Vibration Testing and Dynamic Response Analysis of Aero-Engine Impellers

Aero-engine impellers operate for a long time with severe conditions such as high rotation speed, high temperature and pressure, and strong aerodynamic excitation. Their dynamic response characteristics are closely associated with the stability of performance and safety of the entire engine. Since they usually have long cantilever geometries and thin-walled structures, impellers inherently have low structural stiffness and multi-order vibration modes, thus they are extremely prone to be excited by resonant excitation under operation, leading to severe stress responses and fatigue accumulation. Particularly, if the frequency of periodic excitation approaches one of the natural frequencies of the impeller, it will lead to a vibration response with a huge amplitude, causing crack initiation, blade fracture, and engine failure.

Introduction

The objective of this paper is to have systematic experimental research and simulation modeling to carry out thorough disclosure of vibration characteristics and dynamic response laws of the impellers of aero-engines under usual exciting loads, as well as comprehensive assessment of their fatigue strength and modal response mechanisms, so as to give them structural optimization design and life prediction.

Analysis of Vibration Sources and Typical Characteristics of Impellers

Because they are high-speed rotating components in service, aero-engine impellers are subjected to the coupling effect of multi-excitation sources and possess extremely complicated vibration characteristics with direct effects on the structural stability and service life of the entire engine. To thoroughly understand the dynamic characteristics of impellers, it is necessary to start with vibration sources, combine their structural characteristics and working conditions, and rationally analyze their overall vibration modes and potential dangers. The main reasons and manifestations of current impeller vibration behavior are as follows:

Periodic Aerodynamic Excitation

Impellers periodically experience aerodynamic disturbances caused by upstream combustion chambers and downstream guide vanes when in operation. Specifically, under high-speed flow and shock wave conditions, blades are apt to experience cyclic pressure or dynamic pressure oscillations. All these excitation sources have definite frequencies, are significantly correlated with engine operating conditions, and share common periodic and repetitive characteristics. As long as the frequency of such aerodynamic excitations coincides with the natural frequency of the impeller, it is highly probable to cause resonant responses, resulting in amplitude amplification, stress concentration, and even fatigue damage to the structure. Particularly in combustion instability or multi-stage rotor interference, the intensity of periodic excitation rises to a high enough value to become the predominant external source of impeller vibration.

Structural Asymmetry and Rotor Imbalance

Due to manufacturing tolerances, material nonhomogeneity, or assembly errors, impellers generally possess some degree of structural asymmetry and mass eccentricity. With high-speed rotation, the mass imbalance produces continuous centrifugal excitation forces, increasing exponentially in rotational speed, and particularly tend to produce strong radial or bending vibrations in low-order modes. Furthermore, mass imbalance can initiate the orbital deviation and variation of the motion of the rotor system, further coupling with casing stiffness differences, bearing stiffness differences, and other structural stiffness differences, producing more complex system vibration behavior.

Multi-Order Modal Coupling Effects

General complex continua are aero impellers, and their dynamic modes are not only conventional bending and torsional modes but also higher-order deformation characteristics such as rim warping and localized blade vibration modes. When the rotational speed, load, or geometric boundary conditions of the impeller are varying, individually independent modes turn into nonlinear coupling, e.g., bending-torsion coupling or root-tip coupling. This superposition and coupling of energy transfer across modes will increase local stress response, and even cause large amplitude responses in frequency ranges far from natural frequencies, so the structural safety assessment would become more difficult.

Rotational Speed Dependence and Nonlinear Characteristics

Under high-speed rotation, impellers have huge centrifugal loads and heating effects, causing prestressing and predeformation of the structure and thus changing its equivalent stiffness and boundary conditions. The dynamic frequency shift phenomenon causes the natural frequency of the impeller dynamically change with rotational speed, generally in a stiffening effect (i.e., frequency increases with rotation speed). In addition, under conditions of large displacement and heavy load, the vibration response of the impeller presents nonlinear characteristics, such as drift of response frequency, appearance of second harmonics, or mode jumping. These nonlinear aspects not only cause challenges to the traditional modal analysis methods but also impose stricter demands on resonance detection, fault prognosis, and structural health monitoring.

Design and Application of Impeller Vibration Testing Methods

For good comprehension of impellers’ dynamic response performance under actual operating conditions and ensuring the structural design reliability and service safety, multi-dimensional vibration test verification should be carried out. Following the requirements of test repeatability, representativeness, and accuracy, this paper formulates and practices three most critical vibration testing methods, from static modal identification to operating condition simulation excitation response, a relatively complete technical system of impeller vibration testing.

Modal Testing

Modal testing is one of the foundations of vibration testing, and it is mainly used to obtain the natural modal properties of structures with no working load. In experiments, the structure is generally struck with an impact hammer or an electromagnetic exciter, and frequency-domain response data are measured by accelerometers placed at significant locations (e.g., blade tips, blade roots, and rims) or in conjunction with non-contacting laser vibrometers. The field-measured frequency response functions (FRF) are treated with singular value decomposition (SVD) to accurately remove the natural frequencies, damping ratios, and mode shapes of each order of the structure. This method not only provides high-precision free modal parameters but also test boundary conditions and reference points of comparison that can be relied upon for finite element modal model calibration and harmonic response calculation.

