal.2016.07.002 1350-6307/© 2016 Elsevier Ltd. All rights reserved.
Please cite this article as: V. Hutsaylyuk, et al., Cyclic deformation of aluminium alloys after the preliminary combined ..., Engineering Failure Analysis (2016), http://dx.doi.org/10.1016/j.engfailanal.2016.07.002
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Such a combined load (monotonic tensile load + additional impulse load) applied in one direction, initiates some changes in the behaviour of the material. Under the influence of external factors, the relative balance of the “sample - testing machine” system is disturbed. This in turn causes the initiation in the material of processes hereinafter referred to as dynamic non-equilibrium processes (DNP). The purpose of these processes is to adapt the material to the new conditional balance in response to the value of the parameters of external influences and the conditions for their implementation. As a result of the application (DNP), two variants of the behaviour of the test material are obtained. The first is applied when under the respective conditions of combined load; external energy is completely absorbed by the material. This causes the creation of a new state of the structure, a so-called dissipative structure. The newly formed structure (state of material) obtains mechanical properties different from the input properties, corresponding to a new type of external loads. From the point of view of optimising the absorption of energy, the formation of this structure is optimal in a certain volume of the material. In turn, this should in some way reflect in the general (global) characteristics of mechanical properties of the material [1–4]. The second variant, where the material is not able to absorb adequate amounts of external energy, at the appropriate time, this causes an instantaneous failure regardless of whether the overall load level exceeds the threshold value. In recent years, synergistic concepts were developed in strength physics and fracture mechanics, with a multi-level character and fractal type of strain and failure of the structural materials of different classes. However, the scientific approach and methodological basis for studying the structure and the physico-mechanical properties of materials is not yet able to provide adequate descriptions of actual analytical processes. Determination of the impact of dynamic non- equilibrium processes on the change in mechanical properties of the material, especially fatigue strain and cracking at present is only possible by means of experimental tests. The aim of this work is to: \\ develop a method of experimental testing of the impact of combined preliminary load on the change of the material's mechanical properties; \\ determine the characteristics of material fatigue at strain after the applying preliminary combined load. 2. Material and test procedure Tests were carried out on samples taken from aluminium alloy D16ChATV and 2024-T351, plated on both sides, as delivered. The chemical composition and mechanical properties specified in the manufacturer's certificate are given in Table 1. Flat samples with dimensions as shown in Fig. 1 were cut in the rolling direction from a sheet with thickness of 3 mm. The test of the impact of additional impulse load was performed using a hydraulic machine ZD-100Pu equipped with a mechanical system to apply additional impulse load, as described in [2,3]. A general view of the test setup is shown in Fig. 2. The processes taking place during the deformation and failure of the material during shock load applied during the main load were examined based on the analysis of the applied mechanical load. The system, in the form of a static indeterminate structure, consists of three parallel elements: \\ The main element, which includes: spherical self-adjusting holders, dynamometer, strain gauge and a sample of the tested material; \\ Two lateral, symmetrical elements in the form of bars with brittle samples with separate dynamometers. After reaching a predetermined load level in the system, brittle samples are destroyed, which in turn causes additional impulse load on the sample. In order to ensure simultaneous failure of brittle samples, mounting flanges are combined transversely using four bolts tightened with a torque tool with controlled value. Additional pulse load was applied in elastic and inelastic areas of the static tensile test curve σ = f(ε), in the range ε = 0 … 9%. The time difference for fracture of brittle elements was in the range of 3 … 5 ms, and the difference is the fracture force load at the moment of fracture was less than 5–10%. The value of the additional load (impulse) was determined by corresponding cross-sectional diameter of brittle samples. Its value in the process of fracture for the each element measured via individual Table 1 Chemical composition and mechanical properties of aluminium alloys. Chemical composition [%]
D16ChATV 2024-T351
Cu
Mg
Mn
Fe
Zn
Si
Ti
Cr
Zn
4.4 3.82–4.04
1.4 1.75–1.77
0.63 0.56
0.18 0.08–0.18
0.16 0.16
0.11
0.07
0.01
0.01
Mechanical properties in the rolling direction
D16ChATV 2024-T351
Rm [MPa]
R02 [MPa]
E, GPa
A [%]
442–463 459–466
316–328 339–345
69.35
18.8–21.2 21.5–24.7
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Fig. 1. Sample for experimental tests.
dynamometer mounted at the end of each rod. Detailed shape and size of the brittle specimen was described in our previous articles [16,17]. The characteristics of the load system guarantee obtaining the predetermined material strain rate with a dynamic redistribution of stresses caused by the failure of brittle samples. As a result, there is a significant change in the energy balance during the process taking place at a high strain rate in the material. Most of the energy released during the failure of brittle samples dissipates in the tested material. This phenomenon results in initiating fundamental structural changes that result in “anomalous” plasticising of the material during all stages of deformation. Obtaining the described effect of plastic deformation of materials is possible only when applying additional restrictions on the course of load changes applied in a mechanical load system. These restrictions are formulated in the following way [14]: Basic load should be applied in at least three stages including: - a shock increase in the strain rate during the first stage; - a shock decrease in the strain rate during the second stage; - a repeated increase in the strain rate during the third stage. All three stages of the complex load are carried out continuously without pause to the loading process. Input impulses to the load system in the range of 100–1500 N s should be applied in less than 0.02 s.
Fig. 2. Setup for the application of additional impulse load: 1-tested sample; 2-brittle samples; 3- rods with dynamometers; 4-mounting flanges.
