materials ii properties and mechanics

Materials II Properties and Mechanics

Module 1Properties and Tests

Why Do We Look at Tests

The importance of understanding the test is at the heart of understanding the materials and what they are capable of performing

Data sheets describe the material properties but do not describe the test methods.

Tests must be regulated to ensure accuracy in the testing of the materials.

ASTM Standard

Mechanical properties are the most important properties looked at when considering a given material because virtually all service conditions and the majority of the end-use applications involve some form of mechanical loading

These values are almost always listed on material data sheets, it is important to remember that the recoded values are typically at room temperature and do not represent the different effect of temperature and other environmental changes

Also important to remember that the product is typically subjected to more than one type of deformation at once

Mechanical Properties

Describe how the material acts with applications of force or load

There are three different types of force


Tension Compression Shear

Comprised of three parts Stress, the force over the original cross

sectional area Strain, the deformation in a percentage of the

change in length compared to the original length

Modulus, is the toughness of the material and is the ratio stress/strain

When reviewing stress/strain curves the more area under the curve the tougher the material


Stress-Strain Curve for a ductile plastic


Stress – force applied to a given area to produce deformation

Strain – change in length per unit of the original length

Elongation – the increase in length produced by a tensile load

Yield point – the first point on the stress-strain curve where an increase in strain occurs with out an increase in stress

Yield strength – the stress at the yield point


Proportional limit – the greatest stress a material is capable of without any deviation from proportionality of stress to strain

Modulus of elasticity – the ratio of stress to strain under the proportional limit of the curve

Ultimate strength – the maximum unit of stress a material will withstand when subjected to an applied load in compression, tension, or shear

Secant modulus – ratio of the total stress to corresponding strain a specific point on the curve


Stress-strain curves are used to classify the general properties of a material

Soft and weak Hard and brittle Soft and tough Hard and strong Hard and tough


Tensile Strength Single most important indication of strength in

a material The force necessary to pull apart a specimen

along with how much the material stretches before breaking


Tensometer and Specimen


Flexural Strength Stress-strain in flexure is also important to

designers of plastic parts It is the ability of a material to withstand bending

forces applied perpendicular to the part Stresses are a combination of compression and

tension The result of a flexural test is the maximum stress

and strain that occur on the outer surface that is in tension


Flexural Strength Most polymers do not break therefore many times

the test is maximum stress when the strain for the outer surface is 5%

This test is good because it represents beams or similar structures


Shear Strength Force needed to produce a fracture by a

shearing action, an example is scissors Force over the cross sectional area being

sheared. Expressed in force per area (psi) based on the area of the sheared edge

Test specimen and apparatus The specimen is a round or square washer

approximately two inches in diameter or square with a 7/16 diameter thru hole in the center


Impact strength Indicates the amount of energy required to

break a given material Impact strength is directly related to the ability

of a material to absorb and distribute energy. Impact strength is directly related to the

chemical structure of the polymeric material.


Impact strength Two main types of impact testing

The falling mass test consists of dropping a ball shaped mass onto the test sample

Pendulum test consists of dropping a pendulum into a test specimen

Charpy Izod Notched Izod


Falling Mass Impact strength Impact must be on a flat surface Indicates a good for direct indicator Does not take into consideration the design or

built in stresses. Does not take into account velocity increases.


Notched Izod Impact strength Utilizes a vertically placed specimen in a direct

path to the pendulum A 90 degree is notched into the speciman

where the point of impact is going to occur The 90 degree notch will induce failure,

important for design consideration because of material notch sensitivity.


