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Tearing failure of web–flange junctions in pultruded GRP profiles
Composites: Part A 36 (2005) 309–317 www.elsevier.com/locate/compositesa
Tearing failure of web–flange junctions in pultruded GRP profiles G.J. Turvey*, Y. Zhang Engineering Department, Lancaster University, Bailrigg, Lancaster LA1 4YR, UK
Abstract Details are presented in the preparation of T-section specimens for determining the tensile tearing strengths of web–flange junctions of two sizes of pultruded glass reinforced plastic GRP wide flange (WF) profiles. Two simple test rigs, used to carry out the tension tests on the web– flange junctions, are described. The tensile tearing strengths derived from 43 tests are presented and the characteristic failure mode of the web–flange junction is explained. It is shown that the tearing strengths of the junctions of the smaller WF profile are larger than those of the larger WF profile and, moreover, that they are only about one-quarter to one-third of the minimum transverse tensile strength of the material given the manufacturer’s design manual. q 2004 Elsevier Ltd. All rights reserved. Keywords: B. Mechanical properties; D. Mechanical testing; A. Polymer-matrix composites (PMCs); E. Pultrusion; B. Strength
1. Introduction Moment-rotation tests on bolted beam to column flange cleat joints between pultruded GRP (WF) profiles have shown that when the ultimate moment of the connection is reached the column flange may be torn from its web due to the high tensile stresses set up in the web–flange junction [1]. A similar mode of tensile tearing of the compression flange from the web has been observed in a four-point flexural failure test on a pultruded GRP WF beam [2]. Likewise, a series of axial compression tests on short pultruded GRP WF columns has shown that total collapse is due to tensile tearing of the flanges from the web [3,4]. The tests, referred to in Refs. [1–4], show that tensile tearing failure generally only occurs after buckling of the flange(s) and/or web of the profile is well developed. An example of this type of failure for a short pultruded hybrid fibre column collapsing under uniaxial compression is shown in Fig. 1. In order to promote the use of pultruded GRP structural grade profiles in appropriate infrastructure applications beyond their current relatively low level, it is essential to enhance our understanding of how these profiles fail and to develop analysis and design tools with which to predict their ultimate load capacities. At this point of time, it appears that * Corresponding author. Tel.: C44-1524-593088. E-mail address: [email protected] (G.J. Turvey). 1359-835X/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2004.06.009
very little progress has been made towards the realisation of these important goals. The majority of recent research on structural grade pultruded GRP profiles has been directed towards the development of knowledge and understanding of the onset of buckling when the profiles are used either as beams or as columns. As far as the authors are aware, the work reported in Ref. [2] represents all that has been achieved up to this point in time in developing a prediction capability for assessing the failure of beams in flexure when tearing separation of the web–flange junction is involved. It is significant that in Ref. [2] the tearing strength of the web–flange junction is, perhaps rather arbitrarily, assumed equal to the average tensile strength of coupons cut transversely from the web and flanges of the GRP WF profile. The weakness of this approach is that the coupon strengths, which are dominated by the strength of the continuous filament mat (CFM), are likely to over-estimate the true tensile strength of the web–flange junction. The reasons for this are twofold. Firstly, the fibres in the CFM are continuous around the root of the web–flange junction, i.e. their radii of curvature change from infinite in the flange to finite around the junction and back to infinite in the web. Secondly, close examination of the web–flange junction region of WF profile cross-sections also shows that localised wrinkling may exist in the CFM layers. Moreover, at the centre of the web–flange junction there is an approximately isosceles triangular shaped core of rovings (Fig. 2). These
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Fig. 1. Tensile tearing separation at the web–flange junctions in a pultruded HF column failing in uniform uniaxial compression.
features are not present in tension coupons cut from either the web or the flanges of pultruded GRP WF profiles. Indeed, when preparing such coupons, great care is exercised to ensure that the web–flange junction is avoided. Thus, in general, the distribution of fibre and matrix material within coupons cut from the web or flanges is much more uniform than the distribution of these materials within the web–flange junction. Hence, tension strengths derived from coupon tests are likely to be significantly higher than the actual tension strength of the web–flange junction. Therefore, it is not altogether surprising that the correlation between the FE analysis and test results reported in Ref. [2] was not particularly close.
If progress is to be made with the development of more accurate analysis and design tools for predicting tensile tearing failure in pultruded GRP beams and columns, it is imperative that improved means of determining the tension and other strength properties of web–flange junctions are developed. This implies that it is necessary to test specimen configurations, which include the web–flange junction. Recognition of this need prompted the authors in 2001 to embark on a programme of research aimed at determining the strength and stiffness properties of the web–flange junctions of pultruded GRP WF profiles. In this paper we present the results of an investigation aimed at quantifying the tensile strengths of the web–flange junctions of two sizes of pultruded GRP WF profiles. First, details of the dimensions and preparation of the web–flange junction specimens are presented. This is followed by a description of the test rigs used to carry out the tensile tearing strength tests. Thereafter, the results of a series of seven preliminary tests on web–flange junctions of the smaller profile are presented. A further 36 junction test results for both sizes of profile are then presented, together with observations of the development and mode of failure within the junction. Finally, it is shown that the tensile strength of the web– flange junction appears to be dependent on specimen size and is much lower than the tensile strength of coupons cut transversely from the web and flanges of the GRP profiles.
2. Preparation of web–flange junction specimens EXTRENw 500 series structural grade WF profiles, manufactured by Strongwell at their factory in Bristol, VA, USA, were selected for the tensile tearing strength tests. (Note: Reference to a product is for factual accuracy purposes only and does not imply any endorsement
Fig. 2. Cross-sections of upper and lower web–flange junctions showing the roving-rich isosceles triangular core (outlined): (a) I4 specimens and (b) I8 specimens.
