Application of Dry-Point-Contact Ultrasonic Transducers for Non-Destructive Material Testing

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Dry-point-contact (DPC) ultrasonic transducers send sound into a test object through small, hard contact tips, with no gel, water, or oil between the probe and the surface. That design choice enables generating low-frequent ultrasonic waves that lets ultrasound work on materials where liquid couplant high-frequency transducers fail: concrete, stone, fibre-reinforced composites, wood, and rough, coated, or hot metal.

This paper explains how DPC transducers are built, how they form longitudinal and shear waves at a point contact, how single elements combine into arrays, and how SAFT and TFM turn array data into images. The same dry-point-contact principle sits behind ACS ultrasonic transducers, concrete tomography and pulse-velocity systems, and guided-wave tools. If you inspect concrete, rock, or difficult surfaces, this is the physics behind the readings.

Related reading on acs-international.com: NDT of Concrete in Practice, The Vital Role of Ultrasonic Concrete Testing in Construction, and our Practical Guide to Ultrasonic Transducer Selection for Industrial NDT.

Abstract

This paper discusses the physical principles, design features, and practical applications of low-frequency piezoelectric Dry-Point-Contact (DPC) transducers in non-destructive testing (NDT). It is shown that the principle of discrete mechanical contact provides efficient ultrasonic energy transmission without the use of couplants, making these transducers particularly suitable for the inspection of coarse-grained and porous materials. Special attention is paid to wave field formation, the generation of longitudinal and shear waves, and the analysis of beam directivity patterns that determine the spatial sensitivity of the transducers. Practical applications are considered, including the inspection of concrete, rock materials, and the implementation of ultrasonic tomographic techniques based on SAFT and TFM algorithms. Advantages, limitations, and future development prospects of the technology are also discussed.
Keywords: ultrasonic testing, dry-point-contact, DPC, SAFT, TFM, directivity pattern.

1. Introduction

Ultrasonic non-destructive testing (UT) is one of the principal diagnostic methods used for the evaluation of materials and structures, because of its high sensitivity to internal defects such as cracks, voids, delaminations, and inclusions. The method is based on the excitation of ultrasonic waves within the inspected object and the subsequent analysis of their propagation, reflection, and scattering characteristics.

Conventional ultrasonic testing techniques typically employ contact piezoelectric transducers that require a liquid couplant layer (e.g., water, gel, or oil) to ensure efficient transmission of acoustic energy into the test object. Although this approach has proven highly effective for metallic and other homogeneous materials, it exhibits significant limitations in a number of practical applications.

In particular, the use of liquid couplants is difficult or impractical during field inspection of porous and capillary-active materials such as concrete, stone, glass-fiber-reinforced composites, and wood. Under such conditions, low-frequency Dry-Point-Contact (DPC) transducers offer a promising alternative. Their operating principle is based on the transmission of low-frequent ultrasonic waves through multiple localized contact zones without the need for a liquid couplant.

DPC transducers occupy an intermediate position between conventional piezoelectric contact transducers and air-coupled ultrasonic systems. They combine relatively high acoustic transmission efficiency with reduced sensitivity to surface condition and enable operation in the low-frequency ultrasonic range from approximately 20 to 400 kHz.

The aim of this paper is to present a systematic analysis of the physical principles and engineering aspects of DPC transducers, with particular emphasis on practical applications and signal-processing techniques.

2. Design and Physical Principles of DPC Transducers

2.1 Design of DPC Transducers

The key distinguishing feature of Dry-Point-Contact (DPC) transducers lies in the principle of acoustic coupling formation. Unlike conventional piezoelectric transducers with a continuous contact surface, DPC transducers employ discrete point-contact elements or arrays of such elements.

These contact elements (tips) establish localized mechanical contact with the surface of the inspected object. Although the actual contact area is considerably smaller than the geometric contact area, the high local contact pressure and efficient mechanical coupling between the tip and the surface ensure effective transmission of ultrasonic vibrations into the test object.

