Study of Biaxial Fatigue Behavior of Fiber Reinforced Polymers Under Tensile and Shear Loadings

Fiber reinforced polymers are used in many structural applications in the aerospace and automotive industries because of their high strength to weight and high modulus to weight ratios. In many of these applications, they are used as thin laminated panels comprising of multiple layers of continuous fibers embedded in a polymer matrix. In general, these laminates behave as an orthotropic material and their properties are direction-dependent. While their uniaxial static and fatigue characteristics have been studied extensively, their biaxial static and fatigue characteristics are not well established. One reason for this is the difficulty of conducting biaxial tests, especially under cyclic loading conditions. The objectives of the current research are two folds: (1) develop a biaxial test method that can be applied to a range of normal and shear loadings, and (2) study the biaxial fatigue behavior of a fiber reinforced polymer laminate using the new test method. The test method developed in this research is based on a butterfly-shaped Arcan specimen. The versatility of the Arcan specimen is that it can be utilized for testing materials under uniaxial normal loading, shear loading or a combination of in-plane normal and shear loadings. The laminate considered in this study was a [0/90/04/0]S Eglass/epoxy. Finite element analysis of a butterfly-shaped Arcan specimen was conducted first to establish its optimum geometry and delineate the importance of the stiffness of the test fixture on the stresses in the significant section of the specimen. An Arcan loading fixture was designed with the capability of loading of flat laminate specimens under various combinations of in-plane tensile and shear stresses. Quasi-static and fatigue tests were conducted with four different specimen configurations containing either 0, 30, 45 or 90o fiber orientations in the outer layers. The quasi-static strength followed a quadratic failure envelope on a normal stress-shear stress plane. Biaxial fatigue tests were conducted under combined tensile and shear stresses to determine the effect of biaxiality on the fatigue performance of the laminate. Development of fatigue damage under biaxial loading was also studied. A new fatigue life prediction model was proposed that can be used to account for the effect of biaxiality on the fatigue life of fiber reinforced polymer laminates.Ph.D.Automotive Systems Engineering, College of Engineering and Computer ScienceUniversity of Michigan-Dearbornhttp://deepblue.lib.umich.edu/bitstream/2027.42/136076/1/Mandapati Final Approved Dissertation.pdfDescription of Mandapati Final Approved Dissertation.pdf : Dissertatio