NIST Center for Metal Forming

METALS

Our objective is to develop the measurement methodology, 

standards and analysis necessary for the US auto industry 

and base metal suppliers to transition from a strainbased to a stress-based design system for auto body 

components, and successfully transfer this technology 

to our customers in industry. With this knowledge, the 

US automotive industry will be able to transition to new 

advanced and lightweight materials more easily, as more 

accurate data and material models will lead to more 

accurate die designs, reducing die tryouts and new model 

development costs.

Objective

Impact and Customers

• Reduction of sheet metal forming die tryouts (redesigns) through improved material data and models will save the 

US auto industry a large portion of the $600M wasted per year in these efforts.

• We have spearheaded the adoption this year by ASTM of the springback cup test (below) favored by industry, 

and developed the data on its robustness, allowing industry to move forward in using this test to characterize the 

springback characteristics of new materials.

• The multiaxial testing technique developed in our lab is 

being installed in the GM research lab and at several 

academic sites.

• Our customers include GM, Ford, United States Steel, 

ALCOA, USCAR, Volvo, ALCAN, and LSTC.

Approach

Materials Science and Engineering Laboratory

The US auto industry spends $600M per year fixing and tweaking forming dies that do not make correct parts. The 

primary reason that the dies are inaccurate is that the computer models of the dies use material models that are 

inaccurate. Upon surveying our industrial partners, we determined that a key NIST role in addressing this problem 

lies in developing new mechanical testing methods and metrology, and also 

developing a fundamental understanding of the interplay between multiaxial 

strain behavior and sheet microstructure. 

We have developed a technique where, for the first time, the sheet’s stressstrain response can be measured along non-linear multiaxial paths. This 

provides unique data on how the multiaxial flow surface changes with 

plastic strain, and this can be used to modify materials models used by 

industry. Concurrent with this, we also measure the evolving crystallographic 

texture of the sheet and the surface roughness, to develop a microstructural 

understanding of the materials’ deformation response.

855-12-21b
Materials Science and Engineering Laboratory

The tensile multiaxial yield surface 

of 5754 aluminum alloy sheet was 

measured from initial yield up to failure 

for the first time. The initial shape of the 

surface closely approximates an ellipse, 

as predicted by most plasticity models of 

forming. However, as the strain levels 

increase to levels typical of forming (5 % 

to 20 % plastic strain), the locus evolves 

asymmetrically, and an apex forms in 

the direction of balanced biaxial (BB) 

straining. At the highest strain levels, 

the error between the predicted and the 

measured flow stresses exceeds 25 %. 

On the same samples, the crystallographic 

texture evolves from a rolling texture to 

one of the predominant FCC deformation 

textures, as expected. Unexpectedly, the 

deformation texture that develops when 

the sheet is deformed in the rolling 

direction (RD) differs from that which 

develops during deformation in the 

transverse direction (TD). 

The two deformation textures are 90 

degree rotations with respect to one 

another, but the twofold symmetries of 

the textures leads to an asymmetry in the 

stress needed to form the textures, which 

coincides with the measured asymmetry 

in the yield surface. These results are 

being modeled using a Taylor-type 

description of the flow behavior of 

polycrystalline aggregates undergoing 

evolving textures, which is the basis of 

many common plasticity models, to see 

if this formulation adequately captures 

the behavior. 

Surface roughness, and how it changes 

with plastic strain and contributes to 

forming friction, is another highly 

important issue. Conventionally, roughness is measured with a profilometer. 

However, we have found that these 

measures may be misleading—as 

both measurement uncertainties and 

statistical errors are compounded when 

the 2-D lines are extrapolated to the 

entire surface. 

Our new approach uses data from a 

scanning laser confocal microscope, 

an instrument that builds a point-bypoint image of a surface in 3-D. The 

data from a single image—representing 

an area of about 1000 μm x 800 μm 

x 20 μm in depth—are analyzed using 

mathematical techniques that treat 

every point in the image simultaneously 

to produce a roughness measure that 

considers the entire 3-D surface rather 

than a collection of 2-D stripes. This 

technique will reduce uncertainties in the 

measured surface roughness and allow 

more accurate characterization of the 

evolution of roughness with strain.

Accomplishments

Mark Iadicola

Steve Banovic

Mark Stoudt

Thomas Gnaeupel-Herold (NCNR)

Adam Creuzige

Tim Foecke

(Metallurgy Division)

(301) 975-6592

timothy.foecke@nist.gov

www.nist.gov/metallurgy

Learn More Publications

Iadicola MA, Foecke T and Banovic SW Experimental Observations of Evolving Yield Loci 

in Biaxially Strained AA5754-O International Journal of Plasticity, 24: 2084 (2008)

Banovic SW, Iadicola MA and Foecke T Textural Development of AA 5754 Sheet 

Deformed under In-Plane Biaxial Tension Metallurgical and Materials Transactions A, 

39A: 2246 (2008)

Foecke T, Iadicola MA, Lin A and Banovic SW A Method for Direct Measurement of 

Multiaxial Stress-Strain Curves in Sheet Metal Metallurgical and Materials Transactions 

A, 38A: 306 (2007)

Foecke T and Gnauepel-Herold T Robustness of the Sheet Metal Springback Cup Test

Metallurgical and Materials Transactions A, 37A: 3503 (2006)

Yield locus evolution

Texture evolution with strain

Surface roughness evolution with strain