Efficient Simulation of Crack Propagation in Adhesive Bonds

Sammanfattning

Accurate yet computationally affordable modelling of interface crack propagation isrequired for the development and certification of large bonded assemblies. Conventionalcohesive zone modelling (CZM) resolves the fracture process zone (FPZ) withtraction–separation laws that typically require element edges below 1 mm. This sizelimitation makes it impractical at industrial relevant scales.This thesis investigates an energy-release-rate cohesive (ERRC) approach for largeelementmodelling of adhesive-interface crack propagation. The method combines avirtual crack closure technique (VCCT) propagation criterion with a local cohesiverelease law, so that crack growth is triggered by the energy release rate while thenewly created crack surfaces dissipates the prescribed fracture energy progressively.The main contribution of the work is the reformulation of this approach as a userdefinedsolid element in LS-DYNA, intended for adhesive interfaces discretised withsolid adherends and finite-thickness bondline representation.The implemented interface is represented by lower–upper nodal pairs, where eachpair carries as discrete state: tied, cohesive, or open. In this thesis, two implementationvariants were developed: a single-core reference (SCR) implementation anda multi-core capable (MCC) implementation.For the DCB benchmark, the implemented method reproduced the expected meshalignedMode I crack-growth behaviour. The crack-length evolution followed the correctedbeam theory (CBT) reference within the resolution of one discrete interfaceelementedge, and the interface-energy diagnostics showed that the released pairs followedthe intended fracture-energy target. Representative Mode I propagation wasobtained using an in-plane interface element length of 4mm, substantially largerthan the sub-millimetre element sizes typically required to resolve a conventionalFPZ.The ENF benchmark confirmed that the discrete nodal-pair formulation can producea coherent Mode II-dominated propagation chain. However, the global force–displacement response showed pronounced dynamic oscillations, particularly for theMCC implementation. The dissipation diagnostics showed that the implementeddamage variable limits the release state, but that the reconstructed path-work quantitiescan become unreliable during rapid, single-cycle Mode II release events. TheENF results therefore verify important parts of the formulation but also show thatfurther stabilisation and quasi-static assessment are required before the method canbe considered robust for Mode II-dominated loading.Overall, the work demonstrated the feasibility of extending the ERRC method tosolid-element adhesive-interface formulation in LS-DYNA. The implementation providesa promising route for efficient large-element simulation of interfacial crackpropagation in bonded structures. At the present stage, the method should be interpretedas a proof of concept for regular, mesh aligned benchmark problems ratherthan a fully general industrial fracture-modelling tool.