Contacting and non-contacting extensometry for ultra high temperature testing

Abstract The determination of the mechanical properties of engineering materials at elevated temperature requires the measurement of temperature, force and strain. Of these three parameters the accurate measurement of testpiece strain within an ultra high temperature environment represents the greatest challenge to both the instrument designer and the researcher. The accuracy and resolution requirements for extensometry are related to the test type. For example, a short term tensile test on a monolithic ceramic would need an extensometer with sub-micron resolution and accuracy. Long term tests such as fatigue and creep require similar accuracy along with excellent long term stability. The extensometer should have an adequate frequency response and be free of resonances that could affect the stability of the system in strain control. In both contacting and non-contacting high temperature extensometers the interface between the extensometer and the testpiece is a critical area. When using a contacting approach, the extensometer must have a very low operating force along with excellent kinematics if knife-edge slip is to be avoided whilst minimizing sideways contact force on the testpiece. The contact points between the testpiece and the extensometer should not be subject to any chemical reactions at the test temperature. The extensometer should be easy to load on to the testpiece. This is not a trivial matter, as the testpiece may be inside a furnace and hence not visible. This chapter sets out to review examples of both contacting and non-contacting extensometry suitable for ultra high temperature applications. Examples of contacting types include a low operating and contact force side-entry contacting design, suitable for use in air and in vacuum. The key designs and operating features are outlined, along with issues such as integration into furnace and vacuum systems and chemical compatibility of knife edge materials. The operation of several non-contacting systems, including laser scanning, integrating laser Doppler and electro-optical types, along with techniques for providing optical reference marks on the specimen are reviewed. The advantages and disadvantages of the alternative approaches are compared. Finally, developments in the application of image processing technology to strain measurement are reviewed with regard to the possibility of use in the ultra high temperature regime.