ASTM E228 Linear Thermal Expansion of Solids

ASTM E228 measures the linear coefficient of thermal expansion of solids using a push-rod dilatometer. The test sample is placed inside the furnace tube while the push rod measures the length changes and transfers them to the displacement indicator situated outside the heated region. MatX Lab performs this testing procedure in accordance with ASTM E228 standards.

What Is ASTM E228?

ASTM E228 is the test method used to determine the linear thermal expansion of solids using a push-rod dilatometer. The test involves positioning the test specimen between a fixed reference plane and a push rod, typically made of fused silica or alumina, depending on the temperature. The movement of the push rod, as the temperature increases, is recorded.
The apparatus expands too. That contribution is removed with a system correction determined by running a reference material of certified expansion under identical conditions, which is why calibration quality decides the accuracy of the result.

Two coefficients come out of the data. The mean coefficient of thermal expansion averages behavior across a temperature interval you nominate, and the instantaneous coefficient describes the slope at a single temperature. Both are reported in units of 10⁻⁶/K.

ASTM E228 Scope and Applications

The technique applies to solid specimens that retain their form and are chemically stable to the push rod and the tube. Silica-based systems operate at cryogenic temperatures and attain 900 °C. Alumina-based systems operate at much higher temperatures.

Use cases:

1. Joint and seal design. Thermal expansion matching of materials used in brazing, glazing, or bolting so that the joint will survive thermal cycles.Glass-to-metal and ceramic-to-metal
2. sealing. Confirming that expansion curves track closely enough across the sealing range to avoid residual stress.
3. Thermal model input. Supplying expansion data for stress analysis of components that operate hot.
4. Phase change detection. Catching transformations, sintering, and softening, all of which show as a slope change or a discontinuity in the expansion curve.

Plastics that have been tested in the ambient range window fall into the ASTM D696 category, and thermomechanical analysis ASTM E831 is suitable for handling small samples and polymers. Interferometric dilatometry per ASTM E289 must be used for low-expansion materials, where system correction would overwhelm the effect. Aerospace testing under this technique includes commercial projects.

ASTM E228 Test Procedure

The correction run matters as much as the sample run. A dilatometer measures a difference, and both halves of that difference have to be right.

Limitations:  A fast heating rate creates a thermal gradient between the sample and its thermocouple, moving the entire curve. Force from the push rod causes deformations in soft materials, and thus, they measure as contractions. Soft metals and polymers require special treatment or another method. Samples that undergo oxidation, sintering, or other processes that result in loss of volatile materials will experience a change in length due to chemical reactions, and the curve will describe them. Low-expansion materials like fused silica or certain glass ceramics have such small movement that system correction becomes an important part of the reading, and interferometry is used in such cases. Anisotropic materials have different expansion rates in every direction, and therefore, each of them requires a separate sample. Initial cycles with samples with residual stress or incomplete sintering do not repeat, so we run one more cycle.

ASTM E228 Specimen Requirements and Test Conditions

Tell us the temperature intervals you want the mean coefficient calculated over. A value quoted across 20 °C to 300 °C will not match one quoted across 20 °C to 500 °C, and specifications name the interval for that reason.

ASTM E228 Test Results and Reporting

The expansion curves provided by your report are based on length versus temperature, with mean coefficients being presented within each range of intervals that you have selected.
An instantaneous coefficient is available whenever necessary. Each graph comes with the conditions under which the test is run: heating rate, atmosphere, size and orientation of the specimen, applied pressure, correction material, and its certified value.

Characteristic features are stated on the graphs rather than eliminated. A shift in slope at 570 °C in a steel sample, contraction during sintering, or hysteresis between heating and cooling are informative and are provided in the report.

ASTM E228 FAQs