Stress measurement with non-indentation

The feasibility of using nano-indentation to measure residual stress in glasses was studied. Indents were placed on the side of flexure specimens at four different distances from the neutral axis while the specimens were under load in four-point-bending. Three different glasses (soda-lime, boro-silicate, and fused silica) were indented with a cube-corner indenter using 2 to 30 mN indentation loads. A high resolution scanning electron microscope was used to measure the length of the cracks emanating from the corners of the indents while the specimen remained under load. The measured crack lengths were correlated to the local stress using indentation theory. For the correlation, elastic beam theory was used to calculate the magnitude of the local stress at the indentation sites. Results derived from crack lengths were in good agreement with local stress within experimental scatter. However, this scatter was found to be rather large as a result of the stochastic nature of crack formation. It can be concluded from this study that nano-indentation can be used to measure residual surface stresses with high spatial resolution provided that a sufficient number of indents are used to assure good statistical accuracy