Engineering Materials

Engineering MaterialsHaseeb Ullah Khan JatoiDepartment of Chemical EngineeringUET LahoreRole of Stress & Strain in Engineering MaterialsMaterial TestingNon-Destructive Testing (NDT)Using Physical Material PropertiesRadiography (Radiations); Dye Penetrant (Color); Ultrasonic (Ultrasounds); Magnetic Particle for cracks (Magnetic)

Destructive Testing (DT)Utilizing mechanical propertiesPush, Pull, Indent, Twist etc involvedA 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

StressA force distributed over an area

Stress = = F/A

Measured in N/m2 or Lb/inch2Strength is the materials ability to accommodate stressStress measured at fracture becomes designated strength of material (depending on load application mode used)Room temperature tensile strength of 1133 steel is 55 to 57 MPaShear strength of 1133 steel is 22 to 23 MPaCompressive strength of 1133 steel is 59 to 61 MPaTensile testing

Specimens are pulled, bent, twisted, compressed and sheared until they breakMaterial BehaviorStrain is the change in length occurring in material with applied stressIn the beginning, material reverts back to original shape when stress is lower (Elasticity)As test proceeds, stress increases, and length within gage region becomes longerStress-strain curve is linear hitherto, and slope is called Elastic ModulusMaterial exhibiting linear stress-strain curve in the elastic range are Hookean (after Robert Hooke)Modulus of Elasticity = E = stress/ strainOver a range of stresses this curve begins to deviate from linearityThis transition from linearity occurs at a point called proportional limitMaterial may exhibit non-linear elastic behavior above proportional limit (Non-Hookean)Further stress applied takes material toward plastic deformationReleasing the stress at this point makes the material to be elongated from original length, called Plastic deformationThis 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 measureFurther stress decreases the cross sectional area as length elongatesMaterial continues to harden and gets stronger, at the same time reducing cross sectional area, reducing the load-carrying capacityForce curve reaches a peak, called ultimate tensile strengthAt this point, reduction in cross sectional area occurs in a pronounced localized spot, called NeckingUltimately, sample fractures into two halves

Tensile test sampleTwo measurements made:

Final length of gage area is measuredFinal diameter of the necked-down portion of sample is measuredDuctilityA measure of materials ability to be stretched or drawnIt is typically reported as percent elongation or percent reduction in area

Percent Elongation =More this % elongation, more the ductility Similarly, % reduction in area =

Stress-strain curvesEngineering Stress-strain curve:The stress values in engineering stress-strain curves are calculated by dividing the force measured during tensile test by original cross sectional area of specimen. Similarly, strain is also calculated for original lengthTrue Stress-strain curve:The stress is calculated by dividing the force measured during the tensile test by actual or instantaneous cross-sectional area of the specimen. Similarly, strain is calculated with instantaneous gage length

true = KntrueWork hardening or strain hardeningWhen material is plastically deformed, interactions with dislocations in materials structure can cause the material to become stronger and harder. This phenomenon is Work hardening, or Strain hardening true = Kntrue (True curve on graph)true = true stresstrue = true strainK = strength coefficientn = strain hardening exponentStrain hardening exponent nThe strain hardening exponent n is a parameter that defines a materials tendency to work harden when plastically deformedA material with high n will become very strong when plastically deformed, whereas the strength of material with a low n does not increase significantly with plastic deformation

Parameters obtained from stress-strain testingModulus of elasticityYield strengthUltimate tensile strengthUltimate strength/yield strength (work hardening)Percent elongationPercent reduction in areaGeneral shape of curve to evaluate propertiesResilienceResilience is the property that defines a materials ability to absorb elastic energyArea under the elastic portion of stress-strain curve provides an indication of materials resilience

ToughnessThe ability of the material to absorb energy before fracturingTotal area under the stress-strain curve up to the point of fracture is toughness of the material

Shear propertiesShear stress measured in tensile tester using special gripsShear yield strength 57.7% of tensile yield strength (Mises-Henskey distortion energy theory for ductile material failure) s = Gss = shear stress, s = shear strain, G = shear modulus

The shear modulus is an important property for calculating the stiffness or rigidityNext LectureApplying stress-strain relation in chemical engineering design (Examples)Iron and steel