Fatigue and Flexing Fatigue life is defined as the number of cycles of

deformation required to bring a part to failure under a given set of conditions

Materials strength is greatly reduce by cyclic loading

Since many materials see this kind of force this type of test is a popular one

Failures occur from repeated applications of stress in different directions

Values are normally given in numbers of cycles to failure at a given stress level


Hardness

Resistance to deformation particularly permanent, indentation or scratching

Is a relative term, no units but a scale “relative hardness”

Two main testsRockwell for relative harder materialsDurometer for relatively softer materials


Rockwell hardness For relative harder materials such as nylons,

acetals, polycarbonates, and acrylics M scale very hard R scale hard C scale is used for metals


Durometer hardness For relatively softer materials

Shore scale D for harder Shore scale A for soft rubbery types

Based on the penetration of a specific indenter under certain conditions

Indenter is spring loaded and protrudes from a base Sharper indenter used for harder materials Larger flat on point used for softer materials


Abrasion resistance Abrasion is related to force, load, and area of

contact The hardness of material also has a big affect Abrasion resistance is the ability to withstand

mechanical action such as rubbing, scraping, or erosion

The test is complicated by the fact that as the material is abraded friction will cause the material to heat up which gives it different characteristics


Abrasion resistance Abrasion resistance is typically measured

by a weight loss when a material is abraded with a given abrader


Specific gravity or density Mass per unit volume lbs/in3 or kg/m3 Relative density, ratio of mass of a given

volume of material over the mass of an equal volume of water with a density of 1

Physical Properties

Tensile Creep The mechanical tests that we have noted to this

point measure the strength of plastic in a short period of time

Short time tests are irrelevant due to the fact that most plastics are in continuous use over a long period of time

Creep measures the deformation of a material over a period of time

Physical Properties

Glass transition temperature The temperature when the material looses its

rigidity and becomes pliable, all materials have a glass transition temperature, it is the window in which they exist is vastly different. (crystalline vs. amorphous)

Melt temperature The temperature at which the material

becomes liquid loosing 90% of its’ viscosity.

Physical Properties

Thermal properties are how the temperature affects different mechanical, electrical, optical, and other properties

Different things effect how the temperature effects the properties

Molecular orientation Orientation decreases thermal stability

Molecular weight Low temperature flexibility and brittleness

Thermal Properties

Thermal conductivity The rate at which the material will transmit

heat Given by a k factor, aluminum has a k factor of

122, it transfers heat very well, some plastic foams have a k factor of 0.01

This is important in insulation materials Specific heat or heat capacity

The amount of heat required to raise the temperature of one unit of mass by one degree Celsius

Thermal Properties

Heat Deflection Temperature The highest continuous temperature that a

material can withstand without deforming Softening Point

This test is done by placing a needle against a sample of material, the temperature is increased 50 C per hour and when the needle penetrates the temperature is recorded

Thermal Expansion A coefficient used to determine expansion in

length, area, or volume

Thermal Properties

Thermal Properties

Mold Shrinkage The amount by which a molded part is

smaller than the cavity space where it was produced

Typically given in in/in, mm/mm, or %

Thermal Properties

Brittleness Temperature At low temperatures the material

approaches it’s glass transition temperature it becomes hard and brittle

The temperature at which a material exhibits a brittle failure in an impact test.

Environmental refers to the area that the plastics products are used in

The environment can have drastic effects on the properties and appearance of different materials

The major environmental properties are Solar radiations

Caused from different type of solar energy Ultraviolet radiation can cause fracture of the

molecular chains which promotes thermal oxidative degradation

This degradation results in embrittlement, discoloration and loss of mechanical properties

Environmental Properties

UV stabilizers are used to combat these effects

Absorbers are both organic and inorganic pigments that absorb the harmful radiation and dissipate it, a common one is carbon black

Stabilizers inhibit the rupture of the chains by chemical means, basically dissipating the energy to lower less damaging levels


Microorganisms, bacteria, fungus, and mold Polymers by themselves are typically not

effected by microorganisms but the lower molecular weight additives such as plasticizers, lubricants and stabilizers are

As these additives migrate to the surface of the part they can come under attack

Degradation can also show up as loss of aesthetics, mechanical properties, and increase of embrittlement


