NCAL: Tension-Compression Testing

Summary

We have developed a fully-instrumented tension-compression testing apparatus to measure the evolution of the Bauschinger Effect (tension-compression yield asymmetry) and the ratio of kinematic to isotropic hardening in advanced automotive sheet metals as they experience multiple cycles of tension and compression within the plane of the sheet. These material behaviors must be better understood to accurately model the complex sheet metal forming operations currently used in automobile manufacturing, including the amount of springback observed in formed parts.  Numerical constitutive material models of this complex behavior can be difficult to properly calibrate; therefore, we are also working on developing optimum processes and test matrices to train the model.

Description

The inability to predict the mechanical behavior of new automotive alloys during forming has generated strong demand in industry for more advanced material models and test methods necessary to calibrate them.  Industry is particularly interested in modeling the behavior of advanced sheet metals around draw-beads that partially constrain the edges of the sheets during forming. In the draw-bead region, the sheet metal undergoes fully-reversed loading (from compression to tension), which brings into play both the Bauchinger effect and the changes in yield surface due to the variation in the ratio of kinematic-to-isotropic hardening, which can be  significant for certain materials.  The test method, developed to generate the necessary property data, subjects a tensile sheet specimen to a series of fully reversed (tension-compression) uniaxial load cycles.  Uniaxial loading produces a homogeneous stress/strain distribution in the gauge area of the specimen thereby enabling a better assessment of the level of springback that a formed part will experience due to these complex plastic behaviors.  However, thin sheets buckle easily during the compression step of the cycle which can severely limit the achievable strains in this step. To combat this problem, the out-of-plane motion of the specimen must be constrained by implementing anti-buckling guides on its lateral faces. While many designs for this test have been proposed, our design and method controls the lateral forces required to prevent out-of-plane buckling during compression and measures the ensuing friction. Both the design of the testing fixture and the specimen itself play an important role in obtaining valid measurements at the strain levels typically experienced during real forming operations.

Our approach to address these problems uses a unique testing setup in conjunction with optimization of the specimen geometry.  Our testing setup prevents buckling by using two opposing pneumatic actuators in force control with independent load cells to simultaneously apply and measure the forces on the anti-buckling guides.  The principal advantage of this approach is that the force applied on the anti-buckling guides can be kept constant while adjusting to specimen thickness changes.  The friction is minimized with lubricants and is monitored using the difference in force between the upper and lower longitudinal load cells.  Strain is measured using stereo-DIC on the thickness edge of the specimen since the anti-buckling guides obstruct view of the planar faces of the sheet specimen.  A verified finite element model is used to optimize the specimen design to ensure prevention of various buckling modes while maximizing the uniformity of strain in the gauge area.

Determination of the numerical constitutive law parameters sufficient to train the model from these tests require careful selection of the series of tension-compression cycles tested.  It also requires development of the proper process to determine the constitutive material model parameters from the test data.  We have been working on optimizing this process for the most popular model used by industry (i.e., Yoshida-Uemori) for a broad range of sheet metal materials used in automotive sheet metal forming. The Yoshida-Uemori model is an advanced elasto-plastic constitutive material model, which is widely used to model the deformation behavior in cyclic loading, especially when Bauchinger effect, work hardening and its stagnation, small-scale re-yielding after large prestrain are prevalent.

More details of the test machine are available at: NCAL: Tension-Compression Test Machine.

Major Accomplishments

• Dilip K. Banerjee et al., “Evaluation of Methods for Determining the Yoshida-Uemori Combined Isotropic/Kinematic Hardening Model Parameters from Tension-Compression Tests of Advanced Lightweighting Materials,” MATERIALS TODAY COMMUNICATIONS 33 (December 2022), https://doi.org/10.1016/j.mtcomm.2022.104270; 

• Mark Iadicola, “Data for Numisheet 2020 uniaxial tensile and tension/compression tests”, National Institute of Standards and Technology Public Data Repository, (2020) https://doi.org/10.18434/M32202 (Accessed 2025-07-18)
• M. R. Stoudt, L. E. Levine, and L. Ma, “Designing a Uniaxial Tension/Compression Test for Springback Analysis in High-Strength Steel Sheets,” EXPERIMENTAL MECHANICS 57, no. 1 (January 2017): 155–63, https://doi.org/10.1007/s11340-016-0202-x.