Technical Field

The invention relates to a system for non-destructively testing a specimen by means of elastic waves, a method of operating the system and a use of a coupling element for non-destructively testing a specimen with a testing device by means of elastic waves.

Background Art

In the field of non-destructive testing (NDT), elastic waves are commonly used to probe man-made structures, such as slabs, pavements, walls, decks, piles or shafts, made from e.g. concrete, timber or metal. The structures may be categorized according to their geometry in plate-like structures, e.g. slabs, pavements, walls and decks, and pile-like structures, e.g. piles and shafts. Information retrieved about the structures includes a thickness of the (plate-like) structures, a length of the (pile-like) structures, and in general internal properties of the structures, e.g. flaws.

While the principle of testing a structure by elastic waves may be applied to both, plate-like and pile-like structures, the devices used for measuring the elastic waves and the methods for processing the measured data conventionally differ between the two categories of structures:

Impact-echo (IE) is an NDT method based on the use of impactgenerated stress (elastic) waves that propagate through plate-like structures and are reflected by internal flaws and external surfaces. Impact-echo can be used on thin concrete structures including slabs, pavements, walls, or decks, to assess their thickness or to locate flaws such as cracks, delamination, voids, honeycombing and debonding. Steel ball impactors are generally used to generate the elastic waves. The ball diameter is generally selected according to the structure expected thickness. IE relies on measuring the transient response of plate-like structures under a mechanical impact, and more particularly their thickness resonances. As a consequence, IE probes must be able to measure high frequencies, e.g. above 10 kHz and in particular up to approximately 30 kHz. The structural health state is then determined based on analysing the measured elastic waves in the frequency domain. Pile integrity testing (PIT), also referred to as low-strain dynamic test or low-strain integrity test, is an NDT method used to assess the integrity of piles or shafts. This method is mostly applied in civil engineering for health assessment of, but not limited to, timber shaft and concrete pile such as used in deep foundation. Like impact echo, this method relies on impact-generated stress (elastic) waves. The elastic waves are generally generated with a hammer at the top of the pile next to the probe. The acquired signal shows the time of flight, i.e. the travel time, between the first impact and the waves reflected from the bottom of the pile or from any flaws in the pile. In PIT, the waves are travelling a longer distance than in IE, and only few reflections are generally captured. As a consequence, PIT probes are typically perfor- mant at low frequencies, e.g. between 100 Hz and 1 kHz. The structural health state is then determined based on analysing the time domain waveform.

As disclosed in patent application PCT/EP2021/063910, whose content is incorporated herein by reference, a combined testing device has been introduced for both applications, in particular being configured to conduct measurements for the IE as well as PIT methods. For such testing device, and in general for any NDT device that senses elastic waves, a good coupling of the elastic wave sensor of the device to the structure to-be-tested, i.e. the specimen, is crucial. Typically, the elastic wave sensor is mounted to a contact element, e.g. made from metal, in particular from steel. Conventionally, coupling between the contact element and the specimen is made either directly, in particular without any further coupling means, or by using some putty or gel between the contact element and the specimen. Other known testing devices e.g. comprise a piezo adapted to directly contact the specimen, or a contact element with a tip of softer material permanently attached to the contact element.

Either method is not ideal: A metal contact element, without further ado, suffers from poor signal quality, e.g. caused by spurious resonances because of insufficient coupling. This is in particular observed for rough surfaces, e.g. on coarse grained concrete. Thus, the surface of the specimen is typically ground smooth at each measurement position, which is tedious and time-consuming. Using putty or gel usually leads to a better coupling but is, again, tedious and time-consuming since the putty or gel has to be applied anew to the contact element or the specimen at each measurement position. Further, a considerable amount of putty or gel is used, and the specimen may need cleaning after the measurements. Using a piezo or any soft-mate- rial tip on the contact element, in turn, suffers from durability issues: Through use on abrasive surfaces, such as concrete surfaces, any soft material will degrade. Disclosure of the Invention

The problem to be solved by the present invention is therefore to provide an improved system for non-destructively testing a specimen by means of elastic waves and an improved method of operating the system, in particular which provides high-quality data from elastic wave measurements, in particular in terms of a large signal-to-noise ratio. At the same time, the system shall be easy to handle and facilitate time and cost efficient measurements at a multitude of measurement positions on the specimen.

