A fatigue fracture mechanics methodology was developed and established, employing a slope-based J-integral approach combined with advanced image processing techniques. Adhesively bonded double cantilever beam (DCB) specimens were tested under constant displacement amplitude loading. The beam rotation was tracked by affixing a repetitive pattern on the DCB specimens and capturing images at the maximum displacement amplitude. Using a custom-developed image processing procedure, the beam rotation was deduced. To validate the methodology, DCB fatigue experiments were conducted at 23, 60 and 75 °C on aluminum adherends bonded with a structural 2-K epoxy adhesive. The J-based approach was compared with a conventional, compliance-based linear elastic fracture mechanics (LEFM) method. The epoxy was a rather brittle, high-modulus adhesive with a bond line thickness of 0.25 mm, resulting in predominantly linear elastic material behavior. By analyzing the images taken during fatigue testing, a stiffening effect of the steel load blocks was observed. Excluding pattern elements directly below the load block yielded the best agreement between J-integral and LEFM data. Both approaches were in excellent agreement within the investigated temperature range. The investigated adhesive exhibited a highly temperature-dependent behavior, which was associated with higher crack propagation rates and a lower fatigue threshold at 60 and 75 °C.

中文翻译：

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基于斜率的 J 积分方法和高级图像处理，用于评估粘合接头的循环疲劳分层行为


开发并建立了一种疲劳断裂力学方法，采用基于斜率的 J 积分方法并结合先进的图像处理技术。在恒定位移幅度载荷下对粘合双悬臂梁 （DCB） 试样进行了测试。通过在 DCB 试样上粘贴重复图案并以最大位移幅度捕获图像来跟踪光束旋转。使用定制开发的图像处理程序，推导出光束旋转。为了验证该方法，在 23、60 和 75 °C 的温度下，对用结构性 2-K 环氧树脂粘合剂粘合的铝粘附物进行了 DCB 疲劳实验。将基于 J 的方法与传统的、基于柔度的线性弹性断裂力学 （LEFM） 方法进行了比较。环氧树脂是一种相当脆的高模量胶粘剂，胶层厚度为 0.25 mm，导致材料主要具有线性弹性特性。通过分析疲劳测试期间拍摄的图像，观察到钢制载荷块的硬化效果。排除载荷块正下方的模式单元可产生 J 积分和 LEFM 数据之间的最佳一致性。两种方法在研究的温度范围内都非常一致。所研究的胶粘剂表现出高度依赖于温度的行为，这与 60 °C 和 75 °C 时较高的裂纹扩展速率和较低的疲劳阈值有关。