Previous studies have developed several techniques to enhance the mode II static fracture toughness of adhesive-bonded joints for structural applications. However, in real-world applications, joints are subjected to fatigue loading more frequently than static loading. In a previous study, we demonstrated static mode II toughness improvement of a modified adhesive joint with a microstructured bondline using tailored sacrificial cracks. In this study, we analyzed the mode II fatigue toughness of this joint by employing the end notch flexural test to characterize the mode II fatigue toughness of the modified bonded joints at different loads, i.e., 50%, 60%, and 70% of the maximum static load. The results demonstrated initiation fracture toughness improvement for the joints tested at 70% of maximum static load, and a one-order of magnitude lower crack growth rate than the conventional joint (interfacial crack propagation). The initiation fracture toughness improvement was caused by the presence of the first sacrificial crack, which facilitated the stress redistribution ahead of the crack tip and formed an elongated strain-affected zone. This reduces the stress intensity at the crack tip and, as a result, delays the initiation of the crack. We found that the crack growth rate improvement was associated with the formation of two adhesive ligaments at the sacrificial crack ends, which dissipated large energy due to plastic deformation and ligament breakage.

中文翻译：

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通过定制牺牲裂纹具有微结构粘合层的复合材料粘合接头的 II 型疲劳特性

先前的研究已经开发了多种技术来增强结构应用中胶接接头的 II 型静态断裂韧性。然而，在实际应用中，接头比静态载荷更频繁地承受疲劳载荷。在之前的一项研究中，我们展示了使用定制牺牲裂纹的微结构粘合层改进的粘合接头的静态 II 型韧性改进。在本研究中，我们通过端部缺口弯曲试验来分析该接头的II型疲劳韧性，以表征改进的粘合接头在不同载荷（即50％、60％和70％）下的II型疲劳韧性。最大静载荷。结果表明，在 70% 最大静态载荷下测试的接头的初始断裂韧性得到改善，并且裂纹扩展速率比传统接头（界面裂纹扩展）低一个数量级。初始断裂韧性的提高是由第一牺牲裂纹的存在引起的，它促进了裂纹尖端前方的应力重新分布，并形成了细长的应变影响区。这降低了裂纹尖端的应力强度，从而延迟了裂纹的萌生。我们发现裂纹扩展速率的提高与牺牲裂纹末端形成两个粘合韧带有关，由于塑性变形和韧带断裂而耗散了大量能量。