Throughout this document, the following geometrical definitions shall apply:

“Plate-like” in particular means that two lateral dimensions of the specimen (or other element) are significantly larger, e.g. at least a factor 2 or 3 larger, than a depth dimension of the specimen (or other element). Plate-like specimens are e.g. slabs, pavements, walls and decks. They may typically be made of concrete, reinforced concrete or rock.

“Pile-like” in particular means that the depth dimension of the specimen is significantly larger, e.g. a factor 2 or 3 larger, than the two lateral dimensions of the specimen. Pile-like specimens are e.g. piles and shafts. They may typically be made of concrete, metal, in particular iron or steel, or timber.

Both, plate-like and pile-like specimens may have an arbitrary shape in a horizontal plane, i.e. the plane spanned by the two horizontal dimensions. In particular, the shape in the horizontal plane may be round, rectangular or square. Further, the shape in the horizontal plane, in other words a cross-section in the horizontal plane, may be variable along the depth dimension.

In both cases, on the plate-like and the pile-like specimen, the measurement position, and in particular also an impact position for generating the elastic waves, are located on a measurement side of the specimen, which in particular essentially coincides with a horizontal plane.

NDT system

According to a first aspect of the invention, the problem is solved by a system for non-destructively testing a specimen by means of elastic waves. The system comprises

- a testing device comprising a contact element and an elastic wave sensor in mechanical connection with the contact element, and - a coupling element.

The coupling element is removably mountable to the contact element, in particular for coupling the contact element to the specimen. “Removably” in particular may in particular mean that the coupling element is mountable and replaceable to/from the contact element without damaging the contact element and in particular also without damaging the coupling element. Advantageously, the coupling element is removably mountable to the contact element without tools. In this way, the coupling element may be exchanged quickly if it has degraded, in particular in terms of its coupling properties, e.g. through abrasion.

“Coupling” may in particular denote a mechanical contact between the contact element or a quality of such mechanical contact in terms of its transmission of elastic waves between the specimen and the contact element. In particular, the coupling element is adapted to transmit elastic waves between the specimen and the contact element.

Further, the coupling element comprises a layer made from plastic. “Plastic” shall in particular denote a synthetic or semi- synthetic material that uses polymers as a main ingredient. In an advantageous embodiment, the plastic is polyurethane.

Such system with the described coupling element exhibits a good coupling to a wide variety of specimens, in particular ranging from specimens with a rough surface, such as e.g. coarse grained concrete, to specimens with a smooth surface, e.g. polished concrete. The good coupling leads to a good quality of the acquired measurement data, in particular in terms of a high signal-to-noise ratio. At the same time, the system is simple to handle: The described coupling element avoids additional steps that, otherwise are required at every measurement position, such as grinding the surface of the specimen or applying putty or gel. This saves time and cost when performing measurements, e.g. according to the IE and PIT methods. Further, the coupling element comprising a plastic layer is in particular durable such that it may be used for performing measurements at a multitude of measurement positions. Still, the coupling element may be exchanged easily if required, in particular if its coupling properties have degraded, e.g. through abrasion.

Advantageously, the coupling element is adapted to adhere to the contact element. In particular, the coupling element may comprise an adhesive layer, e.g. comprising acrylic. These features further improve the usability of the system since the coupling element, by itself, stays on the contact element for a multitude of measurements. In general, the coupling element only needs to be replaced when it is damaged and shows a deteriorated coupling. Replacing the coupling element is simplified since it does not require any tools but can simply be done by hand.

Properties of coupling element

In an advantageous embodiment, the coupling element has a hardness Shore A between 60 and 85, in particular between 65 and 80, in particular 72. Such hardness balances a good coupling to a variety of surfaces, in particular a good flexibility and adaptation of the coupling element to different surfaces, with a high transmission of elastic waves and avoids too much attenuation within the coupling element.

In a further embodiment, the coupling element has a resilience, measured according to ASTM-D-2632, between 5 % and 20 %, in particular between 8 % and 11 %. In particular, this is a measure of impact resilience from measurement of a vertical rebound of a dropped mass. Again, a coupling element with such property provides a good flexibility and high transmission of elastic waves.