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During the experimental tests, the load must be transferred to the sample via the self-adjusting holders, without rigid attachment of the tested element. Test of samples under a static load was performed on the Instron 8802 servo hydraulic machine with the registration, respectively of the strain and load values. An extensometer with a 25 mm measurement base was used for measuring strain. Fatigue tests carried out under conditions with a cyclic load with the asymmetry coefficient R = 0.1. Load range was determined in suitability to the stresses in the range between the yield strength and the ultimate strength. Fatigue tests were implemented on the samples in the initial state, after the previous application of static load (up to strain level as in a combined load) and after impulse load. Tests performed until a complete failure. For each level of load (stress, strain) in all three states of the material (the original, after the prior monotone tension-loading, after prior combined loading) 3 to 5 samples were investigated. Basic series of samples for experimental studies were 20 to 25 pieces. For each level of loading (stress, strain) in all three states of the material (the original, after the prior monotone tension-loading, after prior combined loading) 3 to 5 specimens were investigated. Averaged experimental data based on the results of the 3 to 5 test samples are presented on Fig. 7. Realization of the prior combined loading is illustrated by the experimental results of some samples, which in the opinion of the authors most adequately characterize the material behaviour at this level of loading.
3. Results and discussion Results of tests were processed and presented in curves for samples made of aluminium alloy 2024-T351 and D16ChATV under static load and combined load and cyclic load. Fig. 3 shows the results of tests using static loads in the initial state. The mechanical properties of both alloys fully correspond to the manufacturer's certificates. Alloy 2024-T351 has a slightly higher value of yield stress and ultimate stress. This translates into plasticity. As is apparent from the graph, alloy D16ChATV has a higher plasticity compared to 2024-T351. On the other hand, in the picture section before the yield point, the behaviour of both alloys is almost identical. For this reason, it was decided to apply combined load (applying additional impulse force on the static component) on the section up to the yield point for both alloys. We tried to keep the level of overall pre-strain for each of the alloys at which the pulse component is applied at the same value. After applying the combined load, aluminium alloys were subjected to a static load again until failure Fig. 4. Application of pulsed load during the static tensile load test (the area marked by the dashed line in Fig. 4) causes the disturbance of straightness of the load section, changing the angle of inclination of the graph and the local jumps (vibration) of tension as well as deformation of both materials. This behaviour under a complex load is probably the result of a breach by the additional impulse load of the equilibrium in the material, leading to the initiation of dynamic non–equilibrium processes. The evidence of implementation of these processes is the time of realization of the combined load (dashed area on Figs. 4, 5) which is about 40– 55 ms until the comprehension of a given strain moment. Because the time of non-simultaneous fracture of brittle samples is about 1%, then authors believe that the processes shown in the dashed area reflect the dynamic non-equilibrium state of material initiated actually by the fracture of brittle samples. These processes are aimed at restoring the stable condition of the structure with respect to the new external load conditions. As a result of the absorption of energy supplied externally, the material is forced to adapt its own structure to the new state by creating its new forms with new, altered mechanical properties.
Fig. 3. Curves of static failure of the tested aluminium alloys.
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Fig. 4. Combined load and static failure of aluminium alloy 2024-T351 (a) and D16ChATV (b).
At the stage of applying the impulse load, alloy 2024-T351 Fig. 4a does presents a rather delicate weakening, maintaining the general levels of properties close to the base material. Secondary static load results in mechanical properties comparable to the initial state while increasing plasticity. Alloy D16ChATV Fig. 4b presents a much wider range of fluctuations of mechanical properties at the moment of applying an impulse load. Momentary strengthening and a relative stabilization section are clearly marked. Further static load causes a gentle increase in the mechanical properties compared to the combined load stage. Comparison of changes in the mechanical properties of the two alloys with the properties in the initial state is shown in Fig. 5. As can be seen in Fig. 5, the newly created “dissipative structure” contributes to global changes in the mechanical properties of the material. This phenomenon is evident in the second part of the graphs showing the failure of tested samples. Under the same conditions of static failure, the sample after applying a complex load exhibits greater plasticity than the sample after applying a static load. This applies to both alloys. In relation to the mechanical properties of the material in the initial state, at the stage of repeated loading alloy 2024-T351 Fig. 5a reaches almost the same values of the mechanical properties with an evident increase in plasticity. The repeated application of static load on alloy D16ChATV Fig. 5b, in addition to the increase in the plasticity, increases the mechanical properties relative to the material in the initial state. This increase roughly corresponds to the maximum momentary strengthening at the moment of application of additional impulse load. Despite the differences of mechanical properties in both alloys in the initial state, increase in plasticity after the repeated application of the load is comparable and is up to 10%. Analyzing the combined loads applied to the tested alloys (Fig. 6) it must be clearly stated that at the comparable preliminary strain and additional impulse force, the material demonstrates different behaviours in the course of applying dynamic nonequilibrium processes. For alloy 2024-T351 it rather means irregular oscillation of mechanical properties with minimal reduction of the value. D16ChATV actually performs the same process in two stages: a momentary strengthening and reduction of the value of a clear section of regular oscillation. This is probably related to the different mechanisms of implementation of strain at different structural levels of the material. Without a doubt, this requires additional fractographic tests.
Fig. 5. Static load of aluminium alloy 2024-T351 (a) and D16ChATV (b) in the initial state, combined load and repeated static load.
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Fig. 6. Combined load of aluminium alloy 2024-T351 and D16ChATV, repeated and static load.
