Understanding Ultrasonic Transducers
Types of Transducers
Dual (Twin Crystal)
Dual Element transducers use seperate transmitting and receiving elements to create a pseudo-focus, which is advantageous for inspecting materials and parts with rough backwall surfaces.
Delay Line
An additional piece of material either acyrlic or graphite, positioned between the transducer and the test material to seperate excitation pulse recovery from backwall echoes. A major reason for using them is for thin material applications.
Contact
This trasnducer is typically has a hard wear surface, which is optimised for direct contact with the test material.
Angle Beam
Ideal for flaw detection, angle beam transducers can be used with transducers wedges or already set at 45, 60 and 70°.
Immersion
Immersion transducers are designed to be immersed in water and use a column or bath of water to couple sound energy into the test piece.
How to select frequency
As frequency increases, the wavelength decreases, which allows for small flaw detectin and thickness/locatino precision. As frequency decreases, the wavelength increases which allows for greater penetration into thick ad/or attenuating materials. Other factors such as nearfield length, beam spreading and beam diameter also affect frequency selection.
Common pulse echoe contact techniques such as contact tests of steel generally use 2.25MHz to 5MHz frequencies. Medium carbon steel castings are usually tested between 1MHz - 5MHz, with high carbon and alloy steels requiring a low frequency range of 0.5MHz to 1MHz. Thin plastics use a 20MHz or higher