Characteristics of sound reflections from discontinuities?
In this article:
- Reflections from Discontinuities are Key to Ultrasonic NDT: Understanding how ultrasonic waves reflect off imperfections in materials is fundamental for effective non-destructive testing, as these reflections form the basis of the pulse-echo method.
- Acoustic Properties and Geometry Dictate Reflectivity: A discontinuity's ability to reflect sound, and the direction of that reflection, depends on the ratio of acoustic impedances, surface structure, and the size and shape of the reflector.
- Reflector Size and Probe Position Affect Signal Interpretation: Very small reflectors scatter sound broadly, while larger ones behave predictably; however, intermediate sizes and the precise position of the probe significantly complicate reflection behaviour and signal analysis.
- Waygate Technologies: Mastering Complex Ultrasonic Reflections. With a broad portfolio of ultrasonic flaw detectors, probes, and advanced software, Waygate Technologies empowers industries to precisely interpret complex sound reflections, ensuring accurate and efficient detection of discontinuities in any inspection scenario.
Understanding how ultrasonic waves interact with discontinuities is critical in non-destructive testing. The complex behavior of reflections—depending on material properties, probe position, and wave type—forms the foundation of pulse-echo techniques. This article explores how various characteristics of reflectors influence the detectability and interpretation of signals in ultrasonic testing ndt. From simple point reflectors to large, complex interfaces, we examine how acoustic impedance, geometry, and wave interaction define what can—or cannot—be detected by the probe.
Key factors influencing reflectivity
The pulse-echo method requires that waves reflected by discontinuities (impurities) must return to the receiver probe and where it is by no means obvious that the receiver probe is also at the same location as the transmitter probe especially with large reflectors. Discontinuities in materials can only be detected if they are interfaces i. e., they produce a sudden change in the acoustic properties. Whether a discontinuity is either a bad or a good reflector depends upon:
• the ratio of the acoustic impedances Z1/Z2
• the surface structure
• the thickness of the interrupted zone
• the size of the reflector
The direction in which discontinuities reflect depends on:
• the surface structure of the reflector
• the shape of the reflector
• the position of the reflector to the transmitter
• the size of the reflector
Not every reflective spot in a material is a material defect. Which reflector is a defect in terms of the acceptance specification must be determined before the test. Reflectors which are larger than the sound beam can be generally detected quite easily by ultrasonics. They can be treated as unlimited Interfaces (chapter 10).
Understanding signal behaviour from different reflector types
The behaviour of a very small (point- shaped) reflector is also easy to understand. It scatters the sound wave in all directions like a spherical radiator. However, all reflector dimensions in between show a complicated reflection behaviour.
Fig. 32–35 give examples of this behaviour. Fig. 32 shows the position of the probe and the reflector. It represents the moment when an ultrasonic pulse is generated.
Fig. 33 corresponds to the moment when the pulse reaches the reflector. The position of the wave fronts within the pulse is shown. The sound pressure is represented by lines of varying thickness.
Fig.34 shows the condition shortly after reflection. A portion of the pulse passes in a disturbed form (I). Another portion of the pulse is reflected in the form of different individual pulses (II), (III) and (IV) , only pulse (III) returning to the probe.
Fig. 35 shows the moment pulse III is received. The probe only picks up a portion of the pulse (III). The probe forms an average signal corresponding to the sound pressure distribution from this section of the pulse. It is only this average signal which can be routed further as an electrical receiving signal. We do not have more than this average signal from a part of the reflected pulse available for evaluating the nature of the reflector. What makes it more difficult is that all reflected partial pulses are not necessarily of the same type of wave:
A longitudinal wave probe for example would not respond to a reflected shear wave even if it hits directly the probe.
The role of probe positioning and wave interaction
If in fig. 32 the position of the probe to the reflector is changed then the propagating pulse (fig. 33) would strike the reflector differently. As a result, the reflected pulses and the pulses which pass through (fig. 34) will be different according to the direction and pressure distribution. The receiving signal reacts accordingly (fig. 35).
There is then no constant reflection characteristic of a reflector, it de- pends upon the sound field used for the transmitter and the receiver and from the position of the reflector within the sound field. The bigger a reflector is (as compared to the wave length λ) then the more sensitive will be the reaction of the reflected pulse upon the position of the reflector in the sound field. Al- though such a reflector is a good reflector it is overlooked if the reflected pulse does not return to the receiver. Locations which reflect badly can be indirectly evaluated by means of the pulses which pass by the reflector (I) fig. (35), if this pulse, after being reflected by a backwall, returns to the probe (backwall echo shadowing).
Influence of boundaries and advanced detection techniques
Reflectors that are known not to reflect to the transmitting probe can be detected with separate receiving probes (fig. 36 tandem technique, fig. 38 delta technique). Boundary surfaces of the test piece that are close to a reflecting discontinuity interfere with the reflection at this
irregularity, fig 37. The signal from such a reflector should not be analysed without correction (corresponding to the signal distortion caused by the boundary surface). (fig. 38, 39).
In summary
The complexity of interpreting ultrasonic reflections lies in the variability of wave behaviour across different types of reflectors and their orientation within the sound field. Reliable flaw detection requires not only understanding these variations but also applying techniques like tandem or delta configurations to enhance sensitivity where direct reflection is insufficient. Ultimately, accurate evaluation hinges on recognizing how both the physical characteristics of discontinuities and the setup of the ultrasonic inspection influence signal interpretation.
Photos to explain the reflection of pulses on small reflectors show, for example:
[13] V. M. Baborovsky, D. M. Marsh, E. A. Slater: Schlieren and computer studies of the interaction of ultrasound with defects Non-Destructive Testing 6 (1973)
No. 4, S. 200-207