This results in increased plasticity at the stage of repeated plastic load and is as a common feature for the two alloys. Alloy D16ChATV additionally strengthens, even achieving higher mechanical properties than alloy 2024-T351. As a result, very interesting is the behaviour of the tested materials under conditions of cyclic loading after the application of initial combined load. Alloys 2024-T351 and D16ChATV were tested in initial state, after the application of initial static load and combined load. The level of initial load for the static and the combined load was the same. The test results for individual alloys are shown in Fig. 7. A common feature of the tested alloys is an increase in strength relative to the initial state after the application of combined preliminary load. The relatively large increase in the mechanical characteristics was associated with the fact that the structure established during the application of combined preliminary load is permanent and does not relax in its entirety. Shifting the line relative to the strength after the previous application of static load in turn proves that the structural changes were implemented in the whole volume of the material at all structural levels. The increase in strength after the initial static load is reasonably justified by the changes generally taking place at the microstructural level. A detailed answer to these questions will only be possible after a fractographic analysis. Combined load of alloy 2024-T351, despite the fact that it shifts the graphs to the right, increases its inclination and as a result accelerated the decline in strength. For alloy D16ChATV, strength after the combined preliminary load is almost parallel to the initial state with an overall shift to the right. And the strength line after the initial static load behaves similarly to the line of combined load for alloy 2024-T351. Also, for alloy D16ChATV, preliminary static load slightly increases strength. To compare the behaviour of two materials, we use graph lines obtained based on averaged data from strength tests Fig. 8. Despite the fact that alloy D16ChATV showed an increase of the mechanical properties at the stage of repeated static load, however the strength characteristics are lower compared to 2024-T351. In the initial state both alloys are at a similar level. The preliminary static load causes the difference in the strength of alloys. Its impact is most noticeable in 2024-T351. Placing
Fig. 7. Fatigue tests of aluminium alloy 2024-T351 (a) and D16ChATV (b) in various states.
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Fig. 8. Fatigue tests of aluminium alloy 2024-T351 and D16ChATV in various states (no experimental data).
the strength graph almost parallel to the graphs for alloys 2024-T351 and D16ChATV in initial state allows us to conclude about the similar volume character of structural changes at all levels, despite a certain difference in the initial structure. On the other hand, the impact of the initial structure is likely to cause differences in the behaviour of the lines after the initial static load. Fractographic tests were conducted on the fracture surfaces of the samples of aluminium alloy 2024-T351 and D16ChATV after applying high-cycle load in the initial state and after the preliminary application of combined load. For the both alloys after the different states of load a selected surface regions of fracture across the whole surface of the cross section of the sample were analyzed. Analyzed areas schematically marked on Fig. 9 of the sample fracture surface of one of the alloys. Fig. 9a shows a diagram analysis of aluminium alloy 2024-T351 in the initial state (refer to circles) and after previous combined load (refer to rectangles). And Fig. 9b respectively representing the scheme analysis of D16ChATV aluminium alloy in the initial state and after previous combined load (refer pentagon). Since the area of fracture surfaces for the alloy D16ChATV in both states are located almost close then it was decide to take one form for indication. Letter notation corresponds to a photo with selected regions of fracture in Figs. 10-13. The fracture surfaces were studied under magnification of ×250 to ×2500. Fracture surfaces of samples of aluminium alloy 2024-T351 destroyed at a high-cycle load in the initial state were analyzed first. The surface of the basic material in analyzed areas is diverse: with pits, traces of grain groups break out, local areas with crumpled surface. At the boundary of the material and the plated coating, areas of grain group break out with traces of plastic strain on the bottom of the cavity could be observed in Fig. 10. The topography of the surface in Fig. 10 is complex; there are visible traces of break outs after the destruction of grain groups and a large number of pits of different shapes and sizes. The bodies of ridges caused by destroying various types of inclusions gain a “foamy” structure. The surface is characterised by the facets with the relevant smooth surface of the planes Fig. 10b and the directivity of the pits and grooves. Despite its resemblance to local cleavage failure, the authors assume this as evidence of plastic strain characteristic of an offset. Fig. 10c, despite the signs of offsets, shows bands, crystallographic planes and secondary fractures that are not developed or are in an embryonic state. On the other hand, in the cavities in Fig. 10d, especially in the area of large inclusions, smooth areas are clearly seen as well as clear cracks connecting the cavities after inclusions and sharp torn edges of the ridges where secondary cracks are also visible.
Fig. 9. Schematics of region location after fractographical research of fracture surface of aluminium alloys 2024-T351 and D16ChATV: а – aluminium alloy 2024T351 in the initial state (marked by circles) and after previous combined load (marked by rectangle); b- aluminium alloy D16ChATV in the initial state (marked by circles) and after previous combined load (marked by pentagon);
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Fig. 10. The areas of local surface fracture of the basic material of aluminium alloy 2024-T351.
