NDTnet 1998 Aug, Vol.3 No.8

Abstract

A short review is given on attempts to present the changes in material state during fatigue process. The indicators used for this purpose can be determined destructively or nondestructively. The rate of crack growth against the range of stress intensity factor or the rate of crack growth against K_I²/E where K_I is the fracture toughness and E the Young modulus, are determined in destructive tests while testing non-destructively one obtains the characteristics of ultrasonic wave attenuation against fatigue life or the attenuation-cycle gradient in relation to the fatigue life. The last information allows to draw conclusions on state of fatigue as well as to make predictions on expected fatigue life.

It is shown that the investigation of fatigue process may be based on a general model which combines the indicators of material state changes with fatigue life. The appropriate formula can be expressed as G = CNⁿ where G is the indicator of changes in material state, e.g., crack size increase da/dN, - ultrasonic wave attenuation /N - attenuation-cycle gradient, C - constant, N - fatigue life, n - exponent.

Details on the application of this model are presented. They are based on results gained when testing steel, aluminium alloy and grey cast iron samples in bending and torsion. Results obtained are showing the significance of such approach as one is able to estimate with confidence the fatigue life expected basing on measurements carried out in initial stages of fatigue life. Also the life still remained can be in some cases estimated. Some limitations of the approach and their reasons are discussed

The practical advantage of the approach presented is illustrated in Figures. They are showing a great dispersion of results when presented in conventional S/N curves (Wöhler curves) and a very good correlation in presentation of attenuation-cycle gradient against fatigue life. This presentation was obtained without knowing the stress applied to individual specimen.

Abstract Source:
Book of Abstracts, 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN: 87-986898-0-00

Full-Text Source:
Proceedings of the 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN:

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