material properties engineering science 10

7/28/2019 Material Properties Engineering Science 10

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Mechanical Behavior

&

Material Properties

Institute of Civil EngineeringCollege of Engineering

University of the Philippines-Diliman

Engineering Science 10Strength of Materials: Why Things Bend and Break?

How are the values and plots determined?

Stress-Strain


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An extensometer isused to measure theelongation of thespecimen.

Stress-Strain

Measuring stress and strain.

Measuring stress and strain.

Stress-Strain

Universal Testing Machine (UTM)


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Stress-strain diagram for steel (ductile):

Stress-Strain

Stress-Strain

2 TYPES of stress-strain diagrams:

1. Conventional stress-strain diagram usesthe original dimensions of a material.

2. True stress-strain diagram uses the actualdimensions of a material at the instant theload is applied


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Four ways in which steel behaves:

Stress-Strain

1. Elastic behavior:

- Upon removal of load, material returns toits original shape.

- Stress is proportional to strain.

- The upper stress limit to this linearrelationship is called the proportionallimit.

- If the stress slightly exceeds theproportional limit, the material may stillrespond elastically until it reaches theelastic limit.


Stress-Strain

2. Yielding:

- Upon a slight increase from elastic limit,material will deform permanently (alsoknown as plastic deformation).

- the stress that causes yielding is called

the yield stressor yield point.It means: Give way

- the specimen continues to deform withoutany increase in load.


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Stress-Strain

3. Strain Hardening:

- After yielding, load is increased until itreaches the maximum stress referred toas the ultimate stress or ultimate strength.

- This phenomenon is called strainhardening or work hardening.


Stress-Strain

4. Necking:

- At the ultimate stress, the cross-sectionalarea begins to decrease in a localizedregion of the specimen.

- Reduction in the area decreases the load-carrying capacity. The specimen breaks at

thefracture stress.

necking fracture


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Properties from thestress-strain

diagram

Concept of stress

The concept of stress and strainwas formalized by Thomas Young(1779 1829)

Young published the definition of themodulus of elasticity in 1807.


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1.) The Modulus of Elasticity (or Youngs Modulus)

Properties

)(Youngs

Modulus

=E

=E

E is the slope ofthe plot in theelastic region

Normal Normal

Youngs Moduli of some materials:

Properties

Youngsmodulus (N/m2)

Engineering materialsSteel 200 GPa

Concrete 20 GPa

Rubber 7 MPa

Biological materials

Bone 17 GPaCartilage 190 MPa

Tendon 13 MPa


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2.) The Modulus of Rigidity or Shear Modulus

Properties

ShearShear

)(Shear Modulus =G

= G

G is the slope ofthe plot in theelastic region

3.) Modulus of resilience (ur) :

Properties

- Amount of energy a material can takebefore experiencing permanentdeformation.

- It is the area under the stress-straindiagram where stress is proportional tostrain.


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4.) Modulus of toughness (ut):

Properties

- Amount of energy a material can takebefore it fractures/ breaks.

- Represents the entire area under thestress-strain diagram.

5.) Poissons Ratio ():

Properties

The ratio betweenthe lateral strain andlongitudinal strain

long

lat

=


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MaterialClassification

Materials can be grouped into:

Materials

1. Ductile

2. Brittle

- Materials that can be subjected to largestrains before it ruptures.

- Exhibit large deformations before failing.

e.g. steel, brass

- Materials that exhibit little or no yielding

before failure.

e.g. concrete, chalk

Note: Materials can be classified as ductile orbrittle thru experiments (tensile test).


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Ductile vs Brittle materials:

Materials

Failures


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Types:

Failure

1. Elastic failure excessive elastic deformation.

2. Slip failure excessive plastic deformationdue to slip (plastic deformationthat is independent of the time)

3. Creep failure excessive plasticdeformation over a long period

of time under constant stress.4. Fracture failure complete separation of the

material.

Failure - as the state or condition in which amember or structure no longer functionas intended.

Ductile failure is usually specified by theinitiation ofyielding.

Brittle failure is specified byfracture.

Failures


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Slip

Gliding of one plane of atoms to another.

Creep

For many materials, if you apply a stress todeform the material, and that stress ismaintained, the deformation increases overtime rather than hold constant.

Natural fibers and soil creep extensively (whyclothes get baggy and house foundationssettle).


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Fracture

Ductile fracture

Brittle fracture

initialnecking

cavityformation

cavitycoalescence

crackpropagation

(in shear)

Fracture

Ductile fracture:


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Fracture

Brittle fracture:

- Brittle fracture takes place with little priordeformation.

- Surface tend to beflatter and perpendicular tothe stress (as experiments show).

Factor of safety


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Factor of safety

Factor of Safety only a fraction of the strength of the material isused. The remaining strength is kept reserved forsafe performance.

Sometimes called as thefactor of ignorance.

Factor of safety (FS)

Factor of safety considerations:

1.) Uncertainty in material properties.

2.) Uncertainty of loadings.

3.) Uncertainty of analyses.

4.) Importance of member to structures integrity.

5.) Types of failure.

6.) Risk to life and property.

*Note: FS does not take into accountunscrupulous practices of contractors andengineers.


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Factor of safety

Designing for strength:

Factor of safety is:

..SF

ult

ws

=

Where wsis the working stress while

ultis the ultimate stress.

Factor of safety mustalways be greater than 1.

Factor of safety

F.S. Application

1.25-1.5 Matl and operating conditions known in detail. Loads known withhigh certainty. Material testing provided. Low weight is important.

(e.g. Aircrafts)

1.5-2.0 Known materials with certification under reasonably constantenvironmental conditions, loads and stresses that can be

determined using qualified design procedures. (e.g. Steel)

2.0-3.0 For less tried materials or for brittle materials under average

conditions of environment, load and stress. (e.g. Concrete)

3.0-5.0 For untried materials used under average conditions ofenvironment, load and stress. Also, for very unpredictable material

behavior (e.g. Soil)


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Strength of a metal

Strength of a metal

Two primary forms of increasing metal strength:

1. Alloying mixture of one metal to anothermetal or a non-metal.

2. Crystal state alterations of the crystalstructure of the metal increasestrength.

Adding the 24kt Gold to the molten Fine Silver


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Strength of a metal

Metal alloys:

- Adding certain elements in trace amounts to ametal to significantly change its strength.

- Since the alloying elements are present only intrace amounts, they dont significantly alterthe modulus (stiffness) or density.

Strength of a metal

Steel alloy of iron (97.9-99.8%) and carbon(0.2-2.1%) by weight.

Carbon in steel acts ashardening agent,preventing dislocations inthe iron atom crystallattice from sliding pastone another.

Steel with increasedcarbon content can bemade harder and strongerthan iron, but is also lessductile.


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Strength of a metal

Altering crystal state:

- Crystal state of steel can be altered by heattreatmentor cold working.

Strength of a metal

Quenching:

- Heat to a very hightemperature (~1400oF) and cool rathersuddenly in water.

- extremely strong butbrittle.


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Strength of a metal

Tempering:

- Reheat to moderatetemperature and coolslowly.

- Adds ductility at theexpense of decreasedstrength.

Annealing:

- Resets the alloy to low

strength, ductile state.

- Reheat alloy abovecritical temperature andallow to cool slowly.

material properties engineering science 10

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