| The use of materials has always been accompanied by our entire history of human development.Because with the gradual progress of mankind into the deep sea and deep space,as well as the use of various new types of major engineering equipment and new instruments on the surface of our planet,China and even countries in the world today impose stringent requirements on material performance requirements.,and fracture as the most complete failure mode of the material,its in-depth study is very necessary.According to the estimation of the World Steel Association,the annual growth of steel consumption in the world is about 10%.Every country has incurred an expensive price because of the fracture accident of steel materials every year.Therefore,it is of great practical and scientific value to study the failure mode-fracturing in the process of material use.This article is based on the prototype developed by its own research group,based on the study of the fracture problem of typical structural materials,and focuses on the fracture toughness,an important index in the material fracture problem.Based on the background of fracture mechanics and the analysis of its development history,this paper reviews the development status and application fields of fracture toughness at home and abroad.Based on the characteristics of self-developed instruments,special fixtures for fracture toughness were designed and simulated.Using the characteristics of the developed instrument,relevant tests and researches on the fracture toughness of typical materials were carried out.The main research contents of the paper are summarized as follows:In this paper,ANSYS simulation software is used to analyze the static and dynamic simulation of the special fixture designed.The analysis results show that the strength and rigidity of the fixture fully meet the requirements.Based on the design of the fixture and the characteristics of the instrument,the fracture toughness of the typical structural material 45 steel and aviation 7075 aluminum and 20 steel was studied.In this paper,the influence of alternating loads on the fracture toughness of the above-mentioned materials is studied as a whole.The idea of orthogonal design is used in the design of the entire test scheme to ensure that valid experimental results can be extracted from a large number of variables and that invalid experimental data can be avoided.During the test and test process,the acoustic emission system was used to detect the energy density distribution of the acoustic emission source during the entire fracture process of the test piece.It was found that two collision peak areas of energy will occur in the aluminum alloy test specimen during the test of the static fracture toughness.These are the nucleation stage of cracks and the instability expansion stage of cracks,respectively,but the energy collisions are lower in the steady state expansion stage.As the core of this paper,the influence of alternating loads on the fracture toughness of materials is studied.In the whole research process,the influence of different frequency,different average value and different amplitude sinusoidal waveform plus alternating load on the fracture toughness of the specimen is mainly studied.Throughout the experiment process,combined with the characteristics of the instrument,the superficial depth of the metallographic structure of the specimen surface was observed in situ,and the relationship between the micromechanical behavior of the above-mentioned three typical materials and the deformation damage mechanism was analyzed.It was found that when different alternating loads or other variables are applied,the normal direction of the crack plane of the specimen is parallel to the material forming rolling direction,the fracture toughness of the material is low,and the fracture toughness of the material is low when the alternating load is applied.Fracture toughness value in static load mode when no alternating load is applied.In comparison with the in-situ observation of images with deep depth of field,it was found that when the same force control was applied,in the six different opening directions,only the normal direction of the crack plane and the direction of the rolling direction were the fastest.When the same displacement control is applied,it is only when the normal direction of the crack plane is parallel to the rolling direction that the required cracking load is minimal when the crack tip is unstable.In this paper,the effects of different alternating loads on the fracture toughness of the specimens are analyzed and compared.At the same time,the trend of the fracture toughness of the materials under different temperature gradients is compared.It is better to use the auxiliary instrument—acoustic emission and in-situ observation methods.The deep reasons for the gap between the fracture toughness of specimens in different directions were compared,so that in the engineering design,many structural design failures due to improper material sampling directions can be circumvented,which is of great scientific significance and practical value. |
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