SAFETY MUST BE ENSURED BY DESIGNING THE STRUCTURE TO SPECIFIC DAMAGE TOLERANCE REQUIREMENTS , SUCH THAT THE INITIAL DAMAGE IS NEVER ALLOWED TO GROW TO ITS CRITICAL POINT/LEVEL.

Structural design must incorporate considerations of both the strength and durability of a component that contains cracks .The residual strength of load carrying capacity or a part in the presence of defects is a function or the material fracture toughness and the durability of the structural component depends due to subcritical crack growth. The damage growth as a function of time and decrease in residual strength (due to crack growth) as a function of time , at some critical time damage grows to a critical size , beyond which the load carrying capacity of the part is smaller than the load it is designed to carry and catastrophic failure occured .Safety is ensured by designing the structure to specific damage tolerance requirements such that initial damage is never allowed to grow to its critical evaluation and reduce the Design limit loads of the structure below a prescribed level P min through out the life of the structure.The life of the structure is affected by several factors such as material strength, crack growth rate , critical crack size ,usage and the maximum operational loads .