In this article, damage tolerance approach is used for life estimation of the main rotor blade of a helicopter under random loading. At first, fatigue crack growth properties and threshold stress intensity range of the spar is obtained based on ASTM E647 test method. Then, crack growth analysis is done using Zencrack software. In order to calculate the length of the smallest growing cracks and their fatigue crack growth life, no crack growth and slow crack growth approaches were used. A comprehensive investigation was carried out on the effect of initial crack length on growth or no growth of cracks.
