Creep failure Strain-time curve Effect of temperature and applied stress Factors reducing creep rate High-temperature alloys

Transcription

1 Fatigue and Creep of Materials Prof. A.K.M.B. Rashid Department of MME BUET, Dhaka Fatigue failure Laboratory fatigue test The S-N Ncurve Fractography of fractured surface Factors improving fatigue life Creep failure Strain-time curve Effect of temperature and applied stress Factors reducing creep rate High-temperature alloys Reference: 1. W. D. Callister, Jr. Materials Science and Engineering An Introduction, 5 th Edition, Jon Wiley & Sons, 2001, Ch. 7, pp Rashid, DMME, BUET MME 291, Lec 14: Fatigue and creep of metals P 02 1

2 Failure under Fluctuating Load Failure occurs at prolonged application of dynamic and fluctuation stress, the value of which is much lower than tensile or yield stress of material (for a static load) bridges, aircrafts, machine components Single largest cause of material failure ( 90% of all material failure) It is catastrophic and insidious, occurring very suddenly and without warning Brittle-like failure, even in ductile materials Failure process occurs by the initiation and propagation of surface-initiated crack, and the fractured surface is usually perpendicular to the direction of the applied stress. Rashid, DMME, BUET MME 291, Lec 14: Fatigue and creep of metals P 02 Laboratory fatigue test rotating bend test periodic and symmetrical about zero axis LOAD periodic and asymmetrical about zero axis Result is commonly plotted as: S (stress) vs. N (# of cycles to failure) graph random stress fluctuation Low cycle fatigue high loads, plastic and elastic deformation High cycle fatigue low loads, elastic deformation (N > 10 5 ) 2

3 The S-N Curve Fatigue limit, or endurance limit, S fat stress below which fatigue failure would not occur for steels, S fat 35-60% of TS Example: Steel Most nonferrous materials do not show any fatigue limit (i.e., S fat = 0!!) Example: Aluminium Fatigue strength, S f stress to cause fracture after specific # of cycles Fatigue life, N f number of cycles to cause failure at a specific stress S f N f The S-N Curve: An Example Rashid, DMME, BUET MME 291, Lec 14: Fatigue and creep of metals P 06 3

4 Constant Probability Curve There always exist a considerable degree of scatter in fatigue data, which may lead to significant design uncertainties. S-N curves are typically best fit curves, drawn through average-value data points. Statistical technique used to specify fatigue life and fatigue limit in terms of probability. Example: At a stress of 200 MPa, 1% samples fails at ~2x10 6 cycles, 50% fails at ~5x10 7 cycles, etc. S-N probability failure curve for 7075-T6 aluminium alloys. P denotes probability of failure. Crack Initiation and Propagation 3 distinct steps of fatigue failure: 1. Crack initiation Small cracks form at the surface at some point of high stress concentration (microcracks, scratches, indents, interior corners, dislocation slip steps, etc.). Quality of surface is important. 2. Crack propagation Crack advances incrementally with each stress cycle Stage I initially slow, involving few grains Stage II faster propagation perpendicular to the applied stress by repetitive blunting and sharpening of process of crack tip 3. Final failure occurs very rapidly once the advancing crack has reached a critical value The fatigue life: N f = N i + N p contribution of the final step to total fatigue life is insignificant since it occurs so rapidly N f : No. of cycles to failure N i : No. of cycles for crack initiation N p : No. of cycles for crack propagation 4

5 Fractograph of Fractured Surface crack origin smooth circular beachmark practical example of fatigue failure dull, fibrous brittle failure direction of rotation final rupture Rashid, DMME, BUET MME 291, Lec 14: Fatigue and creep of metals P 09 Fractograph of Fractured Surface Transmission electron fractograph showing fatigue striations. Rashid, DMME, BUET MME 291, Lec 14: Fatigue and creep of metals P 10 5

6 Factors Improving Fatigue Life Reducing working stress (magnitude, amplitude) Imposing compressive surface stress (by shot peening, case hardening, etc.) (to suppress crack growing) Improving quality of surface (removing defects e.g., g, sharp edge, notch, groove, etc.; applying surface treatments) Removing environmental effects (thermal fluctuations, corrosive environment) Rashid, DMME, BUET MME 291, Lec 14: Fatigue and creep of metals P 11 Failure under Constant Load At High Temperature Creep is a time-dependent and permanent deformation of materials when subjected to prolonged constant load at a high temperature (T > 0.4 T m ). Objects commonly failed under creep: turbine blades, steam generators, etc. Rashid, DMME, BUET MME 291, Lec 14: Fatigue and creep of metals P 12 6

7 Obtaining creep (ε-t) curve in laboratory experiment Constant load Steady-state creep rate, ε/ t Time of rupture, t r 1 Instantaneous deformation mainly elastic. 2 Primary creep decreasing creep strain with time due to workhardening 3 Secondary (steady-state) creep rate of straining is constant: balance of hardening and recovery (longest stage in duration) 4 Tertiary creep rapidly accelerating strain rate up to failure due to microstructural changes (formation of internal cracks, voids, cavities, grain boundary separation, necking, etc.) Fractograph of Fractured Surface Fractured surface showing oxide films Rashid, DMME, BUET MME 291, Lec 14: Fatigue and creep of metals P 14 7

8 Effect of Temperature and Applied Stress Dependency of steady-state creep rate on σ and T:. ε s = K 1 σ n. ε s = K 2 σ n -Q c RT K 1, K 2 and n = materials constant Q c = activation energy for creep With increasing stress or temperature: The instantaneous strain increases The steady-state creep rate increases The time to rupture decreases Factors reducing creep rate/failure High-melting point of material Increased Young s modulus Coarse-grained structure (reduces grain boundary sliding) (Opposite effect to strength!!) Materials resilient to creep (high temperature alloys) Stainless steels Steels containing Cr and/or Ni. Refractory metals High melting point elements, like Nb, Mo, W, Ta. Superalloys Co, Ni based alloys: solid solution hardening and secondary phases. Directional solidification producing highly elongated grains or single crystals. Rashid, DMME, BUET MME 291, Lec 14: Fatigue and creep of metals P 16 8

9 Failure Type Description Characteristic Property Generalized Dislocation motion at: σ y, TS Yielding σ σ y Fracture Crack growth to rupture at: K c σ < TS (ductile) σ < σ y (brittle) Fatigue Cyclic crack growth at: S N f Curve σ < σ fracture K max < K c Creep High temperature (T > 0.4 T m ) Q c, n deformation by diffusion at: σ < σ y Rashid, DMME, BUET MME 291, Lec 14: Fatigue and creep of metals P 17 Next Class Surface Treatments of Steels 9