Explanation

Under the action of stress and environmental factors (or a combination of both), the process of crack initiation of crack free specimens (or components) is called crack nucleation. It can be divided into microscopic crack nucleation and macroscopic crack nucleation. Cracks can be divided into the following categories: cracks generated during loading in inert media; fatigue crack under alternating load; creep cracks under the combined action of stress and temperature; stress corrosion crack under combined action of stress and chemical medium: hydrogen induced crack caused by hydrogen entering. The nucleation process and mechanism of each type of crack are different and need to be studied separately. The nucleation of cracks during loading is the result of uneven plastic deformation. Uneven deformation can cause stress concentration (such as dislocation packing). When the concentrated stress is equal to the atomic bonding force, materials will separate and form microcracks. At the junction of twin, the inclusion and the second phase should be concentrated, and microcracks tend to form preferentially. Some microcracks may not grow (or may not have enough time to grow) after nucleation, and the propagation and connection of microcracks will lead to the nucleation of macroscopic cracks. Sometimes micro cracks already exist inside the sample, and their growth and connection lead to the nucleation of macroscopic cracks.

Classification