This study provides an experimental investigation on the effect of microcracks on the tensile properties of 3D woven composites. A four-step experimental procedure using the combination of micro-XCT, acoustic emission (AE) and digital image correlation (DIC) is here proposed. Typical tensile damage behaviors were characterized by the stress–strain curves, AE signal analysis and DIC full field strain measurement. Due to a typical four stages stress–strain behavior, phenomena of stiffness degradation and stiffness hardening were successively found during the tensile process. Samples with various damage levels were produced by the in situ AE monitoring. Their 3D microcrack morphologies show the crack initiation, propagation process and the damage modes. Detectable damages initiated during the stress range from 65.98% to 72.93% σs. The cracks volume fraction (CVF) shows a positive correlation relationship with the corresponding tensile load. Moreover, the CVF was used to characterize the degree of damage. The samples with various phased damages were tested again in the fourth step to obtain their residual modulus and residual strength. Detected microcracks have little influence on the residual strength, while the residual modulus witnesses a regular decrease along with the damage increase. The effect of microcracks on the tensile properties is characterized by the relationships between the gradually increased damages and the corresponding residual properties which provide a foundation for damage evaluation of 3D woven structures in service.
Open hole quasi-static and fatigue characterisation of 3D woven composites
