Equipment tests thermo-mechanical fatigue characteristics

Thermo-mechanical fatigue testing machine

Zwick Roell has developed equipment for use in reliability testing to understand the effects of applying both temperature variations and strain on components.

A new thermo-mechanical fatigue testing machine has been developed by engineers at Zwick Roell in Germany, which they claim could have significant benefits over other currently available systems.

Thermo-mechanical fatigue (TMF) is caused by a combination of mechanical and thermal loading where both stresses and temperature vary with time. TMF testing is a variant of fatigue testing, designed to replicate service conditions experienced by a wide range of materials and engineered components. Typically, TMF testing characterises the response of materials to both cyclic mechanical loading and fluctuating temperature and this type of loading can be more damaging, by more than an order of magnitude, compared with isothermal fatigue at the maximum operating temperature. Material properties, mechanical strain range, strain rate, temperature and the phasing between temperature and mechanical strain all play a role in the type of damage formed in the material.

Zwick Roell offers a TMF machine which it says is ideally suited for research in the aerospace and power generation industries. The TMF system simulates the complex effects of thermal cycling combined with mechanical loading, normally experienced by gas turbines and similar equipment during operation. More sophisticated than isothermal fatigue, TMF testing helps to better evaluate the life of many materials and engineered components that are used in high-temperature applications.

Based on a very stiff electro-mechanical testing machine, Zwick Roell points out that unlike a servo-hydraulic system, it does not require an oil supply, uses less energy whilst conducting TMF tests and only requires basic maintenance which helps keep cost of ownership as low as possible.

The machine configuration is based on a four column load frame for maximum stiffness, incorporating a central lead-screw for the load application. The entire deformation of the load frame, including the drive mechanism and the load cell comes in with an impressive stiffness of 165kN/mm.  An inductive heating system facilitates tests up to 1200C and an integrated specimen cooling system is also provided.

TMF tests are most frequently carried out on high temperature alloys and utilise a combination of in-phase and out-of-phase mechanical and thermal cycling. Zwick Roell states that test configurations can be configured via the industry standard, intuitive testXpert ll software to accommodate TMF tests, analysis, report templates and calculations. The company claims that by using pre-configured TMF testing routines and having the ability to generate bespoke programs using its ‘Graphical Sequence Editor’ software, test routines can be set up quickly and test results of the highest integrity can be achieved in the shortest possible time. The Zwick Roell TMF package includes software that can monitor specimen temperature and strain as measured by a bi-axial extensometer as well as calculating and controlling mechanical strain in real-time.

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