In a further embodiment, the coupling element has a plate-like shape. In particular, the coupling element may have a circular, hexagonal or rectangular shape. Such coupling element is well-suited for typical contact elements which have a protruding tip with typically essentially circular cross-section for contacting the specimen. Further, a coupling element with one of the given shapes can easily be manufactured from a sheet of the corresponding plastic material that may be available as rollstock. In general, a multitude of coupling elements may be provided in form of a sheet, e.g. as rollstock, wherein each coupling element is pre-cut or perforated.

In a further embodiment, the coupling element has a thickness between 0. 1 mm and 1.5 mm, in particular between 0.5 mm and 1 mm, in particular 0.8 mm. Such thickness has been found to be a good compromise between the coupling element’s ability to adapt to a wide range of surface roughness and avoiding excessive attenuation of elastic waves. Advantageously, the thickness of the coupling element is chosen according to the surface roughness of the specimen, in particular a lower thickness for smooth surfaces and a higher thickness for rough surfaces.

In a further embodiment, a lateral dimension of the coupling element is between 5 mm and 40 mm, in particular between 10 mm and 20 mm. Such lateral dimensions suits typical contact elements and is easy to handle when replacement is needed.

In a further embodiment, lateral dimensions of the coupling element are essentially equal to dimensions of the tip of the contact element. In particular, the tip may comprise a plane or rounded contact surface for providing a good coupling to the specimen.

In a further embodiment, the coupling element may be adapted to a specific contact element not only in terms of its lateral dimensions but also in the third dimension, in other words, regarding its thickness. In this case, the coupling element comprises a recess that is complementary in shape to the tip of the contact element. Such specific three-dimensional shape of the coupling element may not only improve the coupling for the elastic waves, but it may also improve the adhesion of the coupling element to the contact element.

Generating elastic waves

As described before, a method for non-destructively testing a specimen by means of elastic waves may comprise generating the elastic waves at an impact position on the specimen, in particular by letting an impactor impact on the specimen. The elastic waves may e.g. be generated by manually hitting the specimen, in particular with an impact hammer or steel ball. Such impact hammer may have dimensions of the order of 0.5 to 3 cm, and in particular a weight between 10 g and 3 kg.

Alternatively, generating the elastic waves may include triggering an automatic impactor to hit the specimen. Such impactor may comprise a spring loaded test weight.

In an embodiment, in particular when acquiring data on a pile-like structure, the method further comprises generating the elastic waves at the impact position repetitively, thereby generating several raw signals, in particular several raw signals per measurement position. The several raw signals may then be averaged per measurement position, in particular before determining the travel time. This improves a signal-to-noise ratio of the data, such that arrival times of reflected waves, and hence the travel time, may be more precisely determined.

In an embodiment, in particular when acquiring data on a plate-like structure, the method further comprises changing the impact position and the measurement position between different measurements. In particular, the method comprises generating the elastic waves at a first impact position and measuring elastic waves at a first measurement position, and after that generating the elastic waves at a second impact position and measuring elastic waves at a second measurement position. This means that raw data with the testing device are acquired at different measurement positions on the measurement side of the specimen, e.g. spaced apart from each other by 5 to 100 cm. Usually, the impact position is shifted together with the measurement position. In general, the impact position is in a vicinity of the measurement position, e.g. having a distance between 5 and 100 cm, in particular between 10 and 20 cm, from the measurement position. Advantageously, the distance between impact position and measurement position is below 40% of a wall thickness of the specimen, as specified by standard ASTM C1383-15.

Processing means

In an embodiment, the system further comprises means adapted to execute the steps of a method for non-destructively testing a specimen by means of elastic waves. Such means may include processing means, such as a CPU or an FPGA. Further, the means may be a personal computer, a laptop or a tablet computer in communication with the testing device. Advantageously, the means include a display configured to display a graphical representation of the raw data and/or processed data to a user.

Further, the testing system may comprise an automatic impactor. The automatic impactor advantageously is in communication with the processing means, e.g. with the tablet computer. In particular, the automatic impactor is configured to send a trigger signal to the processing means when the impactor impacts on the specimen. This may be used as a starting time for determining the travel time.

Also, the method described above may be implemented by a computer program. The computer program may then comprise instructions to cause the testing system to execute the steps of the method.

Advantageous features of the device

A further aspect of the invention relates to a testing device, in particular for the above testing system. All features described with respect to the system and the method are also meant to be applicable to the testing device, and vice versa.

