Abstract
Brittle fracture initiation characteristics under combined mode I+II and I+III were examined by shear and bend test of notched specimens. In shear test, by the alternate arrangement of the four support points, there appears a pure shear stress field between of the two inner support points, so that any combination of mode I, II and III is easily realized. Materials used for these tests were PMMA plate and mild steel plate.It was confirmed that the fracture strength and the direction of the crack extension are rather in good agreement with σθ criterion under mode I+II condition and also under mode I+III condition in case where portion of the mode III is not exceeding. When the portion of the mode III becomes surpassing, brittle fracture initiates at one point on the notch tip line and propagates in a plane different from the initial notch plane. This type of fracture has to be evaluated by a new fracture parameter such as normal stress on the fracture plane. It was also confirmed that critical COD δc in the shear stress field is larger than one in the bending stress field if δc is greater than 0.05 mm or so on the mild steel. Fracture from a crack inclined to a principal stress is not always evaluated to the safety side by using a projected crack size on a plane normal to the principal stress, if the crack exists in a shear stress field and fracture initiates under small scale yielding condition. But if δc is larger. than 0.05 mm or so as ordinary structural steel, this flaw evaluation method becoms reasonable because δc in shear stress field becoms larger than one obtained by bend test.
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