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Mode I fracture testing of pultruded glass fiber reinforced epoxy rods: Test development and influence of precracking method and manufacturing
Engineering Fracture Mechanics xxx (2015) xxx–xxx
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Engineering Fracture Mechanics journal homepage: www.elsevier.com/locate/engfracmech
Mode I fracture testing of pultruded glass fiber reinforced epoxy rods: Test development and influence of precracking method and manufacturing Iurii Burda, Andreas J. Brunner ⇑, Michel Barbezat Laboratory for Mechanical Systems Engineering, Empa, Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland
a r t i c l e
i n f o
Article history: Received 31 October 2014 Received in revised form 29 June 2015 Accepted 11 August 2015 Available online xxxx Keywords: Pultruded glass fiber reinforced epoxy rods Delamination resistance Quasi-static mode I Test development Modified Double Cantilever Beam specimen
a b s t r a c t Pultruded glass fiber polymer-matrix composite rods are industrially produced for various applications. The development of a quasi-static mode I delamination test for these rods using a modified Double Cantilever Beam specimen is described. Issues for investigation were size and shape of test specimens, type of load introduction, and different pre-cracking methods. When the test procedure was applied to rods with different lengths, a dependence of the R-curve behavior on the specimen length was revealed. This is attributed to the strong fiber bridging that was observed in the tests. There are also indications for manufacturing effects on scatter. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction In many applications, e.g., transportation, power generation and power lines, cryogenic magnets, biomedical implants and devices, and structural engineering, the fracture toughness or delamination resistance of glass fiber-reinforced polymer-matrix (GFRP) composites is crucial for the mechanical stability and the durability and hence the long-term, reliable use of GFRP materials and elements (see, e.g., [1–5]). While standard toughness or delamination tests for carbon fiber-reinforced (CFRP) and GFRP composites require special size/shape of the test specimens, e.g., the Double Cantilever Beam (DCB) specimen (see, e.g., [6] for details), there are no standard fracture tests for FRP structural elements or material specimens prepared from them. The standard test for mode I opening loading of CFRP and GFRP composites [7] using the DCB specimen at least provides some guidance on how to prepare starter cracks for specimens which do not have the a starter insert film. There is also literature discussing delamination resistance testing of GFRP material taken from structural elements with specific shape. For mode I tensile opening, one example is filament wound GFRP pipes, GFRP cylinders another (see, e.g., [8–10]), and round-rod specimens with grooves subjected to torsion are discussed by [11]. For composite cylinders, there have also been investigations of other loading modes beside tensile opening mode I, e.g., in-plane shear mode II [12] or mixed mode I/II [13]. For GFRP pultruded rods, such as those used as core of insulating elements in power transmission lines [14] no test methodology for fracture toughness or delamination resistance is yet available to the best knowledge of the authors. The
⇑ Corresponding author. Tel.: +41 58 765 44 93. E-mail address: [email protected] (A.J. Brunner). http://dx.doi.org/10.1016/j.engfracmech.2015.08.009 0013-7944/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Burda I et al. Mode I fracture testing of pultruded glass fiber reinforced epoxy rods: Test development and influence of precracking method and manufacturing. Engng Fract Mech (2015), http://dx.doi.org/10.1016/j.engfracmech.2015.08.009
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Nomenclature CFRP D DCB GFRP l l1 l2 NL point Pre-cracking Pre-cracking Pre-cracking Pre-cracking R-curve 2h 5% point
method method method method
A B C D
carbon fiber-reinforced polymer-matrix diameter of GFRP rod Double Cantilever Beam glass fiber-reinforced polymer-matrix length of GFRP rod specimen length of pre-crack obtained from first method (in combinations of pre-cracking methods) length of pre-crack obtained from second method (in combination of pre-cracking methods) point of deviation of the load–displacement curve from initial linear behavior blade cutting with diamond-coated blade diamond wire cutting knife blade tapping wedge opening using a screw driver delamination resistance curve thickness of GFRP rod after machining point of intersection of a linear curve with 5% increase in compliance with the load–displacement curve
brittle failure of such GFRP rods has been investigated by [15], but no quantitative evaluation of fracture toughness or delamination resistance has been performed. Typically, these GFRP rods are manufactured by pultrusion, the glass fiber content is high (estimated to about 60 volume-% determined from cross-section micrographs of one rod from batch #1) and also the epoxy matrix is highly filled with CaCO3 (typically up to about 20 wt.%). Hence, the first aim of this paper is to develop and evaluate a fracture test methodology for quasi-static mode I tensile opening loading for pultruded GFRP rods. Specifically, different methods for creating the initial starter crack (analogous to the wedge pre-cracking method described in Annex B.8 of [7]) have been investigated and compared. Further, the dependence of the delamination propagation behavior on the length of the GFRP rod has been investigated as well. 2. Materials, experimental methods and data analysis Two series of epoxy GFRP rods (designated batches #1 and #2) were manufactured with a proprietary process, called ‘‘pull-pressing”, from glass fiber (type ECR) and epoxy (resin type EP 828LVEL with anhydride hardener Epikure 3601 and amine-based accelerator EKcat 201). The epoxy matrix was filled with 20 wt.% of CaCO3. This process is different from commercial continuous pultrusion and yields single rods with a length up to about 3.5 m. As in rods from continuous pultrusion, the glass fibers are aligned along the length of the rods. Specimens cut from the rods are hence basically axisymmetric, except for microscopic variation of fiber density. Nevertheless, due to the sequential manufacturing, some variation in properties is possible among specimens cut from different rods (even if taken from the same production date). For a comparison of the effects of manufacturing methods, a third series (designated batch #3) of epoxy GFRP rods manufactured by industrial scale pultrusion (supplied by Pfisterer Sefag AG) has been investigated as well. However, in this case, no details on materials and composition, e.g., fiber or matrix type, or filler type and content, are available. The pull-pressed GFRP rods had a nominal diameter of 18.57 mm with a specified tolerance of 0.10 mm and the pultruded GFRP rods one of 18.6 mm (tolerance specification not available). After pull-pressing, the diameter of the rods of batches #1 and #2 was brought into the specified tolerance by surface machining.
Fig. 1. GFRP specimen size and shape for quasi-static mode I fracture testing, numbers indicate fixed dimensions in millimeters. The nominal diameter b was 18.57 mm for batch #1 and #2, and 18.6 mm for batch #3. The total length l was 90 mm for comparison of the different pre-cracking methods, l1 and l2 indicate pre-cracking lengths for combinations of pre-cracking methods (see Table 1 for details). Selected tests with one pre-cracking method were performed on specimens with lengths l of 150 mm, 210 mm and 270 mm.
