Mechanical behavior of hybrid FRP composites with bolted joints

Transcription

1 Mechanical ehavior of hyrid FRP composites with olted joints A.C. Manalo, H. Mutsuyoshi & S. Asamoto Department of Civil and Environmental Engineering, Saitama University, Saitama, Japan T. Aravinthan Centre of Excellence in Engineered Fire Composites, University of Southern Queensland, Toowooma, Queensland, Australia H. Matsui Toray Industries, Incorporated, Tokyo, Japan ABSTRACT: The ehavior of an innovative hyrid Fire Reinforced Polymer (FRP) composite with olted joints was investigated. Coupons and full-size specimens were tested to determine the effect of applied olttorque and the contriution of adhesive onding on the load capacity and failure mode of the hyrid FRP with olted joints. The results showed that at different levels of applied olt torque (1, 15, and 25 N-m), little friction resistance developed. A slight increase on the load capacity was however oserved with increasing tightening torque. On the other hand, the olting accompanied y adhesive onding provided resistance against slipping. The full-size hyrid FRP girder with joints using olts and epoxy exhiited the same strength and stiffness as the girder without joints while olting alone resulted to a eam with only 65% of the stiffness of those without joints. Theoretical analyses were conducted and result showed a good agreement with the experimental results. 1 INTRODUCTION Many infrastructures suffer from corrosion prolems resulting to its reduced service life. This prolem has een severely affecting many ridges especially in the coastal areas of Japan. Maintenance of these infrastructures has increased dramatically hence new materials have een developed to overcome this prolem. FRP composite materials have excellent properties which could e advantageous in harsh environmental conditions ut its high cost has limited its use. A hyrid FRP composite for ridge girder application is now eing developed. The innovative feature of this girder optimizes the comined use caron fire reinforced polymer (CFRP) and glass fire reinforced polymer (GFRP) for maximum structural performance while minimizing production cost (Mutsuyoshi et al., 7). It is elieved that this material can e competitive with other conventional materials in the near future when life cycle cost is taken into account (Nishizaki et al., 6). In view of this, several studies have een undertaken to advance this material for civil infrastructure use. FRP composites are not covered y material standards and information on its material properties is not well known and documented. According to Bank (6), it is generally preferred to determine the material properties of FRP composites experimentally and not rely only on theoretical models. Thus, tests of coupons were initially conducted in order to determine the effective mechanical properties representative of the hyrid FRP composite section. Test method from accepted or existing test standards for FRP composites were adapted and modified to work with thick laminates of the hyrid FRP composites. The design of an efficient connection represents one of the major challenges in the development of composite structures (Magness, 1991). The capacity of a structure is often limited y the capacity of its connection thus a reliale jointing system should e determined in order to use the hyrid FRP girder for practical applications. In this study, fundamental investigation using coupon and full-size specimens was conducted to characterize the strength and ehavior of the hyrid FRP composites with olted joints. The parameters investigated were the effect of applied olt torque and the contriution of epoxy adhesive on the load capacity and failure mode of olted joints. Analytical evaluation of the joint strength using the effective mechanical properties otained from the coupon tests was conducted and compared with the experimental results. 2 MATERIAL PROPERTIES OF HYBRID FRP COMPOSITES 2.1 Test of hyrid FRP coupons A total of 3 tensile and 36 compressive tests using coupon specimens cut from laoratory size panels

2 were conducted to determine the effective mechanical properties of hyrid FRP composites. Steel pipes filled with high strength, low viscosity epoxy was used as anchorage of the hyrid FRP coupons under tensile loading. The FRP coupons were taed over 3 mm at each end with GFRP laminates or inserted into steel pipes filled with epoxy for the compressive strength test to prevent end-crushing. The specimens for the compressive modulus test were identical to the strength test specimens without the tas. The tensile test was conducted in accordance with the ASTM D3916 and ACI 44 specifications as shown in Figure 1a. On the other hand, the hyrid FRP coupons were tested for strength and modulus under compressive load using the test fixture faricated ased on ASTM D as shown schematically in Figure 1. FRP coupon strain gauges a. tensile test. compressive