In order to investigate the effect of beam and crack parameters and also number of cracks on the DI, a two level factorial design with five factors is conducted for a double cracked simply supported beam. The factors and levels are shown in Table 4. The location of first crack xc1/l nominated as factor A is set at two levels of 0.25 and 0.45. The second crack location as factor B with two levels of distance from the first crack set at 0.05 (to resemble near distance) and 0.20 (to resemble far distance), and each crack depth ratio nominated as factors C and D is considered at two values of 0.05 and 0.25 to resemble small and medium cracks. The beam height to length ratio nominated as factor E is set at two levels of 1/30 and 1/15. The DI of the first crack which is the maximum of CWT