PSI - Issue 13

Emre Özaslan et al. / Procedia Structural Integrity 13 (2018) 535–541 M.A. Güler et al./ Structural Integrity Procedia 00 (2018) 000 – 000

536

2

Nomenclature PSC

Point stress criteria SCF Stress concentration factor W Width of the specimen D Diameter of the hole t Thickness of the specimen 1 E

Ply longitudinal Young modulus Ply transverse Young modulus Ply major Poisson’s ratio Ply longitudinal tensile strength Ply longitudinal compression strength Ply transverse tensile strength Ply transverse compression strength Ply shear modulus

2 E 12 G 12 v t X c X

t Y c Y

S

Ply shear strength

Unnotched specimen strength

0 

0 d

Characteristic distance

1. Introduction

Due to their high strength to weight ratio with high stiffness, fiber reinforced composites have a growing appeal in aerospace, automotive and energy industries for many years. Understanding the mechanical behavior and predicting the accurate strength capability are very important issues for a safe design of composite structures. Creating a notch on the composite structure causes stress concentration and complex failure mechanism. The notch can be created as a circular hole in a composite plate as in the case of present study. Opening a hole creates a stress concentration that reduces overall load carrying capacity of the composite structure. Because of the direct connection between stress concentration and strength of the structure, understanding the stress concentration phenomenon is essential. Toubal et al. (2005) investigated the stress concentration characterization of a carbon/epoxy composite plate with a circular hole and showed that the stress concentration is influenced from the loading direction. They also obtained good correlation between experimental, analytical and numerical results in terms of stress distribution away from the hole but not at the hole edge. Kumar et al. (2016) observed the higher stress concentration factor for shear stress than the loading direction stress under uniaxial tension loading. Wu and Mu (2003) proposed a computational method to obtain stress concentration factor for cylinders and finite width plates with circular holes. In their method the stress concentration factor only depends on the hole diameter to plate width ratio. They achieved consistent results for uniaxial loading but not for biaxial loading. Bakhshandeh et al. (2008) studied the effect of orthotrophy ratio on the stress concentration factor for finite-width plates using three-dimensional finite element model. They showed that the hole diameter to plate width ratio does not have a significant effect on the stress concentration factor when orthotropy ratio is low. Similarly, changing the orthotropy ratio does not create considerable deviation on stress concentration when the hole diameter to plate width ratio is high. Hallet et al. (2009) examined the effect of ply stacking sequence on the open hole tensile strength and failure mode of composite laminates. They observed that the different ply stacking sequences with constant thickness may change the strength of the laminate up to %28. In addition, the changing of the ply stacking sequence may transform the failure mode from brittle failure to delamination failure. Wisnom et al. (2008) studied the size effects in unnotched laminates under tension loading. They concluded that the strength of sublaminates might be increased up to %10 by increasing of the laminate thickness. However, the strength of ply level laminates may decrease by increasing of the laminate thickness. They also emphasized that changing of the laminate size not only affect the strength but also may alter the failure mode. In this study, carbon/epoxy composite specimens with central circular hole 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,