ON THE CORRELATION BETWEEN THE C AND M IN THE PARIS EQUATION FOR FATIGUE CRACK-PROPAGATION

Several workers have noticed that, under conditions of Stage II fatigue crack growth, the C and the m parameters in the Paris equation, da/dN equals C DELTA K**m exhibit a correlation of the form ln(C) equals a plus bm. Additional data is presented to confirm that this correlation exists over a wide range of testing conditions. A computer simulation is described which generates random C and m values according to a very simple rule and it is shown that these random values also exhibit the correlation. Thus the C-m correlation, while present, does not possess any fundamental significance and is purely the result of firstly, the logarithmic method conventionally used to plot the data, and secondly, the nature of the dimensions of the physical quantities used in the Paris equation.