Analysis of these images allows us to draw conclusions about the fact that the failure of the material is dominated by plastic strain implemented by different mechanisms. Also, at different levels of the scale, brittle failure is implemented; however its role is secondary. The failure surface of aluminium alloy 2024-T351 after the previous combined load is shown in Fig. 11. A general view of the failure surface in Fig. 11 lets us immediately notice a certain fragmentation and alternation of the areas with the dominance of the mechanisms of ductile and brittle failure. Areas of plastic strain in Fig. 11b present directivity and traces of offsets, occupy a large space in relation to the surface of the image and the fractured surface. The “foam” structure Fig. 11c has different sizes of voids; a single element has both large and small voids. The tops of the ridges show no plate. Minimal plastic strain areas are located on the lateral surfaces of facets in the vicinity of the inclusions, Fig. 11d. Analysis of the surface in the vicinity of holes after the inclusions also shows the change in the nature of failure. The “foam” structure disappears in the pits in Fig. 11. We have homogeneous surfaces with round pits, secondary cracks and other inclusions torn from the surface. As is apparent from Fig. 11f, secondary fractures are located at the base of the ridge and are not always created from the initiator in the form of a pit or inclusion, but develop more on the borders of crystallographic planes. The analysis of aluminium alloy D16ChATV will be carried out in a similar way as it is tested under the same conditions and at the same load. Characteristic pictures of the failure surface are shown in Fig. 12. White bands in Fig. 12a, b are presented using facets with clear traces of the impact of plastic strain. They are formed in the vicinity of voids or break out of separate grains. Local realization of elements of brittle fracture forms surfaces in the vicinity of the pits, merging cavities (Fig. 12c, d), and initiates secondary cracks. In general, plastic strain dominates in case of aluminium alloy D16ChATV in the initial state at cyclic loads, and brittle failure plays a secondary role almost to the breaking point. Analyzing the failure surface of aluminium alloy D16ChATV after the preliminary application of combined load Fig. 13 compared with the previous static load a significant surface changes are not observed. The location in the local area of the base material Fig. 13a allowed to determine changes in the failure process. Plastic strain is present on separate facets scattered across the surface, ridges on the boundaries of the grain break outs are higher and uniform. The area at the bottom of the cells is smooth, with no signs of plastic strain Fig. 13b. Inclusions fell off in most cases, however a few places were observed where they remained in place after failure. Cracks were observed on the failure surface Fig. 13c. They develop by connecting the pits and voids within the same area after grains are broken out as well as between them, on the boundaries of crystalline planes. In Fig. 13d, we can see that the structures of ridges and pits differ. The ridges present a structure with images on the surface in conjunction with the round form of pits. However, we cannot conclude that the structure is “foamy”, in the middle it is obviously uniform. Embryonic rupture on the large cavity after inclusion might also be explained by the level of tension that was almost two times smaller compared to the previous samples. Please cite this article as: V. Hutsaylyuk, et al., Cyclic deformation of aluminium alloys after the preliminary combined ..., Engineering Failure Analysis (2016), http://dx.doi.org/10.1016/j.engfailanal.2016.07.002
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Fig. 11. The failure area surface of a sample of aluminium alloy 2024-T351 after the preliminary application of combined load.
The analysis of the sample indicates that the failure is achieved using a complex failure mechanism with the participation of both components: plastic failure and brittle failure, with the dominance of each at different stages of failure. 4. Conclusions Based on the tests carried out, we can draw further conclusions: 1. Under the conditions of combined load, processes take place in the elements of the mechanical load system of the tested samples, related to the shock exchange of energy, leading to the modification of the mechanical properties of the tested material. During the application of complex loads involving single impulse loads, plasticising zones of various range appear in the material of the tested elements made of aluminium alloys, which revealed during the repeated static load. 2. The increase in the plasticity of alloy 2024-Т351 and the change of the mechanical properties of alloy D16ChATV, observed after the application of the additional impulse load, depends on the type of preliminary strain and the nature of the dynamic non-equilibrium processes. 3. The dissipative structure or the new state of the material resulting from the combined preliminary load is permanent, it does not relax entirely upon releasing the load. It has a decisive impact on the strength properties of the tested alloys and contributes to their increase. 4. Achieving similar results of increased strength for alloy 2024-T351 and D16ChATV after the preliminary application of combined load proves the similar character of the final volume of structural changes at all levels, despite some differences Please cite this article as: V. Hutsaylyuk, et al., Cyclic deformation of aluminium alloys after the preliminary combined ..., Engineering Failure Analysis (2016), http://dx.doi.org/10.1016/j.engfailanal.2016.07.002
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Fig. 12. Failure area surface of the samples of aluminium alloy D16ChATV in initial state.
in the initial structure. On the other hand, the differences clearly visible during the application of additional impulse load, may be caused by the various participation of the deformation processes on the structural level in the overall process of deformation.
Fig. 13. Failure surface of the samples of aluminium alloy D16ChATV after preliminary application of combined load.
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5. The fractographic studies of aluminium alloy 2024 -T351 and D16ChATV in different states confirmed the link between the dissipative and original structures of material. A contribution of a dissipative structure that formed after the previous combined load to the fracture of the material was noted. On this basis, it is stated also the opportunity to explain the behaviour of macro-properties of materials based on realization of corresponding mechanisms of fracture.
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Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Cyclic deformation of aluminium alloys after the preliminary combined loading Volodymyr Hutsaylyuk a,⁎, Lucjan Snieżek a, Mykola Chausov b, Janusz Torzewski a, Andrii Pylypenko b, Marcin Wachowski a a b
Military University of Technology, Gen. S. Kaliskiego Str. 2, 00-908 Warsaw, Poland National University of Life and Environmental Sciences of Ukraine, Geroiiv Oborony 12, 03041 Kyiv, Ukraine
a r t i c l e
i n f o
Article history: Received 12 November 2015 Received in revised form 12 May 2016 Accepted 15 July 2016 Available online xxxx Keywords: Aluminium alloys 2024-T351 and D16CzATW Pre-combined loading Cyclic deformation
a b s t r a c t This work explores sensitivity aluminium alloys under the influence of preliminary combined loading. There were examined 2024-T351 and D16ChATV aluminium alloys. The research was carried out under the conditions of tensile static and constant-amplitude fatigue tests on samples made of virgin material and after pre-combined loading. As a result of the research, it was concluded that the pre-combined loading leads to the modification of the mechanical properties of the tested materials. During the study reached similar results in increased strength for two aluminium alloys after the pre-combined loading, despite some differences in the structure of the output. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction In real operating conditions, the majority of structural elements and machine parts made of aluminium alloys are usually subjected to various types of loads. Almost always, these loads are complex. Conditionally, we can distinguish two basic types of their application. One type is preliminary load and another is the basic load. Real operating conditions make it quite difficult to separate their application into different stages. For most structures made of aluminium alloys, the basic load assumed as cyclic load. Despite the fact that the basic load largely determines the lifetime of the material, preliminary load also has a significant input on the lifetime of the material. In recent years, much attention was paid to research on maintaining mechanical properties of structural materials at high strain rate of ε = 103 … 106 s−1. At the same time, experimental tests have been conducted of the structure of materials that have been subjected to a high strain rate as a result of which, in the course of implementing deformation mechanisms, a new groups of structure elements were created (e.g. macro and micro groups) [1–6]. In order to describe the behaviour of materials under complex loads associated with a shock change in the strain rate, a large number of phenomenological models have been developed, allowing to perform calculations on the strength of structural elements with the shock change of the strain rate [7–11]. There are a number of studies that show that the preliminary load significantly affects the mechanical properties of the material and its strength. Special attention should be paid to the preliminary load applied by monotonic loading with an additional impulse component [12–15]. ⁎ Corresponding author. E-mail addresses: [email protected] (V. Hutsaylyuk), [email protected] (L. Snieżek), [email protected] (M. Chausov), [email protected] (J. Torzewski), [email protected] (A. Pylypenko), [email protected] (M. Wachowski).
