In this research, a machine vision system was developed to measure the contact area and dimensions of agricultural samples in mechanical properties testing. Two probes were made of aluminum and round glass panes. The Universal Testing Machine was equipped with these probes. One camera was positioned inside the probe to monitor the contact area between probe and samples. Other camera was located outside of probe to measure dimension of samples during the test. Onions were selected as study products and those mechanical properties were measured using this machine vision system. Also, to verify the results of this system, the mechanical properties were calculated using conventional methods. The effects of onion cultivars (red and yellow), loading direction (polar and equatorial) and loading speed (15 and 25 mm/min) on the size of contact area, the stress and strain, elasticity modulus and Poisson’s ratio were examined. The results showed that there was no any statistically significant differences between conventional method and our presented method at 99% confidence level. Therefore, a machine vision system can be replaced with conventional method. It was possible to assimilate the shape of contact area to a near perfect circle. The effect of loading direction on Poisson’s ratio and the loading speed on stress and elasticity modulus were significant. Red onions under polar loading with speed 15 mm/min had maximum Poisson’s ratio, stress and modulus of elasticity. The stress was obtained as 0.281 ± 0.044 MPa. The values of elasticity modulus were obtained as 2.56 ± 1.4 and 2.77 ± 1.8 MPa for yellow and red onions, respectively. The Poisson’s ratio along x and y axis were obtained as 0.393 ± 0.05 and 0.375 ± 0.07, respectively. Polar loading samples were easy to deform laterally compared to equatorial loading samples. The contact area, stress and Poisson’s ratio increased with increasing deformation while the modulus of elasticity decreased.

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