material properties engineering science 10
Post on 03-Apr-2018
219 Views
Preview:
TRANSCRIPT
-
7/28/2019 Material Properties Engineering Science 10
1/21
7/17/12
1
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
-
7/28/2019 Material Properties Engineering Science 10
2/21
7/17/12
2
An extensometer isused to measure theelongation of thespecimen.
Stress-Strain
Measuring stress and strain.
Measuring stress and strain.
Stress-Strain
Universal Testing Machine (UTM)
-
7/28/2019 Material Properties Engineering Science 10
3/21
7/17/12
3
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
-
7/28/2019 Material Properties Engineering Science 10
4/21
7/17/12
4
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.
Four ways in which steel behaves:
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.
-
7/28/2019 Material Properties Engineering Science 10
5/21
7/17/12
5
Four ways in which steel behaves:
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.
Four ways in which steel behaves:
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
-
7/28/2019 Material Properties Engineering Science 10
6/21
7/17/12
6
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.
-
7/28/2019 Material Properties Engineering Science 10
7/21
7/17/12
7
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
-
7/28/2019 Material Properties Engineering Science 10
8/21
7/17/12
8
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.
-
7/28/2019 Material Properties Engineering Science 10
9/21
7/17/12
9
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
=
-
7/28/2019 Material Properties Engineering Science 10
10/21
7/17/12
10
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).
-
7/28/2019 Material Properties Engineering Science 10
11/21
7/17/12
11
Ductile vs Brittle materials:
Materials
Failures
-
7/28/2019 Material Properties Engineering Science 10
12/21
7/17/12
12
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
-
7/28/2019 Material Properties Engineering Science 10
13/21
7/17/12
13
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).
-
7/28/2019 Material Properties Engineering Science 10
14/21
7/17/12
14
Fracture
Ductile fracture
Brittle fracture
initialnecking
cavityformation
cavitycoalescence
crackpropagation
(in shear)
Fracture
Ductile fracture:
-
7/28/2019 Material Properties Engineering Science 10
15/21
7/17/12
15
Fracture
Brittle fracture:
- Brittle fracture takes place with little priordeformation.
- Surface tend to beflatter and perpendicular tothe stress (as experiments show).
Factor of safety
-
7/28/2019 Material Properties Engineering Science 10
16/21
7/17/12
16
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.
-
7/28/2019 Material Properties Engineering Science 10
17/21
7/17/12
17
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)
-
7/28/2019 Material Properties Engineering Science 10
18/21
7/17/12
18
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
-
7/28/2019 Material Properties Engineering Science 10
19/21
7/17/12
19
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.
-
7/28/2019 Material Properties Engineering Science 10
20/21
7/17/12
20
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.
-
7/28/2019 Material Properties Engineering Science 10
21/21
7/17/12
21
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.
top related