http://dx.doi.org/10.1016/j.engfailanal.2016.07.002 1350-6307/© 2016 Elsevier Ltd. All rights reserved.
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Such a combined load (monotonic tensile load + additional impulse load) applied in one direction, initiates some changes in the behaviour of the material. Under the influence of external factors, the relative balance of the “sample - testing machine” system is disturbed. This in turn causes the initiation in the material of processes hereinafter referred to as dynamic non-equilibrium processes (DNP). The purpose of these processes is to adapt the material to the new conditional balance in response to the value of the parameters of external influences and the conditions for their implementation. As a result of the application (DNP), two variants of the behaviour of the test material are obtained. The first is applied when under the respective conditions of combined load; external energy is completely absorbed by the material. This causes the creation of a new state of the structure, a so-called dissipative structure. The newly formed structure (state of material) obtains mechanical properties different from the input properties, corresponding to a new type of external loads. From the point of view of optimising the absorption of energy, the formation of this structure is optimal in a certain volume of the material. In turn, this should in some way reflect in the general (global) characteristics of mechanical properties of the material [1–4]. The second variant, where the material is not able to absorb adequate amounts of external energy, at the appropriate time, this causes an instantaneous failure regardless of whether the overall load level exceeds the threshold value. In recent years, synergistic concepts were developed in strength physics and fracture mechanics, with a multi-level character and fractal type of strain and failure of the structural materials of different classes. However, the scientific approach and methodological basis for studying the structure and the physico-mechanical properties of materials is not yet able to provide adequate descriptions of actual analytical processes. Determination of the impact of dynamic non- equilibrium processes on the change in mechanical properties of the material, especially fatigue strain and cracking at present is only possible by means of experimental tests. The aim of this work is to: \\ develop a method of experimental testing of the impact of combined preliminary load on the change of the material's mechanical properties; \\ determine the characteristics of material fatigue at strain after the applying preliminary combined load. 2. Material and test procedure Tests were carried out on samples taken from aluminium alloy D16ChATV and 2024-T351, plated on both sides, as delivered. The chemical composition and mechanical properties specified in the manufacturer's certificate are given in Table 1. Flat samples with dimensions as shown in Fig. 1 were cut in the rolling direction from a sheet with thickness of 3 mm. The test of the impact of additional impulse load was performed using a hydraulic machine ZD-100Pu equipped with a mechanical system to apply additional impulse load, as described in [2,3]. A general view of the test setup is shown in Fig. 2. The processes taking place during the deformation and failure of the material during shock load applied during the main load were examined based on the analysis of the applied mechanical load. The system, in the form of a static indeterminate structure, consists of three parallel elements: \\ The main element, which includes: spherical self-adjusting holders, dynamometer, strain gauge and a sample of the tested material; \\ Two lateral, symmetrical elements in the form of bars with brittle samples with separate dynamometers. After reaching a predetermined load level in the system, brittle samples are destroyed, which in turn causes additional impulse load on the sample. In order to ensure simultaneous failure of brittle samples, mounting flanges are combined transversely using four bolts tightened with a torque tool with controlled value. Additional pulse load was applied in elastic and inelastic areas of the static tensile test curve σ = f(ε), in the range ε = 0 … 9%. The time difference for fracture of brittle elements was in the range of 3 … 5 ms, and the difference is the fracture force load at the moment of fracture was less than 5–10%. The value of the additional load (impulse) was determined by corresponding cross-sectional diameter of brittle samples. Its value in the process of fracture for the each element measured via individual Table 1 Chemical composition and mechanical properties of aluminium alloys. Chemical composition [%]
D16ChATV 2024-T351
Cu
Mg
Mn
Fe
Zn
Si
Ti
Cr
Zn
4.4 3.82–4.04
1.4 1.75–1.77
0.63 0.56
0.18 0.08–0.18
0.16 0.16
0.11
0.07
0.01
0.01
Mechanical properties in the rolling direction
D16ChATV 2024-T351
Rm [MPa]
R02 [MPa]
E, GPa
A [%]
442–463 459–466
316–328 339–345
69.35
18.8–21.2 21.5–24.7
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Fig. 1. Sample for experimental tests.
