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首页> 外文OA文献 >A Stochastic Model Based on Fiber Breakage and Matrix Creep for the Stress-Rupture Failure of Unidirectional Continuous Fiber Composites 2. Non-linear Matrix Creep Effects
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A Stochastic Model Based on Fiber Breakage and Matrix Creep for the Stress-Rupture Failure of Unidirectional Continuous Fiber Composites 2. Non-linear Matrix Creep Effects

机译：基于纤维破裂和基质蠕变的随机模型，用于单向连续纤维复合材料的应力破裂衰竭2.非线性矩阵蠕变效应

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Stress rupture (sometimes called creep-rupture) is a time-dependent failure mode occurring in unidirectional fiber composites under high tensile loads sustained over long times (e. g., many years), resulting in highly variable lifetimes and where failure has catastrophic consequences. Stress-rupture is of particular concern in such structures as composite overwrapped pressure vessels (COPVs), tension members in infrastructure applications (suspended roofs, post-tensioned bridge cables) and high angular velocity rotors (e.g., flywheels, centrifuges, and propellers). At the micromechanical level, stress rupture begins with the failure of some individual fibers at random flaws, followed by local load-transfer to neighboring intact fibers through shear stresses in the matrix. Over time, the matrix between the fibers creeps in shear, which causes lengthening of local fiber overload zones around previous fiber breaks, resulting in even more fiber breaks, and eventually, formation clusters of fiber breaks of various sizes, one of which eventually grows to a catastrophically unstable size. Most previous models are direct extension of classic stochastic breakdown models for a single fiber, and do not reflect the micromechanical detail, particularly in terms of the creep behavior of the matrix. These models may be adequate for interpreting experimental, composite stress rupture data under a constant load in service; however, they are of highly questionable accuracy under more complex loading profiles, especially ones that initially include a brief “proof test” at a “proof load” of up to 1.5 times the chosen service load. Such models typically predict an improved reliability for proof-test survivors that is higher than the reliability without such a proof test. In our previous work relevant to carbon fiber/epoxy composite structures we showed that damage occurs in the form of a large number of fiber breaks that would not otherwise occur, and in many important circumstances the net effect is reduced reliability over time, if the proof stress is too high. The current paper continues our previous work by revising the model for matrix creep to include non-linear creep whereby power-law creep behavior occurs not only in time but also in shear stress level and with differing exponents. This model, thus, admits two additional parameters, one determining the sensitivity of shear creep rate to shear stress level, and another that acts as a threshold shear stress level reminiscent of a yield stress in the plastic limit, which the model also admits. The new model predicts very similar behavior to that seen in the previous model under linear viscoelastic behavior of the matrix, except that it allows for a threshold shear stress. This threshold allows consideration of behavior under near plastic matrix yielding or even matrix shear failure, the consequence of which is a large increase in the length-scale of load transfer around fiber breaks, and thus, a significant reduction in composite strength and increase in variability. Derivations of length-scales resulting from non-linear matrix creep are provided as Appendices in the Supplementary Material.
机译：应力破裂（有时称为蠕变断裂）是在高的拉伸负荷下单向纤维复合材料中发生的时间相关的故障模式持续在较长的时间（例如，许多年），导致高度可变的寿命，并且其中失效具有灾难性的后果。应力断裂是在这种结构特别关注，因为复合物包覆压力容器（COPVs），张力构件在基础设施应用中（悬浮屋顶，后张桥电缆）和高角速度的转子（例如，飞轮，离心机和螺旋桨）的。在微机械水平，应力破裂开始随机缺陷一些单根纤维的失败，接着局部载荷传递到通过在基体中的剪切应力相邻完整的纤维。随着时间的推移，在剪切的纤维蠕变之间的矩阵，这导致延长的周围之前的纤维断裂本地纤维过载区，从而导致甚至更多的纤维断裂，并最终，各种尺寸的纤维断裂的形成簇，其中一个最终增长到一个灾难性的不稳定大小。以往大多数模型是一个单一的光纤经典随机故障模型的直接延伸，并不能反映该微型机械的细节，特别是在基体的蠕变行为的条款。这些模型可以是足够用于服务的恒定载荷下解释实验，复合应力断裂数据;然而，它们是高度可疑精度下更复杂加载配置文件，特别是那些最初包括了一个“证明负荷”的1.5倍选择的服务负载短暂的“验证测试”。这样的模型通常用于预测证明测试存活者的改进的可靠性比没有这样的验证试验的可靠性更高。在我们之前的相关碳纤维工作/环氧复合材料结构我们发现出现了大量的纤维断裂本来不会发生的形式破坏，在许多重要的情况下，最终的效果是降低了可靠性随着时间的推移，如果证明压力过大。当前的纸张通过修改模型矩阵蠕变，从而不仅在时间上，而且在剪切应力水平，然后用不同的指数出现幂律蠕变行为，包括非线性蠕变延续了我们以前的工作。该模型中，因此，可接纳另外两个参数，一个来确定剪切蠕变速率的灵敏度剪切应力水平，另一个充当在塑性极限的屈服应力的阈值剪切应力水平让人想起，其模式也承认。新的模型预测非常相似的行为，以在所述下矩阵的线性粘弹行为以前的模型所示，不同之处在于它允许一个阈值剪切应力。该阈值允许在邻近塑料基质产生甚至矩阵剪切破坏，其中的后果是在长度尺度围绕纤维断裂负荷传递的大的增加，并且因此，在复合材料强度的显著降低和变性增加考虑行为的。从非线性矩阵蠕变所得长度尺度的推导被提供作为附录的补充材料。

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