Non-destructive detection of stress cracking in rice kernel by acoustic emission

Abstract: Rice kernels after being harvested would encounter various stresses in its sequent operations such as drying, milling. These stresses easily cause the stress cracks. Stress cracks are classified into micro cracks, meso cracks and macro cracks. The stress cracks are generally propagated from micro to meso cracks, and even to macro ones, which are called fissuring. Rice fissuring would reduce the head rice yield and thus the economic value of rice. However, there is no effective method to study stress cracks. In this work, the acoustic emission method was first proposed to investigate the cracks. The hot air drying and mechanical compression experiment were performed. The experimental material was the Tianyou 3301 indica hybrid rice with an initial moisture content of 22%. The acoustic emission (AE) pulse signals generated by the stress cracking inside the rice kernel during the drying and mechanical compression processes were monitored by AE instrument respectively. The relationships between drying, stress and strain, AE pulse signal of the rice kernel were analyzed based on the rice drying experimental and numerical results, and AE results. Experimental results show that the AE signals in the drying and mechanical compression processes can be measured effectively. In the air drying process, the timing sequence property of the AE signals generated in the rice kernel agreed well with the drying characteristics and some typical drying phenomena. In the case of the rice kernel with an initial 22% moisture content dried at 60 ℃, the AE signals were active in the initial heating stage of 0-7 min and these signals were believed to be caused by the temperature gradient. In the 7-40 min, the AE signals were weak because the temperature gradient diminished but the moisture gradient increased gradually. The AE signals became active again after 40 min, which was expected to be caused by the moisture content gradient. It was observed that their peaks were corresponding to the maximum moisture content gradient. In the sequent cooling process, the intensive AE signals were observed, which agreed well with the fact that the rice fissuring would increase fast in the cooling process after a high-temperature air drying process. In the mechanical compression process of 15 N, the mechanical stress increased linearly and the dense AE signal areas appeared sequentially, indicating the stress cracks propagated from micro to meso ones gradually during the compression process. No rice fissuring was observed. In the mechanical compression process of 30 N, the peak value of the AE signal appeared simultaneously in the time of stress yielding of the rice kernel; and also the rice fissuring was observed. Compared with the kernel with the initial 22% moisture content, the rice kernel with an initial 14% moisture content had a weaker AE signal distribution in the mechanical compression process of 30 N, which agreed well with the fact that the kernel with a low moisture content has a higher strength. This work demonstrates that the AE method is an effective and promising one to study the rice stress cracking.