声波遇到不连续介质时的特征
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 impodances Z1/Z2
• the surface structure
• the thickness of the interrupted zone
• the size of the reflector
The direction in which discontinuities reflect depends upon:
• the surface structure of the reflector
• the shape of the reflector
• the position of the reflector to the transmitter
• the size of the reflector
A reflector in the material is not necessarily a flaw. Which reflector is to be regarded as flaw depends upon the acceptance instructions and this is to be established before commencing 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 (10).
The behaviour of a very small (point) reflector is.easily understood. lt scatters the sound wave in every direction just like a ball radiator. All reflector sizes in between however show a complicated reflection charactehstic.
Fig. 32-35 give examples of this behaviour.
Fig. 32 shows the position of the probe and the reflector. lt 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. lt 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 reflected transverse wave even if it impinged directly under the probe.
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 depends 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. Although 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). Reflectors of which is known that they do not reflect back to the probe can be detected by separate transmitting and receiving probes (fig. 36 — tandem technique, fig. 37 — delta technique). Test specimen interfaces which lie close to reflecting discontinuities interfere with the reflection from those discontinuities. The signal of this kinri of reflection should not be evaluated without being corrected (according to the signal distortion due to the interface) (fig. 38, 39).
