1. Field of the Invention
This invention relates to the field of inspection, and more particularly, to the field of ultrasonic inspection of metals.
2. Description of the Prior Art
Non-destructive test (NDT) methods, such as ultrasonic, X-ray, die penetrant, and magnetic inspection, are highly developed for inspecting parts during manufacture and prior to their use. A more difficult and a less developed art is the in-place inspection of large structural items, such as gas and oil pipelines, during their lifetime of use. These pipelines carry flammable products under high pressure, frequently near populated areas, and it is important that their integrity be assured during their many years of use.
Conventional ultrasonic inspection techniques utilize a narrow beam of longitudinal or transverse type waves which is generated in a transducer outside the part to be inspected. The narrow ultrasonic beam is injected into the part by actual contact of the transducer with the part or by contact with a transmitting medium, such as water, which also contacts the part. These prior art techniques are not suitable for inspecting installed pipelines because of the inaccessibility of the pipe and because of the tremendous areas of pipe which must be scanned by the relatively narrow ultrasonic beam. Additionally, wear of the transducer as it moves along the pipe is a serious problem.
Recently, techniques have been developed for generating an ultrasonic Lamb wave in a metal as described in U.S. Pat. No. 3,850,028, entitled, "Method for Ultrasonic Inspection", by the same inventors as the present disclosure. Unlike the commonly used longitudinal or transverse wave, the Lamb wave fills the entire cross section of the object being inspected and is not just a narrow beam of ultrasonic energy. Thus, it can be used to rapidly inspect large areas.
In prior ultrasonic inspection with Lamb waves, the amplitude and phase of beams reflected from defects in the material are analyzed to determine the size and location of the defects causing the reflection. No use is made of the beam which is transmitted past the defect. Because of the large wavelength of the Lamb wave, the reflected wave cannot clearly resolve the shape of small defects causing the reflection. Neither can the reflected wave detect a generalized or nonlocalized uniform decrease in the thickness of an object. Thus, it is not possible to determine the type of defect and the effect of the defect on the integrity of the material utilizing only reflected waves.