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It has been explored that thin ply would improve the mechanical performances of carbon fiber reinforced epoxy composites laminate. It has not been known whether there are similar effects on thermoplastic composites. In this article, carbon fiber reinforced polyamide 6 (CF/PA6) unidirectional fabrics with two thicknesses of 0.045 and 0.105 mm were used to fabricate the unidirectional laminate (UDL). Tensile and bending behaviors of the UDL were recorded with a digital image correlation (DIC) system, their damage macro‐morphologies were observed with high‐speed camera during loading. The fracture microstructures of the samples were observed with SEM and optic microscope. The results showed that the tensile and bending strengths of the samples with 0.045 mm thick are 79.8% and 24.8% higher than that with 0.105 mm thick. Thinning ply can inhibit the crack expanding, and the time from crack originating to fracture of the sample is larger than that of the thick ply one. Morphologies of the fractures revealed that the crack mainly expanded in the matrix along the loading direction for thin‐ply laminate, while the crack developed in the fiber cluster for thick‐ply laminate. The results also revealed that the mechanical properties of the laminate can be improved obviously by using thin‐ply. [ABSTRACT FROM AUTHOR]

The representative volume element (RVE) in fiber composites refers to the smallest volume of material that contains all constituents of the composite, i.e. , fiber and matrix, and is a domain in which the homogenization of microscale response will lead to the macroscale behavior. Therefore, the RVE size in fiber composites is the length scale at which micro to macroscale transition occurs. In this work, a multiscale experimental approach is developed to experimentally characterize the length scale of RVE in a carbon fiber composite with a random fiber distribution. The approach is based on combining optical and scanning electron microscope (SEM) digital image correlation (DIC) measurements. A statistical procedure based on spatial averaging of local strain data is applied to the strain fields obtained from optical and SEM DIC measurements. The convergence between local strains averaged over a carefully selected domain and global strains is used as the criterion for identifying the RVE size in the examined composite. The RVE sizes determined using this approach are stress dependent and increase with the evolution of internal damage in the composite. [Display omitted] [ABSTRACT FROM AUTHOR]

This paper focuses on interface properties between the composite laminae, and the failure strength of carbon fiber reinforced polymer (CFRP) T-stiffeners in a pull-off test. Interfacial fracture toughness was studied and optimized based on the theoretical method and experimental observations in the scanning electron microscope (SEM) view, due to the insertion of fibers or fiber tows into interface adhesive layers created in the manufacturing process. Numerical load-displacement curves associated with different interfacial fracture toughness were calculated and compared with experimental load-displacement curves. It was found that the load-displacement curve calculated by the optimized interfacial fracture toughness was in better agreement with experimental records. Additionally, the filler cracking and interface debonding was numerically predicted using the extended finite element method (XFEM) combined with cohesive zone model (CZM) and experimentally recorded using a high-speed camera. The digital image correlation (DIC) technique was further used to capture deformations and strain distributions in the radius region. Strain distributions obtained from the combined numerical calculation and experimental DIC could help in better understanding the failure mechanisms of CFRP T-stiffeners. Image 1 • We proposed a modification of the interfacial fracture toughness induced by the fiber insertion. • It was simulated that noodle cracking induced interface debonding was responsible for the final failure. • Monitoring by digital image correlation indicated high load in flange direction led to the noodle crack. [ABSTRACT FROM AUTHOR]