How to Test Ultrasonic Transducers?

Introduction

Ultrasonic transducer testing converts the returned mechanical vibrations into electrical energy after converting these pulses to mechanical vibrations. Ultrasonic transducers are small probes that generate and receive the high-frequency sound waves used for thickness gaging and flaw detection in ultrasonic nondestructive testing applications. Transducers serve as the initial point for all ultrasonic test structures as devices that convert one energy form into another. These devices have various case styles, sizes, or frequencies to meet inspection requirements, whether paper-thin coatings’ thickness measurement or enormous multi-ton steel forgings’ detection.

Transducers, especially phased array transducers convert the electrical energy’s pulse in ultrasonic NDT from the test instrument into mechanical energy as sound waves that move through the test piece. Likewise, the transducer converts the sound waves reflecting the test piece into the electrical energy pulse that the test instrument can process and display. In effect, the airmar transducer generates and receives sound waves’ pulses at frequencies much higher than the human hearing range, acting as an ultrasonic microphone and speaker.

1. What is Transducer Testing?

The basic physical dimension measurements serve as the starting point for transducer testing. Followed by the performance testing for opens, shorts, continuity, etc. It is essential to ensure that the device has no significant defects after assembly by making simple electrical measurements like capacitance and loss. Then, you can proceed to perform functional testing by using a calibration block. This typically comprises a specified running of pulse-echo size in water or air that reflects a pre-determined target at a certain distance.

Additionally, you may want to record sensitivity, bandwidth, and ring-down. Also, at the time, it is critical to measure Total Acoustic Power output. Ultimately, you will need to put the device under whatever environmental conditions designed for it and test it again.

Based on the proposed use, these performance tests entail driving the transducer with a spike or pulse, square wave, or sine wave. Typically, the industry-standard pulse receiver does this, sometimes using an amplifier. In addition, they use an oscilloscope to view the pulse and return to check the subjective ‘quality’ or ‘cleanliness’ of the waveform or make numerical measurements.

2. What are the Applications of Ultrasonic Transducer Testing?

Experts classify transducers into groups according to the applications. Here are the applications of ultrasonic transducer testing.

Immersion Transducers

They designed these transducers to immerse them in water and couple sound energy into the test piece using a column or bath of water. Immersion transducers integrate an acoustic lens that directs the sound beam into a small space. With that, they boost sensitivity to small reflectors. People often use them for optimizing sound coupling into grooves, channels, or sharp radiuses in test parts with complex geometry, in-process or on-line tests on moving pieces, and for scanned tests.

Delay Line Transducers

These transducers integrate a fused silica, epoxy, or plastic cylinder, a delay line between the test and the active element. Thin material applications such as measuring fragile test parts or testing spot welds in sheet metal are the main reason for using them. The test indicates essential to separate back-wall echoes from the excitation pulse recovery. Typically, you can use a delay line as a thermal insulator to protect the heat-sensitive transducer feature from direct contact with hot test parts. Alternatively, you can shape or contour the delay lines to enhance sound coupling in tight confined or curved spaces.

Angle Beam Transducers

These transducers have the same construction as contact transducers. However, they can generate sound tilted at an angle to the coupling surface since you can use them with angle beam wedges. Essentially, you can configure wedges to generate around 45, 60, or 70 degrees refracted shear waves. Since the most common weld geometry needs to aim at sound waves at an angle, you will find them standard in most weld inspections. All common weld inspection codes reference these transducers.

Contact Transducers

You can use contact transducers in direct contact with the test part. What protects the active element from damage in any typical use is the hard, thin wear plate that you can cut to a one-quarter thickness of the wavelength. Experts use these transducers in flaw detection applications involving straight beam tests like when looking for metal delaminations or ingots voids in composites and various thickness gaging applications.

Dual Element Transducers

You can use dual element transducers or duals primarily for corroded or rough surfaces tests. These transducers integrate distinct transmitting and receiving features mounted on a delay line at a slight angle to focus sound energy to a selected distance beneath the surface of a test part.

3. What are the Types of Ultrasonic Transducer Testing?

• Beam Alignment Measurements

• Beam Profiles

• Electrical Impedance Plots

Here are some of the ultrasonic transducer testing types:

• Beam Alignment Measurements – these types offer information on the alignment degree between the transducer housing and the sound beam axis. Applications that require high certainty regarding beam positioning concerning a mechanical reference surface will find this data particularly useful.
• Beam Profiles – beam profiles inform about critical data on field traits of transducer sound. To create transversed beam profiles, you can scan the transducer around a target at a specific distance. They are also essential to determine beam symmetry or focal spot size.
• Electrical Impedance Plots – you can get crucial data about a transducer’s construction and design through the electrical impedance plots. Additionally, you can get identical transducers from various sources using them.

4. How do you Test Ultrasonic Transducer?

If you want to test an ultrasonic transducer, you can use a similar principle in naval fish finder and SONAR system. In addition, you will need to introduce ultra-high frequency sound into the part you intend to test. You can present the sound on a visual display since some will reflect the sending unit when it hits any material with a different acoustic impedance.

Essentially, you can determine the distance to the reflector when you know the time needed for the sound to revert to the sending unit and sound speed through the acoustic velocity. You have between 1.0 and 10.0 MHz for the typical sound frequencies used in the ultrasonic transducer, which do not travel through the air and are too high to be heard. The higher frequencies can detect more minor indications and may not penetrate as deeply.

5. What Characteristics are used to Evaluate Ultrasonic Transducer Testing?

Some of the procedures listed below will help obtain the electrical and acoustic characteristics.

• Frequency Response – the sinusoidal burst and shock excitation are the two procedures that can help obtain the frequency response.
• Relative Pulse-Echo Sensitivity – you can get this through a sinusoidal burst technique through the frequency response. With that, obtain the value from the pulse-echo signal amplitude received from a specific target and the amplitude relationship of the voltage applied.
• Time Response – you can describe the waveform radio frequency response through the time response. In essence, get the response using a pulse-echo, shock excitation procedure. The chosen specific targets are critical to record the waveform and time responses.
• Frequency Response – there is a peak of 5 MHz for the frequency response above the transducer and can operate over various frequencies. They can measure its bandwidth at 50 percent of the peak frequency and -6 dB points. Typically, quote the broadband transducer usable bandwidth at the -20 dB points.
• Complex Electrical Impedance – the commercial impedance measuring instrumentation can obtain the complex electrical impedance. These measurements can provide the phase and magnitude of the search unit’s impedance over its operating frequency range. So, make these measurements according to specifications required for any instrument manufacturer and under laboratory conditions with minimal external accessories or cable lengths.
• Sound Field Measurements – you can establish parameters through these measurements, including curved and flat transducers and transverse or on-axis sound beam profiles. Use a hydrophone transducer to scan the sound field to achieve these measurements. With that, mapping the sound domain in three-dimensional space becomes easy.

Conclusion

The primary purpose of an ultrasonic transducer is to convert acoustic energy back into electrical power after the initial conversion from electrical to acoustic energy. Therefore, the function of the piezoelectric element is to convert the electrical energy to acoustic (and vice versa). So, the rest of the transducer manipulates the acoustic energy and interfaces with the rest of the world.