Materials Properties

• Definition - normal load, shear load - tension, compression - stress, strain• Stress and Strain Diagram• Material Characteristics - ductility - brittleness - toughness - transition temperature - endurance limit

• 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 in to make the material smaller, the load is called a compression load.

Tension

Compression

1 Classifying Load

• Shear Load : Tangential load

1 Classifying Load (cont)

pulling apart

Pressure

Cargo

2 Stress and Strain

In order to compare materials, we must have measures.

• Stress : load per unit Area

AF

σ

F : load applied in poundsA : cross sectional area in in² : stress in psiσ

A

F F

Stress and Strain (cont)

• Strain : - Ratio of elongation of a material to the original length - unit deformation

oLe

ε

e : elongation (ft)Lo : unloaded(original) length of a material (ft) : strain (ft/ft) or (in/in)ε

Elongation

oLLe

L : loaded length of a material (ft)

Lo e

L

Baldwin Hydraulic Machine for Tension & Compression test

3 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.

Stress-Strain Diagram

Strain ( ) (e/Lo)

41

2

3

5

Str

ess

(F

/A)

Elastic Region

PlasticRegion

StrainHardening Fracture

ultimatetensile strength

Slop

e=E

Elastic region slope=Young’s(elastic) modulus yield strengthPlastic region ultimate tensile strength strain hardening fracture

necking

yieldstrength

UTS

y

εEσ

ε

σE

12

y

ε ε

σE

A36 Steel

Stress and Strain Diagram

Stress-Strain Diagram (cont)

• Elastic Region (Point 1 –2) - 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.

εEσ : Stress(psi)E : Elastic modulus (Young’s Modulus) (psi) : Strain (in/in)

σ

ε

- Point 2 : Yield Strength : a point at which permanent deformation occurs. ( If it is passed, the material will no longer return to its original length.)

ε

σE or

• Plastic Region (Point 2 –3)

- 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 be

the as the slope between Point 1 and 2.

- The distance between Point 1 and 4 indicates the

amount of permanent deformation.

Stress-Strain Diagram (cont)

• Strain Hardening

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

curve will follow back to Point 3 with the same

Elastic Modulus(slope).

- The material now has a higher yield strength of

Point 4.

- Raising the yield strength by permanently straining

the material is called Strain Hardening.

Stress-Strain Diagram (cont)

• Tensile Strength (Point 3) - The largest value of stress on the diagram is called Tensile Strength(TS) or Ultimate 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 stress decreases as necking and non-uniform deformation occur. - Fracture will finally occur at Point 5.

Stress-Strain Diagram (cont)

Example 1.

Mooring line length =100 ft diameter=1.0 in Axial loading applied=25,000 lb Elongation due to loading=1.0 in

222

2

785.0)(0.5 r A

800,31 785.0

000,25

inin

psiin

lb

A

F

mooring line

loading

1) Find the normal stress.

2) Strain?

)/( 00083.0

112

100

1inin

ftin

ft

in

L

e

o

Example 2. - Salvage crane is lifting an object of 20,000 lb. - Characteristics of the cable diameter=1.0 in, length prior to lifting =50 ft

) 785.0) (0.5 r (A

478,25 785.0

000,20

222

2

inin

psiin

lb

A

F

1) Normal stress in the cable?

2) Strain?

)/( 000728.0 1035

478,256

ininpsi

psi

E

psi 1035

psi 000,70

psi 000,60

6

UT

E

y

3) Determine the cable stretch in inches.

inft

inftininLe

L

e

o

o

44.0)1

1250()/ 000728.0(

5.4 Material Properties

• Strength

• Hardness

• Ductility

• Brittleness

• Toughness

Characteristics of Material are described as

5.4 Material Properties

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.

- Ceramic also exhibit high strength characteristics.

5.4 Material Properties

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.

5.4 Material Properties

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

5.4 Material Properties

4) Brittleness - Measure of the material’s inability to deform before failure.

- The opposite of ductility.

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

ceramics

Ductile

Brittle

Str

ess

Strain

5.4 Material Properties

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

unit : (lb/in²) *(in/in) =lb·in/in³

b) Charpy test

- Impact toughness can be measured.

