Data underlying the publication: Multi-material adhesive joints with thick bond-lines: crack onset and crack deflection

Structural bonding of composite materials is being used in shipbuilding and civil industries. Due to manufacturing and in-service constraints, the bonded regions are characterized by adhesive layers with a thickness of up to 10 mm. With such thicknesses, a weakness to be recognized, from both scientific and applied points of view, is the stress gradient at bi-material edges and corners, exacerbated by differences in materials properties. This study aims to investigate the fracture onset and crack deflection in adhesive joints with thick bond-lines (≈ 10 mm) under global mode I loading. More specifically, the role of adherend-adhesive modulus-mismatch and the role of pre-crack length, Δ𝑎, are scrutinized. The parameters controlling the crack path directional stability are also discussed. Single-material (i.e. steel-steel and GFRP-GFRP) and bi-material (i.e. steel-GFRP) double-cantilever beam joints bonded with a structural epoxy adhesive are tested. The tests are aided by a 3D image acquisition system. Moreover, the different joints are modelled analytically, considering a beam on elastic-plastic foundation, to indicate and include characteristic length scales of the problem (e.g. adhesive thickness, yield plastic zone, elastic zone). To link the experimental findings to existing theoretical models, the behaviour of the different joints is also assessed numerically. An empirical relation, in terms of geometrical and material properties of the joints, that defines the transition between non-cohesive and cohesive fracture onset is found - for a given material mismatch (Δ𝑎crit./ℎadher)4 ∼ (𝑡a/ℎadher). In general, for Δ𝑎 < Δ𝑎crit.: the stress singularity near the bi-material corner rules over the stress singularity at the pre-crack tip. The bi-material corner with the highest modulus-mismatch dictates the region of fracture initiation; for Δ𝑎 ⩾ Δ𝑎crit.: the stress singularity at the pre-crack tip is dominant, resulting in cohesive fracture onset. However, the cracking direction rapidly deflects out from the adhesive layer centre-line. Positive 𝑇-stress along the crack tip is found considering 10 mm thick bond-line, being one of the factors for unstable crack path.

复合材料结构胶接技术已应用于造船与民用工业领域。受制造工艺与服役工况限制，胶接区域的胶层厚度可达10 mm。对于此类厚胶层，从科学研究与工程应用视角来看，双材料（bi-material）界面的边缘与拐角处存在应力梯度这一显著缺陷，且材料属性差异会进一步加剧该问题。
本研究旨在探究厚胶层（≈10 mm）胶接接头在整体I型（mode I）加载下的断裂起裂与裂纹偏转行为。具体而言，本研究将详细考察被粘物-胶层模量失配（adherend-adhesive modulus-mismatch）与预制裂纹长度Δ𝑎的作用，同时讨论控制裂纹路径方向稳定性的相关参数。
本试验采用结构环氧胶（structural epoxy adhesive）胶接的单材料（即钢-钢与玻璃纤维增强聚合物（Glass Fiber Reinforced Polymer, GFRP）-GFRP）及双材料（钢-GFRP）双悬臂梁（double-cantilever beam, DCB）接头为研究对象，试验过程采用三维图像采集系统辅助观测。
此外，本研究基于弹塑性地基梁模型对各类接头进行解析建模，以明确并纳入该问题的特征长度尺度（如胶层厚度、屈服塑性区、弹性区）。为将试验结果与现有理论模型相结合，本研究还通过数值方法对各类接头的力学行为进行了评估。
本研究得到了一个基于接头几何与材料属性的经验关系式，该关系式可界定非内聚型断裂（non-cohesive fracture）与内聚型断裂（cohesive fracture）起裂之间的转变边界：在给定材料失配条件下，(Δ𝑎crit./ℎadher)^4 ~ (𝑡a/ℎadher)。
总体而言，当Δ𝑎 < Δ𝑎crit时，双材料拐角处的应力奇异性（stress singularity）主导预制裂纹尖端的应力奇异性；模量失配程度最高的双材料拐角将决定断裂起裂区域。当Δ𝑎 ⩾ Δ𝑎crit时，预制裂纹尖端的应力奇异性占据主导，进而引发内聚型断裂起裂，但此时裂纹方向会快速偏离胶层中心线。对于10 mm厚的胶层，裂纹尖端存在正T应力（T-stress），这是导致裂纹路径失稳的因素之一。