Practical aspects of using Hertzian ring crack initiation to measure surface flaw densities in glasses: influence of humidity, friction and searched areas

Abstract

Ring crack initiation loads on glass, using spherical Tungsten carbide (WC) and glass (G) indenters, are measured and analysed. Our measurements demonstrate that environmental humidity plays a key role in determining the load to fracture; experiments conducted without controlling this variable cannot be used to obtain material properties. The role of friction is explicitly considered for dissimilar (WC–G) elastic contacts. For this material pair, the stresses at fracture are well described by a boundary lubrication value of friction coefficient. The fracture loads are used in a fracture-mechanics formulation to calculate crack sizes on glass surfaces. The ‘searched-area’ concept for dissimilar contacts is described, and used to provide crack density values for these surfaces.

Keywords:

• indentation
• contact mechanics
• crack growth
• fracture
• glass

Acknowledgements

We thank Clay Newton at Sandia National Laboratories (SNL) for helping with the measurements, and for valuable discussions. A useful review by Thomas Buchheit at SNL is gratefully acknowledged. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the US Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

Notes

1. Note that this approximation is used as it simplifies the calculations for the searched area. Calculations without this assumption, i.e. using Equation (6) in cases where the ‘surface stress only’ description is not accurate, are possible.