TAKING INTO ACCOUNT DAMAGE IN STRUCTURES MATERIALS SELECTION

When structures materials where basically aluminium alloys, proper material selection was as always a compromise between properties, basically between ''fatigue'' type alloys presenting better crack growth and critical crack length properties than ''yield strength'' alloys, but with lower static strength. This, combined with other parameters such as available product sires, corrosion resistance, was leading to a relatively simple selection process, all aluminium alloys behaving more or less the same way within a approximate to 10 % bracket of strength allowables. Problem is becoming more complex when material selection is including composite materials as they exhibit a wastly different behaviour : sensitivity of static strength to notches such as fastener holes; quasi insensitivity to fatigue even on notched specimen (for all fiber dominated failure modes); sensitivity to the effects of low speed impacts, particularly so in compressive loading. It means that, contrary to the metals, fatigue is not a design driver, and on the other hand impact damage is becoming a design driver. This paper, after some considerations on impact strength and related design strategies present the influence of impact on two application examples: the case of a fighter airplane wing upper surface, and the pressurized fuselage skin of a transport airplane.