Another important distinction between DPC transducers and conventional piezoelectric probes is that the tip transmitting ultrasonic vibrations from the piezoelectric element to the acoustic contact point has dimensions significantly smaller than the ultrasonic wavelength. Owing to its acoustically small dimensions, the tip behaves as a lumped mechanical body and vibrates with negligible internal deformation.

Figure 1. Different types of DPC transducers

The image below shows the design of a longitudinal-wave DPC transducer. The transducer generates normal stresses at the contact point with the solid surface. For simplicity, the piezoelectric element is represented as a two-layer structure, although a larger number of layers may also be employed. The “+” and “−” symbols schematically indicate the polarization directions of the piezoelectric plates. With this polarization configuration, the piezoelectric element undergoes longitudinal expansion–contraction oscillations.

The vibration directions of the contact tip are indicated by arrows. By changing the polarization configuration of the two-layer piezoelectric element, the vibration mode of the contact tip can be altered from longitudinal (normal) to shear oscillations.

Figure 2. Schematic design of DPC transducers for the excitation of longitudinal and shear ultrasonic waves

2.2 Operating Frequencies and Spectral Characteristics of DPC Transducers

Depending on the design and dimensions of the piezoelectric elements, DPC transducers can operate over a frequency range from 20 to 400 kHz. Whether the transducer is damped also shapes its frequency response.

Undamped, narrow-band transducers are best for velocity measurements. Damped transducers fire short, broadband pulses instead, which gives the resolution needed for pulse-echo imaging. The images below show the pulse and frequency responses of undamped and damped DPC transducers.

The damping element is typically fabricated from a liquid composite material with high ultrasonic attenuation and is applied over the entire free surface of the piezoelectric element.

Figure 3 – Pulse and frequency response of an undamped DPC transducer

Figure 4 - Pulse and frequency response of a damped DPC transducer

2.3 Directivity Patterns of DPC Transducers

The directivity pattern determines the spatial distribution of transmitted and received ultrasonic energy and is one of the principal characteristics of an ultrasonic transducer. Owing to the low operating frequency and the small effective aperture of DPC transducers, their radiation patterns are generally broad.

The images below show the directivity patterns of DPC transducers. A transducer with longitudinal oscillations of the contact tip generates and receives longitudinal bulk waves propagating normal to the surface of the half-space. In contrast, a transducer with shear oscillations of the contact tip enables the generation and reception of shear bulk waves propagating in the normal direction.

Because of the point acoustic contact, each transducer inevitably excites and receives, in addition to the dominant wave mode, other types of ultrasonic waves propagating at specific angles with respect to the surface of the half-space.

Figure 5. Schematic directivity pattern of a longitudinal-wave DPC transducer

Figure 6. Schematic directivity pattern of a shear-wave DPC transducer

Different types of surface waves propagate along the surface from the contact point in various directions. A transducer with longitudinal oscillations of the contact tip acts as an omnidirectional source of Rayleigh waves. A transducer with shear oscillations of the contact tip generates longitudinal subsurface (head) waves and Rayleigh waves in the direction of the displacement vector, as the image below shows. In the direction perpendicular to the displacement vector, the same transducer excites horizontally polarized shear waves (SH waves).

These characteristics of transducers with shear oscillations of the contact tip enable the measurement of longitudinal and shear wave velocities in materials using indirect through-transmission techniques.

Figure 7. Propagation directions of surface waves generated by a shear-wave DPC transducer

3. Multi-Element DPC Systems (Arrays) and Signal Processing Methods

3.1 Transition from Single Transducers to Arrays

Single DPC transducers, owing to their point-like aperture, enable efficient excitation and reception of low-frequency ultrasonic waves. However, their application to flaw detection remains limited due to several inherent drawbacks:

• lack of spatial selectivity;
• difficulty in localizing defects within the inspected volume;
• ambiguity in signal interpretation.

The transition from single-element transducers to multi-element array systems fundamentally expands the capabilities of ultrasonic measurements.

3.2 Passive and Active Arrays Based on DPC Transducers

To increase inspection range, DPC transducers are combined into arrays that form a more directional beam. Each element rides on a spring inside the housing, so it holds steady contact pressure even on an uneven surface.