The testing device comprises

- a housing, e.g. made of plastic. In particular, the housing is a protective enclosure and e.g. splash- and dust-proof. The housing may have an ergonomic shape to be gripped easily and reliably by a user and e.g. comprise grips.

- the contact element protruding from the housing and advantageously comprising a cavity: In particular, the contact element is adapted to be brought in mechanical contact with, e.g. pressed against, the specimen, in particular with the coupling element interposed, at the measurement position. The contact element advantageously has a plate-like shape. In particular, the contact element consists of one piece, in particular of metal, more particularly of stainless steel, which advantageously does not corrode in contact with concrete. In an embodiment, a part of the contact element protruding from the housing has a conical or rounded or flat shape. Each of these features contributes to a good mechanical coupling of the contact element to the specimen in order to measure elastic waves in the specimen accurately, in particular with high sensitivity. In a different embodiment, the contact element may cover a bottom side of the device entirely, in this way “protruding” from the housing.

- the elastic wave sensor advantageously being mounted in the cavity: Advantageously, the elastic wave sensor is mounted, in particular glued, to a wall of the cavity, in particular a wall that extends along an impact direction of the contact element. This enables a good coupling of the wave sensor to the contact element, and thus to the specimen, such that a high sensitivity, in particular over a large frequency range, e.g. between 10 Hz and 30 kHz, in particular between 100 Hz and 20 kHz, may be achieved. Also, this avoids spurious frequencies in this frequency range, e.g. coming from natural oscillations of the sensor or of the system of sensor and contact element.

The elastic wave sensor may be any sensor suitable for measuring oscillations in the mentioned frequency range. Advantageously, the elastic wave sensor is a piezo or MEMS accelerometer, in particular a capacitive MEMS accelerometer. Such MEMS accelerometer has a high sensitivity and covers the required frequency range. In particular, the raw data representing elastic waves measured by the elastic wave sensor may cover a frequency range from zero to at least 15 kHz, in particular to at least 20 kHz.

In an embodiment, the elastic wave sensor is arranged on a flexible carrier or wires, in particular a flex print, in order to avoid a transmission of vibrations e.g. from the housing via the carrier to the sensor. Further, such carrier avoids that natural oscillations of the carrier are located in the required frequency range. In this respect, it is also advantageous that the flexible carrier extends along an impact direction of the contact element.

Further, the contact element may be mechanically decoupled from the housing via a damping element between the contact element and the housing. In particular, the damping element may attenuate an amplitude of oscillations of the housing by at least 50%, in particular at least 90%, before reaching the contact element. In an embodiment, the damping element may comprise at least one O-ring, e.g. made of rubber. In a further embodiment, the damping element may comprise damping glue.

Further, the contact element may comprise a protrusion held in a corresponding notch of the housing. In particular, the protrusion extends from a circumferential side of the contact element. This facilitates a mechanically decoupled, but still splash- and dust-tight mounting of the contact element in the housing. In such embodiment, the protrusion may be clamped in the notch by means of the damping element, in particular by at least one or two O-rings.

In an embodiment, a diameter of the contact element is between 10 and 50 mm, in particular between 20 and 30 mm. A height of the contact element may be between 5 and 15 mm. Such contact element does not have natural oscillations in the relevant frequency range, which otherwise would lead to spurious frequencies in the raw data.

Advantageously, the protruding part of the contact element protrudes from the housing by at least 2 mm. This ensures that only the contact element - and not the housing - is in contact with the specimen or with the coupling element, even when the testing device is pressed against the specimen or coupling element to ensure a good coupling.

The cavity advantageously has a slot-like shape, which in particular is adapted to a form factor of the elastic wave sensor on the carrier. In particular, the cavity may have a width of 5 mm or less, in particular 3 mm or less, and a length of at least 5 mm. Further, the cavity may have a depth between 5 and 10 mm and/or have rounded edges. For an easy and fix mounting of the sensor in the cavity, the elastic wave sensor may be mounted to the cavity by means of a glob-top. These dimensions and features, again, avoid spurious frequencies in the raw data acquired by the sensor, and make the device robust.

In an embodiment, the contact element is mounted to a first end of the housing, and the housing extends between the first and a second end. A part of the housing extending between the first and the second end may then have a diameter or width between 1 and 10 cm, in particular between 4 and 7 cm. A length of the part of the housing extending between the first and second end may be between 5 and 15 cm, in particular between 7 and 10 cm. Such housing has a small form factor, and is easily and safely gripped by one hand.

