Understanding of Crack Growth in Single- and Bi-Material Bonded Joints with “Extra-Thick” Adhesive Bond-Lines

Structural Integrity & Composite

Multi-material adhesive joints with thick bond-lines: Crack onset and crack deflection

This study investigates the fracture onset and crack deflection in multi-material adhesive joints with thick bond-lines (≈10 mm) under global mode I loading. The role of adherend-adhesive modulus-mismatch and pre-crack length 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 joints are modelled analytically, considering a beam on elastic-plastic foundation, to include characteristic length scales of the problem (e.g. adhesive thickness, plastic zone) and numerically using Finite Element Model. 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. Above a specific pre-crack length the stress singularity at pre-crack tip rules over the stress singularity near bi-material corners, resulting in mid-adhesive thickness 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 to be one of the factors for the unstable crack path.Structural Integrity & Composite

From thin to extra-thick adhesive layer thicknesses: Fracture of bonded joints under mode I loading conditions

The fracture behaviour of joints bonded with a structural epoxy adhesive and bond line thicknesses of 0.1–4.5 mm has been studied. However, limited research is found on similar joints with thicker bond lines, which are relevant for maritime applications. Therefore, the effect of the adhesive bond line thickness, varying from 0.4 to 10.1 mm, on the mode I fracture behaviour of steel to steel joints bonded with a structural epoxy adhesive was investigated in this study. An experimental test campaign of double-cantilever beam (DCB) specimens was carried out in laboratory conditions. Five bond line thicknesses were studied: 0.4, 1.1, 2.6, 4.1 and 10.1 mm. Analytical predictions of the experimental load-displacement curves were performed based on the Simple Beam Theory (SBT), the Compliance Calibration Method (CCM) and the Penado-Kanninen (P-K) model. The P-K model was used to determine the mode I strain energy release rate (SERR). The average mode I SERR, G Iav., presented similar values for the specimens with adhesive bond line thicknesses of 0.4, 1.1 and 2.6 mm (G I av.=0.71, 0.61, 0.63 N/mm, respectively). However, it increased by approximately 63% for 4.1 mm (G I av.=1.16 N/mm) and decreased by about 10% (in comparison with 4.1 mm) for the 10.1 mm (G I av.=1.04 N/mm). The trend of the G Iav. in relation to the bond line thickness is explained by the combination of three factors: the crack path location, the failure surfaces features and the stress field ahead of the crack tip. Structural Integrity & CompositesAdhesion Institut

Role of adherend material on the fracture of bi-material composite bonded joints

The aim of this study is to investigate the effect of the adherend material on the mode I fracture behaviour of bi-material composite bonded joints. Both single-material (steel-steel and composite-composite) and bi-material (steel-composite) joints bonded with a structural epoxy adhesive are studied. Additionally, two adhesive bondline thicknesses are considered: 0.4 mm (thin bondline) and 10.1 mm (thick bondline). The Penado-Kanninen reduction scheme is applied to evaluate the mode I strain energy release rate. The results show that the mode I fracture energy, GIc, is independent of the adherend type and joint configuration (single or bi-material). GIc shows average values between 0.60 and 0.72 N/mm for thin bondlines and 0.90–1.10 N/mm for thick bondlines. For thin bondlines, the failure is cohesive and the similar degree of constraint that is imposed to the adhesive by the high-modulus (i.e., steel) and/or relatively thick (i.e., composite) adherends results in similar values of GIc for both single- and bi-material joint types. For thick bondlines, the crack grows closer to one of the adhesive-adherend interfaces, but still within the adhesive. The results show that the adhesive could deform similarly, although the crack has been constrained on one side by different types of adherends, either a steel or composite.Structural Integrity & CompositesAdhesion Institut