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Carbon-fiber-reinforced polymer (CFRP) composites are widely used in lightweight structures because of their high specific strength, specific modulus, and low coefficient of thermal expansion. Additionally, the unidirectionally arrayed chopped strand (UACS) laminates have excellent mechanical properties and flowability, making them suitable for fabricating structures with complex geometry. In this paper, the damage process of UACS quasi-isotropic laminates under tensile load was tested using acoustic emission detection technology. The mechanical properties and damage failure mechanism of UACS laminates were studied combined with finite element calculation. By comparing and analyzing the characteristic parameters of acoustic emission signals such as amplitude, relative energy, and impact event, it is found that acoustic emission behavior can accurately describe the damage evolution of specimens during loading. The results show that the high-amplitude signals representing fiber fracture in continuous fiber laminates are concentrated in the last 41%, while in UACS laminates they are concentrated in the last 30%. In UACS laminates, more of the damage is caused by matrix cracks and delamination with medium- and low-amplitude signals, which indicates that UACS laminates have a good suppression effect on damage propagation. The stress–strain curves obtained from finite element analysis agree well with the experiment results, showing the same damage sequence, which confirms that the model described in this research is reliable. [ABSTRACT FROM AUTHOR]

Variable Angle Tow placement is a way to steer individual curvilinear fibers. This work presents the assessment of tensile behavior of open-hole composite laminates with Variable Angle Tow reinforcement. A new multi-scale finite element method, consisting of a microscale unit cell model and a macroscale gradient property model, is developed to simulate Variable Angle Tow structures with various fiber trajectories. The tensile strength and the failure process of open-hole reinforced laminates with Variable Angle Tow reinforcement under tensile loading are predicted and analyzed. Experiments are also conducted to investigate reinforcing efficiency and failure modes of the open-hole laminates. The comparison of predicted and experimental results for the tensile strength and failure modes of T700/Epoxy laminates demonstrates clearly that the mechanical behavior of Variable Angle Tow structure can be simulated very well by the proposed multi-scale model. Moreover, it is found that the tensile strength of Variable Angle Tow laminates is closely related to the eccentricity and it reaches the maximum value only when the trajectories of curvilinear fibers keeps consistent with maximum principal stress trajectories of the open-hole plate. [ABSTRACT FROM AUTHOR]

The effect of off-axis ply on the tensile properties of unbalanced symmetric [0/θ]ns laminates was explored through experimental and numerical analysis. Six CFRP [0/θ]2s plies with different off-axis angles θ were fabricated for tensile tests. In situ observations of the damage to the laminates were conducted to investigate the initiation and progressive growth of the laminates during the tension tests. The fiber fractures, crack initiation, and progressive propagation were analyzed by observing the free edge of the laminates, and the difference in damage behavior caused by different off-axis angles was investigated. All the six [0/θ]2s plies with off-axis angles θ ranging from 15° to 90° showed approximate linear stress–strain responses in the tensile tests. Matrix cracks were not observed prior to the final catastrophic failure in the off-axis layers of the [0/θ]2s laminates with a θ in the range of 15–60°. Finite element analysis (FEA) of the [0/θ]s plies was conducted using a 3D micromechanical model, in which matrix cracking and fiber-matrix debonding in the off-axis layer were simulated using a cohesive interface element. Three micromechanical crack-free, cohesive interface, and initial crack models were analyzed to predict the influence of the matrix cracks inside the off-axis layer on the damage behavior of the [0/θ]s laminates. The numerical results from the initial crack micromechanical model show a lower bound of the tensile strength of the [0/θ]s plies. A high stress concentration is observed adjacent to the cracked off-axis layer, inducing a tensile strength loss of about 20%. [ABSTRACT FROM AUTHOR]

Laminates with unidirectionally arrayed chopped strands (UACS) are one of the advanced short fiber reinforced polymer composites (SFRP) with significant application prospect, which greatly improves mechanical properties compared to the traditional SFRP, meanwhile ensuring excellent flowability. In practice, composite laminate with an open hole is one of the typical connective components, and it is necessary to clarify the allowable load and damage tolerance performance of notched structures. In the present study, UACS laminates were fabricated using the continuous carbon fiber reinforced polymer (CFRP) prepreg, on which regularly arrayed bi-angled slits were introduced by a commercial numerical control cutter. The tensile strength and strain distribution around the open hole of the notched UACS laminate were experimentally investigated, while the damage progression near the open hole of the notched UACS laminate was analyzed by the finite element method (FEM). The tensile strength of the notched UACS laminate was measured at 298 MPa, which is about 60% of the strength of the unnotched UACS laminate. The simulation results match well with the experimental results, although there is a little overestimate on strength, by about 5% and 7%, for unnotched and notched UACS laminates, respectively. The final critical failure mode for the notched UACS laminate is mainly dominated by the delamination instead of the fiber breakage in the unnotched UACS laminate. [ABSTRACT FROM AUTHOR]