Curved structures made of fibre-reinforced plastics tend to show an out-of-plane failure type, when loaded under bending. One method to determine the related tensile strength in thickness direction, is the standardised unfolding test related to ASTM D6415 with L-profile specimens. However, the standard shall be used for unidirectional materials only, which is not the case for multidirectional laminates. Therefore, this publication presents methods to determine the layer-wise stress and strain state as well as occurring failure types of multidirectional laminates. An extension of the Lekhnitski formulae is presented and its results are validated by comparison with high resolution strain measurements by Digital Image Correlation. Based on the analytical solution, a failure analysis using Cuntze’s Failure Mode Concept is conducted. It was revealed, that the failure load and position can be predicted accurately, if a failure type related strength –in-plane or out-of-plane failure– is used. In the presence of 45° -layers higher deviations occur between test and prediction and therefore limits the validity of the presented analytical failure analysis.

中文翻译：

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在纯弯曲下确定多向层压板的应力、应变和失效类型

当弯曲加载时，由纤维增强塑料制成的弯曲结构往往表现出平面外破坏类型。确定厚度方向相关拉伸强度的一种方法是与 ASTM D6415 相关的标准展开试验，使用 L 型材试样。但是，该标准仅适用于单向材料，不适用于多向层压板。因此，本出版物介绍了确定多层应力和应变状态以及发生的多向层压板失效类型的方法。提出了 Lekhnitski 公式的扩展，并通过与数字图像相关的高分辨率应变测量进行比较来验证其结果。基于解析解，使用 Cuntze 的失效模式概念进行失效分析。据透露，如果使用与失效类型相关的强度（平面内或平面外失效），则可以准确预测失效载荷和位置。在存在 45° 层的情况下，测试和预测之间会出现更高的偏差，因此限制了所呈现的分析故障分析的有效性。