The Coupled Finite Element For Predicting Fatigue Crack Initiation Lives

Fatigue failure of components has been an important factor relating to the safety and the cost of the mechanical equipment and the structure in many engineering fields. The fatigue failure process of the structure can be divided into two stages: fatigue crack initiation and crack spread. Basically, the stage of the crack spread is analysed by fracture mechanics at present. The stage of fatigue crack initiation is studied by the way of experiments combining with statistics. The above way which inevitably needs lots of statistical testing and costs much, as well as wasting lots of time, is not highly feasible. The research of fatigue crack initiation life is with large theoretical and practical significance.The paper studies the fatigue crack initiation lives of the plane stress plate-like components by using fatigue damage mechanics and coupled finite element. Damage coupling effect can be realized by modifying stiffness matrix with coupled finite element. The step of the damage evolution is divided by the damage degree of the critical element when the stress field and the damage field are known, and then the damage evolution is analysised by the simple damage evolution equation. The material constants in the damage evolution equation are obtained by calculating with the existing experimental data. The concept of equivalent stress is introduced into the damage evolution equation. The corresponding stiffness matrix and loading matrix can be obtained continually under the condition of increasing the damage evolution step of the critical element, then the equivalent stress can be calculated, after that the corresponding fatigue crack initiation life can be calculated by using the damage evolution equation, the progress will not stop until the damage degree of the critical element reaches to one. The fatigue crack initiation life of the plate-like component is obtained by adding the fatigue crack initiation lives with different damage degrees.It is proved that the results of the fatigue crack initiation lives are ideal by comparing with the exiting experimental data, and it is feasible to predict the fatigue crack initiation lives of plate-like components by using the above mothed under the condition of large range damage. At last the fatigue crack initiation lives of several kinds of plate-like components are calculated.