Study On Drying Induced Cracking Of The Rice Kernel By Acoustic Emission Method

The rice kernels after harvesting would undergo sequent processing steps such as drying and rice milling.In these processing steps,the rice kernels will be subjected to various stresses such as thermal stress,wet stress and mechanical stress,which would thereby generate stress cracks or fissuring.The generation and propagation of stress cracks inside the rice kernel is the basis for exploring the mechanism of rice fissuring;however,rice fissuring is a big challenge in the field of rice research.Due to the shortage of the effective detection methods for stress cracks in rice kernels,this thesis proposes the application of acoustic emission(AE)technology to study rice cracks.Although AE technology has widely applied in large structures such as bridges and pressure vessels,its application in small biological particles such as kernels has not yet been discovered.In order to verify the feasibility of the above propostion,this paper establishes a rice mechanical compression and hot air drying AE experimental setup,to measure and analyze the acoustic emission characteristics under the compression loading and drying conditions.Experimental results show that the AE technology can effectively detect the AE signal emitted by the Tianyou 3301 rice kernel during the compression loading and drying.For rice kernels with a moisture content of 20%,the maximum stress intensity is about 8 MPa.The maximum value of the acoustic emission ringing is about 100 s to 110 s,which appears at the same time with the rice stress yield.The stress intensity of the rice kernel with 15%moisture content reaches above 10 MPa.The moisture content and temperature influences the generation of stress cracks in the kernel.The acoustic emission signal of the grain with a low temperature and moisture during the drying process is less than that with a high water content and temperature drying;also have a higher maximum signal value which appeared a bit later.In the hot air drying process of the rice kernel with a moisture content of 22%at 60? drying temperature,the acoustic emission timing characteristics are closely related to the drying characteristics:The moisture emission signal generated in first 7 min may caused by the temperature gradient;The dense AE signals generated in around 40 min is mainly due to the moisture content gradient,which is verfied by the simulation result that the maximum moisture content gradient also appeared in about 40 min.The acoustic emission experimental results confirms some typical rice drying phenomena of rice,but also provide a possible explanation for these phenomena.Based on the analysis of the AE signals,it was found that when the grain is completely dried in the glass state or the rubber state,the kernels are not prone to fissuring,and the kernels which undergoes the glass transition is prone to fissuring.When the drying temperature was controlled at 30? or 60?,the rice is always kept in the glass or rubber state,which can effectively inhibit the generation of cracks,so that the whole rice rate can be guaranteed.