Theoretical And Experimental Research On High Temperature Ultrasonic Fatigue Testing Technology

As the requirements for long-service life of high-end equipment such as aero-engines continue to increase,the study of Very-high-cycle fatigue（VHCF）in key component materials has gained more attention.VHCF testing involves long cycles,driving the development of efficient testing technologies.The resonant fatigue testing techniques combine high efficiency and low energy consumption,making them well-suited for VHCF testing.Among these techniques,ultrasonic fatigue testing has been widely applied in VHCF behavior research.However,existing ultrasonic fatigue testing techniques lack a convenient general calculation method at the theoretical design level,and lack loading means and its systematic research at the technology level,especially for bending cyclic loads and high-temperature loads.These technical limitations constrain VHCF behavior research under the aforementioned loads.In addressing these issues,this study establishes the theory of rod/beam resonance design based on the Transfer Matrix Method（TMM）,carries out systematic research on the ultrasonic fatigue test technology of tension-compression/bending at high temperature,and conducts VHCF test analysis of TC4 titanium alloy for blades.The main research work of this study is as follows:（1）For the theoretical support of resonance design and testing technology applied to rod/beam components in ultrasonic fatigue testing machines,vibration analysis was conducted on non-uniform rods and non-uniform Timoshenko beams based on TMM,and then the theory of resonance design for rod/beam was formed.In order to improve the computational accuracy of the non-uniform rod’s vibration analysis,a vibration model considering lateral inertia effect is established,and a calculation method based on the TMM and the modal superposition method is proposed.The calculated results clarified that the displacement and stress distribution of the rod under ultrasonic excitation is independent of the material damping,while the excitation force is directly proportional to the material damping,which explains that the ultrasonic horn should be made of materials with low damping.In order to improve the computational efficiency of the non-uniform Timoshenko beam’s vibration analysis,a new variable-width beam element is used and its eigenfunctions are introduced into the TMM,and then a new algorithm with better convergence and computational efficiency is proposed as compared to the traditional Transfer Matrix Method for uniform beams（UBTMM）.These non-uniform rod/beam vibration models and their algorithms lead to the theory of rod/beam resonance design,which uses the eigenfunctions of the rod/beam elements as interpolating functions in its algorithms,allowing for direct calculation of resonance dimensions.（2）Conducted research on tensile-compressive ultrasonic fatigue test technology at high temperatures.To provide a platform for validating the technology,an ambient ultrasonic fatigue tester was developed,and then extended with a high-temperature ultrasonic fatigue module containing an ambient heating unit and an internal/external air-cooled high-temperature horn.For the design requirements of high-temperature horn with longitudinal vibration characteristic,a steady-state thermal analysis algorithm for variable-section rods is proposed and coupled into the proposed rod resonance design method,forming a resonance design method with coupled temperature field.The proposed rod resonance design method was also introduced into the design and analysis of the tension-compression ultrasonic fatigue specimen,determining the optimal transition radius of the specimen and specifying allowable measurement deviations for elastic modulus,density,and machining tolerances.In view of the importance of stress maintenance and temperature rise control in fatigue testing,the stress and temperature rise tests were carried out,validating the reasonableness of stress estimation based on measured displacements and obtaining intermittent loading parameters for effectively controlling temperature rise.（3）Conducted research on bending ultrasonic fatigue test technology at high temperatures.In order to improve stress distribution on the specimen,a novel bending ultrasonic fatigue specimen with variable thickness and operating in the second-order mode was proposed,and its geometry is adapted to the requirements of room/high temperature tests,then the testing technology developed on the room-temperature bending specimen was applied to the high-temperature bending specimen.For this type of specimen,the proposed beam resonance design theory is introduced for its optimized design,a prescriptive design process is given,followed by the accuracy verification of the theoretical design results.To determine the final machining dimensions of the specimen,an expression for the highly stressed volume(i.e.V₉₅)was derived,and an analysis effect analysis of specimen’s key resonance dimensions on its evaluation index(including V₉₅)was carried out.To further determine the specimen for fatigue tests,the influence of testing and machining deviations of physical parameters on the second-order natural frequency was studied,determining the allowable testing deviations for the elastic modulus and density as well as the allowable machining deviations for the gauge segment’s half-thickness.In response to the requirement for consistent evaluation and stress estimation of the identified specimens during testing,a study on consistent evaluation based on modal testing was conducted,and a methodology for estimating the maximum stress in terms of response displacement amplitude was proposed and applied to high-temperature bending specimen after verification of the room-temperature bending specimen.Furthermore,temperature rise tests were carried out,and the intermittent loading parameters were determined for the fatigue tests at room-temperature and high-temperature,respectively.（4）Experimental researches using the developed testing techniques to investigate VHCF behavior of TC4 titanium alloy for compressor blades.In order to select a comparison material with similar microstructure and mechanical properties from the literature,the microstructural morphology of the test material was obtained,and the tensile mechanical parameters were measured at room temperature and 400°C.The specimens designed in this paper were subjected to VHCF tests by the developed experimental technique on the self-developed ultrasonic fatigue testing machine,in which the symmetrical bending ultrasonic fatigue test at high temperatures corresponding to the high-temperature bending specimens was carried out for the first time in the world.The test results confirmed the feasibility of the developed techniques,and the validity of these results was confirmed through comparisons with data from the selected literature.In addition,the statistical analysis of failure locations and the fatigue fracture analysis of the specimens were carried out,in which the statistical results showed that the specimens were fractured within V₉₅ as designed,and the fracture analysis revealed the crack initiation mechanism under different temperatures and load types.This study consolidates the theoretical and technical foundation in ultrasonic fatigue testing,which is of guiding significance for the development of new resonant fatigue testing technology,and will provide a guarantee for the testing and evaluation of new materials for rotating components such as compressor blades in aircraft engines.