APPROXIMATE METHODS FOR ANALYZING THE GROWTH OF LARGE CRACKS IN FATIGUE DAMAGED AIRCRAFT SHEET MATERIAL AND LAP JOINTS

Several levels of approximation are investigated to account for the ef fect of small fatigue cracks on the residual strength of aircraft shee t materials and fuselage lap joints containing major cracks. A version of the Dugdale model is proposed which accounts for strain hardening of the sheet in an approximate way and which incorporates a criterion for crack advance leading to crack growth resistance. This model build s upon the model proposed by Nilsson and Hutchinson [1] and accounts f or the detailed interaction between the major crack and the small dama ge cracks. A simpler version of the model uses the damage-reduced loca l strength of the sheet or joint in assessing the effect of the major crack on residual strength. The simpler approach thus bypasses the nec essity of a direct determination of the highly complicated details of the interaction of the small cracks in a lap joint with a major crack.