Co-authors​ :

 Amit CHAUDHARY (INDIA), Supratik MUKHOPADHYAY  

The demand for weight reduction in the aerospace, marine, and automotive industries has led to the heavy use of composite materials in making load-carrying primary structures. Interestingly, there is a large asymmetry in the mechanical response of these materials under tension and compression, with the ultimate strength in the latter case being almost 30% less than the former. The main reason for this asymmetry is related to a unique instability-driven failure mechanism known as 'fibre kinking' under compression, which promotes early structural failure. Fibre kinking is triggered by local fibre misalignments, which, when subjected to compressive loading, promote further fibre rotation, resulting in the formation of a narrow kink band of very high localized shear strain. This eventually propagates, causing a sudden and unstable structural collapse. The presence of cut-outs for assembly and inherent manufacturing defects further weakens the structure and acts as an initiation site for this type of failure. In this regard, open-hole compression (OHC) testing has become a benchmark exercise for measuring the compressive strength of components containing notched geometry. In a multi-directional OHC laminate, kinking is likely to initiate and interact with other damage mechanisms, such as delamination and ply splitting, resulting in a complex and nonlinear damage progression. This cannot be predicted using analytical models. Accurate predictive computational models of such progressive damage can provide valuable understanding without the need for an elaborate experimental program that is costly and time-consuming.
In the current work, a novel three-dimensional semi-discrete continuum damage modelling framework, implemented in commercial software as a user-defined material package, is applied to predict failure in open-hole compression laminates. A mesoscale (ply-by-ply) modelling approach is adopted, with each individual ply represented by solid elements separated by layers of cohesive interface elements to model delamination. An improved fibre kinking model that uses a kinematically separated two-scale formulation is used to simulate compressive damage in the axial plies. Mesh-independent onset and propagation of discrete ply cracks are realized using the directed continuum damage mechanics method (D-CDM) recently proposed by Mukhopadhyay and Hallett [1]. This methodology is used to simulate failure under compression in OHC laminates across a varied range of lay-ups. The models are shown to be able to realistically reproduce the progressive and interactive nature of damage growth in all cases and, overall, are in very good agreement with experimental findings in terms of ultimate failure load, damage sequence, and damage pattern.