dynamometer mounted at the end of each rod. Detailed shape and size of the brittle specimen was described in our previous articles [16,17]. The characteristics of the load system guarantee obtaining the predetermined material strain rate with a dynamic redistribution of stresses caused by the failure of brittle samples. As a result, there is a significant change in the energy balance during the process taking place at a high strain rate in the material. Most of the energy released during the failure of brittle samples dissipates in the tested material. This phenomenon results in initiating fundamental structural changes that result in “anomalous” plasticising of the material during all stages of deformation. Obtaining the described effect of plastic deformation of materials is possible only when applying additional restrictions on the course of load changes applied in a mechanical load system. These restrictions are formulated in the following way [14]: Basic load should be applied in at least three stages including: - a shock increase in the strain rate during the first stage; - a shock decrease in the strain rate during the second stage; - a repeated increase in the strain rate during the third stage. All three stages of the complex load are carried out continuously without pause to the loading process. Input impulses to the load system in the range of 100–1500 N s should be applied in less than 0.02 s.
Fig. 2. Setup for the application of additional impulse load: 1-tested sample; 2-brittle samples; 3- rods with dynamometers; 4-mounting flanges.
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During the experimental tests, the load must be transferred to the sample via the self-adjusting holders, without rigid attachment of the tested element. Test of samples under a static load was performed on the Instron 8802 servo hydraulic machine with the registration, respectively of the strain and load values. An extensometer with a 25 mm measurement base was used for measuring strain. Fatigue tests carried out under conditions with a cyclic load with the asymmetry coefficient R = 0.1. Load range was determined in suitability to the stresses in the range between the yield strength and the ultimate strength. Fatigue tests were implemented on the samples in the initial state, after the previous application of static load (up to strain level as in a combined load) and after impulse load. Tests performed until a complete failure. For each level of load (stress, strain) in all three states of the material (the original, after the prior monotone tension-loading, after prior combined loading) 3 to 5 samples were investigated. Basic series of samples for experimental studies were 20 to 25 pieces. For each level of loading (stress, strain) in all three states of the material (the original, after the prior monotone tension-loading, after prior combined loading) 3 to 5 specimens were investigated. Averaged experimental data based on the results of the 3 to 5 test samples are presented on Fig. 7. Realization of the prior combined loading is illustrated by the experimental results of some samples, which in the opinion of the authors most adequately characterize the material behaviour at this level of loading.
3. Results and discussion Results of tests were processed and presented in curves for samples made of aluminium alloy 2024-T351 and D16ChATV under static load and combined load and cyclic load. Fig. 3 shows the results of tests using static loads in the initial state. The mechanical properties of both alloys fully correspond to the manufacturer's certificates. Alloy 2024-T351 has a slightly higher value of yield stress and ultimate stress. This translates into plasticity. As is apparent from the graph, alloy D16ChATV has a higher plasticity compared to 2024-T351. On the other hand, in the picture section before the yield point, the behaviour of both alloys is almost identical. For this reason, it was decided to apply combined load (applying additional impulse force on the static component) on the section up to the yield point for both alloys. We tried to keep the level of overall pre-strain for each of the alloys at which the pulse component is applied at the same value. After applying the combined load, aluminium alloys were subjected to a static load again until failure Fig. 4. Application of pulsed load during the static tensile load test (the area marked by the dashed line in Fig. 4) causes the disturbance of straightness of the load section, changing the angle of inclination of the graph and the local jumps (vibration) of tension as well as deformation of both materials. This behaviour under a complex load is probably the result of a breach by the additional impulse load of the equilibrium in the material, leading to the initiation of dynamic non–equilibrium processes. The evidence of implementation of these processes is the time of realization of the combined load (dashed area on Figs. 4, 5) which is about 40– 55 ms until the comprehension of a given strain moment. Because the time of non-simultaneous fracture of brittle samples is about 1%, then authors believe that the processes shown in the dashed area reflect the dynamic non-equilibrium state of material initiated actually by the fracture of brittle samples. These processes are aimed at restoring the stable condition of the structure with respect to the new external load conditions. As a result of the absorption of energy supplied externally, the material is forced to adapt its own structure to the new state by creating its new forms with new, altered mechanical properties.
Fig. 3. Curves of static failure of the tested aluminium alloys.
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Fig. 4. Combined load and static failure of aluminium alloy 2024-T351 (a) and D16ChATV (b).
At the stage of applying the impulse load, alloy 2024-T351 Fig. 4a does presents a rather delicate weakening, maintaining the general levels of properties close to the base material. Secondary static load results in mechanical properties comparable to the initial state while increasing plasticity. Alloy D16ChATV Fig. 4b presents a much wider range of fluctuations of mechanical properties at the moment of applying an impulse load. Momentary strengthening and a relative stabilization section are clearly marked. Further static load causes a gentle increase in the mechanical properties compared to the combined load stage. Comparison of changes in the mechanical properties of the two alloys with the properties in the initial state is shown in Fig. 5. As can be seen in Fig. 5, the newly created “dissipative structure” contributes to global changes in the mechanical properties of the material. This phenomenon is evident in the second part of the graphs showing the failure of tested samples. Under the same conditions of static failure, the sample after applying a complex load exhibits greater plasticity than the sample after applying a static load. This applies to both alloys. In relation to the mechanical properties of the material in the initial state, at the stage of repeated loading alloy 2024-T351 Fig. 5a reaches almost the same values of the mechanical properties with an evident increase in plasticity. The repeated application of static load on alloy D16ChATV Fig. 5b, in addition to the increase in the plasticity, increases the mechanical properties relative to the material in the initial state. This increase roughly corresponds to the maximum momentary strengthening at the moment of application of additional impulse load. Despite the differences of mechanical properties in both alloys in the initial state, increase in plasticity after the repeated application of the load is comparable and is up to 10%. Analyzing the combined loads applied to the tested alloys (Fig. 6) it must be clearly stated that at the comparable preliminary strain and additional impulse force, the material demonstrates different behaviours in the course of applying dynamic nonequilibrium processes. For alloy 2024-T351 it rather means irregular oscillation of mechanical properties with minimal reduction of the value. D16ChATV actually performs the same process in two stages: a momentary strengthening and reduction of the value of a clear section of regular oscillation. This is probably related to the different mechanisms of implementation of strain at different structural levels of the material. Without a doubt, this requires additional fractographic tests.
