Assessment of the effect of the notch radius on ductile stable crack growth

Mode I stable crack growth SCG behaviour emanating from notches of different radii was investigated. Fracture tests were carried out on compact tension (CT) specimen of 8 mm thickness. The specimen was fabricated from EN 34NiCrMo6 low alloy steel (akin to AISI 4330/4340) with various notch radii, r, ranging from 0.08 mm to 3.0 mm (0.08, 0.16, 0.25, 0.50, 1.00, 2.00, 3.00 mm). The notch was introduced in the specimen by using the EDM wire-cutting technique. Three different ratios of initial crack length ao to width W were used (namely a₀/W = 0.45, 0.50 and 0.55). Experimental results include the load-load line displacement (L-LLD) diagrams and initiation (P _i and P_q) and maximum fracture loads, P_max. P_i was taken as the linear limit of the P-δ_LL curve while P_q was identified as the point where a line with a slope of 95% of the initial slope of the P-δ_LL curve intersects the P-δ_LL curve. Tests show that despite more than 37 folds increase in notch radius, the average maximum load percentage increase (for the three a_o/W ratios) recorded was no more than 21 %. The crack initiation and crack front tunnelling, employing the dye penetrant technique, was observed to diminish with r. Minimal extension around P_max, at the specimen mid thickness was observed, only under magnification of the fracture surface, in a few number of specimens of a₀/W=0.55 and r = 3 mm. The initiation load for specimen ofr > 2 can be predicted satisfactorily by the stress concentration approach also the maximum load may be predicted with a degree of conservatism employing the yield limit load P_L for sharp notches. In general, the results suggest that the fracture mechanics approach is likely applicable up to r ≤ 2 mm in predicting crack initiation and instability load.