Surface acoustic waves (SAWs) are elastic waves that propagate along the surface of a solid.

Surface acoustic waves are a type of sound wave discovered by British physicist Lord Rayleigh in the 1880s during his research on seismic waves; these waves propagate with energy concentrated at the Earth’s surface. 。

            Surface waves are highly effective for inspecting near-surface defects.

As theoretically discussed, because the energy of surface waves is concentrated within two wavelengths below the surface, the sensitivity for detecting surface cracks is extremely high.

The spatial distribution of surface waves can be decomposed into vibrations in two directions—perpendicular to and parallel with the surface—resulting in elliptical particle motion composed of longitudinal and transverse waves.

99% (40 dB) of the total energy is concentrated within a depth range of two wavelengths below the surface.

The velocity of surface waves is approximately half that of longitudinal waves and slightly lower than that of transverse waves (see Table 1).

1. Generation of Surface Waves

There are two fundamentally different methods for generating surface waves: the Y-cut quartz crystal method and the acoustic-transmission wedge method.

Common surface-wave probes use wedges similar to those used in angled probes; when the shear-wave refraction angle is 90°, a surface-wave front is generated.

2. Propagation laws of surface waves at the surface

2.1 Propagation in a Plane

2.1.1 The Influence of Oil

If oil is applied to the surface through which the reflected wave propagates, the wave will be almost completely attenuated. The same effect occurs when the surface is pressed by hand. This is because, in theory, the problem is solved under the assumption of a vacuum on one side of the test specimen, whereas the treatment involving a liquid leads to a fundamentally different approach.

Another viewpoint can be understood as resulting from the attenuation of the vertical component of surface waves within the liquid layer. Pressing on the propagating surface with one’s hand can help determine whether the surface reflects from cracks or edges. This also underscores the importance of ensuring that there is no oil or grease on the probe face during flaw detection.

2.1.2 Reflection of Surface Waves

Surface waves are reflected when they encounter surface or near-surface defects. Such defects include cracks and volumetric defects (such as porosity and slag inclusions). Surface-wave inspection primarily focuses on detecting cracks; volumetric defects will not be discussed here.

(1) Reflections from rectangular slots (similar to cracks) in welds typically indicate rectangular slot defects. The relationship between slot depth and echo amplitude is illustrated in Figure 3.

(2) Edge Reflection

When surface waves propagate and encounter the edge of a workpiece, reflected waves are generated; moreover, a portion of the wave energy passes beyond the edge and continues to propagate as surface waves. Although this phenomenon cannot be explained by existing reflection theories, it can be readily demonstrated experimentally. Specifically, when the edge is a right angle, the reflection coefficient reaches its maximum (see Figure 4).

3) Edge Reflection with Curvature

If the edge is chamfered with a radius of curvature r, the reflected echo decreases. The relationship between reflectivity and the radius of curvature is as follows: as shown in Figure 5, for a right-angled edge (r = 0), the reflectivity is 52%; when r = 5, the reflectivity drops to about 6%. This indicates that more energy can pass through the curved surface.

Between the reflected signals from edges A and D, other echo signals B and C sometimes appear. This may be the result of interference between the incident wave and the echoes. Changing the inspection frequency will yield different results.

2.1.3 Distortion Waves Generated by Surface Waves

(1) Surface waves generate converted shear waves (S) at edges and corners; when the surface exhibits a certain degree of curvature, the generation of these converted waves occurs point by point. The formation of converted waves leads to attenuation of the surface wave, and they can also affect the interpretation of the echo waveform. However, under certain circumstances, this phenomenon can be exploited for practical purposes.

2.1.4 Converted Surface Waves Generated by Shear Waves: During the propagation of shear waves, when the main sound beam is tangent to an internal bore or a fillet, converted surface waves are generated. These converted surface waves exhibit relatively high intensity and can interfere with the accurate interpretation of the flaw-detection waveforms.

An example of flaw detection using transformed surface waves: The root of a steam turbine blade has few and narrow accessible inspection surfaces. By employing an angled wedge probe at an appropriate angle so that the sound beam is tangent to the curved surface, strong surface waves are generated. These surface waves propagate along the surface, thereby making it possible to detect cracks at locations F1 and F2.

Both laboratory experiments and field applications have yielded promising results.

3. Instrument Adjustment for Surface Wave Flaw Detection

3.1 Scan Speed Adjustment

The method for adjusting scan speed differs from that used with conventional angled probes. The incidence point is calculated based on the probe’s leading edge (i.e., the probe tip).

When adjusting the scan speed, align the probe with the edge of the test block and adjust the echo signal to a fixed proportional relationship based on horizontal distance; typically, this is achieved by moving the probe back and forth to change the L value.

For example, set L1 to 20 mm and L2 to 40 mm. Using the depth knob and the horizontal knob, adjust the horizontal scale to the 20-mm mark when L1 is 20 mm, and to the 40-mm mark when L2 is 40 mm; this completes a 1:1 adjustment. Alternatively, the adjustment can be completed in a single step by aligning edges A and B.

