Failure analysis project

Jag rear end

Trevor Manning

10/11/05

Introduction

The test piece is a car part (please excuse my lack of mechanical terminology), which transfers power from the engine to the wheel. Rotating the wheel and moving the car forward. The larger end (right side) of the part is connected to the ?engine side? of the power transfer, while the smaller end is bolted onto the wheel. It failed during service, and split into 2 pieces ( the third piece shown was sectioned after failure). The part is made from Steel (estimated steel number ? 52100)

The Part in action! The sectioned part. (about 20cm long)

Conditions of Stress applied from its working environment.

As the part rotates various forces act upon it. There is the rotational force itself, as well as a tension force from being bolted in place. Also, the suspension system linked with the bouncing of the wheel applies a varying Bending moment.

The rotation and bending forces oscillate over time and hence the shaft would undergo stresses similar to fatigue loading conditions. It is assumed that the tension is a constant, or with minimal amount of variation. Note: Due to the rotation of the shaft any vertical stresses that are applied over time would not be of uniform direction.

The thermal stresses involved would be minimal (if any).

Examination of the fracture surface

Method of analysis

First of all, a test piece containing the fracture surface was sectioned from the shaft. Then photos were taken of the specimen.

The fracture surface piece was leveled before being set in a mold. This was examined under an Scanning Electron Microscope. As well as an analysis of the Elemental content of the fracture surface. The test piece was cleaned and polished before being examined under a microscope and undergoing a hardness test. Followed by an etching process and micrograph images taken of the etched sample.

Vickers Hardness

Results of the hardness test ? With a 100g weight it was found to have a Vickers hardness reading of 327.

Micrograph images of Etched sample

(Note the lack of markings in any particular direction)

In the Scanning Electron Microscope the following pictures were taken.

Images taken at various regions on the sectioned fracture surface and at various resolutions.

- (above) Crack at low relatively low magnification, surrounded by fatigue marks.

- (above) at relatively high magnification. Notice cup/cone structures possibly indicating ductile failure in this region. ( at fracture surface)

Of particular interest is the central crack. It is surrounded by fatigue markings. From loading conditions it is assumed that these were created from a high cycle fatigue. Closer to the edge of the fracture surface it is flat. It is estimated in this zone that the failure was brittle and rapid. The crack sides were still joined together by strands of material.

An examination of the elements at the surface revealed a steel with approximately 1.4 (Wt)% chromium, 1% Manganese and 0.5% Silicon.

This is similar to a 52100 Steel, which has a specified 1% carbon, 1.45% chrome, and 0.35% manganese.

Failure theory

So why did it fail?

Failure occurred at the wheel end of the specimen. There is a noticeable incline at this point, perhaps the smaller radius of this part of the shaft combined with the bending moment applied by the force of the wheel on the part, was large enough to cause the weakened part to fail catastrophically to give a large local stress.

Possibly crack initiated by a local impurity. Stress concentrations around this impurity, combined with the fatigue loading conditions mean that the crack could grow over time.

The crack did not grow all the way to the surface.

One explanation of why this occurred is that possibly the crack grew to a certain critical length, enough to weaken the section to the extent that either the applied tension or bending moment initiated a rapid catastrophic failure.

Conclusion

From the tests done, the most viable conclusion we can draw is that the oscillating stresses applied magnified around an impurity or high stress zone in the material. These magnified stresses initiated the formation of a crack, and over time the crack length increased. Eventually the weakened material could not contain the stresses and failed catastrophically.

References

UTS lab resources.