Study of Local Mechanical Responses in an Epoxy–Carbon Fiber Laminate Composite Using Spherical Indentation Stress–Strain Protocols

Successful deployment of the highly heterogeneous, laminated, polymer matrix composites (PMCs) in high-performance structural applications is currently hindered by the lack of reliable experimental protocols for evaluation of the local mechanical responses at the salient meso-length/structure scales present in these material systems. Our main interest in this paper lies in establishing and demonstrating protocols for high-throughput evaluation of the local mechanical responses in PMCs at a length scale larger than the fiber diameter but smaller than the individual laminate (i.e., ply) thickness. This goal was accomplished in this work through a successful extension of the spherical indentation stress–strain protocols demonstrated recently for metallic samples. Specifically, plies with fibers at 0°, 30°, 60°, and 90° to the indentation direction were tested, and the means and standard deviations of their indentation moduli and the indentation yield strengths were measured and reported in this paper. The measured values of the indentation moduli were validated with finite element (FE) simulations performed using estimated values of the effective single laminate stiffness parameters. Furthermore, the measured variation in the indentation moduli was shown to correlate extremely well with the corresponding FE predictions that accounted for the measured variation in the local fiber volume fractions in the primary indentation deformed zones in the sample. These comparisons provided strong support for the validity of the extended spherical indentation protocols developed in this work for PMC samples.