Stretching

Stretching

Forming Modes

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. In contrast with drawing, significant metal thinning occurs in stretching, especially in the biaxial tension mode. The biaxial increase in surface area reduces the metal thickness, maintaining the constancy of volume. The thinning soon reaches the onset of the local neck and failure as defined by the appropriate forming limit curve. The steel property that improves stretching is the strain hardening exponent, or n-value.

Stretchability, or the ability for a sheet metal to be stretched with no metal flowing from the flange or binder, often is assessed by the hemispherical dome test. Here, a hemispherical punch (usually with a 100 mm diameter) deforms a fully clamped blank. This ensures pure biaxial stretch without metal flowing from the blank into the deformation zone (Figure 1).

Figure 1: Stretch forming generated by a hemispherical punch stretching a locked circular blank.

Figure 1: Stretch forming generated by a hemispherical punch stretching a locked circular blank.

 

Comparing the ratio of maximum dome height to punch diameter (H/d) is one way to view the results. Figure 2 illustrates a typical test output. Note the maximum dome height (H/d) at failure decreased as the yield strength increased and the n-value decreased.

Figure 2: Dome stretch tests of 1mm thick steel using a 100 mm hemispherical punch and a clamped blank.C-9

Figure 2: Dome stretch tests of 1mm thick steel using a 100 mm hemispherical punch and a clamped blank.C-9

 

Additional stretch tests are possible with the hemispherical dome tester other than the dome height at failure shown in Figure 2. The limiting dome height (LDH) test stretches a rectangular steel strip which is locked in the longitudinal direction (Figure 3). Typically, a conventional rust preventive oil coats the blanks. Strips of different width are tested, with a circular lock bead preventing metal flow from the binder in the regions where the blank dimensions are large enough. The output of this test is the maximum dome height at failure. Figure 3 shows the achievable hemispherical dome height is substantially higher for the TRIP steel compared to the HSLA steel grade of equivalent tensile strength.

Figure 3: Limiting Dome Height is greater for TRIP than HSLA at the same tensile strength.T-2

Figure 3: Limiting Dome Height is greater for TRIP than HSLA at the same tensile strength.T-2

 

The same tooling, steels, and lubricant from Figure 3 generated the thinning strains in Figure 4. Instead of forming to failure, the 50 mm radius hemispherical punch stretched the dome height to only 25 mm for both steels. The high n-value of TRIP steels minimizes strain gradients and reduces localizes thinning, helping to delay necking and form more complex geometries.

Figure 4: TRIP steel experiences less thinning than HSLA steel of the same tensile strength when formed to a constant dome height.T-2

Figure 4: TRIP steel experiences less thinning than HSLA steel of the same tensile strength when formed to a constant dome height.T-2

 

The Limiting Dome Height test results for EDDS (vacuum-degassed interstitial-free) steel and three Advanced High Strength Steel grades are in Figure 5. Instead of plotting the various dome heights (as in Figure 3) to find the minimum value, Figure 5 simply shows the minimum value for each steel. TWIP (Twinning Induced Plasticity) steel has unique properties for stretchability and total elongation. Stretchability exceeds even that of EDDS IF steel.

Figure 5: Limiting Dome Height values reflect relative stretchability of three AHSS compared with a low strength IF steel.P-2

 

Bulge and Dome Testing

Bulge and Dome Testing

Testing and Characterization

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

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.

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

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.