1. Principles of Engineering Polymers & Composites Lab
Lecture 11:
Material Properties
Polymers & Composites Lab

2. Engineering Materials
Plastics
Metals
Steel
Stainless steel
Die & tool steel
Cast iron
Ferrous
Non-ferrous
Aluminum
Copper
Zinc
Titanium
Tungsten
Thermosets
Phenolic
Polymide
Epoxies
Polyester
Elastomers
Rubber
Polyurethane
Silicone
Thermoplastics
Acrylic
Nylon
ABS
Polyethylene
Polycarbonate
PVC
Ken Youssefi
SJSU, ME dept.

3. Engineering Materials
Composites
Reinforced plastics
Metal-Matrix
Ceramic-Matrix
Laminates
Ceramics
Glass
Carbides
Nitrides
Graphite
Diamond
Glasses
Glass ceramics
Metals
Plastics
Ken Youssefi
SJSU, ME dept.

4. Mechanical Properties

5. Bulk mechanical properties
Stiffness
Strength
Elasticity
Ductility
Brittleness
Malleability
Toughness
Resilience
Hardness

6. Stiffness The ability to resist deflection
Property that enables a material to withstand high stress without great strain
resistance of a material to deformation
Represented by the slope of the stress-strain curve (steeper slope & higher E – greater stiffness)
E = / (for tension or compression)
G = / (for shear, modulus of rigidity)
Only defined for elastic deformation; (complex for nonlinear responses)
Compliance – opposite (inverse?) of stiffness (flatter slopes & lower E)

7. Strength The ability of a material to resist deformation
Strength The ability of a material to resist deformation. The maximum load before failure.
stiffness ≠ strength
Y = Yield strength
U = Ultimate strength
R = Rupture (failure)
Greatest stress that a material can withstand prior to failure
May be defined by the proportional limit, yield point, or ultimate strength
Depends on the kind of stress and the nature of the loading

8. Elasticity The ability of a material to resume its normal shape after being stretched or compressed
the ability of a material to resume its original size and shape upon removal of applied loads
No known material that is elastic at all stresses
Determination of elastic limit establishes the elastic range of the limit of elasticity

9. Ductility
The ability of a material to undergo considerable plastic deformation under tensile load before rupture
The ability to stretch without breaking
Ductility – characteristic of a material that undergoes considerable plastic deformation under tensile load before rupture
Can be drawn into a long thin wire by a tensile force without failure
Quantified by % elongation (gage length/original gage length) and % reduction in area (area/original area)
High %elongation is high ductile material

10. Brittleness Absence of any plastic deformation prior to failure
Lack of ductility; not necessarily a lack of strength
Brittleness – absence of any plastic deformation prior to failure
Fails suddenly without warning
No yield point or necking
Rupture strength = ultimate strength
Brittle materials typically weak in tension and thus, tested in compression
Do not confuse brittle and ductile with strength.

11. Malleability
The ability of a material to undergo considerable plastic deformation under compressive load before rupture.
The ability to be hammered into shapes.
characteristic of a material that undergoes considerable plastic deformation under compressive load before rupture
Ductile materials are often malleable

12. Toughness A combination of strength and ductility.
The ability to absorb punishment without breaking in two.
Toughness – property of a material enabling it to endure high-impact or shock loads; ability to absorb energy during plastic deformation; measure of the capacity of a material to sustain permanent deformation
If material can be highly stressed & greatly deformed without rupture, it is tough
Area under the curve (1 tougher than 2)
Brittle materials are not very tough (small plastic deformation before fracture occurs)
Fracture toughness – resistance to rapid propagation of crack because of available energy

13. Resilience
The ability of a material to spring back into its original shape.
Property of a material enabling it to endure high impact loads without inducing a stress in excess of the elastic limit
Energy is absorbed during blow is stored and recovered when body is unloaded
Measured by area under elastic portion of curve
Resilience – measure of energy absorbed by a material and returned when load is removed; materials that quickly return to their original shape are called resilient

14. Hardness
The ability of a material to resist scratching, wear, or penetration
Resistance of a material to scratching, wear, or penetration
Measured by a compression test
Not frequently used for biological tissues

15. Strength testing of materials
Tensile testing
Compression testing

16. Stress-Strain curve Unique for every material Pulling force applied
The amount of elongation (stretch)

17. Force is applied until sample fails
Destructive testing
A straight line axial force is applied to a test sample (typically in the y axis)
Force is applied until sample fails
Hounsfield Tensometer
Image courtesy of NSW Department of Education and Training

18. Standard Test Sample (dog bone)
Ensures meaningful and reproducible results
Uniform cross section

19. Dog bone is created to test specifications
Dog bone is secured in tester and slowly stretched

20. A tension force (F) is applied to the dog bone until failure occurs
Simultaneously the applied tension force (F) and dog bone elongation ΔL are recorded F ΔL
A plot is created from the stored load elongation data

