FATIGUE FAILURE

únavový lom, mech. únava

University of Žilina KMI Matej Janega Slovakia

matej.janega@gmail.com

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