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Fig. 3. Pairs of web–flange junction specimens, with strain gauges bonded to the roots of the junctions, ready for testing: (a) I4 specimens and (b) I8 specimens.
whatsoever.) The profiles incorporate two types of E-glass reinforcement, viz. rovings (unidirectional fibre bundles) and CFM, held together in a matrix of polyester resin and filler (chalk or clay). The reinforcement, resin and filler volume percentages are typically 60, 30 and 10%, respectively. The web–flange junction specimens were prepared from two sizes of WF profile. The size of the smaller profile was 102!102!6.4 mm (nominal dimensions) and the size of the larger profile was 203!203! 9.5 mm. These specimens are hereinafter referred to as I4 and I8 profiles, respectively. Web–flange junction specimens were prepared in three stages. In the first stage, the I4 and I8 profiles were cut normal to their axes using a band saw to produce in succession 25, 40, 50 or 60 mm long profiles. The cut ends of each short length of I4 or I8 profile were then smoothened using a belt driven sander, which shortened the overall length of the profile by about 1 mm. Each I8 profile was then cut longitudinally along the centreline of its web to produce two T profiles - one containing the top and the other the bottom web–flange junction of the original short length WF profile. Because the depth of the web of the I4 profile was much smaller than that of the I8 profile, only one T profile could be obtained from each I4 profile. Therefore, the top or bottom flange of the profile was removed by cutting the web longitudinally close to the web–flange junction of the flange to be removed. Fig. 3 shows typical examples of the I4 and I8 specimens ready for testing.
the central slot of the clamping plate, was gripped in the jaws of the upper platen of the testing machine. The width of the central slot in the steel clamping plate was too narrow to be used with the I8 T-section specimens. Therefore, two thick steel strips were used to clamp the flange of the I8 Tsection specimen to the steel base plate. In order to maintain a similar clamping force for the individual specimens and to achieve a uniform clamping pressure each of the four bolts in the clamping plates/strips was tightened to the same torque of 20 N m, which, from the preliminary tests, had proved sufficient to hold the specimen firmly in contact with the lower platen of the testing machine. In order to take account of any unevenness in the thickness of the flange of a T-section specimen and also to achieve a uniform clamping pressure, two 20 mm wide polyvinyl plastic strips were placed between the lower surface of the flange of the specimen and the steel base plate. A similar pair of aluminium strips, 20 mm wide and 1 mm in thickness, was placed between the upper surface of the flange of the T-section specimen and the lower surface of the clamping plate or clamping strips. A sketch of the arrangement for clamping the flange of a T-section specimen is shown in Fig. 4 and photographs of I4 and I8 T-section specimens under test in the universal testing machine are shown in Fig. 5.
3. Details of rigs for tensile tearing failure tests on web–flange junction specimens Two simple test rigs were developed for carrying out the tensile tearing strength tests—one for each size of T-section specimen. The rig for the I4 T-section specimens utilised a thick steel plate with a longitudinal central slot to clamp the flange of the specimen to a thick steel base plate, which was bolted to the lower platen of a universal testing machine. The web of the specimen, which protruded through
Fig. 4. Sketch of the loading arrangement for determining the tensile tearing strength of web–flange junctions of I4 and I8 T-section specimens.
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Fig. 5. Photographs of the rigs used for tensile tearing strength tests on web–flange junction specimens: (a) I4 T-section specimen, showing slotted clamping plate and (b) I8 T-section specimen, showing clamping strips.
4. Preliminary tensile tearing failure tests on small web–flange junction specimens Before carrying out the main series of tests on I4 and I8 T-section specimens to determine the tensile tearing strengths of their web–flange junctions, it was decided to carry out seven preliminary tests on I4 T-section specimens. The nominal lengths of the web–flange junctions of these specimens were 25, 40, 50 and 60 mm. Because the width of the grips of the testing machine was only 50 mm, it was not possible to grip the web of the 60 mm long specimen over the whole of its length. However, this minor difference in the test procedure, compared to that of the other six specimens, was not thought to have affected the test results. The seven test specimens were sub-divided into two groups according to whether the web–flange junction under test was the bottom or top junction of the WF profile. These tests were carried out reasonably quickly and with minimal specimen preparation in order to gain some idea of the failure loads and the development of the failure modes. Therefore, no sanding of the ends of the specimens was undertaken and the torques applied to the four bolts, which clamped the flange of the T-section to the base plate, were not identical for each test. The results of this preliminary series of web–flange junction tests, designated TJ1, are presented in Table 1 together with the actual dimensions of each specimen.
The tensile tearing strength of each web–flange junction has been determined by dividing its failure load by the nominal junction area, i.e. the specimen length multiplied by the web thickness. The strengths obtained are plotted as a vertical bar chart in Fig. 6.
5. Tensile tearing failure test results for I4 and I8 web–flange junction specimens Having completed the preliminary series of tensile tearing strength tests on the I4 web–flange junction specimens, the main series of test specimens were prepared. Eighteen I4 and 18 I8 specimens were prepared as described previously. The specimens were cut into three lengths, viz. 25, 40 and 60 mm. All of the I8 specimens were cut from the same length of WF profile, whereas this was only possible for 16 of the I4 profiles, because the I4 profiles used in the preliminary tests were also cut from this profile. The remaining two I4 profiles were cut from a different WF profile. The latter two specimens were designated TJ2.1-BB and TJ2.2-BB, respectively. The 36 web–flange junction T-section specimens were arranged into six groups and labelled I4-TJ2.1, I4-TJ2.2, I4-TJ2.3, I8-TJ1.1, I8-TJ1.2 and I8-TJ1.3. Except for one specimen in each of groups I4-TJ2.1 and I4-TJ2.2, all of the specimens in a group were cut from the same WF profile
Table 1 Geometrical details, failure loads and tensile tearing strengths obtained from tension tests on the I4-TJ1 series of web–flange junction specimens Specimen
Length (mm)
Web thickness (mm)
Failure load (kN)
Tensile tearing strength (N/mm2)
Remarks
I4-TJ1-B1 I4-TJ1-B2 I4-TJ1-B3 I4-TJ1-T1 I4-TJ1-T2.1
25.66 40.91 51.90 26.12 40.25
6.28 6.29 6.26 6.26 6.26
3.26 4.68 6.17 2.90 4.25
20.23 18.19 18.99 17.74 16.87
Audible cracking noise at 2.65 kN
I4-TJ1-T2.2 I4-TJ1-T3
40.92 59.54
6.26 6.26
5.06 7.36
19.75 19.75
Audible cracking noise at 2.52 kN Audible cracking noise at 3.5 kN and drop in tension force Audible cracking noise at 4.6 kN
Note: B1 denotes the bottom web–flange junction of the WF profile for specimen 1 and T1 denotes the top web–flange junction of the WF profile for specimen 1.
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Fig. 6. Tensile tearing strengths of the I4-TJ1 series of web–flange junctions.
