Applications of laser-spot thermography to thermal diffusivity and crack measurements on static and moving samples

176 p.Esta tesis tiene como objetivo explorar la termografía infrarroja con excitación óptica focalizada para estudiar la difusividad térmica en el plano y para la caracterización de grietas. Por un lado, se investiga la difusividad de todo tipo de materiales, midiendo muestras estáticas y también muestras en movimiento a velocidad constante. Se usa tanto iluminación continua como modulada, y los materiales investigados cubren un amplio rango de valores de conductividad, transparencia y grosor, incluyendo también el estudio de materiales anisótropos. Por otra parte, se utiliza iluminación modulada para caracterizar grietas calibradas con penetración infinita, con el objetivo de determinar su anchura y ángulo de inclinación, así como también para cuantificar la anchura y penetración de grietas finitas presentes en muestras reales

Laser-spot step-heating thermography to measure the thermal diffusivity of solids

In this work we show that laser-spot step-heating thermography allows measuring the thermal diffusivity of solids accurately. It consists in illuminating the sample surface with a continuous-wave focused laser spot and recording the time evolution of the surface temperature with an infrared camera. The inherent noise associated to time domain measurements is reduced by applying a very simple image processing procedure. The methodology has been tested on reference samples. Unlike laser spot lock-in (or pulsed) thermography, a step-heating thermography device can be easily put together combining a low-end laser and an entry level IR camera.This work has been supported by Ministerio de Ciencia e Innovación (PID2019-104347RB-I00, AEI/FEDER, UE), by Gobierno Vasco (PIBA 2018–15) and by Universidad del País Vasco UPV/EHU (GIU19/058)

Determining the thermal diffusivity and the principal directions on anisotropic moving samples with laser-spot thermography

We propose a method to measure the thermal diffusivity of (an)isotropic samples moving at constant speed using laser-spot thermography with continuous illumination. The method does not require previous knowledge of the principal directions and can be applied whichever their orientation with respect to the direction of motion. We demonstrate analytically that any radial temperature profile crossing the center of the laser spot fulfills a linear relation with the distance to the laser spot, whose slope depends on the thermal diffusivity, on the sample speed and on the angle of the profile with respect the sample motion. A multi-linear fitting of the temperature profiles at several angles provides a reliable method to obtain the thermal diffusivity. Experiments performed on reference samples, both isotropic and anisotropic, con- firm the validity of the method to provide accurate and precise thermal diffusivity values and to identify the thermal principal directions.This work has been supported by Ministerio de Ciencia e Innovación (PID2019-104347RB-I00, AEI/FEDER, UE), by Gobierno Vasco (PIBA 2018-15) and by Universidad del País Vasco UPV/EHU (GIU19/058)

Sizing the Depth and Width of Narrow Cracks in Real Parts by Laser-Spot Lock-In Thermography

We present a complete characterization of the width and depth of a very narrow fatigue crack developed in an Al-alloy dog bone plate using laser-spot lock-in thermography. Unlike visible micrographs, which show many surface scratches, the thermographic image clearly identifies the presence of a single crack about 1.5 mm long. Once detected, we focus a modulated laser beam close to the crack and we record the temperature amplitude. By fitting the numerical model to the temperature profile across the crack, we obtain both the width and depth simultaneously, at the location of the laser spot. Repeating the process for different positions of the laser spot along the crack length, we obtain the distribution of the crack width and depth. We show that the crack has an almost constant depth (0.7 mm) and width (1.5 µm) along 0.7 mm and features a fast reduction in both quantities until the crack vanishes. The results prove the ability of laser-spot lock-in thermography to fully characterize quantitatively narrow cracks, even below 1 µm.This work has been supported by Ministerio de Ciencia e Innovación (PID2019-104347RB-I00, AEI/FEDER, UE), by Gobierno Vasco (PIBA 2018-15), and by Universidad del País Vasco UPV/EHU (GIU19/058)

Measuring the in-plane thermal diffusivity of moving samples using laser spot lock-in thermography

In this work we deal with samples that move at constant speed and are illuminated by a modulated and focused laser beam. We have obtained a general expression for the surface temperature of these moving samples: it is valid not only for opaque and thermally thick materials, but also for thermally thin and semitransparent samples. Moreover, heat losses by convection and radiation are taken into account in the model. Numerical calculations indicate that the temperature (amplitude and phase) profiles in the directions parallel and perpendicular to the sample motion are straight lines with respect to the distance to the laser spot. The slopes of these straight lines depend on sample speed, modulation frequency and in-plane thermal diffusivity of the sample. Provided the two first experimental parameters are known, the in-plane thermal diffusivity can be retrieved in a simple manner. Measurements performed on materials covering a wide range of thermal diffusivity values, from insulators to good thermal conductors, confirm the validity of these linear methods.This work has been supported by Ministerio de Economía y Competitividad (DPI2016-77719-R, AEI/FEDER, UE), by Universidad del País Vasco UPV/EHU (GIU16/33) and by Gobierno Vasco (PIBA2018/15). A. Bedoya greatly thanks the support of CONACyT through the Beca Mixta Program for a research stay at the UPV/EHU

Quantifying the width and angle of inclined cracks using laser-spot lock-in thermography

