INDUSTRY:
Automotive vehicle transportation -custom modified trailers.

PROBLEM:
The fasteners securing unique metal structures within the automobile vehicle transporter were breaking. The structures were holding two types of European luxury cars in angled loaded positions on the truck. The trucks took the vehicles from the port of entry to the dealer destinations throughout the USA.

When the failures occurred, some of the cars plunged down on each other, damaging the vehicles, thus incurring insurance claims. Fortunately the situation did not cause any personal injuries, other than the necessity to recover from an incident that need not have occurred.

DAMAGE:
Depending on the car model, five, or even six, vehicles were loaded inside each truck. Luckily, in a large national fleet of trucks, only six cars were damaged due to fastener failure. However, with cars of this value, this was quite a costly issue to address.

The major cost by far was the recall of all the transporter trucks throughout the country. Furthermore, a huge amount of labour was required to take the structures apart and then mechanically lift the cars off the transporter.

Following this, replacement of all the fasteners had to be overseen and then, finally, the carrying units within the trucks had to be reassembled.

OBSERVATIONS:
Safeguarding expensive cars within an enclosed trailer body, completely obscuring them from public view for security reasons, was a sensible concept. However, the complexities involved with the design and fabrication of the interior structure were elaborate.

The tilting mechanism was carried out manually through an actuated screw-drive system enclosed within the steel structural members. Holding the members together were the failing fasteners. They were 5 / 16 ” -18 x 3 /4” alloy steel socket countersunk head cap screws. All failures were at the fillet radius. The products had a manufacturer’s identification mark on the head.

INVESTIGATION:
Examination of the drilled pilot holes showed that care had been taken to protect the fillet radius by chamfering the edges of the holes.

A conversation with the employees revealed that the first indication of trouble was when some of the personnel were in the vicinity of the assembled structural units, which were being stored outdoors, and kept on hearing an occasional pinging sound at high pitch, like metal hitting metal.

Walking through the area, they found dozens of broken heads on the ground. A laboratory evaluation was conducted and it was found that the heat treatment was improper and that the fasteners were carburised, thus having a very high surface hardness. Using the electron microscope small amounts of red corrosion product and an inter-granular type of fractured surface at the failure point was observed.

CONCLUSION:
These fasteners had a high surface hardness, a plain, unprotected finish and the outdoor exposure of the evening’s dew to cause rusting; they were tightened correctly to a normal clamp load. This had created the right conditions for a Stress Corrosion Cracking failure.

RECOMMENDATION:
To maintain the required clamp load, replace the fasteners with correctly heat-treated alloy steel products with controlled procedures when plating, or alternatively, use stainless steel fasteners made to strength Grade 80.

Submitted by Pat Monaghan - Technical Support Manager SPS Unbrako.

Improper plating can lead to Hydrogen Embrittlement and disastrous fastener failures