Research On Prestressed Multilayer Stagger-split Ultrahigh Pressure Die

The ultrahigh pressure technology is a science that researches on the generation of high pressure and the law of physical state of matter under high pressure; it is used more and more widely in scientific research and industrial production. High pressure research relies on various kinds of high pressure apparatuses. Large cavity static high pressure die, which has the advantages of a large sample space and a stable high pressure-temperature environment, has become a research focus. It has become more and more urgent for researchers to have a large cavity die with higher stable pressure. However, the difficulty of manufacturing large volume hard alloy is the main obstacle to the large scale of high pressure die.Prestressed multilayer stagger-split section super high pressure die is a novel type of high pressure die. It is combined with the characteristics of the traditional belt type die and the split cylinder vessel, and is composed of a sector block cylinder, a discrete prestressed ring and a hoop ring. Its core advantage lies in the stable temperature field and stable pressure field which is same as the belt die, and it also has a higher bearing capacity and a larger volume of the cavity. Because of it is the small size of cemented carbide, it solves the difficulty of large-scale die. In this paper, the numerical simulation method is used to analyze the stress distribution of prestressed multilayer stagger-split die. We compare the result with that of belt die, and designed the corresponding dies. According to the stress characteristics of the sector block cylinder, we design a double-beveled sector block cylinder die. The bearing capacity of the cylinder is discussed and the experimental verification is carried out. According to the structural characteristics of the sector block cylinder, the cylinder is optimized; the mechanical characteristics of the sector block cylinder during the actual working process are studied; the extrusion process of pyrophyllite medium in the cylinder blocks is analyzed and the stress distribution of pyrophyllite is studied.The main investigation contents and conclusions are listed as follows:1. Design principle and structure characteristics of prestressed multilayer stagger-split ultrahigh pressure diePrestressed multilayer stagger-split die is designed on the basis of belt die, and it has the advantages of the split cylinder vessel. It is a kind of method that can realize large scale of ultrahigh pressure die. The cylinder is designed according to the principle of massive support. The force acting on the inner wall of the cylinder is diffused and dispersed along the radial direction. The friction force between sector blocks further decreases the pressure. In the end, there is only a small pressure acting on the inner wall of the shrink sleeve. The relationship between the two layers and the size of the cylinder is derived. Based on the cylinder size and the inner wall pressure, the size of the interference fit between the cylinders is calculated. The residual stresses of the shrink sleeve are calculated separately when there is internal pressure and there is no internal pressure.2. Finite element modeling and stress analysis of stressed multilayer stagger-split dieBased on the structure characteristics of prestressed multilayer stagger-split die, this paper discusses the boundary condition needed to be applied in the finite element software of ANSYS/Workbench; establishes a finite element model of prestressed multilayer stagger-split die and select of material model, element type, contact and friction condition in the model. The characteristics of stress distribution are analyzed by using the static implicit algorithm and compared with the belt die. The results show that the the cylinder circumferential stress of prestressed multilayer stagger-split die is very small. The equivalent stress and shear stress also obviously decrease. The stress of the interference fit hoop ring is not changed much after the compression cylinder. By adjusting the number of layers and blocks, we can adjust the force of hoop ring. Even if the pressure inside the cavity is large enough, the hoop ring can still keep safe. The bearing capacity of the prestressed multilayer stagger-split die is determined by the pressure of cylinder and the shear stress of the cylinder reaches the strength limit earlier than the equivalent stress.3. Design and stress analysis of double-beveled sector block cylinderThe internal stress of cylinder can be divided into two parts: one is hydrostatic stress and the other is deviatoric stress. Which leads to the failure of the cylinder is a deviatoric stress. In order to reduce the deviatoric stress on the inner wall of the cylinder, we design chamfers angle at the front of the sector block to investigate the stress distribution of sector block cylinder of double bevel. The results show that: The hypotenuse will give double-beveled sector block a lateral support force when double-beveled sector block is compressed. The deviatoric stress of the cylinder block is reduced under the lateral support, so that the shear stress and the equivalent stress of the cylinder block are also decreased. The bearing capacity of double-beveled sector block cylinder is significantly improved. Compared with sector block cylinder die, the stress of outer hoop ring changes very small when the pressure is on. When the numbers of sector block、angle of hypotenuse and length of hypotenuse are 6, 20° and 0.5 mm, the stress of doubled-beveled sector block has minimum stress value.4. Cylinder optimization of prestressed multilayer stagger-split ultrahigh pressure dieIn order to improve the bearing capacity of multilayer stagger-split die, using optimum design method to calculate the optimal size of the cylinder, and research on the influence of height-to-diameter ratio, height ratio and thickness for cylinder’s bearing capacity. The results show that the bearing capacity of the sector block cylinder is determined by the height-to-diameter ratio of cylinder, height ratio and thickness ratio of cylinder. And the reasonable ranges respectively are 1 to 2, 2.4 to 2.8 and 4.5 to 6.5.5. Experimental stress analysis of prestressed multilayer stagger-split ultrahigh pressure dieIn order to verify the accuracy of simulation results, the experimental verification of belt die, prestressed multilayer stagger-split ultrahigh pressure die and double-beveled prestressed multilayer stagger-split ultrahigh pressure die are carried out respectively, and the corresponding oil pressure is 4.1 MPa, 6.7 MPa and 7.3 MPa, respectively. The bearing capacities of the prestressed multilayer stagger-split die and the improved die are much bigger than the one of the belt die. The experimental result of the prestressed multilayer stagger-split die is much greater than the results of numerical simulation. The numerical simulation results show that the bearing capacity of the prestressed multilayer stagger-split die is about 1.3 times bigger than that of the belt die, but the experimental result is about 1.6 times bigger than that of the belt die. The influence of the "flying edge" on the sector block cylinder is analyzed by numerical simulation, and the results show that the "flying side", which is formed during the extrusion process of anvil hammer, has lateral surpport for the side of the sector block. Under the lateral support force, the sector block cylinder’s bearing capacity is more than 7.5 GPa, and the numerical results are basically the same as the experimental results. With the lateral support force of "flying side", the bearing capacity of the optimized sector block cylinder is more than 8.2 GPa.6. The stress analysis of the pressure transmitting medium in prestressed multilayer stagger-split ultrahigh pressure dieIn order to study the stress distribution of the pyrophyllite medium in the cavity. This paper use the Druker-Prager model to define the elastic-plastic constitutive relation, which is generally applicable to geotechnical materials, and use finite element method to analysis the extrusion process of pyrophyllite in the cavity. The results show that the pyrophyllite on the lower surface of the anvil is the area that has the largest flow, and this region has the highest pressure. The central region of pyrophyllite sample has the second highest pressure, and the pressure is gradually decreased along the radial and axial direction. The pressure in the cavity of belt die is much smaller than that of the presstressed multilayer stagger-split die, but the pressure distribution in the two cavities are the same. In order to verify the simulation results, pyrophyllite sample is used in the experimental verification, which is made by intering pyrophyllite powder. When the cylinder is damaged, the oil pressures are 4.1 MPa and 6.7 MPa, respectively. The simulation results and the experimental results are in agreement with the results above.