Failure mechanism: CREEP

Mumbai University.
Mechanical Engineering
SEM III
Material Technology
Module 2.3

Creep:
Definition and significance of creep, Effect of temperature and creep on mechanical behaviours of materials, Creep testing and data presentation and analysis, Mechanism and types of creep, Analysis of classical creep curve and use of creep rate in designing of products for load-bearing applications, Creep Resistant materials

1. Module 2 FAILURE MECHANISM Fracture Fatigue failure creep Keval K. Patil, M.E.-DESIGN, ([email protected]) 1

2. Creep Keval K. Patil, M.E.-DESIGN, ([email protected]) Part 3 2

3. Creep Materials are often placed in service at elevated temperatures and exposed to static mechanical stresses (e.g., turbine rotors in jet engines and steam generators that experience centrifugal stresses, and high-pressure steam lines). Deformation under such circumstances is termed creep Creep is slow plastic deformation of metal under constant stresses at constant temperature for prolonged period Defined as the time-dependent and permanent deformation of materials when subjected to a constant load or stress, creep is normally an undesirable phenomenon and is often the limiting factor in the lifetime of a part. Keval K. Patil, M.E.-DESIGN, ([email protected]) 3

4. Effect of temperature on mechanical behaviour of materials When temperature of material increases, the mobility of atoms increases rapidly, which changes mechanical properties of material. High temperature will also result in greater mobility of dislocation by the mechanism of dislocation climb. The equilibrium concentration of vacancies increase and new deformation may form at high . In some metals, the slip system changes or additional slip system are introduced with increasing temperature. At high temperature, cold-worked metals will recrystallize and undergo grain coarsening while age hardening alloys may overage by prolonged exposure at high temperature and loose strength. Successful use of metals at high temperature involves a number of problems. But modern technology demand better high temperature strength and oxidation resistance. Keval K. Patil, M.E.-DESIGN, ([email protected]) 4

5. Creep testing The specimen to be tested is placed in the electric furnace where it is heated to a given temperature. The usual method of creep testing consist of subjecting the specimen at constant tensile stress at constant temperature and measuring the extent of deformation or strain with the time. The typical creep testing machine is shown in figure even though, it appears to be simple, it requires considerable laboratory equipment, great care and precision in performance. The time of each test may be matter of hour, weeks, months or even years. Keval K. Patil, M.E.-DESIGN, ([email protected]) 5

6. Creep testing Creep is also determined in compression, shear and bending. The data is presented by plotting creep curve as deformation verses time at constant temperature and stress. The test specimen may be circular, square or rectangular in cross-section. Either a continuous record of deformation with time or sufficient number of deformation readings with time should be taken over the entire period of Keval K. Patil, M.E.-DESIGN, ([email protected]) 6

7. Creep test setup Keval K. Patil, M.E.-DESIGN, ([email protected]) 7

8. analysis It is frequently important to be able to exploit creep or stress-rupture data into regions where data are not available. Therefore, common methods of plotting creep data are based on plots which yield reasonable straight line. Figure shows the common method of presenting the influence of stress on the state or minimum creep rate. Note that a log-log plot is used, so that extrapolation of one log-cycle represents a tenfold change. A change in slope of the line will sometimes occur. It has been shown that the value of the minimum creep rate depends on the length of time the creep test has been carried out. It has been shown for long-time creep test(t> 10,000 hours) that the creep strength based on ‘1%’ creep n true strain is essentially equal to creep strength based minimum creep rate. Keval K. Patil, M.E.-DESIGN, ([email protected]) 8

9. analysis Keval K. Patil, M.E.-DESIGN, ([email protected]) 9

10. Creep curve A creep curve is a plot between the total creep or strain and the time for the entire duration of test. Primary creep: The primary or transient creep is a decreasing creep rate because of the work hardening process resulting from deformation Keval K. Patil, M.E.-DESIGN, ([email protected]) 10

11. Creep curve Secondary creep: During the secondary or steady state creep(i.e. minimum creep rate), the deformation continues at an approximately constant rate. During this process, a balance exist between the rate of work hardening and rate of softening because of recovery or recrystallization. The steady state creep may be essentially viscous or plastic in character, depending upon the stress level and temperature. Tertiary Creep: Creep rate increases with time until fracture occurs in this stage Tertiary creep can occurs due to necking of the specimen or grain boundaries sliding at high temperature. Keval K. Patil, M.E.-DESIGN, ([email protected]) 11

