For high-performance aero-engines and high-power equipment, the high-temperature reliability and dynamic stability of impeller structures are becoming more significant design and test requirements. To break through the performance bottleneck of metallic materials in the high-temperature environment, new composite materials such as metal matrix composites (MMC) and ceramic matrix composites (CMC) have been increasingly and widely applied. These materials have better thermal stability, lower thermal expansion coefficients, and great oxidation resistance, which can significantly increase the life of the impeller and improve working efficiency.
Introduction
The anisotropy and complex interfacial structure of the composite materials themselves make the materials more prone to problems such as non-uniform thermal deformation, micro-crack initiation, and interlayer delamination under high-temperature service. Especially under the compound action of thermal-force-vibration, the operating state of the impeller is extremely difficult to be validly monitored by traditional contact sensors. On one hand, sensors are extremely prone to fail in the high-temperature and high-speed rotating state; on the other hand, the data acquisition process is generally constricted by such factors as thermal radiation and optical interference. Therefore, the development of non-contact, high-precision, and real-time response measure technologies has become an inevitable choice for solving the problem of high-temperature service condition monitoring.
Light wave measurement technology, especially laser interferometry, digital image correlation (DIC), laser Doppler vibrometry (LDV), and infrared imaging, as a new generation of non-contact measurement methods, not only is able to achieve dynamic deformation measurement at the nanometer level but also can be used to identify strain, displacement, and cracks in complex thermal structures, and is gradually playing a core role in the thermal test and life evaluation of composite material impellers.
Measurement Challenges of Composite Material Impellers in High-Temperature Conditions
With the wide application of composite materials (such as ceramic matrix composites and carbon fiber-reinforced resin matrix composites) in aviation impeller structures, measuring the service status of these materials under high-temperature conditions has become a key link in quality control and life prediction. However, complex material physical properties and severe environmental conditions bring tremendous challenges to traditional detection technologies. Especially in the area of impeller thermal deformation, stress distribution, and dynamic response, the existing measurement methods have some limitations in accuracy, stability, and interpretation of measured data.
Thermal Geometric Deformation Cannot Be Ignored
Composite material impellers not only experience tremendous centrifugal force but also, at the same time, the excitation of high thermal loads under the high-temperature working environment. Due to the big difference between the thermal expansion coefficients of the fiber reinforcement phase and matrix material, there is generally non-uniform expansion caused in the interface region and hence nonlinear geometric distortion and local debonding.
Although the small amplitude of this deformation (generally at the micron or even sub-micron level), it will play a big role in dynamic balance, aerodynamic clearance control, and high-cycle fatigue life. Once the correct identification and prediction modeling of thermal geometric changes are lacking, it is easy (prone to) cause assembly error amplification and running yaw augmentation, and ultimately activate failure modes such as fatigue cracks or rotor unbalance. Therefore, the technique of achieving non-contact, high-frequency, and high-precision measurement of geometric size in a high-temperature rotating state has been one of the challenges to modern impeller measurement technology.
Contact Sensors Face Limiting Challenges
Contact sensors such as resistance strain gauges and thermocouples applied in traditional strain testing have experienced the limitation of their material characteristics under ultra-high-temperature (above 800°C) environments. These sensors typically rely on pasting or embedding processes for mounting on the composite material surface, but under the condition of long-term high temperature influence, it is easy for the adhesive layer to age and drop, and the signal output will drift or even distort.
More importantly, because the sensor itself does not share the same thermal expansion as the matrix material, the thermal output signal will generally be superimposed over the effective strain response, providing a non-negligible “thermal error term”. Especially in the design of high-speed rotating impellers, this thermal error will be cumulatively built up repeatedly, drastically affecting the accuracy and reproducibility of strain data. Therefore, when under high-temperature dynamic conditions, not only is it difficult to obtain stable valid data using the traditional contact sensing technology, but it is also possible to generate interference damage to the impeller surface structure.
Background Radiation and Material Reflectivity Interference Are Serious
Strong thermal radiation in high-temperature environments puts extremely high requirements on non-contact optical measuring technologies. When the surface temperature of the impeller increases, its own infrared radiation signal will be significantly enhanced, which will impose a serious impact on optical signal-based measurement methods such as laser interferometry, infrared thermal imaging, and digital image correlation (DIC).
