Glass Fracture Mechanism—A Rethinking

Conventional attitudes within the forensic science community concerning the fracturing of glass center around tensile failure of the glass, frequently depicted as a “bending” of the glass. While this is not conceptually incorrect, it represents only one case of a more universal phenomenon in which the tensile failure of glass does not necessarily involve any significant deflection of the glass. Tensile failure can be achieved with either quasi-static or dynamic loading of the glass. In quasi-static loading, tensile failure will initiate a fracture at the weakest point (that is, the locus of a Griffith crack), but the surfaces of this crack may be in optical contact, and thus no perceptible deformation of the glass would be required before failure. A consideration of dynamic loading is necessary to explain the “cratering” effect observed in moderate- to high-velocity projectile impact. In sharp dynamic loading (for example, a bullet impact) the tensile stress is provided by the reflection and subsequent interference of the compression waves which precede the passage of the projectile; this particular type of stress results in Hopkinson fractures, a multiplicity of which creates a crater. The dimensions and chamfering of projectile craters are a manifestation of the crack velocity propagation, and are not inherently a function of projectile velocity or caliber.