Creep
- 1. BY Dr. K. SENTHILARASAN ASSISTANT PROFESSOR DEPARTMENT OF PHYSICS E.G.S.PILLAY ARTS & SCIENCE COLLEGE NAGAPATTINAM-611002 CREEP
- 2. Creep The creep is defined as the property of a material by virtue of which it deforms continuously under a steady load. Creep is the slow plastic deformation of material under the application of a constant load even for stresses below the proportional limit. This yielding (increase of strain without increase in load) may continue to the point of fracture. Usually creep occurs at high temperature. In designing of I.C. engines, jet engines, boilers and turbines creep is taken into account as an important property. This property is exhibited by iron, nickel, copper and their alloys at elevated temperatures. But zinc, tin, lead and their alloys show creep at room temperature only.
- 3. In metals creep is a plastic deformation caused by slip occurring along crystallographic direction in the individual crystals together with some deformation of the grain boundary of the materials. After complete release of load, a small fraction of this plastic deformation is recovered with time. Thus, most of the deformation is non recoverable.
- 4. Creep limit Creep limit is defined as the maximum static stress that will result in creep at a rate lower than some assigned rate at a given temperature. Creep strength is usually defined as the stress for 1% strain in 10,000 hours in the case of jet turbine design while for steam turbine design, it is the stress for 1% strain in 100000 hours.
- 6. Primary stage (Instantaneous elongation and transient creep) is characterised by the relatively rapid extension but at a decreasing rate. This primary stage is of great interest to the designer since it forms as early part of the total extension reached in a given time and may affect clearances provided between components of a machine. Thermal expansion may also be included in this stage. Secondary stage (viscous or steady creep) is characterised by the period of extension during which the creep occurs at a more or less constant rate having its having minimum creep rate. The is important part of the curve which is used to estimate the service life of the alloy. Tertiary stage is characterised by the accelerated rate of extension which finally lead to rupture. So the use of alloys should be avoided in this stage.
- 7. Factors influencing creep resistance 1. Heat treatment: Creep resistance of steel is influenced by heat treatment. At temperature of 300 C or higher, the maximum creep resistance is usually produced by normalising. 2. Grain size: Creep resistance is also influenced by the grain size of steel. 3. Strain hardening: Strain hardening of steel increase its creep resistance. 4. Alloying additions: At temperature below the lowest temperature of recrystallization, the creep resistance of steel may be improved by the ferrite forming elements like nickel, cobalt and manganese or by the carbide forming elements like chromium, molybdenum, tungsten and vanadium.
- 8. Theories of creep The creep rate for the second stage (steady state) increase with increasing stress at a constant temperature according to the equation 𝑑ε 𝑑𝑡 = A 𝑛 Where “n” is a constant called stress exponent whose value is about 5 and A is also a constant and epsilon and Sikma are creep strain and stress respectively. The creep rate also increases with increasing temperature for a given stress. 𝑑 𝑑𝑡 = B𝑒 −𝑄/𝑅𝑇 Where Q is called the activation energy for creep, R is the gas constant and T is the temperature of the material in Kelvin.
- 9. One can draw a graph between the log stress and a combined parameter of time and temperature called Larson Miller parameter, given by P=T (C+log t) where T is the temperature, t the rupture time and C is a constant. We can get the graph as a straight line. Using this graph, it is possible to predict the repute time for lower stresses and temperatures.