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Technical Paper

Predicting the Radius of a Sheet Bent Around Drawbeads

Drawbeads in production stamping dies often have insufficient penetration of the male bead into the female cavity. With insufficient penetration, the actual bending radii of the sheet metal are larger than the geometrical radii of the drawbead. The actual bending radii in the sheet directly affect the force that restrains sheet movement. To predict the restraining stress due to a drawbead, it is necessary to know the actual bending radii in the sheet as it passes though the drawbead. Data from a previous study are used to develop empirical regression equations for predicting measured radii of the sheet that is bent around the radii in a drawbead. A physical model for the evolution of the sheet radii as the drawbead closes is proposed. This model is consistent with the empirical equations and the mechanics of the sheet bending process.
Technical Paper

Sheet Thinning during Plane-Strain Bending

Knowledge of the net thinning strain that occurs in a sheet as it is bent over a single radius is an important component in understanding sheet metal formability. The present study extends the initial work of Swift on thinning during plane-strain bending to sheet steels with power law stress-strain behavior and with the inclusion of friction. The experimental data come from studies on the enhanced forming limit curve on DQSK steel and analysis of the curl behavior of 590R and DP600 steels. Results for single radius bending from these studies are used in the present investigation. It has been found that the amount of net thinning strain depends on back tension, initial plane-strain yield strength, and the maximum true bending strain calculated for the neutral plane at the mid-thickness of the sheet.
Technical Paper

Effect of Draw Beads on the Mechanical Properties of Sheel Sheet

Draw beads are used in many stamping dies to control the flow of metal into a die cavity. The multiple bends imposed by the draw beads cause a change in the mechanical properties of the sheet before it enters the die cavity. Since the necessary data to completely characterize the full deformation history of a sheet passing through draw beads are not available, it is not possible to use a fundamental approach to determine the effect on subsequent mechanical properties. In the present investigation data from a prior study [1] are used to develop empirical relationships to predict the yield strength, uniform elongation and tensile strength as a function of the cumulative maximum effective bending strain (ε̅max-cum) due to draw beads and the entry radius to a die cavity.
Technical Paper

The Effective Unloading Modulus for Automotive Sheet Steels

In stamping advanced high strength steels (AHSS), the deviations from desired part geometry caused by springback from a radius, curl, twist, and bow are major impediments to successfully producing AHSS parts. In general, the conventional elastic modulus is used to quantify the strain that occurs on unloading. This unloading strain causes deviations from desired part geometry. Considerable evidence in the literature indicates that for tensile testing, the conventional elastic modulus does not accurately describe the unloading strain. The present study uses new data and results from the literature to examine the average slope of tensile stress strain curves on unloading. This slope is termed the effective unloading modulus. The results from this study quantitatively describe how the effective unloading modulus decreases with increasing strength, prestrain, and unloading time.
Technical Paper

Bauschinger Effect Response of Automotive Sheet Steels

In a study of the Bauschinger effect, data were collected from three sources in the published literature. Quantitative stress-strain data were taken from these papers, and the results re-analyzed. The resulting database has 44 lots of sheet steels, including drawing quality, interstitial free, bake hardening, HSLA (and related grades), dual phase, TRIP, recovery annealed, and martensitic grades. In analyzing the data, it is found that use of the 0.05% yield strength on reversal instead of the conventional 0.2% yield strength provides more generality in explaining the results. In this analysis, the Bauschinger effect is characterized by a term (BE), which is the difference between the steel strength just prior to reversal and the 0.05% yield strength on reversal normalized by the strength just prior to reversal. An initial prestrain of 2% is needed to establish a dislocation morphology that can be generalized across many of the steel grades.
Technical Paper

Experimental Evaluation of Curl and Tensile Properties of Advanced High Strength Sheet Steels

The response of HSLA steel, 590R, and dual-phase steel, DP-600, to non-uniform deformation imposed in a laboratory Bending-Under-Tension (BUT) test apparatus was evaluated. Samples were deformed with both low and high back tension forces at bend angles of 45 and 90 degrees, and evaluated to determine the “side-wall curl”, i.e. the curvature in the sheet section in contact with the die. The results indicate that there are no consistent differences between the two steels, 590R and DP-600. It was found that back tension, tensile strength and sheet thickness were the primary factors affecting curl. The bend angle has an influence on curl, with the curl radius at a 90° bend angle being greater than the curl radius at a 45° bend angle.
Technical Paper

The Use of Increased Curvature to Reduce the Weight of Body Panels

Current trends in the automotive industry seem to be to use the minimum panel curvature that is possible, on the theory that a box provides the most space at the least weight. The purpose of this paper is to investigate the premise that increased curvature of body panels results in a net weight reduction, since the contribution of the increase in surface area is less than the reduced thickness resulting from the increased overall stiffness of panels with increased curvature. In assessing this concept, an expression has been developed for calculating the surface area as a function of panel curvature and panel size in two perpendicular directions. Using this relationship and previously published Chrysler work on the effect of these same two curvature and panel size variables on panel stiffness, an expression is developed for percent weight change at constant stiffness.
Technical Paper

Design and Manufacturing Guidelines for Ultra High Strength Steel Bumper Reinforcement Beams

The weight of a bumper system with a plastic fascia is significantly affected by the weight of the bumper reinforcement beam. This paper addresses the design and manufacture of ultra high strength steel bumper reinforcements, and provides an example of a proven design. it is shown that light gauge, ultra high strength steel bumper reinforcement beams can, in some cases, be an economically effective way to save weight.
Technical Paper

Evaluation of a New, Dual-Phase, Cold-Rolled Steel - Mechanical Properties, Aging Responses, and Weldability

Mechanical properties and welding characteristics of a commercial, dual-phase, low-carbon, cold-rolled steel are described. The new steel, HI-FORM 80d, exhibits a total elongation of about 24% as produced and develops a yield strength of about 625 MPa (91 ksi) in a formed and paint-baked part. Property uniformity is excellent and the weldability essentially equivalent to an AISI 1006 steel. In addition, the aging response of HI-FORM 80d is such that yield strengths near 550 MPa (80 ksi) can be achieved with strains of less than 2% and lower paint-bake temperatures than are currently in use.
Technical Paper

An Assessment of the Variability in Machining Behavior in Low Carbon Resulphurized Free-Machining Steels

An assessment has been made of the variability in machining characteristics in several grades of low carbon resulphurized steels. Variability results from the influence of processing on structural features such as the sulfide inclusion morphology, the extent and type of solid solution strengthening, and the amount and distribution of special additives such as lead and tellurium. Close control of ferro alloy and scrap composition, deoxidation, casting temperature, lead and tellurium addition practices, and reheat and rolling temperatures are utilized to minimize variability in the machining behavior of free-machining steels.