test Figure 1. Coupon test set-up and instrumentation 2.2 Effective mechanical properties Tale 1 shows the effective mechanical properties of the hyrid FRP laminates ased on the coupon test. The effective elastic modulus in the longitudinal (E 11 ) and transverse (E 22 ) directions was determined from the linear portion of the stress-strain curve etween and 3 microstrains. The strength was calculated from the maximum load and the average width and thickness of the coupon specimens. Poisson s ratio was computed as the ratio of the lateral strain to that of the axial strain. These mechanical properties were used in the prediction of the ehavior of the olted joints for hyrid FRP composites. Units P top loading platen Tale 1. Effective mechanical properties of hyrid FRP laminates. Parameters Notations cap head fixing olts FRP coupon strain gauges cap head load guide ottom loading platen Tension Compression Flange We Flange We Modulus N/mm 2 E 11 5,67 15,3 48,35 19,78 N/mm 2 E 22 13,4 15,3 16,28 19,78 Poisson s mm/mm υ ratio mm/mm υ Maximum N/mm 2 σ stress N/mm 2 σ Failure strain % P 3 COUPON TEST OF BOLTED JOINTS 3.1 Experimental program A doule-lap olted joint with 2, 4 and 8 olts was considered as the experimental model. This corresponds to the specimens 2, 4 and 8 respectively. Twenty-seven coupons representing the flanges of the hyrid FRP composite girder were tested up to failure. The description of the specimens for coupon test of olted joints is listed in Tale 2. All of the specimens were prepared ased on the minimum recommended geometric configurations for olted lap joints as listed in Bank (6). Stainless steel olts with a nominal diameter of 1 mm and yield strength of 6 MPa were used. The torque was applied to the olts using a presetting torque wrench at 1, 15, and 25 N-m. Tale 2. Specimen description coupon test of olted joints. Specimen name Numer of specimens Numer of olts Applied olt torque Epoxy adhesives 2-1t-wo without 2-15t-wo without 2-t-wo 5 2 without 2-25t-wo without 4-t-wo 2 4 without 8-t-wo 2 8 without 8-25t-wo without 4-t-e 2 4 with 8-t-e 2 8 with Figure 2 shows the geometrical configuration of the 2-olted joint tested in this study. The specimen with epoxy adhesives and those without had the same configuration. The experimental set-up is illustrated in Figure 3. Angular aluminum ars were attached on oth sides of the FRP laminate and the splice plate. Linear variale differential transducers (LVDT) and draw-wire displacement transducers were then provided on these aluminum ars in order to measure the relative displacement etween the FRP laminate and the splice plate. Splice plate (8x9x232) 4 Filled plate (8x13.5x17) Steel olts, 1 mm φ a. Side view FRP laminate 2- Aluminum plate (8x2x17) 1 mm 82 mm 3 5 mm 5 mm 1 mm 17 mm mm 17 mm. Top view Figure 2. Doule lap onded/olted joint (2-olted joints)

3 P splice plate FRP coupon specimen displacement transducer LVDT strain gauges draw wire displacement transducer Aluminum angular ar Figure 3. Schematic diagram of the olted joint test set-up LVDT draw wire displacement transducer t-w o-1 8-t-w o t-w o t-w o-2 Figure 5. Load-displacement of 8-olted joints at and 25 N- m applied olt torque 3.2 Effect of applied olt toque on joint strength The load-displacement ehavior shown in Figure 4 indicated that with the different levels of applied olt torque there was very little friction resistance since slipping of the joints occurred at the initial loading stage. The ehavior is almost linear indicating that the olts slipped into earing with the FRP laminates. This linear ehavior occurred up to around 7 kn with the displacement etween 2 and 3 mm. The curve ecame non-linear until final failure. This could e due to the initiation of earing failure in the FRP comined with ending of the olts. In the range of the investigated torques, it was oserved that the capacity of olted joints increased slightly with increasing torque. The joints with 25 N-m tightening torque has the highest average load capacity while the specimen with 1 N-m has the lowest. The ehavior of 8-olted specimens using and 25 N-m torque was further examined in order to determine the amount of applied olt torque to e used in the succeeding tests. Figure 5 shows similar loaddisplacement ehavior for all tested specimens. Slipping of the joints occurred at initial loading stage. The displacement increased linearly with load up to 3 kn then ecoming non-linear up to failure. However, the specimens with N-m torque showed higher stiffness and failed at a slightly higher load. It was then concluded that an applied torque of N-m is reasonale for hyrid FRP composites t-w o (94.1) 2-25t-w o (98.8) t-w o (89.1) 2-15t-w o (91.2) Figure 4. Load-displacement of the joints at different levels of applied olt torque 3.3 Contriution of epoxy adhesives The contriution of epoxy adhesives on the strength and ehavior of olted joints for hyrid FRP composite was examined using specimens