Category Archive: Fatigue Rigs

The Value of Fatigue Testing

Fatigue Testing

Fatigue testing is a crucial procedure used by engineers and technicians to help predict the durability of a part or component under its operating conditions. To appreciate the value of fatigue testing, it is essential to first understand the phenomenon.

Fatigue is a type of structural damage prevalent in cyclically loaded structures. The fatigue is characterized by an initiation and growth of cracks that will eventually result in a catastrophic fracture of a material. Unlike the structural failure caused by an overload, fatigue damage develops under the magnitude of the stresses below the material’s yield strength, and therefore tends to covertly manifest without deforming or showing obvious “warning signs”. The rate at which a fatigue crack grows is dependent on the material’s properties as well as the intensity and cycle frequency of the applied load.

Fatigue testing is used to evaluate a material’s (and component’s) structural durability by testing and analyzing its ability to withstand cyclic loading conditions.

Fatigue Testing Methods

There are different types of fatigue testing machines with various capabilities, ranging from automated material specimen tensile test machines to large full-scale structural test rigs. In a rotating structure, such as gas turbine disks and blades, accurately capturing the loading conditions in fatigue-prone features can be a complicated challenge. Rotors often have intricate geometries to serve its intended functions and these features are subject to varying degree of multiaxial stresses caused by the CF load, which also changes over the course of the operating cycle of the machine as rotor speed changes. These loading conditions are difficult, if not impossible, to capture in test methods other than spin testing.

Within the rotor specific testing techniques, Test Devices Inc. is specialized in performing different variants of LCF (Low Cycle Fatigue) and HCF (High Cycle Fatigue) tests.

The LCF regime is characterized by a higher load application, within the material’s elastic-plastic range, at low frequency (or cycle rate). For example, the take-off and landing cycle of an airplane. Typically, the HCF regime is characterized by the application of a very high-frequency load which results in a rapid accumulation of fatigue damages in a short span of time. These conditions are found in the vibrating vanes or blades subject to resonance.

Both LCF and HCF are common issues in the operation and safety of critical rotating components. Unexpected failure of rotors, such as high-speed impellers and turbine engines, which operate under high centrifugal forces often end in a catastrophic effect.

Benefits of Spin Fatigue Testing

Fatigue testing is performed to generate the data needed to validate and/or refine a probabilistic model of the components’ life in its operational environment. The way the fatigue damage initiates and develops in a material can be influenced by various factors including the loading field cycle pattern, axiality, temperature and environmental factors (e.g. oxidation and corrosion).

The result of the fatigue test data helps to define the maintenance and repair cycle requirements and the safe operating life of machines. These will have a significant impact on the cost of ownership of the equipment as well as safety-related issues.

Test Devices specializes in providing the most relevant test data to our customers who design and develop the advanced machines that are critical to our transportation, infrastructure, and national defense.

Spin fatigue testing can be used in any industry or application where the durability and integrity of a rotor and rotating component matters. Some industries that regularly use fatigue testing and fatigue testing machines include:

  • Automotive – turbochargers, tires, and wheels
  • Aerospace – jet engine components, turbine rotors, wing materials
  • Medical – prosthetics, implants,
  • Apparel – athletic shoes, textiles
  • Construction – bolts, fasteners, reinforcement bars

Learn More From Test Devices, Inc.

At Test Devices Inc., we offer both low-cycle and high-cycle fatigue spin testing services for clients who work with high-speed rotating components in demanding environments. We are capable of testing various complex and cutting-edge machinery parts for jet engines, electric motors, turbomachinery, and energy storage systems.

Our qualified and experienced engineers are willing to assist you in fatigue testing your unique part or component. If you would like to learn more about our testing services, feel free to contact our support team today.

Low Cycle Fatigue Rigs vs. Dynamic Spin Rigs

To validate the integrity of rotating parts against centrifugally loaded conditions, spin testing is a critical step to ensure rotor product quality. While the principle of the spin testing appears straightforward, it’s very easy to miss the important subtleties of the testing mechanics. In fact, since the early days of the spin testing, millions of dollars have been wasted and many perfectly good parts have been needlessly destroyed due to subpar testing practices.

