Characterization of the Glass and the Glass Fracture
Introduction to Glass Fracture
Glass: Glass is a brittle material that is
characterized by the fact that it deforms solely in an
elastic manner and that it can only fail in tension. In
fact, a crack will always travel in a direction
perpendicular to the maximum principle stress. Because there
is no plastic deformation of the material, it has no ability
to absorb energy during failure. In addition, most glass
failures initiate at a surface flaw; therefore, the failure
characteristics are dominated by surface defects. The
failure stress of normal glass (i.e. annealed) can range
from a minimum of 2000psi to a value as high as 40,000psi.
From this standpoint the failure of glass is much less
predictable than that of a material such as steel.
Tempered Glass: Tempering is special heat treatment
developed to create a compressive stress on the surface of
the glass. Due to the principles of superposition, this
compressive stress must be overcome before the surface can
be put into tension; therefore, tempering increases the
failure stress of glass by 10,000psi or more. It is
important to note that the compressive stresses at the
surface of tempered glass are balanced by tensile stresses
in the center of the glass; therefore, once a crack
penetrates the compressive layer, it is driven by the
residual tensile stresses in the glass. Therefore tempered
glass tends to break up into “blocks” of glass. The size of
the block is an indication of the temper stress; the block
gets smaller as the temper stress increases.
Crack Velocity: At terminal velocity a crack in
glass travels at 6000 fps. Therefore a crack traveling one
foot requires less than 0.2 of a millisecond. Because of the
high tensile stresses in the center of tempered glass, all
tempered glass failures travel at this rate.
Objective
For this study the objective was to determine the
origin of a tempered glass fracture
Procedure and Results
Glass Basketball Backboard Failure
Figure # 1 : An overview of the remaining
glass clamped between the two metal plates that held the
basket to the backboard.
Figure # 2 : A close up of fracture initiation site. The
failure initiated at one of the bolt holes in the glass, and
it spread diagonally from the lower left hole .
Rear Window for a Pickup Truck
Before Impact: A standard Strainoptics Grazing Angle Surface
Polarimeter (i.e. GASP) was used to measure the temper
stress of each of the windows tested (Figure #1). This
instrument introduces a red laser beam at a grazing angle to
the glass surface and uses retardation of the light by the
stressed glass to measure the stress near the surface of the
glass. The stress in the 11 windows examined varied from
17,000 to 18,500 psi. This is well above the 10,000psi
required by DOT.
Figure #3: Photograph of a GASP used to find
the surface stress in tempered glass.
After Impact:
Fracture Pattern: In order to see the crack pattern
generated by the fracture of tempered glass, a window was
covered with transparent pressure sensitive tape before
impact. This holds the glass in place after failure so one
can see the fracture pattern of the glass. Without the tape,
the glass will fall out of the frame and the fracture
pattern will be lost.
Figure #4: Fracture pattern of the tempered
window. One can see that the crack spreads out from the
point of impact (much like the spokes in a wheel).
Understanding this can help one to find the impact point if
enough of the pattern is preserved.
Fracture Surface: Numerous fracture surfaces
were examined after impact, and they all demonstrated that
the glass was tempered. Figure #3 is a representative
fractograph of the window failure..
Figure #4: Representative photomicrograph of a tempered
window fracture surface. It includes two sets of Wallner
lines and mist hackle in the center. This is typical of
tempered glass failure (Mag. 20X) |
