Ultrasound Excited Thermography Research Articles

Multisource ultrasound-excited thermography for detecting concrete microcracks in hydraulic structures

ABSTRACT Hydraulic concrete structures often suffer from cracking that could lead to engineering accidents. Thus, it is important to identify emerging concrete cracks so that the premonitory danger of the components can be comprehensively evaluated, and the disaster management level can be advanced to the initial stage of the hazard. This study investigated the use of ultrasound-excited thermography to detect surface microcracks in concrete engineering components. A detection system and method for the synergistic excitation of multiple excitation sources were developed, and self-adaptive pressure-loading sleeves containing back shafts made of glass fibre-reinforced nylon were developed to exert pressure on the transducers. Eighteen groups of in-situ experiments were conducted on important parts of the key component using a combination of 28-kHz and 40-kHz-transducers with a power of 100 W. The results demonstrate that ultrasound-excited thermography with the synergistic excitation of multiple excitation sources can be applied to detect surface cracks in approximately semi-infinite reinforced concrete members in-situ. The radius of the current investigation of cracks stimulated by one 100-W transducer pressed by the self-adaptive sleeve in reinforced concrete members is as far as 300 mm. The minimum widths of the detected cracks are 0.03 mm.

Sizing the length of surface breaking cracks using vibrothermography

Ultrasound excited thermography is used to determine the length of vertical surface breaking cracks. Two methods are proposed based on the analysis of the maximum temperature reached along the line containing the crack and the time at which the maximum temperature occurs. It is shown that, for short bursts, the full width at half maximum of the maximum temperature curve provides the crack length, and that the time at which the maximum temperature occurs increases beyond the crack tip. The range of application of the methods is analysed and the validity is checked taking data on samples containing artificial calibrated cracks.

Ultrasound-excited thermography for detecting microcracks in concrete materials

This study investigated the use of ultrasound-excited thermography to detect microcracks in concrete materials. Pre-cracked concrete specimens full of standardised microcracks were fabricated to quantify the detectability of cracks. These cracks were independently assessed in terms of width by identifying the separation between two crack faces and measuring their surface widths. Thereafter, excitation experiments were performed. Three sets of specimens were excited with ultrasound frequencies of 20 and 40 kHz. The temperature fields of specimens were observed with a thermal imager and specified through a quantitative analysis with regard to crack width and temperature rise. Detectable widths of cracks excited with a remarkable temperature rise were further analysed based on width statistics of the surface cracks. The analysis showed that ultrasound-excited thermography effectively detected concrete cracks having widths of 0.01–0.09 mm.

Crack detection in oak flooring lamellae using ultrasound-excited thermography

Today, a large number of people are manually grading and detecting defects in wooden lamellae in the parquet flooring industry. This paper investigates the possibility of using the ensemble methods random forests and boosting to automatically detect cracks using ultrasound-excited thermography and a variety of predictor variables. When friction occurs in thin cracks, they become warm and thus visible to a thermographic camera. Several image processing techniques have been used to suppress the noise and enhance probable cracks in the images. The most successful predictor variables captured the upper part of the heat distribution, such as the maximum temperature, kurtosis and percentile values 92–100 of the edge pixels. The texture in the images was captured by Completed Local Binary Pattern histograms and cracks were also segmented by background suppression and thresholding.The classification accuracy was significantly improved from previous research through added image processing, introduction of more predictors, and by using automated machine learning. The best ensemble methods reach an average classification accuracy of 0.8, which is very close to the authors’ own manual attempt at separating the images (0.83).

Fatigue crack detection on structural steel members by using ultrasound excited thermography

Fatigue crack detection on structural steel members by using ultrasound excited thermography

Composite Evaluation Using Contact and Non-Contact Ultrasound Excited Thermography

In ultrasound excited thermography method, high energy ultrasound is used as exciting source which can stimulate the defects such as crack and disband to produce heat. The heat excited would lead to temperature increase. The surface temperature would be captured and recorded by a thermal camera and further analysis would done. In this method, the defect is selectively stimulated, which makes this method suitable for small crack detection. Compared to contact ultrasound excited thermography method in which the sample to be tested is contacted with the high energy ultrasound source, in non-contact ultrasound excited thermography method, the sample has no contact with the horn of the high energy source and the energy is coupled by air. The non-contact method is suitable for the application in which even and not very high ultrasound energy is required or the sample is fragile. CFRPs with impact damage and a composite component with cracks were inspected with these two methods, the results show that ultrasound excited thermography method is suitable for composite evaluation.

Use of Ultrasound Excited Thermography Applied to Massive Steel Components: Emerging Crack Detection Methodology

In the field of nondestructive testing of structural components, active thermography methods are increasingly in demand. The most experience is available in testing of carbon-fiber-reinforced plastic (CFRP) or similar types of composite material. One of the emerging techniques is ultrasound excited thermography, which has not been fully transferred to massive steel members used in constructional steelwork thus far. The idea of ultrasound excitation is to generate elastic waves that propagate inside the investigated structure. In case of internal flaws, such as cracks, the boundary faces move relative to each other. The resulting rubbing and clapping of crack faces generate frictional heat, which is detected by means of an infrared camera. This paper demonstrates the usage of high-frequency mechanical excitation to detect cracks in hot-rolled girders and reduced plate specimens. The ultrasonic lock-in and the ultrasonic sweep thermography approaches are presented. Localized heating of the crack regions and distributed heating patterns caused by material damping can be observed. The influences of the tuned frequency and the crack depth as well as effects of prestressing and repeated excitation are discussed. In addition to experimental results, a finite-element simulation of the thermostructural problem is conducted. The model can be easily adjusted to match the experimental results of a performed ultrasonic sweep thermography.

