Abstract: This paper proposes the back-calculating method for determining the parameters of da / dN equations within the range of relatively small crack length, The (a,t)datu.m within the range of relatively small crack length under the load spectrum is the basis of the method. The appropriate computing program of crack growth is used in tne method. The objective function is the sum of square of error (SSE) between the testing and calculating life, and the direct optimizing method (searching method) is adopted to optimize it. The method can substitute the da / dN test that is very defficult and non-economic. It is very significant to apply the DCGA in durability analysis.

Key words: crack growth, relatively small crack, back-calculating method