Experimental determination of a probabilistic failure envelope for carbon fiber reinforced polymers under combined compression–shear loads

Abstract

The compressive strength of fiber reinforced composites is sensitive to material imperfections. Material imperfections are spread in the volume in an apparently random manner in the form of fiber misalignment, resulting in non-deterministic strength in compression dominated load cases. The main factors dictating failure under compression dominated loads are the fiber misalignment and the nonlinear material behavior. To enable reliable failure prediction, a quantification of the strength uncertainty is required. Therefore, the current investigation considers different experimental aspects of the problem such as measurements of the fiber misalignment and the material characterization. To implement combined compression–shear loading, a newly in-house developed combined loading fixture was used. A statistically significant number of specimens was tested under aforementioned load cases. Macroscopic images of the failed specimens and a micrograph of the fracture surface are shown to provide evidence of microbuckling failure mode under combined compression–shear loads. Using the experimental strain measurements, a probabilistic failure envelope in strain space is presented. Results of the axial compression load case are interpreted in the context of the notion of the effective misalignment angle using an analytical model. A failure envelope in stress space is derived using an analytical solution for the combined compression–shear load cases and the effective global misalignment angle calculated from the measurements. Applied far field stresses of different load cases are compared, and a visual depiction of the strain localization phenomenon leading to microbuckling failure using digital image correlation is presented.