Finite element models of watermelon and their applications

The firmness of watermelon can be nondestructively evaluated by measuring the resonant frequency of the watermelon. The watermelon was excited by a wooden pendulum ball and the response signal was captured by an accelerometer. The FE models of watermelons were established and the optimum location of the force excitation and the suitable response measurement sensor for measuring the resonant frequency of watermelon were determined. The middle part on the fruit surface was suggested to be used while estimating the fruit firmness by the resonant frequency of the first-type longitudinal mode. To measure the flexural mode, the directions of both the exciting force and sensory axis of the response measurements sensor should be kept in tangent to the fruit surface. The comparison between experimental results and the finite element estimated results confirms that the model is in reasonable agreement with real fruit. It was found that the Young's modulus was an adequate indicator for firmness, and the high correlation between the resonant frequency and Young's modulus existed, R²=0.90. The results of this study may be helpful to develop the non-destructive firmness sensor for spherical fruits.