Structural-thermal finite element simulation of vibrothermography applied to cracked steel plates

One of the major drawbacks of ultrasound excited thermography applied to metal components manifests in the pronounced frequency dependence of crack detectability. This leads to an uncertainty in the application of the method. In order to investigate the crack face interaction by which it is constituted whether a certain crack can be detected or not a structural-thermal Finite Element simulation of a massive steel plate is conducted. The roughness of the crack faces is taken into account by generating a random asperity distribution. With this type of modelling the achieved numerical results are in a good agreement with experimental data of a performed ultrasonic sweep thermography (UST). Further investigation of the crack face morphology after repeated ultrasound excitation reveals that the true contact area is very small compared to the apparent contact area. The FE simulation approach allows for a realistic hot-spot size from which frictional heat is generated.

Ultrasound excited thermography of thickwalled steel load bearing members

Ultrasonic lockin and burst-phase thermography establish more and more as failsave NDT methods in the scope of inspection of light-weight composite structures. The methods were not fully transferred to heavy load bearing members used in constructional steelwork so far. The investigation of three massive components, a steel plate, a 3 m long hot rolled girder and an extremely thick-walled welded joint revealed the applicability of ultrasonic lockin, burst and sweep procedures. The influence of the coupling location and the thermal response behaviour of single and multiple defects are discussed. The use of differential laser vibrometry is introduced in order to examine the extent of relative motion of crack faces and to analyse the connection between mode III oscillation velocity and crack detectability.

Ultrasound excited thermography of thickwalled steel load bearing members

Ultrasonic lockin and burst-phase thermography establish more and more as failsave NDT methods in the scope of inspection of light-weight composite structures. The methods were not fully transferred to heavy load bearing members used in constructional steelwork so far. The investigation of three massive components, a steel plate, a 3 m long hot rolled girder and an extremely thick-walled welded joint revealed the applicability of ultrasonic lockin, burst and sweep procedures. The influence of the coupling location and the thermal response behaviour of single and multiple defects are discussed. The use of differential laser vibrometry is introduced in order to examine the extent of relative motion of crack faces and to analyse the connection between mode III oscillation velocity and crack detectability.

DEFECT DETECTION WITHIN A PIPE USING ULTRASOUND EXCITED THERMOGRAPHY

An UET (ultrasound excited thermography) has been used for several years for a remote non-destructive testing in the automotive and aircraft industry. It provides a thermo sonic image for a defect detection. A thermograhy is based On a propagation and a reflection of a thermal wave, which is launched from the surface into the inspected sample by an absorption of a modulated radiation. For an energy deposition to a sample, the UET uses an ultrasound excited vibration energy as an internal heat source. In this paper the applicability of the UET for a realtime defect detection is described. Measurements were performed on two kinds of pipes made from a copper and a CFRP material. In the interior of the CFRP pipe (70mm diameter), a groove (width - 6mm, depth - 2.7mm, and length - 70mm) was engraved by a milling. In the case of the copper pipe, a defect was made with a groove (width - 2mm, depth - 1mm, and length - 110 mm) by the same method. An ultrasonic vibration energy of a pulsed type is injected into the exterior side of the pipe. A hot spot, which is a small area around the defect was considerably heated up when compared to the other intact areas, was observed. A test On a damaged copper pipe produced a thermo sonic image, which was an excellent image contrast when compared to a CFRP pipe. Test on a CFRP pipe with a subsurface defect revealed a thermo sonic image at the groove position which was a relatively weak contrast.

Ultrasound excited thermography - advances due to frequency modulated elastic waves

Ultrasound excited thermography allows for defect selective imaging using thermal waves that are generated by elastic waves. The mechanism involved is local friction or hysteresis which turns a dynamically loaded defect into a heat source which is identified by a thermography system. If the excitation frequency matches to a resonance of the vibrating system, temperature patterns can occur that are caused by standing elastic waves. These undesirable patterns can affect the detection of damage in a negative way. We describe a technique how the defect detectability of ultrasound activated thermography can be improved. With the objective of a preferably diffuse distributed sonic field we applied frequency modulated ultrasound to the material. That way the standing waves can be eliminated or reduced so that the detectability is significantly improved.

Ultrasound excited thermography using frequency modulated elastic waves

Ultrasound excited thermography allows for defect selective imaging using thermal waves that are generated by elastic waves. The mechanism involved is local friction or hysteresis which turns a dynamically loaded defect into a heat source which is identified by a thermography system. If the excitation frequency matches to a resonance of the vibrating system, temperature patterns can occur that are caused by standing elastic waves. These undesirable patterns can affect the detection of damages in a negative way. We describe a technique how the defect detectability of ultrasound activated thermography can be improved. With the objective of a preferably diffuse distributed sonic field we applied frequency modulated ultrasound to the material. This way the standing waves can be eliminated or reduced and the defect detectability is improved.