Windshields just don't crack for no reason... it was either hit with
something... or had some sort or torsional stress.

Professor
Check out FlashAlert at www.telstar-electronics.com

The cause might be reasonable, but hidden.  I had an old Pulsar NX that had
a stone hit the windshield, left a little nick in the glass.  Over the next
day I got a crack over a foot long.  At one point I tapped the glass and it
grew an inch.  Could have happened the same way to her, she may not have
noticed the stone hit.  (Windshields by nature are "pre-stressed" by the
tempering processes used.).

In some cases heat/cooling can trigger a crack also.

Gary K

If you know what to look for, a hammer leaves a different type/size mark at
the start of cracks.

I have seen them crack for no obvious reason.
Clearly there is a scientific reason, but it need not be so obvious as a
hammer, stone peck, etc.

My Reatta split from the top seal about 1 inch.  I 'repaired' it to keep it
from running but there
was no mechanical damage which initiated it.  ( A new replacement, when they
are available, is
about $1800)

My SAAB blew out a rear glass sitting in a parking place.  No rock, no
damage, no nothing.

When you stress a piece of glass, and if it is not perfectly bedded, it can
crack.  And it may be
nobody's fault...

(Now, shitteaux husbands, and wives, have been known to promote events which
stress may
have favored in the first place.)

If you cant prove it, it didnt happen.

Someone reported the spontaneous shattering of a rear windshield
on that silly "Car talk" show on NPR today. It was very hot,
and the guys said that because glass expands, if there is
a flaw in the glass, it will break. Seems weak to me, glass
will expand much less than the metal of the vehicle, so I
dont see where the stress on the glass came from.

Don't know where she lives, but rust in the pinchweld can cause
localised pressure that can lead to a crack.

Has the windshield ever been replaced? If so, failure to patch the
nicks in the paint will cause rapid rusting if the vehicle is in a
Northeastern area of North America.

The glass would have to have been weakened at some point.  A small
stone or something else thrown up by the vehicle in front is the most
likely cause.  A tiny chip can spread into a window spanning crack in
no time.

After working in a glass shop for several years, I've seen some wierd things
that can shatter glass, or even cause laminated to crack and then run.  The
3 basic types of glass are laminated(used in windshields), tempered(used in
the other windows) and plate.  Tempered is very damage resistant, however
when it does break it just explodes into thousands of tiny pieces.  Tempered
has to be manufactured at the size needed, it can not be cut after the fact,
unlike both plate and laminated.  Plate glass when it breaks leaves larger
jagged pieces(many older homes windows are plate). Laminated is just 2
pieces of 1/8" plate glass with an adhesive sandwiched between them.  During
installation, there are rubber shims placed at the edges of the windshield
for proper alignment and to keep it from shifting.  Between the glass and
the lip of the body a rubber based sealant is used to both weatherproof and
provide sufficient expansion.  Its unusual for factory windshields to leak
or crack from the edges, but they can if there is a thin spot in the sealant
allowing the glass to impact the lip.  However, this would most likely
manifest itself within a few days of manufacture, not several years down the
line.

Glass is fairly stable thermally as long as the changes are gradual, however
rapid changes can crack it(or worse in the case of tempered).  Driving out
of a heated garage into sub zero weather can cause a rapid enough rate of
thermal contraction to crack it.  Even running the defrost at the hottest
setting when its below zero can be bad, it causes the inner layer to expand
while the outer layer stays contracted.  When this happens, typically the
inner sheet will buckle and crack.

The other windows on the car are a different matter.  The rear glass is
usually tempered(as are the side windows), especially if it has an electric
defrost.  Laminated won't work here as the heat produced will melt the
adhesive.  Tempered glass usually breaks from the edges inward. The 2
biggest reasons for tempered glass to fail are overpressure caused by high
powered stereo systems, and objects striking at a high rate of speed, both
of which are fairly common problems.

I've had 8 vehicles since I started driving, ranging from a 68 Mustang to a
93 Ram diesel.  I've logged better than 1.5 million miles total, did 740k on
the ram before it got totaled in sept 96.  Out of that I've changed 2
windshields(the mustang and the ram), a couple of side windows, and one rear
window(an 89 Escort I put in a spin on gravel and hit rear first on an
embankment, the rear window came out in one piece and shattered when it hit
the mud).  The ram has been to almost every state in the lower 48(never hit
Maine, Vermont or New Hampshire), and covered a good chunk of Canada also.

In your friends case, the most likely cause was her husband hitting the roof
of the vehicles near the lip of the windshield.  I bet there are slight
dents in the roof, though it could have happened from the inside.  The
chances of 4 out of 6 cracking in the same spot are astronomical, or she has
gremlins in her garage.  I bet that after he leaves, she won't crack anymore
windshields, except for rocks.