Engineering Materials

1. Engineering Materials HaseebUllah Khan Jatoi Department of Chemical Engineering UET Lahore

2. Role of Stress & Strain in Engineering Materials

3. Material Testing • Non-Destructive Testing (NDT) • Using Physical Material Properties • Radiography (Radiations); Dye Penetrant (Color); Ultrasonic (Ultrasounds); Magnetic Particle for cracks (Magnetic) • Destructive Testing (DT) • Utilizing mechanical properties • Push, Pull, Indent, Twist etc involved

4. 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

5. Stress • A force distributed over an area • Stress = σ = F/A • Measured in N/m2 or Lb/inch2 • Strength is the material’s ability to accommodate stress • Stress 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 MPa • Shear strength of 1133 steel is 22 to 23 MPa • Compressive strength of 1133 steel is 59 to 61 MPa

6. Tensile testing • Specimens are pulled, bent, twisted, compressed and sheared until they break

7. Material Behavior Strain is the change in length occurring in material with applied stress In the beginning, material reverts back to original shape when stress is lower (Elasticity) As test proceeds, stress increases, and length within gage region becomes longer Stress-strain curve is linear hitherto, and slope is called Elastic Modulus

8. Material exhibiting linear stress-strain curve in the elastic range are Hookean (after Robert Hooke) Modulus of Elasticity = E = Δstress/ Δstrain Over a range of stresses this curve begins to deviate from linearity This transition from linearity occurs at a point called proportional limit Material may exhibit non-linear elastic behavior above proportional limit (Non-Hookean)

9. 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 point Measured at an offset strain of 0.2% as this point is difficult to measure

10. Further stress decreases the cross sectional area as length elongates Material continues to harden and gets stronger, at the same time reducing cross sectional area, reducing the load-carrying capacity Force curve reaches a peak, called ultimate tensile strength At this point, reduction in cross sectional area occurs in a pronounced localized spot, called Necking Ultimately, sample fractures into two halves

15. Tensile test sample • Two measurements made: • Final length of gage area is measured • Final diameter of the necked-down portion of sample is measured

16. Ductility A measure of material’s ability to be stretched or drawn It is typically reported as percent elongation or percent reduction in area Percent Elongation = More this % elongation, more the ductility Similarly, % reduction in area =

17. Stress-strain curves • Engineering 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 length • True 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

18. σtrue = Kεntrue

19. Work hardening or strain hardening • When material is plastically deformed, interactions with dislocations in material’s structure can cause the material to become stronger and harder. This phenomenon is Work hardening, or Strain hardening • σtrue = Kεntrue(True curve on graph) • σtrue = true stress • εtrue = true strain • K = strength coefficient • n = strain hardening exponent

20. Strain hardening exponent “n” The strain hardening exponent ‘n’ is a parameter that defines a material’s tendency to work harden when plastically deformed A 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

22. Parameters obtained from stress-strain testing Modulus of elasticity Yield strength Ultimate tensile strength Ultimate strength/yield strength (work hardening) Percent elongation Percent reduction in area General shape of curve to evaluate properties

23. Resilience Resilience is the property that defines a material’s ability to absorb elastic energy Area under the elastic portion of stress-strain curve provides an indication of material’s resilience

25. Toughness 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

28. Shear properties • Shear stress measured in tensile tester using special grips • Shear yield strength ≈ 57.7% of tensile yield strength (Mises-Henskey distortion energy theory for ductile material failure) σs = Gεs • σs = shear stress,εs = shear strain, G = shear modulus • The shear modulus is an important property for calculating the stiffness or rigidity

29. Next Lecture Applying stress-strain relation in chemical engineering design (Examples) Iron and steel