21. F ΔL A B
Test sample A and B are 230 red brass. Test sample A has a diameter of in. Test sample B has a diameter of in.
If both samples are tested to failure, will the applied tension force and elongation be the same for both tests?
NO – because one sample is thicker. That’s why “stress” is what we’re really after = Force/Area

22. Load-elongation results are dependent upon sample size.
Larger sample indicates larger load-elongation.
How can test data be manipulated to represent a material and not an individual test sample?
By plotting “stress” on the Y-axis = Force/Area

23. “Stress” is load per unit area = Force/Area
Divide load (F) by the original test sample cross-sectional area (A0)

24. Example: calculate the stress in the dog bone with a 430 lb applied force.

25. Manipulating Elongation Results
To eliminate test results based on sample size, calculate sample strain
Strain = the amount of stretch per unit length

26. Calculate the strain in the dog bone with an elongation of 0.0625in.

27. The result of all that is the “Stress-Strain Curve”

28. Tensile Test Initial response is linear
Stress and strain are proportional to one another
Elastic Range
Proportional Limit (The stress at which proportionality ceases)

29. Tensile Test
Modulus of Elasticity (E) The proportional constant (ratio of stress and strain)
A measure of stiffness – The ability of a material to resist stretching when loaded
An inherent property of a given material

30. Tensile Test
If the load is removed, the test sample will return to its original length
The response is elastic or recoverable
Exaggerated stretch to illustrate principle

31. Tensile Test Elastic Limit Uppermost stress of elastic behavior
Elastic and proportional limit are almost identical, with the elastic limit being slightly higher

32. Tensile Test – Stress-Strain Curve
Resilience The amount of energy per unit volume that a material can absorb while in the elastic range
Area under the stress-strain curve
Why would this be important to designers? Hint: car bumper

33. Tensile Test – Stress-Strain Curve
Yield Point When the elastic limit is exceeded
A very small increase in stress produces a much greater strain
Most materials do not have a well-defined yield point

34. Tensile Test – Stress-Strain Curve
Offset Yield Strength
Defines the stress required to produce a tolerable amount of permanent strain
Common value is 0.2%

35. Tensile Test – Stress-Strain Curve
Plastic Deformation Unrecoverable elongation beyond the elastic limit
When the load is removed, only the elastic deformation will be recovered

36. Tensile Test – Strength Properties
Plastic deformation represents failure
Part dimensions will now be outside of allowable tolerances

37. Tensile Test – Stress-Strain Curve
Deformation
Test sample elongation
Cross-sectional area decreases
Load bearing ability increases – Why?
The material is getting stronger – How?

38. Tensile Test – Stress-Strain Curve
Weakest point is stretched and becomes stronger
New weakest point is stretched and becomes stronger, and so on
This keeps occurring until the decrease in area overcomes the increase in strength

39. Tensile Test – Stress-Strain Curve
Tensile Strength Load bearing ability peaks
Force required to continue straining the test sample decreases
Weakest location at the peak continues to decrease in area – Necking

40. Tensile Test – Stress-Strain Curve
Failure If continued force is applied, necking will continue until fracture occurs
Ductility Amount of plasticity before fracture
The greater the ductility, the more a material can be deformed

41. Tensile Test – Samples
Compare the material properties of these three metal samples

42. Toughness
Work per unit volume required to fracture a material
Total area under the stress-strain curve from test initiation to fracture (both strength and ductility)

43. Compression Testing Up until now we’ve been talking about Tensile Testing…
Stress and strain relationships are similar to tension tests – elastic and plastic behavior
Test samples must have large cross-sectional area to resist bending and buckling
Material strengthens by stretching laterally and increasing its cross-sectional area

44. Website for Virtual Tensometer http://lrrpublic. cli. det. nsw. edu

45. Polymers and Composites

46. What is a polymer? Poly mer
many repeat unit
repeat
unit
repeat
unit
repeat
unit C H
Polyethylene (PE) Cl C H
Polyvinyl chloride (PVC) H
Polypropylene (PP) C
CH3

48. Bulk or Commodity Polymers
Relatively few polymers responsible for virtually all polymers sold – these are the bulk or commodity polymers
“Teflon”

49. Plexiglas, Lucite

51. Composites:
A Composite material is a material system composed of two or more macro constituents that differ in shape and chemical composition and which are insoluble in each other. The history of composite materials dates back to early 20th century. In 1940, fiber glass was first used to reinforce epoxy.
Applications:
Aerospace industry
Sporting Goods Industry
Automotive Industry
Home Appliance Industry

