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Case Study #14 — Fastener Failure Workshop

Custom Trailers
 

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.

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