Research On Fatigue Fracture Of Metal Pipe Under The Load Of Composite Vibration

Metal pipes are in great demand and widely used.Many important parts and components in the machinery manufacturing industry are made of various metal pipes.Various metal pipe structures are often subjected to complex and changeable stresses in the practical application process,which leads to crack initiation and propagation.Finally,the breakage leads to mechanical failures and even engineering accidents.Moreover,the actual stress level of the corresponding parts when fatigue failure occurs is usually less than the theoretical stress calculated from the static fracture of the pipe structure.Therefore,the research on fatigue fracture of metal pipes is the guarantee and foundation of structural safety design,and it is the reference and basis for the life evaluation of its products.It has important significance in theoretical research and engineering application.At present,the existing research is mostly confined to single-frequency vibration fatigue or tension-compression fatigue and torsion fatigue of metal materials.Fatigue fracture research of metal materials under composite vibration load is lacking.And the crack initiation mechanism and fracture law of metal pipes are rarely involved.Aiming at these problems,the fatigue fracture of metal pipes with prefabricated V-notches under composite vibration load was investigated based on the self-developed high and low frequency composite vibration experimental device.The main research contents include:（1）The structure of the composite vibration experimental device is analyzed,including the composition mode and control flow of mechanical system and computer control system.The vibration principle of the experimental device is expounded,and the composite vibration characteristics are analyzed.The output parameters of different variables such as frequency,exciting force and amplitude are given.At the same time,the frequency measurement and control process,the adjustment method of the excitation force and the amplitude variation of the no-load and the loaded vibration are discussed.（2）The final fracture of metal pipes can be regarded as a comprehensive problem of fatigue and crack body.Systematic analysis based on the theory of fracture mechanics and fatigue.Based on the basic concept of fatigue fracture of metal materials,the fatigue fracture type,fatigue fracture process and the characteristics of different cracks and sections are analyzed.The initial part and mechanism of fatigue cracking of metal pipes under composite vibration load are studied.By establishing the composite vibration fatigue damage model and analyzing the stress concentration effect,the interaction of the composite vibration loads is pointed out,and the influence of the frequency and size of superimposed high-frequency vibration load on the fatigue fracture of metal pipes is clarified.（3）Crack initiation and propagation conditions of prefabricated V-notched metal pipes are analyzed.The main contents are as follows:a)The mathematical model of metal pipes under vibration is established,which can be transformed into a second-order vibration system.The composite vibration characteristics are analyzed in detail,and the varying laws of the amplitude are obtained.b)The fracture criterion of stress intensity factor is analyzed,and the relationship between the exciting force and frequency applied in the process of composite vibration of metal pipes is established.c)Combining with the fatigue crack propagation thresholdΔK_th and fracture toughness K_IC,the vibration load conditions under which cracks can initiate and propagate smoothly are calculated and deduced.（4）The same V-notches were prefabricated for 45 steel,304 stainless steel and 6061aluminum alloy pipes,and the experimental research was carried out based on the relevant theoretical analysis.It includes three parts:no-load vibration experiment,vibration fatigue fracture experiment and controllable fatigue fracture experiment.The experimental results show that the cracks of metal pipes initiate at the roots of both sides of V-notches and propagate to the inside of the pipes under vibration loads,resulting in instability and fracture.It makes the sections of metal pipes have flat expansion area and uneven fracture area.At the same time,the experiment verified the acceleration effect of composite vibration loads on fatigue fracture of metal pipes,and the controllability of fatigue fracture of metal pipes under variable frequency vibration load and resonance effect.In addition,according to the experimental results,the influence of composite vibration loads on the fatigue fracture of metal pipes is further analyzed and summarized.A method for measuring and evaluating the quality of pipe section is proposed,and the section characteristics of vibration fatigue fracture of metal pipes are given.