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Abstract Cracking in D6ac steel has played a key role in the development of thedamage tolerant design philosophy. Until recently it had been thought thatcracking in D6ac steel was well understood. However, Forth et. al. [1] haverecently reported that crack growth data obtained by NASA for D6ac steelrevealed that there was little R ratio dependence in the Paris Region and that therewas thus little, if any, crack closure. In this paper we reveal how the NASA dataconforms to the Generalised Frost-Dugdale crack growth law. We then show howthe average block variant of the Generalised Frost-Dugdale law can be used topredict the growth of small cracks in a USAF study into crack growth in D6acsteel specimens representative of the critical region of F-111 wing pivot fitting.