with 4 and 8- olt configurations at N-m tightening torque. The load-displacement relationship of specimens with and without epoxy adhesives is shown in Figure 6. The results showed that slipping of the olted joints in specimens without epoxy occurred at a lower load. On the other hand, the olting accompanied y adhesive onding provided resistance against slipping etween the splice plates and the FRP laminates. The deonding of the epoxy adhesives lead to an arupt drop in load ut the load again increased thereafter indicating that the applied load was transferred to the olts. The connection then ehaved similar to the olted joints without epoxy adhesives until final failure. The 4-t specimens with and without epoxy failed at almost the same load and similar failure mode was oserved on the FRP laminates. All of the 4-t specimens failed at an applied load of 5 kn. On the other hand, the 8-t specimens with epoxy failed at a slightly lower load compared to that without epoxy. The maximum load that the specimen without epoxy carried is 4 kn while the specimen with epoxy is only around 4 kn. The failure mode for oth specimens was also different t-w o (6) 4-t-e (9) 8-t-w o (419) 8-t-e (48) Figure 6. Load-displacement of joints with and without epoxy

4 3.4 Failure mode The splice plates were removed after every test to examine the damage on the FRP laminates. Figure 7a shows that the specimens without epoxy failed due to earing of the FRP laminates in oth the CFRP and the GFRP with initial signs of shearing out in the CFRP. Crushing of the composite material in the vicinity of the olt-hole interface was oserved. In the specimen with olts and epoxy adhesives, failure was due to earing and delamination etween the CFRP and the GFRP (Figure 7). The high energy released during the epoxy deonding might have caused the delamination. Net-tension failure was also oserved in the GFRP after the final failure. This finding is similar to Duthinh () where the laminates containing +45 o GFRP plies showed low tensile strength and failed mostly in nettension. However, the presence of unidirectional CFRP in the hyrid FRP composites prevented the sudden failure of the specimens. P = 2τ A (2) v and A was computed using the Equation 3. A 2 d = n( π / 4) (3) Figure 8 shows the comparison etween the load carrying capacity of the olted joints otained from the experimental investigation and the predicted values using the theoretical equations. In view of the difficulty in determining the load in which failure of the olted joints occurred, the maximum load carried y the specimen was used as the measure of the joint strength. The results showed that a good agreement was achieved etween the values predicted using Equation 1 and the result of the experiment in the specimen with 2-olted joints. However, a higher load was otained in the actual test compared to the predicted values for the 4 and 8-olted joints. Using the Equation 2, the estimated joint capacities ased on the shear strength of the olts are slightly higher than the experimental results for all the numer of olts. This result showed that the load carrying capacity of the hyrid FRP composites with olted joints can e relialy designed ased on the compressive strength properties of the materials determined from the test of coupon specimens. 5 a. earing failure. net tension failure Figure 7. Failure of the hyrid FRP composite with olted joints with and without epoxy 3.5 Analytical evaluation Theoretical mechanics-ased equations were used to analyze the strength limits of the olted connections. The load capacity due to earing, P was calculated using Equation 1. According to Bank (6), if the earing strength of the FRP composites is not availale, the material compressive strength can e used for approximate calculation. P = σ nd t (1) pl where σ = compressive strength of FRP; n = numer of olts; d = diameter of olts; and t pl = thickness of the laminate. On the other hand, the load capacity of olts in doule shear, P v is given as Equation 2. Shear strength was determined using the shear yield stress of the olt, τ, which was approximated as.6f y, where f y is the yield strength of the olt multiplied y the total cross-sectional area of the olt shank. Load capacity (KN) Numer of olts 2 olts 4 olts 8 olts Eq. 1 (earing) Eq. 2 (olt shear) Figure 8. Predicted and actual strength of olted joints 4 FULL-SCALE TEST OF BOLTED JOINTS 4.1 Experimental program The flexural ehavior of full-size hyrid FRP girder with joints using olts alone (J-FRP) and olts and epoxy (J-FRP-epoxy) at midspan was also determined. CFRP and GFRP of 9. mm thickness comprised the top and ottom flanges of the hyrid FRP girder while only GFRP of 9. mm thickness for the we. The girder was faricated with a length of 37 mm and a clear span of 3 mm. The cut eams were joined at the flanges using 1 mm diameter stainless steel olts and 9 mm thick splice plates. A torque of N-m was applied to all the olts.

