1. Mechanical Properties
Chapter 4c
Mechanical Properties

2. Heat Distortion Temperature
The maximum temperature at which a polymer can be used in rigid material applications is called the softening or heat distortion temperature (HDT).
A typical test (plastic sheeting) involves application of a static load, and heating at a rate of 2oC per min. The HDT is defined as the temperature at which the
elongation becomes 2%.
A: Rigid poly(vinyl chloride)
50 psi load.
B: Low-density poly(ethylene)
C: Poly(styrene-co-acrylonitrile)
25 psi load.
D: Cellulose acetate
(Plasticized) 25 psi load.

3. Transient Testing: Resilience of Cured Elastomers
Resilience tests reflect the ability of
an elastomeric compound to store
and return energy at a given
frequency and temperature.
Change of rebound
resilience (h/ho) with
temperature T for:
1. cis-poly(isoprene);
2. poly(isobutylene);
3. poly(chloroprene);
4. poly(methyl methacrylate).

4. Types of Polymers Polymer Family Tree Polyethylene 33% Vinyls 16%
Polypropylene
15%
PMMA
ABS
Nylon
Polycarbonate
Saturated Polyester
PEEK
Polyurethane
Some are thermosets as well.
PVC
Not Cross-Linked
90% of market
Thermoplastics
Will reform when melted
Epoxy
Melamine Formaldehyde
Phenolic
Polyester (unsaturated)
Polyimide
Some are thermoplastic as well.
Silicone
Urea Formaldehyde
Cross-linked
10% of market
Thermosets/Elastomers
Will not reform
Polymer Family Tree

5. Ballpark Comparisons Tensile strengths Polymers: ~ 10 - 100 MPa
Metals: 100’s ’s MPa
Elongation
Polymers: up to 1000 % in some cases
Metals: < 100%
Moduli (Elastic or Young’s)
Polymers: ~ 10 MPa - 4 GPa
Metals: ~ GPa

6. Amorphous v Crystalline Polymers Thermo-mechanical properties

7. Thermal Expansion
If a part is to be produced within a close dimensional tolerance, careful consideration of thermal expansion/contraction must be made.
Parts are produced in the melt state, and solidify to amorphous or semi-crystalline states.
Changes in density must
be taken into account
when designing the mold.

8. Thermal Expansion

9. Stress Strain Studies

10. Anatomy of a Stress Strain Graph
Elongation = 100% x  
Initial slope is the Young’s Modulus (E’ or sometimes G)
TS = tensile strength
y = yield strength
Toughness = Energy required to break (area under curve)

11. Compression and Shear vs. Tensile Tests
Stress-strain curves are very dependent on the test method. A modulus determined under compression is generally higher than one derived from a tensile experiment, as shown below for polystyrene.
Tensile testing is most sensitive
to material flaws and microscopic
cracks.
Compression tests tend to
be characteristic of the polymer,
while tension tests are more
characteristic of sample flaws.
Note also that flexural and shear
test modes are commonly
employed.

12. Stress Strain Graphs  
Chains in neck align along elongation direction: strengthening 
Elongation by extension of neck 

13. Beyond “B”, the yield strength, deformations are plastic

14. Ductility & Elongation (EL)
EL < 5% Brittle
EL > 5% Ductile
Thermosets = strong & brittle
Not Ductile
Thermolastics = depends on T

15. Cold Drawing above the Tg

16. TENSILE RESPONSE: • Compare to responses of other polymers:
Stress-strain curves adapted from Fig. 15.1, Callister 6e. Inset figures along elastomer curve (green) adapted from Fig , Callister 6e. (Fig is from Z.D. Jastrzebski, The Nature and Properties of Engineering Materials, 3rd ed., John Wiley and Sons, 1987.)
• Compare to responses of other polymers:
--brittle response (aligned, cross linked & networked case)
--plastic response (semi-crystalline case) 25

