What is creep, and why is it important?

Creep is the time-dependent, permanent deformation of a material at elevated temperatures. Understanding creep behavior provides insights for design improvements and quality assurance. It can really wreak havoc for manufacturers of metals, refractories, ceramics, and other materials required to withstand high temperatures.

Creep can cause unsafe conditions, reduce manufacturing efficiencies, and cost manufacturers hundreds of thousands of dollars in repair costs and lost time. Materials vulnerable to creep should undergo testing as part of a manufacturer’s product development and quality assurance processes.

Design engineers may select a material with a high-melting temperature, high-elastic modulus, and large grain size in order to resist creep. However, creep testing is necessary to understand dimensional changes over time at elevated temperatures as well as differences in creep resistance due to variations in the material.

So how do we test for creep resistance?

ASTM C832 Standard Test Method of Measuring Thermal Expansion and Creep of Refractories Under Load. This method determines thermal expansion and creep under a compressive stress for 50 hours. The percent linear change is recorded continuously during the heat-up and creep period under load.

What does a typical sample look like after testing?

Creep testing results of firebrick samples (pic below) showed Composition A (on left) had deformed 0.1% at 1600°C while composition B (right) deformed 5% at 1375°C. Why Composition A proved more creep-resistant than B could be due to a number of different factors, including 1) It was made from higher quality raw materials (less Fe2O3, TiO2, or SiO2), 2) It contained a higher concentration of alumina 3) The composition of B was such that it formed liquid at the test temperature.