ESDU 07012 is concerned with the damage-tolerance approach to the design against fatigue and the application of crack propagation models to the estimation of fatigue life of structures. The concept of the stress intensity factor associated with the LEFM approach is discussed in this Item. The crack growth rate is related to the stress intensity factor range. This relationship enables the fatigue life of the component to be found. Several crack growth models are discussed, including the Paris, Walker, Forman, Elber, etc. The principles for crack growth under cyclic loading of constant amplitude can be extended to variable-amplitude loading. Expressions are provided for the fatigue life of a structural component subjected to random excitation in terms of the probability density function for the rainflow cycle stress range (ESDU 06010). The methods outlined in the present Item have been applied to a box made from aluminium alloy. With use of the Paris expression for crack growth and with the aid of spectral methods (ESDU 06011) an estimate of the fatigue life of the box component was made which agreed fairly well with the fatigue life estimate obtained experimentally during acoustic testing.