Successful spin testing requires participants to:

  1. Focus on the overall test objective – Which rotor features will the test assess, and under what conditions?spin rig comparison
  2. Understand the test part – How is the test part being assembled in relation to adjoined parts in the actual operating environment? How will it interact with those related components, how will those interactions mechanically or structurally constrain the test part while under assessment, and how is the spin tooling designed to accurately represent the condition?
  3. Recognize what to test for and what results will indicate – What measurements must be taken? How should these measurements be captured so that the resulting data is useful in understanding the failure mode, validating the engineering models, and updating the part design accordingly?

Beyond the basics, spin testing can also be used to validate the durability of the rotors in more a realistic environment. Doing fatigue assessment in spin testing allows engineers to study part durability by using actual production parts (or modified versions), which would be more representative test specimens than coupons in terms of geometry, size and manufacturing process. Additionally, the loading condition resulting from the spinning would create realistic stress conditions, specifically related to the multi-axial loading field and the loading cycle.

Test Devices, Inc. (TDI) and Schenck offer two different types of spin rigs for advanced fatigue assessment tests: a Low Cycle Fatigue Rig (LCF) and a Dynamic Spin Rig (DSR). Each style of rig is designed to accommodate specific testing requirements.

  • Low Cycle Fatigue Rigs are designed to perform various low cycle fatigue (LCF) tests, which are typically used to evaluate the overall durability of parts in terms of usage or operational cycles. For example, for a jet engine for a commercial airliner, this would be based on the number takeoff and landing cycles for a specified flight path.
  • Dynamic Spin Rigs, on the other hand, are designed to test the dynamic properties of the rotor parts and its high cycle fatigue (HCF) related failure modes. The HCF is a fatigue failure mode driven by the resonance of a part; for example, turbine blades may have resonances within an operating speed range that have modeshapes tied to incipient and rapid fatigue damages/failures.

Low Cycle Fatigue (LCF) Rigs for Dependable and Realistic Testing

test devices and schenck logosTDI offers unmatched performance and reliability in its advanced LCF test spin rigs. Featuring robust armor cylinders to ensure safety, a drive system with the rapid cycle time to expedite test schedules, and accurate cycle speed control system to hit cycle targets, the rigs have been continuously proven over Test Devices’ more than 40 years of testing experience.

Combined with various automated testing features, Test Devices’ LCF rigs boast the highest-productivity drive systems available. These rigs typically incorporate a range of standard features, including:

  • Custom data acquisition system to record all test data
  • TDI’s proprietary crack detection system technology
  • Containment chamber designed to withstand high-energy burst failures
  • Vacuum system to eliminate aerodynamic losses and friction heat
  • Automated control system to perform various LCF test speed profiles
  • High performance drive systems with RPM capabilities of up to 250,000 rpm

In addition to these features, LCF rigs can also be equipped with elevated temperature test capabilities, such as:

  • Isothermal – Uniform temperature on the test rotor
  • Thermal gradient – Capability to map the realistic “engine-like” rim-to-the bore temperature profile on the rotor

Dynamic Spin Rig (DSR) for Advanced Turbine System Research

High cycle fatigue (HCF) failure typically occurs during the critical phase of an aircraft’s operations, such as during the take-off for commercial airliners or rapid throttle changes in military aircraft. While the duration of an HCF-inducing condition exposure could be brief, the damage often develops rapidly before reaching a critical level. In most cases, there is no opportunity for correction once the condition reaches this point.

DSR was developed to allow engineers to test and validate the dynamic property of bladed rotors at the component level, therefore minimizing test costs while still retaining the realism in the test. DSR are equipped with the capability to excite the bladed rotor with a desired dynamic load (of an Engine Orders, or EOs). The spin testing condition incorporates the effect of the centrifugal load on the test parts, which is known to affect its damping and resonance properties – this unique test element cannot be accurately replicated by traditional table-top methods.

Test Device’s DSRs are equipped with unique testing capabilities, which include but are not limited to:

  • Proprietary high-precision tachometer & the rpm control system to “lock on” to the target resonances
  • Oil-jet and aero-pulse blade excitation systems
  • Multi-point strain gage & slip ring (or telemetry) systems
  • No-contact Stress Measurement System (NSMS), which is also known as the Blade Tip Timing system

Testing Rigs from Test Devices

With years of experience working with cutting-edge spin test rigs, Test Devices, Inc. is proud to provide testing services utilizing both low cycle fatigue and dynamic spin rigs to customers. To learn more about our LCF and DSR spin rigs, or to discuss options for your specific application, request a quote from the team today.