# Mechanical properties of metal 2.pptx

1. Jun 2023
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### Mechanical properties of metal 2.pptx

• 1. MECHANICAL PROPERTIES OF METALS (Ⅱ) Dr. Aneela Wakeel 1
• 2. Resilience, Ur • Ability of a material to store energy • Energy stored best in elastic region   y d Ur    0 2 If we assume a linear stress- strain curve this simplifies to Adapted from Fig. 6.15, Callister & Rethwisch 8e. y y r 2 1 U   @ Resilience is the capacity of a material to absorb energy when it is deformed elastically and then, upon unloading, to have this energy recovered. The associated property is the modulus of resilience, which is the strain energy per unit volume required to stress a material from an unloaded state up to the point of yielding.
• 3. Resilience 3
• 4. Problem A steel alloy to be used for a spring application must have a modulus of resilience of at least 2.07 MPa (300 psi). What must be its minimum yield strength? 4
• 5. 5 Toughness • Energy to break a unit volume of material • Approximate by the area under the stress-strain curve. Brittle fracture: elastic energy Ductile fracture: elastic + plastic energy Adapted from Fig. 6.13, Callister & Rethwisch 8e. very small toughness (unreinforced polymers) Engineering tensile strain,  Engineering tensile stress,  small toughness (ceramics) large toughness (metals)
• 6. Toughness, Ut Engineering Strain, e = DL/Lo) Engineering Stress, S=P/Ao Ut  Sde o ef   (Sy  Su ) 2 EL% 100       Su Sy 6
• 7. Toughness & Resilience • Toughness: A measure of the ability of a material to absorb energy without fracture. (J/m3 or N.mm/mm3= MPa) • Resilience: A measure of the ability of a material to absorb energy without plastic or permanent deformation. (J/m3 or N.mm/mm3= MPa) • Note: Both are determined as energy/unit volume 7
• 8. True stress and strain 8 From Figure, the decline in the stress necessary to continue deformation past the maximum, point M, seems to indicate that the metal is becoming weaker. This is not at all the case; as a matter of fact, it is increasing in strength. However, the cross-sectional area is decreasing rapidly within the neck region, where deformation is occurring. This results in a reduction in the load-bearing capacity of the specimen. The stress is taken on the basis of the original cross sectional area before any deformation, and does not take into account this reduction in area at the neck.
• 9. True Stress & Strain • True stress • True stress • True strain 9 i T A F     o i T   ln                1 ln 1 T T Adapted from Fig. 6.16, Callister & Rethwisch 8e.
• 10. Problem 10
• 11. Hardening 11 • Curve fit to the stress-strain response:  T  K  T  n “true” stress (F/A) “true” strain: ln(L/Lo) hardening exponent: n = 0.15 (some steels) to n = 0.5 (some coppers) • An increase in y due to plastic deformation.   large hardening small hardening y 0 y 1
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• 13. Problems 13 Ans: 23.7mm(P.6.38), 66,400Psi (P.6.39), 343MPa (P.6.40)
• 14. Fracture 14
• 15. Fracture modes 15
• 16. Fractographic studies 16 Photograph showing V-shaped “chevron” markings characteristic of brittle fracture. Arrows indicate origin of crack.
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• 18. Fatigue failure Fatigue failure is brittle like in nature even in normally ductile metals, in that there is very little, if any, gross plastic deformation associated with failure. The process occurs by the initiation and propagation of cracks, and ordinarily the fracture surface is perpendicular to the direction of an applied tensile stress. 18
• 19. Cyclic stresses The applied stress may be axial (tension-compression), flexural (bending), or torsional (twisting) in nature. In general, three different fluctuating stress–time modes are possible. 19
• 20. Range of stress Characteristics of fluctuating stress cycle 20 Mean stress Stress amplitude Stress Ratio
• 21. Problem 21 A fatigue test was conducted in which the mean stress was 50 MPa and the stress amplitude was 225 MPa. •Compute the maximum and minimum stress levels. (275MPa, -175MPa) •Compute the stress ratio. (-0.64) •Compute the magnitude of the stress range. (450MPa)
• 22. The S-N curve 22 Schematic diagram of fatigue-testing apparatus for making rotating bending tests.
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• 25. Fatigue failure in Ductile materials (Al) 25
• 26. Fatigue failure in brittle materials 26
• 27. Methods to improve Fatigue life 27
• 28. Problem 28
• 29. Creep 29 Materials are often placed in service at elevated temperatures and exposed to static mechanical stresses (e.g., turbine rotors in jet engines and steam generators that experience centrifugal stresses, and high-pressure steam lines). Deformation under such circumstances is termed creep. Defined as the time-dependent and permanent deformation of materials when subjected to a constant load or stress, creep is normally an undesirable phenomenon and is often the limiting factor in the lifetime of a part. It is observed in all materials types; for metals it becomes important only for temperatures greater than about 0.4𝑇𝑚 ( absolute melting temperature).
• 30. Definition Creep is the tendency of a solid material to slowly move or deform permanently under the influence of stresses. It occurs as a result of long term exposure to high level of stress that are below the yield strength of the materials Creep is more severe in materials that are subjected to heat for long periods, near melting point. Creep always increase with temperature. 30
• 31. Factors affecting Creep • The rate of this deformation is the function of 1. The material properties (melting point, young modulus, grain size) 2. Exposure time 3. Exposure temperature 4. the applied structural load 31 Creep deformation is important not only in systems where high temperature are endured such as nuclear power plants, jet engines and heat exchanger, but also in the design of many everyday objects. For example, metal paper clips are stronger than plastic ones because plastics creep at room temperature.
• 32. Creep 32
• 33. Creep curve • Stage 1: Transient or primary creep – Creep rate decreases with time due to strain hardening. • Stage 2: Secondary steady –state creep- Costant rate or plot becomes linear. Balance between competing strain hardening and recovery(softening) of the material. • Stage 3: Tertiary Creep- Creep rate increases with time leading to necking and fracture. Accelerated rate leading to creep rupture or failure 33
• 34. Creep- Temperature dependence 34
• 35. Creep failure 35
• 36. To Increase Creep Rupture Resistance 36
• 37. Problem 37
• 38. Factor of safety • The factor of safety is the structural capacity of a system which determines the load-carrying capacity beyond its actual load. In other words, how strong the system is then what is required is called Factor Of Safety (FOS) 38 Factor of safety is determined by the following formula Factor of safety = Actual Load / Working Load Design factor is basically how much load a part is REQUIRED to withstand, and the safety factor is the amount of load a part could Be able to withstand. So, the design factor is the minimum requirement and safety factor is the limit beyond which the part will fail. At the minimum, the safety factor can be equal to the design factor.
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