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.
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.
Grit blasting is a low cost surface modification treatment widely used to enhance mechanical fixation of implants through increasing their roughness. As a result of the severe surface plastic deformation, beneath the surface it produces additional effects such as grain size refinement, work hardening and compressive residual stresses, which are generally evaluated with destructive techniques. In this research work, the blasting induced effects by Al 2 O 3 and ZrO 2 particles and their evolution after annealing at 700ºC were evaluated in 316LVM (Low Vacuum Melting) stainless steel specimens using two non-destructive thermoelectric techniques (NDTT), the non-contacting and contacting thermoelectric power measurements. Microstructural analysis and microhardness measurements performed beneath the blasted surface reveals that the non-contact NDTT results correlate well with the reversion of the α'-martensite developed during blasting, whereas the contact NDTT results are closely related to the grain size refinement and work hardening and the expected evolution of compressive residual stresses. Potential of these techniques to monitor subsurface changes in blasting processes and others severe surface plastic deformation techniques are clearly envisaged.
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