Rotational Excitation Testing

In order to more closely approximate the modal response behavior under real operating conditions, the following article introduces rotational excitation testing. This test positions the test impeller on a high-speed rotational test bed in vacuum or low-interference conditions and applies controllable excitation forces (e.g., electromagnetic coils, aerodynamic disturbances, or unbalance loading) over a range of specified rotational speeds. Vibration response of the structure to rotation conditions is recorded by a rotation speed-synchronized response measuring system (e.g., contact strain gauges or wireless acceleration modules). The test can identify “modal migration” and resonance periods caused by rotational speed variation, of inestimable significance to the interpretation of gyroscopic effect, softening/hardening property, and stability limits of the structure. It is also widely used as a major test process before the validation of high-speed impeller structural design.

Laser Vibrometry Scanning (SLDV)

To address the issue of calculating multi-order coupled modes and local response peak positions in complex impellers, this paper also introduces a scanning laser Doppler vibrometry (SLDV) system for full-field mode shape testing. This method can be used to achieve high-speed scanning of multiple response points on the impeller surface at non-contact mode with the output of full 3D dynamic response maps. Compared with traditional point measurement methods, SLDV can clearly reconstruct the modal coupling behavior of complex structures and the local high-stress concentration areas, providing valuable reference values for structural design optimization and precision improvement of fatigue life prediction. In addition, the system supports collaborative operation with revolving platforms for dynamic modal visualization analysis in all kinds of operating conditions and is a major technical means in the vibration testing field of high-end revolving machinery.

Finite Element Modeling and Harmonic Response Simulation Analysis

To comprehensively study the dynamic performance of high-speed rotation titanium alloy turbine impellers, in this paper, a numerical simulation model is constructed based on the finite element method (FEM) and combined with harmonic response analysis to find main frequency points and structural response characteristics. The accuracy and applicability of the simulation model are ensured through comparison and calibration with experimental modal test data. This analysis not only suggests potential resonance risk but also provides authentic reference for structural optimization and fatigue life prediction.

Finite Element Modeling Strategy

This paper uses ANSYS and Suochen general structural simulation platform to build a 3D solid model based on the titanium alloy turbine impeller, fine meshing, and introduction of material properties (E=1.1×10¹¹Pa, ρ=4500kg/m³). The prestress, boundary conditions, and rotational inertia are considered for accurate modeling of the rotation effect, and the modal analysis and harmonic response analysis combined is used.

Verification of Modal Simulation Results

Simulation and measured data show:

Error is kept in the range ±2%, indicating high reliability of the model, and simulation results can provide accuracy support for fatigue life prediction.

Dynamic Response Simulation Analysis

According to modal analysis, typical harmonic loads (frequency range 0–3000 Hz) are reintroduced in order to simulate unbalanced force excitation during operating conditions, and frequency domain response analysis of the impeller is done. The simulation analysis shows that at the 1st (at about 763 Hz) and 2nd (at about 1485 Hz) bending mode frequencies, structural response amplitude turns drastically large, with local stress concentration points, indicating potential resonance risks. These areas are mainly concentrated at the blade root and hub transition areas, which are the initial source of fatigue cracks. With continued increase in excitation frequency beyond 2000 Hz, the response curve settles down slowly, indicating that the structural dynamic gain by high-frequency excitation is small, and the mid-low frequency band is the most critical focus area for structural design and vibration prevention control.

Vibration Fatigue Testing and Stress Assessment

For additional testing of the vibration-induced fatigue response, the following test techniques are used:

Strain Measurement and Stress Calibration

12 strain gauges are located at maximum stress locations established through finite element analysis. Resistance strain gauges measure strain data for different amplitudes, and linear fit is obtained with amplitude output through laser displacement sensors to find the following functional relationships:

Δσ/2 = a⋅A + b or Δε = c⋅A + d

where the good fitting correlation coefficient R²=0.9999 and levels of stress may indirectly be controlled by controlling the amplitude.

Fatigue Strength Testing

Sinusoidal dwell vibration tests are conducted under conditions of resonance frequency, and amplitude variations are monitored. Fatigue damage is considered to occur first when frequency drift is greater than 1%. The stress-up-down technique is utilized to examine the endurance of 10 samples, acquiring valid fatigue life data to act as the basis of engineering application.

Conclusion

As critical rotational components with simultaneous coupled excitations, the vibration response behavior and dynamic properties of impellers are the focus in ensuring the safe operation of aero-engines. Along with clarifying the mechanism of modal responses and stress concentration positions of impellers under different excitations, the effectiveness of different test techniques and simulation technologies is also validated by the strict experimental and simulation investigation in this paper.