Structural Design And Finite Element Analysis Of Double-layer Split Ultrahigh Pressure Die

Ultrahigh pressure apparatus plays an irreplaceable role in solid geoscience,condensed matter physics,material chemistry and and industrial production of superhard materials.With the deepening of scientific research and the improvement of industrial production level,the requirements for ultrahigh pressure apparatus are also increasing.At present,there is an urgent need for ultrahigh pressure apparatus with a high pressure capacity and large cavity simultaneously.However,this is limited by the pressure-bearing capacity of ultrahigh pressure die and the difficulty in manufacturing large-size cemented carbide.Therefore,it is of great significance to carry out structural design of novel ultrahigh pressure die for both high pressure scientific research and industrial production of diamond.Split-type die is a novel ultrahigh pressure die based on the idea of “dividing before cracking”.By dividing the traditional belt-type die,the circumferential tensile stress on the inner wall of cylinder can be effectively reduced and the pressure-bearing capacity of die can be improved.At the same time,the size of the cylinder is reduced significantly,which solves the difficulty in manufacturing large-size cemented carbide and makes the large-scale of sample cavity available.Although some research works have been done on split-type ultrahigh pressure dies,there are still some respective shortcomings,which need further reasonable design and deeper investigation.In this paper,a novel double-layer split ultrahigh pressure die(DLSD)was studied.The stress and deformation analysis and pressure-bearing capacity prediction of the DLSD were carried out by the finite element method(FEM).The double-layer split structure was optimized.Relevant experimental verifications were carried out.The aim is to provide theoretical basis for the rational design and practical application of split-type ultrahigh pressure die.The main contents and conclusions are summarized as follows:1.Structural design and mechanical analysis of double-layer split ultrahigh pressure dieThe design principle and technical characteristics of the DLSD were analyzed.Based on the thick-walled cylinder theory,the mechanical analysis of DLSD was carried out.The optimum design principle of the multi-layer composite cylinder was discussed and the calculation method of the interference was deduced.The geometric dimensions of DLSD were determined.2.Finite element model and interference assembly analysis of double-layer split ultrahigh pressure dieThe material and failure criterion,interference contact algorithm,analysis unit and the simplification of loading conditions were discussed.The finite element model of DLSD was established.Finite element analysis of interference assembly process of the DLSD was carried out by using static implicit algorithm.The deformation and stress distribution characteristics of cylinder and supporting rings were studied.3.Finite element analysis of double-layer split ultrahigh pressure die under high pressure loadStress distributions of the DLSD under high pressure were studied and compared with the belt-type die by FEM.The results show that the circumferential tensile stress on the cylinder of DLSD was obviously less than that of the belt-type die.The stress state of cylinder was improved,and the Mises stress and maximum shear stress were reduced.But the stresses on supporting rings were similar to those of the belt-type die.The stress and displacement distribution characteristics of cylinder and supporting rings for single-layer split die,double-layer split die and multi-layer split die were compared.The results show that the stress of the double-layer split die was similar to that of the single-layer split die.Considering the manufacturing difficulty,the die should be split as many layers as possible.However,the stress of the outer supporting ring increases obviously when the die was multi-layer split,and the radial displacements of cylinder blocks increased.This indicated that the double-layer split die structure was more reasonable.Pressusre-bearing capacities of the belt-type die and DLSD was predicted.The maximum pressusre-bearing capacities are 5.66 GPa and 6.57 GPa,respectively.Destructive experiments were conducted to test the pressure-bearing capacity.The experimental results show that the pressure-bearing capacity of DLSD increased by 19.1% with respect to the belt-type die.4.Optimum design of double-layer split ultrahigh pressure die structureThree composite structures,i.e.: inner quadrate and outer round,inner round and outer quadrate,inner quadrate and outer quadrate,were designed for the split layer.The stress distributions of DLSD with three composite structures were compared by FEM.The results show that the stress of the cylinder decreased significantly when the inner quadrate and outer round structure was adopted,but the stress of the first layer supporting ring increases slightly.When the inner round and outer quadrate structure was adopted,the stress of the cylinder VII basically remains unchanged,while obvious stress concentration generated on the first layer supporting ring.When the inner quadrate and outer quadrate structure was adopted,the stress of the cylinder decreased significantly,while the stress concentration on the first layer supporting ring also generated.A structural improvement method based on variable interference fit was proposed to improve the stress concentration.The effects of four different forms of variable interference on reducing stress concentration were studied.The results show that the stress concentration decreased significantly when an arc structure or multi-segment continuous plane structure was adopted for the inner wall of supporting block.The maximum stress reduced by 18.1% compared with that when the round cylinder was used.The maximum pressure-bearing capacity of the DLSD with inner quadrate and outer quadrate structure was 8.91 GPa,which was 35.6% higher than that of the DLSD when both the cavity and cylinder were round.The finite element analysis results were verified by destructive experiments of the cylinder and supporting ring blocks.The experimental results are basically consistent with the finite element analysis results.5.Research on influence of the numbers of split blocks on double-layer split ultrahigh pressure dieThe double-layer split structures with different numbers of split blocks were designed.The influences of the number of split blocks on stress distributions and cavity deformation were studied.The results show that when the number of split blocks was 3,the stress of the cylinder was larger and the deformation of the cavity was serious,which indicated that the double-layer split structure with 3 blocks was unreasonable.As the numbers of blocks increase from 4 to 10,the stress of the cylinder increased gradually,and the pressure-bearing capacity decreased accordingly.At the same time,the radial displacement of the cylinder blocks decreased gradually,thus the deformation of cavity decreased.With the increase of numbers of blocks,the stress of the first layer supporting ring can be maintained at a lower level.But the stress of the second supporting ring increased gradually,and the stress differences between four layers of supporting rings increased.The above results show that the number of split blocks was not the more or less the better.There are respevtive advantage and weaknesses when more or fewer blocks were adpoted.The pressure-bearing capacities of DLSDs with different numbers of split block were predicted by FEM.The DLSDs with 4 and 8 blocks were tested to selectively validate the finite element analysis results and a good agreement was obtained.6.Research on structure design of 4-8 type double-layer split ultrahigh pressure dieBased on the study on numbers of split blocks,a 4-8 type DLSD was designed.Two types of layout for split plane with parallel split and stagger split were studied.The distributions of stress and displacement of cylinder blocks were studied by FEM.The results show that the layout for split plane has little effect on die stress and cylinder block displacement.Meanwhile,the stress and displacement levels of the cylinder can be basically the same as those of when the cylinder and first layer supporting ring were split into 8 blocks.The stress levels of the supporting rings can be basically the same as those of when the cylinder and first layer supporting ring were split into 4 blocks.The effects of height-diameter ratio of cavity,cylinder size,interference,cylinder cone angle and die end angle on the stress and cavity deformations of 4-8 type DLSD were studied.Afterward,a compact DLSD was designed with three layers of supporting rings used.With the material performances of supporting rings being fully utilized,the die structure can be more compact,the size of the supportring can be reduced,and the manufacturing difficulty and cost of ultrahigh pressure die can be reduced.