Please cite this article in press as: Burda I et al. Mode I fracture testing of pultruded glass fiber reinforced epoxy rods: Test development and influence of precracking method and manufacturing. Engng Fract Mech (2015), http://dx.doi.org/10.1016/j.engfracmech.2015.08.009
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Table 1 Summary of combinations of pre-cracking methods, pre-crack lengths and batches as well as number of specimens tested (see text for a description of the precracking methods A–D). Pre-cracking type
Precracking length [mm]/ method
Precracking length [mm]/ method
Precracking length [mm]/ method
Number of specimens tested, batch number and specimen length
Blade cutting 3 mm thick and diamond wire 0.3 mm diameter and knife blade tapping (A + B1 + C) Blade cutting 3 mm thick and screw driver wedging (A + D) Diamond wire saw only 0.3 mm diameter (B1) Diamond wire saw 0.3 mm diameter and knife blade tapping (B1 + C) Diamond wire saw 0.3 mm diameter and knife blade tapping (B1 + C) Diamond wire saw 0.3 mm diameter and knife blade tapping (B1 + C) Diamond wire saw 0.3 mm diameter and knife blade tapping (B1 + C)
35/A
5/B1
5/C
3 specimens, batch #1, l = 90 mm
30/A
3/D
–
5 specimens, batch #1, l = 90 mm
30/B1 5/B1
– 30/C
– –
30/B1
3/C
–
30/B1
3/C
–
5 specimens, batch #1, l = 90 mm 5 specimens, batch #1, l = 90 mm, long precrack from tapping 5 specimens, batch #1, l = 90 mm, short precrack from tapping 6 specimens, batch #2, l = 90 mm
30/B1
3/C
–
30/B1
3/C
–
30/B1
3/B2
Diamond wire saw 0.3 mm diameter and knife blade tapping (B1 + C) Diamond wire saw 0.3 mm followed by 0.13 mm diameter (B1 + B2)
15 specimens, batch #2, l = 150 mm, 210 mm, 270 mm (5 specimens each per length) 5 specimens, batch #3, l = 90 mm 5 specimens, batch #2, l = 90 mm
The specimen shape and size cut from these GFRP rods which should emulate a DCB-type specimen is schematically shown in Fig. 1, where l1 and l2 indicate the length of the pre-cracks generated with the different methods (see Table 1 for details). The shape of the specimen with top and bottom side machined plane and parallel arose from the necessity to have sufficient grip and sufficiently accurate center points for drilling the pin-holes. For that, about 1 mm was machined off on each side (top and bottom) resulting in a nominal thickness 2 h of about 16.5 mm. For specimens with other diameters, however, this value may have to be adjusted. Putting pin-holes directly into the GFRP rod eliminated the use of complex shaped load introductions and respective corrections for stiffening of the half beams (see [7] for details). Cutting specimens to length and pre-cracking was performed at room-temperature, but without temperature and humidity control of the workshop. Specimens were then stored at +23 °C and 50% relative humidity (controlled within ±2 °C and ±5% relative humidity) before testing and tested in this climate. The duration of conditioning at the test climate before testing varied between about one and several hundred hours. The total pre-crack length was set around 33 mm with a margin of about 10% in order to use relatively short specimens (90 mm long) and simultaneously to avoid use of large deflection corrections (again see [7] for details). The specific cross-section of the GFRP rod specimen required a correction factor for the different moment of inertia compared to a rectangular beam cross section in the modified compliance calibration formula [7] used for calculating GIC for a standard DCB specimen with rectangular cross-section. Four different methods were used for manufacturing pre-cracks: (A) blade cutting (diamond-coated blade with a thickness around 3 mm), (B) wire cutting (diamond wire with a diameter around 0.3 mm labeled B1 and 0.13 mm labeled B2, respectively), (C) knife blade tapping (knife thickness around 0.45 mm), and (D) wedge opening using a screw driver (in combination with an initial crack obtained by blade cutting). In order to restrict crack propagation during pre-cracking by methods C and D, and thus to have a better control of the pre-cracking length, the sample was additionally clamped in a vice. For the comparison of the different pre-cracking methods, selected combinations of the four basic methods A–D were also investigated, which also included different relative lengths of the pre-cracks. Using GFRP rods from batch #2 of pull-pressed GFRP rods, the effects of specimen length were investigated. This included specimens of 90, 150, 210, and 270 mm length, all with pre-cracking with diamond wire saw (0.3 mm diameter) up to about 30 mm and short knife tapping (with the specimen clamped in a vice) for an additional 3 mm, labeled ‘‘B1 + C (3 mm)”. Table 1 summarizes all combinations that have been tested. Please note that there was one method, where a short wire pre-crack (5 mm) was followed by knife blade tapping for 30 mm. This will be referred to as ‘‘B1 + C (30 mm)” to distinguish it from the same combination with long diamond wire (30 mm) and short knife blade tapping pre-crack of 3 mm, i.e., ‘‘B1 + C (3 mm)”. Fracture toughness testing was performed on a tensile test machine (type Z010 from Zwick, with a 1 kN load cell) at a cross-head speed of 1 mm/min. All specimens were tested directly from the respective pre-crack (Table 1) without additional pre-cracking, unloading and reloading (i.e., different from the requirements of [7]). Delamination initiation and propagation was observed visually without the use of a traveling microscope. A cold light source was used to highlight the crack tip in the semi-transparent GFRP composite. Visual observation typically was from one side of the specimen with occasionally checks observing the crack from the top (using a light source, see Fig. 3a) in order to identify possible asymmetric crack tip development across the specimen width. It is well known that visual determination of delamination length is operator-dependent [16] and hence, most tests were performed by one operator, and a few complementary tests by a second operator. All tests were evaluated by one operator, however. A round robin exercise had shown that choosing the onset of nonlinearity (NL) points for delamination initiation from the load–displacement curves to be operator-dependent as well [17]. Please cite this article in press as: Burda I et al. Mode I fracture testing of pultruded glass fiber reinforced epoxy rods: Test development and influence of precracking method and manufacturing. Engng Fract Mech (2015), http://dx.doi.org/10.1016/j.engfracmech.2015.08.009
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For the comparative analysis of the test data, the modified compliance calibration method (MMC), i.e., method B according to [7] was used, however with an additional correction factor for the difference in cross-section between the rod-based and the standard beam specimens. The critical energy release rate, GIC, is determined from Eq. (1) where the value of ‘‘m” is the slope of a plot of the width-normalized cube-root of the specimen compliance C (i.e., (C/b)1/3) versus the thickness-normalized delamination length (a/2h),
2 2 1 3m P ðbCÞ3 2ð2hÞ b CI 2 23 3m P bC GIC ¼ F 2ð2hÞ b N
GIC ¼
ð1Þ ð2Þ
Further, in Eq. (1) 2h denotes the specimen thickness, P the load, b the specimen width, C the specimen compliance calculated from the machine recording of load and displacement, and CI the correction factor for the difference in moment of inertia between the specimens machined from the rod and a beam with rectangular cross-section (same thickness and width equal to the radius of the rod). Compared with Eq. (2) for the MCC method for DCB specimens (given in [7]), the load-block correction factor (denoted N) and the large displacement correction factor (denoted F) have been set equal to 1, whereas the multiplicative inertia correction factor (1/CI) has been added. Since Eq. (1) without the correction factor CI applies to specimens with a rectangular cross-section (thickness 2h and width b), a correction for the different moment of inertia of the cross-section of the GFRP rods was derived taking the width b equal to the nominal radius of the GFRP rods. A numerical comparison resulted in a correction CI of 0.78 determined from comparing the moments of inertia of the specimen used here with that of a beam with rectangular cross-section (width b times thickness 2h). Considering the measured, slight variations in thickness and radius of the different GFRP rod specimens, the variation in this correction factor CI was shown to be between 0.76 and 0.81, i.e., a variation of less than 5%. Hence, all GIC-values were multiplied by a constant correction factor 1/CI amounting to (1/0.78), i.e., neglecting the effect of small variations in specimen geometry on the correction factor (but using the effective width and thickness for each specimen for the other factors in Eq. (1)). Another possible effect to be accounted for is machine and test set-up compliance. For the comparative evaluation of the different pre-cracking methods, the machine and setup compliance was neglected. GIC initiation values were calculated using the loads indicating the NL- and the 5% increase in compliance (C5%) point, respectively determined from the load–displacement plot. Propagation values for determining the resistance curve (R-curve) were also determined from Eq. (1), again see [7] for details. Considering the effects discussed above, the GIC initiation and the propagation values provide a relative comparison between batches of GFRP rods and specimens with different pre-crack types, but shall not be used for design before validation. 3. Test results and discussion 3.1. Comparison of different pre-cracking methods The sharpness of the crack tip produced with the different pre-cracking methods was not measured directly. The initiation values of GIC do depend on a number of factors, the sharpness of the pre-crack being one among others. Relevant additional factors are, e.g., the position of the crack tip with respect to the mid-plane and its orientation, but also the damage created around the crack tip by the pre-cracking method. Based on experience from fracture toughness testing of unreinforced and nanocomposite epoxies [18], blade tapping according to [19] seems to be an effective method for creating a pre-crack approximating a ‘‘natural” crack. The fact that GIC values (presented below) from combination of diamond wire
Fig. 2. Photographs of a 270 mm long GFRP rod specimen (batch #2) during the mode I DCB test (left) showing full specimen length, load introduction and light source used to highlight the position of the crack tip, (right) enlarged view of the part inside the rectangle shown on the left. Note the strong and large-scale fiber bridging.
Please cite this article in press as: Burda I et al. Mode I fracture testing of pultruded glass fiber reinforced epoxy rods: Test development and influence of precracking method and manufacturing. Engng Fract Mech (2015), http://dx.doi.org/10.1016/j.engfracmech.2015.08.009
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sawing and tapping tend to be lower than those from other pre-cracking methods, e.g., A and D is evidence that tapping yields consistent initiation values of GIC. Nevertheless, the data obtained from the rod-based specimens have to be validated. The approaches used for this will be discussed in more detail below. Specimens typically developed large-scale fiber bridging (see Figs. 2 and 3 for examples), extending up to 10 cm or more behind the crack tip. Fiber bridging close to the crack tip (Fig. 4) made clear identification of the tip position sometimes difficult. However, from observation, it is estimated that the accuracy of the delamination length measurement was within ±0.2 mm or less in all cases (see Fig. 4). No significant deviation of the delamination from the mid-plane of the specimens was observed. Fracture initiation and propagation data of selected pre-cracking methods for GFRP rod specimens are shown in Fig. 4. The data were evaluated with a spreadsheet implementing the modified compliance calibration analysis [7] according to Eq. (1) shown above and selected data were checked with another spreadsheet incorporating three methods for analysis (corrected beam theory, experimental compliance method and modified compliance calibration [20]). Initiation values NL and C5% (Fig. 4a) are averages with standard deviations from the number of specimens listed in Table 1. Columns in Fig. 4b show propagation values (excluding initiation data) averaged over the first 25 mm of delamination propagation beyond the tip of the pre-crack of the number of specimens listed in Table 1. Clearly, some pre-cracking methods yield significantly larger initiation and propagation values than others. It can further be noted that average propagation data are larger than the initiation values (both, NL and C5%) for each pre-cracking method. The respective standard deviations (from testing several specimens under nominally identical conditions) seem to increase with increasing initiation or average propagation value. In addition to pre-cracking methods ‘‘B1 + C (30 mm)” and ‘‘A + D”, also all other pre-cracking methods or combinations from Table 1 not shown in Fig. 2 yielded higher initiation and average propagation values than methods ‘‘B1”, ‘‘B1 + B2”, and ‘‘B1 + C (3 mm)”. From visual observation, all specimens, independent of the pre-cracking method used, show evidence of significant fiber bridging (Figs. 2 and 3). Assessing the different pre-cracking methods is based on the assumption that pre-cracking should yield minimal damage at the crack tip or in the fracture process zone surrounding the crack tip and hence result in comparatively ‘‘low” initiation values. Based on this criterion, the pre-crack using only the diamond wire (0.3 mm diameter, i.e., ‘‘B1”), does yield the lowest initiation values (NL and C5%). However, the combination of two diamond wires of different diameter (first 0.3 mm for 30 mm then 0.13 mm for 3 mm, i.e., ‘‘B1 + B2”) and the combination of diamond wire (0.3 mm diameter) and ‘‘short” knife blade tapping, i.e., ‘‘B1 + C (3 mm)” yield slightly higher, but comparable initiation values. This is based on a comparison of the standard deviation from averaging values from five specimens each (NL 240 ± 31 J/m2 and C5% 292 ± 40 J/m2 for thick diamond wire (0.3 mm) only, NL 275 ± 26 J/m2 and C5% 366 ± 37 J/m2 for the combination of thick and thin diamond wire (0.3 mm followed by 0.13 mm) and NL 272 ± 37 J/m2, and C5% 337 ± 38 J/m2 for diamond wire with knife tapping). It is also observed that the methods compared in Fig. 4 yielding higher initiation values also yield higher average propagation values. The reason for this is not clear, in view of the massive fiber bridging observed (Figs. 2 and 3) it could be hypothesized that these effects would dominate over differences in initiation. It can be noted that the correction values D (which were calculated for selected specimens with a spreadsheet analysis developed for comparison of the corrected beam method with the modified compliance calibration method) turned out to be in the range between about 27 mm and 37 mm. This indicates strong fiber bridging or damage based on the analysis discussed by [21]. The data indicate strongly rising R-curves for all pre-cracking methods tested (Fig. 5). There is no clear indication of a plateau in the R-curves, although a maximum value is reached at delamination lengths around 70 mm, i.e., about 20 mm from the end of the specimens, after which the values tend to decrease. This is a potential indication of end effects. It was hence decided to investigate specimens with different lengths (90 mm, 150 mm, 210 mm and 270 mm) all with a pre-crack made by the combination of diamond wire (0.3 mm diameter) and knife blade tapping ‘‘B1 + C (3 mm)”.
