Experimental Investigation of Mode II Fatigue and Size Effect in Unidirectional Carbon/Epoxy Composite Beams

Abstract

This paper investigates mode II quasibrittle behavior and fatigue characteristics within laminated composite structures and examines these through the lens of size effect. The primary focus of this study is mode II quasi-static fatigue. Employing a closed-molding technique, composite beam specimens of varying dimensions were fabricated in-house, utilizing Toray T800S carbon/epoxy prepregs. An initial pre-crack was deliberately introduced at the midplane of each specimen using a Teflon insert, with predefined crack lengths. An experimental quasi-static fatigue investigation was also conducted using load-controlled tests on geometrically scaled specimens. The results of these fatigue tests enabled the formulation of the threshold and stable crack growth regions of the fatigue Paris Curve for three different specimen sizes. This identified increasing values for threshold Stress Intensity Factors, ∆Kth, with increasing specimen size. It also showed increasing slopes for the linear fatigue crack growth for increasing specimen size, demonstrating the size effect influence for both values. The fatigue energy of initiation was also analyzed using both Linear Elastic Fracture Mechanics (LEFM) and Bažant’s Type II Size Effect Law. It was found that calculating these values using LEFM resulted in different values for the smaller specimen sizes, which contradicts the energy of initiation being an intrinsic material property. To address this issue, Bažant’s Type II Size Effect Law was applied to the experimental data which allowed the determination of the fatigue energy of initiation as a consistent material property.