Advantageously, the device does not comprise an actuator or impactor. This avoids that vibrations generated by the actuator or impactor propagate inside the device, e.g. along the housing, and affect the measured data. However, the testing system may comprise an actuator or impactor in addition to the testing device. The actuator or impactor may be mounted in an impactor housing. In an embodiment, the impactor housing may then be mounted to the testing device, advantageously in a defined distance, and in particular with a damping element between impactor housing and the testing device’s housing in order to avoid that vibrations from the impactor reach the sensor, which would be detrimental to the measured data.

In an advantageous embodiment, the testing device further comprises a communication module, in particular a Bluetooth transmitter or other wireless transmitter, configured to transmit raw data representing elastic waves measured by the elastic wave sensor. The raw data may then be received by processing means, such as a laptop or tablet computer, and processed according to the above method.

Method of operating the system

A further aspect of the invention relates to a method of operating the above system. The method comprises the following steps:

- abutting the testing device to the specimen at a first measurement position with the coupling element interposed between the contact element and the specimen: The coupling element in particular leads to a better coupling of the contact element to the specimen such that elastic waves from the specimen are better picked up by the elastic wave sensor on the contact element. This facilitates a higher sensitivity of the elastic wave measurements and suppresses spurious resonances due to suboptimal coupling;

- performing at least one elastic wave measurement at the first measurement position: A measurement time window may be triggered e.g. manually or automatically in dependence of the impactor impacting on the specimen;

- abutting the testing device to the specimen at a second measurement position with the coupling element interposed between the contact element and the specimen: With the described coupling element, this is easily done by transferring the coupling element together with the testing device to the second measurement position. In comparison to using putty or gel for coupling the contact element to the specimen, the described procedure saves time and cost;

- performing at least one elastic wave measurement at the second measurement position, and in particular performing further elastic wave measurements at further measurement positions while using the same coupling element;

- replacing the coupling element by another coupling element and in particular repeating the above steps: A time when the coupling element is replaced may depend on an integrity of the coupling element. In particular, the coupling element is advantageously replaced when it shows visual damages or when its elastic properties have changed significantly.

Yet another aspect of the invention relates to a use of a coupling element, in particular of the above coupling element, for non-destructively testing a specimen with a testing device, in particular with the above testing device, by means of elastic waves. Such use comprises interposing the coupling element between the contact element of the testing device and the specimen, in particular for coupling the contact element to the specimen. As described above, the testing device comprises an elastic wave sensor in mechanical connection with the contact element, and the coupling element comprises a layer made from plastic, in particular from polyurethane.

Features described in relation to one aspect of the invention (system, device, method, use) are meant to be disclosed in relation to the other aspects as well. Further advantageous embodiments are listed in the dependent claims as well as in the description below.

Brief Description of the Drawings

The invention will be better understood and objects other than those set forth above will become apparent from the following detailed description thereof. Such description makes reference to the annexed drawings, wherein:

Fig. 1 shows a perspective view of a testing device of the system according to an embodiment of the invention;

Fig. 2 shows elements of the testing device of Fig. 1 in an open state;

Fig. 3 shows a cut through the testing device of Fig. 1;

Fig. 4 shows a part of the testing device of Fig. 1 in an open state;

Fig. 5 shows a detail of a cut through the testing device of Fig. 1;

Fig. 6 shows a testing device and system according to an embodiment of the invention;

Figs. 7a, 7b and 7c show (a) a side view, (b) a vertical cut and (c) a perspective view on a vertical cut, respectively, through a contact element of a testing device and system according to an embodiment of the invention;

Fig. 8 shows a perspective view on a contact element and a coupling element of a system according to an embodiment of the invention; Figs. 9a, 9b and 9c show diagrams with the frequency content of exemplary measurement data acquired using (a) a conventional bare metal contact element without additional coupling element, (b) putty as coupling element, and (c) a coupling element according to an embodiment of the invention, respectively.

Modes for Carrying Out the Invention

Fig. 1 shows a perspective view of an embodiment of the testing device of the above system. The device comprises a housing 1, e.g. made of plastic. For easy usability, in particular for a better grip, the housing 1 comprises knobs 12, e.g. in the form of Swiss crosses, which are advantageously made of plastic or rubber. Further, the housing 1 comprises a fixation 11, e.g. in form of a loop, for mounting a ribbon, which may be used to safely hold the device.