Fig. 3. Photograph of the fiber bridging near the crack tip, the crack tip visually determined on the side of this GFRP rod is indicated by the white arrow. The delamination length in this case corresponds to 103.75 mm (with an estimated uncertainty of ±0.05 mm).
Please cite this article in press as: Burda I et al. Mode I fracture testing of pultruded glass fiber reinforced epoxy rods: Test development and influence of precracking method and manufacturing. Engng Fract Mech (2015), http://dx.doi.org/10.1016/j.engfracmech.2015.08.009
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Fig. 4. Comparison of initiation values (a) non-linear (NL) and 5% increase in compliance (C5%) and of (b) average propagation values for selected precracking methods or combinations. Average propagation values are compared for the same delamination length increment of 25 mm beyond the tip of the pre-crack for each specimen (see Table 1 and text for a description of the pre-cracking methods A–D and of the averaging used).
3.2. Investigation of specimen length effects The R-curves of GFRP rod specimens with different lengths are shown in Fig. 6. The data indicate that a minimum delamination length between about 120 mm and 150 mm is required to obtain a maximum in the R-curve. The maximum values seem to range from about 2500 to 3500 J/m2. Again, a drop in the R-curve values is observed at delamination lengths within about 20 mm from the end of the specimens. This is clear evidence that either damage (fracture process zone, possibly extending 10–20 mm) ahead of the visually observed crack tip, or reduced bending stiffness affects delamination propagation toward the end of the rod. Both graphs show scatter among the different R-curves for the individual specimens tested. Among the 270 mm long specimens (Fig. 6b), there is one that yields clearly higher values than the other specimens and a strongly corrugated R-curve. From visual observation during the tests and inspection of the specimen after the test, no indication of a behavior that would invalidate the test has been found. The corrugated R-curve possibly arises from fiber bridging that is larger or a higher amount of fiber bridges failing at about the same time (visually not identified) than in the other specimens. The load and hence the propagation values drop when large fiber bridges break. Typical fiber bridging for the GFRP rods is shown in Figs. 3 and 4. The specimen is retained in the analysis and hence significantly contributes to the scatter in the results. The data analysis revealed an interesting effect, namely a dependence of the initiation values (NL and C5%) and of the average propagation values on the specimen length. The average NL initiation values and standard deviations obtained when considering different delamination lengths for the analysis are summarized in Table 2. The first column in Table 2 shows the average values of four or five specimens with all data points for each specimen included. The second column shows the same data, when data points from the last 20 mm of nominal specimen length are excluded (i.e., the end effects discussed above are eliminated). The third column shows the data with data points from the first 70 mm of the delamination (including Please cite this article in press as: Burda I et al. Mode I fracture testing of pultruded glass fiber reinforced epoxy rods: Test development and influence of precracking method and manufacturing. Engng Fract Mech (2015), http://dx.doi.org/10.1016/j.engfracmech.2015.08.009
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Fig. 5. R-curves for specimens from (a) batch #1 and (b) batch #2 of GFRP rod specimens comparing pre-cracking by wire cutting (diamond wire diameter 0.3 mm) and knife blade tapping with different tapping lengths (‘‘B1 + C (30 mm)” versus ‘‘B1 + C (3 mm)”), i.e., different relative length of method ‘‘B1” and ‘‘C”. Note the significant difference in initiation values GIC and the strong increase in R-curves.
pre-crack). Hence, for specimens of 90 mm length, the values in the second and third column are identical for each batch. There is a steady increase in the average NL initiation value for the specimens from batch #2 when the specimen length is increased from 90 mm to 270 mm. The values for 210 and 270 mm are significantly higher than those from batch #1 to #3 (for a nominal length of 90 mm). With increasing specimen length, the end effects become less important (compare values in the first and second column) as expected. It can also be noted that batch #3 yields the lowest value of all three batches, and consistent values independent of the delamination length that is analyzed. Comparing the three batches of GFRP rods, there is a clear ranking with decreasing toughness values when going from batch #1 to #2, and then to #3. When analyzing data from the same delamination length (70 mm) for specimens from batch #2, the values are consistent independent of the total specimen length. Hence, for comparative testing, data from the same delamination length for all specimens shall be analyzed in order to get consistent results. The reason for this length dependent behavior observed in the GFRP rods is not fully understood yet. It can be hypothesized that it relates to fiber bridging, possibly strongly increasing until a plateau in delamination propagation is observed. For comparative testing, this effect can be taken into account by analyzing specimens of comparable delamination lengths and by choosing specimen lengths 20–30 mm longer than the delamination length. In order to observe the full R-curve behavior, specimen lengths of at least 250 mm are recommended. However, this value may also depend on the type of glass and on the matrix composition of the GFRP rod. These effects will be investigated in a later publication. When data from the first 70 mm delamination length are analyzed with the spreadsheet incorporating corrected beam theory, comparable D-values (delamination length correction, see [7] for details) between about 32 mm and 36 mm are obtained for all specimens independent of their total length. In an earlier analysis of standard GFRP beam specimens [20] a trend for increasing D-values with decreasing number of data points analyzed (and hence with decreasing delamination length) was only observed for very short total delamination lengths (on the order of about 10 mm or less) whereas at longer delamination lengths the D-values were roughly constant independent of the number of data points. Fig. 7 shows the results of the crack speed for GFRP rod specimens with different lengths from batch #2, obtained from point-wise analysis of the crack speed (namely from visually observed delamination length increments and time intervals between load–displacement records from the test machine). The crack speed initially (short crack lengths) drops rapidly and then levels off after about 150–200 mm. For each specimen length, a marked increase in crack speed is observed for Please cite this article in press as: Burda I et al. Mode I fracture testing of pultruded glass fiber reinforced epoxy rods: Test development and influence of precracking method and manufacturing. Engng Fract Mech (2015), http://dx.doi.org/10.1016/j.engfracmech.2015.08.009
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Fig. 6. R-curves for specimens from (a) batch #1 and batch #2 (b) of GFRP rods comparing pre-cracking by diamond wire and knife blade tapping (precracking methods ‘‘B1 + C (30 mm)” with ‘‘B1 + C (3 mm)”, i.e., with different relative length of method ‘‘B1” to method ‘‘C”, note the significant difference in initiation values and the strong increase in R-curves in both cases.
the last data points within about 20 mm from the end. This end effect is also reflected in the drop in propagation values toward the end of the specimen length. These observations are consistent with the assumption of strongly increasing fiber bridging, and simultaneously, strongly increasing R-curves before a maximum in GIC is reached after 150–200 mm crack length.
