Thomas Gnäupel-Herold

Crystallographic Texture Evolution in 1008 Steel Sheet During Multi-axial Tensile Strain Paths

AbstractThis paper considers the crystallographic texture evolution in a 1008 low carbon steel. The texture evolution along uniaxial, plane strain and balanced biaxial strain states were measured. For uniaxial testing, grains tend to rotate such that the {111}〈11̄0〉 slip directions are aligned with the loading axis. For plane strain and balanced biaxial strain states, the majority of grains are distributed with the {111} plane parallel to the sample normal direction. Accompanying visco-plastic self consistent (VPSC) predictions of the texture evolution were made along same strain paths and strain increments. Comparing between the measured texture evolution and computational texture evolution indicate that the VPSC model qualitatively predicts the measured texture evolution, but the rate at which the texture evolution occurs is over predicted.

Neutron measurements of stresses in a test artifact produced by laser-based additive manufacturing

A stainless steel test artifact produced by Direct Metal Laser Sintering and similar to a proposed standardized test artifact was examined using neutron diffraction. The artifact contained a number of structures with different aspect ratios pertaining to wall thickness, height above base plate, and side length. Through spatial resolutions of the order of one millimeter the volumetric distribution of stresses in several was measured. It was found that the stresses peak in the tensile region around 500 MPa near the top surface, with balancing compressive stresses in the interior. The presence of a support structure (a one millimeter high, thin walled, hence weaker, lattice structure deposited on the base plate, followed by a fully dense AM structure) has only minor effects on the stresses.

A monolithic polycapillary focusing optic for polychromatic neutron diffraction applications

We have conducted measurements at five different thermal neutron wavelengths to determine the transmission characteristics of a tapered monolithic focusing lens with a focal length of 100 mm, suitable for time-of-flight diffraction. Both the width of the focused beam and the intensity gain of the optic increase as a function of wavelength. We have performed similar measurements on a polychromatic beam on a pulsed neutron source, where the results are subject to background from short wavelength neutrons. The use of a beryllium filter shows the increased effective gain for the longer wavelengths at the expense of an increased focused beam width by a factor of 2.

Measurement and Calculation of Elastic Properties in Low Carbon Steel Sheet

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.

Multiaxial constitutive behavior of an interstitial-free steel: Measurements through X-ray and digital image correlation

Constitutive behaviors of an interstitial-free steel sample were measured using an augmented Marciniak experiment. In these tests, multiaxial strain field data of the flat specimens were measured by the digital image correlation technique. In addition, the flow stress was measured using an X-ray diffractometer. The flat specimens in three different geometries were tested in order to achieve 1) balanced biaxial strain, and plane strain tests with zero strain in either 2) rolling direction or 3) transverse direction. The multiaxial stress and strain data were processed to obtain plastic work contours with reference to a uniaxial tension test along the rolling direction. The experimental results show that the mechanical behavior of the subjected specimen deviates significantly from isotropic behavior predicted by the von Mises yield criterion. The initial yield loci measured by a Marciniak tester is in good agreement with what is predicted by Hills yield criterion. However, as deformation increases beyond the vonMises strain of 0.05, the shape of the work contour significantly deviates from that of Hills yield locus. A prediction made by a viscoplastic self-consistent model is in better agreement with the experimental observation than the Hill yield locus with the isotropic work-hardening rule. However, none of the studied models matched the initial or evolving anisotropic behaviors of the interstitial-free steel measured by the augmented Marciniak experiment.

