Notched Specimen Of The Local Multi-axis Strain Analysis With The Random Fatigue Life Prediction

In this paper, both theories and experiments concerning the analysis of multiaxial stress and strain of noches as well as the prediction of the multiaxial fatigue life of the notched specimen subjected to random loading were investigated.Firstly, some representative approaches for multiaxial local stress and strain analysis were reviewed and evaluated briefly.By analyzing the local stress and strain of the notched specimen with a centre hole subjected to monotonic and cyclic loading using 3-D elasto-plastic finite element, the multiaxial mechanical responses of the notched specimen as well as their changes with the nominal stress (amplitudes) were investigated. The changing characteristic features of the stress and strain concentration factor as well as their geometric average both at the notch surface and in the middle of the specimen were obtained. By comparing the results obtained using FEM with those obtained using experiments, it is shown that the former is more agreement with the later under the monotonic cases. In addition, the variation of the multiaxial local stress and strain with the thickness of the notched specimens was also studied. It was shown that with increasing thickness the position of the maximum strain in y-direction transferred from the middle plan to the surface. Two methods for describing the cyclic stress-strain curve, namely CST and IST, were used to investigate the effect of the constitutive equation of the material on the calculated multiaxial stress and strain at the notch root. The results showed that the calculated values using IST accorded well with the measured values.The multiaxial local strain of the notched specimens with a certain thickness as well as its response with the cyclic loading was obtained using electrical measurements. These results were used to compare with those obtained using FEM and approximate approaches. The plastic slip characterizations around notch were obtained using Metallographical observations. The initiation and propagation of fatigue cracks at the notch roots were also observed by using SEM method. It was indicated that the crack firstly initiated from the surface of notch tip.Using three uniaxial local stress-strain approximate methods and three multiaxial local stress-strain methods, we calculate the local stress and strain of the notched specimens subjected to both monotonic and cyclic loading. These results were compared with those determined by both FEA and experiments. It was shown that the prediction precision of the methods with correction of multiaxial was better than that with uniaxial. Both the multiaxial ESED method and multiaxial Neuber method got some satisfied prediction results. The above three uniaxial and two multiaxial local stress-strain approximate methods were further used to predict the life of the notched specimens subjected to random loading. The predicted lifetimes was compared with the measured results, indicating that the multiaxial Neuber method was the best among these methods. As a consequence, the multiaxial Neuber method was suggested to be applied into the fatigue life prediction of the engineering component under multiaxial random loading.