In this work we introduce a method to determine the width and the orientation of tilted cracks. This method combines laser-spot lock-in thermography and calculations of the sample temperature by means of finite elements modelling. The fitting of the surface temperature, calculated from the numerical model, to experimental lock-in thermography data obtained by focusing the laser spot close to an artificial calibrated inclined crack delivers its width and angle. The agreement between nominal and retrieved values proves the ability of the method to size inclined cracks, even those of micrometric width.This work has been supported by Ministerio de Ciencia e Innovación (PID2019-104347RB-I00, AEI/FEDER, UE), by Gobierno Vasco (PIBA 2018–15) and by Universidad del País Vasco UPV/EHU (GIU19/058)

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.This work has been supported by Ministerio de Economía y Competitividad (DPI2016-77719-R, AEI/FEDER, UE), by Gobierno Vasco (PIBA2018/15) and by Universidad del País Vasco UPV/EHU (GIU16/33)

Measurement of the thermal conductivity of fluids using laser spot lock-in thermography

In this work, we propose a new method to retrieve the thermal conductivity of fluids, kf, using laser spot lock-in infrared thermography. The measurement cell consists in two paralepidid blocks separated by a narrow gap of variable and known width, L. This gap can be filled with a gas or a liquid, producing a thermal contact resistance, Rth, which quantifies the thermal barrier to heat propagation. By focusing the laser spot close to the interface between the two solids, this thermal contact resistance can be obtained by fitting the temperature field recorded by the infrared camera to its analytical expression. Using the wellknown relationship Rth = L/kf, the thermal conductivity of the fluid is obtained from a simple linear relation. Measurements performed in test samples show a good agreement between determined thermal conductivities and literature reported values, demonstrating the validity of the method.This work was partially supported by research grants from SIP-IPN (20181764, 20196720) and CONACyT (205640). The support of COFAA-IPN by the SIBE and BEIFI programs is also acknowledged. A.B. greatly thanks the support of CONACyT through the Beca Mixta Program for a research stay at the UPV/EHU and UPV/EHU support through “Ayuda para facilitar la estancia en la UPV/EHU de personas investigadoras en formación de países latinoamericanos matriculadas en las enseñanzas de doctorado de la UPV/EHU (2019)”. Authors are also grateful for support by Ministerio de Economía y Competitividad (DPI2016-77719-R, AEI/FEDER, UE), by Gobierno Vasco (PIBA2018/15) and by Universidad del País Vasco UPV/EHU (GIU16/33

Diseño y producción de vidrios metálicos granulares con magnetorresistencia gigante

A granular metallic glass is an amorphous metall with nanocristalline phases incrusted on it. The effect of giant magnetoresistance (decrease of electrical resistance due to the presence of a magnetic field) is a quantum phenomenon related to electronic spins. This effect has been observed in systems formed by ferromagnetic layers alternated with paramagnetic layers, and also in granular materials, but never in metallic glasses. The objective of this project is to produce a granular metallic glass with the proper composition to study the presence of giant magnetoresistance on it.Un vidrio metálico granular es un metal amorfo con fases nano-cristalinas incrustadas. El efecto de magnetorresistencia gigante (disminución de la resistencia eléctrica ante la presencia de un campo magnético) es un fenómeno cuántico relacionado con los spins electrónicos. Este efecto ha sido observado en sistemas formados por capas ferromagnéticas alternadas con capas magnéticas, y también en materiales granulares, pero no en vidrios metálicos. El objetivo de este proyecto es producir un vidrio metálico granular con la composición adecuada para poder estudiar la presencia de magnetorresistencia gigante en él.Un vidre metàl·lic granular és un metall amorf amb fases nano-cristal·lines incrustades. L'efecte de magnetoresistència gegant (disminució de la resistència elèctrica en front d'un camp magnètic) és un fenomen quàntic relacionat amb els spins electrònics. Aquest efecte s'ha observat en sistemes formats per capes ferromagnètiques alternades amb capes paramagnètiques, i també en materials granulars, però no en vidres metàl·lics. L'objectiu del projecte és produir un vidre metàl·lic granular amb la composició adequada per tal d'estudiar-ne la presència de magnetoresistència gegant

Diseño y producción de vidrios metálicos granulares con magnetorresistencia gigante

A granular metallic glass is an amorphous metall with nanocristalline phases incrusted on it. The effect of giant magnetoresistance (decrease of electrical resistance due to the presence of a magnetic field) is a quantum phenomenon related to electronic spins. This effect has been observed in systems formed by ferromagnetic layers alternated with paramagnetic layers, and also in granular materials, but never in metallic glasses. The objective of this project is to produce a granular metallic glass with the proper composition to study the presence of giant magnetoresistance on it.Un vidrio metálico granular es un metal amorfo con fases nano-cristalinas incrustadas. El efecto de magnetorresistencia gigante (disminución de la resistencia eléctrica ante la presencia de un campo magnético) es un fenómeno cuántico relacionado con los spins electrónicos. Este efecto ha sido observado en sistemas formados por capas ferromagnéticas alternadas con capas magnéticas, y también en materiales granulares, pero no en vidrios metálicos. El objetivo de este proyecto es producir un vidrio metálico granular con la composición adecuada para poder estudiar la presencia de magnetorresistencia gigante en él.Un vidre metàl·lic granular és un metall amorf amb fases nano-cristal·lines incrustades. L'efecte de magnetoresistència gegant (disminució de la resistència elèctrica en front d'un camp magnètic) és un fenomen quàntic relacionat amb els spins electrònics. Aquest efecte s'ha observat en sistemes formats per capes ferromagnètiques alternades amb capes paramagnètiques, i també en materials granulars, però no en vidres metàl·lics. L'objectiu del projecte és produir un vidre metàl·lic granular amb la composició adequada per tal d'estudiar-ne la presència de magnetoresistència gegant