5.4 Material Properties

- Charpy V-Notch Test

5.4 Material Properties

• 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 toughness measurement test

because the energy is absorbed by the specimen very

rapidly.

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

temperature

• Charpy V-Notch Test

(continued)

Temperature (°F)

Cha

rpy

Tou

ghne

ss(l

b·in

)

BrittleBehavior

Ductile Behavior

Transition Temperature

5.4 Material Properties

• Charpy V-Notch Test

(continued)- 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 ductile

and stronger, greater energy is required to fracture the

material-The transition temperature is the boundary between brittle

and ductile behavior.

The transition temperature is an extremely important

parameter in selection of construction material.

5.4 Material Properties

High Carbon Steel

Charpy Test

Stainless Steel

6) Fatigue

5.4 Material Properties

• The repeated application of stress typically produced by an oscillating load such as vibration.• Sources of ship vibration are engine, propeller and waves.

Cycles N at Fatigue Failure

Str

ess

(psi

) 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

Evaluation of fatigue curve

A

- Endurance limit of each material :- Case 1) stress level= 30x103 psi, max cycles=104 :- Case 2) stress level= 30x103 psi, max cycles=106 :- Case 3) stress level= 30x103 psi, max cycles=106 :- Case 4) stress level= 50x103 psi, max cycles=106 :

Number of cycles

0

20

40

60

80

Str

ess

(x10

³) p

si

B

C 103 104 105 106 107

Factors effecting Material Properties

5.4 Material Properties

• Temperature : Increasing temperature will decrease - Modulus of Elasticity - Yield Strength - Tensile Strength Decreasing temperature will: - Increase ductility - Reduce brittleness• Environment - Sulfites, Chlorine, Oxygen in water, Radiation

5.5 Non-Destructive Testing (NDT)

• NDT : Inspections for material defects

• External Inspection Technique

- Visual Test (VT)

- Dye Penetrant Test (PT)

- Magnetic Particle Test (MT)

• Internal Inspection Technique

- Radiographic Test (RT)

- Ultrasonic Test (UT)

- Eddy Current test

- Hydrostatic Test

- Can be used to examine only the surface of a material.

- Should be done during the all phases of maintenance (QAI).

- Can be performed quickly and easily and at no virtually cost.

- Often performed under some magnification to locate defects.

- Sometimes photographs are needed for a permanent record.

Visual Testing (VT)

- Can be used for location and identification of

only surface defects : cracks, seams, laps, laminations

or porosity

- Uses dyes to make surface flaws visible to naked eye.

- Can be used as a field inspection for glass, metal, castings,

forgings and welds.

- Simple and inexpensive

Dye Penetrant Test (PT)

Dye Penetrant Test (PT) (contd.)

Magnetic Particle Test (MT)

- Method that can be used to find surface and near surface flaws

in ferromagnetic materials such as steel and iron.

- The technique uses the principle that magnetic fields (flux) will

be distorted by the presence of a flaw.

Radiographic Test (RT)

- The x-ray (gamma) rays are used. - The rays pass through the material and exposes film. - RT requires trained technicians. - RT may have large effect on ship access and watchstanding.

The picture shows the integrity of welding for the 2.5mm thick steel plate

(Ultrasonic Test UT)

• UT uses high frequency sound waves to detect flaws,

measure material thickness, or level in a tank or vessel.

• Can be used on all metals and nonmetals.

• Excellent technique for detecting deep flaws in tubing, rods,

adhesive-joined joints.

• It is used on aircraft to detect cracks in structure

• Ultrasonic Test (UT)

Eddy Current Test

- Involves the creation of a magnetic field in a specimen and

reading the field variations on an oscilloscope.

- Can only be used on conductive materials and is only good for

limited penetration depth.

- Used for measurement of wall thickness, cracks of tubes, wire,

or ball bearings.

• Eddy Current Test

Elliptical Crack

Hydrostatic Tests

• System being tested is isolated and pressurized by a pump.

• System is inspected for leaks at welds, valve bodies, valve seats, etc.

• Automatic and manual pressure reliefs are used to prevent overpressurizing system beyond desired test pressure.

Hydrostatic Test Pump