Damage mechanics method to predict the fatigue life of 3-D structural members under spectrum load

A conservative integral of three dimensional structures in damage mechanics was established. The expression of the stress and the strain considering the effect of damage coupling was deduced by means of the path independent property of this integral and the condition of small scale damage. Based on the damage evolution equations expressed by the damage driving force, a closed form solution to determine the fatigue crack formation life under the spectrum load was obtained. According to the above analysis, the damage evolution equation with equivalent stress of non variable separated type was established, and a closed form solution of integral type was obtained to determine the fatigue crack formation life under the short period load. The fatigue life of the landing gear of certain airplane as the representative of 3-D structural member is predicted.