Ultrasonics may be the NDT wave of the future.

Ultrasonics May Be the NDT Wave of the Future

Nondestructive testing (NDT) of materials offers many advantages over destructive testing, including cost savings and improved product quality, since more pieces can be inspected without significantly increasing operating costs. As a result, many NDT techniques have been developed over the years, and today almost every foundry performs some type of NDT.

A few of the more popular techniques presently used include X-ray, pressure, ultrasonic, eddy current and electrical conduction tests. It is surprising, however, that ultrasonic NDT methods have not been more fully exploited in the casting industry, since they offer the following advantages: * the ability to locate types of defects that cannot be detected by other methods, such as thin horizontal cracks typical of cold shuts; * accurate determination of defect depth; * accurate determination of part thickness, which can be used to verify that tolerances are being met before production of a casting begins; * no danger to the operator; * lower equipment costs that other comparable methods; * portability, permitting field inspections; * potential classification of defects, i.e., sharp, smooth, planar, volumetric, etc.

Standard ultrasonic methods permit reliable detection of gas bubble location(s), volumetric shrink holes, sand or other inclusions, cold shuts, cracks and other defects. Under development are advanced ultrasonic techniques that will permit the measurement of material properties affecting a casting's tensile or compressive strength as well as other mechanical parameters. Even anisotropic stiffness coefficients (a function of the axial alignment of a metal's constituents) can be estimated, along with the presence of inhomogeneity.

Physics of Ultrasonic NDE 

Ultrasonic NDE uses high frequency mechanical waves (above 20 Khz) to probe the inner structure of a specimen. When an ultrasonic wave encounters a discontinuity, i.e., an inclusion, void, crack, sudden change in density, etc, part of the wave is reflected.

A reflected signal contains much information about the discontinuity which produced it, including its depth (proportional to the time it takes the reflection to return), its size (proportional to the amplitude of reflection) and even its shape (pulse shape variations).

Because different defects interact differently with the ultrasonic wave, each generates a "signature" reflected signal. By extracting different features of this signal, e.g., peak frequency, pulse duration, shape characteristics, etc, it is sometimes possible to determine type of defect.[1]

One commonly asked question is, "How small of a defect can be detected using ultrasonics?" Unfortunately there is no general answer, since many factors, other than defect size, influence detectability. These include: defect shape; content, i.e., air-filled or solid; as well as the "cleanliness" of the host material.

In Fig. 1, the three prominent red areas are drilled holes of 0.014 in. diameter, although they appear somewhat larger, which were introduced for reference purposes. The blue and gray areas are "clean" regions where almost no reflected signal was present. The green, yellow and red regions represent anomolies of increasing size, respectively. As can be seen from the figure defects considerably smaller than 14 mils (green regions), which typically exist in castings, can be detected using ultrasonic techniques.

Advanced Signal Processing

When trying to locate extremely small defects, the problem of material cleanliness becomes a very important factor. What happens is that the reflected signal from the defect is imbedded within the "noise" that is produced by the material itself, thus producing a very low signal to noise ratio (S/N), and masking the defect.

To overcome this problem, some sophisticated signal processing algorithms, such as split spectrum processing (SSP),[2] have been developed. SSP is based on the fact that the noise from the material is frequency dependent: a 1 Mhz transducer causes different amounts of noise than a 10 Mhz transducer, but the amplitude of the echo from a defect is not significantly changed.

SSP analyzes the reflected signal, removes the frequency dependent portions, leaving only the "true" anomolies. The combination of advanced ultrasonic NDT techniques with suitable signal processing software should prove capable of solving even the most difficult casting inspection problems.

In summary, ultrasonics can be an extremely versatile and useful tool for the nondestructive testing of all types of castings. Its uses range from defect inspection and determination of material cleanliness, to verifying dimensional tolerances, and for on-site monitoring and process control. [Figure 1 Omitted]

References [1]Rose, J.L., J.B. Nestleroth and K. Balasubramaniam, "Utility of Feature Mapping in Ultrasonic Non-Destructive Evaluation," Ultrasonics, vol 26 (May 1988). [2]Rose, J.L., P. Karpur and V.L. Newhouse, "Utility of Split-Spectrum Processing in Ultrasonic Non-Destructive Evaluation," Materials Evaluation, vol 46 (Jan 1988).