Low-cycle fatigue (LCF) in centrifugal impellers of air compressors is a critical failure mechanism driven by high stress amplitudes and a relatively low number of cycles (typically less than 10⁵). Unlike high-cycle fatigue (HCF), which results from resonant vibrations, LCF is primarily governed by bulk plastic strain at stress concentration points.
Here are the primary causes of low-cycle fatigue in these components:
1. Start-Stop Cycles (Transient Operations)
The most common cause of LCF is the cyclic loading experienced during startup and shutdown.
Centrifugal Loads: When the rotor accelerates from rest to operating speed, the impeller blades and hub experience immense radial tensile stresses due to centrifugal force. Each start-stop cycle constitutes a massive stress cycle from zero to maximum (or near-maximum) stress.
Plastic Deformation: If the material’s yield strength is exceeded at stress concentration points (such as fillets or blade roots) during these transients, plastic strain accumulates. Repeated cycling leads to crack initiation.
2. Aerodynamic Excitation and Resonance
While often associated with high-cycle fatigue, aerodynamic forces can induce LCF if the amplitude is high or if the machine operates frequently in resonant conditions.
Flow Instabilities: Phenomena such as rotating stall, surge, or inlet distortion create fluctuating aerodynamic pressures on the blades. These fluctuations impose alternating bending stresses on the blades.
Off-Design Operation: Running the compressor frequently at off-design points (low flow or high flow) can induce high alternating stresses that, combined with the mean centrifugal stress, result in LCF.
3. Thermal Gradients (Thermal-Mechanical Fatigue)
In high-pressure or high-speed air compressors (including integrally geared compressors or those with high compression ratios), significant temperature differentials occur.
During Transients: During startup, the hub (center) heats up faster than the outer shroud or casing. This thermal gradient induces compressive thermal stresses on the hub and tensile stresses on the periphery.
Mismatch in Expansion: Dissimilar materials (e.g., a steel shaft with an aluminum or titanium impeller) or uneven temperature distribution cause constrained thermal expansion, leading to cyclic plastic strain in the impeller bore or blade attachments.
4. Material Defects and Manufacturing Residual Stresses
The initiation site for LCF is almost always a location of stress concentration.
Surface Roughness/Notches: Sharp radii at blade root fillets, machining marks, or weld repairs act as stress raisers. Under cyclic centrifugal load, these notches concentrate the stress, causing local yielding.
Residual Stresses: Manufacturing processes (such as welding, heat treatment, or machining) leave residual stresses. If these residual stresses are tensile and combine with operational tensile stresses, the effective mean stress increases, accelerating LCF crack growth.
Inclusions: Material impurities (non-metallic inclusions) in the forging or casting act as initiation points for micro-voids and cracks under cyclic loading.
5. Corrosion and Environmental Effects
Although air compressors typically handle ambient air, specific environments can accelerate LCF.
Corrosion Fatigue: If the intake air contains moisture, salt, or industrial pollutants (e.g., H2S or chlorides), pitting occurs. Pits act as stress concentrators. The combination of a corrosive environment and cyclic stress lowers the fatigue strength significantly compared to pure mechanical fatigue.
Hydrogen Embrittlement: In rare cases, if improper plating or cleaning processes are used during manufacturing, hydrogen embrittlement can cause sudden cracking under sustained tensile loads combined with cycling.
6. Mechanical Damage and Fretting
Fretting Fatigue: In assembled impellers (where blades are inserted into a disk, or where the impeller is mounted on a shaft via interference fit), micro-motion occurs under cyclic load. This fretting wears the surface, removes protective oxide layers, and creates local stress concentrations that serve as nucleation sites for LCF cracks.
Foreign Object Damage (FOD): Impact from debris (scale, tools left in piping, or condensate slugs) creates small dents or cracks. These discontinuities act as stress risers, dramatically reducing the LCF life.
7. Overspeed Events
Although not a "cycle" in the traditional sense, occasional overspeed events (exceeding the maximum continuous speed) can cause localized plastic deformation. Subsequent normal start-stop cycles will then cause this plastically deformed zone to experience low-cycle fatigue failure much earlier than the design life.
Summary of Failure Progression
In centrifugal impellers, LCF typically follows this pattern:
Initiation: At a stress concentration (fillet, inclusion, FOD) due to high mean stress (centrifugal) + alternating stress (start/stop or vibration).
Micro-crack Growth: Propagation along planes of maximum shear stress (Stage I).
Macro-crack Growth: Propagation perpendicular to the maximum principal stress (centrifugal force) (Stage II), leading to through-cracking of the blade or hub.
Mitigation strategies typically involve precise control of startup/shutdown cycles (avoiding rapid starts), using finite element analysis (FEA) to optimize fillet radii to reduce stress concentration factors, applying surface treatments (shot peening) to induce compressive residual stresses, and strict material quality control.
