Research On J_IC Test Method And Failure Assessment Of A Shipbuilding Steel Welded Structure

Several problems in current JIC test method were studied, and the difference between Elastic-Plastic Fracture Mechanics (EPFM) residual strength analysis and the CEGB procedures for the "Assessment of the Integrity of Structures Containing Defects" (R6) was discussed, and the failure assessment of a shipbuilding steel welded structure was presented. The main results are as follows.(1) The relationship between the ratio of stretch zone width to crack tip opening displacement value (SZW/δ) and mechanical properties of materials suggested by Prantl does not have a good agreement with experimental data. The major reason is that some assumptions used in the deduction process are contradictory. The SZW /δ value seems to be linear with instead of proportional to flow stress.(2) Analyzing the blunting process of cracked specimen according to the D-B model, a blunting line equation has been obtained on the basis of the path independence of J-integral. Experimental results of three states of a low alloy steel show that this equation is more precise to describe the blunting process than those in current standards.(3) In current JIC test method, the slope of the blunting line is too low, restrictions on △amin and Jmax are too conservative to obtain valid data points for highly tearing resistance materials. Hence, it is suggested that the new blunting line should be recommended and restrictions on △amin and Jmax should be reduced: 0.03mm offset line of the blunting line is taken as the left exclusion line, and Jmax = b0σY /6 is taken as the upper exclusion line.(4) When JIC/σY is used to characterize the minimum ligament, the coefficient a is strength-dependent, that is, a increases with the increasing strength. However, When Jk/E instead of JIC/σY is used, the effect of strength on the coefficient aE is negligible, and aE of these materials are morecoincide with each other than a. It is indicated that Jk/E is more suitable to characterize the validity criterion. A new validity criterion is recommended. Experimental results show that this criterion is more precise to determine the critical ligament than that in current standard.(5) Residual strength diagram of the panel determined by EPFM residual strength analysis is much different from that calculated by R6. It is suggested that the reason should be that failure criterions used in two methods are different. The failure criterion of EPFM residual strength analysis using ductile fracture toughness JIC is actually the initiation of crack growth, while that of R6 using plain strain fracture toughness KIC (or plain stress fracture toughness KC) is really the instability of crack growth.(6) When the loading rate is lower, fracture property of weld metal in the welded structure is a bit lower than that of shipbuilding steel. While, when the loading rate is higher, the fracture property of weld metal is far lower than that of shipbuilding steel, which indicated that the welded structure is more inclined to failure in weld metal under dynamic load. In addition, fracture property and failure model of the welded structure both have loading rate sensitivity, that is, with the increasing loading rate, fracture property of the structure gets lower and the structure is more prone to destroy in brittle fracture.(7) The relationship between critical crack size and stress is related to failure model. When collapse is dominant, the critical crack size is nearly linear with stress. While, when brittle fracture is dominant, the stress- critical crack size curve is nearly a hyperbola.