Crack growth in D6ac steel structural components

Cracking in D6ac steel has played a key role in the development of the damage tolerant design philosophy. Until recently it had been thought that cracking in D6ac steel was well understood. However, Forth et. al. [1] have recently reported that crack growth data obtained by NASA for D6ac steel revealed that there was little R ratio dependence in the Paris Region and that there was thus little, if any, crack closure. In this paper we reveal how the NASA data conforms to the Generalised Frost-Dugdale crack growth law. We then show how the average block variant of the Generalised Frost-Dugdale law can be used to predict the growth of small cracks in a USAF study into crack growth in D6ac steel specimens representative of the critical region of F-111 wing pivot fitting.