Measuring and testing – Specimen stress or strain – or testing by stress or strain... – By loading of specimen
Patent
1986-12-05
1988-08-16
Myracle, Jerry W.
Measuring and testing
Specimen stress or strain, or testing by stress or strain...
By loading of specimen
73577, G01N 1908
Patent
active
047635282
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method for detecting the shape and/or the position of a crack front within a part, especially a metallic one, subjected to stresses which cause its fracture or ductile rupture.
The importance of being able to track the evolution of the crack in a test piece under fatigue conditions or monotonic loading is well known. The definition of the shape of the crack front and the determination of its position at any given time and of its growth rate is useful in the assessment of the mechanical properties of the test-piece material. In particular, the toughness of the material that is to say, its resistance to sudden crack growth can be deduced from the calculation of the Rice integral, which is the (line) integral of the energy density over a closed circuit around the crack (see in this respect the article by J. R. Rice in "Fracture" Vol. II p. 191 ed. Leibowitz 1968, Academic Press). It is therefore important to be able to monitor with precision the shape and the instantaneous position of the crack front in a test piece.
Several methods are used for this determination including the so-called multi-specimen method: according to this method, several specimens are used which are exposed to the desired stress levels for a longer or shorter time, and once the stressing of the test piece has been stopped, the cracked region is marked, usually by oxidation after which the specimen is broken to allow the observation and measurement of the position and shape of the crack front. This destructive testing technique has essentially two disadvantages: on the one hand, it requires the use of a large number of specimens which leads to increases in the cost of the study and the necessary time; and on the other hand, one is confronted with variations between the structures of the different test-pieces because of the number of specimens used.
It has been suggested that the progression of crack growth fronts could be studied using non-destructive methods which are based either on the observation of the growth of the crack at the edge of the part, or on some method, in particular mechanical or electrical, allowing a rectilinear line to be positioned which constitutes the median line of the real crack front. The disadvantage with these methods is that it is not possible to determine either the exact shape of the crack front or the development of this shape in the course of the crack formation, so that it is necessary to make a hypothesis on the shape of the curves corresponding to the crack front in order to be able to calculate the elements serving for the toughness determination of the material. These non-destructive methods thus have the drawback of being imprecise by reason of the lack of the determination of the shape and exact position of the crack front.
Among the non-destructive methods of the type described above, the use of an ultrasonic generator "transducer" has been suggested (see in the respect the article "An Ultrasonic Crack Growth Monitor" by W. G. Clark and L. J. Ceschini, Materials Evaluation, August 1969, pages 180 to 184). According to this method, an ultrasonic beam generated from a flat "transducer" is injected into the test piece subjected to cracking; the axis of the diverging beam is disposed perpendicularly to the fracture plane and the displacement of the crack front with respect to the axis of the ultrasonic beam entails a modification in the reception amplitude. The principle of the method is to displace the "transducer" in such a way as to maintain a constant reception and it is then assumed that the displacement of the "transducer" is equal to the displacement of the crack front. In fact, this method would only give an accurate displacement of the crack front if the latter were to have a constant shape throughout the displacement in the material, which is not the case. Moreover this method permits the definition of neither the effective shape of the crack front, nor the precise position of the tip or end of the crack.
The object of the present invention is the proosal
REFERENCES:
patent: 3332278 (1967-07-01), Wood
patent: 4292848 (1981-10-01), Rainey
patent: 4441369 (1984-04-01), Lessard et al.
Nakazawa et al., "Proceedings of 4th Symposium on Ultrasonic Electronics", Japanese Journal of Applied Physics, vol. 23, suppl. 23-1.
Nakazawa, H. et al., Ultrasonic Monitoring . . . Materials, Japanese Journal of Applied Physics, vol. 23, Suppl. 23-1, 1983, pp. 12-16.
Andrews, E. H. et al., An Ultrasonic Technique . . . Velocities, Journal of Materials Science, No. 8, 1971, pp. 1093-1099.
Bouami Driss
De Vadder Daniel
Ecole Centrale Des Arts Et Manufactures
Myracle Jerry W.
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