3.3. Different manufacturing methods The R-curves for specimens from batches #1, #2 and #3 are compared in Fig. 8. With respect to initiation, a difference between batches #pp and #2 of pull-pressed GFRP rods (Fig. 8b) can be noted (average NL of 272 ± 37 J/m2 versus 243 ± 24 J/m2). This difference indicates batch to batch variation that likely exceeds the rod to rod variation from pull-pressing. Batches #1 and #2 were produced about one year apart, from separately prepared matrix formulations. Batches #pp and #2 of pull-pressed GFRP rods, on the other hand, yield somewhat higher initiation values than the GFRP rods
Table 2 Summary of non-linear (NL) initiation values and standard deviations of GIC for different batches and sample lengths (pre-cracking method B1 + C, average of four or five specimens each). Specimen length l [mm] 90
150 210 270
Batch number
NL from full delamination length
NL from delamination length excluding last 20 mm of specimen length
NL from 70 mm total delamination length
1 2 3 2 2 2
271.7 ± 36.7 239.8 ± 25.6 202.7 ± 43.3 250.8 ± 10.6 326.7 ± 14.2 313.9 ± 61.9
263.2 ± 34.9 247.0 ± 15.1 205.7 ± 54.8 289.4 ± 12.0 326.2 ± 13.7 313.5 ± 61.9
263.2 ± 34.9 247.0 ± 15.1 205.7 ± 54.8 246.2 ± 10.0 245.3 ± 16.1 233.7 ± 47.4
Please cite this article in press as: Burda I et al. Mode I fracture testing of pultruded glass fiber reinforced epoxy rods: Test development and influence of precracking method and manufacturing. Engng Fract Mech (2015), http://dx.doi.org/10.1016/j.engfracmech.2015.08.009
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Fig. 7. Crack speed from point-wise analysis of delamination length increments for selected specimens (batch #2) with different length (see insert).
Fig. 8. R-curves including initiation values for GFRP rod specimens from batches #1, #2 and #3. Pre-cracking was performed by wire sawing (diamond wire 0.3 mm diameter) and short knife blade tapping (see insert).
from commercial pultrusion (batch #3, average NL initiation of 186 ± 26 J/m2). With respect to delamination propagation, the R-curves clearly indicate a larger scatter among the specimens of batches #1 and #2 from pull-pressing in comparison to batch #3 from pultruded GFRP rods. Scanning electron microscopy (SEM) micrographs from cross-sections of pull-pressed GFRP rods (batch #1 and #2) shown in Fig. 9a and b indicate rather inhomogeneous distributions in fiber density compared with that from pultrusion shown in Fig. 9c. The resin-rich areas between the fibers and the resulting, more inhomogeneous fiber–matrix distribution in Fig. 9a and b may induce additional delaminations or fiber bridging, resulting in the higher observed scatter in R-curves for the pull-pressed GFRP rods.
Please cite this article in press as: Burda I et al. Mode I fracture testing of pultruded glass fiber reinforced epoxy rods: Test development and influence of precracking method and manufacturing. Engng Fract Mech (2015), http://dx.doi.org/10.1016/j.engfracmech.2015.08.009
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Fig. 9. SEM micrographs for a comparison of fiber distributions in the cross-sections of GFRP rods from pull-pressing of (a), batch #1, (b), batch #2, and from pultrusion (c) batch #3.
3.4. Validation of test method and data analysis The spread-sheet for the calculation of the critical energy release rate, GIC, for the GFRP rod specimens was validated by using a second spreadsheets that had been developed independently. The results calculated by the two spreadsheets differed at most by 2% due to rounding errors in the line fitting and the calculations. The validation of the test method is still on-going. One approach is to use standard DCB-specimens according to [7] cut from GFRP laminated plates with film (about 10 lm thick) as starter cracks manufactured with the same epoxy matrix as the rods. If a correlation between the standard DCB test results and those from the GFRP rods for the same material type is achieved, this will be an indication that the rod test method is at least capable of providing comparative results. Further, the rod and laminates data will be compared with GIC data from unreinforced epoxy specimens tested according to [19]. These investigations are currently being performed and will be reported later. 4. Summary and outlook A modified Double Cantilever Beam (DCB) specimen for mode I tensile opening fracture toughness testing of GFRP rods with circular cross-section has been developed and evaluated in comparative tests of three batches from two different production methods (continuous pultrusion and discontinuous pull-pressing). The circular cross-section had to be reduced for sufficiently accurate drilling of pin holes in the rods for load introduction. This further required a correction factor for data analysis based on the modified compliance calibration method (compared with the rectangular beam cross-section of standard DCB specimens). The data analysis was verified by using two independently developed spread-sheets. A major aim of the investigation was the development of a method for pre-cracking the GFRP rods. It turned out that a pre-crack from a diamond wire saw (two diameters were used), possibly combined with a short crack extension via knife blade tapping yielded sufficiently reproducible and comparatively low initiation values. Applying this pre-cracking method to different batches of GFRP rods yielded indications of effects from the manufacturing process and the resulting fiber distribution on the reproducibility and scatter of the data. In all tests, significant fiber bridging was visually observed and this, quite likely, also yielded delamination length dependent effects in the data analysis. Therefore, in order to characterize the full R-curve behavior of the GFRP rods, sufficiently long specimens (at least 250 mm long, if pre-cracks are limited to less than 35 mm) have to be used. From an application point of view, it may be asked whether the GFRP rod specimens could also be tested under mode I tensile opening cyclic fatigue fracture loading. This has not been attempted yet. The strong fiber bridging could yield rather low crack propagation speeds in cyclic fatigue and hence result in long test duration. It is also questionable whether the pin-holes drilled into the GFRP rods would be sufficiently wear resistant against bearing stress for long-term cycling. In that case, the pinholes could possibly be reinforced with metal inserts, but this has not been evaluated in the present research. Nano-scale fillers have received significant attention for their potential to improve fracture toughness or delamination resistance of glass or carbon fiber-reinforced polymer composites (see, e.g., [22–25]), in spite of some recent critical assessments of the perspectives [26] which, however, referred to carbon nanotube fillers. In view of the special application of the GFRP rods in electrical insulators, several commercial, inorganic (i.e., electrically non-conducting) nano-scale fillers have been added to the epoxy matrix and been investigated with respect to their performance in quasi-static mode I testing of the GFRP rods and compared with the CaCO3-filled commercial product as a first application of the test methodology. These tests indicate that the methodology presented here is capable of discriminating between fracture toughness values obtained from various types of nano-scale fillers in GFRP rods. These results will be presented and discussed in a separate publication. Acknowledgments The assistance by Mr. Michael Buhagiar and Mr. Dietmar Haba for testing and data analysis, by Mr. Lorenzo De Boni, Mr. Marcel Rees and Mr. Kurt Ruf for specimen machining and technical support is gratefully acknowledged. We thank the Swiss Please cite this article in press as: Burda I et al. Mode I fracture testing of pultruded glass fiber reinforced epoxy rods: Test development and influence of precracking method and manufacturing. Engng Fract Mech (2015), http://dx.doi.org/10.1016/j.engfracmech.2015.08.009
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Commission for Technology and Innovation (CTI) for financial support (project No. 11238-1 PFIW-IW), and Dr. Frank Schmuck, Dr. Konstantin O. Papailiou and Mr. Anton Furrer (Sefag Pfisterer AG) for material supply, technical support and discussions. References [1] Gand AK, Chan T-M, Mottram JT. Civil and structural engineering applications, recent trends, re-search and developments on pultruded fiber reinforced polymer closed sections: a review. Frontiers Struct Civil Engng 2013;7(3):227–44. http://dx.doi.org/10.1007/s11709-013-0216-8. [2] Bank LC. Progressive failure and ductility of FRP composites for construction: review. J Compos Constr 2013;17:406–19. http://dx.doi.org/10.1061/ (ASCE)CC.1943-5614.0000355. [3] Stewart A, Douglas EP. Accelerated testing of epoxy-FRP composites for civil infrastructure applications: property changes and mechanisms of degradation. Polym Rev 2012;52(2):115–41. http://dx.doi.org/10.1080/15583724.2012.668152. [4] Hollaway LC. A review of the present and future utilisation of FRP composites in the civil infra-structure with reference to their important in-service properties. Constr Build Mater 2010;24:2419–45. http://dx.doi.org/10.1016/j.conbuildmat.2010.04.062. [5] Brunner AJ. Fracture mechanics of polymer composites for aerospace applications. In: Irving PE, Soutis C, editors. Chapter 8 in polymer composites in the aerospace industry. Woodhead Publishing, Series in Composites Science and Engineering: Number 50; 2015. p. 191–230 [ISBN 978-0-85709-5237]. [6] Brunner AJ, Blackman BRK, Davies P. A status report on delamination resistance testing of poly-mer-matrix composites. Engng Fracture Mech (Spec Issue) 2008;75(9):2779–94. http://dx.doi.org/10.1016/j.engfracmech.2007.03.012. [7] ISO 15024 fibre-reinforced plastic composites – determination of mode I interlaminar fracture toughness, GIC, for unidirectionally reinforced materials. International Organization for Standardization; 2001. [8] Beckert W, Lauke B, Friedrich K. Delamination toughness computation for curved thermoplastic composites. Appl Compos Mater 1995;1:395–400. http://dx.doi.org/10.1007/BF00568044. [9] Davies P, Rannou F. The effect of defects in tubes: Part 1. Mode I delamination resistance. Appl Compos Mater 1995;1:333–49. http://dx.doi.org/ 10.1007/BF00568040. [10] Ozdil