Glass Expert Performs Glass Failure Analysis of Glass Pie Plate
Introduction: A glass pie plate was being used to melt
caramel. When being removed from the oven, it fractured and injured the user.
Objective: Perform a failure analysis to determine the
root cause of the fracture.
Description: Figure #1 shows the failed glass cookware in
the as received condition. All the pieces are held together by the hardened
caramel.
Figure #1: Broken glass dish in the as received condition. All
the pieces are covered with the hardened caramel.
Procedure:
1. Rinse all the pieces with warm water to dissolve the hardened
sugar solution.
2. Reassemble the broken dish in order to find the failure
origin.
3. Trace the fracture back to the origin.
4. Microscopically examine the origin to determine the cause of
failure..
5. Evaluate other anomalies with respect to the dish failure.
Results: Figure #2 shows the re-assembled glass dish. The
origin is at the top of the photo.
Figure #2: Photograph of the re-assembled dish. The failure
origin is at the top of the photo. The "wandering" nature of the cracks leaving
the origin indicates that the failure was driven by thermal stresses.
Figures 3 through 5 summarize the findings. The root cause of
the failure was an oval shaped defect inside the glass.
Figure #3: Close-up of the region of the fracture origin.
Figure #4: Low power micrograph of the failure origin. This
failure originated in the interior of the glass (Mag. 12X).
Figure #5: Low power micrograph of the failure origin. This
failure originated in the interior of the glass. The Wallner lines emanate in
all directions from an oval shaped defect. It appears that the defect is a
collapsed bubble (seed) that did not "re-weld" (Mag. 40X).
Conclusion: The failure was a thermal crack that
initiated at a flaw in the glass. The flaw appears to be a collapsed bubble in
the glass. This is a manufacturing defect.
Additional Observation: There was a "cluster" of straight
cracks. The fracture surfaces indicated that they were in regions where the
glass surface compressive stresses in tensile stresses in the center (See Figure
#6).
Figure #6: Photomicrograph of the type of fracture surface found
on the straight cracks. The mist hackle in the center indicates significant
tensile stresses in the center of glass. In addition, there are two sets of
Wallner lines. This is similar to a "tempered glass" fracture surface, This
crack traveled from right to left (Mag. 20X).
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