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In materials science, fatigue is the progressive and localised structural damagethat occurs when a material is subjected to cyclic loading. The maximum stressvalues are less than the ultimate tensile stress limit, and may be below the yieldstress limit of the material.

fatigue of materials

Stress vs. Strain curve typical of aluminum.1 Ultimate Strength2 Yield Strength3 Proportional Limit Stress 4 Rupture5 Offset Strain (usually 0.002)

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Lecture Power Points to accompanyBeer/Johnston/DeWolfMECHANICS OF MATERIALS, 3rd ed.

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Characteristics of fatigue

● The process starts with dislocation movements, eventually forming persistent slip bands that nucleate short cracks.

● The greater the applied stress, the shorter the life.

● Damage is cumulative. Materials do not recover when rested.

● Fatigue is a stochastic process, often showing considerable scatter. Fatigue life scatter tends to increase for longer fatigue lives.

● Fatigue life is influenced by a variety of factors, such as temperature, surface finish, presence of oxidizing or inert chemicals, residual stresses, contact (fretting), etc.

● Some materials (e.g., some steel and titanium alloys) exhibit a theoretical fatigue limit below which continued loading does not lead to failure.

● In recent years, researchers have found that failures occur below the theoretical fatigue limit at very high fatigue lives (109 to 1010 cycles). An ultrasonic resonance technique is used in these experiments with frequencies around 10-20 kHz.

● High cycle fatigue strength (about 103 to 108 cycles) can be described by stress based parameters. A load-controlled, servo-hydraulic test rig is commonly used in these tests, with frequencies of around 20-50 Hz. Other sort of machines like resonant magnetic machines can also be used, achieving frequencies up to 250Hz.

● Low cycle fatigue (typically less than 103 cycles) is associated with widespread plasticity, thus a strain based parameter should be used for fatigue life prediction. Testing is conducted with constant strain amplitudes at 1-5 Hz.

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The process starts with dislocation movements,eventually forming persistent slip bands that nucleate short cracks.

Micrographs showing how surface fatigue cracks grow as material is further cycled.From Ewing & Humfrey (1903)

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The greater the applied stress, the shorter the life.

Fatigue properties are shown on S-N diagrams.Lecture Power Points to accompanyBeer/Johnston/DeWolfMECHANICS OF MATERIALS, 3rd ed.

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Complex loadings

In practice, a mechanical part is exposed to a complex, often random, sequence of loads, large and small.

In order to assess the safe life of such a part:1. Reduce the complex loading to a series of simple cyclic loadings using a technique

such as rainflow analysis; 2. Create an histogram of cyclic stress from the rainflow analysis; 3. For each stress level, calculate the degree of cumulative damage incurred from the S-

N curve; and 4. Combine the individual contributions using an algorithm such as Miner's rule.

Miner's rule is also called the Palmgren-Miner linear damage hypothesis. It is used to calculate cumulative damage due to k different stress magnitudes each contributing ni cycles at stress magnitude Si. Failure is assumed to occur when the summation of the damage fractions for all the events experienced by the structure is equal to or larger than a damage criterion, C . Mathematically this theory is stated as,

Where n is the number of cycles at stress S applied to the specimen and Ni is the life corresponding to stress magnitude Si . The constant C is determined by experiments and is usually 0.7 < C < 2.2. For most analysis, the value of C = 1 is used.

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Creep / FluênciaCreep is the term used to describe the tendency of a 'solid' material to slowly deform permanently to relieve stresses. It occurs as a result of long term exposure to levels of stress that are below the yield strength or ultimate strength of the material.Creep is more severe in materials that are subjected to heat for long periods, and near the melting point.

Typical creep curve for steel

http://www.twi.co.uk/j32k/protected/band_3/jk81.html

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Typical creep curve of a single crystal Ni-base super alloy

http://sakimori.nims.go.jp/MP/creep.html

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Atrito entre materiaise

Desgaste ou erosão de materiais

atrito: friction

desgaste: wear = loss of material from a surface by means of some mechanical action

erosão: erosion = wear due to mechanical interaction between a surface and a fluid, a multicomponent fluid, or impinging liquid or solid particles

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Em física, o atrito é uma força natural que actua apenas quando um objecto estáem contacto com outro e sofre a acção de outra força que tende a colocá-lo em movimento.

