Presentation structure1. Basic terms2. Creep curve3. Effect of Temperature & Stress 4.Creep Mechanisms (Deformation Types) 5. Creep Failure6. Creep Test 7. Compare of Creep and Stress Rupture Test8. Prevention of creep failure
1. Basic terms -Slow long time - dependent
deformation under a constant applied load,
-generally at high temperature (thermal creep),
but can also happen at room temperature
in certain materials (glass), albeit much slower.
> 0,3 – 0,5 Ni-superalloy ≈ 0,75
2. Classical creep curve
εo
Elastic strain
Primary - elastic strain and fast extension
Secondary – steady strain,
Tertiary – associated with necking and formation of grain boundary voids .
2.1 Real Creep Curve- Differences between individual creep stages
3. Effect of Temperature & Stress
- is usually affected by changing conditions of loading and temperature
- creep mechanisms is often different between metals, plastics, rubber..
Creep – caused of atomic diffusion in atomic lattice at elevated strenght and temperature.METALS -Bulk Diffusion (Nabarro-Herring creep) Grain Boundary Diffusion (Coble creep) Dislocation climb/creep
POLYMERS-Thermally activated glide in polymers and other viscoelastic materials
4. Creep MechanismsMechanism map
G – shear modulus
-It happens at lower strenght and higher temperature,
-intense diffusion of vacancies against the movement of intense interstitial diffusion with the main atoms,
- Creep rate decreases as grain size increases.
4.1 Bulk Diffusion (Nabarro-Herring)
Changed lenght of the grain size
- Diffusion on the grain boundaries at lower strenght and lower temperature,
-intense vacancy migration on the grain boundaries,
-dislocations climbs up to boundaries and transitions to grain boundaries area, then moves with the grain boundaries.
4.2 Grain Boundary Diffusion (Coble creep)
Changed lenght of the grain size
- It happens at higher strenght and higher temperature
-strong dependence on applied stress
- Slip and climb of dislocations
4.3 Dislocation climb/creep
-Creating of cavities and cracks in material, which are forming into micro and macro – cracks,
-fracture characteristics are temperature and deformation speed dependent ( / t) ,
Transgranular fracture creates at low T and high speed of deformation,
Intergranular fracturecreates at high T and low speed of deformation.
5. Creep Failure
-at low T and high speed of deformation,
- voids and cavities near the inclusions,
- stress is concentrated on inclusion situated in matrix of grain.
5.1 Transgranular fracture
5.2 Intergranular fracture-at high T and low speed of deformation,
a) cavity nucleationb) cavity connectionc) gap creationd) micro-crack forming
-Intergranular fracture surface
5.3 CREEP FAILURE pictures
- Measure of strain on time and rupture time
- relatively low loads and creep rate ,
- long duration 2000 to 10,000 hours,
- not always fracture,
- Strain typically less than 0.5%,
-average 50 reading should be taken.
6. Creep Test
RUPTURE TEST - material result in failure under a overload. - rupture cannot be tolerated in .
CREEP TEST- Predicts rupture lives usingLarson Miller Parameter
LMP= T(C+log tr)
Where T = temperature (K or ºC) tr = time before failure (hours) C= material specific constant
7. Comparison of Creep and Stress Rupture Test
8. Prevention of creep failure
Reduce the effect of grain boundaries: -Use single crystal material with large grains. -Addition of solid solutions to eliminate vacancies. Employ materials of high melting temperatures. Consult Creep Test Data during materials Selection -Set right inspection intervals according to life expectancy.
Using materials which are specially resistant to creep:- Stainless steels,- „Super Alloys“ – Co, Ni based (solid solution hardening and
secondary phases).
THANKS FOR YOUR ATTENTION
Don´t be a creep, respect the creep