Creep is a plastic deformation process that occurs when materials are subjected to elastic loading for a long period of time, often at high temperature. Engineering materials do not plastically deform when loaded within the elastic regime for short times. However, when the elastic load is applied for a sufficient period the material eventually deforms plastically by creep. The amount of creep experienced by structural and engine materials used in aircraft is negligible at room temperature. However, the creep rate increases rapidly with temperature (usually above 40% of the absolute melting temperature for metals) and therefore creep is an important engineering property for materials required to operate at high temperature, such as jet and rocket engine materials.
The creep properties of materials are determined using the creep test. The test involves measuring the elongation of a material specimen while under a constant applied tensile stress at high temperature (Fig. 5.23). The specimen is heated inside a thermostatically controlled furnace attached to a creep testing machine that applies the tensile load. The level of applied stress is below the yield strength of the material. The elongation of the specimen over time is measured using an extensometer or other device capable of measuring the strain. Creep tests are usually performed over long periods, with the loading times typically ranging from 1000 to 10 000 h. The extension of the specimen must be measured using a very sensitive device because the actual amount of deformation before failure may be less than a few percent. Ideally, the test should be performed under conditions that replicate, as close as practical, the temperature, stress and time-scale of the aircraft material when used in service. For example, creep tests performed on materials for engine turbine blades should be performed close to the operating stress and temperature of the blades, which is about 180 MPa and 1200 °C.
The test results are plotted as strain against time to give a creep graph, as shown in Fig. 5.24. The curve for most materials can be divided into three regions: primary creep, secondary creep, and tertiary creep. A series of creep tests performed at different stress levels and temperatures are performed to obtain a complete assessment of the creep properties for a material. The creep properties of aerospace metals and composites are described.
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