Date post: | 21-Jan-2017 |
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FATIGUE FAILURE
únavový lom, mech. únava
University of Žilina KMI Matej Janega Slovakia
Presentation structure1. Fatigue basics2. Requirements for Fatigue Failure of Metals3. Stress Cycles4. Fatigue Testing 5. Construction of S-N curve6. Stages of Fatigue Process 7. Analysis of Fatigue Failure8. Prevention of Fatigue Failure
1. Fatigue basics- Repeated cyclic loadings - ..Weakness/tiredness
- 2 types of loadings: 1. )low-cycle loading
N < 105
- high loads, el. + also some plastic strain
2. )high-cycle loading N > 105 - lower stresses- totally elastic strain
FL- fatigue limit (endless number of cycles) 105 106 Cycles to
failure N
STRESS↑
107
FL
2. Requirements for Fatigue Failure of Metals3 necessary factors :
1. Sufficient amount of tensile stresses,
2. enough variations in the applied stress (cycling),
3. Sufficient large number of cycles.
3. Stress cycleCycle types:
1. Completely Reversed Stress cycle, (amplitude is symmetrical about mean zero)
2. Repeated Stress Cycle,(stresses are not equal)
3. Irregular or random stress cycle.(real conditions, wings etc.)
Finding fatigue properties- laboratory simulation tests.
-duplicate as nearly as possible the service stress conditions (stress level, time frequency..)
Rotating- bending test machine - compression and tensile stresses are charged on the specimen as it is bent and rotated at the same time.
Presenting in S – N curve
4. Fatigue testing
- Requires 8 – 12 specimens,
- first testing on high level of stress (2/3 of Rm),
- applying lower levels of stress until specimen reaches and define Fatigue Limit.
5. Construction of S-N curve
Three stages of Fatigue propagation :
I stage – crack initiation, cyclic hardening (annealed steelRm/Re> 1,4), cyclic softening (hardened steel, martensite, work hardened Rm/Re< 1,2),
II stage - crack nucleation and growth of crack on small material volume (surface),
III stage – crack propagation and final failure on overload.
6. Stages of Fatigue ProcessI. CRACK INITIATION (growth 0,25nm/cycle)
Three stages of Fatigue propagation :
I stage – crack initiation, cyclic hardening (annealed steelRm/Re> 1,4), cyclic softening (hardened steel, martensite, work hardened Rm/Re< 1,2),
II stage - crack nucleation and growth of crack on small material volume (surface),
III stage – crack propagation and final failure on overload.
6.1 Stages of Fatigue ProcessII. CRACK NUCLEATION (stable growth) Fatigue
striations
∆l/cycle
Three stages of Fatigue propagation :
I stage – crack initiation, cyclic hardening (annealed steelRm/Re> 1,4), cyclic softening (hardened steel, martensite, work hardened Rm/Re< 1,2),
II stage - crack nucleation and growth of crack on small material volume (surface),
III stage – crack propagation and final failure on overload.
6.2 Stages of Fatigue ProcessIII. CRACK PROPAGATION (unstable growth)
III. Unstable stage
overload
INITIATION SITE
- Differences of Low and high nominal stress,
- differences of stress types,
- differences of fracture surface.
7. Analysis of Fatigue failure
Typical fatigue failure caused by fatigue forces
LOW σ
HIGH σ
- Difference of Low and high nominal stress,
- differences of stress types,
- differences of fracture surface.
7.1 Analysis of Fatigue failure
Stress type dependence
Torsional fatigue
Rotating bending fatigue
- Differences of Low and high nominal stress,
- differences of stress types,
- differences of fracture surface.
7.2 Analysis of Fatigue failure
Ductile and brittle material differences
SMOOTH
NOTCHED
8. Prevention of Fatigue Failure
High velocity
1. SURFACE TREATMENTS-Surface rolling –(compressive stress applying between the rollers and material)-Polishing – (reducing surface scratches)- Shot peening- (putting surface into compression)-Carburizing - (surface treatment)
2. REMOVE STRESS CONCENTRATORS-Design- (removing of sharp edges)-TIG dressing – (weld treatment)
Crankshaft polishing
High velocity shooting against the surface
Surface rolling
8. Prevention of Fatigue Failure1. SURFACE TREATMENTS-Surface rolling –(compressive stress applying between the rollers and material)-Polishing – (reducing surface scratches)- Shot peening- (putting surface into compression)-Carburizing - (surface treatment)
2. REMOVE STRESS CONCENTRATORS-Design- (removing of sharp edges)-TIG dressing – (weld treatment)
TIG – Dressing :Removing of weld toe intrusions
THANKS FOR YOUR ATTENTION