mechanical properties part 1

7/27/2019 Mechanical Properties Part 1

1/14

Mechanical Properties of Materials

This section will deal with the mechanical properties of materials i.e. howmaterials react under loading

Compression Tension Torsion Shearing Bending Pressure

Material PropertiesWhen choosing a material for a specific design, you will be given a list ofproperties. This section will familiarise you with these properties. Take forexample ABS (Acrylonitrile-butadiene-styrene).

-

-

-

--

-

3e22

90

1900

1900230

0.34

ohm.cm2.3e21ResistivityMPa.m 1/24.3-1.2Fracture toughness

Electrical Properties MPa22-11Endurance Limit

oC62Max Service Temp.%1e2-1.25Elongation

oC88Glass TemperatureMPa55-28Tensile Strength

J/kg.K1400Specific HeatMPa51-19Elastic Limit1/

oC85Thermal expansionHV15-5.6Hardness - Vickers

W/m.K0.19Thermal conductivityGPa2.9-1.1Youngs modulus

Thermal Properties Mechanical Properties

USD/kg2.7-2Price

Kg/m 31.2e3-1e3Density

General Properties


2/14

Stress & Strain

Overview The mechanical performance of a material is often assessed based on how

it reacts to applied loading.

If you are a human, the ability to cope with stress without undue strain iscalled resilience. If you are a material, it is called stiffness.

Stress and strain are quantities often used to assess and compare themechanical behaviour different materials.

Stress

Overview If we were to compare the force required to break two different sized objects

made from the same material, which would require more force to pull apartand break? Obviously the larger object would be more difficult to pull apartand break.

However, if we took the size of the object into consideration we could bettercompare the strengths of difference materials of different sizes. Stress istherefore used to compare the strengths of different materials

Stress is a measurement that can help us to quantify the point at which amaterial begins to fail. The strength of a material is often quantified by theamount of stress it can withstand before breaking.

Force ForceForce Force


3/14

Stress

Measurement Specimens of different size will withstand different loads and amount of

stretching. Stress is used to compare how materials with different cross-sectional area react to loads.

Stress ( ) is measured as a force or load applied perpendicular to aspecimen divided by the cross-section Area.

Stress ( ) is measured in N/m 2 or Pascal Pa

Area

Force=

Force

r

ForceArea of circle r2

Stress

Example A load of 500N is applied to a cylinder with radius, r, of 10mm. A 500N load

is applied to a bar with a rectangular cross section, width 8mm x breath6mm. Which experiences more stress?

Force

r

Force Force

Force

w

B

Area of circle r2

Area A x B


4/14

StressSolution.

Which experiences a greater stress?Cylinder 1.6MPa or Bar 10.4MPaWhy? Because the cylinder had a greater cross-sectional area. The load isdistributed over a grater area thus reducing stress.

Strain

Overview When a material is loaded it will experience stress, which causes strain -

strain is another of saying how something stretches. If we were to apply an equal pulling force to two different rods of the same

material, where one is longer than the other, which would stretch more?

It might not be immediately obvious, but the longer rod will stretch more

under the same load as the shorter rod. Rather than comparing how much different materials stretch, we calculate

strain which takes account of the original length of the specimen or objectthat we intend to load.

Stress has shown us how to quantify how the size (or cross-sectional area)of a component spreads the loads. However, we now want to know howthe stress causes a component to stretch or change in length.

Force ForceForce Force


5/14

StrainMeasurement

The strain, , is calculated as

Units: (mm/mm) Dimensionless, often expressed as a percentage Percentage strain

LenghtOriginalLengthinChange=Strain

Lo

Clamped in place

Force

Lo original length

L change in length o L

LStrain

=

100%

=

o L L

Strain

Strain

Example A cylinder 0.5m long is subjected to a tensile force which causes to become

0.502m long. What is the strain?


6/14

Relationship between stress and strain

The definitions of stress and strain allow one to compare specimens ofdifferent cross sectional areas and of different lengths.

When we apply a load, the component will experience stress and will alsobecome strained (stretched). In material science the relationship betweenstress and strain is of great importance.

To understand how materials behave under applied loading, firstly we mustdiscuss standard test methods used to describe the behaviour of materials.

This is typically done using a tensile testing machine

The Tensile testA test specimen of a specific material with a standardised geometry isclamped into a set of jaws. The specimen is slowly pulled apart whilethe force and amount of stretch is recorded and displayed on a graph.


7/14

Tensile test specimen

The tensile testing machine measures force and displacement.

Only the central portion of the test specimen is considered. The end caps(green) are what is used to clamp the specimen into the testing machine.

The force is converted into stress, , since the cross sectional area, A o, isknown. The amount of stretching is converted into strain, , since theoriginal length, L o, of the specimen is known. L o is referred to as the gaugelength.

