ii Fatigue Life of Hybrid FRP Composite Beams

(ABSTRACT) As fiber reinforced polymer (FRP) structures find application in highway bridge structures, methodologies for describing their long-term performance under service loading will be a necessity for designers. The designer of FRP bridge structures is faced with out-of-plane damage and delamination at ply interfaces. The damage most often occurs between hybrid plys and dominates the life time response of a thick section FRP structure. The focus of this work is on the performance of the 20.3 cm (8 in) pultruded, hybrid double web I-beam structural shape. Experimental four-point bend fatigue results indicate that overall stiffness reduction of the structure is controlled by the degradation of the tensile flange. The loss of stiffness in the tensile flange results in the redistribution of the stresses and strains, until the initiation of failure by delamination in the compression flange. These observations become the basis of the assumptions used to develop an analytical life prediction model. In the model, the tensile flange stiffness is reduced based on coupon test data, and is used to determine the overall strength reduction of the beam in accordance the residual strength life prediction methodology. Delamination initiation is based on the out-of-plane stress σ z at the free edge. The stresses are calculated using two different approximations, the Primitive Delamination Model and the Minimization of Complementary Energy. The model successfully describes the onset of delamination prior to fiber failure and suggests that out-of-plane failure controls the life of the structure. iii ACKNOWLEDGEMENTS This work could have never been completed without the patience, assistance and friendship of so many people. The past two years have been better than I could have ever anticipated. If I thanked everyone for everything, the people who bind this thing wouldn't be very happy. Hopefully I'll hit most of the important points….. First and foremost, Dr. John " Jack " Lesko-I remember meeting you, walking out of your office and being 100% sure that you were the right advisor for me. Now, two years later, I'm still certain I made the right choice. Your patience, encouragement and advice were essential to my success here. I appreciate all the confidence and respect you had for my work. Beyond the research, I think we had some pretty good laughs. More than once I've had people comment on how well I got along with my advisor. Maybe we weren't always super professional, but I think we still got …