Evaluation of low cycle fatigue life for fork-type blade root attachments of steam turbine

摘要

We have developed a new evaluation method for low-cycle fatigue life for fork-type root attachments of long steam turbine blades. In evaluating low-cycle fatigue strength at the root attachment area, the major difficulty is the contact stress problem. The conventional method, which is based on the local peak stress or strain range and fatigue data for small, plain specimens, has several problems when it is applied at the contact area of pin joints because (a) it does not include the effect of a steep stress or strain distribution along the crack extension direction, and (b) the local peak stress at the contact area depends on the element size of finite element method (FEM) analysis, due to the stress singularity problem. In this study, low-cycle fatigue life was estimated by calculating the micro crack growth life for a range of 0.04 to 1 mm in depth under a steep strain distribution, using the micro fatigue crack growth rate, which is proportional to the effective crack length according to non-linear fracture mechanics. To verify this evaluation method, low-cycle fatigue tests on pin joint models were conducted, in which the crack growth behavior at the inner surface of the pin hole was observed by interrupting the tests. We confirmed that the estimated results for micro crack growth behavior agreed well with the measured results. To obtain the local strain range distribution for fork-type root attachments, we suggest a two-step FEM analysis, including three-dimensional analysis of a whole-blade model and two-dimensional elastic-plastic contact analysis of a single blade-fork model.