UNIT 2UNIT 2

0utcomes0utcomesIn this unit you will be able to:In this unit you will be able to: Explain the term stress.Explain the term stress. Explain the term strainExplain the term strain.. Present graphically the relationship between Present graphically the relationship between

stress and strain.stress and strain. Use the stress strain diagram to identify:Use the stress strain diagram to identify:

– The limit of proportionality.The limit of proportionality.– The limit of elasticityThe limit of elasticity..– The yield point.The yield point.– The yield strength.The yield strength.– The ultimate strength.The ultimate strength.– The breaking strengthThe breaking strength..

Carry out the tensile test for ductile & brittle Carry out the tensile test for ductile & brittle material.material.

I Wants to UnderstandI Wants to Understand

PlasticityPlasticity The ability of a material to undergo The ability of a material to undergo permanent deformation under load.permanent deformation under load.

VitalVital ImportantImportant

ForgingForging To shape a metal by plastic deformationTo shape a metal by plastic deformation

RollingRolling Process of thinning a material through Process of thinning a material through

passing it between to pressing wheelspassing it between to pressing wheels

FundamentalFundamental BasicBasic

SpecimenSpecimen Piece of material used to study the entire Piece of material used to study the entire properties of this material.properties of this material.

StressStress

StressStress is the internal distribution of is the internal distribution of forceforce per unit per unit areaarea that balances and reacts to external loads that balances and reacts to external loads applied to a body. applied to a body.

Stress σ = F/AStress σ = F/AWhere:Where: F = Applied Load in Newton (N).F = Applied Load in Newton (N).

A = Cross-sectional area in meter square (m²)A = Cross-sectional area in meter square (m²)

-Solids, liquids and gases have stress fields-Solids, liquids and gases have stress fields . .

--Static fluids support normal stress but will flow under Static fluids support normal stress but will flow under shear stress.shear stress.

--Moving viscous fluids can support shear stress. Moving viscous fluids can support shear stress.

-Solids can support both shear and normal stress.-Solids can support both shear and normal stress.

StrainStrain

strainstrain is the geometrical expression of deformation is the geometrical expression of deformation caused by the action of caused by the action of stressstress on a physical body. on a physical body.

Strain is calculated by:Strain is calculated by: 1st, assuming a change between two body 1st, assuming a change between two body

states: states: -the beginning state -the beginning state -and the final state-and the final state 2nd, the difference in placement of two points in 2nd, the difference in placement of two points in

this body in those two states, expresses the this body in those two states, expresses the numerical value of strain. numerical value of strain.

Strain ε=Strain ε=

Where: Where: εε is strain in measured direction. is strain in measured direction.

δl δl is the extension of material.is the extension of material.

l0 l0 is the original length of the material.is the original length of the material.

Strain has no units of measure because in the formula Strain has no units of measure because in the formula the units of length are cancelled.the units of length are cancelled.

0l

l

Stress-Strain RelationshipStress-Strain Relationship

1-Modulus of Elasticity1-Modulus of Elasticity stress is proportional to load and strain is stress is proportional to load and strain is

proportional to deformation, this implies that stress proportional to deformation, this implies that stress is proportional to strain. Hooke’s law is the is proportional to strain. Hooke’s law is the statement of that proportionality.statement of that proportionality.

Stress (σ) / Strain (ε) = EStress (σ) / Strain (ε) = E

If a material obeys the Hooke’s law, it is elastiIf a material obeys the Hooke’s law, it is elasti

2- Limit of proportionality2- Limit of proportionality The limit of proportionality is where the applied load The limit of proportionality is where the applied load

is proportional to the extension.is proportional to the extension.

3- Limit of Elasticity3- Limit of Elasticity

A material is said to be A material is said to be elasticelastic if it deforms under if it deforms under stress, but then returns to its original shape when stress, but then returns to its original shape when the stress is removed.The material’s limit of the stress is removed.The material’s limit of elasticity is at the point where the material deforms elasticity is at the point where the material deforms due to a stress and does not return to its original due to a stress and does not return to its original shape no more.shape no more.

4- Yield strength or the yield point4- Yield strength or the yield point Yield strength, or the yield Yield strength, or the yield

point, is defined as the stress point, is defined as the stress at which a material begins to at which a material begins to plastically deformplastically deform..

Knowledge of the yield point is Knowledge of the yield point is vital when:vital when:

1- designing a component since 1- designing a component since it generally represents an it generally represents an upper limit to the load that upper limit to the load that can be applied.can be applied.

2- It is also important for the 2- It is also important for the control of many materials control of many materials production techniques such production techniques such as forging, rolling, or as forging, rolling, or pressing.pressing.

5-Ultimate strength5-Ultimate strength The maximum strength a material can withstands The maximum strength a material can withstands

when subjected to an applied load.when subjected to an applied load.

6-Breaking strength6-Breaking strength The stress coordinates on the stress-strain curve at The stress coordinates on the stress-strain curve at

the point of rupture.the point of rupture.

7-Graphic representation7-Graphic representationA stress-strain curve is a graph derived from A stress-strain curve is a graph derived from

measuring load (stress - σ) versus extension measuring load (stress - σ) versus extension (strain - ε) for a sample of a material.(strain - ε) for a sample of a material.