F, Carlsson LA. Characterization of mode I delamination growth in glass/epoxy composite cylinders. J Compos Mater 2000;34(5):398–419. http:// dx.doi.org/10.1177/002199830003400503. [11] Tan T, Ren F, Wang JJ-A, Lara-Curzio E, Agastra P, Mandell J, et al. Investigating fracture behavior of polymer and polymeric composite materials using spiral notch torsion test. Engng Fract Mech 2013;101:109–28. http://dx.doi.org/10.1016/j.engfracmech.2012.07.007. [12] Ozdil F, Carlsson LA, Li X. Characterization of mode II delamination growth in glass/epoxy composite cylinders. J Compos Mater 2000;34(4):274–98. http://dx.doi.org/10.1177/002199830003400402. [13] Ozdil F, Carlsson LA. Characterization of mixed mode delamination growth in glass/epoxy composite cylinders. J Compos Mater 2000;34(5):420–41. http://dx.doi.org/10.1177/002199830003400504. [14] Papailiou KO, Schmuck F. Silicone composite insulators, power systems. Berlin Heidelberg: Springer-Verlag; 2013. http://dx.doi.org/10.1007/978-3642-15320-4. [15] Liang XD, Wang CS, Fan J. Research on brittle fracture of FRP rods and the test method. Power Syst Technol 2003;27:34–7 [in Chinese]. [16] Brunner AJ. Experimental aspects of Mode I and Mode II fracture toughness testing of fibre-reinforced polymer-matrix composites. Comput Methods Appl Mech Engng 2000;185:161–72. http://dx.doi.org/10.1016/S0045-7825(99)00257-1. [17] Davies P. Round robin analysis of GIc interlaminar fracture test. Appl Compos Mater 1996;3:135–40. http://dx.doi.org/10.1007/BF00135051. [18] Hsieh TH, Kinloch AJ, Taylor AC, Sprenger S. The effect of silica nanoparticles and carbon nanotubes on the toughness of a thermosetting epoxy polymer. J Appl Polym Sci 2011;119:2135–42. http://dx.doi.org/10.1002/app.32937. [19] ISO 13586, Plastics – determination of fracture toughness (KIC and GIC) – linear elastic fracture mechanics (LEFM) approach. International Organization for Standardization; 2000. [20] Brunner AJ. Towards standardized interlaminar fracture tests for unidirectional fiber-reinforced polymer-matrix composites. In: Mota-Soares CA, Mota-Soares CM, Freitas MJM, editors. NATO science series: E applied science, vol. 361. Kluwer Academic; 1999. p. 279–87 [ISBN 0-7923-5870-8]. [21] Brunner AJ, Blackman BRK, Williams JG. Calculating a damage parameter and bridging stress from GIC delamination tests on fibre composites. Compos Sci Technol 2006;66(6):785–95. http://dx.doi.org/10.1016/j.compscitech.2004.12.040. [22] Sprenger S. Epoxy resin composites with surface-modified silicon dioxide nanoparticles: a review. 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Please cite this article in press as: Burda I et al. Mode I fracture testing of pultruded glass fiber reinforced epoxy rods: Test development and influence of precracking method and manufacturing. Engng Fract Mech (2015), http://dx.doi.org/10.1016/j.engfracmech.2015.08.009
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Engineering Fracture Mechanics journal homepage: www.elsevier.com/locate/engfracmech
Mode I fracture testing of pultruded glass fiber reinforced epoxy rods: Test development and influence of precracking method and manufacturing Iurii Burda, Andreas J. Brunner ⇑, Michel Barbezat Laboratory for Mechanical Systems Engineering, Empa, Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland
a r t i c l e
i n f o
Article history: Received 31 October 2014 Received in revised form 29 June 2015 Accepted 11 August 2015 Available online xxxx Keywords: Pultruded glass fiber reinforced epoxy rods Delamination resistance Quasi-static mode I Test development Modified Double Cantilever Beam specimen
a b s t r a c t Pultruded glass fiber polymer-matrix composite rods are industrially produced for various applications. The development of a quasi-static mode I delamination test for these rods using a modified Double Cantilever Beam specimen is described. Issues for investigation were size and shape of test specimens, type of load introduction, and different pre-cracking methods. When the test procedure was applied to rods with different lengths, a dependence of the R-curve behavior on the specimen length was revealed. This is attributed to the strong fiber bridging that was observed in the tests. There are also indications for manufacturing effects on scatter. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction In many applications, e.g., transportation, power generation and power lines, cryogenic magnets, biomedical implants and devices, and structural engineering, the fracture toughness or delamination resistance of glass fiber-reinforced polymer-matrix (GFRP) composites is crucial for the mechanical stability and the durability and hence the long-term, reliable use of GFRP materials and elements (see, e.g., [1–5]). While standard toughness or delamination tests for carbon fiber-reinforced (CFRP) and GFRP composites require special size/shape of the test specimens, e.g., the Double Cantilever Beam (DCB) specimen (see, e.g., [6] for details), there are no standard fracture tests for FRP structural elements or material specimens prepared from them. The standard test for mode I opening loading of CFRP and GFRP composites [7] using the DCB specimen at least provides some guidance on how to prepare starter cracks for specimens which do not have the a starter insert film. There is also literature discussing delamination resistance testing of GFRP material taken from structural elements with specific shape. For mode I tensile opening, one example is filament wound GFRP pipes, GFRP cylinders another (see, e.g., [8–10]), and round-rod specimens with grooves subjected to torsion are discussed by [11]. For composite cylinders, there have also been investigations of other loading modes beside tensile opening mode I, e.g., in-plane shear mode II [12] or mixed mode I/II [13]. For GFRP pultruded rods, such as those used as core of insulating elements in power transmission lines [14] no test methodology for fracture toughness or delamination resistance is yet available to the best knowledge of the authors. The
⇑ Corresponding author. Tel.: +41 58 765 44 93. E-mail address: [email protected] (A.J. Brunner). http://dx.doi.org/10.1016/j.engfracmech.2015.08.009 0013-7944/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Burda I et al. Mode I fracture testing of pultruded glass fiber reinforced epoxy rods: Test development and influence of precracking method and manufacturing. Engng Fract Mech (2015), http://dx.doi.org/10.1016/j.engfracmech.2015.08.009
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Nomenclature CFRP D DCB GFRP l l1 l2 NL point Pre-cracking Pre-cracking Pre-cracking Pre-cracking R-curve 2h 5% point
method method method method
A B C D
carbon fiber-reinforced polymer-matrix diameter of GFRP rod Double Cantilever Beam glass fiber-reinforced polymer-matrix length of GFRP rod specimen length of pre-crack obtained from first method (in combinations of pre-cracking methods) length of pre-crack obtained from second method (in combination of pre-cracking methods) point of deviation of the load–displacement curve from initial linear behavior blade cutting with diamond-coated blade diamond wire cutting knife blade tapping wedge opening using a screw driver delamination resistance curve thickness of GFRP rod after machining point of intersection of a linear curve with 5% increase in compliance with the load–displacement curve
brittle failure of such GFRP rods has been investigated by [15], but no quantitative evaluation of fracture toughness or delamination resistance has been performed. Typically, these GFRP rods are manufactured by pultrusion, the glass fiber content is high (estimated to about 60 volume-% determined from cross-section micrographs of one rod from batch #1) and also the epoxy matrix is highly filled with CaCO3 (typically up to about 20 wt.%). Hence, the first aim of this paper is to develop and evaluate a fracture test methodology for quasi-static mode I tensile opening loading for pultruded GFRP rods. Specifically, different methods for creating the initial starter crack (analogous to the wedge pre-cracking method described in Annex B.8 of [7]) have been investigated and compared. Further, the dependence of the delamination propagation behavior on the length of the GFRP rod has been investigated as well. 2. Materials, experimental methods and data analysis Two series of epoxy GFRP rods (designated batches #1 and #2) were manufactured with a proprietary process, called ‘‘pull-pressing”, from glass fiber (type ECR) and epoxy (resin type EP 828LVEL with anhydride hardener Epikure 3601 and amine-based accelerator EKcat 201). The epoxy matrix was filled with 20 wt.% of CaCO3. This process is different from commercial continuous pultrusion and yields single rods with a length up to about 3.5 m. As in rods from continuous pultrusion, the glass fibers are aligned along the length of the rods. Specimens cut from the rods are hence basically axisymmetric, except for microscopic variation of fiber density. Nevertheless, due to the sequential manufacturing, some variation in properties is possible among specimens cut from different rods (even if taken from the same production date). For a comparison of the effects of manufacturing methods, a third series (designated batch #3) of epoxy GFRP rods manufactured by industrial scale pultrusion (supplied by Pfisterer Sefag AG) has been investigated as well. However, in this case, no details on materials and composition, e.g., fiber or matrix type, or filler type and content, are available. The pull-pressed GFRP rods had a nominal diameter of 18.57 mm with a specified tolerance of 0.10 mm and the pultruded GFRP rods one of 18.6 mm (tolerance specification not available). After pull-pressing, the diameter of the rods of batches #1 and #2 was brought into the specified tolerance by surface machining.
Fig. 1. GFRP specimen size and shape for quasi-static mode I fracture testing, numbers indicate fixed dimensions in millimeters. The nominal diameter b was 18.57 mm for batch #1 and #2, and 18.6 mm for batch #3. The total length l was 90 mm for comparison of the different pre-cracking methods, l1 and l2 indicate pre-cracking lengths for combinations of pre-cracking methods (see Table 1 for details). Selected tests with one pre-cracking method were performed on specimens with lengths l of 150 mm, 210 mm and 270 mm.
