Computational service life estimation of contacting mechanical elements in regard to pitting Z. Ren * , S. Glodez University of Maribor, Faculty of Mechanical Engineering, Smetanova ul. 17, SI-2000 Maribor, Slovenia Received 14 November 2000; accepted 4 July 2002 Abstract A computational model for determining the service life of contacting surfaces in regard to surface pitting is pre- sented. The model considers the material fatigue process leading to pitting, i.e. the conditions required for the short fatigue crack propagation originating from the initial crack in a single material grain. In view of small crack lengths observed in surface pitting, the simulation takes into account the short crack growth theory. The stress field in the contact area and the required functional relationship between the stress intensity factor and the crack length are de- termined by the finite element method. An equivalent model of two contacting cylinders is used for numerical simu- lations of crack propagation in the contact area. On the basis of numerical results, and with consideration of some particular material parameters, the probable service life period of contacting surfaces is estimated for surface curvatures and loadings that are most commonly encountered in engineering practice. Ó 2002 Civil-Comp Ltd. and Elsevier Science Ltd. All rights reserved. Keywords: Surface pitting; Material fatigue; Service life; Fracture mechanics; Computational simulation; Finite element method 1. Introduction Mechanical elements subjected to rolling and sliding contact conditions fail by several mechanisms and the most prominent among these is surface pitting [1–4]. Pitting is a fatigue phenomenon and is characterised with a gradual deterioration of the contacting surfaces. Failure occurs either when the surface has deteriorated so much that the component no longer functions as designed, or when the damage becomes severe enough to lead to failure by another mechanism, such as breakage. The process of surface pitting can be visualised as the formation of small, surface-breaking or subsurface ini- tial cracks, which grow under repeated contact loading. Eventually, the crack becomes large enough for unstable growth to occur, which causes the material surface layer to break away. The resulting void is a pit. The service life of contacting mechanical elements consists of the crack initiation phase and the crack propagation period required for pit formation. The number of required stress cycles N for the pitting occurrence on contacting surfaces can than be deter- mined as N ¼ N o þ N f ð1Þ where N o is the number of stress cycles required for the initial crack appearance in the material and N f is the number of stress cycles required for a crack to propagate from the initial to the critical crack length. In single crystals the initial phase of fatigue life can take a con- siderable time. However in polycrystalline metals the crack initiation phase is usually very small, if compared to the fatigue life of mechanical elements [5]. There are also many possible stress concentration positions on or under the contact surfaces, like grain boundaries, triple * Corresponding author. Tel.: +386-2-220-7702; fax: +386-2- 220-7994/7990. E-mail address: ren@uni-mb.si (Z. Ren). 0045-7949/02/$ - see front matter Ó 2002 Civil-Comp Ltd. and Elsevier Science Ltd. All rights reserved. PII:S0045-7949(02)00263-8 Computers and Structures 80 (2002) 2209–2216 www.elsevier.com/locate/compstruc