National Physical Laboratory

Thermal Expansion

The thermal expansion characteristics of a material control dimensional changes with temperature, and thus the suitablity for purpose in many applications where materials are mismatched, from the buckling of railway lines to the focus control of satellite antennae. The characteristics for a brittle material are perhaps much more important to know than in more ductile materials, because thermally induced strains radically increase risks of failure, either as monolithic components or in joining to other materials. Mechanical dilatometer facilities are available for the measurement of fractional length change as a function of temperature, from which the derived mean linear expansion coefficient over a temperature range or the ‘expansivity’ at a given temperature can be computed. Effects such as annealing or softening temperatures can be determined. Phase changes can be detected, and the magnitude of their effect on overall dimensions determined.

Test equipment is of two types:

  1. Low temperature tests: This facility employs a vertically standing low-expansion fused silica test-piece support and permits measurement over the range -140 °C to 200 °C. It has the capability to accept small components should the need arise. It has been widely used for all types of materials, including plastics, and composites. Calibration is made using a drum micrometer for apparatus sensitivity, a fused silica reference material for the apparatus correction, and a run with no test-piece for the baseline shift correction. The typical uncertainty in calibration is ± 0.2 x 10-6 °C-1 in mean coefficient over a 100 °C temperature interval, or ± 20 parts per million in fractional length change, whichever is the greater. Test-pieces may be up to 100 mm in length with flat, square ends, but need to be stable while standing upright.

  2. High temperature tests: This facility is a conventional horizontal system using an alumina test-piece support. It has a capability of 1400 °C, with a controlled flushed gas environment. It has been used for a wide range of material types, including ceramics, fine grained refractories, minerals, slags, glasses and glass-ceramics, hardmetals, metals and alloys of all types. Metals melting behaviour can sometimes be obtained satisfactorily using an alumina double piston cell to retain a test sample while it melts. Calibration is undertaken similarly to the low temperature system, with the exception that a platinum reference is used to obtain the apparatus expansion correction.

Tests are made as closely as possible to the procedures in EN 821-1 and ASTM E228, but may be varied to suit the particular material being investigated. Normally at least three thermal cycles are performed to ensure that the test-piece is behaving stably and repeatably, and the mean of the second and third cycles, heating and cooling is taken as the determination unless the material undergoes phase changes or shows progressive irreversible behaviour.

Thermal Expansion Measurements

Dimensional changes parallel and perpendicular to geological strata during
firing of a South African pyrophyllite mineral used as a machinable ceramic

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E-mail: materials_enquiries@npl.co.uk

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