一、引言实践证明,COD方法对于中低强度钢结构的断裂安全分析是比较实用和有效的。它的优点之一在于可以用比较小的三点弯曲试样方便地测定材料的断裂韧性参量δ_c。然而,裂缝顶端张开位移(δ)不是裂缝体应力应变场直接的参量,它的大小与裂缝尖端钝化及伸张区的形成相联系。目前还没有比较方便可靠的方法 I. Introduction Practice has proved that the COD method is more practical and effective for fracture safety analysis of medium and low strength steel structures. One of its advantages is that the fracture toughness parameter δ_c of the material can be conveniently measured with a relatively small three-point bending test specimen. However, the crack tip displacement (δ) is not a direct parameter of the stress-strain field of the fracture body. Its size is related to the crack tip tip formation and the formation of the tension zone. At present there is no more convenient and reliable method