CREEP FAILURE lom pri tečení

University of Žilina KMI Matej Janega Slovakia

matej.janega@gmail.com

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