Experimental and numerical investigation of stress concentration and strength prediction of carbon/epoxy composites

Abstract

Unidirectional composites are very popular structural materials used in aerospace, marine, energy and automotive industries, thanks to their superior material properties. However, the mechanical behavior of composite materials is more complicated than isotropic materials because of their anisotropic nature. Also, stress concentration presence on the structure, like a hole, makes the problem further complicated. Therefore, enormous numbers of tests are required in understanding the mechanical behavior and strength of composites which contain stress concentration. Accurate finite element analysis and analytical models enable us to understand mechanical behavior and predict the strength of composites without enormous number of tests which cost serious time and money. In this study, unidirectional Carbon/Epoxy composite specimens with central circular holes were investigated in terms of stress concentration factor and strength prediction. The composite specimens which had different specimen width (W) to hole diameter (D) ratio were tested to investigate the effect of hole size on the stress concentration and strength. Also, specimens which had same specimen width to hole diameter ratio, but different sizes were tested to investigate the size effect. Finite element analysis were performed to determine the stress concentration factor for all specimen configurations. Also, Point stress criteria (PSC) was used to predict strength of the specimens. For quasi-isotropic laminate, it was found that the stress concentration factor at the hole tip increases approximately %15 as W/D ratio decreased from 6 to 3. Also, it is observed that as W/D ratio quadrupled with constant specimen width the failure strength increases by %62.4. For the specimens which had same width to hole diameter ratio but different (scaled) dimensions, the stress concentration factor at the hole tip was seen to be identical as expected. For these specimens, as W/D ratio doubled the specimen failure strength reduces by %13.2 because of the size effect. For all type of specimens, the PSC method could predict the strength of specimens with maximum %8 error.