ALTERNATING STRESS 2, Developing process of the fatigue failure Failure of materials under the action of an alternating stress, in custom, is called the fatigue failure 1). Sub-structures and microscopic structures produce changes and the core of the permanent damage is formed 2). Microcosmic cracks are produced 3). Microcosmic cracks expand merge and form the leading crack 4). The macroscopic leading crack expand steadily 5). The structure losts its stability or ruptures completel2、Developing process of the fatigue failure: 1).Sub-structures and microscopic structures produce changes and the core of the permanent damage is formed. 2).Microcosmic cracks are produced. 3).Microcosmic cracks expand 、 merge and form the“leading” crack. 4).The macroscopic “leading” crack expand steadily. 5).The structure losts its stability or ruptures completely. Failure of materials under the action of an alternating stress, in custom, is called the fatigue failure