This document discusses creep failure, which is the slow, time-dependent deformation of materials under constant stress, especially at high temperatures. It defines key creep terms and concepts, including the classic creep curve with primary, secondary, and tertiary creep stages. The main creep mechanisms are described as bulk diffusion, grain boundary diffusion, and dislocation climb/creep. Creep failure occurs through the nucleation and growth of cracks and cavities. Methods to prevent creep failure include using single crystal materials, solid solution strengthening, high melting temperature alloys, and consulting creep test data for lifetime expectations under loading conditions.

1. CREEP FAILURE
lompritečení
University of Žilina KMI Matej Janega
Slovakia matej.janega@gmail.com

2. Presentation structure
1. Basic terms
2. Creep curve
3. Effect ofTemperature & Stress
4.Creep Mechanisms (Deformation Types)
5. Creep Failure
6. CreepTest
7. Compare of Creep and Stress RuptureTest
8. Prevention of creep failure

3. 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.
Operating temperature
melting point temperature
> 0,3 – 0,5
Ni-superalloy ≈ 0,75

4. 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 .

5. 2.1 RealCreepCurve
- Differences
between individual
creep stages

6. 3. Effect ofTemperature & Stress
- is usually affected by
changing conditions of
loading and
temperature
- creep mechanisms is
often different between
metals, plastics, rubber..

7. Creep–causedofatomic
diffusionin atomiclattice
atelevated strenght and
temperature.
METALS -
Bulk Diffusion(Nabarro-
Herringcreep)
Grain Boundary Diffusion
(Coble creep)
Dislocation climb/creep
POLYMERS-
Thermally activated glide
in polymersandother
viscoelastic materials
4.Creep Mechanisms
Mechanism map
G – shear modulus

8. -Ithappens atlowerstrenght
andhighertemperature,
-intense diffusionof vacancies
againstthemovement of
intense interstitialdiffusion
withthemainatoms,
-Creepratedecreasesasgrain
sizeincreases.
4.1 Bulk Diffusion (Nabarro-Herring)
Changed lenght of the grain size

9. -Diffusiononthegrain
boundariesatlowerstrenght
andlowertemperature,
-intensevacancymigrationon
thegrainboundaries,
-dislocationsclimbsupto
boundariesandtransitionsto
grainboundariesarea,then
moveswiththegrain
boundaries.
4.2Grain Boundary Diffusion (Coble creep)
Changed lenght of the grain size

10. - It happens at
higher strenght and
higher temperature
-strong dependence
on appliedstress
-Slip and climb of
dislocations
4.3 Dislocation climb/creep

11. -Creating ofcavities and
cracks in material, which
areforming intomicro and
macro –cracks,
-fracture characteristics
aretemperature and
deformation speed
dependent ( /t),
Transgranularfracture
creates atlowTandhigh
speed ofdeformation,
Intergranularfracture
creates athighTandlow
speed ofdeformation.
5.Creep Failure

12. -at lowT and high
speed of deformation,
- voids and cavities
near the inclusions,
- stress is
concentrated on
inclusion situated in
matrix of grain.
5.1Transgranular fracture

13. 5.2 Intergranular fracture
-at highT and low
speed of deformation,
a) cavity nucleation
b) cavity connection
c) gap creation
d) micro-crack forming

14. -Intergranular
fracture surface
5.3CREEP FAILURE pictures

15. -Measureofstrain on
time and rupturetime
-relatively low loads and
creeprate ,
-longduration2000 to
10,000 hours,
-notalways fracture,
-Strain typically less than
0.5%,
-average 50reading
shouldbetaken.
6.CreepTest

16. RUPTURE TEST-materialresult
infailureunderaoverload.
-rupturecannotbetoleratedin.
CREEP TEST- Predictsrupture
livesusing
LarsonMillerParameter
LMP=T(C+logtr)
WhereT=temperature(KorºC)
tr=timebeforefailure(hours)
C=materialspecificconstant
7.Comparison ofCreep andStress RuptureTest

17. 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 CreepTest 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).

18. THANKS FOR YOUR ATTENTION
Don´t be a creep, respect the creep