52. Composite Survey

53. Composite Survey: Particle-I
Particle-reinforced
Fiber-reinforced
Structural
• Examples:
(copyright United States Steel Corporation, 1971.)
- Spheroidite
steel
matrix:
ferrite (a)
(ductile)
particles:
cementite ( Fe 3 C )
(brittle)
60 mm
(courtesy Carboloy Systems, Department, General Electric Company.)
- WC/Co
cemented
carbide
matrix:
cobalt
(ductile)
particles: WC
(brittle,
hard) V m :
5-12 vol%!
600 mm
(courtesy Goodyear Tire and Rubber Company.)
- Automobile
tires
matrix:
rubber
(compliant)
particles: C
(stiffer)
0.75 mm

54. Composite Survey: Particle-II
Particle-reinforced
Fiber-reinforced
Structural
Concrete – gravel + sand + cement
- Why sand and gravel? Sand packs into gravel voids
Reinforced concrete - Reinforce with steel rebar or remesh
- increases strength - even if cement matrix is cracked
Prestressed concrete - remesh under tension during setting of concrete. Tension release puts concrete under compressive force
- Concrete much stronger under compression.
- Applied tension must exceed compressive force
threaded
rod
nut
Post tensioning – tighten nuts to put under rod under tension
but concrete under compression

55. Composite Survey: Fiber
Particle-reinforced
Fiber-reinforced
Structural
Fibers themselves are very strong
Provide significant strength improvement to material
Ex: fiber-glass
Continuous glass filaments in a polymer matrix
Strength due to fibers
Polymer simply holds them in place and environmentally protects them

56. Composite Survey: Fiber
Particle-reinforced
Fiber-reinforced
Structural
Fiber Materials
Whiskers - Thin single crystals - large length to diameter ratio
graphite, SiN, SiC
high crystal perfection – extremely strong, strongest known materials!
very expensive
Fibers
polycrystalline or amorphous
generally polymers or ceramics
Ex: Al2O3 , Aramid, E-glass, Boron, UHMWPE
Wires
Metal – steel, Mo, W

57. Fiber Loading Effect under Stress:

58. Composite Survey: Fiber
Particle-reinforced
Fiber-reinforced
Structural
• Critical fiber length (lC) for effective stiffening & strengthening:
fiber strength in tension
fiber diameter
shear strength of
fiber-matrix interface
• Ex: For fiberglass, a fiber length > 15 mm is needed since this length provides a “Continuous fiber” based on usual glass fiber properties
• Why? Longer fibers carry stress more efficiently!
Shorter, thicker fiber:
Longer, thinner fiber:
Poorer fiber efficiency
Better fiber efficiency s
(x)

59. Fiber Alignment aligned continuous aligned random discontinuous
Adapted from Fig. 16.8, Callister 7e.
aligned
continuous
aligned random
discontinuous

60. Composite Survey: Structural
Particle-reinforced
Fiber-reinforced
Structural
• Stacked and bonded fiber-reinforced sheets
-- stacking sequence: e.g., 0º/90º or 0º/45º/90º
-- benefit: balanced, in-plane stiffness
• Sandwich panels
-- low density, honeycomb core
-- benefit: light weight, large bending stiffness
honeycomb
adhesive layer
face sheet

61. Polymers – Composites Lab

62. Starch bioplastic

63. Starches/polysaccharides: polymers of glucose
Amylose is strait-chain
Amylopectin is branched
Glycogen is an intermediate energy storage form of glucose. It is stored in liver and muscle tissues.
Corn starch is 30% amylose and 70% amylopectin
Amylose molecule

64. Cornstarch bioplastic

65. Starch and Cellulose – two forms of glucose.
Starch is the intermediate energy storage molecule in animals.
Cellulose is a structural polysaccharide in plants.

66. Rayon is a manufactured regenerated cellulose fiber
Rayon is a manufactured regenerated cellulose fiber. It is made from purified cellulose, primarily from wood pulp, which is chemically converted into a soluble compound.

67. Vinyl plastic from glue and Borax

68. Polystyrene Plastic

69. Protein plastic Casein is milk protein.
There are about 200 amino acids making up casein.
The Chemical Formula of casein is C47-H48-N3-O7-S2-Na
Protein plastic

70. Pykrete is a frozen composite material made of approximately 14 percent sawdust or some other form of wood pulp (such as paper) and 86 percent ice by weight (6 to 1 by weight). Its use was proposed during World War II by Geoffrey Pyke to the British Royal Navy as a candidate material for making a huge, unsinkable aircraft carrier. Pykrete has some interesting properties, notably its relatively slow melting rate (because of low thermal conductivity), and its vastly improved strength and toughness over ice; it is closer in form to concrete.
Pykrete is slightly more difficult to form than concrete, as it expands during the freezing process. However, it can be repaired and maintained using seawater. The mixture can be moulded into any shape and frozen, and it will be extremely tough and durable, as long as it is kept at or below freezing.
Ice Alloy
A composite of ice and pulp