In some of the ultrasonic nondestructive damage evaluation of solids, why receiving probes having twice the frequency of transmitting probes are used ? What will be the drawback if both the probes have same frequencies ?
Not sure this is the case in your application, but some types of damage give rise to significant non-linear behavior, causing higher harmonics in the received signal. Thus the presence of higher harmonics (particularly at twice the excitation frequency) is sometimes used as an indicator for damage. If your measurement is based on this effect it makes sense to choose the receiving probe to have twice the frequency of the transmitter.
As mentioned above,double frequency probes are used when you are looking for higher harmonics generated due to the nonlinearity in the material.
Say, you are using a 5MHz probe which generates 5MHz,10MHz,15MHz frequency components due to the nonlinearity in the material. In order to effectively capture the 10MHz you need to use a receiver which is having sufficient bandwidth and better frequency response at those higher frequencies.
Laser based receivers having wide bandwidth can also be used.They have wider bandwidth compared to transducers commercially available.
Doubling the frequency allows for the detection of flaws causing non-linear effects which would produce harmonics such as kissing bonds or closed cracks. If one is looking for linear flaws, porosity or lack of side wall fusion for example, then it is advantageous to have your transmit and receive probes at the same frequency, as the majority of the energy in ultrasound reflected from the beam will exist in the fundamental frequency.
On the topic of laser based receivers they are usually operated with a DC to 20MHz bandwidth, however despite this advantage they are highly dependent on the optical properties of the sample you are inspecting and may not be suitable for all applications.
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