Fig. 5. Static load of aluminium alloy 2024-T351 (a) and D16ChATV (b) in the initial state, combined load and repeated static load.
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Fig. 6. Combined load of aluminium alloy 2024-T351 and D16ChATV, repeated and static load.
This results in increased plasticity at the stage of repeated plastic load and is as a common feature for the two alloys. Alloy D16ChATV additionally strengthens, even achieving higher mechanical properties than alloy 2024-T351. As a result, very interesting is the behaviour of the tested materials under conditions of cyclic loading after the application of initial combined load. Alloys 2024-T351 and D16ChATV were tested in initial state, after the application of initial static load and combined load. The level of initial load for the static and the combined load was the same. The test results for individual alloys are shown in Fig. 7. A common feature of the tested alloys is an increase in strength relative to the initial state after the application of combined preliminary load. The relatively large increase in the mechanical characteristics was associated with the fact that the structure established during the application of combined preliminary load is permanent and does not relax in its entirety. Shifting the line relative to the strength after the previous application of static load in turn proves that the structural changes were implemented in the whole volume of the material at all structural levels. The increase in strength after the initial static load is reasonably justified by the changes generally taking place at the microstructural level. A detailed answer to these questions will only be possible after a fractographic analysis. Combined load of alloy 2024-T351, despite the fact that it shifts the graphs to the right, increases its inclination and as a result accelerated the decline in strength. For alloy D16ChATV, strength after the combined preliminary load is almost parallel to the initial state with an overall shift to the right. And the strength line after the initial static load behaves similarly to the line of combined load for alloy 2024-T351. Also, for alloy D16ChATV, preliminary static load slightly increases strength. To compare the behaviour of two materials, we use graph lines obtained based on averaged data from strength tests Fig. 8. Despite the fact that alloy D16ChATV showed an increase of the mechanical properties at the stage of repeated static load, however the strength characteristics are lower compared to 2024-T351. In the initial state both alloys are at a similar level. The preliminary static load causes the difference in the strength of alloys. Its impact is most noticeable in 2024-T351. Placing
Fig. 7. Fatigue tests of aluminium alloy 2024-T351 (a) and D16ChATV (b) in various states.
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Fig. 8. Fatigue tests of aluminium alloy 2024-T351 and D16ChATV in various states (no experimental data).
the strength graph almost parallel to the graphs for alloys 2024-T351 and D16ChATV in initial state allows us to conclude about the similar volume character of structural changes at all levels, despite a certain difference in the initial structure. On the other hand, the impact of the initial structure is likely to cause differences in the behaviour of the lines after the initial static load. Fractographic tests were conducted on the fracture surfaces of the samples of aluminium alloy 2024-T351 and D16ChATV after applying high-cycle load in the initial state and after the preliminary application of combined load. For the both alloys after the different states of load a selected surface regions of fracture across the whole surface of the cross section of the sample were analyzed. Analyzed areas schematically marked on Fig. 9 of the sample fracture surface of one of the alloys. Fig. 9a shows a diagram analysis of aluminium alloy 2024-T351 in the initial state (refer to circles) and after previous combined load (refer to rectangles). And Fig. 9b respectively representing the scheme analysis of D16ChATV aluminium alloy in the initial state and after previous combined load (refer pentagon). Since the area of fracture surfaces for the alloy D16ChATV in both states are located almost close then it was decide to take one form for indication. Letter notation corresponds to a photo with selected regions of fracture in Figs. 10-13. The fracture surfaces were studied under magnification of ×250 to ×2500. Fracture surfaces of samples of aluminium alloy 2024-T351 destroyed at a high-cycle load in the initial state were analyzed first. The surface of the basic material in analyzed areas is diverse: with pits, traces of grain groups break out, local areas with crumpled surface. At the boundary of the material and the plated coating, areas of grain group break out with traces of plastic strain on the bottom of the cavity could be observed in Fig. 10. The topography of the surface in Fig. 10 is complex; there are visible traces of break outs after the destruction of grain groups and a large number of pits of different shapes and sizes. The bodies of ridges caused by destroying various types of inclusions gain a “foamy” structure. The surface is characterised by the facets with the relevant smooth surface of the planes Fig. 10b and the directivity of the pits and grooves. Despite its resemblance to local cleavage failure, the authors assume this as evidence of plastic strain characteristic of an offset. Fig. 10c, despite the signs of offsets, shows bands, crystallographic planes and secondary fractures that are not developed or are in an embryonic state. On the other hand, in the cavities in Fig. 10d, especially in the area of large inclusions, smooth areas are clearly seen as well as clear cracks connecting the cavities after inclusions and sharp torn edges of the ridges where secondary cracks are also visible.
Fig. 9. Schematics of region location after fractographical research of fracture surface of aluminium alloys 2024-T351 and D16ChATV: а – aluminium alloy 2024T351 in the initial state (marked by circles) and after previous combined load (marked by rectangle); b- aluminium alloy D16ChATV in the initial state (marked by circles) and after previous combined load (marked by pentagon);
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Fig. 10. The areas of local surface fracture of the basic material of aluminium alloy 2024-T351.
