1. Materials II Properties and Mechanics
Module 1
Properties and Tests

2. 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.

3. ASTM Standard

4. ASTM Standard

5. Mechanical Properties
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

6. Mechanical Properties
Describe how the material acts with applications of force or load
There are three different types of force
Tension
Compression
Shear

7. Mechanical Properties
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

8. Mechanical Properties
Stress-Strain Curve for a ductile plastic

9. Mechanical Properties
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

10. Mechanical Properties
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

11. Mechanical Properties
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

12. Mechanical Properties
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

13. Mechanical Properties
Tensometer and Specimen

14. Mechanical Properties
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

15. Mechanical Properties
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

16. Mechanical Properties
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

17. Mechanical Properties
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.

18. Mechanical Properties
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

19. Mechanical Properties
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.

20. Mechanical Properties
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.

21. Mechanical Properties
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

22. Mechanical Properties
Hardness
Resistance to deformation particularly permanent, indentation or scratching
Is a relative term, no units but a scale “relative hardness”
Two main tests
Rockwell for relative harder materials
Durometer for relatively softer materials

23. Mechanical Properties
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

24. Mechanical Properties
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

25. Mechanical Properties
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

26. Mechanical Properties
Abrasion resistance
Abrasion resistance is typically measured by a weight loss when a material is abraded with a given abrader

27. Physical Properties 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

28. 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

29. Physical Properties Glass transition temperature Melt 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.

30. Thermal 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

31. Thermal Properties Thermal conductivity Specific heat or heat capacity
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

32. Thermal Properties Heat Deflection Temperature Softening Point
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

33. 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 %

34. 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.

35. Environmental Properties
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

36. 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

37. Environmental Properties
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

38. Environmental Properties
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

39. Environmental Properties
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

40. Environmental Properties
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

41. Optical Properties Specular gloss Luminous transmittance
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

42. 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

43. Optical Properties Color Color defined by three terms Value Hue Chroma
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

44. Optical Properties
Value

45. Optical Properties
Hue

46. Optical Properties
Chroma

47. 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

48. Optical Properties Color L*=100 white + b* = yellow - a* = green
Tristimulus system or L, a, b,
L*=0 black
+ a* = red
- a* = green
- b* = blue
+ b* = yellow
L*=100 white

49. Optical Properties = 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
red
object =
reflectance %
wavelength (nm.)

50. Optical Properties daylight incandescent 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
daylight
incandescent

51. 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

52. 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

53. 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

54. 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

55. 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

56. 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

57. Electrical Properties
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

58. 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

59. 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