Please cite this article in press as: Burda I et al. Mode I fracture testing of pultruded glass fiber reinforced epoxy rods: Test development and influence of precracking method and manufacturing. Engng Fract Mech (2015), http://dx.doi.org/10.1016/j.engfracmech.2015.08.009
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Table 1 Summary of combinations of pre-cracking methods, pre-crack lengths and batches as well as number of specimens tested (see text for a description of the precracking methods A–D). Pre-cracking type
Precracking length [mm]/ method
Precracking length [mm]/ method
Precracking length [mm]/ method
Number of specimens tested, batch number and specimen length
Blade cutting 3 mm thick and diamond wire 0.3 mm diameter and knife blade tapping (A + B1 + C) Blade cutting 3 mm thick and screw driver wedging (A + D) Diamond wire saw only 0.3 mm diameter (B1) Diamond wire saw 0.3 mm diameter and knife blade tapping (B1 + C) Diamond wire saw 0.3 mm diameter and knife blade tapping (B1 + C) Diamond wire saw 0.3 mm diameter and knife blade tapping (B1 + C) Diamond wire saw 0.3 mm diameter and knife blade tapping (B1 + C)
35/A
5/B1
5/C
3 specimens, batch #1, l = 90 mm
30/A
3/D
–
5 specimens, batch #1, l = 90 mm
30/B1 5/B1
– 30/C
– –
30/B1
3/C
–
30/B1
3/C
–
5 specimens, batch #1, l = 90 mm 5 specimens, batch #1, l = 90 mm, long precrack from tapping 5 specimens, batch #1, l = 90 mm, short precrack from tapping 6 specimens, batch #2, l = 90 mm
30/B1
3/C
–
30/B1
3/C
–
30/B1
3/B2
Diamond wire saw 0.3 mm diameter and knife blade tapping (B1 + C) Diamond wire saw 0.3 mm followed by 0.13 mm diameter (B1 + B2)
15 specimens, batch #2, l = 150 mm, 210 mm, 270 mm (5 specimens each per length) 5 specimens, batch #3, l = 90 mm 5 specimens, batch #2, l = 90 mm
The specimen shape and size cut from these GFRP rods which should emulate a DCB-type specimen is schematically shown in Fig. 1, where l1 and l2 indicate the length of the pre-cracks generated with the different methods (see Table 1 for details). The shape of the specimen with top and bottom side machined plane and parallel arose from the necessity to have sufficient grip and sufficiently accurate center points for drilling the pin-holes. For that, about 1 mm was machined off on each side (top and bottom) resulting in a nominal thickness 2 h of about 16.5 mm. For specimens with other diameters, however, this value may have to be adjusted. Putting pin-holes directly into the GFRP rod eliminated the use of complex shaped load introductions and respective corrections for stiffening of the half beams (see [7] for details). Cutting specimens to length and pre-cracking was performed at room-temperature, but without temperature and humidity control of the workshop. Specimens were then stored at +23 °C and 50% relative humidity (controlled within ±2 °C and ±5% relative humidity) before testing and tested in this climate. The duration of conditioning at the test climate before testing varied between about one and several hundred hours. The total pre-crack length was set around 33 mm with a margin of about 10% in order to use relatively short specimens (90 mm long) and simultaneously to avoid use of large deflection corrections (again see [7] for details). The specific cross-section of the GFRP rod specimen required a correction factor for the different moment of inertia compared to a rectangular beam cross section in the modified compliance calibration formula [7] used for calculating GIC for a standard DCB specimen with rectangular cross-section. Four different methods were used for manufacturing pre-cracks: (A) blade cutting (diamond-coated blade with a thickness around 3 mm), (B) wire cutting (diamond wire with a diameter around 0.3 mm labeled B1 and 0.13 mm labeled B2, respectively), (C) knife blade tapping (knife thickness around 0.45 mm), and (D) wedge opening using a screw driver (in combination with an initial crack obtained by blade cutting). In order to restrict crack propagation during pre-cracking by methods C and D, and thus to have a better control of the pre-cracking length, the sample was additionally clamped in a vice. For the comparison of the different pre-cracking methods, selected combinations of the four basic methods A–D were also investigated, which also included different relative lengths of the pre-cracks. Using GFRP rods from batch #2 of pull-pressed GFRP rods, the effects of specimen length were investigated. This included specimens of 90, 150, 210, and 270 mm length, all with pre-cracking with diamond wire saw (0.3 mm diameter) up to about 30 mm and short knife tapping (with the specimen clamped in a vice) for an additional 3 mm, labeled ‘‘B1 + C (3 mm)”. Table 1 summarizes all combinations that have been tested. Please note that there was one method, where a short wire pre-crack (5 mm) was followed by knife blade tapping for 30 mm. This will be referred to as ‘‘B1 + C (30 mm)” to distinguish it from the same combination with long diamond wire (30 mm) and short knife blade tapping pre-crack of 3 mm, i.e., ‘‘B1 + C (3 mm)”. Fracture toughness testing was performed on a tensile test machine (type Z010 from Zwick, with a 1 kN load cell) at a cross-head speed of 1 mm/min. All specimens were tested directly from the respective pre-crack (Table 1) without additional pre-cracking, unloading and reloading (i.e., different from the requirements of [7]). Delamination initiation and propagation was observed visually without the use of a traveling microscope. A cold light source was used to highlight the crack tip in the semi-transparent GFRP composite. Visual observation typically was from one side of the specimen with occasionally checks observing the crack from the top (using a light source, see Fig. 3a) in order to identify possible asymmetric crack tip development across the specimen width. It is well known that visual determination of delamination length is operator-dependent [16] and hence, most tests were performed by one operator, and a few complementary tests by a second operator. All tests were evaluated by one operator, however. A round robin exercise had shown that choosing the onset of nonlinearity (NL) points for delamination initiation from the load–displacement curves to be operator-dependent as well [17]. Please cite this article in press as: Burda I et al. Mode I fracture testing of pultruded glass fiber reinforced epoxy rods: Test development and influence of precracking method and manufacturing. Engng Fract Mech (2015), http://dx.doi.org/10.1016/j.engfracmech.2015.08.009
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For the comparative analysis of the test data, the modified compliance calibration method (MMC), i.e., method B according to [7] was used, however with an additional correction factor for the difference in cross-section between the rod-based and the standard beam specimens. The critical energy release rate, GIC, is determined from Eq. (1) where the value of ‘‘m” is the slope of a plot of the width-normalized cube-root of the specimen compliance C (i.e., (C/b)1/3) versus the thickness-normalized delamination length (a/2h),
2 2 1 3m P ðbCÞ3 2ð2hÞ b CI 2 23 3m P bC GIC ¼ F 2ð2hÞ b N
GIC ¼
ð1Þ ð2Þ
Further, in Eq. (1) 2h denotes the specimen thickness, P the load, b the specimen width, C the specimen compliance calculated from the machine recording of load and displacement, and CI the correction factor for the difference in moment of inertia between the specimens machined from the rod and a beam with rectangular cross-section (same thickness and width equal to the radius of the rod). Compared with Eq. (2) for the MCC method for DCB specimens (given in [7]), the load-block correction factor (denoted N) and the large displacement correction factor (denoted F) have been set equal to 1, whereas the multiplicative inertia correction factor (1/CI) has been added. Since Eq. (1) without the correction factor CI applies to specimens with a rectangular cross-section (thickness 2h and width b), a correction for the different moment of inertia of the cross-section of the GFRP rods was derived taking the width b equal to the nominal radius of the GFRP rods. A numerical comparison resulted in a correction CI of 0.78 determined from comparing the moments of inertia of the specimen used here with that of a beam with rectangular cross-section (width b times thickness 2h). Considering the measured, slight variations in thickness and radius of the different GFRP rod specimens, the variation in this correction factor CI was shown to be between 0.76 and 0.81, i.e., a variation of less than 5%. Hence, all GIC-values were multiplied by a constant correction factor 1/CI amounting to (1/0.78), i.e., neglecting the effect of small variations in specimen geometry on the correction factor (but using the effective width and thickness for each specimen for the other factors in Eq. (1)). Another possible effect to be accounted for is machine and test set-up compliance. For the comparative evaluation of the different pre-cracking methods, the machine and setup compliance was neglected. GIC initiation values were calculated using the loads indicating the NL- and the 5% increase in compliance (C5%) point, respectively determined from the load–displacement plot. Propagation values for determining the resistance curve (R-curve) were also determined from Eq. (1), again see [7] for details. Considering the effects discussed above, the GIC initiation and the propagation values provide a relative comparison between batches of GFRP rods and specimens with different pre-crack types, but shall not be used for design before validation. 3. Test results and discussion 3.1. Comparison of different pre-cracking methods The sharpness of the crack tip produced with the different pre-cracking methods was not measured directly. The initiation values of GIC do depend on a number of factors, the sharpness of the pre-crack being one among others. Relevant additional factors are, e.g., the position of the crack tip with respect to the mid-plane and its orientation, but also the damage created around the crack tip by the pre-cracking method. Based on experience from fracture toughness testing of unreinforced and nanocomposite epoxies [18], blade tapping according to [19] seems to be an effective method for creating a pre-crack approximating a ‘‘natural” crack. The fact that GIC values (presented below) from combination of diamond wire
Fig. 2. Photographs of a 270 mm long GFRP rod specimen (batch #2) during the mode I DCB test (left) showing full specimen length, load introduction and light source used to highlight the position of the crack tip, (right) enlarged view of the part inside the rectangle shown on the left. Note the strong and large-scale fiber bridging.
