Life Prediction of Three-Dimensional Fatigue Crack Propagation in Residual Stress Fields with PMMA

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An extended method for predicting fatigue lives of a 3-dimensional crack propagating in residual stress fields proposed by the present authors was examined with PMMA as follows.(1) Basic data of 2-dimensional cracks, a da/dN-DELTAK_<eff> and U-R relation, were obtained, which were to be employed in the 3-dimensional U prediction method. (2) During loading, Newton's rings could not be generated owing to difficulty of the experiments. In stead, observations of the crack closure phenomenon were made by making use of total reflection on the crack surfaces of sodium-vapor lamp developed by the present authors. However, by this method deformation aspects of the crack surfaces can not be observed in theoretically, but aspects in which the crack was closing from its crack front could be observed clearly. (3) Same experiments as (2) were conducted for specimens with residual stresses, and aspects that a crack was closing from the compressive residual stress field could be observed clearly. In n … More o-loading, Newton's rings could be generated, and it was confirmed that a crack surface bent in the tensile residual stress field. Consequently many informations on the crack closure phenomenon of 3-dimensinal cracks could be obtained. (4) A influence coefficient method by Shiratori et al and Newman-Raju's equations were mainly employed in calculating K values. F. E. M. was employed only for verifying those K values because a great deal of work it required. Without reference to this original program, the present life prediction method was applyed to PMMA, and its prediction results were in good agreement with experimental results.On the basis of these informations, this life prediction method could be applyed to metal specimens with residual stresses, and their results were compared with experimental results. Two kinds of residual stress distributions were employed in the experiments. One was caused by electronic beam welding of SM41B(instead of HT80 which was not available) and the other was in SUS-clad SPV steel. The experiments were conducted in the same way as PMMA.Consequently it was confirmed that the extended method for predicting fatigue lives of a 3-dimensional crack propagating in residual stress fields proposed by the present authors was in valid. Less