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and had the same nominal length. Three of the specimens included the bottom web–flange junction and three the top web–flange junction of the WF profile. The I4-TJ2.1–2.3 specimens were loaded in tension at a rate of about 1 kN/min until failure. Lower loading rates, ranging from about 0.25 to 0.5 kN/min, were used for the majority of the tests on the I8-TJ1.1–1.3 web–flange junction specimens, though two specimens, I8-TJ1.1-B3 and I8-TJ1.3-B1 were loaded at much lower rates, viz. 0.09 and 0.04 kN/min, respectively. A typical tension load versus displacement plot obtained from a test on an I8 web–flange junction specimen is shown in Fig. 7. It is evident that the response is only mildly nonlinear up to about 90% of the ultimate failure load. The detailed results of the tensile tearing strength tests on the I4 and I8 web–flange junction specimens are presented in Tables 2 and 3, respectively. In order to compare the tensile failure strengths of the I4 and I8 web–flange junctions the strengths obtained for all of the 36 specimens tested are presented in the form of a bar chart in Fig. 8. It appears that overall there is slightly more scatter in the tensile tearing strengths obtained from the tests on the I8 than the I4 specimens. There is also some evidence that the average tensile tearing strength of the bottom web–flange junction (right-hand set of bars in each group) is lower than that of the top web–flange junction (left-hand set of bars in each group). Also the tensile tearing strengths of the web– flange junctions of the I8 specimens are lower than those of the I4 specimens.
6. Development of failure in web–flange junctions Fig. 7. Typical load versus displacement response for a tension test on an I8 web–flange junction specimen.
A sketch of the cross-section of the idealised fibre architecture (roving and CFM layers) of the web–flange
Table 2 Geometrical details, failure loads and tensile tearing strengths obtained from tension tests on the I4-TJ2 series of web–flange junction specimens Specimen
Length (mm)
Web thickness (mm)
Failure load (kN)
Failure load per unit length (N/mm)
Tensile tearing strength (MPa)
I4-TJ2.1-T1 I4-TJ2.1-T2 I4-TJ2.1-T3 I4-TJ2.1-B1 I4-TJ2.1-B2 I4-TJ2.1-BB I4-TJ2.2-T1 I4-TJ2.2-T2 I4-TJ2.2-T3 I4-TJ2.2-B1 I4-TJ2.2-B2 I4-TJ2.2-BB I4-TJ2.3-T1 I4-TJ2.3-T2 I4-TJ2.3-T3 I4-TJ2.3-B1 I4-TJ2.3-B2 I4-TJ2.3-B3
25.36 25.27 25.36 25.63 25.32 25.03 40.01 39.81 39.40 40.32 40.42 39.52 61.00 60.39 59.34 60.48 60.16 59.64
6.34 6.32 6.33 6.33 6.32 6.32 6.34 6.32 6.32 6.31 6.31 6.33 6.32 6.30 6.34 6.30 6.33 6.32
3.64 3.64 3.64 3.32 3.57 3.76 5.44 5.70 5.31 5.38 4.50 5.65 7.40 7.60 8.06 6.92 7.34 6.80
143.5 144.0 143.5 129.5 141.0 150.2 136.0 143.2 134.8 133.4 111.3 143.0 121.3 125.8 135.8 114.4 122.0 114.0
22.64 22.79 22.68 20.46 22.31 23.77 21.45 22.66 21.32 21.15 17.64 22.59 19.19 19.98 21.42 18.16 19.27 18.04
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Table 3 Geometrical details, failure loads and tensile tearing strengths obtained from tension tests on the I8-TJ1 series of web–flange junction specimens Specimen
Length (mm)
Web thickness (mm)
Failure load (kN)
Failure load per unit length (N/mm)
Tensile tearing strength (MPa)
I8-TJ1.1-T1 I8-TJ1.1-T2 I8-TJ1.1-T3 I8-TJ1.1-B1 I8-TJ1.1-B2 I8-TJ1.1-B3 I8-TJ1.2-T1 I8-TJ1.2-T2 I8-TJ1.2-T3 I8-TJ1.2-B1 I8-TJ1.2-B2 I8-TJ1.2-B3 I8-TJ1.3-T1 I8-TJ1.3-T2 I8-TJ1.3-T3 I8-TJ1.3-B1 I8-TJ1.3-B2 I8-TJ1.3-B3
25.40 24.70 24.68 25.28 24.73 24.70 40.30 39.53 40.60 40.42 39.66 40.62 60.03 60.00 59.97 60.31 59.78 59.92
9.65 9.64 9.62 9.50 9.50 9.50 9.63 9.67 9.62 9.51 9.56 9.58 9.67 9.67 9.63 9.59 9.56 9.54
4.12 4.75 3.85 3.80 4.50 4.13 5.84 7.62 6.02 5.60 6.81 5.67 9.24 7.48 7.43 8.85 7.64 8.85
162.2 192.3 156.0 150.3 182.0 167.2 144.9 192.8 148.3 138.5 171.7 139.6 153.9 124.7 123.9 146.7 127.8 147.7
16.81 19.95 16.22 15.82 19.15 17.60 15.05 19.93 15.41 14.57 17.96 14.57 15.92 12.89 12.87 15.30 13.37 15.48
Fig. 8. Tensile tearing strengths of web–flange junctions of I4 and I8 WF profiles. (Note: ‘LZ25’ denotes that the nominal length of the specimen is 25 mm and ‘Av. 22.4’ denotes that the average tensile tearing strength is 22.4 MPa).
junction of an I8 profile is shown in Fig. 9. Observation of the web–flange junction area of the specimens’ cross-sections during testing showed that failure initiated as a small interfacial crack in the form of an inverted ‘v’ at the upper vertex of the triangular shaped core of rovings at the centre of the junction. As the tensile load on the junction increased, the crack increased in length and its shape changed to an inverted ‘y’ due to the formation of a vertical crack at the upper vertex of the triangular shaped core of rovings. Further increase in the tensile load caused the cracks to extend outwards along the middle of the flange and upwards along the middle of
Fig. 9. Sketch of the cross-section of the web–flange junction of an I8 WF profile showing the roving/CFM fibre architecture.
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Fig. 10. Sketch of crack initiation and development in the web–flange junction of a test specimen: (a) initial inverted ‘v’ shaped crack, (b) change to an inverted ‘y’ shaped crack as the tensile load increases and (c) crack shape at failure with flexural-tension cracks in the outer CFM layers.
the web until the junction failed. At failure there was also evidence of flexural-tension cracking in the outer CFM layers in the transition radii between the flange and the web. A sketch of this failure progression is shown in Fig. 10 and a photograph of the crack pattern at failure in the web–flange junction of an I8 specimen is shown in Fig. 11.
