Equivalent damage and residual strength for impact damaged composite structures

Determining structural durability and damage tolerance of aircraft-composit e 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 o f damage, e.g., fatigue cracking, environmental degradation, or damage intr oduced by foreign objects. When occurring, all types of damage need immedia te attention for determination of the effect on aircraft performance or fun ctionality. There is a need in other words for simplified predictive method s for rapid assessment of occurring damage, where impact damage is the most important damage mode. In this paper two residual strength models are pres ented, the so-called soft inclusion and delamination buckling theory, and c ompared to experimental results on impact damaged composite structures. Tho se experiments span a variety of impact events, from 8 J to 55 J and differ ent layups. The investigation has been supported by FE-technique for determ ination of the stress distribution in the buckled state and for characteris ation of the damaged region. It can be concluded that for low-energy impact , through conservative assumptions on stiffness reduction, that the soft in clusion is unconservative for residual strength prediction. In contrast the delamination buckling theory shows good agreement for various impact energ y levels, thicknesses and layups, The importance of repeated loading for co mposite structures with artificial delaminations is also demonstrated. If r epeated loaded, the local buckling strain is strongly reduced as compared t o a non-repeated loaded structure. It can also be shown that artificial del aminations, though deep-lying, can grow in a stable manner if pre-buckled.