Fracture modes in curved brittle layers subject to concentrated cyclic loading in liquid environments
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E. Dianne Rekow Basic Science Department, New York University College of Dentistry, New York, New York 10010
Yeon-Gil Jung School of Nano and Advanced Materials Engineering, Changwon National University, Changwon, Korea
Yu Zhanga) Department of Biomaterials and Biomimetics, New York University College of Dentistry, New York, New York 10010 (Received 30 July 2008; accepted 14 September 2008)
Damage response of brittle curved structures subject to cyclic Hertzian indentation was investigated. Specimens were fabricated by bisecting cylindrical quartz glass hollow tubes. The resulting hemicylindrical glass shells were bonded internally and at the edges to polymeric supporting structures and loaded axially in water on the outer circumference with a spherical tungsten carbide indenter. Critical loads and number of cycles to initiate and propagate near-contact cone cracks and far-field flexure radial cracks to failure were recorded. Flat quartz glass plates on polymer substrates were tested as a control group. Our findings showed that cone cracks form at lower loads, and can propagate through the quartz layer to the quartz/polymer interface at lower number of cycles, in the curved specimens relative to their flat counterparts. Flexural radial cracks require a higher load to initiate in the curved specimens relative to flat structures. These radial cracks can propagate rapidly to the margins, the flat edges of the bisecting plane, under cyclic loading at relatively low loads, owing to mechanical fatigue and a greater spatial range of tensile stresses in curved structures.
I. INTRODUCTION
Brittle layers on polymeric substrates are relevant to a wide range of engineering structures, including biomedical prostheses.1–3 Full-coverage ceramic dental restorations (crowns) are a special case in point. Although there has been an immense amount of study concerning the fracture of crowns, the bulk of the work reported in the literature has focused on model flat ceramics/polymer bilayers loaded on the top surface with a spherical indenter.1,3 Teeth and crowns exhibit pronounced curvature and shape irregularities, which can substantially alter stress state and thereby influence critical loads to initiate and propagate cracks in the brittle layers. Recently, several studies have extended to domelike polymer-filled glass shells using single-cycle load-to-fracture Hertzian indentation in dry environments.4–9 However, crowns are subject to repeated chewing function in liquid environments, a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0081 J. Mater. Res., Vol. 24, No. 3, Mar 2009
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which this study takes into consideration. Accordingly, we are investigating the initiation and failure conditions of curved surfaces as a function of number of cycles using hemicylindrical shells (by bisecting cylindrical tubes through their diameter along the cylinder axis) supported by compliant bases cyclically loaded on the outer circumference with a sphe
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