requires small specimens, it is unlikely that any were cut from the flanges of I4 WF profiles, as they would have had an overall length of about 40 mm. This length is probably too short to allow the coupons to be gripped adequately and provide reliable strength values. It would, however, be practical to cut transverse coupons from the web of an
7. Specimen size effects and comparison with coupon tension strengths It is clear from Fig. 8 that the tensile tearing strengths of the web–flange junctions of the I4 and I8 WF profiles appear to reduce as the length of the specimen increases, i.e. there is a length effect. This is also shown in Fig. 12, where the test data are plotted against specimen length. This length effect may well be attributable to the fact that longer specimens are more likely to contain larger defects. Because failure in web–flange junctions tends to initiate from defects, which are randomly distributed, the longer specimens are likely to exhibit lower tensile strengths. The Weibull weakest link model [5] is widely used to determine the strength of brittle materials from test data. However, insufficient data were obtained from the present tests to establish whether or not the Weibull model is valid for the pultruded GRP material of the web–flange junctions of the two sizes of WF-profiles. Even though the strength at full-scale may be overestimated if size effects are ignored, conventional design procedures continue to ignore such effects, often through lack of data, particularly where pultruded GRP composites are concerned. A recent assessment of the current position regarding size effects in laminated composites has been published by Wisnom [6]. It is of interest to compare the tensile tearing strengths obtained in the present tests with the tensile strengths of coupons cut transversely from the webs and flanges of I4 and I8 profiles. According to the manufacturer’s design handbook [7], the minimum value of the transverse tensile strength of the EXTRENw 500 series I4 and I8 WF profiles is 48.3 MPa. This value has been determined according to ASTM standard D638. It is not clear, however, whether the coupons were cut from both the web and flanges of the I4 and I8 profiles. Even though ASTM standard D638 only
Fig. 11. Typical crack pattern at failure in the web–flange junction of an I8 specimen.
Fig. 12. Tensile tearing strength versus specimen length for web–flange junctions of I4 and I8 WF profiles.
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Table 4 Comparison of the average tensile tearing strengths of web–flange junction specimens and the tensile strengths of coupons cut transversely from WF flange profiles Specimen I4-TJ2.1 I4-TJ2.2 I4-TJ2.3 I8-TJ1.1 I8-TJ1.2 I8-TJ1.3 Transverse coupons (I4 and I8 WF profiles) [7] Transverse coupons (I8 WF profile) Three-point flexure tests on simply supported webs (I8 WF profile) a b
Average length (mm) 25.3 39.9 60.2 24.9 40.2 60.0
Average thickness (mm)
Average tensile strength (MPa)
Comments
6.33 6.32 6.32 9.57 9.60 9.61
22.4 21.1 19.3 17.6 16.3 14.3 48.3
Average of six tests Average of six tests Average of six tests Average of six tests Average of six tests
170a
9.61
63.2
Minimum value derived from coupon tests [ASTM D638] Average of six tests
184b
9.59
92.0
Average of five tests
This is the nominal overall length of the coupons. Their average width was 25.4 mm and the grip length was nominally 30 mm. This is the nominal span dimension. The nominal widths were 25 and 40 mm.
I4 WF profile and the web and flanges of an I8 WF profile. It is of interest to note that the tensile transverse strength of the pultruded GRP web and flange material used in the failure model of Bank and Yin’s analysis [2] was 68.9 MPa. This value is identical with the value of the tensile transverse strength of EXTRENw 500 series plate material given in Ref. [7]. Tension tests carried out recently at Lancaster University under room temperature conditions on coupons cut transversely from the web of an EXTRENw 500 series I8 WF profile gave an average strength of 63.2 MPa. In addition, three-point flexural failure tests, also conducted at Lancaster University, on simply supported webs of I8 WF profiles gave an average transverse flexural strength of 92.0 MPa. These values are compared with the tensile tearing strengths of the web–flange junctions in Table 4. It is evident that the tensile tearing strengths of the web–flange junction specimens range from less than one-third to nearly one-half of the transverse tensile strength given in the Strongwell design manual [7]. They are even lower when compared with the Lancaster University test data. This suggests that it is not adequate to use coupon strength data in modelling failure in pultruded GRP WF profiles where final failure is due to tearing separation between the flange(s) and the web.
8. Concluding remarks A total of 43 tension tests have been carried out on the web–flange junctions of two sizes of pultruded GRP WF profiles in order to quantify the tensile tearing strength of the junction. It has been shown that the tearing strength reduces slightly with specimen length. Moreover, the tensile tearing strengths of the web–flange junctions of the smaller
WF profiles are about one-quarter to one-third greater than those of the larger WF profiles. Comparison of the tensile tearing strengths of web– flange junctions with the tensile strength of coupons cut transversely from the flanges and web of pultruded WF profiles shows that the latter vary from about two to four and a half times the former strengths. It has been shown that failure of the web–flange junction under tension loading arises in a characteristic manner, i.e. as a small inverted ‘v’ shaped crack at the interface of the CFM and the upper vertex of the triangular core of rovings at the centre of the junction. As the load increases the shape of the crack changes from an inverted ‘v’ to an inverted ‘y’ as a crack starts to extend up to the middle of the web. Further increase in the tension load causes the cracks to extend into and along the middle of the flange and up to the middle of the web until failure of the junction occurs. Finally, the results of the present study show that it is not adequate to use simple failure criteria, based on tensile transverse strength data, obtained from simple flat or dog bone coupons, in numerical modelling studies of the failure behaviour of pultruded GRP WF profiles where tearing separation between a flange and a web may arise. The coupon strength values are simply too large. It is essential to test specimen configurations, which include the actual region of failure, and may, therefore, be expected to produce more representative material strength data.
Acknowledgements The results presented in this paper were obtained during the course of a research investigation funded by the UK’s
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EPSRC (Engineering and Physical Sciences Research Council) under Grant No. GR/R28386/01 as part of its Structural Integrity Managed Research Programme. The authors wish to record their appreciation to EPSRC for supporting their research.
References [1] Bank LC, Mosallam AS, Gonsior HE. Beam-to-column connections for pultruded FRP structures. In Serviceability and durability of construction materials. Proceedings of the First Materials Engineering Congress. Materials Engineering Division, ASCE; 1990. p. 804–13.
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[2] Bank LC, Yin J. Analysis of progressive failure of the web–flange junction in post-buckled I-beams. ASCE J Compos Construct 1999; 3(4):177–84. [3] Stubbs D. Buckling tests on pultruded GRP short columns in axial compression. Third Year Project Report. Engineering Department, Lancaster University; 1998. p. 1–159. [4] Yates R. Buckling tests on pultruded HF short columns in axial compression. Third Year Project Report. Engineering Department, Lancaster University; 1999. p. 1–94. [5] Weibull A. Statistical distribution function of wide applicability. J Appl Mech 1951;18(3):293–7. [6] Wisnom MR. Size effects in the testing of composite materials. Compos Sci Technol 1999;59(13):1937–57. [7] Anon. EXTRENw Fiberglass structural shapes design manual, Strongwell, Bristol, VA; 1989.
