Optical measurement techniques for structures and systems
The elasticity parameters of rubber-like materials can be obtained from tensile experiments on strips if adequate quantities of the material are available. For biomedical specimens, it is not always possible to harvest enough material to produce strips of manageable size for tensile tests. In that case, elasticity parameters need to be measured in situ. Correct quantitative parameters to describe tympanic membrane (TM) elasticity are an important input for realistic modeling of middle ear mechanics. Up till now, only tensile experiments on cut-out strips have been carried out, which result in inaccurate and incomplete parameters. In the future, we want to determine the elasticity parameters more accurately and completely by inverse finite element modeling of in situ point indentation measurements. In the study presented here, the method of measuring and model optimization is validated on a scaled phantom model of the TM with known elasticity parameters. The phantom model had approximately eight times the size of a human TM and it was made of natural latex used for medical gloves. The elasticity parameters of this rubber, needed for validation, were calculated using a uniaxial tensile test. The measurement method consisted of a point indentation perpendicular on the membrane surface. carried out on different positions. The local surface normal was determined from the Moire shape measurement, and the indentation needle was positioned along this direction. The indentation length and the resulting force on the needle were measured with an LVDT and a load cell. The three-dimensional shapes were also measured with the LCD-Moire profilomeny technique. Afterwards, the experiment was simulated with a finite element model. The rubber type was modeled as a Mooney-Rivlin material. The difference between experimental - and finite element model force-indentation data was optimized using a surrogate optimization routine. This resulted in a range of acceptable parameters, using the experimental - and model shape data, a small range could be filtered out. The resulting parameters describe both force-indentation data and three-dimensional shape measurements, thus describing the rubber type. Our results show that the proposed technique allows us to determine rubber membrane elasticity parameters quite well. The next step will be TM elasticity.