Development of an Evaluation Method for Fretting Fatigue at the Mating Surface between a Cylinder Block and Main Bearing Cap with Temperature Fluctuations

Abstract

<div class="section abstract"><div class="htmlview paragraph">Fretting is a phenomenon in which a fatigue crack is initiated by a small relative slip between two objects, resulting in crack propagation and fracture at stresses far below the fatigue limit [<span class="xref">1</span>, <span class="xref">2</span>]. Since the mechanism behind fretting is complex and covers multiple disciplines, it is not easy to develop a consistent evaluation method. In the field of engine development, fretting events can also pose an issue due to the complexity of the mechanism [<span class="xref">3</span>]. In particular, it has been a challenge to help predict changes in the presence and severity of fretting events, as the engine temperature fluctuates with operating conditions. As one method for evaluating fretting, Sato, et al. have made predictions using analytical models based on the finite element method (FEM) [<span class="xref">4</span>, <span class="xref">5</span>]. However, their predictions did not take into account temperature fluctuations in the system, and they were unable to predict events in which the occurrence of fretting fatigue changed with temperature fluctuations.</div><div class="htmlview paragraph">In this study, first, tests were conducted on an actual engine to examine the effect of temperature on the occurrence of fretting. In addition, the mating surface was observed in detail after the tests. Specifically, the microcracks on the mating surface were measured, and the damaged areas were analyzed for their composition. As a result, it was found that the main factors influencing the occurrence of fretting were the relative slip velocity caused by thermal expansion and the relative slip rate and stress amplitude caused by the crank load. Traditionally, fretting has been evaluated based on the stress amplitude at the mating surface and the relative slip caused by the crank load [<span class="xref">4</span>]. In this study, the relative slip caused by thermal expansion due to the difference in thermal expansion coefficients between the two objects was incorporated into the evaluation, making it possible to evaluate fretting while taking the effect of temperature into account. The influence of each factor was quantified using a three-dimensional nonlinear FEM. Simulations included bolt-tightening force, cyclic crankshaft force, and temperature variation. The simulation results were combined with test results from various engine types to develop experimental criteria.</div></div>