These arrays can be realized either as passive systems, without integrated transmitting and receiving electronics, or as active systems incorporating transmitter and receiver circuitry directly within the DPC transducer housing.

In active systems, the signal-to-noise ratio of the received ultrasonic echo signals is significantly improved. This enhancement is particularly important for synthetic-aperture-based imaging techniques, enabling the reconstruction of tomographic images with substantially higher detection capability and spatial resolution.

Figure 8. Different types of DPC arrays

Figure 9. DPC transducer with active transmitting and receiving channels

3.3 Features of DPC Array Applications

A DPC array consists of a set of independent dry-point-contact transducers, each of which can operate either synchronously with the other elements as part of a common physical aperture using a shared acoustic channel, or independently in transmit–receive mode as an element of a synthetic aperture.

In the latter case, multichannel electronics are required to acquire pitch-catch data for all possible transmitter–receiver combinations. This approach enables the recording of the complete synthetic aperture dataset, commonly referred to as Full Matrix Capture (FMC).

Compared with conventional piezoelectric phased arrays, DPC arrays exhibit several distinctive features:

• Low operating frequency and correspondingly large ultrasonic wavelength. This significantly limits the effectiveness of conventional phase-based beam steering techniques.
• Extremely broad directivity patterns of individual elements. Each element behaves approximately as a point source, simultaneously generating multiple types of bulk and surface waves.

So with DPC arrays, the work shifts from steering the beam to processing the signals and reconstructing the image.

3.4 Synthetic Aperture Focusing Technique (SAFT)

The Synthetic Aperture Focusing Technique (SAFT) is not the primary subject of this paper; therefore, only those aspects most relevant to DPC arrays are briefly considered here.

SAFT is based on the coherent summation of signals acquired at different transmitter and receiver positions. In effect, it reconstructs the echo as if it came from a single point inside the part.

For DPC transducers, SAFT is particularly effective because it:

• fully uses the broad directivity patterns of the transducers;
• improves spatial resolution;
• effectively suppresses stochastic noise caused by structural heterogeneity of the material.

In practical applications, SAFT enables:

• localization of reflecting objects;
• enhancement of image contrast.

3.5 Total Focusing Method (TFM)

The Total Focusing Method (TFM) represents a further development of the Synthetic Aperture Focusing Technique (SAFT) and uses the complete Full Matrix Capture (FMC) dataset acquired with both linear and matrix array apertures (Figure 10).

For each point of the reconstructed image, the following operations are performed:

• calculation of the ultrasonic travel time from each transmitting element to the image point and subsequently to each receiving element;
• coherent summation of the corresponding signals using appropriate delay laws;
• in the case of heterogeneous propagation media, the application of correlation-based signal superposition techniques is recommended to improve image quality and suppress incoherent noise.

Figure 10. TFM image formation scheme for a matrix DPC aperture

4. Application Areas of DPC Transducers and DPC Transducer Arrays

DPC transducers and arrays based on them, similarly to other types of ultrasonic piezoelectric probes, are used both for investigating the physical properties of materials and for flaw detection applications. What they can do depends on the inspection task and on the the material properties.

A characteristic feature of DPC transducers is that, because of their design, they generate low-frequency ultrasonic waves and are therefore well suited for the inspection of structurally heterogeneous materials exhibiting high ultrasonic attenuation. In particular, they provide stable acoustic coupling under the following conditions:

• high surface roughness and irregularity;
• presence of surface coatings;
• high surface curvature and limited probe aperture dimensions;
• elevated or reduced operating temperatures.

Depending on the application, two principal operating modes are commonly employed:

• through-transmission mode – for evaluating the mechanical properties of the medium, including strength assessment;
• pulse-echo mode – for localization of material discontinuities and detection of embedded metallic structural elements.

4.1 Inspection of Materials without Liquid Acoustic Coupling

DPC transducers earn their place wherever a liquid couplant is impractical or would distort the reading.

Typical applications include:

• Non-destructive testing of heterogeneous / highly scattering materials (concrete & rock, GFRP, wood)
• in-situ inspection of civil engineering structures, including vertical and overhead surfaces;
• inspection of porous and hygroscopic materials, such as carbon electrodes;
• inspection of objects with contaminated, rough, or abrasive surfaces;
• operation under low- and high-temperature conditions.

