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Non-destructive testing (NDT) with piezoelectric transducers

Non-destructive testing (NDT) with piezoelectric transducers

Last Updated on May 26, 2023 by You Ling

As an important non-destructive testing device, piezoelectric ultrasonic transducers have been widely used in various fields of the national economy. With the development of modern industrial technologies, especially aerospace, nuclear power energy, and intelligent manufacturing, various industrial and mining environments have put higher demands on the performance of piezoelectric transducers. It is urgently expected that piezoelectric transducers can monitor the operation of equipment more stably, provide early warnings, and prevent structural damage to important equipment. This article focuses on introducing piezoelectric ultrasonic transducers and their structures, and elaborates on the unique advantages of piezoelectric ultrasonic transducers in non-destructive testing.

1、What is non-destructive testing?

Non-destructive testing refers to the detection of various engineering materials, parts, structures, and surface defects without causing damage to the object being tested, by utilizing changes in physical quantities such as heat, sound, light, electricity, and magnetism caused by abnormalities or defects in the internal structure of the material.

2、Types of non-destructive testing

(1) Ultrasonic testing

Ultrasonic testing is based on the propagation and reflection principles of high-frequency sound waves. It can be used for defect detection/evaluation, dimensional measurement, material characterization, etc. Ultrasonic receivers and transmitters are used for testing.

Ultrasonic sound waves are transmitted through the material being tested. The sound propagates through the component and reflects from a rigid surface located at the other end of the transmitter. The time required to measure the emission and reception of sound waves is recorded. The time differences in different parts of the component can be used to identify defects in the material.

Different types of ultrasonic testing modes can be used to identify different defects, voids, material degradation, etc. Heavy mechanical components undergo periodic ultrasonic testing. A good example of ultrasonic testing is the detection of defects and deformations in railway car wheels and axles.

(2) Vibration analysis

Vibration analysis is a common method used to monitor the condition of rotating components during operation. The basic principle of vibration analysis is that different materials have different vibration characteristics.

In addition to vibration meters, different types of sensors can be installed to measure vibrations. They are designed to measure displacement, velocity, and acceleration that rotating equipment may encounter, as well as misalignment, looseness, and similar faults.

Like all other technologies discussed here, vibration analysis provides valuable data for condition monitoring and predictive maintenance.


(4) Magnetic Particle Testing (MT)

Magnetic particle testing is used to detect near-surface defects in ferromagnetic materials. The specimen is held between the two poles of an electromagnet, and a suspension of magnetic particles is poured onto the specimen. This testing method is based on the influence of a magnetic field on ferromagnetic materials.

When magnetic particles gather near defects and cracks, the defects on the material surface are highlighted. To achieve better visibility, ultraviolet light is used to observe the defects.

Magnetic particle inspection can be carried out using wet horizontal machines or handheld devices such as magnetic yokes. MT is commonly used to inspect the following:

Inner and outer surfaces of boilers and pressure vessels

Components damaged by fire

Locomotives and historical boilers

Yawning drying machines

Cargo holds

Ships for liquefied petroleum gas service

Modifications to welding repairs and pressure projects

(3) Penetrant Testing

When magnetic particle testing is not feasible, penetrant testing can be used. Performing penetrant testing requires a clean working surface.

During penetrant inspection, a liquid dye penetrant is sprayed onto the area to be tested and allowed to dwell on the surface. The required dwell time for the penetrant to work on the surface may range from 10 minutes to one hour, depending on the characteristics of the material being tested.

The liquid penetrant is then removed from the working surface using a dry, lint-free cloth. A small amount of developer is sprayed onto the tested surface. If there are defects on the surface being tested, the liquid dye will be brought to the surface after the application of the developer.

Liquid penetrant testing is commonly used to test welding surfaces and operates on the principle of capillary action.

(5) Eddy Current Testing

Eddy current testing is a common non-destructive testing technique used in both manual and automated testing scenarios. It is based on the principle of electromagnetic induction.

When voltage is applied to a coil, it generates a strong magnetic field. When a metal is introduced into the coil, the magnetic field fluctuates, and the current flowing through the circuit increases. This is due to the eddy currents flowing within the metal.

When there are defects or voids in the material, the current consumption increases. Eddy currents must propagate over a longer distance, increasing resistance, which is manifested as increased current consumption. The differences in current consumption across different cross-sections of the material can be used to identify the location and size of defects.

This type of non-destructive testing is performed using eddy current testing equipment, including electromagnetic probes, current flow detectors, ECT conductivity meters, and other accessories. These tools are used to perform different types of electromagnetic testing, such as surface scans, sub-surface detection, weld inspection, fastener hole inspection, tube inspection, heat treatment verification, and metal grade classification.

(6) X-ray Testing and Industrial Computerized Tomography (CT)

X-rays and other computed tomography techniques are widely used in the medical field. However, some of the same techniques are also employed in industrial applications as part of non-destructive testing.

X-ray and CT scanning are used for industrial radiography to visualize detailed images of the tested material. X-rays pass through the component, and the images can be printed on film or viewed in real-time using computers.

Computerized tomography techniques can also color-code various objects based on composite metals or existing voids. X-rays can be sent through the test object from different angles to obtain higher-detail images. X-ray testing and computerized tomography belong to a broader category of radiographic testing, where different types of ionizing radiation can be utilized.

2、Principles of Ultrasonic Nondestructive Testing

In ultrasonic testing, high-frequency sound waves are transmitted into the material to detect defects or locate changes in material properties. The most commonly used ultrasonic testing technique is pulse-echo, where sound is introduced into the test object, and the reflections (echoes) from internal defects or the geometry of the surface of the part are received by a receiver. Below is an example of shear wave weld inspection. Note the indication that extends to the upper screen limit, which is produced by the reflection of sound from defects within the weld.

