Notch effects in fatigue and fracture

摘要

The notch effect in fracture is characterised by the fact that the critical gross stress of a notched structure is less that the critical net stress which acts on the remaining ligament area under the notch tip. The notch effect in fracture is sensitive to structure geometry, scale effects and loading mode. This cannot be explained by the hot spot approach (i.e. the value of the maximum stress) but can be by the volumetric approach. In this approach we assume that the fracture process needs a physical volume in which acts an average weighted stress. Two parameters are needed to describe the fracture criterion; the effective distance which corresponds to the diameter of the fracture process volume assuming a cylindrical shape, and the effective stress. A procedure to determine the effective distance from the value of the minimum of the relative stress gradient is proposed and checked. The influence of notch radius on fracture toughness is described in terms of the energy J parameter. The existence of a critical notch radius below which the fracture toughness remains constant is verified. The notch effect in fatigue is characterised by the fact that the notched specimen Wohler curve is below the smooth specimen curve. For high cycle fatigue, the concept of effective stress range is able to represent the notch influence. For low cycle fatigue, the description is done in terms of effective strain energy density.