Analysis of these images allows us to draw conclusions about the fact that the failure of the material is dominated by plastic strain implemented by different mechanisms. Also, at different levels of the scale, brittle failure is implemented; however its role is secondary. The failure surface of aluminium alloy 2024-T351 after the previous combined load is shown in Fig. 11. A general view of the failure surface in Fig. 11 lets us immediately notice a certain fragmentation and alternation of the areas with the dominance of the mechanisms of ductile and brittle failure. Areas of plastic strain in Fig. 11b present directivity and traces of offsets, occupy a large space in relation to the surface of the image and the fractured surface. The “foam” structure Fig. 11c has different sizes of voids; a single element has both large and small voids. The tops of the ridges show no plate. Minimal plastic strain areas are located on the lateral surfaces of facets in the vicinity of the inclusions, Fig. 11d. Analysis of the surface in the vicinity of holes after the inclusions also shows the change in the nature of failure. The “foam” structure disappears in the pits in Fig. 11. We have homogeneous surfaces with round pits, secondary cracks and other inclusions torn from the surface. As is apparent from Fig. 11f, secondary fractures are located at the base of the ridge and are not always created from the initiator in the form of a pit or inclusion, but develop more on the borders of crystallographic planes. The analysis of aluminium alloy D16ChATV will be carried out in a similar way as it is tested under the same conditions and at the same load. Characteristic pictures of the failure surface are shown in Fig. 12. White bands in Fig. 12a, b are presented using facets with clear traces of the impact of plastic strain. They are formed in the vicinity of voids or break out of separate grains. Local realization of elements of brittle fracture forms surfaces in the vicinity of the pits, merging cavities (Fig. 12c, d), and initiates secondary cracks. In general, plastic strain dominates in case of aluminium alloy D16ChATV in the initial state at cyclic loads, and brittle failure plays a secondary role almost to the breaking point. Analyzing the failure surface of aluminium alloy D16ChATV after the preliminary application of combined load Fig. 13 compared with the previous static load a significant surface changes are not observed. The location in the local area of the base material Fig. 13a allowed to determine changes in the failure process. Plastic strain is present on separate facets scattered across the surface, ridges on the boundaries of the grain break outs are higher and uniform. The area at the bottom of the cells is smooth, with no signs of plastic strain Fig. 13b. Inclusions fell off in most cases, however a few places were observed where they remained in place after failure. Cracks were observed on the failure surface Fig. 13c. They develop by connecting the pits and voids within the same area after grains are broken out as well as between them, on the boundaries of crystalline planes. In Fig. 13d, we can see that the structures of ridges and pits differ. The ridges present a structure with images on the surface in conjunction with the round form of pits. However, we cannot conclude that the structure is “foamy”, in the middle it is obviously uniform. Embryonic rupture on the large cavity after inclusion might also be explained by the level of tension that was almost two times smaller compared to the previous samples. Please cite this article as: V. Hutsaylyuk, et al., Cyclic deformation of aluminium alloys after the preliminary combined ..., Engineering Failure Analysis (2016), http://dx.doi.org/10.1016/j.engfailanal.2016.07.002
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Fig. 11. The failure area surface of a sample of aluminium alloy 2024-T351 after the preliminary application of combined load.
The analysis of the sample indicates that the failure is achieved using a complex failure mechanism with the participation of both components: plastic failure and brittle failure, with the dominance of each at different stages of failure. 4. Conclusions Based on the tests carried out, we can draw further conclusions: 1. Under the conditions of combined load, processes take place in the elements of the mechanical load system of the tested samples, related to the shock exchange of energy, leading to the modification of the mechanical properties of the tested material. During the application of complex loads involving single impulse loads, plasticising zones of various range appear in the material of the tested elements made of aluminium alloys, which revealed during the repeated static load. 2. The increase in the plasticity of alloy 2024-Т351 and the change of the mechanical properties of alloy D16ChATV, observed after the application of the additional impulse load, depends on the type of preliminary strain and the nature of the dynamic non-equilibrium processes. 3. The dissipative structure or the new state of the material resulting from the combined preliminary load is permanent, it does not relax entirely upon releasing the load. It has a decisive impact on the strength properties of the tested alloys and contributes to their increase. 4. Achieving similar results of increased strength for alloy 2024-T351 and D16ChATV after the preliminary application of combined load proves the similar character of the final volume of structural changes at all levels, despite some differences Please cite this article as: V. Hutsaylyuk, et al., Cyclic deformation of aluminium alloys after the preliminary combined ..., Engineering Failure Analysis (2016), http://dx.doi.org/10.1016/j.engfailanal.2016.07.002
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Fig. 12. Failure area surface of the samples of aluminium alloy D16ChATV in initial state.
in the initial structure. On the other hand, the differences clearly visible during the application of additional impulse load, may be caused by the various participation of the deformation processes on the structural level in the overall process of deformation.
Fig. 13. Failure surface of the samples of aluminium alloy D16ChATV after preliminary application of combined load.
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5. The fractographic studies of aluminium alloy 2024 -T351 and D16ChATV in different states confirmed the link between the dissipative and original structures of material. A contribution of a dissipative structure that formed after the previous combined load to the fracture of the material was noted. On this basis, it is stated also the opportunity to explain the behaviour of macro-properties of materials based on realization of corresponding mechanisms of fracture.
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Please cite this article as: V. Hutsaylyuk, et al., Cyclic deformation of aluminium alloys after the preliminary combined ..., Engineering Failure Analysis (2016), http://dx.doi.org/10.1016/j.engfailanal.2016.07.002