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Fatigue and fracture

Failure of rate-dependent adhesive joints

Rate dependence of crack initiation and propagation within materials and interfaces can be a critical issue to address, for example in the numerical assessment of crash-worthiness of automotive and aerospace structures, or in the case of delamination along rubber interfaces, such as those in bonded flexible risers used in the off-shore industry.

Rate-dependent cohesive-zone models have been developed, implemented in user subroutines for the FE code ABAQUS and experimentally validated at Brunel, within the rigorous theoretical framework of thermodynamics with internal variables and fractional viscoelasticity. The latter allowed us to capture the rate-dependence of crack propagation along a rubber interface between two steel adherents across the entire range of tested speeds, spanning 5 logarithmic decades, with excellent correlation between numerical and experimental results and with only 7 parameters to be identified in the model. Other models have been developed that capture very different types of rate dependence, typical of different types polymeric materials, whereby for an increasing crack speed the total fracture resistance can monotonically increase, monotonically decrease or initially increase and then decrease.




Numerical and experimental load-displacement curves at different tested speeds for the above shown double-cantilever beam made of steel adherents and a rubber interface.

Failure of interfaces involving mixed-mode and frictional effects

Structural failure of interfaces is a common problem across a very wide variety of engineering problems at very different scales, from cracks in concrete dams, mortar-joint failure in brick masonry, frictional slipping of steel bars in concrete, delamination of composites, de-bonding of adhesive joints, fibre-matrix de-bonding in composites, or de-bonding of nanoparticles in nanocomposites. Unlike cracks propagating in a bulk material, which tend to quickly orient themselves as mode-I (opening) cracks, cracks propagating along structural interfaces are often forced to have significant and sometimes predominant mode-II and more-III components. In all these cases, the mode-mixity of crack propagation is a crucial aspect that needs to be taken into account. This makes the experimental characterisation of the fracture resistance a challenging task, which also lead to great difficulties to define reliable test standards for industrial applications. Coupling between damage and friction is also important and certainly cannot be ignored when sliding of crack faces occurs in presence of compressive stresses.

For all these cases, a number of cohesive-zone models have been developed at Brunel and implemented as user subroutines in the FE code ABAQUS, with a number of unique features. Coupling between damage and friction is ensured via a physically sound damage-mechanics formulation which ensures that the progressively increasing importance of friction and unilateral contact is captured during the damage process rather than only at the end of it. Furthermore, using a simple but effective multiscale approach, a number of models have been developed to capture role played by the microgeometry of the fracture surface in determining the mixed-mode dependency of the crack propagation. This research has resulted in models where the physical parameters are very limited in number and have a clear physical meaning. For example, one adhesion energy only (rather than one in each mode) can be defined, together with an average inclination of the asperities of the fracture surface and one friction coefficient, to fully determine the mixed-mode resistance to fracture.

All models have been successfully validated against experimental results for a range of engineering problems, with two examples shown below among many others.



Failure of a masonry wall subject to compression and shear.


Push-out test of a polyester fibre from an epoxy matrix