FATIGUE DAMAGE ESTIMATION FROM PSEUDO-RANDOM LOAD SEQUENCE GENERATED FOR METALS AND FIBER REINFORCED COMPOSITES

Composite materials are widely used in many applications in view of tailor suiting of properties that include stiffness, toughness, and strength. Fatigue loading of components made of composites is inevitable. The theory to predict fatigue damage and fatigue life is well established for metals, while such a clear definition is not available for composites. The damage in composite can be due to matrix cracking, delamination, fiber fracture, matrix crazing, etc. The problem is complex when one considers pseudo-random load sequences like an aircraft spectrum. This study evaluates the damage under a typical European Standard spectrum for Fighter Aircraft Loading STAndard for Fatigue and Fracture (FALSTAFF) as well as its programmed version. The phenomenological model proposed by Broutman and Sahu and empirical models proposed by Palmgren-Miner; Bond and Farrow; and Hashin-Rotem are used for calculating damage and fatigue life. Constant life diagrams are used for calculating the number of cycles to failure. The Range-Damage-Exceedance curve is employed to omit cycles from the FALSTAFF load sequence. It is observed that the Programmed FALSTAFF load sequence derived for metals is applicable for composites as both the full FALSTAFF spectrum and programmed version provide similar damage estimates. An increase in reference stress resulted in a decrease in fatigue life. Hashin-Rotem model is used to study sequence effect from the Programmed version of FALSTAFF. It is estimated that fatigue life under low-high load sequence is more as compared to high-low load sequence.