Question

You, a car accident investigation expert, are assigned a case where the insurance company suspects that the car company did not properly account for the fatigue failure when designing the axle of their car. You are given a sample of a broken front-right axle to determine if the axle failure was a fatigue failure. You were told that the axle broke when the car traveling at 45 mph hit a pothole in the road. How will you most efficiently determine if the failure caused was a fatigue failure or the pothole effect? You are only given the broken axle.

Answer 1

Axle is the central rod or shaft that connects the wheels of a vehicle. Wheels move by the functioning of axles also it bear the weight of passengers, body of vehicle and other cargos.

Fatigue failure

Fatigue failure is a major design criterion in industries like aerospace, transportation and oil & gas machineries etc.

How to know whether failure is fatigue or not?

When we examine the broken part, check whether it is a brittle or ductile failure mode on the fracture surface? In metals, ductile fracture indicates a static failure. Fatigue in metals generally exhibits a brittle nature. There will be a large deformation or deflection within the part for ductile failure. This deformation gives a prior warning of failure.

The surface of the fracture is bright and crystalline in case of fatigue failure. Crystalline appearances of the fracture lead to the assumption that under repeated stress metal crystallized by changing its internal structure from fibrous form to crystalline form. It was assumed that crystalline metal become brittle. When examined under microscope minute cracks can be seen to form in metal. A metal fails in fatigue when these cracks spread until there is not sufficient sound metal left to carry the load. In such cases two zones can be seen: (1) a comparatively smooth surface where the crack has spread and where the faces of the crack have battered each other smooth as the crack opened and closed; and (2) a rough crystalline surface which represents the sudden final failure of the metal.

Brinell hardness test of steel seems to be correlated with tensile strength and is indication of fatigue limit. Fatigue cracks developed under repeated stress could be detected with high-power microscope.

Pothole damage

Potholes forms when moisture passes through small cracks in road surface. As temperature changes, moisture expands and contracts due to thawing and freezing. Damages to vehicles from potholes are not that much severe. The most obvious pothole damage affects vehicles tires. The tires of a vehicle are in direct contact with road. Driving over large potholes can cause extensive damage in axle and can sometimes even bend the axles. If axle is damaged by hitting in pothole it shows a clear indication of loud and frizzy noises during car motion.