Application: Using nondestructive testing to evaluate the integrity of bonds between dissimilar materials of low acoustic impedance and high acoustic impedance such as plastic, fiberglass, rubber or epoxy bonded to metal, glass, or ceramic.
The specific application that suggested this note involved a manufacturer of automobile clutch assemblies who needed to check the integrity of bonding in a clutch plate consisting of a disk of carbon/fiber/resin composite bonded to a steel supporting plate. Other applications would include bonding of plastic liners in metal pipes and tanks, plastic safety coatings over glass, rubber coatings on metal rollers, and epoxy coatings on metal parts. The technique has also been used on metal bearings where soft, low impedance metal such as babbit is bonded over hard, high impedance metal (steel or brass).

Problem: A technique commonly for ultrasonically evaluating the integrity of a bond or weld is to look at the amplitude of the signal reflected from the interface between the two materials. This method works well when the two joined materials have similar acoustic impedance such as metal to metal or plastic to plastic. However, when the two bonded materials differ greatly in acoustic impedance such as a plastic to metal bond, the change in the amplitude of reflected signals between good and bad bonds is less dramatic. In these cases, the most readily visible difference between bonds and disbonds is a reversal in the phase or polarity of the echo from the boundary.

Equipment: This test may be performed with any ultrasonic instrument that provides an unrectified RF waveform display such as the EPOCH XT, EPOCH LTC, EPOCH 600, and EPOCH 1000 portable flaw detectors. If portability is not required, any one of a number of Pulser/Receivers (Models 5072PR, 5073PR, 5077PR, 5058PR) may be used along with an oscilloscope or digitized waveform display. Select the transducer type and frequency that suit the application, however, broadband transducers are recommended in all cases to maximize the effect.

Procedure: Both the phase (or polarity) and amplitude of a reflected signal at the boundary between two materials are determined by the relative acoustic impedance of the two materials. The phase or polarity of the echo from the boundary between the materials is reversed or inverted when the order of relative acoustic impedances is reversed (low to high versus high to low).

Figures 1 and 2 show echoes from a plastic/air and plastic/metal boundary respectively, demonstrating phase inversion. Photos made with an EPOCH flaw detector and V109 5 MHz broadband contact transducer, using F214 test block.

Figure 1 shows the echo from the back side of a 0.2 in/5 mm flat piece of Plexiglas plastic, as seen by a V109 5 MHz broadband transducer with an EPOCH 4 flaw detector set up in the RF mode. In this case the sound energy is reflecting from a boundary between a relatively high impedance material (plastic) and a very low impedance material (air). With this setup, the returning echo from this high to low impedance boundary is seen as a negative polarity echo.

Figure 1: Echoes from Plexiglas Plastic

In Figure 2, the same piece of Plexiglas is coupled to a piece of aluminum. The sound energy is now reflecting from a boundary between a relatively low impedance material (plastic) and a high impedance material (aluminum). Plastic has higher acoustic impedance than air, but lower than aluminum, steel, or other materials and we are concerned not with the absolute acoustic impedances but rather the high to low or low to high relationship at the boundary. The result is a positive echo, inverted with respect to the signal from the plastic to air boundary.



Figure 2: Echoes from Plexiglas Plastic Coupled to Aluminum

In general, there is a clear difference in phase or polarity between bonded and air-backed materials that can be quickly seen through the use of calibration standards. It is best to optimize echoes on a bonded sample and then observe the phase change from an unbonded sample. The same pattern described above would result from using a pulser-receiver such as a Model 5072PR with an analog or digital oscilloscope. Again, a lack of bonding creates a plastic/air boundary that produces a negative echo, while a solid bond inverts the echo to positive.

Note : Displayed echo polarity is arbitrary depending on signal processing within the instrument. While Panametrics-NDT flaw detectors and pulser/receivers produce displays of the type shown here, some equipment from other manufacturers display signals that are the reverse; a test block can help determine this factor. The basic principle of phase inversion at a low to high impedance boundary applies in any case.