Dropping the couplant isn't just convenient. It also makes measurements more reproducible:

• absence of a variable intermediate layer between the transducer and the test object;
• reduced dependence of signal characteristics on couplant layer thickness;
• improved repeatability and reproducibility of serial measurements.

At the same time, several additional factors become critically important:

• stability of the contact pressure;
• monitoring of the condition and wear of the contact tips, since their dimensions directly affect the transmission and reception delay times.

The image below shows the thickness measurement of a heavy steel plate at elevated temperature using a dual-element DPC array.

Figure 11. Thickness measurement of heavy steel plate using a dual-element DPC array at 150 KHz

4.2 Inspection of Concrete and Reinforced Concrete Structures

Concrete is one of the most characteristic application areas for DPC transducers. Its structure consists of:

• coarse aggregates (gravel, crushed stone);
• cement matrix;
• Reinforcement elements such as steel rebars, tendon ducts.

From the viewpoint of ultrasonic wave propagation, concrete represents a strongly scattering and structurally heterogeneous medium. When the ultrasonic wavelength becomes comparable to the characteristic aggregate size:

• multiple scattering phenomena occur;
• the ultrasonic signal rapidly loses coherence.

Under such conditions, high-frequency ultrasonic inspection methods become ineffective. So low-frequency ultrasonic techniques, typical of DPC systems, are used instead.

Typical concrete inspection tasks include:

4.2.1 Thickness Measurement of Structures

In the case of single-sided access, measurement of ultrasonic wave travel time in pulse-echo mode enables thickness evaluation, as the image below shows. Accurate thickness determination critically depends on proper calibration of the ultrasonic wave velocity, which should preferably be performed directly on a region of the test object with known thickness.

For inspection of objects with unknown thickness under single-sided access conditions, modern ultrasonic instruments provide the capability for automatic ultrasonic velocity calibration using surface-wave-based techniques.

Figure 12. Detection of back-wall spalling in a concrete structure using one-sided ultrasonic thickness measurements

4.2.2 Localization of Metallic Structural Elements (Reinforcement Bars)

Although electromagnetic methods are often more sensitive for detecting metallic elements located within near-surface concrete layers, ultrasonic tomography also constitutes a highly informative inspection technique for this purpose, particularly in massive reinforced concrete structures containing multilayer reinforcement (Figure 13).

Figure 13. Detection of multilayer reinforcement in a massive 1.1 m thick concrete structure

4.2.3 Detection of Voids and Honeycombing

Voids generate strong ultrasonic reflections due to the significant contrast in acoustic impedance between the defect and the surrounding material. However, because of multiple scattering effects and the relatively low reflectivity of certain discontinuities, the received ultrasonic signals are often blurred and difficult to interpret directly. Under such conditions, spatial signal-processing techniques, such as the Synthetic Aperture Focusing Technique (SAFT), are required to improve defect localization and image quality.

Figure 14: Three-dimensional representation of volumetric discontinuities in a concrete structure

4.2.4 Crack Depth Measurement

Cracks act as acoustic reflectors with pronounced directivity characteristics and are therefore most effectively detected using shear-wave transducers. In DPC transducers, the generation of shear waves occurs naturally as a consequence of the point-contact excitation mechanism, which constitutes a significant advantage of this technology.

The image below shows how the depth of surface-opening cracks (perpendicular to the surface) is measured with a matrix DPC array, using the diffraction signals from the crack tip.

Figure 15. Measurement of open-crack depth using diffraction-signal visualization

4.2.5 Evaluation of Concrete Strength Using UPVT

The Ultrasonic Pulse Velocity Test (UPVT) method is based on measuring the propagation velocity of ultrasonic waves in concrete to indirectly evaluate its mechanical and structural properties. The method enables the detection of defects, cracks, low-density zones, and material heterogeneities, while also providing an integral assessment of concrete quality. For quantitative strength evaluation, UPVT results are typically calibrated using reference samples with known compressive strength or correlated with data obtained from destructive testing methods.

