Open hole size effects on tensile properties of 3D braided composites

Owing to the excellent integrated structure, notch-insensitivity, delamination-free characteristics, 3D braided composites have a broad range of engineering applications. In this paper, the notch size effects on two types of 3D braided composites were experimentally examined. Style I incorporated 40% of longitudinal lay-in yarns. Style II was the pure braids. The Point Stress Criterion (PSC) was applied to predict the open-hole strength of 3D braided composites. It is found the 3D braided composites can keep higher proportion residual strength after involving the different circular hole sizes compared to plain woven laminates. The open-hole pure braided specimen shows better performance than that the braids with longitudinal yarns, the lay-in longitudinal yarns improve neither specimens’ un-notched strength, nor the modulus. The predicted open-hole strength were compared with experimental results. The traditional analytical method can predict the open-hole strength of 3D braided composite to some extent. Under uniaxial tensile stress, the failure behaviour of two types of 3D braided specimens are different. For un-notched specimen, clear cracks usually show up on the Style II specimen, while it is not true for Style I coupon. For notched specimen, the crack of both notched specimens will propagate along the notch and finally render the specimen to fail

Effect of Hole Arrangement on Failure Mechanism of Multiple-Hole Fiber Metal Laminate under On-Axis and Off-Axis Loading

Mechanical joints are commonly required in structures made of fiber metal laminate (FML), which pose a threat due to multi-site stress concentrations at rivet or bolt holes. Thus, for a reasonably designed FML joint, it is essential to characterize the failure mechanism of multiple-hole FML; however, little information about this has been found in open literature. In the present work, influences of hole arrangement and loading strategy (on-axis or off-axis) on the failure mechanism of multiple-hole FML were investigated, by performing finite element analyses and energy dissipation analyses with elastoplastic progressive damage models that took curing stress into account. Six types of specimens with holes arranged in parallel and staggered forms were designed, whose geometrical parameters were in strict accordance with those specified for composites joints. It indicated that the stress distribution, gross/net notched strength, critical fracture path, and damage evaluation process were only slightly influenced by the hole number and hole arrangement. On the other hand, they were strongly influenced by the loading strategy, due to the transition of failure domination. Results presented here can provide evidence for introducing design regulations of composite joints into the more hybrid FML, and for reasonably determining its multiple-hole strength merely based on the sing-hole specimen.

A Macro-Meso Correlation Model for Numerical Simulation of CFRP Tensile Notched Strength

In this paper, the tensile mechanical behavior of Carbon Fiber Reinforced Plates (CFRP) with central open-holes was studied by experimental and simulation ways. A correlation model was built by macromechanical analysis and mesodamage analysis to form a progressive analysis architecture. Composite laminates were disassembled into two kinds: Representative Volume Element (RVE), which were 3D intralayer orthotropic, and the 2D interlayer cohesive element. The macromechanical analysis built connections between external loading cases and RVE stress distribution filed. The mesodamage analysis aimed to determine multimode damage initiation and evolution inside RVE. By comparing simulation results with experimental data, the prediction accuracy on failure mode, ultimate load, and fracture morphology were good enough to show the effectiveness and rationality of this new model. In addition, this model’s applicability to different material and geometrical parameters was also verified by simulating the experiment results in the literature.

Notched strengths of asymmetric woven hybrid laminates

In this article, the notched strengths of asymmetric woven hybrid laminates were studied by using both experiment and the finite element-based point stress criterion (FEMPSC). It was experimentally shown that the notched strength decreases with the increase of hole radius for all the laminates machined by water jet cut, drilling, and milling. Particularly, it is noted that the notched strength of specimens of the same hole size made by water jet cut is obviously higher than those made by both drilling and milling, due to stress redistribution. The results also indicate that the notched strengths predicted by FEMPSC agree well with the experimental data, with only a 4% margin of error.