Fracture Mechanics Study on Subsurface Median Crack in Layered Structure Subjected to Mechanical Contact

The subsurface transverse median crack system due to spherical contact in brittle coating-bond -substrate sandwich structure is investigated with an emphasis on stress intensity factor near the crack front using finite element method. This crack system emanates from the lower surface of the brittle layer and extends transversely towards the top of contact surface, resulting in ultimate failure of the structures. In this paper, finite element mesh is designed, and the crack front mesh designing solution for a surface crack subjected to remote tension is validated with Newman and Raju empirical equation. Then the proposed crack front mesh design solution is applied to a typical sandwich structure of various modeling parameters to obtain the SIF with 3D-J integral.

Surface Cracking and Degradation of Dense Hydroxyapatite through Vickers Microindentation Testing

Surface degradation and cracking of dense hydroxyapatite were evaluated through Vickers micro indentation using indentation loads ranged from 25 gf to 2000 gf. Crack lengths, imprint diameters and the number of lateral cracks and chips were measured using SEM. The crack length-indentation load data were analyzed with regard to the specific relations of Palmqvist and fully developed radial cracks. Crack type transition load from Palmqvist to median crack was experimentally assessed through serial sectioning technique. The analytical estimated transition load, based on the theoretical relation of the indentation load and crack lengths showed a good agreement with one obtained from experimental itinerary. Palmqvist and median cracks were identified in low and medium indentation loads, respectively. High indentation load could also lead to the formation of lateral cracks and chips. The tendency for lateral cracking was evaluated taking into account the number of lateral cracks and chips. The chips were found to be appeared just after test in higher indentation load, whereas in medium loads they could be detectable only after several weeks.