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SummaryThe effect of the notch depth on the critical CTOD value at cleavage fracture initiation was analysed by the local approach. Three-point bend CTOD specimens with different notch depth extracted from a high strength steel were tested at the lower shelf temperature. The critical CTOD values showed large dependence on the notch depth. Three-dimensional elastic-plastic finite element analyses were performed to investigate the crack tip stress fields for all specimen geometries. The numerical results showed that shallow and deep notch specimens exhibit very different near tip stress fields at the same CTOD level, which leads to the specimen geometry dependence of the fracture behaviour. The analysis of the notch depth dependence of CTOD results was conducted on the basis of two formulations of the local approach. In the first formulation, cleavage fracture was assumed to be controlled by the maximum principal stress. In the second formulation, the stress tensor ahead of the crack tip was fully described in terms of all principal stresses and cleavage fracture was assumed to be controlled by an equivalent stress for mode I loading. It was shown that both formulations of the local approach lead to geometry independent parameters, especially when the stress triaxiality effect is taken into account. Prediction of the notch depth dependence of CTOD results based on the local approach presented good agreement with the experimental data, particulary when stable crack growth prior to cleavage was not significant .

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Several precracking methods for crack-tip opening displacement (CTOD) testing of weldments were studied, including as local precompression, reverse bending, and high R-ratio fatigue. The precracking procedure could influence the validity of the precrack shape. However, it was found that an irregular precrack front had little effect on CTOD values of the heat-affected zone (HAZ). Results indicate that the current limitations on the fatigue precrack shape can be relaxed. CTOD values comparable to those of the standard B × 2B specimen were obtained from the subsidiary B × B specimen with a through-thickness notch of a length-to-specimen-width ratio of 0.5. Hence, it can be possible to use B × B specimens instead of B × 2B specimens in the standard CTOD testing of weldments. The CTOD test results for weld HAZ were strongly affected by the notch positioning in the welded joint. By analyzing the weld thermal history, HAZ microstructures were classified. Then, it was revealed that CTOD values were closely related to the maximum size of the local brittle zone (LBZ) at the crack tip.

Three kinds of 780 MPa grade ultra-fine grained steels with different chemical composition were produced by warm rolling. The steels were characterized by ultra-fine ferrite grains (less than 1 mm). With the steels strength-overmatching welded joints were prepared, and their mechanical properties were investigated. It is found that softening occurred in the heat-affected zone (HAZ) because of the coarsening of ferrite grains due to welding heat input. However, by using low welding heat input and strength-overmatching weld metal, the detrimental effect of softening on strength was restrained, and welded joints with strength equivalent to that of base metal were obtained. The deformability of welded joints was found to be related to the yield ratio (yield strength / tensile strength) of base metal. Low yield ratio is desirable to the deformability of welded joints. The HAZs in the welded joints of low welding heat input of 10 kJ/cm have good impact toughness for all the steels. Except for 0.14C-0.30Si-1.46Mn steel, the HAZs in the welded joints formed from the other two steels also have good impact toughness for welding heat input of 20 kJ/cm, and their fracture appearance transition temperature (vTrs) of the HAZs is lower than Ϫ40°C, and their Charpy impact energy at Ϫ40°C exceeds 200 J.KEY WORDS: ultra-fine grained steel; welded joint; mechanical property; tensile property; Charpy impact test. Table 1. Chemical composition of steels examined (mass%).