METALLIC FAILURES

This category is divided into five sub-groups.  Each sub-group consists of a type of failed metallic item which the company has investigated.  The sub-groups are as follows:

• structures
• components
• off-road vehicles
• bicycles
• chairs

In each case, the failure investigation consists of the following tasks:

• visual examination and photography
• chemical analysis of the broken item
• hardness testing of the broken item
• chemical analysis of any corrosion products
• examination of the fracture surface to determine type and direction of crack propagation
• examination of the broken item’s microstructure for defects which may have contributed to the failure         

Structures

A typical example of a structural failure is shown in the photograph.  In this case, a conveyor used to construct a dam failed.  The other photo shows that the failure occurred at a weld.  Close examination of the weld found that the weld failure was due to lack of penetration (LOP).

Components

1. Crankshaft

A crankshaft from an unmanned air vehicle (UAV) fractured after approximately 100 hours of service.  The investigation found that a lack of oil on one of the bearing surface created a very high temperature due to friction.  The high temperature reduced the surface hardness. enabling a fatigue crack to form at the bearing/journal interface.

2. Socket Extension

The male end of a socket extension failed in torsion.  The fracture started at the radius between the socket base and square insert.  The socket had been chrome plated.  Failure was due to hydrogen embrittlement which occurred during the plating operation.

3. Cutoff Wheel

A portion of a carbide tip on the end of a cutoff wheel broke off during use.  The tip was composed of tungsten carbide particles sintered together using a cobalt binder.  An insufficient amount of cobalt at the fracture origin caused the tip to break off as the blade rotated.

4. Wrench

A wrench failed just below the head.  The wrench body was cast from aluminum.  A very large porosity defect existed within the casting.  During use, a crack initiated at the defect and propagated through the body.

5. Gear

A tooth from a reduction gear in an aircraft engine broke off.  The fracture was found to originate from a large inclusion (i.e., foreign particle) within the gear tooth.

6. Leg Prosthetic

The aluminum support rod of a leg prosthetic fractured.  The fracture was found to initiate at a forming impression which had been left in the support rod.  A fatigue crack initiated at that location and propagated over time through the rod.

7. Off-Road Vehicles

A steel strut failed on an off-road vehicle.  The fracture began at an outside edge and propagated through the strut by cyclic or fatigue loading.  A stronger strut material was recommended.

8. Bicycles

Numerous bicycle failures have been examined, two of which are shown here.  In both cases, failure occurred in or started from a weld.  Typically in these instances, welding thin-walled tubing creates a defect or weakened area from which a crack can originate.

9. Chairs

Numerous chair failures have been examined, three of which are shown here.  In the first two cases, the fracture originated at a weld.  In the third case, the fracture occurred in the metallic support designed to hold the back in place.  The support was cast from aluminum.  No defects were noted in the casting, nor were there signs of misuse.  The fracture initiated at a location where there was a reduction in thickness of the support.  The thickness reduction resulted in a stress concentration from which a fatigue crack initiated and propagated.