Tearing failure of web–flange junctions in pultruded GRP profiles G.J. Turvey*, Y. Zhang Engineering Department, Lancaster University, Bailrigg, Lancaster LA1 4YR, UK
Abstract Details are presented in the preparation of T-section specimens for determining the tensile tearing strengths of web–flange junctions of two sizes of pultruded glass reinforced plastic GRP wide flange (WF) profiles. Two simple test rigs, used to carry out the tension tests on the web– flange junctions, are described. The tensile tearing strengths derived from 43 tests are presented and the characteristic failure mode of the web–flange junction is explained. It is shown that the tearing strengths of the junctions of the smaller WF profile are larger than those of the larger WF profile and, moreover, that they are only about one-quarter to one-third of the minimum transverse tensile strength of the material given the manufacturer’s design manual. q 2004 Elsevier Ltd. All rights reserved. Keywords: B. Mechanical properties; D. Mechanical testing; A. Polymer-matrix composites (PMCs); E. Pultrusion; B. Strength
1. Introduction Moment-rotation tests on bolted beam to column flange cleat joints between pultruded GRP (WF) profiles have shown that when the ultimate moment of the connection is reached the column flange may be torn from its web due to the high tensile stresses set up in the web–flange junction [1]. A similar mode of tensile tearing of the compression flange from the web has been observed in a four-point flexural failure test on a pultruded GRP WF beam [2]. Likewise, a series of axial compression tests on short pultruded GRP WF columns has shown that total collapse is due to tensile tearing of the flanges from the web [3,4]. The tests, referred to in Refs. [1–4], show that tensile tearing failure generally only occurs after buckling of the flange(s) and/or web of the profile is well developed. An example of this type of failure for a short pultruded hybrid fibre column collapsing under uniaxial compression is shown in Fig. 1. In order to promote the use of pultruded GRP structural grade profiles in appropriate infrastructure applications beyond their current relatively low level, it is essential to enhance our understanding of how these profiles fail and to develop analysis and design tools with which to predict their ultimate load capacities. At this point of time, it appears that * Corresponding author. Tel.: C44-1524-593088. E-mail address: [email protected] (G.J. Turvey). 1359-835X/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2004.06.009
very little progress has been made towards the realisation of these important goals. The majority of recent research on structural grade pultruded GRP profiles has been directed towards the development of knowledge and understanding of the onset of buckling when the profiles are used either as beams or as columns. As far as the authors are aware, the work reported in Ref. [2] represents all that has been achieved up to this point in time in developing a prediction capability for assessing the failure of beams in flexure when tearing separation of the web–flange junction is involved. It is significant that in Ref. [2] the tearing strength of the web–flange junction is, perhaps rather arbitrarily, assumed equal to the average tensile strength of coupons cut transversely from the web and flanges of the GRP WF profile. The weakness of this approach is that the coupon strengths, which are dominated by the strength of the continuous filament mat (CFM), are likely to over-estimate the true tensile strength of the web–flange junction. The reasons for this are twofold. Firstly, the fibres in the CFM are continuous around the root of the web–flange junction, i.e. their radii of curvature change from infinite in the flange to finite around the junction and back to infinite in the web. Secondly, close examination of the web–flange junction region of WF profile cross-sections also shows that localised wrinkling may exist in the CFM layers. Moreover, at the centre of the web–flange junction there is an approximately isosceles triangular shaped core of rovings (Fig. 2). These
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Fig. 1. Tensile tearing separation at the web–flange junctions in a pultruded HF column failing in uniform uniaxial compression.
features are not present in tension coupons cut from either the web or the flanges of pultruded GRP WF profiles. Indeed, when preparing such coupons, great care is exercised to ensure that the web–flange junction is avoided. Thus, in general, the distribution of fibre and matrix material within coupons cut from the web or flanges is much more uniform than the distribution of these materials within the web–flange junction. Hence, tension strengths derived from coupon tests are likely to be significantly higher than the actual tension strength of the web–flange junction. Therefore, it is not altogether surprising that the correlation between the FE analysis and test results reported in Ref. [2] was not particularly close.
If progress is to be made with the development of more accurate analysis and design tools for predicting tensile tearing failure in pultruded GRP beams and columns, it is imperative that improved means of determining the tension and other strength properties of web–flange junctions are developed. This implies that it is necessary to test specimen configurations, which include the web–flange junction. Recognition of this need prompted the authors in 2001 to embark on a programme of research aimed at determining the strength and stiffness properties of the web–flange junctions of pultruded GRP WF profiles. In this paper we present the results of an investigation aimed at quantifying the tensile strengths of the web–flange junctions of two sizes of pultruded GRP WF profiles. First, details of the dimensions and preparation of the web–flange junction specimens are presented. This is followed by a description of the test rigs used to carry out the tensile tearing strength tests. Thereafter, the results of a series of seven preliminary tests on web–flange junctions of the smaller profile are presented. A further 36 junction test results for both sizes of profile are then presented, together with observations of the development and mode of failure within the junction. Finally, it is shown that the tensile strength of the web– flange junction appears to be dependent on specimen size and is much lower than the tensile strength of coupons cut transversely from the web and flanges of the GRP profiles.
2. Preparation of web–flange junction specimens EXTRENw 500 series structural grade WF profiles, manufactured by Strongwell at their factory in Bristol, VA, USA, were selected for the tensile tearing strength tests. (Note: Reference to a product is for factual accuracy purposes only and does not imply any endorsement
Fig. 2. Cross-sections of upper and lower web–flange junctions showing the roving-rich isosceles triangular core (outlined): (a) I4 specimens and (b) I8 specimens.
