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Chapter 1:
Crack Basics
The theoretical strength of amorphous materials is in excess of 7000 MPa (~1,000,000 psi), due to strong covalent bonds. For example, silica glass (SiO2) has strong tetrahedral bonds between the silicon and oxygen atoms. Indeed, strengths of 3500 to 14,000 MPa (500,000 to 2,000,000 psi) have been attained by chemical polishing with hydrofluoric acid solutions and by flame-polishing small silica rods. One might consider the only “defect” in such materials to be the interstitial molecular spacing, which is on the subnanometer level. Recent advances in nanotechnology further bear this out. However, manufacturing processes, such as those involving generating, grinding, or lapping, are such that defects of much larger proportion - on the micron level - are introduced, which greatly reduces strength. A. A. Griffith, noting that failure of glass occurred orders of magnitude below its theoretical atomic strength, was the first to postulate that there were microscopic cracks in every material, and that these cracks were larger than the interatomic distance. Griffith further hypothesized that these cracks lowered the overall strength of the material. He presented experimental results on glass to prove this by introducing defects of various sizes and showing that it was these effects that determined the strength of the glass. Griffith’s work is the basis of modern fracture mechanics, which describes the failure of glass, ceramics, and other materials.
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