classifying load

8/10/2019 classifying load

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Normal Load (Axial load) : Load is perpendicular to the

supporting material.

- Tension Load: As the ends of material are pulled apart

to make the material longer, the load is called a tension

load.

- Compression Load: As the ends of material are pushed into make the material smaller, the load is called

a compression load.

Tension

Compression

Classifying Load


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Shear Load : Tangential load

Classifying Load (cont)

pulling apart

Pressure

Cargo


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Stress-Strain Diagram

A plot of Strain vs. Stress.

The diagram gives us the behavior of the material and

material properties.

Each material produces a different stress-strain

diagram.


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Strain ( ) (e/Lo)

41

2

3

5

Elastic

Region

Plastic

Region

Strain

Hardening Fracture

ultimatetensile

strength

Elastic region

slope=Youngs(elastic) modulus

yield strength

Plastic regionultimate tensile strength

strain hardening

fracture

necking

yieldstrength

UTS

y

12

y

E


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Stress-Strain Diagram (cont)

Elastic Region(Point 12)

- The material will return to its original shape

after the material is unloaded( like a rubber band).

- The stress is linearly proportional to the strain in

this region.

E

: Stress

E : Elastic modulus(Youngs Modulus)

: Strain (in/in)

E or


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Plastic Region(Point 23)

- If the material is loaded beyond the yield strength,

the material will not return to its original shape

after unloading.- It will have some permanent deformation.

- If the material is unloaded at Point 3, the curve will

proceed from Point 3 to Point 4. The slope will bethe as the slope between Point 1 and 2.

- The distance between Point 1 and 4 indicates the

amount of permanent deformation.



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Strain Hardening

- If the material is loaded again from Point 4, the

curve will follow back to Point 3 with the same

Elastic M odulus(slope).- The material now has a higher yield strength of

Point 4.

- Raising the yield strength by permanently strainingthe material is called Strain Hardening.



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Tensile Strength(Point 3)- The largest value of stress on the diagram is called

Tensi le Strength(TS)orUltimate Tensile Strength

(UTS)

- It is the maximum stress which the material can

support without breaking.

Fracture(Point 5)

- If the material is stretched beyond Point 3, the stressdecreases as necking and non-uniform deformation

occur.

- Fracture will finally occur at Point 5.



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A36 Steel

Stress and Strain Diagram


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Material Properties

Strength

Hardness

Ductility

Brittleness

Toughness

Characteristics of Material are described as


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1) Strength

- Measure of the material property to resist deformation

and to maintain its shape

- It is quantified in terms of yield stress or ultimate tensile

strength.

- High carbon steels and metal alloys have higher strength

than pure metals.

- Ceramicalso exhibit high strength characteristics.


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2) Hardness

- Measure of the material property to resist indentation,

abrasion and wear.

- It is quantified by hardness scale such as Rockwell and

Brinell hardness scale.

- Hardness and Strength correlate well because both

properties are related to in-molecular bonding.


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3) Ductility- Measure of the material property to deform before failure.

- It is quantified by reading the value of strain at the

fracture point on the stress strain curve.

- Example of ductile material :

low carbon steel

aluminum

bubble gum


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4) Brittleness

- Measure of the materials inabilityto deform before failure.

- The opposite of ductility.

- Example of brittle material : glass, high carbon steel,

ceramics

Ductile

Brittle

Strain


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5) Toughness-Measure of the material ability to absorb energy.

-It is measured by two methods.

a) Integration of stress strain curve

- Slow absorption of energy

-Absorbed energy per unit volume

b) Charpy test

- Impact toughnesscan be measured.


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- Charpy V-Notch Test


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Charpy V-Notch Test (continued)

- The potential energy of the pendulum before and after

impact can be calculated form the initial and final location

of the pendulum.

- The potential energy difference is the energy it took to

break the material. absorbed during the impact.

- Charpy test is an impact toughnessmeasurement test

because the energy is absorbed by the specimen very

rapidly.

- Purpose : to evaluate the impact toughness as a function of

temperature


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Temperature (F)Charp

yToughness(lbin)

Brittle

Behavior

Ductile

Behavior

Transition

Temperature


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- At low temperature, where the material is brittle and

not strong, little energy is required to fracture the material.

-At high temperature, where the material is more ductileand stronger, greater energy is required to fracture the

material

-The transition temperatureis the boundary between brittle

and ductile behavior.

The transition temperature is an extremely important

parameter in selection of construction material.


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High Carbon Steel

Charpy Test

Stainless Steel


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6) Fatigue

The repeated application of stress typically produced by

an oscillating load such as vibration.

Sources of ship vibration are engine,propeller andwaves.

Cycles N at Fatigue Failure

Steel

Aluminum

Endurance Limit: A certain threshold

stress which will not cause the fatigue

failure for the number of cycles.

Aluminum has no endurance limit


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Factors effecting Material Properties

Temperature :

Increasing temperature will decrease

- Modulus of Elasticity

- Yield Strength

- Tensile Strength

Decreasing temperature will:

- decrease ductility

- increase brittlenessEnvironment

- Sulfites, Chlorine, Oxygen in water, Radiation


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classifying load

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