13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Flat-tipped Indentation Fracture Mechanics Xiaozhi Hu1*and Yujun Xie2*, 1 School of Mechanical and Chemical Engineering, University of Western Australia, Perth, WA 6009, Australia 2 Department of Mechanical Engineering, Liaoning Shihua University, Fushun, 113001, P. R. China * Corresponding author: xiao.zhi.hu@uwa.edu.au, yjxie@lnpu.edu.cn Abstract A singular stress field exists in an elastic substrate around the contact edge with a rigid flat-tipped indenter. This surface-contact-induced singular stress field can also be described by the stress intensity factor concept, if the indentation stress intensity factor Kind is introduced for indentation cracking analysis. The Kind–controlled singular stress field is almost identical to that of the Mode-I tensile singular stress field around the crack tip, except the negative sign due to the compressive nature of surface contact loading. This study presents an energy-based fracture mechanics analysis for the indentation stress intensity factor (ISIF) Kind and the indentation-induced boundary cracking within the Kind-dominant region around the contact edge. It is found that the critical indentation stress intensity factor exists, and the relation between the indentation fracture toughness KIC-ind and the common Mode-I fracture toughness KIC is established analytically, showing KIC-ind = 2.5KIC. The indentation-cracking angle at the contact edge is also determined. The fracture mechanics model on surface contact cracking induced by a flat-tipped indenter provides a useful alternative for measuring the fracture toughness KIC, which can be useful for characterization of surface fracture properties of bulk elastic bodies and coating fracture properties of layered structures such as MEMS. Keywords Indentation, Indentation fracture toughness, Contact mechanics, Surface cracking 1. Introduction Frictional sliding induced surface damage in the form of micro-crack initiation can be the precursor for potentially much severe structural damage. Fracture mechanics modeling concerning this important structural integrity issue has been an interesting topic for years [1-3]. It is noted from the work [1,2,4,5] that a mixed-mode singular stress field exists in an incompressible substrate at the sliding contact edge of a rigid flat-ended indenter pressing down onto the substrate. This study will examine closely the process of micro-crack initiation on the contact surface, which is controlled by the singular stress field at the sharp corner of the flat-ended indenter, and the cracking angle. The familiar singular stress fields are those associated with sharp cracks in elastic solids, through which the Mode-I fracture toughness KIC at the critical loads can be determined from the common stress intensity factors. Although the singular stress field in an incompressible substrate at the sliding contact edge of a rigid flat-ended indenter is not due to the presence of a sharp crack tip, this contact-induced singular stress field does share some similar features with those of a crack-induced singular stress field, which implies the well-known concepts of linear elastic and elastic-plastic fracture mechanics such as the stress intensity factor K and the J-integral [6-11] can be adopted for modeling of the crack initiation from a crack-free surface under frictional sliding wear. The significance of fracture mechanics modeling of contact crack initiation can be seen from fretting fatigue and other engineering applications such as rock fracture mechanisms in rock cutting. This study will present a new methodology based on the aforementioned singular stress field generated at the sliding contact edge of a rigid flat-ended indenter, which will connect the contact mechanics together with the fracture mechanics by using energy-based modeling.
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