Failure Analysis: Manufacturing Engineer Examines the Root Cause of
Automobile Rear View Mirror Failures
The root cause of the failure of customer returns of heated
automobile rear view mirrors was investigated.
Theory: Glass fracture theory suggests that the condition
of the glass surface is the dominant factor. Theoretically, glass should fail at
1 million pounds per square inch (PSI). In reality the failure stresses are
measured in the range from 2500 to 30,000 PSI. These values are much lower than
the theoretical strength, and their variation is quite broad. The explanation
for this behavior is that the strength is controlled by surface defects.
Description: The mirrors in this study are heated from
the backside to remove snow and ice, and they showed a
peak in customer returns in the winter months. This is the time the mirror
heater is heavily used. Since the mirror is heated from its backside; the front
surface is put in tension and causes the failures. The mirrors in question have
a convex curvature that is put into the coated glass before it is "scribed and
broken" to size. The scribing operation occurs on the front surface (i.e.
on the coated surface). The scribing operation is effected by driving a hard
carbide wheel on the coated glass surface. The wheel creates a damage perimeter
in the shape of the mirror. This damage is driven through the glass thickness to
separate the mirror from the glass sheet. The start point of the scribing
operation is critical, and if not done properly, the wheel can cause a "crush"
point.
Analysis: A failure analysis was performed on samples of
these mirrors to determine the cause of failure. Figure #1 is a sample of a
failed mirror. It has a horizontal crack in the middle that goes across the
entire mirror.
Figure #1: Photograph of a representative cracked mirror. The
horizontal crack traverses it just below the "4". The crack is
indicated by the arrow.
Failure analysis revealed that these failures resulted from a
"crush" that occurred at the scribe "start" point (Figure #2). Because the
height of the curved mirror surface is variable, the scribe wheel hits the
surface too hard in some instances. The resulting impact from the scribe tool
causes the crush.
Figure #2: Photo micrograph of a typical crack initiation point.
The crack originated at a crush point seen on the lower right corner. The coated
side of the mirror is on the bottom of the photograph (Mag.
30X).
Figure #3: Photo micrograph some of the Wallner lines on the
same crack shown in Figure #1.. The crack travels from right to left The coated
side of the mirror is on the bottom of the photograph (Mag. 20X).
Figure #4: Higher magnification photo micrograph of a
representative crush found at the failure origin (Mag 200X)
Crush
Note: The following photographs are of the edges of newly
scribed and broken mirrors. They have not failed. A later failure would be a
crack separating the piece in at the point of the scribe start. The crack would
go into the photograph and away from the reader. In these photographs the coated
surface is at the top of the photograph.
Figure #5:Start point on an a new mirror that shows a crush at the
top of the starting point. The starting point is on the top of the "winged"
shaped crack in the middle of the picture. The arrow indicates the crush (Mag 30X).
Figure #6:Start point on an a mirror that has no crush at the
top of the starting point. The starting point is on the top of the "winged"
shaped crack in the middle of the picture (Mag. 30X).
Conclusion: The delayed cracking of these rear view mirrors was
caused by thermal tensile stresses generated by the backside heater that acted
on a weak spot on these mirrors. The weak spot is a "crush" caused by the scribe wheel when it
hit the glass surface to start the scribing
process. The crush is a scribe process defect that can be eliminated with
adjustment of the process.
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