ONE WAY TO VERIFY MATERIAL MODELS
For instance, initial simulations of an electrical seal revealed that material properties necessary for good analysis (compressive stress-strain data and planar tension data) were unavailable and could not be generated by the material vendor. Both values were needed for analysis, along with strain-energy constants.
The analysts turned to DatapointLabs, Ithaca, N.Y. (www.datapointlabs.com), a firm that characterizes physical properties of materials, especially nonmetals, for finite-element modeling. The material lab works with the vendor of the seal manufacturer's FEA software, Ansys Inc., Canonsburg, Pa. (www.ansys.com), so properties are delivered in a file easily read by the Mooney-Rivlin material model used in Ansys. Mooney-Rivlin simulates products that undergo large strains and small volume changes, such as silicon rubber.
"Ansys V7 has curve-fitting capability for material models," says Hubert Lobo, founder and president of DatapointLabs. "We tested the validity of the material-model parameters by recreating tensile and compressive test setups within Ansys."
Lobo verified the compressive model for silicon rubber using Ansys Solid185 elements. Measured and predicted stresses were compared at a strain of 30%. Coefficients for the Mooney-Rivlin model were generated using an Ansys curve-fitting utility and a generic curve-fitting program.
"Most FEA programs have limited means for fitting materialmodel parameters to test data," adds Lobo. "In many cases, the curve fitting is complex and requires expertise typically not available to the analyst. Variability in the process and the quality of the material-model parameters will differ, depending on who does the regression and the tools being used."
Compressive stress-strain data are measured by placing a disk of the material between lubricated platens and compressing it. "Tests can be performed at different strain rates and temperatures, even in the melt state," notes Lobo. Planar-tension data approximates a shear state for hyperelastic materials. Tests are performed in tension on thin specimens. Constants are derived by fitting raw data to the Mooney-Rivlin equation.