Lo

Ao

End caps

The Tensile Test

Video of tensile test:http://www.doitpoms.ac.uk/tlplib/mechanical-testing/results2.php


8/14

Stress Strain Response

Animation found at:http://www.doitpoms.ac.uk/tlplib/mechanical-testing/pageseq.php?seqno=1&total=12&pageno=1&popup=&return=results2

0.15 0.3 0.45 0.6

Strain

Atomic level

Electronic bonds between atoms keep structure stable (left image). If thematerial is loaded above yield stress, those bonds break causing permanentdeformation (right image).

Elastic deformation

(Atomic bonds remain intact)

Permanent or plastic deformation

(Atomic bonds detach)


9/14

Stress-Strain Curve

Ultimate Tensile Strength

Yield Stress Failure

Below is an idealised stress-strain curve for steel. This idealisation willbe used to identify different parts of the curve and what they represent.

Many of the mechanical properties for ABS, shown previously can befound from this curve

Strain,

S

t r e s s ,

Youngs Modulus

Overview The stiffness of a component means how much it deflects under a given

load, however, some materials are stiffer than others. Firstly lets concentrate on the linear portion of the curve. This linear portion

is described as the Youngs Modulus or Modulus of Elasticity or ElasticModulus. This portion of the curve describes the elasticity or stiffness of amaterial. At this stage the material when unloaded will spring back into itsoriginal state (Elastic recovery).

A stiff material will have a high Youngs modulus and will deform onlyslightly under applied stress (diamond).

A flexible material will have a low Youngs modulus and will deform greatlyunder applied stress (rubber).

Linear-Elastic region


10/14

Youngs Modulus

Design Issues The stiffness or rigidity of a material is important if we dont want a product

to deflect too much under loading, such as bicycles or furniture. In thesecases we would require materials with a high Youngs modulus, E.

In other cases we do want a product to deflect considerably, as in the caseof rubber bushings which allow motion between components and alsoreduce vibrations. In this case we would require materials with a lowYoungs modulus, E.

Rubber bushings

Youngs Modulus

Measurement The Youngs modulus, E, it is calculated by finding the slope of the line

E has the same units as stress, N/m 2 or Pa A steeper slope means a higher E and a greater stiffness

Strain

S t r e s s

( N / m 2 )

Slope = Stress/Strain = E

12

12

x x y y

Slope

=

Strain

S t r e s s

( N / m 2 )

Steel

Glass

Wood

Rubber


11/14

Youngs Modulus

Comparison with other materials

Youngs Modulus

Example Using the stress strain curve shown below. Determine the modulus of

elasticity, E

0,0

190MPa

0.001


12/14

Yield StrengthOverview The strength of a material is its resistance to failure or permanent

deformation. The point at which a material stops behaving elastically andbegins to behave plastically (permanently deforms) is known as the yieldstress of yield strength, y. If the specimen is stressed above y then plasticor permanent deformations occur. This deformation is irreversible.

A strong material requires high loads to permanently deform it.

PlasticElastic

S t r e s s

Strain

Yield StrengthMeasurement The problem with measuring the yield strength

is that some stress-strain curves dont alwayshave a definite transition point between linear(elastic) portion and the yield point.

The y is found from the by drawing a lineparallel to the linear elastic portion of the curvefrom the 0.002 strain point. This is called theproof strain, proof

The yield stress or yield strength ( y) ismeasured in N/m 2 or Pa.


13/14

Elastic Recovery

If a material is deformed plastically to a point A and the stress is thenreleased, the material ends up with a permanent strain. However, duringunloading, the curve traces a straight-line path that is parallel to the Youngsmodulus.

The material therefore recovers some of its elasticity, this is known asElastic strain recovery

Strain

S t r e s s

yo

Permanentstrain Elastic strainrecovery

A

u n

l o a

d i n

g

Strain Hardening

If the stress is reapplied, the material again responds elastically up until thepoint A. This point is now our new yield point ( y1), which is higher than theoriginal yield point ( yo).

The material now requires more stress to reach the new yield point thusmaking it stronger.

Strain hardening is also known as work hardening as the strength of thematerial is increased by force rather than heat treatment (discussed later).

y1

yo

l o a

d i n

g

u n

l o a

d i n

g

A

Strain


14/14

Ultimate Tensile Strength

The UTS is the highest point of the stress strain curve. After this point,strain hardening is ineffective as the stress reduces with increasing strain.Also, the material is approaching the point of failure.

For structural applications, the yield stress is usually a more importantproperty than the tensile strength, since once the yield stress has passed,the structure has deformed beyond acceptable limits.

UTS

Failure

mechanical properties part 1

Documents