Especially since the surface of composite materials contains several heterogeneous materials such as ceramic coatings and carbon fiber reinforced areas, their surface reflectivity and emissivity differ significantly, which will easily bring about speckle enhancement, discontinuous interference fringes, and imaging artifacts in the optical system, and reduce the image contrast and measurement signal-to-noise ratio. In several experiments, it was found that the reflectivity drift of carbon/ceramic hybrid surfaces at elevated temperatures is more than 15%, which directly influences the precision of infrared temperature measurement and the stability of strain field calculation. Therefore, how to achieve effective optical compensation in the thermal radiation background, select suitable bands and coatings, and build extremely powerful image processing algorithms is the direction in which breakthroughs need to be made in future high-temperature measurement technology.
Difficulty in Strain Decoupling and Thermal Compensation Is High
In the actual impeller thermal test process, strain and thermal output are generally mixed and superposed. If these two cannot be decoupled successfully, it will result in serious distortion of strain data, thereby perplexing the process of strength check and life prediction. Traditional thermal compensation methods (e.g., virtual strain gauges and dual-channel compensation circuits) are not straightforward to deal with dynamic changes under non-linear thermal gradients and transient thermal shocks.
Especially in high-temperature composite materials, due to the inhomogeneous heat capacity and thermal conductivity, there is a spatial lag effect in the temperature field distribution, which renders the decoupling model based on the homogeneous temperature assumption inapplicable. In addition, the change of thermal conductivity of the materials can also deflect the stress wave propagation path, further leading to complicated data interpretation. Therefore, the creation of a strain-temperature calculation algorithm based on a multi-physical coupling model and the introduction of high-frequency thermal field feedback are the key supports for realizing real service condition measurement.
High-Temperature Light Wave Measurement Technology System and Principles
At present, for composite material impeller high-temperature measurement, optical methods have formed a diversified technical roadmap. The comprehensive analysis of existing technical means can be summarized as follows:
Laser Interferometry Measurement Technology
By making a comparison of the phase shift between the reference light and the target reflected light with an interferometer (e.g., Michelson interferrometer or Fabry-Perot interferometer), nanometer-level displacement and deformation can be measured. The extrinsic Fabry-Perot interferometric (EFPI) fiber optic sensor, which is resistant to high temperature and radiation, possesses excellent dynamic response in high-temperature structures.
Laser Doppler Vibrometry (LDV)
Based on the Doppler principle, LDV is able to accurately measure the vibration amplitude and frequency of high-speed rotating impellers in non-contact mode and has been widely applied to natural frequency identification, unbalance measurement, and resonance analysis.
Digital Image Correlation Technology (DIC)
DIC realizes the real-time measurement of the full-field surface strain field and deformation field through the application of high-resolution image matching algorithms. To enhance environmental adaptability, researchers have developed high-temperature resistant sprayed speckles, thermal flow disturbance resistance technology, and added ultraviolet light illumination and optical filtering and other measures, effectively improving imaging quality under extreme high temperatures.
Infrared Thermal Imaging and Spectral Measurement
By the surface temperature distribution obtained from a multi-band infrared imaging system, it can assist in ascertaining the thermal stress concentration area and material abnormal points, and then merge with DIC or interferograms to ascertain the potential hazards such as cracks and material deterioration.
Engineering Application Examples of High-Temperature Light Wave Measurement
Using the high-pressure compressor impeller of a ceramic matrix composite of a specific aero-engine as an example, the laser interferometry, DIC, and infrared thermal imaging technologies are integrated in the ground thermal test to realize the dynamic measurement of the multi-physical field coupling response during its service:
- Test temperature: 1150℃
- Rotation speed: Up to 38,000 rpm
- Measurement objects: Dynamic balance error, axial thermal displacement, crack propagation trend
The analysis results show that the impeller’s radial displacement under the thermal load is about 45 μm, and the main frequency of 420 Hz does not cause resonance risk; infrared imaging reveals that there is an abnormal zone of temperature increase in the middle of the blade, and the interference pattern also confirms that it is an interface stress concentration area. Then (Subsequently), the thermal response consistency was significantly improved through local material and process adjustment.
This case fully validates the trueness and applicative value of high-temperature light wave measurement technology for composite material thermal structure tests and provides important data support for subsequent design iterations.
Conclusion
With the increasingly wide application of composite materials in modern power systems, how to further grasp their structural behavior under high-temperature, high-speed, and multi-field coupling conditions has become the focus of industrial attention. High-temperature light wave measurement technology with the characteristics of non-contact, high resolution, and multi-dimensional coordination is gradually becoming the key technique for high-performance impeller structure test, monitoring, and design optimization. Through in-depth analysis of its technical system, principles, and engineering practices, this paper displays the broad prospects of this technology in the recognition of high-temperature complex structure states.