5 The eam was simply supported and tested under four-point loading as shown in Figure 9. The load was applied manually y hydraulic jack at third points through a spreader eam. Displacement transducers were installed to measure the displacement and strain gauges were attached on the FRP girder to evaluate the strain during loading until final failure. Data logger was used to record the data with the load-deflection curve displayed during testing to monitor the eam ehavior. A similar full-size eam without joints (FRP-9mm) was initially sujected to a four-point ending test to verify the difference in ehavior of the hyrid FRP composite girder with and without joints. The load and midspan deflection relationship of FRP-9mm, J-FRP and J-FRP-epoxy under four-point loading test is shown in Figure 1. According to the result, specimen FRP-9mm ehaves linear elastic up to failure. The eam failed at an applied load of 94.9 kn and a midspan deflection of 39.1 mm. The slope of the load-midspan deflection of J-FRP is the same with that of FRP-9mm only up to an applied load of 5 kn. With the continuous application of load, the eam showed a slight yet steady decrease in stiffness. This could e due to the slipping of the olts and the gap provided etween the eam end faces which allowed it to rotate. The specimen was loaded up to 97.9 kn and a midspan deflection of 56.3 mm was recorded. At this point, the test was stopped and the load was released. There was no sign of failure oserved in the olts and in the FRP laminates when the splice plates were detached. It was then concluded that if the test was continued, the specimen J-FRP might have carried a higher load efore its final failure. Using the same specimen, epoxy adhesives were provided in the olted joints and the flexural ehavior of this girder was examined under the same loading condition. In the specimen with olted and epoxy onded joints, the load increases linearly with deflection until final failure. The eam failed at an applied load of 93.9 kn with a midspan deflection of 36.9 mm. It is noteworthy that the stiffness of J-FRP-epoxy is slightly higher than that of FRP-9 mm even though the load levels are identical for the two specimens. The difference in oserved stiffness can e attriuted to the delay of epoxy deonding and the contriution of the splice plates on the ending capacity of the eam. The epoxy filling the gap etween the eam ends might have prevented it to rotate. This result suggests that the use of comined olts and epoxy adhesives provided a reliale connection method for hyrid FRP composites. 1 hydraulic jack load cell spreader eam safety rig Support lock FRP Girder FRP Girder olted joints mm Support lock FRP-9mm J-FRP J-FRP-epoxy Figure 1. Load and midspan deflection of hyrid FRP with and without olted joints Figure 9. Test set-up for full-size hyrid FRP girder with joints 4.2 Load-displacement relationship 4.3 Load-strain relationship Figure 11 shows the relationship etween the load and strain of the top and ottom flanges at midspan section of the hyrid FRP girder with and without joints. Result showed that the strains in oth tension and compression increased linearly with load for all specimens. An average maximum strain of 4,5 microns in tension and 4,6 in compression were measured for all specimens. It was oserved that the failure in all specimens occurred at a compressive strain comparale with the recommended value of 4, microstrains in a maximum strain type failure criterion in the compressive flanges of hyrid FRP estalished from the test of coupons. However, the maximum tensile strains measured in all specimens were only 3% of the recommended failure strain Strain (microns) FRP-9 mm, T J-FRP, T J-FRP-epoxy, T FRP-9 mm,c J-FRP, C J-FRP-epoxy, C Figure 11. Load and strain relationship hyrid FRP composite girder with and without olted joints

