Numerical crack nucleation and propagation analyses of bonded joints

Delamination and debonding are typical interface failures that may endanger the structural integrity of laminated and adhesively bonded structures. Two numerical fracture analysis methods, the virtual crack closure technique (VCCT) and the cohesive zone model (CZM), are commonly used tools to criticality analyse such failures. The objective of this thesis work is to study the limitations of the VCCT and the CZM and to extend their applicability.

The work consists of four case studies. The first two studies focus on the VCCT that is typically used in crack propagation analyses when material plasticity can be ignored. The possibility to use the method in analyses of adhesively bonded joints with a yielding adhesive and with yielding adherends is studied with analysis cases of double cantilever beam (DCB) and wedge peel test specimens. The experimental results provided a reference for the analyses. In the third study, a method combining the CZM and the VCCT is developed for crack nucleation and propagation analyses. The applicability of the method is studied with analyses of the DCB test. The final study concentrates on fracture models of hybrid laminates under mixed-mode loading and thermal stresses. A hybrid laminate cracked lap shear (CLS) specimen was analysed in the study by using
the VCCT, the CZM and their combination.

The thesis work indicates that the VCCT is a feasible analysis method for crack propagation analyses of a DCB specimen with a yielding adhesive. Comparing to analytical (initial crack length) solutions, the VCCT also improves correlation with experimental data when plasticity exists in the adherends of a wedge peel test specimen. The developed combined CZM-VCCT method is feasible for analysing the crack nucleation and propagation phases of the DCB specimen. The combined method is also able to estimate crack onset for a hybrid laminate CLS specimen. The hybrid laminate study further indicates that the CZM is feasible and the VCCT is unfeasible for analyses of hybrid laminates with thermal loading.