Characteristics of acoustic emission signals of Hertzian and unloading cracks in glass

Reproducible acoustic emission (AE) signals can be observed when a spherical diamond indenter is used to generate cone‐ or ring‐shaped Hertzian cracks on the surface of a glass specimen. The signals which are emitted during the loading of the specimen have features virtually indentical to those resulting when a glass capillary is broken on the surface of the specimen. This paper describes the first clear identification of Hertzian cracks which act as point sources of acoustic emission with features corresponding to a vertical force. The temporal characteristics of the force correspond to a Heaviside function with risetime less than 0.1 μs and an amplitude ranging from 1.8×10⁻³ to 4.2×10⁻³ N. The unloading of the diamond indenter from the glass also produces characteristic AE signals. These are distinguishable from the loading signals and they are shown to correspond to the development of unloading cracks on the surface of the specimen. Quantitative information about the unloading cracks is obtained from the AE signals by measuring the amplitude of the first pressure or P wave of the unloading signals. It is shown that this feature is directly related to the maximum applied load attained prior to unloading. A load threshold is found below which no unloading signals are detected.