National Physical Laboratory

Miniaturised Testing

Electrothermal Mechanical Test (ETMT) System

NPL has developed the electrothermal mechanical testing machine (ETMT) in order to enable complex mechanical tests on very small test-pieces. An electrically conducting test-piece is held between a pair of grips aligned on linear bearings. It is heated by passing through it a controlled d.c. current and peak temperature controlled by a thermocouple on the centre of the sample.

One grip is connected to the rigid base of the machine through a load cell and the other moved by a speed controlled motor (load or by displacement control). Displacement of the grips is monitored by capacitative transducers. Test-piece resistivity in the hot zone is measured by spot-welded contacts on the test-piece, and plastic strain can usually be linked to changes in resistivity.

This arrangement means that the test-piece can be subjected, for example, to:

  • Monotonic tension or compression tests
  • Creep tests under controlled load
  • Rising temperature deformation tests – useful for finding the temperature at which a material radically weakens
  • Cyclic loading/fatigue tests
  • Cyclic thermomechanical fatigue, with the temperature cycling in phase or out of phase with the loading cycles
  • Thermal diffusivity measurement (subject to limitations of a thermal cooling model)
  • Thermal expansion measurement (subject to limitations of a thermal gradient model)
  • Emissivity as a function of temperature and oxide film development (using a radiation camera sighted on the test-piece)
    via the flexible LabVIEW® based software developed by NPL for controlling the test and recording the output data

The system has a maximum force capability of 4 kN, with no limit on test temperature. In fact tests can be performed up to the melting point, so long as the test-piece does not sag significantly. Phase changes are readily detected by changes in resistivity.

So far the system, which was originally design to make high temperature tests on hardmetals, has been developed to be able to cope with a wide range of materials and test requirements. It has been used on a very wide range of hardmetals, cermets, and metal alloys, including nickel, titanium and iron based systems.

Related publications


Bryan Roebuck

Last Updated: 9 Dec 2016
Created: 15 Jun 2010


Please note that the information will not be divulged to third parties, or used without your permission