12. Mechanism of creep failure Creep is a deformation process in which three main features to be involved are: The normal movement of dislocation along slip planes. Process ‘dislocation climb’ which is responsible for rapid creep at temperatures above 0.5 times of melting temp. Slipping at grain boundaries. The following mechanism are known to be responsible for creep in crystalline materials. Dislocation climb. Vacancy diffusion. Grain boundary sliding. Keval K. Patil, M.E.-DESIGN, ([email protected]) 12

13. Dislocation climb: In the primary stages of creep, dislocation move quickly at first but soon becomes pilled up at various barriers. At temperature in excess of 0.5 tm, thermal activation is sufficient to promote a process known as ‘dislocation climb’. It is shown in figure this would bring into use new slip planes and so reduce the rate of work hardening. Keval K. Patil, M.E.-DESIGN, ([email protected]) 13

14. Dislocation climb: In addition to plastic deformation by dislocation movement, deformation by a form of slip at grain boundaries also occurs during the secondary stage of creep. These movements possibly leads to the formation of ‘vacant sites’, that is lattice position from which atoms are missing and this in turn makes possible ‘ dislocation climb’. In the tertiary stage of creep micro-cracks are initiated at grain boundaries due to the movement of dislocations. In some cases, there is migration of vacant site, as a result necking and consequent rapid failure follows. Keval K. Patil, M.E.-DESIGN, ([email protected]) 14

15. Vacancy diffusion: the diffusion of vacancies control creep rate. In this mechanism, grain boundary acts as a source and sink for vacancies. The mechanism depends on the migration of vacancies from one side of a grain to another. Referring to figure a grain ABCD is under stress ‘p’, the atoms moved from faces ‘BC’ and ‘AD’, along the path shown and the grain creeps in the direction of stress Movement of atoms creating vacancies on face ‘AB’ & ‘DC’ and destroying them on the other faces. Keval K. Patil, M.E.-DESIGN, ([email protected]) 15

16. Grain boundary: the sliding of neighbouring grains with respect to the boundary that separates them. Figure shows that, grain boundaries lose their strength at lower temperature than the grains themselves. This effect arises from non crystalline structure of the grain boundary Keval K. Patil, M.E.-DESIGN, ([email protected]) 16

17. Analysis of classical creep curve Andrade’s work on analysis of classical creep curve is focused on topic of creep. He considered that the constant stress creep curve represents the superposition of two separate creep processes which occur after the sudden strain which results from applying the load. The first component of the creep curve is a ‘transient creep’ with a creep rate decreasing with time. Added to this is a constant-rate ‘viscous-creep’ component figure shows Andrade's analysis of the classical creep curve. Andrade found that the creep urve could be represented by the following empirical equation. e = e [1+ t^(1/3)] e^( kt ) Where, e= strain, t= time, & k = constant, e = instantaneous strain. Keval K. Patil, M.E.-DESIGN, ([email protected]) 17 c

18. Creep resistance materials Creep resistant material are required for structural and machine components used at elevated temperatures. They should be capable of withstanding these temperatures without undergoing creep beyond the specified limit, which may cause dimensional changes beyond permissible limit used in the design. The following are the requirements of a creep resistance material. It should have high melting point because, the creep becomes significant above 0.4 tm (tm=melting point). If the melting point is high, material can be used at higher temperature, e.g. iron, nickel, cobalt. Keval K. Patil, M.E.-DESIGN, ([email protected]) 18

19. Creep resistance materials It should have coarse grained structure. The grain boundary region becomes quasi-viscous at creep temperature . Since in coarse grained materials grain boundary area is less, so that less amount of quasi- viscous region is formed with a less tendency to flow, reducing the creep deformation. It should be precipitation hardenable . It should have fine insoluble precipitates at the operating temperature. If coherent precipitates are present, maximum creep resistance is obtained e.g. in nickel base and iron-nickel-base superalloy coherent precipitates of Ni3 (Al, Ti) is formed. Dispersion hardening improves creep resistance. It should have high oxidation resistance i.e. the oxide film should follow either a logarithmic or a cubic law of growth. Keval K. Patil, M.E.-DESIGN, ([email protected]) 19

20. Conclusion Keval K. Patil, M.E.-DESIGN, ([email protected]) 20