Controversy about the Origin of the Grain Boundary Internal Friction Peak and Its Clarification

In the seventies some scientific workers from France and Italy suggested that the grain boundary internal friction peak (named the K (e) over cap peak in the literature) widely accepted as a grain boundary process, is originated from the motion of lattice dislocations. Since this problem is one of fundamental importance, this controversy has drawn much international attention. Started from 1982, the Hefei research group made a critical analysis of the large amount of literature concerning this problem and,performed a series of crucial experiments to clarify the controversy. It is concluded that the irrelevant evidence suggested by the controverters comes from the farfetched interpretation and the mis-identification of the internal friction peaks appeared under various experimental conditions and different states of the specimens.

Method for measuring rice grain internal damage degree undergoing threshing force

During the threshing process of rice,the grains fall off the head of the rice ear due to the impact of the threshing bar.At the same time,the impact force of the threshing element causes a certain degree of damage to the grain.However,there are relatively few methods to analyze the internal damage of rice grains during the threshing process.In this study,the connection force between rice grains and stalks and the compressive bearing capacity of the grains were tested on a push-pull test machine,and then the critical impact force and velocity of rice grains during plastic deformation and brittle fracture were obtained by Hertz theory.On this basis,the quantitative evaluation model of grain internal damage was established through the extraction and calculation of the damaged area inside the grain,and the damage degrees inside the grain under different loading times and loading forces were analyzed.The results showed that the average threshing force required for rice grains is 1.57 N(variance is 0.0529),and the critical impact forces for plastic deformation and brittle fracture of the grains during threshing are 138.79 N and 145.77 N.Since the threshing force during the threshing process was 43.9-71.9 N,it could be known from the internal damage model that the grain is in the safe loading area.Under the same load,the vertical pressure causes the most damage,the lateral pressure takes second place,and the positive pressure was the least.The results of this study can provide a basis for the development of combine harvester and rice grains damage evaluation.

Material flow behavior modeling with consideration of size effects

Size effects make traditional forming theories infeasible in analyzing the micro-forming process, so it is necessary to develop an accurate material model to describe the material flow behavior with consideration of size effects. By studying the size effects of the flow behavior of H80 foils experimentally, it is found that the foil flow stress and strain hardening ability reduce significantly with the decrease of foil thickness. The reduction of the proportion of internal grains which own complete grain boundaries is the main cause of size effects of foil flow behavior. Moreover, grain refinement can reduce the size effects on material flow behavior. On these bases, a phenomenological material model has been developed to mathematically describe the material flow behavior with consideration of the effects of geometry size, grain size and strain hardening behavior. The reasonability and accuracy of this new model are verified by comparing the calculation values with experimental results in metal foil tensile and micro-bulk upsetting experiments. These experimental results and the proposed model lay a solid foundation for understanding and further exploring the material flow behavior in the micro-forming process.