Surface Stress Effects on Indentation Fracture Sequences

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Abstract
Ion exchange was used to introduce surface compression in soda‐lime‐silica glass (SLS) and aluminosilicate glass (ALS). In situ observations of the indentation cracking sequences on the modified surfaces were made and compared with those on the base glasses. The initiation point for each crack type formed during the loading and unloading cycle was monitored on the indenter load‐displacement traces. On the base glasses, median (in the SLS) and cone (in the ALS) cracking was observed during the loading cycle. “Modified” median, radial, and lateral cracking was observed in both glasses during unloading. The introduction of the residual compressive surface stress was found to completely suppress all cracking during the loading cycle (up to 100 N) in both the SLS and ALS. In addition, the initiation of the cracks formed during the unloading cycle was shifted to further down in the cycle, i.e., to lower values of the initiation load. A pronounced tendency for a shift in the cracking from median to radial was also observed on the residually stressed surfaces. A similar shift was observed on stress‐free surfaces during unloading as the value of the initiation load decreased for a given peak load.

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