Design And Experimental Study Of Combined Loads Mechanical-thermal Coupled Fatigue Testing Device

Fatigue failure is one of the typical failure modes of all kinds of solid materials under service conditions.Based on the cumulative effect of cyclic alternating loads,the occurrence of fatigue failure is often sudden and hidden,which is difficult to be accurately predicted.Therefore,accidents caused by fatigue failure often bring significant safety risks,and failure accidents are numerous.Under actual working conditions,materials are often affected by multi-axis alternating load and temperature field.In order to ensure the safe and reliable service of materials and components under alternating loads,it is important to study the high temperature fatigue properties and fatigue behavior evolution mechanism of materials under composite stresses.In this paper,the research on mechanical-thermal coupling fatigue testing of composite loads at home and abroad is reviewed and analyzed.The basic theory of material fatigue testing and the fatigue failure mechanism are introduced.Based on a three-degree-of-freedom driving principle of fatigue experiment,the corresponding theoretical models of tensile fatigue and tensile-bending fatigue are proposed,which provide a theoretical basis for the design of multi-degree-of-freedom fatigue device.By adjusting calibration force sensor,laser displacement sensor and three-dimensional digital speckle strain measurement system,the alternating displacement and load of specimens under different loading voltage are detected,and the performance of mechanical loading module is simulated and tested.By adjusting the loading voltage of several groups of parallel ceramic heating tubes,regulating the loading temperature in the sample area of the device,and detecting the temperature distribution in each part of the loading module,the temperature loading performance of the device was analyzed.Through the functional division of the fatigue driving module,loading module and testing module,the working flow of the whole set of mechanical-thermal coupling fatigue device is explained.Then,according to the proposed design index,the piezoelectric stack and mechanical sensor are selected.By testing and optimizing the size,shape and structure of piezoelectric stack,flexible hinge and spherical pair,the static analysis and steady-state heat transfer analysis of the mechanical loading module are carried out.The movement and deformation characteristics of the whole machine under uniaxial tension fatigue and tension-bending fatigue modes were obtained,and the dynamic characteristics of the device were verified.Furthermore,uniaxial tension fatigue test and tension-bending composite fatigue test were carried out on copper specimens with double V-notches.In uniaxial tension fatigue test,combined with the analysis of three-dimensional digital speckle strain measurement system,it is found that the strain in the standard interval increases monotonously with the increase of cyclic load.In tension-bending fatigue tests,a large number of shear dimples caused by shear stress can be observed on the fatigue fracture morphology,and the direction of fatigue crack propagation can be determined from the parabolic projection of tear dimples.It is found from the tensile-bending fatigue test of copper under variable temperature that the strength of the material decreases under high temperature environment,and the ability of material to resist elastic-plastic deformation decreases,which ultimately leads to rapid fatigue failure.In the high-entropy alloy test,the five high-entropy alloys exhibited good strength and toughness,and the strength of the materials after rolling annealing heat treatment showed different degrees of improvement,and the toughness showed different degrees of reduction.In the high temperature fatigue test,the fatigue life of the Fe₅₀Mn₃₀Co₁₀Cr₁₀ high-entropy alloy decreases with increasing temperature.