The objective of this study was to characterize the early structural degradation behavior of tubular braided composites (TBCs) under quasi-static tension through direct quantification of the surface strains developed in their yarn-reinforced and unreinforced (resin-rich) material regions. Kevlar-epoxy TBCs with varying braid angles and numbers of yarns were analyzed through quasi-static testing and stereo digital image correlation (DIC) analysis to understand how the yarn-reinforced and resin-rich regions deformed under tensile loads up to the initiation of structural degradation within the braid structure. Structural weakening was confirmed to initiate in the unreinforced, resin-rich regions of the TBC specimens by way of a normal tensile failure mode, as the strains developed within these regions were the first to deviate from a linear-elastic growth trend and also exceed the expected strain to failure of their constituent material. Through two-way full-factorial ANOVA analysis with a 95% confidence interval, the stress required to initiate degradation within these regions was found to depend significantly upon both the quantity and angular orientation of the preform yarns, the former to a markedly lower extent. The maximum strain accumulated in the resin-rich regions upon their degradation was found to not statistically depend on either of these parameters, affirming that it is purely defined by the strength properties of the constituent neat resin material. To provide conservative and repeatable estimates of the stress to failure in TBC structures, it is recommended that this quantity be estimated from both the global stress-strain data of the entire specimen and the local stress-strain data measured exclusively within its resin-rich regions, with the lower of the two taken. Through these means, the relationship between a TBC’s preform geometry and its stress to failure may be optimally modeled using a polynomial regression trend.

中文翻译：

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通过逐区应变分析检查管状编织复合材料的初始结构退化

本研究的目的是通过直接量化纱线增强和未增强（富含树脂）材料区域中产生的表面应变，表征管状编织复合材料 (TBC) 在准静态张力下的早期结构降解行为。通过准静态测试和立体数字图像相关 (DIC) 分析，对具有不同编织角度和纱线数量的 Kevlar-epoxy TBCs 进行了分析，以了解纱线增强和富含树脂的区域在拉伸载荷下如何变形直到结构开始编织结构内的降解。通过正常的拉伸破坏模式，结构弱化被证实在 TBC 试样的未增强、富含树脂的区域开始，因为在这些区域内产生的应变首先偏离了线性弹性增长趋势，并且还超过了其组成材料的预期失效应变。通过具有 95% 置信区间的双向全因子 ANOVA 分析，发现在这些区域内引发降解所需的应力显着取决于预制纱线的数量和角度取向，前者的程度明显较低。发现在富含树脂的区域降解时积累的最大应变在统计上不依赖于这些参数中的任何一个，确认它纯粹是由组成纯树脂材料的强度特性定义的。为 TBC 结构中的失效应力提供保守且可重复的估计，建议根据整个试样的整体应力应变数据和仅在其富含树脂的区域内测量的局部应力应变数据来估计该数量，取两者中的较低者。通过这些方式，可以使用多项式回归趋势对 TBC 的预制件几何形状与其失效应力之间的关系进行最佳建模。