A força de atrito é causada pelo contacto dos dois corpos ou meio em que se move o corpo em movimento (neste caso especial, pela viscosidade do meio).

http://pt.wikipedia.org/wiki/Atrito

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The coefficient of friction is a dimensionless quantity used to calculate the force of friction (static or kinetic).

The coefficient of static friction is defined as the ratio of the maximum static friction force (F) between the surfaces in contact to the normal (N) force.

The coefficient of kinetic friction is defined as the ratio of the kinetic frictionforce (F) between the surfaces in contact to the normal force.

F = µ.N

Coefficient of frictionCoeficiente de atrito

http://en.wikipedia.org/wiki/Coefficient_of_friction

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Pin-on-Disk Tribometer

Pin-on-Disk Tribometer operates on the following principal:- a flat, pin or sphere is loaded onto the test sample with a precisely known weight- the highly stiff elastic arm insures a nearly fixed contact point and thus a stable position in the friction track- the friction coefficient is determined during the test by measuring the deflection of the elastic arm of by direct measurement of the change in torque- wear coefficients for the pin and disk material are calculated from the volume of material lost during the testThis simple method facilitates the study of friction and wear behaviour of almost every material combination with or without lubrication.

Users can easily control test parameters such as: speed, frequency, contact pressure and varying time. Real-life conditions of a practical wear situation can be closely simulated by precise control of humidity, temperature and gas composition. Specialized options have been developed for operations at high/low temperature, high load and high rotational speed. Other options include a unique tri-mode lubrication system, a real-time depth of track, an electrical conductivity detector and a motorized radial positon.Multiple test configurations are available to extend the capabilities of the instrument including Linear Reciprocating, Block-on-Ring, Thrust Washer Testing and Four Ball Wear.

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The study of the processes of erosion is part of the discipline of tribology.

The principal erosion processes are:

• Adhesive wear (desgaste adesivo)

• Abrasive wear (desgaste abrasivo)

• Surface fatigue: a process by which the surface of a material is weakened by cyclic loading, which is one type of general material fatigue)

• Fretting (or fretting corrosion): a special wear process that refers to corrosion damage at the asperities of contact surfaces. This damage is induced under load and in the presence of repeated relative surface motion, as induced for example by vibration

• Erosion wear : wear due to mechanical interaction between a surface and a fluid, a multicomponent fluid, or impinging liquid or solid particles

Erosion wear

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Adhesive wear occurs when two solid surfaces slide over one another under pressure.

Surface projections, or asperities, are plastically deformed and eventually welded together by the high local pressure. As sliding continues, these bonds are broken, producing cavities on the surface, projections on the second surface, and frequently tiny, abrasive particles, all of which contribute to future wear of surfaces.

For adhesive wear to occur it is necessary for the surfaces to be in intimate contact with each other. Surfaces which are held apart by lubricating films, oxide films etc. reduce the tendency for adhesion to occur.

http://www.gordonengland.co.uk/wear.htm

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Abrasive wear occurs when material is removed by contact with hard particles.

The particles either may be present at the surface of a second material (two-body wear) or may exist as loose particles between two surfaces (three-body wear).

The abrasive wear mechanism isbasically the same as machining, grinding, polishing or lapping thatwe use for shaping materials. Twobody abrasive wear occurs whenone surface (usually harder thanthe second) cuts material awayfrom the second, although thismechanism very often changes to three body abrasion as the weardebris then acts as an abrasivebetween the two surfaces.

Abrasives can act as in grindingwhere the abrasive is fixed relativeto one surface or as in lappingwhere the abrasive tumblesproducing a series of indentationsas opposed to a scratch.

http://www.gordonengland.co.uk/wear.htm

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Examples ofReducing Wear by using Thermal Spray Coatings

• for Soft Bearing Surfaces, the following coatings are commonly used: - aluminium bronze - phosphor bronze - white metal or babbitt (an alloy of tin, lead, copper, and antimony, used to reduce friction in

bearings, developed by the US inventor Isaac Babbit in 1839)

- aluminium bronze/polymer composites

• for Hard Bearing Surfaces, the following coatings are commonly used: - cermet coatings like tungsten carbide/cobalt and chromium carbide/nickel

chromium- oxide ceramics like chromium oxide and alumina- molybdenum- various hard alloys of iron, nickel, chromium or cobalt