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A New Precracking Method Based On Strain Introduced And Fracture Toughness Evaluation Of Ceramics

Posted on:2021-05-11Degree:MasterType:Thesis
Country:ChinaCandidate:Y J WeiFull Text:PDF
GTID:2381330605455534Subject:Materials Science and Engineering
Abstract/Summary:PDF Full Text Request
Accurately evaluating the fracture toughness,which is one of the important basic mechanical parameters of ceramics,or the ability to resist crack propagation,and the quantitative index to measure toughness of ceramics is closely related to the design,preparation and safe service of ceramic materials.The key of single edge precracked beam(SEPB)method which is recognized as the most accurate test method to determine the fracture toughness of ceramic due to its samples with intrinsic cracks is how to introduce a direct crack in the ceramic sample.Because of the intrinsic brittleness and low limit strain for ceramics,the crack in ceramic material will propagate rapidly once the crack occurs in the precracking testing.It is difficult to obtain the precracked samples,which is regarded as an international problem.In this paper,based on the fact that SEPB method can be used to evaluate the fracture toughness of ceramics accurately,in order to solve the difficult problem of preparing SEPB samples,a new method named as strain induced cracking method was proposed to precrack,and related research was carried out on the evaluation of fracture toughness of ceramics materials.Firstly,based on the excellent tensile strength for the metal and strain dependence of crack propagation in brittle materials,a new simple and universal precracking method of ceramics materials,e.q.strain induced cracking method,was proposed.After adhesive bonding brittle material and ductile material together for loading,the interface constraint of the ductile material on the brittle material and the stress redistribution of the two materials at the interface can effectively restrain crack propagation in the brittle material,which is used to realize the slow propagation of the crack.Strain induced cracking method was used to precrack on alumina,silicon carbide and silicon nitride.The crack length was closely related to the tensile zone height of the ceramic material.Compared with traditional bridge indentation method,the advantages of strain induced cracking method,such as high success rate,low cost,small load and so on,are included.The fracture toughness of alumina,silicon carbide and silicon nitride measured were 4.45±0.41 MPa·m1/2,3.45±0.34 MPa·m1/2,5.23±0.49 MPa·m1/2,respectively.Secondly,to monitor and analyze acoustic emission signals during the precracking tests of SEPB method,which can improve the successful rate and simplicity of preparing SEPB sample,thus obtain the precracked samples of specific length,it was combined strain induced cracking method and acoustic emission technology in this paper.In addition,acoustic emission can also be used to characterize the damage evolution of ceramic materials.Finally,compared with single edge notched beam(SENB)method and single edge V-notch beam(SEVNB)method,SEPB method was analyzed.SEPB,SENB and SEVNB samples were prepared by strain induced cracking method,mechanical processing and laser processing technology,respectively.The simplicity and accuracy of the three test methods were analyzed according to the morphology characteristics of notches or cracks,fracture morphology characteristics,load displacement curves and fracture toughness results.SEVNB method using laser processing technology can improve the simplicity of SEVNB samples,but it also brought high cost.SEPB method using strain induced cracking method can greatly improve the simplicity and economy.
Keywords/Search Tags:Fracture toughness, Single edge precracked beam(SEPB) method, Strain induced cracking method, Acoustic emission technology, Single edge V-notch beam(SEVNB) method
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