Failure Behavior of Pultruded GFRP Members Under Combined Bending and Torsion

Glass fiber reinforced polymer (GFRP) composite sections, manufactured through pultrusion process, are known for their high strength-to-weight ratio, corrosion resistance, low thermal conductivity, electric and magnetic transparency, low life cycle cost and ease of fabrication. They are being widely applied in infrastructure systems.;The load response of thin-walled GFRP composite sections is different from that of isotropic slender members. For instance, stress variation exists across the wall thickness and the FRP members are more prone to warping and local buckling including shear-lag. The literature survey revealed that the response of pultruded GFRP sections under combined bending and torsion has not yet been studied both in terms of the strength and stiffness.;Thru this research, the behavior of full scale sections was investigated under bending, torsion and combined bending and torsion. The shapes under investigation include circular, square and wide-flange with dimensions from 2 to 6 and lengths from 24 to 144 . The analytical part is based on modified flexural and torsional theories for anisotropic materials.;Due to the absence of any formulation on combined bending and torsion of orthotropic sections, the formulae for isotropic sections were extended for orthotropic ones. Some finite element analysis models were also included to make a comparison.;The experimental work consists of (i) determining the laminate properties at coupon level through tension-tests, shear-tests, burnout-tests and microscopy, and (ii) investigating the behavior of full-length samples under 3-point bending, pure torsion and combined bending and torsion. Under torsion and combined loading, a dedicated apparatus was designed, fabricated, instrumented and calibrated at WVU-CFC Major Units laboratory. This apparatus is capable of incorporating samples of cross-sections up to 6 x6 and lengths up to 144 ; with the angle of twist measurement ranging from +60° to -60°.;It was found from coupon tests that strength and modulus values are measurable with a reasonable range of accuracy, but fiber alignment and fiber volume fraction may vary along the cross-section. The bending behavior of full-length closed-sections was controlled by strength and that of wide-flange sections was due to flange-buckling. The torsional behavior of closed sections was also strength controlled, while the wide-flange section practically showed no torsional strength. The behavior under combined bending and torsion was influenced by principal stresses and maximum shear stresses under the effect of compressive bending and torsional shear stresses. On the T/Tmax - M/Mmax interaction curve, some of the data-points agreed-well with the ideal curve, while the others lay beyond that. The reasons of deviation were investigated to be load rate, variations in fiber content and different types of losses in the apparatus