J-Integral Test And Residual Strength Analysis For Steel 907A And Its Weld

Applying the current elastic-plastic fracture toughness testing method (GB2038-91) to determinate the JIC -values of steel 907A and its weld, it cannot be obtained reasonable measured results. The main reason of these difficulties brought should be attributed to too low slope of blunting line and little strict zone of valid data. It is suggested that the contribution of strain hardening should be estimated fully in the depiction of blunting -process. On the base of experiments and analysis, a new form of blunting line is proposed. It is as follows:J = 2K(0.5εF)"â–³aWhich has a good agreement with results measured experimentally, and can describe the blunting process of the crack truly. The choice of valid data and the determination of JIC -value in J -integral test were briefly discussed on the basis of testing the JIC -values of steel 907A and its weld.The process of the crack blunting, initiation and stable extension after loaded was also analyzed in this thesis. It is pointed that this process should be such a process from quantitative change to qualitative. So-called crack initiation point is actually the J-value at which the blunting at crack tip evolves to fracture with micro-void coalescence, and by which the macroscopic effect induced presents first. Eventually, a method of determining crack initiation point of ductile metals by testing-curve-analysis is put forward.In the second section of this thesis, it is discussed that the results of fracture mechanics test how to be applied in the safe analysis of shipbuilding structures. The results show that the smooth partion of residual strength curve of steel 907A and its weld was singularly higher than yielding stresss of the steel when K -criterion based on linear-elastic fracture mechanics was applied to analyze their plane structures with center through crack. It is shown that it is not reasonable to apply this criterion for the analysis of above structures whose material has higher fracture toughness. Then the elastic-plastic fracture criterion,which was used to deal with the same issue. The effect of loading rate on the residual strength of plane structures with center through crack for steel 907A and its weld was studied through threee cases of K = 10⁰MPa m/s(static state), k = 10⁵MPa m/s and K =10⁶⁷MPam/s, respectively. At the same time, the effect of plane width on the residual strength of above structures also was discussed. It is conclued that reducing plane width and increasing loading rate both will lower the residual strength of structures, and possibly result in the transition of failure mode. The reduction of plane width changes the failure mode through changing the geometry fator, while the increase of loading rate lead to the change of failure mode through decreasing the fracture toughness of material, which are two different effected mechanisms in essence. In the same loading rate, the residual strength of weld is always lower than that of matrix for steel 907A.On the base of an engineering approach for elastic-plastic fracture analysis, crack impetus maps of plane structures with center through crack for steel 907A and its weld have been built. The maps in engineering could be conveniently used to forecast and determine buckling load and stable extension of the structure with original different length cracks.