Effect of multiple crack interactions on the strength of glass

Objectives: To test the hypothesis that the strength will be less if there are two or more cracks of approximately the same size within a critical distance of each other.

Methods: Commercial soda-lime-silica glass bars (5 mm x 10 mm x 40 mm) were indented using a 1 kg Vickers diamond at single and multiple sites. The multiple sites consisted of two and three indents which were 500 microns and 1000 microns apart. The bars were then fractured in four-point flexure with the indents on the tensile surface. The strengths were recorded and the fracture surfaces examined for crack sizes. Fracture mechanics equations were used to estimate the effect of interaction. These values were compared to the measured values.

Results: The average strength of bars with single indents was 79 ± 7 MPa , for bars with two cracks at 500 microns apart was 72 ± 3 MPa and for bars with two cracks at 1000 microns apart was 70 ± 3 MPa. There is a significant statistical difference between the strengths from a single indent and the strengths from multiple indents.

Conclusions: Strengths are lowered due to stress field interactions if multiple crack sizes of comparable sizes are within 1000 microns of each other. Indents must be greater than 1mm apart to avoid interaction of the residual stress fields. The suggested distance is at least 1.5 mm between the crack tips.

Support: Sandia National Laboratories, Albuquerque NM 87122