The image below shows through-transmission testing of a massive concrete structure, using an instrument with two DPC arrays that operate at 100 kHz and send and receive longitudinal waves.

Figure 16. Application of through-transmission testing to a massive concrete structure

4.3 Guided Wave Applications

Due to the generation of shear-wave components, DPC transducers can effectively excite guided waves in bounded media. This capability can be utilized for:

• inspection of extended structural elements;
• diagnostics of structures with limited accessibility, such as lighting poles;
• detection of defects located remotely from the excitation point, for example corrosion beneath supports.

A practical advantage of guided-wave inspection is that, when ultrasonic waves propagate in bounded media where the propagation path is determined by the geometry of the object, inspection of hidden or inaccessible regions becomes possible, including areas located at considerable distances from the ultrasonic excitation point.

An additional advantage of guided-wave excitation using DPC transducer systems, apart from the absence of liquid couplants, is the capability to generate horizontally polarized shear (SH) waves in both electrically conductive and non-conductive materials, including inspection through paint and protective coatings.

The image below shows hidden corrosion detection in lighting poles, using a guided-wave system built on an array of 32 DPC transducers. During inspection, circumferential scanning of the pole is performed with a step size of 20 mm, while real-time reconstruction of a B-scan image is carried out using the Synthetic Aperture Focusing Technique (SAFT).

Figure 17. Detection of hidden corrosion in metallic utility poles using a guided-wave inspection method

5. Conclusion

This paper has reviewed the physical principles, design features, and application areas of low-frequency Dry-Point-Contact (DPC) piezoelectric transducers for ultrasonic non-destructive testing. It has been shown that DPC technology enables efficient transmission of ultrasonic energy without the use of liquid couplants, which is particularly important for the inspection of coarse-grained, porous, and structurally heterogeneous materials such as concrete, stone, and composite materials.

The transition from single-element probes to multi-element DPC arrays, combined with advanced imaging algorithms such as SAFT and TFM, significantly improves image quality, spatial resolution, and defect detection reliability in highly scattering media. Practical examples demonstrate the effectiveness of the technology for thickness measurement, reinforcement detection, identification of voids and cracks, and ultrasonic characterization of materials.

Thus, DPC transducers represent a promising direction in the development of advanced ultrasonic testing systems, particularly for the inspection of civil engineering and industrial structures operating under challenging environmental and coupling conditions.

Frequently Asked Questions

What is a dry-point-contact (DPC) transducer?
A DPC transducer couples ultrasound into a material through small, hard contact tips instead of a flat face and a liquid couplant. The tips make localized mechanical contact, so the probe transmits sound without gel, water, or oil. DPC transducers operate in the low-frequency range of roughly 20 to 400 kHz.

Do DPC transducers need couplant?
No. The point-contact design transmits ultrasound dry, with no liquid couplant. This is the main reason DPC transducers suit field inspection of porous and rough materials, where a couplant is impractical or would distort the reading.

Which materials suit DPC ultrasonic testing?
Coarse-grained, porous, and strongly scattering materials such as concrete, stone, and fibre-reinforced composites, plus surfaces that are rough, curved, coated, or at high or low temperature. These are the cases where conventional high-frequency contact probes lose signal.

How is DPC different from EMAT and air-coupled ultrasound?
All three avoid a liquid couplant, but they work differently. EMAT generates ultrasound electromagnetically and works on conductive materials only. Air-coupled systems transmit across an air gap and lose a lot of energy, therefore they are not suitable for pulse-echo testing. DPC uses direct mechanical point contact, which gives higher transmission efficiency at low frequency and works on non-conductive materials such as concrete in both pulse-echo and through-transmission technique.

Can DPC transducers measure concrete strength?
Indirectly. With the Ultrasonic Pulse Velocity Test (UPVT), DPC arrays measure how fast sound travels through the concrete. That velocity correlates with compressive strength once it is calibrated against reference samples or destructive test data.

What can DPC arrays detect in concrete?
Thickness from one-sided access, reinforcement and embedded metal, voids and honeycombing, crack depth, and strength by UPVT. Combined with SAFT and TFM imaging, the arrays reconstruct tomographic images of the interior.