Raymond F.      Mignogna,P.E.

A series devoted to considerations of basic technical topics related to metallurgy and statistics. 

ELEVATED TEMPERATURE TESTING

The mechanical behavior of all materials is affected by temperature.  Normally, whenever we speak of mechanical properties, the underlying assumption is that we’re talking about room temperature conditions unless we specify otherwise.  For test purposes, “room temperature” generally means 25 degrees Centigrade (77 degrees Fahrenheit).  Aside from precise research measurements, adjustments are generally not made for the normal indoor temperature range under which we all work and live.

However, there are many circumstances where materials have to function well above that range, and indeed, some materials operate at temperatures over  2000 degrees (F).

It would be all too easy (and lead to serious, possibly catastrophic errors) to simply extrapolate room temperature results and sensitivity to those very much higher temperatures (the same can be said for extreme cold conditions). Research over many years has shown that the mechanism of deformation in metals changes as the metal is subject to elevated temperatures, thus changing the way in which it responds to applied loads.  Therefore, it has become common practice to test materials at elevated temperatures, and use that data to evaluate the properties of the material at relevant elevated temperatures. 

In addition to the usual tensile test performed at elevated temperatures, compression tests may also be performed, with attention being paid to both heating and strain rates either before or during the test.

Another series of tests, designed to measure the load carrying capacity of a metal at elevated temperatures, are also commonly used to assess performance.  These tests are called creep, creep-rupture and stress rupture tests.   The creep test measures the deformation of a material over some fixed time period.  It’s usually performed with a relatively light load, and is not generally carried out to fracture.  Creep-rupture and stress-rupture tests, on the other hand, are designed to measure deformation and load carrying capacity at loads that will eventually cause the specimen to fracture.

ASTM has developed a series of standard test methods for testing materials at elevated temperatures.  These documents contain detailed procedures so that valid comparisons can be made between tests run at different laboratories, and on differing materials.  Some of the more relevant methods are:

E-21; Elevated Temperature Tensile Testing

E-139; Creep, Creep-Rupture and Stress-Rupture Testing

E-209; Elevated Temperature Compression Testing

E-633; Guide for Thermocouple Use in Creep & Stress-Rupture Testing