Recently, interesting phenomena have been reported to occur in the region very near to a surface-breaking defect when a surface acoustic wave is incident on the defect. These near-field effects have been observed for interactions of Rayleigh waves with defects that propagate perpendicular to the sample surface when an ultrasonic source or detector directly illuminates the defect[23,128,130,234–236]. The ef- fect is known as near-field ultrasonic enhancement and manifests as a large increase in the peak-to-peak amplitude of the Rayleigh wave as either the laser source or detector is scanned over the defect.
The enhancement is observed by time windowing the received ultrasonic sig- nal about the arrival time of the incident Rayleigh wave for each scan position as
Figure 2.13: An example of near-field ultrasonic enhancement of the peak-to-peak amplitude a of Rayleigh wave as a scanned laser detector passes over a 2 mm deep normal defect.
either the source or the detector is scanned over the defective region. The peak-to- peak amplitude of the Rayleigh wave is measured at each position, and the variations in this amplitude are shown in figure 2.13 for scanning laser detection, plotted as a function of the laser detector position as it is scanned over a 2 mm deep defect propagating normal to the surface. For negative detector positions the detector and the source, which is located a fixed distance away from the detector, are on the same side, of the defect, and the peak-to-peak amplitude of the Rayleigh wave when the detector is far away from the defect is seen to have a steady value. As the detector passes over the defect a large increase in the amplitude of the Rayleigh wave is ob- served; this is the Rayleigh wave enhancement. As the detector continues to move, such that the defect lies between it and the source, the peak-to-peak amplitude is reduced as part of the wave energy is blocked by the defect[107].
For defects that propagate perpendicular to the material surface the increase in the peak-to-peak amplitude as the detector passes over the defect has been re- ported to be caused by constructive interference between the incident Rayleigh wave, the reflected Rayleigh wave and a mode converted surface skimming longitudinal wave that occurs at the defect[104,236,237]. Provided that the detector is close enough to the defect that these waves, which travel at different speeds (vr = 2940ms−1and
vL = 6300ms−1 [141]) arrive within the time window that was used to monitor the peak-to-peak amplitude, a superposition of the waves occurs, which, for constuctive
Figure 2.14: An example of near-field ultrasonic enhancement of the peak-to-peak amplitude a of Rayleigh wave as a scanned laser source passes over a 2 mm deep normal defect.
interference, produces a Rayleigh wave with increased peak-to-peak amplitude. The amplitude of the reflected and mode converted waves dictates how much they will contribute to the increased peak-to-peak amplitude, and, as the extent of the re- flection and mode conversion experienced by the Rayleigh wave is dependent on the defect depth, the extent of the enhancement varies as a function of defect depth[104]. The position of the defect is also obtained to within approximately±0.25 mm from this technique as the point of enhancement for Rayleigh waves occurs when the de- tector is approximately 0.25 mm from the defect[104].
A similar enhancement of the peak-to-peak amplitude of Rayleigh waves is observed when a laser source is passed over a surface-breaking defect[23,128,234,235].
The peak-to-peak amplitude of the ultrasonic signal recorded in a time window that corresponds to the incident Rayleigh wave as a laser source is scanned over a 2 mm deep defect propagating perpendicular to the material surface is shown in figure 2.14. Again, for the negative positions the laser source and the detector are on the same side of the defect, and the peak-to-peak amplitude is steady in the region in which the source is away from the defect. As the laser source passes over the defect a large increase in the peak-to-peak magnitude of the windowed Rayleigh wave is seen, which then drops off again when the defect lies between the source and the detector.
served as the laser source passes over an artificial surface-breaking defect propagat- ing normal to the material surface has been attributed to arise from the contributions of several mechanisms[23,235]. An increase in the magnitude of the high frequency content of the Rayleigh wave has been observed to occur simultaneously with the increase in the peak-to-peak amplitude, and this frequency enhancement has been attributed to changes in the ultrasonic generation process. The maximum frequency contentfmax of an ultrasonic wave generated by a laser source is dependent upon the wave speed,v, and the laser spot diameter,w, from[23,185],
fmax =
r
2v
πw . (2.71)
Therefore, as the laser source passes over the lip of an artificial defect a reduction in the dimension, w, on the material surface is observed, which in turn increases
fmax, thereby contributing to the enhancement of higher frequency components of the signal[235].
A contribution to the enhancement from changes in the boundary conditions of generation at the artificial defect has also been identified when the laser source is directly over the defect. At the defect the source no longer causes a uniform expansion of the surface material into the unheated material surroundings, and the material at the defect can expand out into the void of the defect. This alters the character of the generated ultrasound and has been shown to cause the enhancement of Rayleigh waves, although its influence is difficult to quantify[235]. A contribution to the enhancement as the source passes over an artificial defect has also been re- ported to arise from constructive interference between the incident Rayleigh wave and a reflected Rayleigh wave at the detector, when the source is close enough to the defect that both incident and reflected waves arrive within the time window that the peak-to-peak amplitude is studied in[235]. Surface wave enhancements have also been reported to exist for Lamb waves[22,64].
These mechanisms have been identified to contribute to the enhancement observed for artificial surface-breaking slots, however, an additional mechanism has been identified to contribute to the enhancement observed for Rayleigh waves as a laser source is passed over a real partially-closed defect. For a defect with partially contacting faces the transient heating of the defect stimulates the crack to open and close, bringing the opposing faces of the defect together. This clapping of the defect faces generates higher harmonics of the surface wave, leading to an enhancement of the Rayleigh wave for real defects[234,238].
its ability to detect defects that are arbitrarily aligned with respect to the scan di- rection and for its ability to provide good estimates of defect positions. However, near-field enhancement inspection does have the limitation that the entire sample must be scanned in order to find a defect, and if the scan step is too large, the correct position required to observe the maximum enhancement may be missed.