Towards understanding residual strength and damage evolution in damaged composite laminates

icaf2023 Tracking Number 36

Composite materials are increasingly used in aerostructures, making up about 50% of the structural weight of the latest airliners, and up to 90% in helicopters. Managing the damage such structures will inevitably sustain is therefore increasingly important in both design and sustainment. Understanding the significance of damage findings requires answering two questions: (1) what is the impact on the residual strength, and (2) how will the damage evolve under fatigue loading? Unfortunately, prediction tools to answer these questions remain lacking. Available models mainly rely on experimental data gathered during development of the aircraft, e.g. to substantiate that the design meets the ‘no-growth’ criterion. However, the complexity of the damage mechanisms and their lay-up dependence, makes it difficult to generalise these data, to predict the behaviour of damage scenarios that don’t precisely match the test conditions. This results in the application of very strict, and likely over-conservative, damage limits to composite structures. We will show that one of the reasons for current limitations of predictive models, is that damage characterisation is driven more by the limitations of inspection techniques, than by an understanding of the physical damage processes. Additionally, investigations into fatigue delamination growth rely on specimens that are not representative of actual composite structures. We will present ongoing efforts in our lab to address these challenges. This includes using acoustic emission sensors to distinguish damage mechanisms during quasi-static and fatigue compression after impact testing, measuring of fatigue delamination growth in multidirectional interfaces, and using pulse-echo ultrasonic scanning to build up a 3D image of the delamination configuration and propagation, under either in-plane or out-of-plane loading. Based on the results we conclude: • Projected delamination area or delamination width are insufficient to properly quantify damage severity; a more detailed damage description is necessary. • Planar (2D) delamination growth in multidirectional laminates involves delamination migration processes that are similar to those seen in 1D delamination tests in multidirectional laminates, but which are not represented in standard unidirectional delamination growth coupons. • Further research is necessary to understand what data is needed to enable accurate prediction of residual strength and remaining life when damage is detected.