Cleavage fracture origins under static and dynamic loading

According to the RKR [1] model, the process of cleavage fracture is considered to be governed by the attainment of a critical tensile stress, called the cleavage fracture stress, in the material. Griffiths and Owen [2] analyzed the distribution of tensile stress ahead of a notch root by the finite element method (FEM), and found that the peak tensile stress locates at the site beneath the notch surface. McMeeking's FEM calculation [3] also shows a similar result for a smoothly-blunting crack specimen. All these mean that cleavage microcrack should initiate at the location beneath the notch or crack surface. In this report, we concluded the fractographic SEM observation results of notch (HQ80) and precracked (HQ60) specimens fractured under static and dynamic loading, and the relation between the location of cleavage microcrack initiation and that of critical tensile stress was analyzed by means of the FEM calculations.