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• It is important for engineers to understand how the various mechanical properties are measured and what these properties represent.
• The engineers may be called upon to design structure/components using pre-determined materials such that unacceptable levels of deformation and failure will not occur.
INTRODUCTION(page 209)
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• It is necessary to know the characteristics of the material and to design the member from which it is made such that any resulting deformation will not be excessive and fracture will not occur.
• The mechanical behavior of a material reflects its response or deformation in relation to an applied load of force.
of bond with original atom neighbors then re-forming with new neighbors.
• Most structures are designed to ensure that only elastic deformation occur when stress is applied.
• Importance to know the stress at which plastic deformation begin (yielding) which may be determined as the initial departure from linearity (proportional limit) of stress-strain curve.
Yielding and Yield Strength (σy):
PLASTIC DEFORMATION(page 220-230)
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Yielding and Yield Strength (σy):
PLASTIC DEFORMATION(page 220-230)
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Example Problem 8.3:
PLASTIC DEFORMATION(page 220-230)
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• Ductility measures the degree of plastic deformation that has been sustained at fracture.
• Brittle very little of no plastic deformation at fracture
(Maximum 5% of fracture stain).
• Ductility as % elongation:
Ductility:
• Ductility as % Reduction in Area:
• Knowledge of ductility is important due to:
• It indicates the degree to which a structure will deform plastically before fracture,
• It specifies the degree of allowable deformation during fabrication.
PLASTIC DEFORMATION(page 220-230)
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• Resilience (J/m3) capacity of a material to absorb energy
when it is deformed elastically and then, upon unloading to have this energy recovered.
Resilience:
• Modulus of Resilience (U) strain energy per unit volume required to
stress a material from unloading state upto the point of yielding.
Area under stress-strain curve taken toyield point.
PLASTIC DEFORMATION(page 220-230)
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• Toughness (J/m3) ability of a material to absorb energy and
plastically deform before fracturing.
• Measure of toughness is ascertained by the area under stress-strain curve up to the fracture point.
Toughness:
PLASTIC DEFORMATION(page 220-230)
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• True stress:
True Stress and Strain:
• True strain:
• Conversion of engineering stress to True stress:
• Conversion of engineering strain to true strain:
• True stress-strain relationship in the plastic region to the point of necking:
PLASTIC DEFORMATION(page 220-230)
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True Stress and Strain:
PLASTIC DEFORMATION(page 220-230)
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Elastic Recovery after Plastic Deformation:
HARDNESS(page 230-236)
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• Hardness measure of a material’s resistance to localized plastic deformation.
• It is performed more frequently than any other mechanical test due to:
Simple and inexpensive
Non-destructive
Other mechanical properties values may be estimated from hardness data:
HARDNESS(page 230-236)
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DESIGN/SAFETY FACTOR(page 239-240)
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• Design stress (σd) calculated stress (σc) multiplied by
design factor N’:
• Safe stress or working stress(σw) yield strength (σy)