Component failure due to fatigue from repeated stress cycles is a critical concern across industries dealing with rotating machinery like aerospace, automotive, and power generation. Whether it is turbine blades, axles, or other power transmission parts, understanding and testing for cyclic fatigue during design is key to preventing in-service failures.
As specialists in testing and developing drive systems involving high-speed rotating components, we at Barbour Stockwell recognize the importance of fatigue testing. In this article, we will provide an overview of cyclic fatigue, testing methodologies, and how fatigue data guides design and material selection for optimal reliability.
The Risks of Cyclic Fatigue
Cyclic fatigue occurs when components are subjected to repeated cyclic stress from vibration, thermal cycles, or fluctuating loads. With enough cycles, cracks initiate at the surface of parts and components. As stress cycling continues, these cracks spread across the cross section, ultimately leading to sudden and catastrophic failure when remaining material can no longer sustain operation loads.
Unlike yielding or fracture from a single overload, fatigue failures can be triggered at stress levels well below a component’s static strength. Tracking stress cycles and identifying when fatigue may occur is not always straightforward. Therefore specialized testing is required to characterize fatigue behavior. The risks of fatigue failure elevate for mission-critical components which sustain millions of operating cycles without opportunity for inspection or replacement.
Capturing Cyclic Fatigue Through Testing
To evaluate durability against fatigue, engineers perform accelerated life testing on prototypes. This testing applies aggressive cyclic stress to simulate an entire product lifetime within a condensed test timeline. Two common approaches include:
- High-Cycle Fatigue (HCF) Testing: This focuses on high numbers of lower stress cycles typical of components like aircraft turbine fan blades. Testing applies successive stress cycles until complete failure, with samples lasting anywhere from thousands to over a billion cycles during testing.
- Low-Cycle Fatigue (LCF) Testing: Typically for structures experiencing higher-strain cycling like engine pistons and casings. Components are tested over relatively few stress cycles, but at higher load amplitudes.
In cyclic fatigue tests, specialized servo-hydraulic and electrodynamic test systems precisely apply cyclic loads and stresses necessary to evaluate product life cycles in an accelerated manner. High stiffness fixturing and precision control allow test labs to run even complex load histories tailored to application parameters.
Advanced instrumentation like strain gages and thermocouples provide real-time monitoring as cyclic stresses initiate cracking and drive crack growth within samples. Testing continues until complete fracture provides an opportunity for failure analysis of fatigue characteristics. Metallurgic analysis examines fracture surface patterns for crack initiation points and rates of propagation.
With insight on number of cycles to failure for a given test stress, engineers extrapolate results to make lifetime predictions under application loading conditions. Testing multiple samples provides data to statistically model how manufacturing variability impacts component fatigue limits.
Optimizing Designs for Fatigue Resistance
Accurately establishing fatigue strength of candidate materials and designs guides engineers toward optimal reliability. Cyclic fatigue testing provides critical data for design decisions by revealing:
- Fatigue limits: Maximum cyclic stresses which can be sustained without failure at a target lifecycle. This guides design to stay within proven safety factors.
- Failure probability: Statistical models based on test results identify chances of failure from variability in materials and manufacturing quality.
- Sensitivity studies: Evaluating different designs, surface finishes, or materials shows the influence of parameters like corrosion resistance and stress concentrations around holes, fillets, and complex geometry.
- Leak-before-break analysis: For components containing hazardous fluids, identifying if cyclic crack growth precedes catastrophic rupture enables fail-safe mechanisms through early leak detection.
Engineering teams collaborate with clients to continuously advance mechanical testing capabilities. Investments in specialized servo-hydraulic and resonant test systems with precise digital control and hardened fixturing to withstand tremendous forces from high-speed loading. These facilities accumulate petabytes of high-speed sensor data from samples, allowing deep analysis of fatigue phenomena as design validation progresses.
By accurately simulating real-world operating environments and extreme usage conditions, our spinning component test centers repeatedly validate durability of innovative powertrain, transmission, and industrial designs before they ever see field use. State-of-the-art testing practices demonstrate factors of safety confirming cyclic stress limits over target service lifetimes spanning millions of revolutions.
Driving Reliability Through Fatigue Data
As systems evolve toward light-weighting and sustainability, engineers require deeper understanding of fatigue to avoid overdesign while upholding safety. Continued progression of analytic tools and testing capabilities provides the missing link to confidently deploy emerging materials, additive manufacturing methods, and complex designs. Fundamentally, cyclic fatigue testing anchors predictive engineering models to physical limits, giving components the best chance of meeting reliability goals through inevitable stress cycles over long working life expectancy.
Barbour Stockwell Inc. specializes in low-cycle fatigue testing. They offer services such as spin testing, drive systems, and various test facilities. Their expertise spans industries like aerospace, automotive, power generation, oil and gas, and abrasives, dealing with complex and high-speed machinery testing.
