Nondestructive evaluation of low-velocity impact-induced damage in basalt-carbon hybrid composite laminates using eddy current-pulsed thermography

Recently, basalt-carbon hybrid composite structures have attracted increasing attention due to their better damage tolerance, if compared with carbon fiber-reinforced polymer composites (CFRP). Low-velocity is considered as one of the most severe threats to composite materials as it is usually invisible and it occurs frequently in service. With this regard, nondestructive testing (NDT) techniques, especially emerging modalities, are expected to be an effective damage detection method. Eddy current-pulsed thermography (ECPT), as an emerging NDT technique, was used to evaluate the damage induced by low-velocity impact loading in a CFRP laminate, as well as in two different-structured basalt-carbon hybrid composite laminates. In addition, ultrasonic C-scan and x-ray computed tomography were performed to validate the thermographic results. Pulsed phase thermography, principal component thermography, and partial least squares thermography were used to process the thermal data and to retrieve the damage imagery. Then, a further analysis was performed on the imagery and temperature profile. As a result, it is concluded that ECPT is an effective technique for hybrid composite evaluation. The impact energy tends to create an interlaminar damage in a sandwich-like structure, while it tends to create an intralaminar damage in an intercalated stacking structure

Flying-spot lock-in thermography and its application to thickness measurement and crack detection

A heating laser beam was scanned periodically along a testing path on the surface of a test object. The thermal response of the beam was recorded by an infrared camera. Using a lock-in thermography algorithm, amplitude and phase images were generated. The phase image is corrected for effects due to the beam movement. A first application shows the contact-free determination of the steel sheet thickness at forming edges. Calibration of phase values to thickness was achieved by using an analytical model of thermal wave transmission. A second application is the detection of a perpendicular surface crack in steel

Theoretical and Experimental Analysis of the Thermal Response in Induction Thermography in the Frequency Range of 2.5 Hz to 20 kHz

The one-dimensional propagation of electromagnetic waves and the propagation of the resulting thermal waves in conducting material are analysed in a coherent way. The heat release due to resistive losses has a static and an oscillating part. Both are considered as heat source terms for the thermal diffusion equation. The time dependence of the temperature is described by analytical solutions. Electrically and thermally conducting materials are classified by the ratio of thermal penetration depth to the skin depth. Experiments performed on ferritic steel, stainless steel and carbon-fibre-reinforced polymer show the time dependence of the thermal signal after heating begins, as described by the theory. At low induction frequencies, an oscillating part of the surface temperature at the double of the induction frequency is detected in accordance with the theory. The results point out new opportunities for induction thermography

Nondestructive Testing, 4. Thermography

In thermal nondestructive testing, interactions of heat flows with the internal structures of test objects are used to detect defects or to characterize coatings. The dominant concept is active thermography, in which additional heat is applied to the test object in an appropriate way to generate thermal contrast. Various heat sources and detectors are employed The most useful information is obtained when heat is applied as a short single pulse or by periodic modulation (dynamic thermography). Then the characteristic thermal penetration is governed by material properties such as thermal diffusivity. Contrast formation from defects is determined by the thermal effusivity, which is the square root of the product of thermal conductivity, mass density, and specific heat capacity. As defects like cracks, inclusions, and pores usually have a different effusivity than the matrix, application of thermal testing is versatile. The techniques can be applied to nearly all materials

Theoretical and Experimental Analysis of the Thermal Response in Induction Thermography in the Frequency Range of 2.5 Hz to 20 kHz

The one-dimensional propagation of electromagnetic waves and the propagation of the resulting thermal waves in conducting material are analysed in a coherent way. The heat release due to resistive losses has a static and an oscillating part. Both are considered as heat source terms for the thermal diffusion equation. The time dependence of the temperature is described by analytical solutions. Electrically and thermally conducting materials are classified by the ratio of thermal penetration depth to the skin depth. Experiments performed on ferritic steel, stainless steel and carbon-fibre-reinforced polymer show the time dependence of the thermal signal after heating begins, as described by the theory. At low induction frequencies, an oscillating part of the surface temperature at the double of the induction frequency is detected in accordance with the theory. The results point out new opportunities for induction thermography.</jats:p

Nondestructive testing of electric contacts by time-resolved infrared radiometry

The bonding of small electric contacts consisting of small platelets of silver alloys welded to thin copper sheets has been tested by time-resolved photothermal radiometry with step-function surface heating. The contact platelet can be regarded as a heat sink coupled to the substrate by the thermal resistance of the welding joint. Identification of good and poor bonding is possible within 50 ms by analysing the normalized signal of the temperature response