On a top side 17 of the housing, which is situated opposite to a bottom side 18 with the contact element (not shown in Fig. 1), various control and in- put/output elements are located. In general, the device may comprise a button 13 configured to switch on and off the device. The button 13 may further be configured to control the elastic wave sensor (not shown in Fig. 1), in particular the button 13 may be configured to trigger the elastic wave sensor to start a measurement. This is advantageous, compared to other ways of triggering the measurement, e.g. measuring during the whole time in which the device is switched on, since only relevant data are acquired, which simplifies a processing of the data. In particular, it avoids the necessity to define a time window, wherein the relevant data are situated, during processing.

In general, the device may comprise an indicator, e.g. an LED, configured to indicate an operation state of the device, such as “on” or “measuring”. Such indicator may be included in the button 13. Advantageously, the button 13, facilitates a one-button control of the device as described before.

At the top side 17, the device further comprises a Bluetooth module 14 and a USB connector 15, both serving as communication modules, e.g. for transmitting measured raw data to a processing unit. The USB connector 15 may also be used to recharge a rechargeable battery forming a power supply of the device. Such rechargeable battery (see Fig. 2) may be replaced by opening the housing 1 via a knob 16.

The button 13, the Bluetooth module 14 and the USB connector 15 are arranged at the top side 17 of the housing in Fig. 1 for easy accessibility and usability of the device. However, each of these elements, in particular the button 13, may alternatively be arranged differently, e.g. on a side wall of the housing 1, located between top side 17 and bottom side 18.

Fig. 2 shows the testing device of Fig. 1 with a side wall of the housing 1 removed. In particular, only a bottom part 23 and a top part 24 of the housing 1 are shown. Thus, interior elements of the device are visible. The device comprises a contact element 2, which, in this embodiment, has a plate-like shape and has an essentially round cross-section in a horizontal plane. The contact element 2 is made of steel and configured to mechanically couple to the specimen for measuring elastic waves.

The contact element 2 is mounted to the housing 1, in particular to the bottom part 23, by means of a fixation element 21, which may be screwed to the housing. Advantageously, the contact element 2 is mounted to the housing via a damping element (see e.g. Fig. 5), such that it is mechanically decoupled from the housing as described above.

The device further comprises a battery 22, e.g. a rechargeable battery as mentioned before, as power supply.

In Fig. 3, a cut through the device is shown with the same elements as described above. In particular, Fig. 3 shows how the fixation 11 is formed by a recess in the housing 1 and a small rod through the recess, thereby forming a loop for e.g. mounting a ribbon.

The contact element 2 comprises a protruding part, which protrudes from the housing 1 and in particular from the bottom part 23 of the housing, e.g. by at least 2 mm. The protruding part has a rounded shape, or alternatively may be conical. This ensures a good coupling of the contact element to the specimen.

Further, the contact element 2 comprises a cavity 25 in form of a slot for mounting the elastic wave sensor (not shown in Fig. 3). A shape and dimensions of the cavity 25 are advantageously adapted to a shape and dimensions of the elastic wave sensor. Typical dimensions are given above. An alternative contact element, which may replace the contact element 2 of Figs. 3 to 5, is depicted in Figs. 7a to 7c, see below.

Fig. 4 shows the bottom part 23 of the housing with the contact element 2 and the elastic wave sensor 3 mounted on a flexible carrier 31 in the slot of the contact element 2. The sensor 3 is a MEMS accelerometer, which has a high sensitivity and a wide frequency range as described before. Alternatively, the sensor 3 may be of a different type, e.g. a piezoelectric sensor, configured to sense elastic waves in the relevant frequency range, e.g. for testing plate-like and pile-like specimens, as described before.

Advantageously, the sensor 3 is mounted entirely in the cavity 25.

In other words, no part of the sensor 3 protrudes from the cavity 25. Further, the sensor 3 is fixedly glued into the cavity, in particular to at least one side wall of the cavity parallel to the impact direction of the contact element 2. This may in particular be achieved by a glob-top over the sensor 3 and the cavity 25. Such way of mounting the sensor 3 in the contact element 2 avoids that the sensor 3 may pick up spurious frequencies, i.e. signals not representing elastic waves in the specimen, e.g. from oscillations of the sensor 3 itself or of the carrier 31.

