Mechanical Properties Chapter 4c Mechanical Properties

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.

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).

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

Ballpark Comparisons Tensile strengths Polymers: ~ 10 - 100 MPa Metals: 100’s - 1000’s MPa Elongation Polymers: up to 1000 % in some cases Metals: < 100% Moduli (Elastic or Young’s) Polymers: ~ 10 MPa - 4 GPa Metals: ~ 50 - 400 GPa

Amorphous v Crystalline Polymers Thermo-mechanical properties

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.

Thermal Expansion

Stress Strain Studies

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)

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.

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

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

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

Cold Drawing above the Tg

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. 15.14, Callister 6e. (Fig. 15.14 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

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

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

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?

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) = 0. So, In terms of width, A = w2, then A/A = 2 w w/w2 = 2w/w = –L/L. Hence, Incompressible solid. Water (almost).

Poisson Ratio: materials specific Metals: Ir W Ni Cu Al Ag Au 0.26 0.29 0.31 0.34 0.34 0.38 0.42 generic value ~ 1/3 Solid Argon: 0.25 Covalent Solids: Si Ge Al2O3 TiC 0.27 0.28 0.23 0.19 generic value ~ 1/4 Ionic Solids: MgO 0.19 Silica Glass: 0.20 Polymers: Network (Bakelite) 0.49 Chain (PE) 0.40 Elastomer: Hard Rubber (Ebonite) 0.39 (Natural) 0.49

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.

Negtive poisson’s ratio foams

Compression Radial n = -1.24 Axial n = 0 Lakes, R. S., "No contractile obligations", Nature, 1992, 358, 713-714.

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

Mechanical properties are sensitive to temperature FIGURE 10.9 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, 2008.

Poly(methyl methacrylate)

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

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

Polymers: Thermal Properties Tg Tm semicrystalline log(Modulus) crosslinked linear amorphous Temperature

Polymers: Thermal Properties Stress Strain decreasing temperature or increasing crystallinity

Properties depend on amount of cross-linking Increasing cross-linking Figure 8.13 M. P. Groover, “Fundamentals of Modern Manufacturing 3/e” John Wiley, 2007