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Creep and creep testing

   

Job Knowledge 81

The use of metals at high temperatures introduces the possibility of failure in service by a mechanism known as creep.

As the name suggests this is a slow failure mechanism that may occur in a material exposed for a protracted length of time to a load below its elastic limit (see Connect article No. 69), the material increasing in length in the direction of the applied stress. At ambient temperature with most materials this deformation is so slow that it is not significant, although the effect of low temperature creep can be seen in the lead on church roofs and in medieval glazing, where both materials have slumped under the force of gravity.

For most purposes such movements are of little or no importance. Increasing the temperature, however, increases the rate of deformation at the applied load and it is vitally important to know the speed of deformation at a given load and temperature if components are to be safely designed for high temperature service. Failure to be able to do this may result in, for example, the premature failure of a pressure vessel or the fouling of gas turbine blades on the turbine casing.

The drive for the more efficient use of fuels in applications such as power generation plant and gas turbines demands that components are designed for higher and higher operating temperatures, requiring new creep resistant alloys to be developed. To investigate these alloys and to produce the design data the creep test is used.

In metals, creep failure occurs at the grain boundaries to give an intergranular fracture. Fig.1 illustrates the voids that form on the grain boundaries in the early stages of creep. The fracture appearance can be somewhat similar to a brittle fracture, with little deformation visible apart from a small amount of elongation in the direction of the applied stress.

Fig.1. The voids that form on the grain boundaries in the early stages of creep a)
Fig.1. The voids that form on the grain boundaries in the early stages of creep a)
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b)

The creep test is conducted using a tensile specimen to which a constant stress is applied, often by the simple method of suspending weights from it. Surrounding the specimen is a thermostatically controlled furnace, the temperature being controlled by a thermocouple attached to the gauge length of the specimen, Fig.2. The extension of the specimen is measured by a very sensitive extensometer since the actual amount of deformation before failure may be only two or three per cent. The results of the test are then plotted on a graph of strain versus time to give a curve similar to that illustrated in Fig.3.

Fig.2. Schematic of a creep test
Fig.2. Schematic of a creep test
Fig.3. Typical creep curve for steel
Fig.3. Typical creep curve for steel

The test specimen design is based on a standard tensile specimen. It must be proportional (see Connect Article No. 69) in order that results can be compared and ideally should be machined to tighter tolerances than a standard tensile test piece. In particular the straightness of the specimen should be controlled to within some ½% of the diameter. A slightly bent specimen will introduce bending stresses that will seriously affect the results. The surface finish is also important - the specimen should be smooth, scratch free and not cold worked by the machining operation. The extensometer should be fitted on the gauge length and not to any of the other load carrying parts as it is difficult to separate any extension of these parts from that in the specimen.

Testing is generally carried out in air at atmospheric pressure. However, if it is necessary to produce creep data for materials that react with air these may be tested in a chamber containing an inert atmosphere such as argon or in a vacuum. If the material is to operate in an aggressive environment then the testing may need to be carried out in a controlled environment simulating service conditions.

Fig.3 shows that creep failure occurs in three distinct phases - a rapid increase in length known as primary creep where the creep rate decreases as the metal work hardens. This is followed by a period of almost constant creep rate, steady state or secondary creep and it is this period that forms the bulk of the creep life of a component. The third stage, tertiary creep, occurs when the creep life is almost exhausted, voids have formed in the material and the effective cross sectional area has been reduced. The creep rate accelerates as the stress per unit area increases until the specimen finally fails. A typical failed specimen is illustrated in Fig.4.

Fig.4. Fractured test specimen
Fig.4. Fractured test specimen

The creep test has the objective of precisely measuring the rate at which secondary or steady state creep occurs. Increasing the stress or temperature has the effect of increasing the slope of the line ie the amount of deformation in a given time increases. The results are presented as the amount of strain (deformation), generally expressed as a percentage, produced by applying a specified load for a specified time and temperature eg 1% strain in 100,000hrs at35N/mm 2 and 475°C.

This enables the designer to calculate how the component will change in shape during service and hence to specify its design creep life. This is of particular importance where dimensional control is crucial, in a gas turbine for instance, but of less importance where changes in shape do not significantly affect the operation of the component, perhaps a pressure vessel suspended from the top and which can expand downwards without being compromised.

There are therefore two additional variations on the creep test that use the same equipment and test specimen as the standard creep test and that are used to provide data for use by the designer in the latter case. These are the creep rupture test and the stress rupture test. As the names suggest both of these tests are continued until the specimen fails. In the creep rupture test the amount of creep that has occurred at the point of failure is recorded. The test results would be expressed as %age strain, time and temperature eg rupture occurs at 2% strain at 450°C in 85,000 hours. The stress rupture test gives the time to rupture at a given stress and temperature eg 45N/mm2 will cause failure at 450°C in 97,000 hrs. This data, if properly interpreted, is useful in specifying the design life of components when dimensional changes due to creep are not important since they give a measure of the load carrying capacity of a material as a function of time.

Relevant Specifications

BS EN 10291 Metallic Materials - Uniaxial Creep Testing in Tension.
BS 3500 Methods for Creep and Rupture testing of Metals.
ASTM E139 Conducting Creep, Creep Rupture and Stress Rupture Tests of Metallic Materials.
BS EN ISO 899 Plastics - Determination of Creep Behaviour.
BS EN 761 Creep Factor Determination of Glass
- Reinforced Thermosetting Plastics
- Dry Conditions.
BS EN 1225 Creep Factor Determination of Glass
- Reinforced Thermosetting Plastics
- Wet Conditions.

This article was written by Gene Mathers.