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Fig. 3. Pairs of web–flange junction specimens, with strain gauges bonded to the roots of the junctions, ready for testing: (a) I4 specimens and (b) I8 specimens.
whatsoever.) The profiles incorporate two types of E-glass reinforcement, viz. rovings (unidirectional fibre bundles) and CFM, held together in a matrix of polyester resin and filler (chalk or clay). The reinforcement, resin and filler volume percentages are typically 60, 30 and 10%, respectively. The web–flange junction specimens were prepared from two sizes of WF profile. The size of the smaller profile was 102!102!6.4 mm (nominal dimensions) and the size of the larger profile was 203!203! 9.5 mm. These specimens are hereinafter referred to as I4 and I8 profiles, respectively. Web–flange junction specimens were prepared in three stages. In the first stage, the I4 and I8 profiles were cut normal to their axes using a band saw to produce in succession 25, 40, 50 or 60 mm long profiles. The cut ends of each short length of I4 or I8 profile were then smoothened using a belt driven sander, which shortened the overall length of the profile by about 1 mm. Each I8 profile was then cut longitudinally along the centreline of its web to produce two T profiles - one containing the top and the other the bottom web–flange junction of the original short length WF profile. Because the depth of the web of the I4 profile was much smaller than that of the I8 profile, only one T profile could be obtained from each I4 profile. Therefore, the top or bottom flange of the profile was removed by cutting the web longitudinally close to the web–flange junction of the flange to be removed. Fig. 3 shows typical examples of the I4 and I8 specimens ready for testing.
the central slot of the clamping plate, was gripped in the jaws of the upper platen of the testing machine. The width of the central slot in the steel clamping plate was too narrow to be used with the I8 T-section specimens. Therefore, two thick steel strips were used to clamp the flange of the I8 Tsection specimen to the steel base plate. In order to maintain a similar clamping force for the individual specimens and to achieve a uniform clamping pressure each of the four bolts in the clamping plates/strips was tightened to the same torque of 20 N m, which, from the preliminary tests, had proved sufficient to hold the specimen firmly in contact with the lower platen of the testing machine. In order to take account of any unevenness in the thickness of the flange of a T-section specimen and also to achieve a uniform clamping pressure, two 20 mm wide polyvinyl plastic strips were placed between the lower surface of the flange of the specimen and the steel base plate. A similar pair of aluminium strips, 20 mm wide and 1 mm in thickness, was placed between the upper surface of the flange of the T-section specimen and the lower surface of the clamping plate or clamping strips. A sketch of the arrangement for clamping the flange of a T-section specimen is shown in Fig. 4 and photographs of I4 and I8 T-section specimens under test in the universal testing machine are shown in Fig. 5.
3. Details of rigs for tensile tearing failure tests on web–flange junction specimens Two simple test rigs were developed for carrying out the tensile tearing strength tests—one for each size of T-section specimen. The rig for the I4 T-section specimens utilised a thick steel plate with a longitudinal central slot to clamp the flange of the specimen to a thick steel base plate, which was bolted to the lower platen of a universal testing machine. The web of the specimen, which protruded through
Fig. 4. Sketch of the loading arrangement for determining the tensile tearing strength of web–flange junctions of I4 and I8 T-section specimens.
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Fig. 5. Photographs of the rigs used for tensile tearing strength tests on web–flange junction specimens: (a) I4 T-section specimen, showing slotted clamping plate and (b) I8 T-section specimen, showing clamping strips.
4. Preliminary tensile tearing failure tests on small web–flange junction specimens Before carrying out the main series of tests on I4 and I8 T-section specimens to determine the tensile tearing strengths of their web–flange junctions, it was decided to carry out seven preliminary tests on I4 T-section specimens. The nominal lengths of the web–flange junctions of these specimens were 25, 40, 50 and 60 mm. Because the width of the grips of the testing machine was only 50 mm, it was not possible to grip the web of the 60 mm long specimen over the whole of its length. However, this minor difference in the test procedure, compared to that of the other six specimens, was not thought to have affected the test results. The seven test specimens were sub-divided into two groups according to whether the web–flange junction under test was the bottom or top junction of the WF profile. These tests were carried out reasonably quickly and with minimal specimen preparation in order to gain some idea of the failure loads and the development of the failure modes. Therefore, no sanding of the ends of the specimens was undertaken and the torques applied to the four bolts, which clamped the flange of the T-section to the base plate, were not identical for each test. The results of this preliminary series of web–flange junction tests, designated TJ1, are presented in Table 1 together with the actual dimensions of each specimen.
The tensile tearing strength of each web–flange junction has been determined by dividing its failure load by the nominal junction area, i.e. the specimen length multiplied by the web thickness. The strengths obtained are plotted as a vertical bar chart in Fig. 6.
5. Tensile tearing failure test results for I4 and I8 web–flange junction specimens Having completed the preliminary series of tensile tearing strength tests on the I4 web–flange junction specimens, the main series of test specimens were prepared. Eighteen I4 and 18 I8 specimens were prepared as described previously. The specimens were cut into three lengths, viz. 25, 40 and 60 mm. All of the I8 specimens were cut from the same length of WF profile, whereas this was only possible for 16 of the I4 profiles, because the I4 profiles used in the preliminary tests were also cut from this profile. The remaining two I4 profiles were cut from a different WF profile. The latter two specimens were designated TJ2.1-BB and TJ2.2-BB, respectively. The 36 web–flange junction T-section specimens were arranged into six groups and labelled I4-TJ2.1, I4-TJ2.2, I4-TJ2.3, I8-TJ1.1, I8-TJ1.2 and I8-TJ1.3. Except for one specimen in each of groups I4-TJ2.1 and I4-TJ2.2, all of the specimens in a group were cut from the same WF profile
Table 1 Geometrical details, failure loads and tensile tearing strengths obtained from tension tests on the I4-TJ1 series of web–flange junction specimens Specimen
Length (mm)
Web thickness (mm)
Failure load (kN)
Tensile tearing strength (N/mm2)
Remarks
I4-TJ1-B1 I4-TJ1-B2 I4-TJ1-B3 I4-TJ1-T1 I4-TJ1-T2.1
25.66 40.91 51.90 26.12 40.25
6.28 6.29 6.26 6.26 6.26
3.26 4.68 6.17 2.90 4.25
20.23 18.19 18.99 17.74 16.87
Audible cracking noise at 2.65 kN
I4-TJ1-T2.2 I4-TJ1-T3
40.92 59.54
6.26 6.26
5.06 7.36
19.75 19.75
Audible cracking noise at 2.52 kN Audible cracking noise at 3.5 kN and drop in tension force Audible cracking noise at 4.6 kN
Note: B1 denotes the bottom web–flange junction of the WF profile for specimen 1 and T1 denotes the top web–flange junction of the WF profile for specimen 1.
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Fig. 6. Tensile tearing strengths of the I4-TJ1 series of web–flange junctions.
