Perhaps the most difficult task when Variable Amplitude ( VA ) loading Fatigue Crack Propagation ( FCP ) tests are made is the precise determination of crack size ( a ). The Back Face Strain ( BFS ) is a powerful approach to misleading this problem. For C(T) or DC(T) type specimens (see Figure 1), crack size versus BFS calibration curves are available in the normalized form EBCW = A(beta) , where E is the Young Modulus [MN/m2], B and W are geometrical dimensions of C(T) or DC(T) specimens [m], C is the compliance (inverse of stiffness), equal to the ratio BFS/P ( P is the applied load) [usn/N], and beta is the ratio a/W . The aN procedure of the present Maple worksheet take as input a cycle and peaks data file acquired during the test (variable m of string type), an initial value of crack size (variable a0 of float type) and the goal graphic introduced by the variable G of symbol type. Graphics can be of two types, crack size versus number of cycles ( N ) (in this case give the AN value to G variable) or crack size less initial crack size versus N (use AA0N in G variable).