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