Equivalent Damage and Residual Strength for Impact Damaged Composite Structures

Abstract

Determining structural durability and damage tolerance of aircraft-composite structures is an important task, not only in the design process but also when the aircraft is in operational use. There are many sources and types of damage, e.g., fatigue cracking, environmental degradation, or damage introduced by foreign objects. When occuffing, all types of damage need immediate attention for determination of the effect on aircraft performance or functionality. There is a need in other words for simplified predictive methods for rapid assessment of occurring damage, where impact damage is the most important damage mode. In this paper two residual strength models are presented, the so-called soft inclusion and delamination buckling theory, and compared to experimental results on impact damaged composite structures. Those experiments span a variety of impact events, from 8 J to 55 J and different layups. The investigation has been supported by FE-technique for determination of the stress distribution in the buckled state and for characterisation of the damaged region. It can be concluded that for low-energy impact, through conservative assumptions on stiffness reduction, that the soft inclusion is unconservative for residual strength prediction. In contrast the delamination buckling theory shows good agreement for various impact energy levels, thicknesses and layups. The importance of repeated loading for composite structures with artificial delaminations is also demonstrated. If repeated loaded, the local buckling strain is strongly reduced as compared to a non-repeated loaded structure. It can also be shown that artificial delaminations, though deep-lying, can grow in a stable manner if pre-buckled.