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The effects of two different braiding parameters, the braiding angle and the axial yarn content, on the crush failure characteristics and specific energy absorption capability of two-dimensional (2D) triaxially braided composite circular tubes were studied. Quasi-static axial compression was employed for these crush tests. The crushing appearance and failure modes in the crushing zones were examined by optical microscopy. The results showed that the triaxially braided carbon composite tubes were crushed by a progressive splaying mode. The average width of the splaying fronds increased with increasing braiding angle, but decreased with increasing axial yarn content. The tube with 200 braiding angle showed the highest specific energy absorption among the tubes with various braiding angles. And the tubes using 6k-12k fiber tows as the axial yarn exhibited higher specific energy absorption among the tubes with various axial yarn content.

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In this study, a novel procedure has been developed for predicting the notched strengths of composite plates each with a center hole. In this approach, the stress distribution of a composite plate with a center hole is first obtained by a finite element analysis, in which the experimental notched strength is applied at the boundary of the finite element model. Secondly, the point stress criterion (PSC) is used to find the characteristic length for each plate with different size of hole by an interpolation of the finite element analysis results. The characteristic length is then expressed as an empirical function of the hole size as well as the width of the plate. Finally, the notched strengths of composite plates are predicted based on the empirical function and the finite element analysis results incorporated with the principle of superposition in elasticity. For validation, three different cases from the literatures are adopted for comparison. It is shown that the predicted notched strengths by this new methodology agree well with both the experimental results and the results from analytical solutions based PSC.

In this study, the notched strengths of three kinds of braided composite plates with a center hole were investigated. Both point stress criterion (PSC) and average stress criterion (ASC) were first applied to find the characteristic length of each braided composite plate with different sizes of the hole. It was found that the characteristic lengths predicted by both criteria are not constants. Based on the concept of three-parameter criterion, the characteristic lengths obtained by both criteria were then proposed to be expressed as functions of hole size as well as width of the plate. The notched strength of braided composite plates was then predicted. Finally, it was shown that both modified PSC and modified ASC gave better predictions of the notched strength than traditional PSC and ASC for all the kinds of braided composite plates.

In the past, the fiber bundles were mostly meshed for computing the mechanical properties of the unit cell in fabric composites. Consequently, not only the degrees of freedom but also the complexities in computation are increased. To improve the efficiency in analysis, in this study, a previously developed spring model was extended to predict the effective elastic moduli of three-dimensional (four-step) braided tubes. In the meanwhile, a related compression test was conducted to validate the computational results. It is shown that the results from the computation agree well with the experimental results. In addition, the effects of fabric parameters (such as surface braid angle and fiber volume fraction in a fan-shaped unit cell) on the effective elastic moduli of three-dimensional (four-step) braided tubes were also investigated.