FINITE-ELEMENT MODELING OF TRANSIENT RESPONSES OF APPLES TO IMPULSE EXCITATION

Finite element models were developed using MARC to study transient res ponses of apples to impulse excitations under a real instrument config uration. Apples were modeled as an axisymmetric composite material com prised of skin, cortex, and core. The modeling approach was validated using a homogeneous elastic material. Simulation results show that the first resonant frequency was not affected by the geometry, size, and mechanical properties of the fruit and was related to the contact mate rial. The square of the second resonant frequency was linearly related to Young's modulus of the fruit (r(2) = 0.999). Apple skin had a smal l effect on the second and third resonant frequencies (less than 2%). Core had a large effect on the second resonant frequency (up to 15%) b ut had a negligible effect on the third one. Firmness measurement usin g the second resonant frequency was affected by apple shape (up to 11% ) but not by apple size. Shape did not affect the third resonant frequ ency. Contact material and area and the input signal affected the ampl itude of the frequency spectrum but did not affect the resonant freque ncies. Resonance measurement was sensitive to sensor's location on the fruit's equator Resonance identification was enhanced by using a cont act material slightly softer than the apple, using a larger contact ar ea and using an input acceleration with the frequency approximately eq ual to the highest output frequency of interest.