Many demands are placed on automotive body structures which influence the material selection process. The impact on safety, manufacturability, and longevity are among the most critical, with each of these balanced against cost and environmental concerns.
Formed sheet metal products experience a complex series of deforming, cutting, and joining before being placed in a body structure, where these components will be subjected to complex loading conditions during the product life cycle. Understanding the failure limits and the conditions which produce failure allows for the design of body structures which can withstand these demands.
Testing helps determine whether a metal is suitable for its intended use. Different tests characterize specific performance aspects. Historically, manufacturers relied on tensile testing to understand metal flow. However, new tests help us understand the behavior of new steel grades and their interactions more thoroughly with new manufacturing technologies.
Stamping and die designers are interested in the forming capabilities of the steels they specify. Complex stampings are created from several different basic forming modes, which are sensitive to different mechanical properties. For this reason, steel formability, especially for AHSS grades, cannot be characterized by a single number, but instead must be done for each basic forming mode.
Forming modes and key points include:
- Stretching is the sheet metal forming process where the punch which creates the part shape forces the sheet metal to thin since lock beads prevent metal flow inward from the flange area.
- The steel property that improves stretching is the strain hardening exponent, or n-value.
- Dome testing characterizes stretchability.
- Higher n-values flatten strain gradients, reduce localized thinning that leads to early failure, and allows the forming of more complex parts.
- Tensile testing cannot be used to determine bendability, since these are different failure modes. Different bend tests characterize bendability.
- The failure strain in the bend is related to the total elongation of conventional steel, but are limited by other issues in AHSS grades.
- For equal strengths, most AHSS grades have higher total elongations than conventional HSLA steels. However, several AHSS grades have limited local formability based on their microstructure, and may be at risk for cracking during edge expansion. AHSS grades with multiphase microstructures such as DP and TRIP experience shear fracture that severely reduces the bendability before failure occurs.
- Drawing is the sheet metal forming process where the punch that creates the part shape forces the sheet metal to pull in from the flange area.
- The steel property that improves cup drawing or radial drawing is the normal anisotropy or rm value.
- The Limiting Draw Ratio (LDR) characterizes cup drawability.
- Higher r-value increases the LDR, but the absolute value of the LDR also depends on the lubrication, blank holder load, die radius and other system inputs.
Local necking during uniaxial tensile testing limits the characterization of the stress-strain response to true strain values below uniform elongation. Extrapolating the true stress – true strain curve beyond uniform elongation requires selecting a hardening law on which to base the extrapolation. However, the chosen hardening law dramatically affects the extrapolated the true stress – true strain curve. Figure 1L-20 shows an example of this extrapolation using a bake hardenable steel. Deviation from the real performance leads to inaccurate thinning and fracture predictions, inaccurate springback predictions, and inaccurate predictions of press force and press energy requirements.
Figure 1: The selected hardening law leads to vastly different stress-strain responses extrapolated beyond uniform elongation.L-20
Bulge testing is one method to generate stress-strain data at higher strains, minimizing the need for extensive extrapolation. Another benefit is that the deformation occurs in two directions (biaxial), which is similar to the metal motion seen in most forming operations and in contrast to uniaxial tensile testing.
In biaxial bulge testingK-17, V-7, a circular sample is clamped around its periphery and pressurized from one side using a viscous incompressible medium, forcing the metal to bulge and expand into a cavity as the pressure increases. Figure 2 shows a typical testing configuration.F-12 Flow stress is calculated from the dome height of the bulging blank and the pressure in the viscous medium. A non-contact system equipped with Digital Image Correlation (DIC) measures strain. The ISO 16808 Test Standard details the requirements.I-12
Figure 2: Bulge testing configuration.F-12
For various reasons, flow stress data at the lowest strains are not as accurate as what is generated at higher strains. This leads to practitioners combining curves, using tensile data at low strains through uniform elongation, and bulge data after that. These blended curves result in a thorough true stress – true strain characterization over a wide range of strains, making it applicable to a variety of formed parts. Figure 3 shows a blended curve for the same alloy highlighted in Figure 1.
Figure 3: Flow curves for a bake hardenable steel generated by combining tensile testing with bulge testing.L-20
Biaxial bulge testing provides two critical inputs for advanced material characterizations required for simulation: biaxial anisotropy and biaxial yield stress.