In situ micromechanical analysis of discontinuous fiber-reinforced composite material based on DVC strain and fiber orientation fields

The influence of fiber orientation and its distribution on the global and local mechanical behavior of a short fiber-reinforced thermoplastic (SFT) composite is studied based on an in situ tensile test using synchrotron X-ray tomography. The in situ test data are utilized to compute three-dimensional (3D) strain fields inside the material at various loading steps through an in-house digital volume correlation (DVC) code. The DVC analysis finds that, although the global strain fields seem relatively uniform, the internal strain fields are locally non-uniform because of the complex reinforcement architecture. The local non-uniformity is further investigated at a single fiber level to find a correlation between the orientation of fibers, fiber volume fraction, and strain field. The investigation is performed on micro-scale subvolumes, in which all the fibers are individually segmented from the 3D tomographic data. It is found that the fibers aligned in the direction transverse to loading impede transversal shrinkage, resulting in a locally small compressive strain in that direction. The in situ test has revealed that the average fiber orientation does not change noticeably even after the global strain reaches 18%. This small change implies that initially-determined fiber orientation is crucial to the locally anisotropic mechanical behavior of a SFT-like material even undergoing large deformation. On the other hand, the loading-directional strain is more affected by the local volume fraction than the fiber orientation. The local volume fraction also mainly determines the stiffness of the subvolume that can be estimated from an analytical homogenization model. The effect of the local volume fraction on the subvolume stiffness is identified from multivariate analysis