An efficient preselection approach to determine critical parameters for slow delamination growth in postbuckled composites

The presence of delaminations can severely undermine the structural integrity of composite structures. These delaminated sites within a structure are vulnerable to sublaminate buckling events due to their reduced bending stiffness. This can in turn progress the delamination growth resulting in loss of load carrying capacity of the structure. The current study presents insights into expected growth behaviour of delaminations in composite laminates under service fatigue loads. Delamination growth can be classified as unstable when the growth rate increases with delamination size or when an abrupt, instantaneous propagation occurs. Conversely, growth is considered stable when the growth rate decreases as the delamination size increases. To gain deeper insights into these growth characteristics, a comprehensive study on composite laminates with through-width delamination has been carried out using an established analytical modelling approach from literature, which is based on the variational principle of total potential energy. For the analysis of delamination growth, the Griffith criterion is used, which postulates that energy release rate (G) governs the process. It has been shown that G begins to increase following thin sublaminate buckling, with the rate of increase with respect to applied strain being higher for a laminate with short delamination than for one with long delamination. The expected delamination growth characteristics under service fatigue loading has been analysed, and strain allowables corresponding to the fatigue limits have been determined. Similarly for static loading scenarios, conditions for stable and unstable growth have been presented and linked to fatigue growth characteristics. A critical discussion is subsequently presented on the effects of imperfection magnitude and delamination depth on the growth behaviour. By presenting these insights into growth behaviour, this study proposes an efficient approach for identifying key parameters that would ensure slow growth of delaminations in postbuckled composite laminates.