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Multiaxial fatigue analysis 

Multiaxial fatigue analysis replicates the load and stress conditions of real-world service environments in controlled laboratory settings to develop safer, more reliable, and dependable materials and components. 

These mechanical tests play a critical role in evaluating complex design structures, components, and materials that will, during their service lifetime, be subjected to complex multiaxial loads in a broad range of industries from aerospace, agriculture, and transportation to power generation and civil engineering.

The complex stresses that occur during operation are examined under static and dynamic operational loads to assess the impact of fatigue and to determine accurate in-service life predictions. The testing is crucial for effectively simulating the mixed-mode loading environments and measuring in-plane stresses in both the X and Y axes of advanced materials and components.

The Rolls-Royce multiaxial testing heritage

Through the Rolls-Royce and ºÃÉ«TV partnership, the team of engineers is now providing high-quality multiaxial testing capabilities, using state-of-the-art technology supported by unique in-depth expertise, to manufacturers across multiple industries.

The team at the Rolls-Royce test facilities in Dahlewitz, Germany, has been using multiaxial testing for many years as a crucial part of the design and qualification process for critical aerospace components. The testing is frequently used to support a high-profile lifting program for high-pressure turbine discs and features associated with Group ‘A’ Critical Parts using hot multiaxis rigs. The primary purpose is to test a sub-element of the fir-tree connection between the turbine disc and the turbine blade. 

The loads in the fir-tree connections have a direct effect on both the blade and disk’s lifespans. When accurately designed, these rotating parts of the turbine can result in improved performance;

• Lower operating costs
• Longer operating life
• Reduction of aircraft emissions
• Reduced environmental impact

The disk is the engine component at greatest risk of cracking, this rigorous safety testing is a critical part of the design and qualification process.

Using a multiaxis rig, forces are applied to the test article from rectangular vectors with additional torque to create a complex load pattern. The load patterns are then analyzed with optical strain measurement methods. This is then used to replicate the fretting fatigue mechanisms acting on the fir-tree connection.

Redefining the safety limits and enabling a better design

The multiaxial test rigs are often used to analyze in-service issues, helping to provide a better understanding of the risks associated with an in-service issue and, therefore the ability to go on to redefine the safety limits. In consequence, it enables a better design, which can be analyzed and approved through its effectiveness on the test rig.

Energy, aerospace, and transportation customers can use the rigs for a wide variety of applications, including determination of frictional break forces with various normal forces and tests under various environmental conditions. 

The testing is suitable for a wide variety of materials, including metals such as high-tensile strength steels, metal alloys such as aluminum alloys, magnesium alloys, and titanium alloys, polymers, ceramics, and elastomers composites, textiles, concrete, and advanced construction materials.

These materials can be tested for a wide range of applications depending on the industry.

The automotive industry frequently uses multiaxial testing to validate new material models for sheet metal or composite components to meet demanding fuel economy targets and increased safety requirements. The construction industry uses it to study advanced materials to allow for more complex designs and meet challenging environmental needs. The power generation industry uses multiaxial testing to test large-scale wind turbine structures that are to be positioned in harsh offshore environments whilst the oil and gas industry uses it to test for pipeline components that will allow exploration in areas that are difficult to reach.

State-of-the-art multiaxial analysis equipment simulating real-world conditions

The Rolls-Royce Dahlewitz site has heavily invested in state-of-the-art multiaxial testing equipment, including MTS Planar Biaxial Servohydraulic systems. The Planar biaxial mechanical equipment simulates real-world conditions, stressing the specimen in multiple directions while allowing the test engineers to exert a high level of control over the process to produce accurate, repeatable results.

The MTS technology allows for the testing to be tailored as required. This could, for example, be focusing on fracture mechanics and fatigue crack growth, examining how quickly a crack will grow and how soon it will reach a critical size under various loading conditions. Or it may be used to run damage tolerance tests to measure crack growth from a large hole in the material, then repair the damage to see how the repair holds up, or test how many times the same hole can be repaired.

To enable the testing to take place under a wide variety of in-service conditions, environmental simulation systems complement the planar biaxial equipment to apply extreme temperatures, vacuum, and humidity to specimens under test.

For non-ambient testing, the planar biaxial frames can be outfitted with environmental chambers for testing at cold temperatures using liquid nitrogen (or other liquefied gases if required), inert atmospheres, vacuums, and both vacuum and high temperature using furnaces or induction heating inside of a vacuum chamber. High-temperature testing can also be accomplished in the air using either furnaces or induction heating with the available temperature range in the environmental furnace ranging from room temperature to 800°C.

The Rolls-Royce and ºÃÉ«TV advantage

Rolls-Royce engineers ensure every step in the testing program is carefully planned before being executed, enabling the manufacturer to be ready for any potential issues. When preparing to test, the team make sure all steps have been taken to ensure a smooth process, taking into consideration issues such as the exact definition of the required load axis and their alignment requirements to each other, as well as available space for the test article, and any potential environmental requirements such as vacuum and temperature uniformity/gradients and main fixture interfaces. 

The MTS technology ensures superior alignment preventing unintentional bending stress that can put the specimen at higher risk of buckling or early failure, minimizing potential specimen damage.

As part of the testing process, the Rolls-Royce experts will verify the failure mechanism and recommend how to identify resolutions. The team can then work closely with the component or material manufacturer to start proof testing an improved design to provide a tailored solution that will prevent future in-service failures.

For more information on multiaxial testing, please contact ºÃÉ«TV today.