Cycles of sub-critical tensile and shear alternating fracturing in diminishing dimensions, under tensile loading

A microscopic study reveals that when the curvature of striae that mark the fracture surface of PMMA glass with a chevron pattern increase beyond the critical angle, μc = 3◦ ± 2◦, a breakdown into alternating tensile dark zones, and bright, ragged shear zones occurs. This breakdownwas repeated in primary, secondary and tertiary cycles in diminishing scales. The secondary and tertiary breakdowns occurred exclusively in the shear zones. Similar breakdowns were found in chevron patterns on the fractured surface of a silicate glass ceramics. Due however to their different properties, certain differences were identified between the two materials in their breakdown characteristics, e. g. in the glass ceramic μc = 20◦ ± 2◦. A similar primary breakdown was also identified on tensile fractures cutting rocks in geological outcrops. In the glass ceramic the interface angle φ, which the striae formwith the fracture boundary, decreased from 32◦ ±2◦in the early stage of the striae growth at relatively low velocity, to 13◦ ± 2◦ during their advanced growth, at greater velocity, demonstrating that φ is a good tool for monitoring the change in fracture velocity in a given material. It was found that four interconnected factors determine the geometries and breakdown styles of the chevron pattern: (1) the curvatures of the fracture front and that of the striae which intersect each other orthogonally, (2) the influence of the fracture boundaries, (3) the material properties, such as stiffness, and (4) the fracture velocity in the material.