Page 31 - Failure Analysis Case Studies II

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Fx250mm = 3.04~ lO’Nmm, (9)
giving F = 12.4 tf. A force of this magnitude can readily be generated in a major collision. If we
assume a vehicle weight of 20 t and a modest deceleration of 5g, then the collision force is 100 tf.
This is 8 times the force required to shear the shaft, so the steering arm need only be subjected to
one-eighth of the collision force to cause the failure.

5. CONCLUSIONS

The steering shaft had been subjected to a large axial torque, sufficient to cause gross yielding of
the cross-section and fracture by a ductile mechanism. There were no indications that failure was
promoted by prior defects or inadequate mechanical properties. If a smal fraction of the likely
collision force had been applied to the end of the steering arm, this would L ve been sufficient to
cause failure. We therefore conclude that the failure was a consequence of the accident, and not its
cause.

REFERENCES

1. Smithells’ Metals Reference Book, 7th edn. Buttenvorth-Heinemann, Oxford, 1992.
2. Atkins, M., Atlas of Continuous Cooling Transformarion Diagrams for Engineering Steel.. . British Steel Corporation,
Swindcn, 1980.
3. Parrish, G. and Harper, G. S., Producfion Gus Carburising. Pergamon Press, Oxford, 1985.
4. Jones, D. R. H., Engineering Materials 3. Pergamon Press, Oxford, 1986.