Weathering Test sample are exposed to heat, sunlight,

and humidity Samples are rated on color change, gloss

level, and loss of physical properties Two main types environmental weathering and

accelerated weathering


Ultraviolet resistance (ASTM D-2565, G-23) Going with weatherability it’s the resistance or

the effects of sunlight Stress cracking

Stress cracking may be caused by solvents, radiation, or strain


Moisture content All plastic materials either collect moisture or

absorb it (hygroscopic) from the atmosphere A moisture analyzer is a piece of equipment

used for this test Materials require drying prior to processing

otherwise you will have poor properties and difficulty processing


Specular gloss Directs light at different angles (20, 45, 60)

and compares the results to the reflection of a mirror

Luminous transmittance Measure the clarity of the plastic

Optical Properties

Color Color perception requires three things

Light source Object Observer

Color is created by the selective reflection and absorption of specific light waves

When light strikes an object the light waves that are reflected is the color that we see

Example an object absorbs all colors accept blue so we see the object as blue

Optical Properties

Color Color defined by three terms

Value Referred to as neutral colors, ranging from white to

black Also called lightness

Hue The attribute of color perception Red – blue – green – yellow

Chroma Also referred to as saturation How far the color is from the neutral axis

Optical Properties

Value

Optical Properties

Hue

Optical Properties

Chroma

Optical Properties

Color Note that color is affected by the light

source or the illuminant CIE has standard illuminates

Daylight Noon light Florescent light

Ways to measure color Tristimulus system or L, a, b,

L is where it lies on the neutral axis, 100 = perfect white and 1 = black

a is green verses red b is blue verses yellow

Optical Properties

Color Tristimulus system or L, a, b,

Optical Properties

L*=0 black

+ a* = red

- a* = green

- b* = blue

+ b* = yellow

L*=100 white

Color Note that all different colors produce different

series of light waves Instrumented color measurement done with a

spectrophotometer Uses a specific light source Gives a spectral read out

Optical Properties

redobject

=re

flect

ance

%wavelength (nm.)

Color Also must do visual color evaluation due to

different light sources Color appears different depending on light

source Referred to as Metamerism Use a light booth for this evaluation

Optical Properties

daylight incandescent

Flammability

Need to understand how polymers burn When exposed to a flame or when it starts to

burn the material decomposes or the molecular chains start to come apart

This produces volatile polymer fragment or short polymer chains on the surface of the part

These short chains are fuel which goes to the flame front

At the flame front it mixes with the oxygen in the atmosphere and produces more heat and more fire

Flammability

Flammability of a material is reduced by breaking the cycle

Additives to disrupt the flame generation Additives to promote the retention of the fuel Additive that act as a heat sink like hydrated

alumina

Flammability

Materials flammability is based on the following criteria

Ease of ignition Flame spread – spreads across a surface Fire endurance - penetrates Rate of heat release – how much and quickly Ease of extinction Smoke evolution Toxic gas generation

Analytical

Rheology As stated earlier melt index measure at a

given temperature and a specific flow rate Rheology is the study of flow and viscosity

is the resistance to flow due to friction between layers

The more friction between layer the greater the resistance to flow

The more force is needed to “move” the material, this force is referred to as shear

Analytical

Rheology Shearing occurs when the fluid is poured or

mixed The rate of speed that the layers move is called

the velocity gradient and is called the shear rate

Shear stress is the stress caused from the layers moving

When the shear rate increases at the same rate as the shear stress the fluid is considered a Newtonian fluid

Water is Newtonian Plastic materials are non-Newtonian

Analytical

Rheology Rheology takes into account this shear

stress and measures flow of a material at different shear rates (variations in injection pressure)

Different types of rheometers Torque rheometers Rotational rheometers Capillary rheometers

Arc resistance Measure of time for the plastic material to arc

or to short Resistivity

The resistance of two conductors with an insulator of the given material between them

Dissipation factor Measures the power lost in the in the plastic

insulator

Electrical Properties

Dielectric strength The electrical voltage required to break down

or arc through a test sample of plastic Dielectric constant

Measure the ability of the material to store electricity

Electrical Properties

Data Sheets Contain the property values for the specific

materials along with other pertinent information concerning the material Melt flow rate Specific gravity Melt temperature Drying conditions

Typically received from the material supplier but can also be found on the internet and other sources

materials ii properties and mechanics

Documents

material properties

important properties

stress force

stressstrain curves

ratio of stress

specimenmechanical properties

curvemechanical properties

general properties