17. Elastomer Molecules High entropy Low entropy Low energy High energy
Model of long elastomer molecules, with low degree of cross‑linking: (a) unstretched, and (b) under tensile stress.
Low entropy
Low energy
High energy

19. YOUNG’S MODULI: COMPARISON
Graphite
Ceramics
Semicond
Metals
Alloys
Composites
/fibers
Polymers
E(GPa)
Based on data in Table B2,
Callister 6e.
Composite data based on
reinforced epoxy with 60 vol%
of aligned
carbon (CFRE),
aramid (AFRE), or
glass (GFRE)
fibers. 13

21. Linear Elasticity: Possion Effect
• Hooke's Law:  = E 
• Poisson's ratio, :
metals:  ~ 0.33
ceramics: ~0.25
polymers: ~0.40
Units:
E: [GPa] or [psi]
n: dimensionless
Why does  have minus sign?

22. Poisson Ratio • Poisson Ratio has a range –1    1/2
Look at extremes
No change in aspect ratio:
Volume (V = AL) remains constant: V =0.
Hence, V = (L A+A L) = So,
In terms of width, A = w2, then A/A = 2 w w/w2 = 2w/w = –L/L.
Hence,
Incompressible solid.
Water (almost).

23. Poisson Ratio: materials specific
Metals: Ir W Ni Cu Al Ag Au
generic value ~ 1/3
Solid Argon: 0.25
Covalent Solids: Si Ge Al2O3 TiC
generic value ~ 1/4
Ionic Solids: MgO 0.19
Silica Glass: 0.20
Polymers: Network (Bakelite) Chain (PE) 0.40
Elastomer: Hard Rubber (Ebonite) (Natural) 0.49

24. Example: Poisson Effect
Tensile stress is applied along cylindrical brass rod (10 mm diameter). Poisson ratio is  = 0.34 and E = 97 GPa.
Determine load needed for 2.5x10–3 mm change in diameter if the deformation is entirely elastic?
Width strain: (note reduction in diameter)
x= d/d = –(2.5x10–3 mm)/(10 mm) = –2.5x10–4
Axial strain: Given Poisson ratio
z= –x/ = –(–2.5x10–4)/0.34 = +7.35x10–4
Axial Stress: z = Ez = (97x103 MPa)(7.35x10–4) = 71.3 MPa.
Required Load: F = zA0 = (71.3 MPa)(5 mm)2 = 5600 N.

26. Negtive poisson’s ratio
foams

27. Compression
Radial
n = -1.24
Axial
n = 0
Lakes, R. S., "No contractile obligations", Nature, 1992, 358,

28. Anisotropic Materials
1) Compaction of UHMWPE powder
2) Sintering
3) Extrusion

29. Mechanical properties are sensitive to temperature
FIGURE Effect of temperature on the stress-strain curve for cellulose acetate, a thermoplastic. Note the large drop in strength and increase in ductility with a relatively small increase in temperature. Source: After T.S. Carswell and H.K. Nason. Manufacturing Processes for Engineering Materials, 5th ed. Kalpakjian • Schmid Prentice Hall,

30. Poly(methyl methacrylate)

31. Lower elastic modulus, yield and ultimate properties
Stress
Strain
Polymers
Metals
Ceramics
Lower elastic modulus, yield and ultimate properties
Greater post-yield deformability
Greater failure strain

33. Polymers: Thermal Properties
In the liquid/melt state enough thermal energy for random motion (Brownian motion) of chains
Motions decrease as the melt is cooled
Motion ceases at “glass transition temperature”
Polymer hard and glassy below Tg, rubbery above Tg

34. Polymers: Thermal PropertiesTg Tm
semicrystalline
log(Modulus)
crosslinked
linear amorphous
Temperature

35. Polymers: Thermal Properties
Stress
Strain
decreasing temperature or increasing crystallinity

36. Properties depend on amount of cross-linking
Increasing cross-linking
Figure M. P. Groover, “Fundamentals of Modern Manufacturing 3/e” John Wiley, 2007