Gerd Busse

Thermal wave imaging with phase sensitive modulated thermography

Thermography and thermal wave techniques can be combined to provide in a short‐time low‐frequency phase angle images where nonthermal features can be suppressed. The principle is optical thermal wave generation simultaneously on the whole sample surface and sequential monitoring of all pixels using both thermographic techniques and lock‐in data analysis. Due to parallel stationary excitation one can use low modulation frequencies allowing for a depth range that is of relevance for applications.

High‐resolution photoacoustic thermal‐wave microscopy

Experiments on photoacoustic thermal‐wave microscopy are presented, for the first time, at high resolution. Using piezoelectric detection and a modulation frequency of 185 kHz, we obtain a thermal‐wave resolution of ∼7 μm. We show that the photoacoustic phase signal provides true thermal‐wave imaging even in the presence of surface features with strong optical contrast.

Lock-in thermography for nondestructive evaluation of materials

Abstract Photothermal radiometry allows for remote measurement of local harmonic heat transport where the phase angle (between remote optical energy deposition and resulting temperature modulation) is sensitive to subsurface features or defects. Phase sensitive modulation thermography (or ‘lock-in thermography’) combines the advantages of photothermal radiometry with the fast technique of infrared imaging thereby revealing hidden defects in a short time. In this paper the principle and various applications are described and analyzed. While this lock-in thermography is based on remote optical heating of the whole area of interest, one can heat defects selectively with modulated ultrasound which is converted into heat by the mechanical loss angle effect which is enhanced in defect regions. This ‘ultrasonic lock-in thermography’ provides images showing defects in a way that is similar to dark field imaging in optical microscopy.

Subsurface imaging with photoacoustics

We have performed thermal‐wave imaging of subsurface features in a metal using, for the first time, piezoelectric detection. Photoacoustic magnitude and phase images are presented.

Amplitude-Modulated Lock-In Vibrothermography for NDE of Polymers and Composites

We present a nondestructive testing method based on lock-in thermography with mechanical heat excitation. Stresses are generated in the sample by vibrating it with a mechanical shaker. The mechanical energy is converted to thermal energy due to the acoustical damping. The defected regions have a stronger damping and also a stress concentration next to them, both of which result in a higher temperature generation. Because of the changes of the thermal properties, the defects also affect the heat conduction. These phenomena result in thermal anomalies due to the defects. The high-frequency vibration used for excitation is amplitude-modulated with a low frequency. The magnitude and phase of the sample temperature with respect to the modulation are measured with an infrared camera and a software lock-in technique. The use of phase information increases the reliability of the defect detection, and the application of high vibration frequencies results in a good thermal signal even at low stress levels, which helps to keep the test truly nondestructive. The suitability of the method was proved with samples of CFRP and aramid composites, and different polymers. The measurements included detection of impact damages, inclusions, voids, and cracks, and the evaluation of stress level distributions, paint thicknesses, and quality of bondings.

Optoacoustic and photothermal material inspection techniques.

Infrared photothermal material inspection in a transmission arrangement is compared to optoacoustic imaging with respect to depth range, resolution, and interpretation of results. The photothermal method seems to be advantageous, but depth information is not obtained.

Nonlinear air-coupled emission: The signature to reveal and image microdamage in solid materials

It is shown that low-frequency elastic vibrations of near-surface planar defects cause high-frequency ultrasonic radiation in surrounding air. The frequency conversion mechanism is concerned with contact nonlinearity of the defect vibrations and provides efficient generation of air-coupled higher-order ultraharmonics, ultrasubharmonics, and combination frequencies. The nonlinear air-coupled ultrasonic emission is applied for location and high-resolution imaging of damage-induced defects in a variety of solid materials.

A local defect resonance to enhance acoustic wave-defect interaction in ultrasonic nondestructive evaluation

It is experimentally shown that, to provide maximum acoustic wave-defect interaction, the concept of a local defect resonance should be applied. The model of a resonant defect is used for the selection of the wave frequency to enhance the excitation of the defect in nonlinear acoustics and ultrasonic thermography. An increase in nonlinear response of the defect at its local resonance exceeds substantially the one at natural frequencies of the specimen. The strong wave-defect interaction is confirmed by resonance induced rise of local temperature of the defect in the frequency band of its local resonance.

Lockin thermography with eddy current excitation

A new lockin thermography method is presented where inductive heating by eddy current is used for periodical sample excitation. In conventional eddy current testing damages like cracks in metals are detected with a coil which changes its impedance over defect areas. The disadvantage of this method is its time consuming point-by-point scanning over the surface of the sample. Induction-Lockin-Thermography (ILT), however, uses a thermography camera with a detector array to monitor induction heated areas. Temperature patterns and their time dependence responding to the coded excitation allow for fast phase angle imaging of defects in larger areas.

Resonant ultrasound spectroscopy of defects: Case study of flat-bottomed holes

Unlike conventional resonant ultrasonic spectroscopy aimed at determining elastic constants and related parameters of solids, resonant ultrasound spectroscopy of defects (RUSOD) addresses an opportunity to detect, visualize, and classify mechanical defects in materials. The approach is based on the resonant ultrasonic wave-defect interaction due to local defect resonance. RUSOD is shown to be defect- and frequency selective imaging technique capable of distinguishing between different defects by variation of ultrasonic frequency.