mechanical prop

21
MECHANICAL PROPERTIES OF MATERIALS Elastic limit Yield Strength Ultimate Tensile Strength Stress strain curve for brittle material Flexural strength Ductility Malleability Resilience Hardness Toughness Stiffness Strength

Upload: mannu057

Post on 20-Nov-2015

256 views

Category:

Documents


2 download

DESCRIPTION

1234

TRANSCRIPT

NTL Lemnis

MECHANICAL PROPERTIES OF MATERIALSElastic limitYield StrengthUltimate Tensile StrengthStress strain curve for brittle materialFlexural strengthDuctilityMalleabilityResilienceHardnessToughnessStiffnessStrength

STRESS STRAIN CURVE FOR DUCTILE MATERIALDUCTILE MATERIAL

ELASTIC LIMIT

The elastic limit is the highest stress at which all deformation strains are fully recoverable. For most materials and applications this can be considered the practical limit to the maximum stress a component can withstand and still function as designed. Beyond the elastic limit permanent strains are likely to deform the material to the point where its function is impaired.Elastic limitis maximum stress or force per unit area within a solid material that can arise before the onset of permanent deformation. When stresses up to the elastic limit are removed, the material resumes its original size and shape. Stresses beyond the elastic limit cause a material to yield or flow. For such materials the elastic limit marks the end ofelastic behavior and the beginning of plastic behavior For most brittle materials, stresses beyond the elastic limit result in fracture with almost no plastic deformation.

YIELD STRENGTHAyield strengthoryield pointof a material is defined inengineeringandmaterials scienceas thestressat which a material begins todeform plastically.Prior to the yield point the material will deformelastically and will return to its original shape when the applied stress is removed.Once the yield point is passed, some fraction of the deformation will be permanent and non-reversible.Somewhere between the proportional limit and the ultimate strength of the material is the yield strength.

ULTIMATE TENSILE STRENGTHUltimate tensile strength(UTS), is often shortened totensile strength(TS) orultimate strength.It is the maximumstressthat a material can withstand while being stretched or pulled before failing or breaking.Some materials will break sharply, withoutplastic deformation, in what is called abrittle failure.Others, which are moreductile, including most metals, will experience some plastic deformation and possiblyneckingbefore fracture.The UTS is usually found by performing atensile testand recording theengineering stressversusstrain.The highest point of thestress-strain curve(see point E on the engineering stress/strain diagrams below) is the UTS.It is an intensive property; therefore its value does not depend on the size of the test specimen.

FRACTURE STRENGTHFracture strength, also known asbreaking strength, is the stress at which a specimen fails via fracture.This is usually determined for a given specimen by atensile test, which charts thestress-strain curve(see image). The final recorded point is the fracture strength.Ductilematerials have a fracture strength lower than theultimate tensile strength(UTS), whereas inbrittlematerials the fracture strength is equivalent to the UTS.If a ductile material reaches its ultimate tensile strength in a load-controlled situation, it will continue to deform, with no additional load application, until itruptures. If the stress-strain curve is plotted in terms oftrue stressandtrue strain,the curve will always slope upwards and never reverse, as true stress is corrected for the decrease in cross-sectional area. The true stress on the material at the time of rupture is known as the breaking strength. This is the maximum stress on the true stress-strain curve, given on curve shown below.

STRESS - STRAIN CURVE FOR BRITTLE MATERIALAmaterialisbrittleif, when subjected to stress, it breaks without significant deformation.In brittle material, there is no specific yield point, so we can not predict the actual behaviour of material. For these types of materials, yield strength is taken by drawing a line (parallel to the slope at origin )at 0.2% offset on the strain axis.

FLEXURAL STRENGTH / MODULUS OF RUPTUREIt is a mechanical property for brittle material.It is defined as the ability of a material to resist deformation under bending load.If an object is subjected to flexural stress, it will undergo both tension and compression behaviour (compression in upper section & tension in lower section ).Flexural strength of a material will depend on either its tensile strength or compression strength, whichever is lesser.

DUCTILITYDuctility is the property of a material due to which it can be drawn into wires under tensile stress.Gold and silver are ranked as the most ductile metals. Other examples are copper , platinum and tungsten.Ductility is directly proportional to the toughness. Higher the ductility , higher the toughness.

