Creep test

What is a creep test?

A creep test, also known as a stress-relaxation test, is used to determine the degree of deformation a material undergoes over time at a constant temperature under a continuous tensile or compressive load. Creep testing is fundamental for materials that need to withstand certain operating temperatures under load. For materials such as metals or alloys, material properties change significantly at higher or lower temperatures.

By examining the results of a creep test, engineers can determine the expected deformation of a material and avoid failure when designing new systems for different environmental conditions.

The use of metals at high temperatures introduces the possibility of failure during use by a mechanism called creep.

As the name suggests, this is a slow failure mechanism that can occur in a material subjected to a load below its elastic limit for an extended period of time, where the material increases longitudinally in the direction of the applied stress.

In most materials, this deformation at ambient temperature is so slow that it is not significant, although the effect of creep at low temperatures can be observed in lead on church roofs and in medieval glazing, where both materials have collapsed under gravity.

For most purposes, such movements are of little or no importance. However, increases in temperature increase the deformation rate at the applied load, and it is vital to know the deformation rate at a given load and temperature to safely design components for high-temperature operation. Failure to do so could lead to premature failure of a pressure vessel or fouling of gas turbine blades on the turbine housing, for example.

The drive for more efficient use of fuels in applications such as power plants and gas turbines requires components to be designed for ever-higher operating temperatures, necessitating the development of new creep-resistant alloys. The creep test is used to investigate these alloys and produce design data.

In metals, creep fracture occurs at grain boundaries and intergranular fracture occurs. The fracture appearance may somewhat resemble brittle fracture, with little deformation visible apart from a small elongation in the direction of the applied stress.

The creep test is performed using a tensile specimen on which a constant tension is applied, often by suspending weights from it. Surrounding the specimen is a thermostatically controlled furnace, with the temperature controlled by a thermocouple attached to the length of the specimen.

The elongation of the specimen is measured by a highly sensitive extensometer, as the actual amount of deformation before collapse can be as little as two or three per cent. The results of the test are then plotted on a graph of elongation versus time to obtain a curve.

The design of the specimen is based on a standard tensile specimen. It should be proportional so that results can be compared and should ideally be machined to tighter tolerances than a standard tensile specimen. In particular, the straightness of the specimen should be checked to within about 1/2% of its diameter. A slightly bent specimen will introduce bending stresses that will seriously affect the results. Surface finish is also important - the specimen should be smooth and scratch-free and not cold worked by machining. The extensometer should be mounted on the length of the specimen and not on any of the other load-bearing parts, as it is difficult to separate any extension of these parts from those in the specimen.

Tests are generally performed in air at atmospheric pressure. However, if it is necessary to produce creep data for materials that react with air, they can be tested in a chamber with an inert atmosphere such as argon or in a vacuum. If the material is to be used in an aggressive environment, testing may need to be carried out in a controlled environment that simulates operating conditions.

A creep fracture occurs in three distinct phases - a rapid increase in length, known as primary creep where the creep rate decreases as the metal hardens. This is followed by a period of almost constant creep, stable creep or secondary creep, and it is this period that accounts for most of a part's creep life. The third phase, tertiary creep, occurs when the creep life is almost exhausted, voids have formed in the material and the effective cross-sectional area has decreased. The creep rate accelerates as the stress per unit area increases until the specimen finally fails.

The creep test aims to accurately measure the rate at which secondary or stationary creep occurs. Increasing the voltage or temperature has the effect of increasing the slope of the line, i.e. increasing the amount of deformation in a given time. The results are presented as the amount of strain (deformation), usually expressed as a percentage, produced by applying a given load for a given time and temperature, e.g. 1% strain in 100,000 hours at 35N/mm 2 and 475°C.

This allows the designer to calculate how the part will change shape during operation and thus specify the design creep life. This is especially important if dimensional control is crucial, for example in a gas turbine, but less important if shape changes do not significantly affect the operation of the part, for example a pressure vessel that hangs at the top and can expand downwards without being endangered.

There are therefore two additional variations on the creep test that use the same equipment and test sample as the standard creep test and are used to provide data that can be used by the designer in the latter case. These are the creep rupture test and the stress rupture test. As the names suggest, both tests are continued until the specimen fails. The creep rupture test records the amount of creep that has occurred at the point where the specimen fails. The test results are expressed as % strain, time and temperature, e.g. failure occurs at 2% strain at 450°C in 85,000 hours. The stress fracture test gives the time to failure at a given stress and temperature, e.g. 45N/mm2 causes failure at 450°C in 97,000 hours. These data, if interpreted correctly, are useful in specifying the design life of components when dimensional changes due to creep are not important because they give a measure of a material's load capacity as a function of time.

Relevant standards

BS EN 10291	Metals Materials - Uniaxial creep testing in tensile.
BS 3500	Methods of creep and rupture testing on metals.
ASTM E139	Performance of creep, creep rupture and stress rupture tests on metals.
BS EN ISO 899	Plastics - Determination of creep behaviour.
BS EN 761	Determination of the creep factor of glass fibre-reinforced thermosetting plastics. Glass reinforced thermosetting plastics. Dry conditions.
BS EN 1225	Creep factor determination of glass-reinforced thermosetting plastics. Glass-reinforced thermosetting plastics. Wet conditions.

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