Mode III translaminar U-notch fracture toughness assessment of unidirectional composite laminates: effectiveness of the virtual isotropic material concept

In this study, a set of fracture tests are conducted on the U-notched specimens made of E-glass/epoxy composite laminates under mode III loading by using the anti-symmetric four-point bend (ASFPB) configuration. The laminates have four different unidirectional lay-up configurations, and the notched specimens have three various notch tip radii. By using the Virtual Isotropic Material Concept (VIMC), the composite laminate is first modeled as an isotropic plate, enabling one to use brittle fracture criteria that exist for the notch fracture toughness prediction of isotropic materials. Then, two stress based brittle fracture criteria, namely the mean stress (MS) criterion and point stress (PS) criterion, are employed to estimate the fracture toughness of the tested U-notched samples. To do so, a novel approach is proposed to obtain the critical stress and critical distances of the composite laminates under mode III loading, which are seriously required in both criteria. A satisfactory agreement is revealed between the predictions of the VIMC-MS and VIMC-PS criteria and the experimental results, confirming their suitability for simply, rapidly, and accurately estimating the out-of-plane translaminar fracture toughness of U-notched composites without the need for layer-wise analysis.