Abstract :
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.
