MME 323: MATERIALS SCIENCE WEEK 6 :

MECHANICAL PROPERTIES*

Adhi Primartomo, PhDEmail: primartomo_a@jic.edu.saOffice: Room 191 – JIC Academic Building* Source: Materials Science and Engineering; 9th Edition; W.D.Callister;

Wiley; 2011

https://sites.google.com/site/primartomo/file-cabinet

~ LECTURE OUTLINE ~

Chapter 8: Mechanical Properties (page 208 -

241)*

• Why Study Mechanical Properties?

• Introduction,

• Tension Tests, Shear & Torsional Test,

• Elastic Deformation,

• Elastic Properties of Materials,

• Plastic Deformation,

• Ductility, Resilience, Toughness,

• Elastic Recovery after Plastic Deformation,

• Hardness,

• Design/Safety Factors.2

WHY STUDY MECHANICAL PROPERTIES?(page 209)

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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.

• Key mechanical design properties are: stiffness, yield strength, hardness, ductility and toughness.

CONCEPTS OF STRESS AND STRAIN(page 210-213)

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• Tension/Tensile test one of the most common

mechanical tests, used to ascertain several mechanical properties that are important in design.

• Standard tensile test specimen:

This shape of specimen is chosen because of:

• During the test, deformation is confined to the “narrow center” region,

• Reduce the possibility of fracture at the end of the specimen.

CONCEPTS OF STRESS AND STRAIN(page 210-213)

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• The output of tensile test is recorded as load vs. elongation or stress vs. strain.

• Engineering Stress (σ):

• Engineering Strain (ε):

• Shear Stress (ε):

ELASTIC DEFORMATION(page 214-218)

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• Elastic deformation (non-permanent deformation)

deformation in which stress and strain are proportional.

• Hooke’s law relationship between stress and strain in

elastic deformation:

• Modulus of Elasticity stiffness: material’s resistance to

elastic deformation

• The greater the modulus, the stiffer the material and smaller elastic strain.

• Modulus of Elasticity is an important design parameter for computing elastic deflections.

E = Modulus of Elasticity

ELASTIC DEFORMATION(page 214-218)

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ELASTIC DEFORMATION(page 214-218)

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• Shear stress and shear strain are proportional to each other:

• Poisson’s Ratio (v) ratio of the lateral and linear (axial)

strain:

G = Shear Modulus

• Relation among elastic parameters:

ELASTIC DEFORMATION(page 214-218)

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• Example Problem 8.2:

PLASTIC DEFORMATION(page 220-230)

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• Plastic deformation (permanent deformation) breaking

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)

divided by factor of safety, N:

SUMMARY OF MECHANICAL PROPERTIES(page 237)

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MECHANICAL PROPERTIES(page 241)

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Design Example 8.2: