A STUDY ON THE OFFSET MEASUREMENT NUMBERS FOR AVERAGE CRACK SIZE ESTIMATION IN ELLIPTIC CRACK FRONT

During continued loading the slow crack growth regions at the crack ti p assume different shapes and sizes. Three different crack front profi les may be identified depending on the extent of crack growth, namely, parabolic, elliptical and semi-circular type. The crack front profile may be described by a parabola for relatively small crack growth, a l arge crack growth size leads to a semi-circular crack front profile wh ile the intermediate stages of crack growth can be described by an ell iptical crack front profile. An analytical optimization of offset meas urement number (N) for realistic crack growth size estimation for elli ptical crack front profile is carried out considering two approaches, namely, offset measurement and area consideration. The variation of av erage crack size (Delta a(av)) as well as convergence of (Delta a(av)) with the offset measurement number (N) is studied. The difference in Delta a(av) obtained by both the methods is also investigated. Experim ental verification of the crack growth front profile is carried out. A ttempts are also made to study the effect of thickness, extent of crac k growth, etc. on the value of N. In general 8-12 measurements are fou nd to be adequate to have a realistic estimation of slow crack growth size for commonly used specimen thickness.