E. Apiñaniz

Study of the enhancement of the magnetic properties of Fe70Al30 in the order-disorder transition

The order-disorder transition produced by ball milling in the intermetallic Fe70Al30 has been systematically studied by x-ray diffraction, calorimetry, and Mossbauer experiments. These techniques show a monotonous transformation that ends after 6 h of milling. In the transition, the lattice parameter increase amounts to 0.7% and there is a large enhancement of the alloy’s magnetism.

Simultaneous measurement of thermal diffusivity and optical absorption coefficient using photothermal radiometry. II Multilayered solids

coefficient using photothermal radiometry. II Multilayered solids Agustı́n Salazar, Raquel Fuente, Estibaliz Apiñaniz, Arantza Mendioroz, and R. Celorrio Departamento de Fı́sica Aplicada I, Escuela Técnica Superior de Ingenierı́a, Universidad del Paı́s Vasco, Alameda Urquijo s/n, Bilbao 48013, Spain Departamento de Matemática Aplicada, EINA/IUMA, Universidad de Zaragoza, Campus Rı́o Ebro, Edificio Torres Quevedo, Zaragoza 50018, Spain

Extending the flash method to measure the thermal diffusivity of semitransparent solids

In this work, we extend the classical flash method to retrieve simultaneously the thermal diffusivity and the optical absorption coefficient of semitransparent plates. A complete theoretical model that allows calculating the rear surface temperature rise of the sample has been developed. It takes into consideration additional effects such as multiple reflections of the heating light beam inside the sample, heat losses by convection and radiation, transparency of the sample to infrared wavelengths and finite duration of the heating pulse. Measurements performed on calibrated solids, covering a wide range of absorption coefficients from transparent to opaque, validate the proposed method.

Influence of disorder on the magnetic properties of FeAl alloys: theory

Abstract Self-consistent electronic calculations are performed by means of the tight binding linear muffin tin orbital (TB-LMTO) method to study the influence of disorder on the magnetic properties of ordered Fe50Al50 and Fe75Al25 alloys. The calculations indicate that in these alloys the disordering of the structure increases the mean magnetic moment of the alloys, which is larger in the case of the Fe50Al50 alloy. The disordering of the alloy also causes an increase of the lattice parameter in relation to the ordered structures.

Accurate reconstruction of the thermal conductivity depth profile in case hardened steel

The problem of retrieving a nonhomogeneous thermal conductivity profile from photothermal radiometry data is addressed from the perspective of a stabilized least square fitting algorithm. We have implemented an inversion method with several improvements: (a) a renormalization of the experimental data which removes not only the instrumental factor, but the constants affecting the amplitude and the phase as well, (b) the introduction of a frequency weighting factor in order to balance the contribution of high and low frequencies in the inversion algorithm, (c) the simultaneous fitting of amplitude and phase data, balanced according to their experimental noises, (d) a modified Tikhonov regularization procedure has been introduced to stabilize the inversion, and (e) the Morozov discrepancy principle has been used to stop the iterative process automatically, according to the experimental noise, to avoid “overfitting” of the experimental data. We have tested this improved method by fitting theoretical data gene...

Analysis of the Tikhonov regularization to retrieve thermal conductivity depth-profiles from infrared thermography data

We analyze the ability of the Tikhonov regularization to retrieve different shapes of in-depth thermal conductivity profiles, usually encountered in hardened materials, from surface temperature data. Exponential, oscillating, and sigmoidal profiles are studied. By performing theoretical experiments with added white noises, the influence of the order of the Tikhonov functional and of the parameters that need to be tuned to carry out the inversion are investigated. The analysis shows that the Tikhonov regularization is very well suited to reconstruct smooth profiles but fails when the conductivity exhibits steep slopes. We check a natural alternative regularization, the total variation functional, which gives much better results for sigmoidal profiles. Accordingly, a strategy to deal with real data is proposed in which we introduce this total variation regularization. This regularization is applied to the inversion of real data corresponding to a case hardened AISI1018 steel plate, giving much better antico...

A thermal paradox: which gets warmer?

In this paper we address a common misconception concerning the thermal behaviour of matter, namely that the front surface of a very thin plate, uniformly illuminated by a constant light beam, reaches a higher temperature than the front surface of a very thick slab made out of the same material. We present analytical solutions for the temperature rise above the ambient of thin and thick samples, after a heating lamp is switched on. It is shown that the temperature rise at the illuminated surface of the thick sample is twice that of the thin plate. However, the temperature rise of the thin plate is much faster (minutes) than the heating of the thick one (hours). This explains why our intuition, which learns from what happens at the very beginning of the process, leads us to the feeling that the front surface of a thin plate will get warmer than the illuminated surface of a thick slab, i.e. the above-mentioned misconception.

Reconstruction of radial thermal conductivity depth profile in case hardened steel rods

In this work the surface thermal-wave field (ac temperature) of a solid cylinder illuminated by a modulated light beam is calculated first in two cases: a multilayered cylinder and a cylinder the radial thermal conductivity of which varies continuously. It is demonstrated numerically that, using a few layers of different thicknesses, the surface thermal-wave field of a cylindrical sample with continuously varying radial thermal conductivity can be calculated with high accuracy. Next, an inverse procedure based on the multilayered model is used to reconstruct the radial thermal conductivity profile of hardened C1018 steel rods, the surface temperature of which was measured by photothermal radiometry. The reconstructed thermal conductivity depth profile has a similar shape to those found for flat samples of this material and shows a qualitative anticorrelation with the hardness depth profile.

Thermal characterization of rods, tubes and spheres using pulsed infrared thermography

In this work we analyse the accuracy of an extension of the flash method to measure the thermal diffusivity of rods, tubes and spheres, which was recently proposed by the authors. We have performed measurements in a wide set of calibrated samples of different sizes and we have found that a lower limiting size of the radius can be established for the validity of the method. On the other hand, a procedure to retrieve the thermal conductivity of tubes, based on filling them with a contrast liquid (water), is proposed. Moreover, the thermal contact resistance between the two layers of coated cylinders is also obtained. Measurements on calibrated samples confirm the validity of the two latest methods.

Characterization of delaminations by lock-in vibrothermography

Vibrothermography has proven to be a useful technique for the detection of buried defects, which reveal themselves as heat sources when mechanically excited with ultrasounds. In this work we present a method to evaluate the depth of delaminations in opaque samples, from lock-in vibrothermography measurements. It is theoretically demonstrated that the phase and the natural logarithm of the surface temperature above the delamination behaves linearly as a function of the square root frequency. The slope of this linear relation is directly proportional to the delamination depth. Measurements performed on composite plates with calibrated delaminations confirm the validity of the method, provided the width of the delamination is higher than the thermal diffusion length.