6 4.4 Failure mode Figure 12 shows the failure mode of specimen FRP- 9 mm. The mode of failure was the delamination etween the CFRP and the GFRP in the compression flange at the constant moment region followed y shear failure at the we. The specimen J-FRP exhiited large amount of deflection when the test was stopped. No signs of failure in the olts and the olt holes of the specimen was seen when the splice plates were removed. For specimen J-FRP-epoxy, failure occurred at the compression flange etween the loading point and the splice plate. During the application of the load, a loud sound was heard following a sharp increase in eam deflection. It was found that the eam failed due to the delamination etween the stiff composites of the comined CFRP and GFRP and the comparatively soft GFRP as shown in Figure 13. Visual inspection showed no sign of deonding failure in the epoxy adhesives. This type of failure was expected since the eam was designed to fail outside the joints. Accordingly, if the joint is sufficiently designed, the loading capacity of the hyrid FRP girder with and without joints is governed y the onding strength at the interface of the CFRP and GFRP. It is therefore recommended that the hyrid FRP composites not e sujected to high compressive stress or further enhancement is needed in the layer composition to prevent such delamination. 5 CONCLUSIONS The test of coupons has proven to e invaluale in the determination of elastic modulus and strength properties of the hyrid FRP composites. Preliminary investigation on the ehavior of olted joints for hyrid FRP composites showed that at different levels of investigated olt torque, little friction resistance developed etween the splice plates and the FRP laminates ut increased significantly with the addition of epoxy adhesives. Bearing failure with initial signs of shearing out was the final failure mode in all olted joint configurations. Furthermore, it was verified that the strength of hyrid FRP composites with olted joints can e approximated using the mechanical properties otained from the coupon test. The hyrid FRP composite girder with joints connected using olts and epoxy adhesives exhiited the same strength and stiffness as the girder without joints while olting alone resulted to a eam with only 65% of the stiffness of those without joints. 6 ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial support provided y the MLIT grant-in-aid for scientific research of construction technology. The assistance of Mr. Nguyen Duc Hai, Ms. Ikumi Yamamoto and Mr. Kensuke Shiroki in conducting the experiment is likewise acknowledged. 7 REFERENCES Figure 12. Failure mode of hyrid FRP girder without joints Figure 13. Failure mode of hyrid FRP girder with olted and epoxy onded joints ACI 44.3R Guide test methods for Fier Reinforced Polymers (FRPs) for reinforcing or strengthening concrete structures. ACI Committee 44. ASTM D 341/ D 341 M-95, Standard test method for compressive properties of polymer matrix composite materials with unsupported gage section y shear loading. American Society for Testing and Materials. Philadelphia, PA. pp Bank, L.C. 6. Composite for Construction: Structural Design with FRP Materials, John Wiley and Sons Ltd., New Jersey. Duthinh, D.. Connections of fier-reinforced polymer (FRP) structural memers: A review of the State of the Art. National Institute of Standards and Technology, Maryland. Magness, F.H Joining of polymer composite materials: A survey. Lawrence Livermore National Laoratory. Mutsuyoshi H., T. Aravinthan, S. Asamoto and K. Suzukawa. 7. Development of new hyrid composite girders consisting of caron and glass fiers. Proceedings of the CO- BRAE Conference 7, 28-3 March. Stuttgart, Germany. Nishizaki, I., N. Takeda, Y. Ishizuka and T. Shimomura. 6. A case study on life cycle cost ased on a real FRP ridge. Proceedings of the Third International Conference on FRP Composite in Civil Engineering (CICE 6), Decemer, Florida, USA.