3.2 Flaw Detection Sensitivity Adjustment

Align the probe with the right-angle edge, adjust the edge echo amplitude to the specified reference level—e.g., 60% of full screen—and then increase the gain by 21 dB to set the flaw detection sensitivity.
For a 5 MHz probe, this sensitivity is equivalent to detecting a crack 0.1 mm deep.

4. Application Examples

4.1 Measuring the Depth of Cracks Shallower Than 1 mm

At the flaw detection sensitivity, when the echo amplitude reaches 21 dB, it indicates that the crack depth exceeds 1 mm. At this point, even if the crack becomes deeper, the echo amplitude will no longer increase.

When the echo amplitude is less than 21 dB, the crack depth can be determined by consulting the curve (see Figure 10).

For example, with a 10-dB (5 MHz) probe and a crack depth of 0.3 mm, practical verification has shown that this method provides relatively accurate depth measurements.

4.2 Depth Measurement of Cracks with a Depth Greater Than 1 mm

When the depth exceeds 1 mm, the echo becomes saturated and no longer increases with further crack depth; at this point, two methods are available for crack-depth measurement.

4.2.1 Single-Probe Method

The principle of depth measurement is illustrated in Figure 11: the echo B represents the reflection from the crack tip. The difference in acoustic path length between A and B corresponds to the crack depth. On the test block, with a smooth notch surface, the B wave is clearly visible, and the measurement accuracy for artificial cracks can reach ±1 mm. In practical applications, however, the crack surface is often uneven, making it difficult or impossible to observe the B wave.

4.2.2 Dual-Probe Method

When no crack is present, the surface wave travels directly to the receiving transducer (one-way); when a crack is present, the direct wave from the actual crack still exists. The wave that bypasses the crack travels in a two-way path, with the crack depth t = L/2. This method is more effective than using a single transducer.

4.3 Utilization of Converted Waves Generated by Shear Waves

In certain special applications, employing conventional probes alone may fail to detect cracks; in such cases, the use of variant surface waves can be considered. For some T-shaped blade roots, it is exceedingly difficult to inspect for root cracks F1 and F2 using other methods.

At this point, a angled shear probe is selected and moved back and forth along the side of the blade root so that the main sound beam is tangent to the arcuate surface at the radius R. This generates a guided surface wave, which propagates along the surface and can be used to detect cracks F1 or F2.

F1 is located in the shadow (blind zone) of the r-angle, making it undetectable by shear-wave inspection; however, experiments have demonstrated that the transformed surface wave exhibits extremely high sensitivity.

4.4 Non-Planar Flaw Detection Techniques

A common condition in ultrasonic testing of steam turbine blade roots: the inspection surface protrudes above the blade root plane by a height of h0.

A common condition in ultrasonic testing of steam turbine blade roots: the inspection surface protrudes above the blade root plane by a height of h0.

As previously mentioned, when the surface wave propagates past edge A, the acoustic energy loss is relatively small (approximately a 6 dB attenuation); however, upon reaching corner B, it generates a converted shear wave, resulting in substantial acoustic energy loss. Consequently, by the time the wave reaches crack F, the acoustic energy is very low, making it impossible to detect crack F.

When h0 ≤ 0.5 mm, sufficient surface-wave energy reaches location F, enabling detection of crack F.

To discuss the underlying reasons, the wavelength of a 5 MHz surface wave is 0.65 mm. Under these conditions, sufficient surface-wave energy can propagate directly to crack F. Detection of cracks at the arcuate portion represents another scenario in turbine-blade flaw detection. In this case, the surface-wave probe is positioned on the blade’s side; the surface wave continues to propagate after passing over edge A and reaches the fillet B (the curved surface). Due to the relatively large radius of curvature at this location, crack F1 can be detected with high sensitivity. Wave-mode conversion occurs at the arcuate section; consequently, when no crack is present, the inherent waveform becomes rather complex (following F1), which can interfere with the accurate identification of crack F2. Changing the probe frequency will result in shifts in both the position and amplitude of the inherent echo. Therefore, during testing on a reference block, it is essential to select the flaw-detection frequency that yields the fewest inherent echoes and delivers the best inspection performance.

5. Requirements for Surface Wave Probes

Surface-wave probes offer high flaw-detection sensitivity; a 5×5 mm piezoelectric element typically provides sufficient sensitivity. To broaden the application scope of surface waves, the key is to reduce the probe’s front-edge dimension. The technical specifications of the surface-wave probes we have developed and manufactured are as follows: Frequency: 2.5 MHz and 5 MHz; Element size: 5×5, 6×6, 8×8, and 10×10 mm; Front-edge dimension: ≤3 mm; Overall dimensions (length × width × height): 18:10:15 (standard model); Sensitivity: ≥40 dB (measured at a distance of 20 mm from the edge); Element sizes and overall dimensions can also be customized according to customer requirements.

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