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and had the same nominal length. Three of the specimens included the bottom web–flange junction and three the top web–flange junction of the WF profile. The I4-TJ2.1–2.3 specimens were loaded in tension at a rate of about 1 kN/min until failure. Lower loading rates, ranging from about 0.25 to 0.5 kN/min, were used for the majority of the tests on the I8-TJ1.1–1.3 web–flange junction specimens, though two specimens, I8-TJ1.1-B3 and I8-TJ1.3-B1 were loaded at much lower rates, viz. 0.09 and 0.04 kN/min, respectively. A typical tension load versus displacement plot obtained from a test on an I8 web–flange junction specimen is shown in Fig. 7. It is evident that the response is only mildly nonlinear up to about 90% of the ultimate failure load. The detailed results of the tensile tearing strength tests on the I4 and I8 web–flange junction specimens are presented in Tables 2 and 3, respectively. In order to compare the tensile failure strengths of the I4 and I8 web–flange junctions the strengths obtained for all of the 36 specimens tested are presented in the form of a bar chart in Fig. 8. It appears that overall there is slightly more scatter in the tensile tearing strengths obtained from the tests on the I8 than the I4 specimens. There is also some evidence that the average tensile tearing strength of the bottom web–flange junction (right-hand set of bars in each group) is lower than that of the top web–flange junction (left-hand set of bars in each group). Also the tensile tearing strengths of the web– flange junctions of the I8 specimens are lower than those of the I4 specimens.
6. Development of failure in web–flange junctions Fig. 7. Typical load versus displacement response for a tension test on an I8 web–flange junction specimen.
A sketch of the cross-section of the idealised fibre architecture (roving and CFM layers) of the web–flange
Table 2 Geometrical details, failure loads and tensile tearing strengths obtained from tension tests on the I4-TJ2 series of web–flange junction specimens Specimen
Length (mm)
Web thickness (mm)
Failure load (kN)
Failure load per unit length (N/mm)
Tensile tearing strength (MPa)
I4-TJ2.1-T1 I4-TJ2.1-T2 I4-TJ2.1-T3 I4-TJ2.1-B1 I4-TJ2.1-B2 I4-TJ2.1-BB I4-TJ2.2-T1 I4-TJ2.2-T2 I4-TJ2.2-T3 I4-TJ2.2-B1 I4-TJ2.2-B2 I4-TJ2.2-BB I4-TJ2.3-T1 I4-TJ2.3-T2 I4-TJ2.3-T3 I4-TJ2.3-B1 I4-TJ2.3-B2 I4-TJ2.3-B3
25.36 25.27 25.36 25.63 25.32 25.03 40.01 39.81 39.40 40.32 40.42 39.52 61.00 60.39 59.34 60.48 60.16 59.64
6.34 6.32 6.33 6.33 6.32 6.32 6.34 6.32 6.32 6.31 6.31 6.33 6.32 6.30 6.34 6.30 6.33 6.32
3.64 3.64 3.64 3.32 3.57 3.76 5.44 5.70 5.31 5.38 4.50 5.65 7.40 7.60 8.06 6.92 7.34 6.80
143.5 144.0 143.5 129.5 141.0 150.2 136.0 143.2 134.8 133.4 111.3 143.0 121.3 125.8 135.8 114.4 122.0 114.0
22.64 22.79 22.68 20.46 22.31 23.77 21.45 22.66 21.32 21.15 17.64 22.59 19.19 19.98 21.42 18.16 19.27 18.04
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Table 3 Geometrical details, failure loads and tensile tearing strengths obtained from tension tests on the I8-TJ1 series of web–flange junction specimens Specimen
Length (mm)
Web thickness (mm)
Failure load (kN)
Failure load per unit length (N/mm)
Tensile tearing strength (MPa)
I8-TJ1.1-T1 I8-TJ1.1-T2 I8-TJ1.1-T3 I8-TJ1.1-B1 I8-TJ1.1-B2 I8-TJ1.1-B3 I8-TJ1.2-T1 I8-TJ1.2-T2 I8-TJ1.2-T3 I8-TJ1.2-B1 I8-TJ1.2-B2 I8-TJ1.2-B3 I8-TJ1.3-T1 I8-TJ1.3-T2 I8-TJ1.3-T3 I8-TJ1.3-B1 I8-TJ1.3-B2 I8-TJ1.3-B3
25.40 24.70 24.68 25.28 24.73 24.70 40.30 39.53 40.60 40.42 39.66 40.62 60.03 60.00 59.97 60.31 59.78 59.92
9.65 9.64 9.62 9.50 9.50 9.50 9.63 9.67 9.62 9.51 9.56 9.58 9.67 9.67 9.63 9.59 9.56 9.54
4.12 4.75 3.85 3.80 4.50 4.13 5.84 7.62 6.02 5.60 6.81 5.67 9.24 7.48 7.43 8.85 7.64 8.85
162.2 192.3 156.0 150.3 182.0 167.2 144.9 192.8 148.3 138.5 171.7 139.6 153.9 124.7 123.9 146.7 127.8 147.7
16.81 19.95 16.22 15.82 19.15 17.60 15.05 19.93 15.41 14.57 17.96 14.57 15.92 12.89 12.87 15.30 13.37 15.48
Fig. 8. Tensile tearing strengths of web–flange junctions of I4 and I8 WF profiles. (Note: ‘LZ25’ denotes that the nominal length of the specimen is 25 mm and ‘Av. 22.4’ denotes that the average tensile tearing strength is 22.4 MPa).
junction of an I8 profile is shown in Fig. 9. Observation of the web–flange junction area of the specimens’ cross-sections during testing showed that failure initiated as a small interfacial crack in the form of an inverted ‘v’ at the upper vertex of the triangular shaped core of rovings at the centre of the junction. As the tensile load on the junction increased, the crack increased in length and its shape changed to an inverted ‘y’ due to the formation of a vertical crack at the upper vertex of the triangular shaped core of rovings. Further increase in the tensile load caused the cracks to extend outwards along the middle of the flange and upwards along the middle of
Fig. 9. Sketch of the cross-section of the web–flange junction of an I8 WF profile showing the roving/CFM fibre architecture.
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Fig. 10. Sketch of crack initiation and development in the web–flange junction of a test specimen: (a) initial inverted ‘v’ shaped crack, (b) change to an inverted ‘y’ shaped crack as the tensile load increases and (c) crack shape at failure with flexural-tension cracks in the outer CFM layers.
the web until the junction failed. At failure there was also evidence of flexural-tension cracking in the outer CFM layers in the transition radii between the flange and the web. A sketch of this failure progression is shown in Fig. 10 and a photograph of the crack pattern at failure in the web–flange junction of an I8 specimen is shown in Fig. 11.
