Measuring and testing – Vibration – By mechanical waves
Patent
1997-11-07
2000-08-22
Moller, Richard A.
Measuring and testing
Vibration
By mechanical waves
73629, 73596, G01N 2904
Patent
active
061054313
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a method and to an apparatus for inspecting an object such as a wall or plate using ultrasonic waves, more particularly using lateral waves.
By lateral waves are meant compression waves propagating close to a surface of an object; such waves may also be referred to as longitudinal surface waves or longitudinal subsurface waves. A lateral wave alone does not satisfy the boundary conditions at a stress-free surface, and so it continually generates a shear wave propagating into the bulk of the object, which may be referred to as a head wave. The lateral wave is thus attenuated. The properties of lateral waves have been studied for example by L. V. Basatskaya et al (Sov. J. NDT 16(7) July 1980) and by L. V. Yuozonene (Sov. J. NDT 16(8) August 1980). Basatskaya et al report that their analysis confirms previous experimental observations that lateral waves have little sensitivity to surface defects, whereas Yuozouene suggests lateral waves may be used to examine surface layers for surface defects. In a wall or plate with two opposed surfaces the head wave will generate another lateral wave at the opposite surface, so that lateral waves propagate along each surface.
According to the present invention there is provided a method for inspecting an object with two opposed substantially parallel major surfaces, the method comprising coupling a transmitter ultrasonic transducer to one of the major surfaces of the object so as to generate shear waves in the object propagating in a direction inclined from the normal to the surface at an angle substantially equal to the shear-compression critical angle; coupling a receiver ultrasonic transducer to one of the major surfaces of the object so as to detect shear waves incident at an angle of incidence equal to the shear-compression critical angle; energising the transmitter transducer to generate at least one pulse of ultrasound; and detecting by means of the receiver transducer a series of ultrasonic peaks corresponding to each generated pulse, the series of peaks being due to ultrasonic waves which have propagated through the object partly as lateral waves on the major surfaces and partly as head waves traversing the object between the major surfaces.
The receiver ultrasonic transducer may be the same transducer as is used to generate the ultrasonic waves, for example where the waves are reflected back to the transducer by an edge surface of the object or by a crack, flaw or defect on either surface or within the object. Alternatively, or additionally the transmitter transducer and the receiver transducer may be separate transducers, which are spaced apart from each other; they may be coupled to the same or to opposite major surfaces.
In general whenever a shear wave is incident at a boundary between media of differing elastic properties it is reflected as both a compression wave and a shear wave. The shear-compression critical angle is the angle of incidence for a shear wave for which the reflected compression wave is at 90.degree. to the normal; if a shear wave is incident at an angle greater than this critical angle only shear waves are reflected. If the speed of compression waves is C.sub.p and the speed of shear waves is C.sub.s then the shear-compression critical angle .theta..sub.c is given by:
The transmitter and the receiver transducers may be more than 0.25 m apart, and may be as much as 1.0 m apart or more. The object may be a plate or wall, for example of a tank or a pipe, and might be for example between 5 and 25 mm thick. Substantially the entire volume of the object between the transducers is inspected, as lateral waves propagate along both the surfaces, and head waves (which propagate across to the opposite surface) are generated at all parts of a surface along which lateral waves are propagating. The exact nature of the received signals depends upon the thickness of the object, and on the material of which it is made, as well as being affected by the presence of any defects. The lateral wave can be expected to travel at substantial
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Duffill Colin
Silk Maurice Geoffrey
AEA Technology PLC
Holt William H.
Moller Richard A.
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