Which piezoelectric elements can be used for ultrasonic nondestructive testing?

Ultrasonic NDT applications typically use single-layer piezoelectric sensors. The ultrasonic transducers used for NDT applications can be thin disks or plates made of our PZT-40 material, which operates at very high frequencies to generate very short pulses for good depth resolution.

3、Design Considerations for Piezoelectric Ultrasonic Transducers

For piezoelectric ultrasonic transducers used in nondestructive testing, the choice of the core piezoelectric material is particularly important. The piezoelectric material can convert electrical signals into sound signals (transmitting transducer) to emit ultrasonic testing waves to the object under test, and it can also receive sound signals reflected back from the object under test and convert them into electrical signals (receiving transducer). Therefore, piezoelectric materials usually require high piezoelectric constants, electromechanical coupling coefficients, and resistivity (to reduce sound losses) as prerequisites for preparing high-performance piezoelectric transducers. In addition, for piezoelectric transducers operating over a wide temperature range, the selection of piezoelectric materials also needs to consider the temperature stability of their piezoelectric properties, ensuring that the piezoelectric constants, electromechanical coupling coefficients, and other electrical parameters change minimally with temperature. Piezoelectric materials that meet these performance requirements are ideal choices for designing piezoelectric transducers for wide temperature range applications.

Usually, there is a certain degree of mismatch between the acoustic impedance of the load under test and the piezoelectric layer, so the selection of a suitable matching layer plays an important role in transducer design.

(1) Selection of the matching layer material. At room temperature, a mixture of epoxy resin and metal powder in a certain proportion can be uniformly mixed to obtain the desired acoustic impedance, achieving the effect of acoustic impedance matching by adjusting the ratio of epoxy resin and metal powder in the mixture.

(2) Transducers operating over a wide temperature range. High-purity alumina and other materials can be selected to design the matching layer, which can provide good acoustic coupling and protect the piezoelectric layer to some extent.

(3) Thickness of the matching layer. The thickness of the matching layer also affects the performance of the device. Usually, the thickness is chosen to be 1/4 of the wavelength of the sound wave, and it satisfies the impedance matching condition Zm = (Z1 × Zt)^(1/2) (where Zm is the acoustic impedance of the matching layer, and Z1 and Zt are the acoustic impedances of the load and the piezoelectric layer, respectively).

The backing layer is a cushion layer behind the piezoelectric transducer, and it also has a certain influence on the nondestructive testing performance of the transducer. To obtain a piezoelectric transducer with wide bandwidth, narrow pulse, and high detection sensitivity, a backing layer with appropriate impedance value is essential. Under external signal excitation, the piezoelectric transducer radiates sound waves in both forward and backward directions simultaneously. Typically, we are interested in receiving the echo signals only from the forward direction. Therefore, it is important to minimize the interference from the reflected waves in the backward direction. In this case, the backing layer acts like an infinitely large sound-absorbing medium, allowing the backward radiated sound waves to be almost completely absorbed in the backing layer. Therefore, the backing layer is often made of materials with high sound attenuation coefficients to improve detection resolution. At room temperature, a mixture of epoxy resin and metal powder, such as tungsten, iron, copper, mixed with epoxy resin, can be used to create a high-impedance sound-absorbing material. Wood chips, glass, cork, mixed with epoxy resin, can also be used as excellent backing materials. For high-temperature applications, zirconia ceramics with a certain porosity are a good choice for sound-absorbing materials.

Finally, for piezoelectric transducers operating in wide temperature range environments, as the external operating temperature increases, the differences in thermal expansion between the internal bonding structures can cause stress damage, leading to a decrease in the detection sensitivity of the piezoelectric transducer. Therefore, when selecting materials for different parts of the piezoelectric transducer, it is preferable to choose structural materials with matched thermal expansion coefficients.


Based on the information provided above, here is a summary:

Ultrasonic nondestructive testing (NDT) involves transmitting high-frequency sound waves into materials to detect defects or locate changes in material properties. The most commonly used technique is pulse-echo, where sound is introduced into the test object and the echoes from internal defects or surface reflections are received. Piezoelectric transducers, typically composed of a single-layer piezoelectric sensor, are commonly used in ultrasonic NDT applications. These transducers rely on materials with high piezoelectric constants, electromechanical coupling coefficients, and low acoustic losses to achieve optimal performance.

The design of piezoelectric ultrasonic transducers requires careful consideration of several factors. The choice of core piezoelectric material is crucial, as it should efficiently convert electrical signals into sound waves for transmission and convert reflected sound signals back into electrical signals for reception. Additionally, the material should possess high piezoelectric constants, electromechanical coupling coefficients, and low resistivity to minimize losses. Temperature stability of these properties is also important for wide temperature range applications.

Matching layers play a significant role in transducer design as they help address the impedance mismatch between the transducer and the tested material. Materials such as epoxy resin mixed with metal powder or high-purity alumina can be used as matching layers, providing good acoustic coupling and protection for the piezoelectric layer.

The thickness of the matching layer affects device performance and is typically chosen to be a quarter-wavelength of the sound wave, satisfying the impedance matching condition. The backing layer, located behind the piezoelectric element, also influences the NDT performance by reducing interference from backward reflected waves. Selecting a backing material with high sound attenuation coefficients improves detection resolution. Materials such as epoxy resin mixed with metal powder or porous zirconia ceramics are suitable choices for backing layers.

In conclusion, piezoelectric ultrasonic transducers are essential in ultrasonic NDT applications. The selection of appropriate piezoelectric materials, matching layers, and backing materials is crucial for achieving high-performance transducers with wide temperature range capabilities. These considerations contribute to the effective detection and characterization of defects in materials.