Dynamic Analysis Of Adhesively Bonded Single Lap Joints Using Full-Field Measurement Techniques

Digital Image Correlation (DIC) is used to provide full-field analysis of the strains generated within an adhesively bonded composite single lap joint. Testing is conducted both quasi-statically at 2 mm/min and an intermediate strain rate, focusing on the root of the discontinuity between adherends where failure initiation is observed. The intermediate rate loading is applied using an Instron VHS 80/20 servo hydraulic test machine operated at 2.5 m/s. High speed cameras are used to capture images at frame rates of 25 kHz. These are used in the DIC to provide the axial, peel and shear strain distributions at the root of the discontinuity between adherends, hence identifying the mechanisms for damage initiation and growth.A significant strengthening effect was observed in the high rate loading; increasing the failure load from 4.5 kN to 9.7 kN. This increase indicates possible viscoelastic strengthening of the epoxy matrix in the matrix dominated through-thickness direction, resisting the high peel stresses generated around the geometric discontinuity. Damage initiation is observed to occur at approximately 50% of the failure load for both quasi static and high strain rate loading conditions. The strains evaluated prior to initiation are observed to be very similar for both cases, indicating that the generation of cracks at the root of the discontinuity is not strain rate dependent. The failure surfaces of the specimens loaded quasi statically and at high rate are markedly different, indicating a strengthening effect from the CSM face sheet material, producing crack blunting and fibre bridging, limiting damage until a critical strain is reached and ultimate failure occurs.