Role of anisotropy in noncontacting thermoelectric materials characterization

Abstract

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 hard alpha inclusion in Ti–6Al–4V by magnetic sensing

“…The measured magnetic signature of highly textured materials such as Ti-6Al-4V often exhibit a significative intrinsic thermoelectric background due to macrostructural features. The strong effect of the intrinsic material background magnetic signature limits the detectability of small, weak or subtle imperfections by the thermoelectric method [2,4,6]. Therefore the measured magnetic signature reported in Fig.…”

“…verified by author and et al This holds in particular for the cases of surface-breaking and subsurface tin inclusions embedded in a copper specimen [2,4]. Also, it was pointed out in [2,4,6,7] that the detectability of small inclusions and subtle imperfections by the non-contacting thermoelectric method is limited by the adverse background (intrinsic thermoelectric anisotropy) of the material to be inspected. This intrinsic background signal in thermoelectric measurements is due to macrostructural features such as texture induced anisotropy, residual stress, cold work, heat treatment, etc.…”

“…The ends of the copper supporters were simultaneously heated and cooled by running water to temperatures of 85 and 10 • C, respectively. However, the actual temperature difference between the ends of the Ti-6Al-4V specimen was monitored during the measurements by thermocouple thermometers and the temperature gradient was kept at 12 • C/cm, which is more than sufficient to produce detectable magnetic signals in high-strength, high-temperature engine materials such as titanium-base alloys [6]. A pair of fluxgate sensors configured in a differential arrangement was used to detect the thermoelectric signals from the specimen.…”

On the role of material property gradients in noncontacting thermoelectric NDE

“…This technique will be discussed in more detail in the experimental part of this paper. At this point, it is sufficient to say that in many cases we found that the shape of the background signature essentially remained the same but flipped its sign when the specimen was rotated by 1808 around the direction of heat propagation, which clearly indicates that the observed signal cannot originate from the anisotropy of the specimen unless it is also inhomogeneous [12]. We will also show that this rotational symmetry can be exploited not only to separate anisotropic effects from those of inhomogeneity, but also to further separate the two principal inhomogeneity components in the thickness and width-directions from each other.…”

“…In a recent paper, we studied the effect of anisotropic texture in a homogeneous material and developed an analytical model capable of quantitatively predicting the resulting thermoelectric signature for simple inspection of geometries [12]. In this paper, our goal is to develop and experimentally verify an analytical model capable of predicting the thermoelectric background signature caused by weak material inhomogeneity for the simplest and most common inspection geometry, namely, in the case of an axially heated slender rectangular bar.…”