engineering materials lec 05

7/29/2019 Engineering Materials Lec 05

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Haseeb Ullah Khan JatoiDepartment of Chemical Engineering

UET Lahore


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Non-Destructive Testing (NDT) Using Physical Material Properties

Radiography (Radiations); Dye Penetrant (Color);

Ultrasonic (Ultrasounds); Magnetic Particle forcracks (Magnetic)

Destructive Testing (DT) Utilizing mechanical properties Push, Pull, Indent, Twist etc involved


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A material with the highest electrical

conductivity in the world is of little utility if

its mechanical properties are not adequate

to allow it to be formed into wire, bent

round a switch lug, and held with a screw


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Specimens are pulled, bent, twisted, compressed and sheared until

they break


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Strain is the change in length occurring inmaterial with applied stress

In the beginning, material reverts back to

original shape when stress is lower(Elasticity)

As test proceeds, stress increases, andlength within gage region becomes longer

Stress-strain curve is linear hitherto, andslope is called Elastic Modulus


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Material exhibiting linear stress-strain curve inthe elastic range are Hookean (after RobertHooke)

Modulus of Elasticity = E =stress/ strain

Over a range of stresses this curve begins todeviate from linearity

This transition from linearity occurs at a pointcalled proportional limit

Material may exhibit non-linear elastic behaviorabove proportional limit (Non-Hookean)


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Further stress applied takes material toward

plastic deformation

Releasing the stress at this point makes the

material to be elongated from original length,called Plastic deformation

This point of transition from elastic to plastic

is termed Elastic limit, or Yield pointMeasured at an offset strain of 0.2% as this

point is difficult to measure


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Further stress decreases the cross sectionalarea as length elongates

Material continues to harden and getsstronger, at the same time reducing crosssectional area, reducing the load-carryingcapacity

Force curve reaches a peak, called ultimatetensile strength

At this point, reduction in cross sectional areaoccurs in a pronounced localized spot, calledNecking

Ultimately, sample fractures into two halves


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Two measurements made:

Final length of gage area is measured

Final diameter of the necked-down portion ofsample is measured


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A measure of materials ability to bestretched or drawn

It is typically reported as percent

elongation or percent reduction in area

Percent Elongation =More this % elongation, more the ductilitySimilarly,

% reduction in area =

gthInitialLen

gthInitialLenhFinalLengt

aInitialAre

FinalAreaaInitialAre


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Engineering Stress-strain curve: The stress values in engineering stress-strain curves

are calculated by dividing the force measured duringtensile test by original cross sectional area of

specimen. Similarly, strain is also calculated fororiginal length

True Stress-strain curve: The stress is calculated by dividing the force

measured during the tensile test by actual orinstantaneous cross-sectional area of the specimen.Similarly, strain is calculated with instantaneous gagelength


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true = Kntrue


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When material is plastically deformed,

interactions with dislocations in materials

structure can cause the material to become

stronger and harder. This phenomenon isWork hardening, or Strain hardening true = K

ntrue (True curve on graph)

true

= true stress

true = true strain

K = strength coefficient

n = strain hardening exponent


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Modulus of elasticity

Yield strength

Ultimate tensile strength

Ultimate strength/yield strength (work

hardening)

Percent elongation

Percent reduction in areaGeneral shape of curve to evaluate

properties


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Resilience is the property that defines a

materials ability to absorb elastic energy

Area under the elastic portion of stress-

strain curve provides an indication of

materials resilience


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The ability of the material to absorb energy

before fracturing

Total area under the stress-strain curve up

to the point of fracture is toughness of the

material


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Shear stress measured in tensile tester usingspecial grips

Shear yield strength 57.7% of tensile yield

strength (Mises-Henskey distortion energy theory for ductile materialfailure)s = Gs

s = shear stress,s = shear strain, G = shear modulus

The shear modulus is an importantproperty for calculating the stiffness orrigidity


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Applying stress-strain relation in chemical

engineering design (Examples)

Iron and steel

engineering materials lec 05

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