To predict fatigue-driven delamination growth in real structures, it is inevitable to understand delamination growth under real load spectra, also known as in-service loads, which most commonly are variable amplitude load patterns. In this work, delamination growth in glass fibre-reinforced polymer laminates subjected to two-level block amplitude loading is investigated using a pure moment loaded DCB test configuration for G-controlled cyclic testing. The variable amplitude load patterns consist of single- and periodic repeated load amplitude changes. The method utilises a digital image-based technique that allows for precise tracking of delamination fronts in translucent materials. The work characterises transient crack growth phenomena following load amplitude changes with emphasis on the actual fatigue crack growth rate. The load amplitude changes may increase the crack growth rate by factors of 5–6 in comparison to the crack growth rate in constant amplitude base-line tests. Periodic repeated load amplitude changes particularly increase the fatigue crack growth. For example, after just 10 periodic repeated low- and high-load blocks, the delamination has extended more than a factor of 2 times the delamination extension predicted from a non-interaction model based on constant amplitude data. The effect of the frequency of load amplitude changes on the fatigue crack growth is also investigated. Frequent load amplitude changes are proven to increase the fatigue crack growth significantly because of the transient crack growth responses following the load amplitude changes.

为了预测真实结构中疲劳驱动的分层增长，不可避免地要了解真实载荷谱下的分层增长，也称为使用载荷，最常见的是可变振幅载荷模式。在这项工作中，使用纯力矩加载 DCB 测试配置进行 G 控制循环测试，研究了在两级块振幅加载下玻璃纤维增​​强聚合物层压板的分层增长。变幅载荷模式包括单次和周期性重复载荷幅值变化。该方法利用基于数字图像的技术，可以精确跟踪半透明材料中的分层前沿。这项工作描述了载荷振幅变化后的瞬态裂纹扩展现象，重点是实际的疲劳裂纹扩展速率。与恒定振幅基线测试中的裂纹扩展速率相比，载荷幅度变化可能会使裂纹扩展速率增加 5-6 倍。周期性重复的载荷幅度变化尤其会增加疲劳裂纹的增长。例如，仅在 10 次周期性重复的低负荷和高负荷区块之后，分层扩展的范围是基于恒定振幅数据的非相互作用模型预测的分层扩展的两倍多。还研究了载荷幅度变化频率对疲劳裂纹扩展的影响。由于负载振幅变化后的瞬态裂纹扩展响应，频繁的负载振幅变化被证明会显着增加疲劳裂纹扩展。