Please cite this article in press as: Burda I et al. Mode I fracture testing of pultruded glass fiber reinforced epoxy rods: Test development and influence of precracking method and manufacturing. Engng Fract Mech (2015), http://dx.doi.org/10.1016/j.engfracmech.2015.08.009
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sawing and tapping tend to be lower than those from other pre-cracking methods, e.g., A and D is evidence that tapping yields consistent initiation values of GIC. Nevertheless, the data obtained from the rod-based specimens have to be validated. The approaches used for this will be discussed in more detail below. Specimens typically developed large-scale fiber bridging (see Figs. 2 and 3 for examples), extending up to 10 cm or more behind the crack tip. Fiber bridging close to the crack tip (Fig. 4) made clear identification of the tip position sometimes difficult. However, from observation, it is estimated that the accuracy of the delamination length measurement was within ±0.2 mm or less in all cases (see Fig. 4). No significant deviation of the delamination from the mid-plane of the specimens was observed. Fracture initiation and propagation data of selected pre-cracking methods for GFRP rod specimens are shown in Fig. 4. The data were evaluated with a spreadsheet implementing the modified compliance calibration analysis [7] according to Eq. (1) shown above and selected data were checked with another spreadsheet incorporating three methods for analysis (corrected beam theory, experimental compliance method and modified compliance calibration [20]). Initiation values NL and C5% (Fig. 4a) are averages with standard deviations from the number of specimens listed in Table 1. Columns in Fig. 4b show propagation values (excluding initiation data) averaged over the first 25 mm of delamination propagation beyond the tip of the pre-crack of the number of specimens listed in Table 1. Clearly, some pre-cracking methods yield significantly larger initiation and propagation values than others. It can further be noted that average propagation data are larger than the initiation values (both, NL and C5%) for each pre-cracking method. The respective standard deviations (from testing several specimens under nominally identical conditions) seem to increase with increasing initiation or average propagation value. In addition to pre-cracking methods ‘‘B1 + C (30 mm)” and ‘‘A + D”, also all other pre-cracking methods or combinations from Table 1 not shown in Fig. 2 yielded higher initiation and average propagation values than methods ‘‘B1”, ‘‘B1 + B2”, and ‘‘B1 + C (3 mm)”. From visual observation, all specimens, independent of the pre-cracking method used, show evidence of significant fiber bridging (Figs. 2 and 3). Assessing the different pre-cracking methods is based on the assumption that pre-cracking should yield minimal damage at the crack tip or in the fracture process zone surrounding the crack tip and hence result in comparatively ‘‘low” initiation values. Based on this criterion, the pre-crack using only the diamond wire (0.3 mm diameter, i.e., ‘‘B1”), does yield the lowest initiation values (NL and C5%). However, the combination of two diamond wires of different diameter (first 0.3 mm for 30 mm then 0.13 mm for 3 mm, i.e., ‘‘B1 + B2”) and the combination of diamond wire (0.3 mm diameter) and ‘‘short” knife blade tapping, i.e., ‘‘B1 + C (3 mm)” yield slightly higher, but comparable initiation values. This is based on a comparison of the standard deviation from averaging values from five specimens each (NL 240 ± 31 J/m2 and C5% 292 ± 40 J/m2 for thick diamond wire (0.3 mm) only, NL 275 ± 26 J/m2 and C5% 366 ± 37 J/m2 for the combination of thick and thin diamond wire (0.3 mm followed by 0.13 mm) and NL 272 ± 37 J/m2, and C5% 337 ± 38 J/m2 for diamond wire with knife tapping). It is also observed that the methods compared in Fig. 4 yielding higher initiation values also yield higher average propagation values. The reason for this is not clear, in view of the massive fiber bridging observed (Figs. 2 and 3) it could be hypothesized that these effects would dominate over differences in initiation. It can be noted that the correction values D (which were calculated for selected specimens with a spreadsheet analysis developed for comparison of the corrected beam method with the modified compliance calibration method) turned out to be in the range between about 27 mm and 37 mm. This indicates strong fiber bridging or damage based on the analysis discussed by [21]. The data indicate strongly rising R-curves for all pre-cracking methods tested (Fig. 5). There is no clear indication of a plateau in the R-curves, although a maximum value is reached at delamination lengths around 70 mm, i.e., about 20 mm from the end of the specimens, after which the values tend to decrease. This is a potential indication of end effects. It was hence decided to investigate specimens with different lengths (90 mm, 150 mm, 210 mm and 270 mm) all with a pre-crack made by the combination of diamond wire (0.3 mm diameter) and knife blade tapping ‘‘B1 + C (3 mm)”.
Fig. 3. Photograph of the fiber bridging near the crack tip, the crack tip visually determined on the side of this GFRP rod is indicated by the white arrow. The delamination length in this case corresponds to 103.75 mm (with an estimated uncertainty of ±0.05 mm).
Please cite this article in press as: Burda I et al. Mode I fracture testing of pultruded glass fiber reinforced epoxy rods: Test development and influence of precracking method and manufacturing. Engng Fract Mech (2015), http://dx.doi.org/10.1016/j.engfracmech.2015.08.009
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Fig. 4. Comparison of initiation values (a) non-linear (NL) and 5% increase in compliance (C5%) and of (b) average propagation values for selected precracking methods or combinations. Average propagation values are compared for the same delamination length increment of 25 mm beyond the tip of the pre-crack for each specimen (see Table 1 and text for a description of the pre-cracking methods A–D and of the averaging used).
3.2. Investigation of specimen length effects The R-curves of GFRP rod specimens with different lengths are shown in Fig. 6. The data indicate that a minimum delamination length between about 120 mm and 150 mm is required to obtain a maximum in the R-curve. The maximum values seem to range from about 2500 to 3500 J/m2. Again, a drop in the R-curve values is observed at delamination lengths within about 20 mm from the end of the specimens. This is clear evidence that either damage (fracture process zone, possibly extending 10–20 mm) ahead of the visually observed crack tip, or reduced bending stiffness affects delamination propagation toward the end of the rod. Both graphs show scatter among the different R-curves for the individual specimens tested. Among the 270 mm long specimens (Fig. 6b), there is one that yields clearly higher values than the other specimens and a strongly corrugated R-curve. From visual observation during the tests and inspection of the specimen after the test, no indication of a behavior that would invalidate the test has been found. The corrugated R-curve possibly arises from fiber bridging that is larger or a higher amount of fiber bridges failing at about the same time (visually not identified) than in the other specimens. The load and hence the propagation values drop when large fiber bridges break. Typical fiber bridging for the GFRP rods is shown in Figs. 3 and 4. The specimen is retained in the analysis and hence significantly contributes to the scatter in the results. The data analysis revealed an interesting effect, namely a dependence of the initiation values (NL and C5%) and of the average propagation values on the specimen length. The average NL initiation values and standard deviations obtained when considering different delamination lengths for the analysis are summarized in Table 2. The first column in Table 2 shows the average values of four or five specimens with all data points for each specimen included. The second column shows the same data, when data points from the last 20 mm of nominal specimen length are excluded (i.e., the end effects discussed above are eliminated). The third column shows the data with data points from the first 70 mm of the delamination (including Please cite this article in press as: Burda I et al. Mode I fracture testing of pultruded glass fiber reinforced epoxy rods: Test development and influence of precracking method and manufacturing. Engng Fract Mech (2015), http://dx.doi.org/10.1016/j.engfracmech.2015.08.009
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Fig. 5. R-curves for specimens from (a) batch #1 and (b) batch #2 of GFRP rod specimens comparing pre-cracking by wire cutting (diamond wire diameter 0.3 mm) and knife blade tapping with different tapping lengths (‘‘B1 + C (30 mm)” versus ‘‘B1 + C (3 mm)”), i.e., different relative length of method ‘‘B1” and ‘‘C”. Note the significant difference in initiation values GIC and the strong increase in R-curves.
pre-crack). Hence, for specimens of 90 mm length, the values in the second and third column are identical for each batch. There is a steady increase in the average NL initiation value for the specimens from batch #2 when the specimen length is increased from 90 mm to 270 mm. The values for 210 and 270 mm are significantly higher than those from batch #1 to #3 (for a nominal length of 90 mm). With increasing specimen length, the end effects become less important (compare values in the first and second column) as expected. It can also be noted that batch #3 yields the lowest value of all three batches, and consistent values independent of the delamination length that is analyzed. Comparing the three batches of GFRP rods, there is a clear ranking with decreasing toughness values when going from batch #1 to #2, and then to #3. When analyzing data from the same delamination length (70 mm) for specimens from batch #2, the values are consistent independent of the total specimen length. Hence, for comparative testing, data from the same delamination length for all specimens shall be analyzed in order to get consistent results. The reason for this length dependent behavior observed in the GFRP rods is not fully understood yet. It can be hypothesized that it relates to fiber bridging, possibly strongly increasing until a plateau in delamination propagation is observed. For comparative testing, this effect can be taken into account by analyzing specimens of comparable delamination lengths and by choosing specimen lengths 20–30 mm longer than the delamination length. In order to observe the full R-curve behavior, specimen lengths of at least 250 mm are recommended. However, this value may also depend on the type of glass and on the matrix composition of the GFRP rod. These effects will be investigated in a later publication. When data from the first 70 mm delamination length are analyzed with the spreadsheet incorporating corrected beam theory, comparable D-values (delamination length correction, see [7] for details) between about 32 mm and 36 mm are obtained for all specimens independent of their total length. In an earlier analysis of standard GFRP beam specimens [20] a trend for increasing D-values with decreasing number of data points analyzed (and hence with decreasing delamination length) was only observed for very short total delamination lengths (on the order of about 10 mm or less) whereas at longer delamination lengths the D-values were roughly constant independent of the number of data points. Fig. 7 shows the results of the crack speed for GFRP rod specimens with different lengths from batch #2, obtained from point-wise analysis of the crack speed (namely from visually observed delamination length increments and time intervals between load–displacement records from the test machine). The crack speed initially (short crack lengths) drops rapidly and then levels off after about 150–200 mm. For each specimen length, a marked increase in crack speed is observed for Please cite this article in press as: Burda I et al. Mode I fracture testing of pultruded glass fiber reinforced epoxy rods: Test development and influence of precracking method and manufacturing. Engng Fract Mech (2015), http://dx.doi.org/10.1016/j.engfracmech.2015.08.009
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Fig. 6. R-curves for specimens from (a) batch #1 and batch #2 (b) of GFRP rods comparing pre-cracking by diamond wire and knife blade tapping (precracking methods ‘‘B1 + C (30 mm)” with ‘‘B1 + C (3 mm)”, i.e., with different relative length of method ‘‘B1” to method ‘‘C”, note the significant difference in initiation values and the strong increase in R-curves in both cases.
