Fatigue [6]

1>

Fatigue is the lowering of strength or failure of a material due to repetitive stress, which may be above or below the yield strength. For fatigue to occur at least part of the stress in the material has to be tensile.

Many engineering materials such as those used in cars, planes, turbine engines, machinery, shoes, etc are subjected constantly to repetitive stresses in the form of tension, compression, bending, vibration, thermal expansion and contraction or other stresses.

There are typically 3 stages to fatigue failure.1) a small crack is initiated or nucleates at the surface and can include

scratches, pits, sharp corners due to poor design or manufacture, inclusions, grain boundaries or dislocation concentrations.

2) the crack gradually propagates as the load continues to cycle.3) a sudden fracture of the material occurs when the remaining cross-

section of the material is too small to support the applied load.

Fatigue under cyclic/repeated loading

2>

Cracks generally grow under repeated loading

Trucks passing over bridges, Sailboat ruddersBicycle pedalsShift gears

May result failure or fracture: fatigue fracturePeriodic inspections required for fatigue critical systems

Thermal fatigue: repeated heating and cooling can cause a cyclic stress due to differential thermal expansion and contraction

Fatigue

3>

Repeated, also called cyclic loads resulting in cyclic stresses can lead to microscopic physical damage.

Accumulation of this microscopic damage with continued cycling is possible until it develops into a macroscopic crack such as cracks that may lead to failure

Fatigue: Damage progression to failure due to repeated or cyclic loading at amplitudes considerably lower than lower than tensile or yield strengths of material under a static loadtensile or yield strengths of material under a static load

Estimated to causes 90 % of all failures of metallicstructures (bridges, aircraft, machine components, etc.)

Fatigue failure is brittle-like (relatively little plasticdeformation) - even in normally ductile materials. Thussudden and catastrophic!

Dynamic Loading and Fatigue

4>

Definitions and Concepts

5>

Constant amplitude stressingMean stressStress amplitude (half of the range)variation about the meanStress ratio R, Amplitude ratio Completely reversed stressing, R = -1

Fatigue Tests

6>

Types of stresses for fatigue tests:axial (tension – compression)flexural (rotating/bending)torsional (twisting)

33 ..232..

.32.

dlP

dMS b

bππ

==

Flexural stress Sb *

* for round specimens

S-N Curves

7>

The most important fatigue data for engineering designs are the S-N curves, which is the Stress-Number of Cycles curves.

In a fatigue test, a specimen is subjected to a cyclic stress of a certain form and amplitude and the number of cycles to failure is determined.

The number of cycles, N, to failure is a function of the stress amplitude, S.

A plot of S versus N is called the S-N curve.

S-N Curves

8>

(a) Typical S-N curves for two metals. Note that, unlike steel, aluminum does not have an endurance limit. (b) S-N curves for common polymers

S-N Curves

9>

Fatigue Limit:For some materials such as BCC steels and Ti alloys,

the S-N curves become horizontal when the stress amplitude is decreased to a certain level.

This stress level is called the Fatigue Limit, or Endurance Limit.

Fatigue Strength:For materials, which do not show a fatigue limit such as

Al, Cu, and Mg (non-ferrous alloys), and some steels with a FCC structure, fatigue strength is specified as the stress level at which failure will occur for a specified number of cycles, where 107 cycles is often used.

Fatigue Strength vs Tensile Strength

10>

Fatigue Life

11>

Fatigue strength is applicable to a component with No endurance limit. It is the maximum stress for which fatigue will not occur at a particular number of cycles, in general, 108 cycles for metals.

Endurance ratio: the endurance limit is approximately ¼ to ½ the tensile strength.

5.025.0 −≈=strength tensile

strenght) (fatiguelimit endurance ratio Endurance

Fatigue life: indicates how long (number of cycles) acomponent survives a particular stress.

Factors Affecting Fatigue Life

12>

Magnitude of stress (mean, amplitude...)

Quality of the surface (scratches, sharp transitions andedges).

Solutions:Polishing (removes machining flaws etc.)Introducing compressive stresses (compensate forapplied tensile stresses) into thin surface layer by “ShotPeening”- firing small shot into surface to be treated.High-tech solution - ion implantation, laser peening.Case Hardening - create C- or N- rich outer layer insteels by atomic diffusion from the surface. Makesharder outer layer and also introduces compressivestressesOptimizing geometry - avoid internal corners, notchesetc.

Fatigue failures

13>

The fracture surface near the origin is usually smooth (Beach mark-crack initiation point). The surface becomes rougher as the crack increases in size.

Striations (concentric line patterns): the slow cyclic build up of crack growth from a surface intrusion. Striations are on a much finer scale and show the position of the crack tip after each cycle.

Granular portion of the fracture surface: rapid crack propagation at the time of catastrophic failure.

Fatigue failures

14>

Typical fatigue-fracture surface on metals, showing beach marks. Magnification: left, 500x; right, 1000x. Source: Courtesy of B.J. Schulze and S.L. Meiley and Packer Engineering Associates, Inc.

Fatigue Crack Propagation (FCP) Testing

15>

Cambridge Polymer Group has developed an automated FCP imaging system that images the sample every 500 cycles of cyclic loading, without any user input required. The FCP imaging system is independent of the load frame performing the cyclical loading, and is therefore simple to transfer and set up at a new location. In the video above, every second is equal to 2000 cycles of loading. Each frame is analyzed to measure the crack length, using the black dots to guide the measurement.

http

s://w

ww

.you

tube

.com

/wat

ch?v

=GPY

w8h

Rky

VA

Standards relating to Fatigue Test

16>

American Society for Testing and Materials (ASTM):

• ASTM E466 - Standard Practice for Conducting Force Controlled Constant Amplitude Axial Fatigue Tests of Metallic Materials (2015)

• ASTM E606 - Standard Test Method for Strain-ControlledFatigue Testing (2012)

• ASTM E1823 - Standard Terminology Relating to Fatigue and Fracture Testing (2013)

Fatigue Test Video

17>https://www.youtube.com/watch?v=LhUclxBUV_E

Notas de aula preparadas pelo Prof. Juno Gallego para a disciplina Lab. Materiais de Construção Mecânica II.® 2016. Permitida a impressão e divulgação. http://www.feis.unesp.br/#!/departamentos/engenharia-mecanica/grupos/maprotec/educacional/

References

https://en.wikipedia.org/wiki/Fatigue_(material)Kelly, S. M. Fatigue.

http://sv.rkriz.net/classes/MSE2094_NoteBook/97ClassProj/anal/kelly/fatigue.htmlASTM International. ASTM E466 - Standard Practice for Conducting Force

Controlled Constant Amplitude Axial Fatigue Tests of Metallic Materials. West Conshohocken, 6 p., 2015.

ASTM International. ASTM E606 - Standard Test Method for Strain-ControlledFatigue Testing. West Conshohocken, 16 p., 2012.

ASTM International. ASTM E1823 - Standard Terminology Relating to Fatigueand Fracture Testing. West Conshohocken, 25p., 2013.

Kalpakjian, S.; Schmid, S. R. Manufacturing Engineering and Technology, 6th Edition. Prentice-Hall, 1197p. (2009).

Metals Handbook, ASM. Mechanical Testing and Evaluation, volume 8.ASM, 9th edition, 1981.

18