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Case Study #11 — Fastener Failure Workshop

Concrete Truck
 

Industry:

Heavy Construction
Concrete Mixer Truck

Problem:

The fasteners bolting the supporting structure that holds the mixer drum failed. The timing of the failure was such that the drum, filled with a full load of concrete, broke off the truck as it was turning a corner in a busy suburban area.

Injuries:

A woman was injured by broken glass from her rear car windows.

Damages:

Two cars were significantly damaged, as the drum flattened the back of the woman's car first, then careened on, taking off part of another car's fender and bumper. The drum finally rolled to a stop in a residential yard and started spewing concrete into a huge puddle. The concrete truck was also heavily damaged, with partial remaining drum support members twisted and various other truck components destroyed.

Observations:

By distinction of the markings, the bolting products used to hold the drum mounting unit together were high-strength, proprietary hex head cap screws with matching flat washers and nuts, and were zinc plated. These particular bolts are known to be of 180,000psi (1220MPa) minimum tensile strength.

 

Some of the failed bolts were distended through stretching. Other bolt failures were not stretched in appearance at all. All of the fasteners were designed as bolted connections with flat washers on each side of the clamped joint, and having a non-locking hex nut of matching strength. There was notable corrosion on most fasteners.

Investigation:

The remains of the fasteners were submitted for failure analysis to an independent laboratory. The lab found two basic failure modes: ductile tensile to failure (literally pulled apart) and intergranular brittle failures with corrosion products on the fracture surfaces and on the edges of the fasteners. An analysis of the corroded areas showed it was hydrochloric acid that caused the damage. In discussions with the concrete company regarding the acid, they were not surprised. They said it was standard practice at their firm to use acid to wash down the trucks after a concrete delivery. They felt this was the best way to get off the concrete smears and droppings from the truck.

Conclusion:

The bolts in these applications are carrying very large loads, and as proven with this accident, they are critical to the success of the vehicle. In order for the fasteners to live and act normally, they must be kept in a reasonably clean condition and they cannot be subjected to chemicals that cause failure. While the acid wash down might seem to be the best way to maintain truck cleanliness, inevitable failures for the future will be the result.

Recommendations:

Three choices can be offered: stop the acid use, protect the fasteners by a resistant coating, or choose a special material with the resistant properties.

 

As appeared in Fastener Technology International magazine.

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