Industry:
Vehicle Transportation
Custom Modified Trailers
Problem:
The fasteners securing unique metal structures within standard truck
semi
trailers were breaking. The structures were holding two brands of European
luxury cars into angular loaded positions on the truck. These trucks
took the cars from the ports of entry to the dealer destinations throughout
the USA. When the failures occurred, some of the cars dropped on each
other, damaging the bodies of these very expensive vehicles.
Injuries:
None.
Damages:
Depending on the model of the car, five or six cars are fitted inside
each truck. Fortunately, out of a large national fleet
of trucks, only
six cars were damaged. The biggest cost was the recall of all these trucks
throughout the nation. Further, a huge amount of labor was required to
take apart the structures that mechanically raised the cars, then replace
all of the fasteners, and then finally to reassemble the units within
the trucks.
Observations:
Enclosing expensive cars within a very common appearing trailer
body,
and thus completely obscuring them from public view is a smart concept.
The complexities involved with the design and fabrication of the interior
structure was elaborate. Then tilting was done by a manually actuated
screw-drive system enclosed within the steel structural members. Holding
these members together were the failing fasteners. There were 5/16" —
18x 3/4" alloy steel socket flat countersunk head
cap
screws. All failures were at the fillet radius. These products had a manufacturer's
identification mark on the head.
Investigation:
Examination of the holes showed that care had been taken to protect
the
fillet radius by chamfering the holes' edges. A conversation with
the workers uncovered that the first indication of trouble was when some of
the men had lunch outdoors. Their picnic table was near the area where
the assembled structural units were kept prior to being installed within
the trucks. While they were eating, they kept on hearing an occasional
pinging sound; a high pitch, like metal hitting metal. Walking through
the area, they found dozens of heads on the ground. A laboratory evaluation
was conducted. It was found that the heat treatment was improper, in that
the surface was carburized, and thus had a very high surface hardness.
An electron microscope found small amounts of red corrosion products and
an intergranular type of fracture surface.
Conclusion:
These fasteners had a high surface hardness, a plain, unprotected
finish,
and the outdoor exposure of the evening's dew to cause rust, and
they were tightened to a normal clamp load. This created the right conditions
for Stress Corrosion Cracking (SCC) failure.
Recommendations:
To maintain the strength, replace the fasteners with properly made
alloy
steel products with a safely applied plating, or use fasteners made from
Type 410 stainless steel.
As appeared in Fastener Technology
International magazine. |
