Numerical Simulation of Soil Resistance in Ripping: A case study of a Sandy Clay Soil

Abstract

In this study, a numerical model based on field experiments, laboratory experiments and simulation was developed to predict the soil resistance to ripping in a sandy clay soil. The study identified the soil and operational parameters that influence soil cutting and that are pertinent to the development of a discrete element model. Field experiments were conducted to collect soil resistance datasets. Draft data was measured using the MSI 7300 digital dynamometer logging data directly to a laptop through the serial port. The ripper tines tested were attached to a tool carriage attached to the three point hitch of the towed tractor (i.e. gear lever in neutral position); the dynamometer was attached between the rear towed tractor and the front towing tractor via steel shackles. A 234 factorial experiment in a Completely Randomized Block design was the statistical technique used to investigate the effects of the operating speed, ripping depth and rake angle on the draft force requirement of a 5cm wide ripper tine. The rake angle was the blocking factor in four levels (30o, 45o, 60o and 75o), tillage depth in three levels (15, 25 and 45 cm) and operating speed in 2 levels (3 and 5 km/hr.). Four replications were used to give a total of 96 treatments. The EDEM Academic™ software from DEM Solutions Limited was applied to simulate the soil resistance to ripping using the different ripper tines. The model was calibrated using the angle of repose test; the forces arising due to particle and boundary contact during simulation were calculated using an inbuilt contact constitutive relation and displayed using EDEM Academic™ inbuilt query feature. The draft datasets obtained from the experiments and the simulation were subjected to Analysis of variance (ANOVA) and the student t-test; the coefficient of determination (R2) was determined from linear regression to be 0.986 indicating a good degree of fit of the measured draft datasets to the predicted draft datasets. ANOVA indicated that the rake angle, operating speed and ripping depth significantly influenced the value of the draft force at the 95% level of confidence. The draft force was found to decrease from a rake angle of 30o to attain a minimum value at 45o then increased to attain a maximum at 75o. The rake angle of 45o was thus found to give the minimum amount of draft force while the rake angle of 75o was found to give the maximum amount of draft force. The draft force increased linearly as the tillage depth and operating speed increased suggesting that they are directly proportional. It was established that the discrete element method was applicable in modeling soil-tool interaction processes and thus could be applied in research activities, product development activities for rapid prototyping and in an actual farm setting to swiftly and reliably establish the expected draft forces and thus in extension aid in the establishment of energy requirements for a particular ripping operation. Key Words: Soil resistance; discrete element method; numerical simulation; specific draft; rake angle; ripping depth; operational speed