DUCTILE TO BRITTLE TRANSITION TEMPERATURE (DBTT)

Many materials experience a shift from ductile to brittle behaviour if the temperature is lowered below a certain point. It is commonly known as the ductile-to-brittle-transition temperature . or It is defined as the temperature at which the material absorbs 15 ft*lb of impact energy during fracture. The temperature at which this shift occurs varies from material to material.Metals such as aluminum, gold, silver, and copper have an FCC (face-centred cubic) crystal lattice structure, and do not experience a sharp shift from ductile to brittle behaviour. Other metals, such as iron, many steels, chromium, and tungsten, have a BCC (body-centred cubic) crystal structure and experience a sharp shift in ductility.MALLEABILITYMalleability is the property of a material, usually metals , due to which it can be converted into thin sheets under compressive stress. Example gold, copper , aluminum etc.Malleable metals usually bend and twist in various shapes.Differences in malleability amongst metals are due to variances in their crystal structures.Compression stress forces atoms to roll over each other into new positions without breaking their metallic bond. When a large amount of stress is put on a malleable metal, the atoms roll over each other, permanently staying in their new position.

RESILIENCEResilienceis the ability of a material to absorb energy when it isdeformedelastically, and release that energy upon unloading.Resilience (Ur) is measured in a unit ofjoulepercubic metre(Jm3) in theSIsystem.It can be calculated byintegrating the stress-strain curvefrom zero to the elastic limit. In uniaxial tension,

Proof resilienceis defined as the maximum energy that can be absorbed within the elastic limit, without creating a permanent distortion.Themodulus of resilienceis defined as the maximum energy that can be absorbed per unit volume without creating a permanent distortion. Modulus of Resilience = (Yield strength)2/ 2 Modulus of elasticity.

HARDNESSHardness is the property of a material that enables it to resist plastic deformation, penetration , indentation and scratching.Hardness is important from an engineering standpoint because resistance to wear by either friction or erosion by steam, oil, and water generally increases with hardness.Hardnessis a measure of how resistantsolidmatteris to various kinds of permanent shape change when a compressiveforceis applied.Hardness is dependent onductility,elasticstiffness,plasticity,strain,strength,toughness, visco-elasticity, andviscosity.There are three main types of hardness measurements:Scratch hardness: Scratch hardness is the measure of how resistant a sample is tofractureor permanentplastic deformationdue to friction from a sharp object.Indentation hardness: Indentation hardness measures the resistance of a sample to material deformation due to a constant compression load from a sharp object.Rebound hardness: Rebound hardness, also known asdynamic hardness, measures the height of the "bounce" of a diamond-tipped hammer dropped from a fixed height onto a material. This type of hardness is related toelasticity.

TOUGHNESSToughness describes a material's resistance to fracture. It is often expressed in terms of the amount of energy a material can absorb before fracture.

Tough materials can absorb a considerable amount of energy before fracture while brittle materials absorb very little.

Either strong materials such as glass or very ductile materials such as taffy can absorb large amounts of energy before failure.

Toughness is not a single property but rather a combination of strength and ductility.

Materials with high yield strength and high ductility have high toughness.The toughness of a material can be related to the total area under its stress-strain curve.

The most common test for toughness is the Charpy impact test.

In many materials the toughness is temperature dependent.

Generally materials are more brittle at lower temperatures and more ductile at higher temperatures.

The temperature at which the transition takes place is known as the DBTT, or ductile to brittle transition temperature.

STIFFNESSStiffnessis the rigidity of an object the extent to which it resistsdeformationin response to an appliedforce. The complementary concept isflexibility: the more flexible an object is, the less stiff it is.

The stiffness,k,of a bodyis a measure of the resistance offered by an elastic body to deformation.

For an elastic body with a singledegree of freedom(for example, stretching or compression of a rod), the stiffness is defined as : K=F/L Fis the force applied on the body. is thedisplacementproduced by the force along the same degree of freedom.

STRENGTHA measure of the maximum load that can be placed on a material before it permanently deforms or breaks. Engineers often use this as yield stress, y, as a measure of a material's strength.

The ability to withstand the stress of physical forces. Cable and wire systems, for example, must be designed in consideration of the amount of twisting and bending they can tolerate and the amount of weight or longitudinal stress a cable or wire can support (tensile strength) without suffering deformation or breaking.

STIFFNESS V/S STRENGTHStiffness and strength are two of the most commonly confused terms in the bicycling world, and many people use them interchangeably. But these are two different things, and building the best-performing frame requires knowing the difference.

Strength:A measure of the maximum load that can be placed on a material before it permanently deforms or breaks. Engineers often use this as yield stress, y, as a measure of a material's strength.

Stiffness:A measure of the amount of deflection that a load causes in a material. Engineers use a value called Young's modulus, E, for stiffness.

It is easy to see why these two terms would be confused. Something that is 'flimsy' might break when a small load is placed on it it has low strength and it might also deflect a large amount with the same load it has low stiffness.

But these two terms aren't interchangeable.

A piece of rubber surgical tubing has very low stiffness because it deflects a lot under load, but it is relatively strong. A piece of glass filament is the opposite it deflects very little under load but might not carry a huge load before it breaks.

Thank You