Search Results

High strain rate test methods to obtain strain-rate dependent sheet steel tensile properties are considered. A tensile test method for sheet steels was developed to obtain accurate stress-strain data over the strain rate range from 0.001 s-1 to 500 s-1 using a servo-hydraulic test machine and tensile samples instrumented with strain gages. Results on several different automotive sheet steels, including interstitial free (IF), high strength low alloy (HSLA), dual phase (DP), and transformation induced plasticity (TRIP) steels, are presented. The results show that strain rate response differs between the various alloy systems. These results are compared with previously published data on strain-rate dependent steel properties. The importance of stress-strain curve shapes, which depend on alloy system, on energy absorption calculations using areas under stress-strain curves are also described.

Knowledge of high strain-rate deformation behavior of automotive body structural materials is of importance for design of new vehicles with improved crash-energy management characteristics. Since a large range of plastic strains is encountered during the forming process prior to assembly, the mechanical behavior of sheet steels under high strain rate deformation conditions must be understood after pre-straining, in addition to the as-produced condition. This paper presents the compression testing methodology employed to examine these properties, and focuses on the effects of quasi-static pre-strains (from 0 to 20%) on the subsequent behavior of a low carbon interstitial free steel tested over a broad range of strain rates (from 10−2 to 103s−1). The results suggest that the increase in yield stress associated with increasing strain rate is not substantially influenced by prior cold work.

The effects of deep rolling were evaluated by reviewing the fatigue performance of three medium-carbon (0.4 C) bar steels representing microstructural classes characteristic of forging steels used for crankshaft and other automotive applications. Deep rolling is a surface deformation process whereby a radially symmetric work piece undergoes a surface deformation operation. The steel grades included a quenched and tempered alloy steel (4140) that demonstrated a high yield stress and low strain hardening rate, a non-traditional bainitic experimental grade (1.2 Mn, 0.72 Si) containing high amounts of retained austenite with low yield stress and high strain hardening rate, and a ferritic/pearlitic grade (1.3 Mn, 0.56 Si) with a low yield stress and medium strain rate hardening rate. A reproducible test methodology to assess fatigue behavior was developed, based on flex-beam, fully reversed, S-N type laboratory fatigue testing.

Bake-hardening steels used for exposed auto-body panels provide low yield strengths before forming, and increased strength and dent resistance after the forming and paint-baking processes. Room temperature aging can alter the sheet properties before forming, after forming, or after baking. Knowledge of the evolution of mechanical properties is important, and the effects of room temperature and simulated room temperature aging (at 50°C and 100°C) on the yielding behavior and the bake hardening response of two different bake-hardening steel grades were studied. The steels included a low strength ultra-low carbon steel and a dual-phase steel with higher strength and greater bake-hardening index. Neither steel exhibited a substantial response to aging prior to tensile pre-straining, although both steels exhibited strength increases after either aging or baking following straining.

The effects of prestrain and zinc coating type on the bending fatigue behavior of titanium-stabilized interstitial free steel were evaluated. From a single zinc bath chemistry, coated sheet steel samples were prepared with either a hot dip galvanized or galvannealed coating. Uniaxial tensile prestrains of 2 and 4 pct. were introduced parallel to the rolling direction on 12.7 cm wide strips. Krouse-type fatigue samples were machined both parallel and transverse to the rolling/prestrain direction. Reversed bending S-N fatigue data showed that the fatigue resistance depended on a complex interaction between the strength increase due to work hardening and fatigue crack development as altered by the presence of the coatings. For both coating types the fatigue resistance increased with prestrain. During prestrain, coating cracks oriented perpendicular to the tensile prestrain direction developed and the crack density was greater in the galvannealed materials.

The quasi-static denting behavior of sheet steels has been analyzed in a systematic FEA study, which considers material properties, panel radius, sheet thickness, panel length and boundary conditions. Results from a full factorial experimental matrix have been analyzed statistically to identify those variables and variable interactions that influence dent performance. The primary factors which control dent performance are material properties, panel radius and sheet thickness, while panel length and boundary conditions are not significant. Based on the results of this study, two commonly used dent criteria (loading energy and visible dent load) are analyzed, and previously reported opposite effects of radius of curvature on dent performance are clarified.

A large set of bending fatigue data on carburized steels has been statistically analyzed to quantitatively describe the effects of process and microstructural variables. Increasing demands on gear steels require a broad examination of past bending fatigue research to reveal the primary factors that determine fatigue performance and guide future gear steel design. Fatigue performance was correlated to specimen characteristics such as retained austenite content, case and core grain size, extent of intergranular oxidation, surface roughness, and the case profiles of residual stress, hardness, and carbon content. Prior austenite grain size in the case and surface residual stress were found to most strongly influence bending fatigue endurance limit. A multiple regression model to predict endurance limit achieved an R-squared value of 0.56.

This study examined the evolution of the surface topography of imperfections in response to painting. Critically-sized imperfections on industrially produced coated sheet steel surfaces were sampled, and carefully characterized. Changes in the surface topography were measured after painting simulations, and visibility after painting was assessed. Three-dimensional optical profilometry was demonstrated to be a powerful technique for assessing imperfection topographies and their evolution during painting; several imperfections that were completely invisible through human inspection after painting were still measurable by 3-D topography. Two primer surfacer systems were used, to compare a conventional liquid system with a high-build anti-chip powder system.