Residual Stresses and Elastic Constants in Thermal Deposits

In this study we investigate experimentally and theoretically the influence of various aspects of the pore structure on residual stress, coating adhesion to the substrate and elastic properties of the coating. For that purpose, feedstock powders of Inconel 625 were prepared with four different particle size distributions. The coatings were prepared by air-plasma spraying under nearly identical spray conditions on grit blasted steel substrates. Neutron diffraction was used to determine the average in-plane residual stresses in the coatings. The in-plane Young’s modulus was determined using a four-point bending apparatus. The porosity and the pore distribution were characterized using small-angle neutron scattering as well as precision density measurements. It was found that both the residual stresses and the elastic modulus of the coatings sprayed with coarse powder were considerably lower than the stresses in the coatings sprayed with the smaller particles. As the particle size decreases, a rising oxide content in the coating as well as a change in the pore distribution elevate the elastic modulus and, as a consequence of that, the residual stresses. The most pronounced effect on the pore distribution is a lower fraction of connected porosity which effectively decreases the pore aspect ratio. With no ductility left due to the embrittlement effect of the oxide particles, these coatings exhibit a low strain tolerance and the residual stresses are close to their maximum level.

Interpretation of Diffraction Data from In Situ Stress Measurements during Biaxial Sheet Metal Forming

Biaxial yield behavior is determined in-situ through X-ray lattice strain measurements. The distributions of d-spacings in different sample directions is affected both by the changes in diffraction elastic constants (DEC) from evolving texture and by the intergranular (IG) strains. Model predictions were found to be lacking, thus, a hybrid approach was developed based on measurements of DEC and IG strains at selected biaxial deformations. In order to convert measured lattice strains to stress for any given biaxial plastic strain a theoretical approximation was fitted to the experimental data, thus allowing the estimation of the evolution of DEC and IG strains with plastic deformation.

Uncertainty in flow stress measurements using X‐ray diffraction for sheet metals subjected to large plastic deformations

X-ray diffraction techniques have been developed to measure flow stresses of polycrystalline sheet metal specimens subjected to large plastic deformation. The uncertainty in the measured stress based on this technique has not been quantified previously owing to the lack of an appropriate method. In this article, the propagation of four selected elements of experimental error is studied on the basis of the elasto-viscoplastic self-consistent modeling framework: (1) the counting statistics error; (2) the range of tilting angles in use; (3) the use of a finite number of tilting angles; and (4) the incomplete measurement of diffraction elastic constants. Uncertainties propagated to the diffraction stress are estimated by conducting virtual experiments based on the Monte Carlo method demonstrated for a rolled interstitial-free steel sheet. A systematic report on the quantitative uncertainty is provided. It is also demonstrated that the results of the Monte Carlo virtual experiments can be used to find an optimal number of tilting angles and diffraction elastic constant measurements to use without loss of quality.

Through-Thickness Stresses in Automotive Sheet Metal after Plane Strain Channel Draw

A series of samples from four automotive materials - AKDQ, HSLA50, DP600, and AA6022-T43 - were deformed in a channel draw processes with different levels of draw bead penetration. As a result, varying magnitudes of deformations in plane strain mode and residual stresses were obtained. Through-thickness stress profiles were obtained non-destructively using a novel, high resolution X-ray diffraction technique.

Through-Thickness Residual Stress Measurement by Neutron Diffraction in Cu+W Plasma Spray Coatings

A range of different spraying techniques can be used to coat the surfaces of engineering components. These techniques are based on different principles and can involve high temperature (plasma spray), high kinetic energy (cold spray) or both (HVOF spray – High-Velocity Oxi-Fuel). Resultant residual stress in such coatings, being a characteristic of the spraying process, can reveal details of the stress formation mechanism. When its dependence on the physical parameters and conditions of the spraying process is established, this knowledge can be used for the prediction and control of stress that occurs in applications. Neutron diffraction is a suitable method for obtaining stress distribution in such coatings. Residual stresses in two-phase Cu+W coatings made by water stabilized plasma spraying were studied. Two-phase coatings develop both significant microstress (inter-phase stress) and the stress dependence on phase content of the coating constituents. Through-thickness residual stress profiles have been measured by neutron diffraction with spatial resolution of 0.5 mm for a series of Cu+W coatings with varying volume fractions. Measurements were made in both phases in order to separate micro- and macro-stresses. Comprehensive sample characterization, measurements of the residual stresses, mechanical and thermal properties of the composite coatings enabled quantitative modeling and interpretation of the experimental data.