Papers by Author: Henry Prask

Abstract: Cold-drawing is employed to fabricate wires and rods, which are mainly used as structural reinforcements in construction as well as in the tyre industry. As a consequence of processing, a residual stress profile is developed. In this paper, residual stress profiles are measured by neutron diffraction in cold-drawn pearlitic steel rods subjected to different deformations (true strain from 0.3 to 1.7). The results show that the residual stress profile produced by cold-drawing is similar in all the samples, irrespective of the degree of deformation.

Abstract: Low carbon steel (usually in sheet form) has found a wide range of applications in industry due to its high formability. The inner and outer panels of a car body are good examples of such an implementation. While low carbon steel has been used in this application for many decades, a reliable predictive capability of the forming process and “springback” has still not been achieved. NIST has been involved in addressing this and other formability problems for several years. In this paper, texture produced by the in-plane straining and its relationship to springback is reported. Low carbon steel sheet was examined in the as-received condition and after balanced biaxial straining to 25%. This was performed using the Marciniak in-plane stretching test. Both experimental measurements and numerical calculations have been utilized to evaluate anisotropy and evolution of the elastic properties during forming. We employ several techniques for elastic property measurements (dynamic mechanical analysis, static four point bending, mechanical resonance frequency measurements), and several calculation schemes (orientation distribution function averaging, finite element analysis) which are based on texture measurements (neutron diffraction, electron back scattering diffraction). The following objectives are pursued: a) To test a range of different experimental techniques for elastic property measurements in sheet metals; b) To validate numerical calculation methods of the elastic properties by experiments; c) To evaluate elastic property changes (and texture development) during biaxial straining. On the basis of the investigation, recommendations are made for the evaluation of elastic properties in textured sheet metal.

Abstract: Thermal spray is a well-established, versatile method of producing protective and functional coatings. As with most thin- or thick-film structures, residual stresses developed during processing play an important role in determining the performance and life of sprayed coatings. Diffraction methods (X-ray or neutron) and in situ curvature measurement have been widely used to measure stresses nondestructively, yet results in coating stress measurements seen in the literatures are sometimes ambiguous or conflicting. This is due not only to the experimental error associated with the measurement and simplifying assumptions, but also the complexity and heterogeneity of the coating structure. During deposition, molten, semi-molten or solid particles successively impinge onto a substrate surface, thus forming a layered structure comprised of ‘splats,’ separated by interfaces, cracks and pores. In this study, X-ray micro-diffraction with a 2-D detector has been used to determine the stress magnitude of both splats and coatings on substrates. Neutron diffraction stress measurements have been made through the entire coating thickness. The process of depositing and cooling has been monitored by in situ curvature measurement. Micro- and macro stresses have been examined. The relation between process and splat and coating residual stresses has been evaluated and interpreted by recourse to microstructural and morphological observations under SEM. This study bridges the behaviors of microscopic single splats and macroscopic coatings, hence helps to fundamentally understand the stress generation during thermal spray process.