Ultrasound Lock-in Thermography Research Articles

In-Plane Heatwave Thermography as Digital Inspection Technique for Fasteners in Aircraft Fuselage Panels

The inspection of fasteners in aluminium joints in the aviation industry is a time consuming and costly but mandatory task. Until today, the manual procedure with the bare eye does not allow the temporal tracking of a damaging behavior or the objective comparison between different inspections. A digital inspection method addresses both aspects while resulting in a significant inspection time reduction. The purpose of this work is to develop a digital and automated inspection method based on In-plane Heatwave Thermography and the analysis of the disturbances due to thermal irregularities in the plate-like structure. For this, a comparison study with Ultrasound Lock-in Thermography and Scanning Laser Doppler Vibrometry as well as a benchmarking of all three methods on one serviceable aircraft fuselage panel is performed. The presented data confirm the feasibility to detect and to qualify countersunk rivets and screws in aluminium aircraft fuselage panels with the discussed methods. The results suggest a fully automated inspection procedure which combines the different approaches and a study with more samples to establish thresholds indicating intact and damaged fasteners.

Study of inspection on metal sheet with subsurface defects using linear frequency modulated ultrasound excitation thermal-wave imaging (LFM-UTWI)

The linear frequency modulated ultrasound excitation thermal wave imaging (LFM-UTWI) was investigated on detection of subsurface defects of metal sheet. A numerical finite element analysis is carried out to calculate thermal wave signal dependence of time by linear frequency modulated ultrasonic wave excitation. Cross-correlation operation in time domain and frequency domain are used to extract the main peak value and the corresponding delay time, respectively. Fourier transform (FT) is applied to calculate the amplitude and phase angle of harmonic component of thermal wave. Experimental results show that various deep subsurface defects are readily detected using LFM-UTWI with once excitation, and LFM-UTWI has an advantage of better defect detectability compared to ultrasound lock-in thermography (ULIT).

Research on the Quantitative Analysis of Subsurface Defects for Nondestructive Testing by Ultrasound Lock-In Thermography

This paper describes the quantitative analysis of the shape, boundary, and depth of subsurface defects by ultrasound lock-in thermography. The phase difference between defective areas and non-defective areas illustrates the qualitative analysis of the shape and the boundary of the subsurface defect. In order to accurately estimate the shape, boundary and depth of the defects, the optimal threshold value method is proposed to identify the shape and boundary of the subsurface defects based on the canny operator of image processing. A self–adaption artificial neural network (ANN) with Takagi-Sugeno modeling is proposed to determine the depth of the subsurface defect. Experimental results for a steel plate with artificial subsurface defects show good agreement with actual values.

Detection of Unbonds in Honeycomb Sandwich Structure Using Lock-In Thermography

Lock-in thermography (LT), that is active infrared testing technology, mainly includes optical lock-in thermography (OLT) and ultrasound lock-in thermography (ULT). LT can be used to detect unbonds between honeycomb core and face sheet of sandwich structures. However, modulation frequency is an important influencing factor. In this paper, the principles of LT are represented, in experimental detections of simulated unbonds in honeycomb sandwich structures with Al-face sheet and CFRP-face sheet using OLT and ULT, detectability of OLT and ULT is compared and analyzed, effect of modulation frequency is researched and the optimal frequencies are obtained.

Improved ultrasound activated thermography using frequency analysis

Ultrasound activated thermography is based on intense acoustic wave interaction with defects. Internal friction in defect areas results in a local temperature rise. The lockinapproach is applied by using a low-frequency amplitude modulation of acoustic excitation to enhance the sensitivity of detection of weak heat sources (ultrasound lockin-thermography). A new approach to detect regions of damage includes the frequency dependence of heat generation: defects like cracks show characteristic frequencies where mechanical losses have a maximum. In order to activate all defects, a wideband frequency-sweep is used. Intact regions of the material under inspection do not significantly change their properties with frequency, so that a defect selective methodology is obtained.

Ultrasound lock-in thermography for advanced depth resolved defect selective imaging

Ultrasound lock-in thermography ('attenuation mapping') is a defect selective 'dark field' NDT technique with a high probability of defect detection (PoD), since only defects produce a signal while other features are suppressed. The basic contrast mechanism is the enhanced local mechanical loss turning a variably loaded defect into a heat source. The method is being applied for quality maintenance, for example in the aerospace and automotive industry to monitor the integrity of thermal features. A variety of examples will be presented to illustrate how well the method is suited to locate defects and to distinguish their depths.

Ultraschall-Burst-Phasen- Thermografie

Phased Ultrasonic Burst Thermography. Phased ultrasonic burst thermography is a new nondestructive technique originating from ultrasound lock-in thermography. It provides thermal wave imaging of defects by ultrasonic stimulation. Relatively short ultrasound bursts yield faster measurements and better reproducibility in relation to the well established lock-in method. The advantages of phase images remain valid: the recognition of defect depth and the suppression of temperature gradients. Application of phased ultrasonic burst thermography to typical defects of aircraft components and ceramic turbine blade coatings provide specific information on their nature.