DETERMINATION OF THE ULTIMATE STRENGTH OF A COMPOSITE MATERIAL BY CONSIDERATION OF CRITICAL MANUFACTURING FLAWS

THE FAILURE CRITERIA PRESENTLY USED FOR PREDICTION OF THE ULTIMATE STRENGTH OF A COMPOSITE MATERIAL ARE BASED UPON THE ASSUMPTION THAT THE COMPOSITE BEHAVES AS AN IDEAL ORTHOTROPIC MATERIAL. THIS IDEALIZATION OF COMPOSITE MATERIALS IS INCONSISTENT WITH THE WAY BRITTLE COMPOSITE MATERIALS FAIL. SINCE FAILURE GENERALLY BEGINS AT A SMALL CRACK OR VOID A FAILUTE CRITERIA BASED UPON A CRITICAL FLAW SIZE AND THE LIKELIHOOD OF CRACK PROPAGATION IS PROPOSED. DEVELOPMENT OF THE CRITERIA WILL BE IMPLEMENTED TO TWO PHASES. DURING PHASE I, THE ANALYTIC METHODS REQUIRED TO PREDICT CRACK PROPAGATION IN COMPOSITE MATERIALS UNDER A VARIETY OF LOADING CONDITIONS WILL BE DEVELOPED. THE TECHNIQUE IS BASED UPON THE "FICTITIOUS CRACK" METHOD AND WILL BE DEVELOPED FOR USE WITH GENERAL-PURPOSE FINITE ELEMENT PROGRAM. THIS METHOD CAN PREDICT CRACK INITIATION AND PROPAGATION AROUND A VARIETY OF EXISTING FLAW SHAPES. THE TECHNIQUE WILL BE DEVELOPED AND TESTED FOR EXTERNAL FORCES PARALLEL AND PERPENDICULAR TO THE FIBER DIRECTION. PHASE II WILL INCLUDE DEVELOPMENT AND VERIFICATION OF AN ULTIMATE STRENGTH CRITERIA BASED UPON THE RESEARCH ACCOMPLISHED BY PHASE I.