Measuring and testing – Vibration – By mechanical waves
Patent
1982-04-19
1984-11-13
Kreitman, Stephen A.
Measuring and testing
Vibration
By mechanical waves
G01N 2904
Patent
active
044818181
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method of detecting preferably continuously, the presence of cracks, the formation of cracks and the growth of cracks in constructions by recording and analysis of acoustic emission from a crack in the construction.
The need for a continuous monitoring of crack formation and crack growth is particularly obvious in the case of complex steel constructions with construction elements which are difficult of access for visual inspection and testing by conventional testing methods. One example of this is off-shore platforms which are exposed for a long time to heavy loading and fatigue stresses while at the same time the platforms are in a corrosive medium, which increases the risk of crack formation. Large cracks can lead to breakage and in the worse case to wrecking with serious damage as a consequence. Because of this, the requirements of authorities and classification societies are comprehensive when it is a question of preventive checking and measures to avoid wrecking. The annual checks of underwater constructions in the North Sea for example comprise primarily cleaning and visual inspection. Crack checks and repairs are carried out as necessary. Every five years, larger checks are carried out but for reasons of cost they are limited to parts of the platform where the risks of corrosion and crack formation are regarded as being greatest, that is to say, to about 10% of the platform construction. A continuous monitoring of crack formation and crack growth which covers the whole platform would naturally be desirable so as to prevent wrecking caused by crack formation in a more reliable manner than has hitherto been the case. As a result of the wrecking of platforms which has occurred in recent times, it may be expected that the regulations regarding preventive checking may be sharpened.
A known crack detecting system which renders possible continuous detection of crack formation and crack growth utilizes the known phenomenon that crack formation and crack growth gives rise to acoustic emission in the construction material. By fitting a plurality of sensors, which are sensitive to the acoustic emission caused by the crack, on the surface of the construction element and recording and analyzing the signals which the sensors deliver depending on acoustic emission, as well as the differences in time between the recordings of the various sensors, an accurate detection of the position and growth of even very small cracks in the construction element has been rendered possible.
A disadvantage of said known crack detecting system is that the sensors must be fitted to the surface of the construction element which is to be tested, so that it is necessary to determine in advance which parts of the construction shall form the subject of crack detecting. Also, when this known method is applied to constructions with a complex formation, for example off-shore platforms, it has therefore been necessary to select in advance a limited number of observation points which are judged to be particularly exposed from the crack point of view. It would actually be far too expensive and difficult to carry out in practice to utilize the said known method to provide an overall crack detection on all the load-bearing construction elements included in the platform.
The object of the present invention is to provide a method of detecting cracks which is admittedly based on recording and analysis of acoustic emission from a crack but which does not suffer from the disadvantages mentioned above and which thus renders possible a continuous and comprehensive crack detection even in complex constructions, using only a few sensors.
The method according to the invention detects the presence of cracks, the formation of cracks and the growth of cracks in constructions by recording and analysis of acoustic emissions from a crack in the construction using at least two sensors sensitive to the acoustic vibrations emitted from the crack to the medium adjacent to the construction. The sensors are mounted in the medium, spa
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Chapman, Jr. John E.
Kockums AB
Kreitman Stephen A.
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