Hector Carreon

Role of anisotropy in noncontacting thermoelectric materials characterization

Inclusions and other types of imperfections in metals can be nondestructively detected by noncontacting magnetic measurements that sense the thermoelectric currents that appear when the specimen is subjected to directional heating and cooling. The detectability of small imperfections is ultimately limited by the intrinsic thermoelectric anisotropy and inhomogeneity of the material to be inspected. This article presents an analytical method for calculating the magnetic field produced by thermoelectric currents in anisotropic materials under two-dimensional directional heating and cooling. Experimental results from a textured Ti–6Al–4V titanium-alloy plate are shown to be in very good agreement with the predictions of this model. The described analytical method can be used to optimize thermoelectric inspection procedures and to evaluate the macroscopic texture of metals from their characteristic magnetic signatures.

Thermoelectric detection of spherical tin inclusions in copper by magnetic sensing

Inclusions and other types of imperfections in metals can be nondestructively detected by noncontacting magnetic measurements that sense the thermoelectric currents around such flaws when the specimen is subjected to directional heating and cooling. This article presents experimental data for the magnetic field produced by thermoelectric currents around surface-breaking spherical tin inclusions in copper under external thermal excitation for different lift-off distances between the sensor and the surface of the specimen. The diameter of the inclusions and the lift-off distance varied from 2.4 to 12.7 mm and from 12 to 20 mm, respectively. A fairly modest 0.7 °C/cm temperature gradient in the specimen produced peak magnetic flux densities ranging from 1 to 250 nT. These results were found to be in good agreement with recently published theoretical predictions [P. B. Nagy and A. H. Nayfeh, J. Appl. Phys. 87, 7481 (2000)].

On the role of material property gradients in noncontacting thermoelectric NDE

Abstract Inclusions and other types of imperfections in nonmagnetic metals can be nondestructively detected by noncontacting magnetic measurements that sense the thermoelectric currents produced by directional heating and cooling of the specimen. The detectability of small and/or weak imperfections is ultimately limited by the intrinsic anisotropy and inhomogeneity of the material to be inspected. This paper investigates the spurious magnetic signature produced by the simplest type of macroscopic inhomogeneity when the material properties exhibit a linear spatial variation in the cross section of a slender bar. An analytical method has been developed for calculating the normal and tangential magnetic fields produced by the resulting thermoelectric currents. Experimental results from a highly inhomogeneous artificial copper/brass sintered specimen were found to be in very good quantitative agreement with our theoretical predictions and fully verified our analytical model. Similar measurements on a weakly inhomogeneous Ti–6Al–4V titanium-alloy bar were also shown to be in very good qualitative agreement with the predictions of the analytical model although the unexpectedly high magnitude of the observed signatures could not be verified by conventional contact measurements, therefore further efforts are needed to better understand the underlying physical phenomenon and clarifying the relationship between the strength of the signature and the very complex microstructural features of this popular high-strength alloy.

Monitoring of the Level of Residual Stress in Surface‐Treated Specimens by a Noncontacting Thermoelectric Technique

We have recently initiated the development of a noncontacting thermoelectric method based on magnetic detection of local thermoelectric currents in the compressed near‐surface layer of surface‐treated metals when a temperature gradient is established throughout the specimen. Beside the primary residual stress effect, the thermoelectric method is also sensitive to the secondary “material” effects of shot peening (local texture, increased dislocation density, hardening), but it is entirely insensitive to its “geometrical” by‐product, i.e., the rough surface topography. This method measures only the weighted average of the near‐surface residual stress, which is sufficient for quantitatively evaluating the degree of thermally‐induced stress release, but, in its present form, it is not suitable for detailed mapping of the residual stress profile. Preliminary results are presented for shot‐peened and low‐plasticity‐burnished IN100 nickel‐base superalloy specimens to show that the technique is also applicable to...

Thermoelectric nondestructive evaluation of residual stress in shot-peened metals

This paper describes a novel noncontacting thermoelectric method based on magnetic detection of local thermoelectric currents in the compressed near-surface layer of shot-peened metals when a temperature gradient is established throughout the specimen. Beside the primary residual stress effect, the thermoelectric method is also sensitive to the secondary “material” effects of shot peening (local texture, increased dislocation density, hardening), but it is entirely insensitive to surface roughness. Our preliminary experimental results in copper indicate that the proposed method is more sensitive to the primary residual stress effect than to the secondary material effects. This method measures only the weighted average of the near-surface residual stress, which is sufficient for quantitatively evaluating the degree of thermally-induced stress release, but, in its present form, it is not suitable for detailed mapping of the residual stress profile.

Noncontacting thermoelectric detection of spherical inclusions in copper by magnetic sensing

This paper presents experimental data for the magnetic field produced by thermoelectric currents around surface-breaking spherical tin inclusions in copper under external thermal excitation for different lift-off distances. The diameter of the inclusions and the lift-off distance varied from 2.4 to 12.7 mm and from 12 to 20 mm, respectively. A fairly modest 0.7 °C/cm temperature gradient in the specimen produced peak magnetic flux densities ranging from 1 to 250 nT, that could be easily measured by a commercial fluxgate magnetometer. The experimental results were found to be in very good agreement with predictions published in an accompanying paper.

On the exploitation of thermoelectric coupling for characterization of inclusions in metals

This paper presents a theoretical model capable of predicting the magnetic field produced by thermoelectric currents around inclusions under external thermal excitation. We investigated how the magnetic signal to be detected depends on the relevant physical properties of the host and the inclusion, the size of the inclusion, the polarization of the magnetometer, the lift-off distance of the magnetometer from the specimen, and the direction and strength of the external heating or cooling applied to the specimen. The presented analytical model is numerically evaluated for comparison to experimental results that are presented in an accompanying paper.

Noncontacting Thermoelectric Detection of Material Imperfections in Metals

This project was aimed at developing a new noncontacting thermoelectric method for nondestructive detection of material imperfections in metals. The method is based on magnetic sensing of local thermoelectric currents around imperfections when a temperature gradient is established throughout a conducting specimen by external heating and cooling. The surrounding intact material serves as the reference electrode therefore the detection sensitivity could be very high if a sufficiently sensitive magnetometer is used in the measurements. This self-referencing, noncontacting, nondestructive inspection technique offers the following distinct advantages over conventional methods: high sensitivity to subtle variations in material properties, unique insensitivity to the size, shape, and other geometrical features of the specimen, noncontacting nature with a substantial stand-off distance, and the ability to probe relatively deep into the material. The potential applications of this method cover a very wide range from detection metallic inclusions and segregations, inhomogeneities, and tight cracks to characterization of hardening, embrittlement, fatigue, texture, and residual stresses.

Anisotropic effects in noncontacting thermoelectric material characterization

Inclusions and other types of imperfections in metals can be nondestructively detected by noncontacting magnetic measurements that sense the thermoelectric currents that appear when the specimen is subjected to directional heating and cooling. The detectability of small imperfections is ultimately limited by the intrinsic thermoelectric anisotropy and inhomogeneity of the material to be inspected. This paper presents an analytical method for calculating the magnetic field produced by thermoelectric currents in anisotropic materials under two-dimensional directional heating and cooling. Experimental results from a textured Ti-6Al-4V titanium-alloy plate are shown to be in very good agreement with the predictions of this model. The described analytical method can be used to optimize thermoelectric inspection procedures and to evaluate the macroscopic texture of metals from their characteristic magnetic signatures.