Damage tolerant evaluation of cracked stiffened panels under fatigue loading

This paper presents the methodologies for damage tolerant evaluation of stiffened panels under fatigue loading. The two major objectives of damage tolerant evaluation, namely, the remaining life prediction and residual strength evaluation of stiffened panels have been discussed. Concentric and eccentric stiffeners have been considered. Stress intensity factor for a stiffened panel has been computed by using parametric equations of numerically integrated modified virtual crack closure integral technique. Various methodologies for residual strength evaluation, namely, plastic collapse condition, fracture toughness criterion and remaining life approach have been described. Effect of various stiffener sizes and stiffener type (concentric and eccentric stiffeners) on remaining life and residual strength has been studied under constant amplitude load. From the studies, it has been observed that the predicted life is significantly higher with concentric and eccentric stiffener cases compared to the respective unstiffened cases. The percentage increase in life is relatively more in the case of concentric stiffener compared to that of eccentric stiffener case for the same stiffener size and moment of inertia. From the studies, it has also been observed that the predicted residual strength using remaining life approach is lower compared to other methods, namely, plastic collapse condition and fracture toughness criterion and hence remaining life approach will govern the design. It is noted that residual strength increases with the increase of stiffener size.