Non-contact Stress-strain Monitoring Research In Special Environment

Strain refers to the relative deformation that occurs in a material at a localized level when subjected to external forces,non-uniform temperature fields,and other physical phenomena.It is a crucial indicator for assessing the strength and stability of objects.Real-time monitoring of stress and strain in critical components is essential for the testing,inspection,and operation of equipment.However,critical components are often exposed to extreme conditions such as high temperature and pressure,presenting significant challenges for stress and strain monitoring technology.This also places high demands on the stability and reliability of monitoring instruments.Therefore,achieving successful stress and strain monitoring of critical components in such special environments is a challenging and highly significant engineering mission.This dissertation focuses on the high-temperature,enclosed working environment inside high-bypass ratio aeroengines and the strain measurements of their internal core blades.It summarizes key issues with existing digital image measurement techniques,such as the difficulty in strain measurement due to missing speckle feature points and the challenge of obtaining optical blade images in engine environments.A comprehensive analysis of the real operating conditions of aircraft engines is conducted,with the goal of strain field measurement on blade replicas.In-depth research is carried out to optimize non-contact strain measurement techniques and design high-temperature,small-sized strain measurement probes,resulting in the following achievements:1.This dissertation conducts a detailed analysis of the mechanical model of blades and the mapping relationship between deformation displacement and strain.Building upon digital image correlation techniques and integrating finite element analysis technology,it investigates a finite element-digital image strain measurement method using the characteristic points of the blade’s built-in air film holes.This measurement method tackles the problem of ineffective measurements in digital image correlation arising from the lack of speckle feature points in blade strain measurement by introducing an equivalent elastic modulus and applying parameter correlation inversely in the finite element model.In an ambient temperature environment,mechanical loads are applied to the blade model,and a detailed comparison is made between the finite element-digital image measurement method and resistive strain measurement as well as digital image correlation strain measurement.The percentage difference in measurement results with the resistance strain method is within 6.6%.2.In a high-temperature environment ranging from 500 °C to 750 °C,the relationship between the comprehensive deformation displacement and strain of air film holes was analyzed using the superposition principle.An infrared thermal imager was introduced to assist in measuring the temperature field distribution of blade replicas.Using the parameter projection method,the temperature field was metricized and incorporated into the finite element model.By imposing constraints on the thermal expansion coefficient,equivalent elastic modulus,and Poisson’s ratio of the specimen,the finite element digital image measurement method was successfully applied to hightemperature strain measurement of the specimens.Compared to DIC,the percentage difference in strain measurement is within 4.3%.Through a comparative experiment of strain measurement under room temperature and high-temperature conditions,the accuracy and reliability of the finite element digital image measurement method for strain field measurement are demonstrated.3.In accordance with the narrow and high-temperature conditions at the turbine end of aircraft engines,a high-temperature,small-sized measurement probe for strain measurement was designed.Several aspects of the simulated optical system,including optical transfer function,distortion and field curvature,and array lattice,were evaluated and optimized.The overall outer diameter of the optical system was optimized to 16 mm.By directly establishing the relationship between strain and imaging quality based on the measurement results of virtual strain under room temperature conditions and the thermal expansion coefficient of materials under high-temperature conditions,the assessment of the imaging quality of the optical system becomes more intuitive.Through a combination of simulation analysis and high-temperature durability tests,it was proven that the designed thermal protection structure can effectively safeguard the optical system.The overall outer diameter of the designed measurement probe is 22 mm,and it can be inserted into the interior of the engine through a through-hole connection for image data collection.Finally,the combination of the finite element and digital image measurement methods with the strain measurement probe was used to achieve strain measurement on blade replicas in high-temperature environments.Compared to DIC,the percentage difference is 7.01%.In this dissertation,through research on non-contact strain methods and hightemperature strain measurement instruments,the potential of the proposed strain measurement technology to achieve online monitoring of blade stress and strain in the special environment of aeroengines is demonstrated.This holds the promise of providing valuable data support for the prediction of turbine blade life and research into operational mechanisms,thereby offering a technical reserve for measurement technology in the field of aircraft engines.