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

Abstract

This paper investigates mode II quasi-brittle fracture and fatigue in laminated compositestructures and characterizes the phenomena in the form of size effect and Cohesive Zone Models (CZMs). This research focuses primarily on mode II quasi-static fracture, but also includes a preliminary experimental investigation of quasi-static fatigue and numerical investigation of forced-dynamic fatigue. For these case studies, composite beam specimens were manufactured in-house using a closed-molding process to create End-Notched Flexure (ENF) test specimens of different sizes, using Toray T700G and T800S carbon/epoxy prepregs. An initial pre-crack was induced using a Teflon insert at the midplane of each specimen, with predetermined crack lengths. Each case aimed to study crack crowth via fracture or fatigue using geometrically scaled sizes to study the size effect phenomena. For the quasi-static fracture case, an additional analysis was applied to identify the influence of Teflon thickness on the fracture energy using two Teflon films with thicknesses of 76 um and 12.5 um, respectively. Experimental data was obtained through ENF tests following American Society for Testing and Materials (ASTM) D7905 parameters, while utilizing microscopic 2D and full-field 3D Digital Image Correlation (DIC) technology to obtain both local and global behavior of the beams. The fracture energy for each Teflon case was analyzed using both Linear Elastic Fracture Mechanics (LEFM) and Bažant’s Type II Size Effect Law (SEL). It was found that using LEFM resulted in different values for differing scaled geometric specimen sizes, contradicting fracture energy as a material property. To address this issue, Bažant’s Type II SEL was applied to the experimental data which produced a single value for fracture energy as a material property. The impact of Teflon size was also observed under this analysis. It was found the thick Teflon, 76 um, created a larger resin pocket in the front of the crack tip, causing a more complex stress profile at the crack tip. Thus, a larger fracture energy was predicted for the thick insert case by size effect analysis. From these experimental results, a CZM was formulated based on the 12.5 um thin Teflon case. This removed the impact of the resin pocket and complex stress distribution found with the 76 um Teflon insert. The CZM parameters were characterized based on microscopic DIC data of the crack tip region obtained from the small size fracture testing. The 3D DIC data was used for validation of global behavior simulated using the CZM, while the experimental result of the thick Teflon insert case provided an additional data set for robust validation of the CZM. The results of this investigation suggest a piece-wise CZM shape to account for the size effect influence. A preliminary experimental quasi-static fatigue investigation was also conducted using displacement controlled tests on size affected 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, and increasing slopes for the linear fatigue crack growth for increasing specimen size, further demonstrating the size effect influence. Future work is outlined to develop all regions of the fatigue Paris Curve using both displacement and load controlled testing methods.