IFMASS10

- Fatigue crack propagated under the influence of the same cyclic-stresses. The fatigue

crack growth rate is so high that it is practically impossible to monitor it.

- Unstable quasi-static propagation is caused by maximal tensile stress in a single

braking cycle. The required remote tensile stress level (282.5 MPa) is achievable with the

applied braking system.

The general conclusion based on the above analysis is that in the present combination

of material with low resistance to fatigue crack growth and a braking system producing

high tensile stresses a reliable monitoring of crack growth for fracture prevention is not

possible. High susceptibility to stress concentration of fatigue crack initiation, i.e. a small

number of cycles before crack appearance, indicates a need to avoid wheel fastening by

chucks during machining procedure. The following actions have been recommended:

- change machining practices;

- change braking system; and

- perform a detailed analysis of wheel stress state.

Detailed analysis means the calculation of stress-state caused by mechanical loading

and thermal shocks using finite element analysis, as well as the measurement of residual

stresses by ultrasonic and magnetic methods.

4. FINAL REMARKS

It is clear from presented crack parameters and their application that three aspects

have to involved for the analysis of in-service structural failures: theoretical, numerical

modelling and experimental. The scope of necessary knowledge and skill is large. In

theoretical sense at least mechanics, mathematics, physics, material science, theory of

elasticity, theory of plasticity, fracture mechanics are inevitable. Examples show that not

only basic knowledge, but also skill and experience are required to solve complicated

problems of stress and strain analysis, including simplifications and assumptions which

must be introduced when exact solution is not possible. Finally, all obtained solutions and

results must be verified by experiments, far from being simple, especially in the case of

full scale testing. This is also to add the complex standard testing of cracked specimens

and metallography, demanding in performance and result analysis.

Fracture mechanics and its parameters significantly contributed to improve structure

integrity by crack significance and residual life assessment, but also in design, including

materials properties improvement and development of new materials, /5/.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the financial support from the Serbian Ministry of

Science under the project OI 144027 “Special topics in fracture mechanics of materials”.

REFERENCES

McEvily, A.,

Metal Failures: Mechanisms, analysis, prevention

, ISBN 0-471-41436-0

Harper, R. F.,

Code of Hammurabi

, University of Chicago Press, (1904).

Easterling, K.

Introduction to the Physical Metallurgy of Welding

2nd edition. Butterworths,

London, (1992).

Report 12-29-12.03/2000:

Report on causes of failure of mono block railroad wheel

, Faculty of

Mechanical Engineering, Belgrade (2000) (in Serbian

Izveštaj o stručnom nalazu uzroka

pucanja monoblok točka na putničkim kolima, ŽTP Beograd

)

Nichols, R. W.,

Use of Fracture Mechanics as an Engineering Tool

, The 1984 ICF Honour

Lecture. Sixth International Conference on Fracture, ICF 6, New Delhi, India (1983).