Ultrasound activated Lockin-Thermography ("ultrasound attenuation mapping") is a defect selective NDT-technique. Its main advantage is a high probability of defect detection ("POD") since only defects produce a signal while all other features are suppressed. The mechanism involved is local sound absorption which turns a variably loaded defect into a heat source. Thermographic monitoring of elastic wave attenuation in defects was reported for the first time in 1979 by Henneke and colleagues for continuous and pulsed ultrasound injection. Later, amplitude modulated ultrasound was used to derive frequency coded phase angle images combining defect-selectivity with robustness of measurement. With mono-frequent ultrasound excitation a standing wave pattern might hide defects. With additional modulation of the ultrasound frequency such a misleading pattern can be minimized. Applications related to quality maintenance (aerospace, automotive industry) will be presented in order to illustrate the potential of frequency modulated ultrasound excitation and its applications.
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. This 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. That way the standing waves can be eliminated or reduced and the detectability is improved.
Defect selective non-destructive testing methods are increasingly being used for safety relevant applications in aerospace and related fields. The main advantage is the clarity in interpretation of such methods: defects are revealed while non- relevant structural information is suppressed. One way to achieve defect selectivity is the combination of mechanical load to generate heat inside a sample and thermography to detect the resulting thermal waves. Ultrasound Lock-in thermography is one example of an established defect selective testing method described previously. In this paper, several kinds of excitation sources and modulation shapes are discussed.
Ultrasound Phase Thermography (UPT) is a non-destructive technique derived from Ultrasound Lock-In Thermography (ULT) which was established a few years ago. UPT provides defect selective imaging using thermal waves generated by elastic waves. The use of short ultrasound bursts instead of sinusoidal signals for excitation allows for faster measurements and better reproducibility as compared to ULT. However, the advantages of phase images are the same: recognition of defect depth and suppression of temperature gradients. Application of UPT to typical defects of aircraft materials and components provide specific information on their nature.
Aerospace structures are subjected to variable loads over long periods with rapid changes of conditions (e.g. humidity, temperature). Therefore the materials and components made out of them may suffer from aging and deterioration, especially since the weight of such structures is an important quantity. On the other hand, any failure of a component may cause costs that exceed by many orders of magnitude the cost of the component itself. On this background it is important to identify defects reliably and early enough during production or maintenance inspections in order to avoid catastrophic failure. This is the general and important task of nondestructive evaluation. We present a method where thermal effects are selectively activated in defects so that defects reveal themselves selectively even in the presence of complicated intact features. The mechanism involved is local friction or hysteresis which turns a variably loaded defect into a heat source which is identified by thermography. Loading is achieved by an elastic wave or oscillation with a suitable time dependence. The method is presented together with results obtained on aerospace structures.
Elastic waves sent into the component propagate inside the sample until they are converted into heat. A defect causes locally enhanced losses and consequently selective heating up. Therefore amplitude modulation of the injected elastic wave turns a defect into a thermal wave transmitter whose signal is detected at the surface by lock-in thermography synchronized to the frequency of amplitude modulation. This way ultrasound lock-in thermography allows for selective defect detection which enhances the probability of defect detection in the presence of complicated intact structures. Measurements were performed on various kinds of mechanical joints. The results of our feasibility study indicate that both optical (OLT) and ultrasound exited lock-in thermography (ULT) are reliable tools for the rapid identification of damaged riveted structures. We demonstrate that one can locate a screw joint or a riveting which provide only a reduced stress. Investigations on airplane components were performed which confirmed the applicability of lock-in thermography for remote maintenance inspection within a few minutes.
Lockin thermography is currently being used for the rapid and remote identification of subsurface structures and defects such as impact damages, delaminations, and hidden corrosion. The purpose of this paper is to show that lockin thermography is also a reliable tool to inspect in a remote way the tightness of mechanical joints in safety relevant structures (e.g. aerospace equipment and vehicles). For example, the rapid identification of loose rivets is a major concern for airlines and manufacturers in order to monitor the structural integrity of their aircraft. Our measurements aimed at the early detection of loose rivets. We analyzed the phase image signature obtained on two metal plates pressed together by screws fastened at various torque levels. A clear relationship was established between phase angle and torque level at which the screws had been fastened. Based on these results our investigations were extended to riveted samples where two aluminum plates were pressed together by an array of ten blind rivets. Also in this case the level of tightness of rivet joints can be detected. In addition to these feasibility studies on model samples, we performed investigations on airplane components which confirmed the applicability of lockin thermography for remote maintenance inspection within a few minutes.
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