The carrier 31 comprises leads to contact the elastic wave sensor 3, and in particular connects the sensor 3 with the communication module and the battery. It is advantageous that the carrier 31 is flexible, e.g. a flex print, since this minimizes spurious oscillations, e.g. from the housing, propagated to the sensor 3 via the carrier.

Fig. 5 shows a cut through the lower part of the device of the previous figures. The contact element 2 is mounted to the bottom part 23 of the housing via the fixation element 21. For this purpose, the contact element 2 comprises a circumferential protrusion 33, which is clamped to the housing, in particular to the bottom part 23, by the fixation element 21. In general, the protrusion 33 may not extend around the entire circumference of the contact element 2, but only cover a part of the circumference. Alternatively, the contact element 2 may comprise several protrusions adapted to mount the contact element 2 to the housing.

In Fig. 5, an O-ring 32, e.g. made from rubber, is arranged between the contact element 2 and the housing, in particular the bottom part 23 of the housing. Such O-ring represents a damping element and is adapted to attenuate oscillations propagating through the damping element, in particular oscillations coming from the housing. In other words, the damping element mechanically decouples the contact element 2 from the housing, at least to a certain degree, e.g. at least 50% or 90%. Further, the O-ring 32 serves as sealing element for protecting the interior of the device, in particular making the housing splash- and dust-tight.

On an opposite side of the protrusion 33, another O-ring 34 is arranged, separating and mechanically decoupling the contact element 2 from the fixation element 21. The other O-ring 34, again, avoids spurious frequencies from external vibrations, e.g. from the housing, to be measured by the elastic wave sensor. In general, the other O-ring 34 may not be necessary, i.e. one O-ring 32 may be sufficient as damping element, e.g. if the fixation element 21 itself exhibits damping properties.

In general, each of the above described features contributes to making the testing device sensitive to elastic waves over a wide range of frequencies, e.g. between 0 and 35 kHz, in particular between 100 Hz and 20 kHz. Thus, such device may be used to probe plate-like specimens, e.g. with an IE method and algorithm, as well as pile-like specimens, e.g. with a PIT method and algorithm, as described above.

Fig. 6 shows a testing system according to an embodiment. The testing system comprises a testing device 5, e.g. as described above. In general, the testing device 5 comprises an elastic wave sensor 51 configured to measure elastic waves in mechanical contact with a specimen 4. As described above, the specimen 4 may have various shapes, e.g. plate-like or pile-like, and it may be of different materials, e.g. concrete, metal or timber. For acquiring data by means of the sensor 51, the device 5 is typically held against a surface of the specimen, e.g. by hand, in particular to establish a good coupling.

As described before, a coupling element according to an embodiment of the invention is advantageously interposed between the contact element with the elastic wave sensor 51 and the specimen 4. Such coupling element further improves the coupling and, thus, sensitivity and data quality. Further such coupling element avoids the necessity to use putty or gel for a good coupling. Exemplary measurement data are shown in Figs. 9a to 9c, see below.

The system further comprises a processing unit 6 configured to receive and process raw data from the device 5. Such processing unit may e.g. be a laptop or a tablet computer, which is convenient to use in the field or on a test site. Alternatively, the processing unit 6 may be comprised in the device 5. Advantageously, the device 5 comprises a communication module 52, e.g. a Bluetooth transmitter, configured to transmit the raw data to a communication module 63 of the processing unit 6. Alternatively, device 5 and processing unit 6 may be connected by a cable, or the processing unit 6 may be an integral part of the testing device 5, i.e. arranged in the same housing.

Advantageously, the processing unit 6 comprises first processing means 61 for carrying out a first processing algorithm, such as an IE algorithm, and second processing means 62 for carrying out a second processing algorithm, such as a PIT algorithm. The first and second processing means 61, 62 may be implemented e.g. as two separate or one common CPU or FPGA.

Further, the processing unit 6 may comprise a display 64 connected to the first and second processing means 61, 62 and configured to display a graphical representation of the raw data or processed data to the user. In particular, the processing unit 6 may be configured to carry out the method as described above.