requires small specimens, it is unlikely that any were cut from the flanges of I4 WF profiles, as they would have had an overall length of about 40 mm. This length is probably too short to allow the coupons to be gripped adequately and provide reliable strength values. It would, however, be practical to cut transverse coupons from the web of an
7. Specimen size effects and comparison with coupon tension strengths It is clear from Fig. 8 that the tensile tearing strengths of the web–flange junctions of the I4 and I8 WF profiles appear to reduce as the length of the specimen increases, i.e. there is a length effect. This is also shown in Fig. 12, where the test data are plotted against specimen length. This length effect may well be attributable to the fact that longer specimens are more likely to contain larger defects. Because failure in web–flange junctions tends to initiate from defects, which are randomly distributed, the longer specimens are likely to exhibit lower tensile strengths. The Weibull weakest link model [5] is widely used to determine the strength of brittle materials from test data. However, insufficient data were obtained from the present tests to establish whether or not the Weibull model is valid for the pultruded GRP material of the web–flange junctions of the two sizes of WF-profiles. Even though the strength at full-scale may be overestimated if size effects are ignored, conventional design procedures continue to ignore such effects, often through lack of data, particularly where pultruded GRP composites are concerned. A recent assessment of the current position regarding size effects in laminated composites has been published by Wisnom [6]. It is of interest to compare the tensile tearing strengths obtained in the present tests with the tensile strengths of coupons cut transversely from the webs and flanges of I4 and I8 profiles. According to the manufacturer’s design handbook [7], the minimum value of the transverse tensile strength of the EXTRENw 500 series I4 and I8 WF profiles is 48.3 MPa. This value has been determined according to ASTM standard D638. It is not clear, however, whether the coupons were cut from both the web and flanges of the I4 and I8 profiles. Even though ASTM standard D638 only
Fig. 11. Typical crack pattern at failure in the web–flange junction of an I8 specimen.
Fig. 12. Tensile tearing strength versus specimen length for web–flange junctions of I4 and I8 WF profiles.
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Table 4 Comparison of the average tensile tearing strengths of web–flange junction specimens and the tensile strengths of coupons cut transversely from WF flange profiles Specimen I4-TJ2.1 I4-TJ2.2 I4-TJ2.3 I8-TJ1.1 I8-TJ1.2 I8-TJ1.3 Transverse coupons (I4 and I8 WF profiles) [7] Transverse coupons (I8 WF profile) Three-point flexure tests on simply supported webs (I8 WF profile) a b
Average length (mm) 25.3 39.9 60.2 24.9 40.2 60.0
Average thickness (mm)
Average tensile strength (MPa)
Comments
6.33 6.32 6.32 9.57 9.60 9.61
22.4 21.1 19.3 17.6 16.3 14.3 48.3
Average of six tests Average of six tests Average of six tests Average of six tests Average of six tests
170a
9.61
63.2
Minimum value derived from coupon tests [ASTM D638] Average of six tests
184b
9.59
92.0
Average of five tests
This is the nominal overall length of the coupons. Their average width was 25.4 mm and the grip length was nominally 30 mm. This is the nominal span dimension. The nominal widths were 25 and 40 mm.
I4 WF profile and the web and flanges of an I8 WF profile. It is of interest to note that the tensile transverse strength of the pultruded GRP web and flange material used in the failure model of Bank and Yin’s analysis [2] was 68.9 MPa. This value is identical with the value of the tensile transverse strength of EXTRENw 500 series plate material given in Ref. [7]. Tension tests carried out recently at Lancaster University under room temperature conditions on coupons cut transversely from the web of an EXTRENw 500 series I8 WF profile gave an average strength of 63.2 MPa. In addition, three-point flexural failure tests, also conducted at Lancaster University, on simply supported webs of I8 WF profiles gave an average transverse flexural strength of 92.0 MPa. These values are compared with the tensile tearing strengths of the web–flange junctions in Table 4. It is evident that the tensile tearing strengths of the web–flange junction specimens range from less than one-third to nearly one-half of the transverse tensile strength given in the Strongwell design manual [7]. They are even lower when compared with the Lancaster University test data. This suggests that it is not adequate to use coupon strength data in modelling failure in pultruded GRP WF profiles where final failure is due to tearing separation between the flange(s) and the web.
8. Concluding remarks A total of 43 tension tests have been carried out on the web–flange junctions of two sizes of pultruded GRP WF profiles in order to quantify the tensile tearing strength of the junction. It has been shown that the tearing strength reduces slightly with specimen length. Moreover, the tensile tearing strengths of the web–flange junctions of the smaller
WF profiles are about one-quarter to one-third greater than those of the larger WF profiles. Comparison of the tensile tearing strengths of web– flange junctions with the tensile strength of coupons cut transversely from the flanges and web of pultruded WF profiles shows that the latter vary from about two to four and a half times the former strengths. It has been shown that failure of the web–flange junction under tension loading arises in a characteristic manner, i.e. as a small inverted ‘v’ shaped crack at the interface of the CFM and the upper vertex of the triangular core of rovings at the centre of the junction. As the load increases the shape of the crack changes from an inverted ‘v’ to an inverted ‘y’ as a crack starts to extend up to the middle of the web. Further increase in the tension load causes the cracks to extend into and along the middle of the flange and up to the middle of the web until failure of the junction occurs. Finally, the results of the present study show that it is not adequate to use simple failure criteria, based on tensile transverse strength data, obtained from simple flat or dog bone coupons, in numerical modelling studies of the failure behaviour of pultruded GRP WF profiles where tearing separation between a flange and a web may arise. The coupon strength values are simply too large. It is essential to test specimen configurations, which include the actual region of failure, and may, therefore, be expected to produce more representative material strength data.
Acknowledgements The results presented in this paper were obtained during the course of a research investigation funded by the UK’s
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EPSRC (Engineering and Physical Sciences Research Council) under Grant No. GR/R28386/01 as part of its Structural Integrity Managed Research Programme. The authors wish to record their appreciation to EPSRC for supporting their research.
References [1] Bank LC, Mosallam AS, Gonsior HE. Beam-to-column connections for pultruded FRP structures. In Serviceability and durability of construction materials. Proceedings of the First Materials Engineering Congress. Materials Engineering Division, ASCE; 1990. p. 804–13.
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[2] Bank LC, Yin J. Analysis of progressive failure of the web–flange junction in post-buckled I-beams. ASCE J Compos Construct 1999; 3(4):177–84. [3] Stubbs D. Buckling tests on pultruded GRP short columns in axial compression. Third Year Project Report. Engineering Department, Lancaster University; 1998. p. 1–159. [4] Yates R. Buckling tests on pultruded HF short columns in axial compression. Third Year Project Report. Engineering Department, Lancaster University; 1999. p. 1–94. [5] Weibull A. Statistical distribution function of wide applicability. J Appl Mech 1951;18(3):293–7. [6] Wisnom MR. Size effects in the testing of composite materials. Compos Sci Technol 1999;59(13):1937–57. [7] Anon. EXTRENw Fiberglass structural shapes design manual, Strongwell, Bristol, VA; 1989.