the last data points within about 20 mm from the end. This end effect is also reflected in the drop in propagation values toward the end of the specimen length. These observations are consistent with the assumption of strongly increasing fiber bridging, and simultaneously, strongly increasing R-curves before a maximum in GIC is reached after 150–200 mm crack length.
3.3. Different manufacturing methods The R-curves for specimens from batches #1, #2 and #3 are compared in Fig. 8. With respect to initiation, a difference between batches #pp and #2 of pull-pressed GFRP rods (Fig. 8b) can be noted (average NL of 272 ± 37 J/m2 versus 243 ± 24 J/m2). This difference indicates batch to batch variation that likely exceeds the rod to rod variation from pull-pressing. Batches #1 and #2 were produced about one year apart, from separately prepared matrix formulations. Batches #pp and #2 of pull-pressed GFRP rods, on the other hand, yield somewhat higher initiation values than the GFRP rods
Table 2 Summary of non-linear (NL) initiation values and standard deviations of GIC for different batches and sample lengths (pre-cracking method B1 + C, average of four or five specimens each). Specimen length l [mm] 90
150 210 270
Batch number
NL from full delamination length
NL from delamination length excluding last 20 mm of specimen length
NL from 70 mm total delamination length
1 2 3 2 2 2
271.7 ± 36.7 239.8 ± 25.6 202.7 ± 43.3 250.8 ± 10.6 326.7 ± 14.2 313.9 ± 61.9
263.2 ± 34.9 247.0 ± 15.1 205.7 ± 54.8 289.4 ± 12.0 326.2 ± 13.7 313.5 ± 61.9
263.2 ± 34.9 247.0 ± 15.1 205.7 ± 54.8 246.2 ± 10.0 245.3 ± 16.1 233.7 ± 47.4
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Fig. 7. Crack speed from point-wise analysis of delamination length increments for selected specimens (batch #2) with different length (see insert).
Fig. 8. R-curves including initiation values for GFRP rod specimens from batches #1, #2 and #3. Pre-cracking was performed by wire sawing (diamond wire 0.3 mm diameter) and short knife blade tapping (see insert).
from commercial pultrusion (batch #3, average NL initiation of 186 ± 26 J/m2). With respect to delamination propagation, the R-curves clearly indicate a larger scatter among the specimens of batches #1 and #2 from pull-pressing in comparison to batch #3 from pultruded GFRP rods. Scanning electron microscopy (SEM) micrographs from cross-sections of pull-pressed GFRP rods (batch #1 and #2) shown in Fig. 9a and b indicate rather inhomogeneous distributions in fiber density compared with that from pultrusion shown in Fig. 9c. The resin-rich areas between the fibers and the resulting, more inhomogeneous fiber–matrix distribution in Fig. 9a and b may induce additional delaminations or fiber bridging, resulting in the higher observed scatter in R-curves for the pull-pressed GFRP rods.
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Fig. 9. SEM micrographs for a comparison of fiber distributions in the cross-sections of GFRP rods from pull-pressing of (a), batch #1, (b), batch #2, and from pultrusion (c) batch #3.
3.4. Validation of test method and data analysis The spread-sheet for the calculation of the critical energy release rate, GIC, for the GFRP rod specimens was validated by using a second spreadsheets that had been developed independently. The results calculated by the two spreadsheets differed at most by 2% due to rounding errors in the line fitting and the calculations. The validation of the test method is still on-going. One approach is to use standard DCB-specimens according to [7] cut from GFRP laminated plates with film (about 10 lm thick) as starter cracks manufactured with the same epoxy matrix as the rods. If a correlation between the standard DCB test results and those from the GFRP rods for the same material type is achieved, this will be an indication that the rod test method is at least capable of providing comparative results. Further, the rod and laminates data will be compared with GIC data from unreinforced epoxy specimens tested according to [19]. These investigations are currently being performed and will be reported later. 4. Summary and outlook A modified Double Cantilever Beam (DCB) specimen for mode I tensile opening fracture toughness testing of GFRP rods with circular cross-section has been developed and evaluated in comparative tests of three batches from two different production methods (continuous pultrusion and discontinuous pull-pressing). The circular cross-section had to be reduced for sufficiently accurate drilling of pin holes in the rods for load introduction. This further required a correction factor for data analysis based on the modified compliance calibration method (compared with the rectangular beam cross-section of standard DCB specimens). The data analysis was verified by using two independently developed spread-sheets. A major aim of the investigation was the development of a method for pre-cracking the GFRP rods. It turned out that a pre-crack from a diamond wire saw (two diameters were used), possibly combined with a short crack extension via knife blade tapping yielded sufficiently reproducible and comparatively low initiation values. Applying this pre-cracking method to different batches of GFRP rods yielded indications of effects from the manufacturing process and the resulting fiber distribution on the reproducibility and scatter of the data. In all tests, significant fiber bridging was visually observed and this, quite likely, also yielded delamination length dependent effects in the data analysis. Therefore, in order to characterize the full R-curve behavior of the GFRP rods, sufficiently long specimens (at least 250 mm long, if pre-cracks are limited to less than 35 mm) have to be used. From an application point of view, it may be asked whether the GFRP rod specimens could also be tested under mode I tensile opening cyclic fatigue fracture loading. This has not been attempted yet. The strong fiber bridging could yield rather low crack propagation speeds in cyclic fatigue and hence result in long test duration. It is also questionable whether the pin-holes drilled into the GFRP rods would be sufficiently wear resistant against bearing stress for long-term cycling. In that case, the pinholes could possibly be reinforced with metal inserts, but this has not been evaluated in the present research. Nano-scale fillers have received significant attention for their potential to improve fracture toughness or delamination resistance of glass or carbon fiber-reinforced polymer composites (see, e.g., [22–25]), in spite of some recent critical assessments of the perspectives [26] which, however, referred to carbon nanotube fillers. In view of the special application of the GFRP rods in electrical insulators, several commercial, inorganic (i.e., electrically non-conducting) nano-scale fillers have been added to the epoxy matrix and been investigated with respect to their performance in quasi-static mode I testing of the GFRP rods and compared with the CaCO3-filled commercial product as a first application of the test methodology. These tests indicate that the methodology presented here is capable of discriminating between fracture toughness values obtained from various types of nano-scale fillers in GFRP rods. These results will be presented and discussed in a separate publication. Acknowledgments The assistance by Mr. Michael Buhagiar and Mr. Dietmar Haba for testing and data analysis, by Mr. Lorenzo De Boni, Mr. Marcel Rees and Mr. Kurt Ruf for specimen machining and technical support is gratefully acknowledged. We thank the Swiss Please cite this article in press as: Burda I et al. Mode I fracture testing of pultruded glass fiber reinforced epoxy rods: Test development and influence of precracking method and manufacturing. Engng Fract Mech (2015), http://dx.doi.org/10.1016/j.engfracmech.2015.08.009
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Commission for Technology and Innovation (CTI) for financial support (project No. 11238-1 PFIW-IW), and Dr. Frank Schmuck, Dr. Konstantin O. Papailiou and Mr. Anton Furrer (Sefag Pfisterer AG) for material supply, technical support and discussions. References [1] Gand AK, Chan T-M, Mottram JT. Civil and structural engineering applications, recent trends, re-search and developments on pultruded fiber reinforced polymer closed sections: a review. Frontiers Struct Civil Engng 2013;7(3):227–44. http://dx.doi.org/10.1007/s11709-013-0216-8. [2] Bank LC. Progressive failure and ductility of FRP composites for construction: review. J Compos Constr 2013;17:406–19. http://dx.doi.org/10.1061/ (ASCE)CC.1943-5614.0000355. [3] Stewart A, Douglas EP. Accelerated testing of epoxy-FRP composites for civil infrastructure applications: property changes and mechanisms of degradation. Polym Rev 2012;52(2):115–41. http://dx.doi.org/10.1080/15583724.2012.668152. [4] Hollaway LC. 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Please cite this article in press as: Burda I et al. Mode I fracture testing of pultruded glass fiber reinforced epoxy rods: Test development and influence of precracking method and manufacturing. Engng Fract Mech (2015), http://dx.doi.org/10.1016/j.engfracmech.2015.08.009