Such testing system has the advantage that it may be used to probe specimens of different shapes and materials, in particular by applying different types of analysis to the raw data, e.g. in time domain as well as in frequency domain. Thus, the user only needs one testing system, i.e. one testing device plus processing unit, for performing non-destructive testing in the field or on a test site. This is more convenient than conventional solutions requiring at least two testing devices, and it saves time and money.

Figs. 7a, 7b and 7c show different views of an alternative advantageous contact element 72. Such contact element may be implemented in a testing device as described before, e.g. as replacement of the contact element 2 in Figs. 3 to 5. The contact element 72 has a bottom side 71, which, in the intended use, forms the contact surface to the specimen to-be-tested. The contact surface may be flat as shown, or it may have a different shape, e.g. rounded or conical.

As illustrated in Figs. 7b and 8, a coupling element 81 as described above is arranged at the bottom side 71 of the contact element 72 for a better coupling to the specimen. The coupling element 81, in the shown embodiment, is made from a sheet of plastic, e.g. polyurethane, and has a circular shape. In other embodiments, the shape of the coupling element 81 may e.g. be rectangular or hexagonal. Advantageously, the coupling element 81 comprises an adhesion layer on one side for removable adhesion to the contact surface of the contact element 72. For an easier transport and better adhesion, the coupling element 81 may additionally have a transfer film on the adhesion layer before the measurements, which is removed for adhering the coupling element to the contact surface.

The contact element 72 further comprises a circumferential protrusion 73 for clamping the contact element 72 to the housing as described above with respect to Fig. 5. Further, the contact element 72 comprises at least one circumferential indentation 74 adapted to receive an O-ring for mechanically decoupling the contact element 72 from the housing, as similarly described with respect to Fig. 5. Further, the contact element 72 may comprise a cavity, e.g. a slot 75, in which the elastic wave sensor is mounted. This, again, facilitates a high sensitivity and signal-to-noise ratio of the device and the measured elastic wave data.

Figs. 9a to 9c show the frequency content of exemplary measurement data, in particular the amplitude of the Fourier-transformed measurement data over the frequency for three different measurement setups. In each case, the same testing device and contact element with a flat tip made of stainless steel is used. The desired frequency component from which information about the specimen is retrieved, also called “signal”, is around 7.5 to 8 kHz in the example data, see the dashed line in Figs. 9a to 9c. Further, in each of the three cases, the data are shown for three different impacts, which naturally differ from each other, e.g. due to changes in the coupling of the contact element to the specimen but also due to changes in the impacts themselves.

In recording the data of Fig. 9a, the contact element of the testing device directly abuts on the specimen, i.e. without any coupling element interposed. It is observed that, while the “signal” at around 7.5 to 8 kHz has the largest amplitude, also frequency components at lower frequencies, e.g. at around 4 kHz and 6 kHz, have significant amplitudes. Further, it is observed that the frequency components significantly differ in amplitude between the different Impacts 1, 2 and 3. This means that, without any coupling element, the signal-to-noise ratio of the measured data is rather low, and that the repeatability of the measurements is poor, in particular due to a bad and inconstant coupling.

Fig. 9b shows comparable measurement data for the case that putty is applied for a better coupling between the contact element and the specimen. It is observed that the “signal” amplitude at around 7.5 to 8 kHz is significantly larger than other frequency components (“noise”), and that the frequency spectra of the three impacts have a similar course. This means that the conventional method of applying putty at each measurement position leads to a good signal-to-noise ratio and a good repeatability of the measurements.

Fig. 9c shows comparable measurement data when using a coupling element according to an embodiment of the invention. The coupling element is made from 0.8 mm thick polyurethane, as e.g. manufactured by 3M under the name SJ5632. It is observed that the “signal” amplitude at around 7.5 to 8 kHz is significantly larger than other frequency components (“noise”), leading to a good signal-to- noise ratio. Further, the frequency spectra of the measured data from the three impacts have a similar course but different amplitudes, meaning that the measurements show a reasonably good repeatability. In conclusion, using a contact element according to the invention (Fig. 9c) leads to measurement data with a good signal-to-noise ratio and a good repeatability. In particular, signal-to-noise ratio and repeatability are much better than without any coupling element (Fig. 9a) and comparable to using putty (Fig. 9b). At the same time, the coupling element according to the invention is simpler to use than putty and may, in contrast to putty, in particular be re-used at a multitude of measurement positions. This saves material, time and cost.