ASTM E228 specifies the method for obtaining linear thermal expansion coefficient of rigid solids using a push-rod dilatometer. A rigid solid has negligible creep or elastic strain effects. This test is used in thermal modelling, design of equipment and components handling wide variation in operating temperatures, quality control and development of new materials for thermal applications.
The standard method ASTM E228 is generally applicable to rigid solids having coefficient of linear expansion (CTE) greater than 0.5 µm/m 0C. The push-rod dilatometry principle involves expansion of heated specimens causing displacement of a push-rod. Sensor mechanisms are employed to record the corresponding linear displacement and temperature. The precision of ASTM E228 is proven over a temperature range of -180 0C to 900 0C, using vitreous silica type of push-rods. Alumina push rods can extend the range to 1600 0C and graphite push rod systems upto 2500 0C. The CTE of a wide range of metals, ceramics and polymers can be tested by this method. The CTE value is used in thermal modelling, design of equipment and components handling wide variation in operating temperatures, quality control and development of new materials for thermal applications.
The ASTM E228 test can be conducted in horizontal or vertical push rods dilatometers, using a push rod type suitable for the temperature range. A sample of specified size is placed in the holder and then a cylindrical furnace is moved to enclose the sample uniformly on all sides. The push rod is always in contact with the end of the sample, extending through the sample holder, to connect with a displacement sensor system outside the furnace. The enclosed sample is subjected to pre-programmed heating rate and the linear expansion corresponding to temperature rise is measured. A reference sample with a push rod can also be provided in a dual channel type apparatus.
The specimen size for the ASTM E228 test is dictated by the dilatometer used and temperature range. Typically, for tests upto 1000 0C, maximum diameter is 12.7 mm (1/2 inch) and lengths are 12.7 mm, 25.4 mm (1 inch) or 50.8 mm (2 inch). Specimens to be tested above 1000 0C, due to more expansion, maximum diameter is 15.875 mm (5/8 inch) and length 25.4 mm (1 inch). Note that tests can be made on some non-standard lengths and shapes as well based on agreed protocol and criteria according to clients’ needs.
The result of the E228 CTE test includes the CTE value of the material and the temperature range in SI units.