Experimental validation and test structure design for an orthotropic constitutive model

A constitutive model for the elastic response of an aluminum honeycomb material had been developed by performing virtual tests on a detailed finite element model of a unit cell of the honeycomb material. The derived constitutive model treats the honeycomb as a continuous material rather than the detailed structure of the unit cell. The orthotropic model of the honeycomb requires nine distinct parameters that were determined from these cell-level computational simulations. To validate the elastic constants predicted by the simulations, tests were required which would fully exercise the orthotropic model. Modal tests have the advantage of deforming a structure in multiple strain patterns simultaneously, thus exercising multiple aspects of the model. For these tests, we designed and fabricated several test structures, including the honeycomb and other materials, and measured their modal frequencies and mode shapes. The design objective was to create test structures in which all the material parameters participated in affecting the modal frequencies, so that the parameters would be observable in the test data. As part of the design and validation process, detailed finite element analyses of the test structures were performed using the elastic constants from the cell-level simulations. Using these structural analyses, one could compute the sensitivities of the modal frequencies with respect to the elastic constants and determine which parameters would actually affect (be observable) the modal frequencies, The adequacy of the computed elastic constants and the assumed orthotropic model was judged based on whether the model of the test structures predicted the observed experimental data and whether the measured data was sensitive to all the material constants.