ME260 Mechanical Engineering Design II Instructor notes

Mechanical Properties of Materials Slope is Young’s Modulus, E, indicates stiffness Ultimate Tensile Strength(UTS) Or simply Tensile Strength (TS) Yield stress, sometimes  y

Mechanical Properties of Materials

Compression is the opposite of tension, i.e. you put pressure on the surface or push on it as opposed to pull on it The stress-strain curve/diagram of a material subjected to compression usually looks similar to a tension test F F Area = A l F F Area = A o lolo (left) Before deformation, and (right) after deformation

Simple tension test: PLASTIC (PERMANENT) DEFORMATION Strain Stress Permanent Strain Unloading Loading/ Unloading Loading

Metals Alloys Graphite Ceramics Semicond Polymers Composites /fibers 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. YOUNG’S MODULI: COMPARISON

Room T values Based on data in Table B4, Callister 6e. a = annealed hr = hot rolled ag = aged cd = cold drawn cw = cold worked qt = quenched & tempered YIELD STRENGTH: COMPARISON

Room T values Based on data in Table B4, Callister 6e. a = annealed hr = hot rolled ag = aged cd = cold drawn cw = cold worked qt = quenched & tempered AFRE, GFRE, & CFRE = aramid, glass, & carbon fiber-reinforced epoxy composites, with 60 vol% fibers. TENSILE STRENGTH: COMPARISON

strain at failure: Another ductility measure: Adapted from Fig. 6.13, Callister 6e. DUCTILITY, %EL Aluminum/structural steels are examples of ductile materials whereas glass/ceramics are not

Effect of Temperature on the Stress-Strain Diagram

Energy to break a unit volume of material Approximate by the area under the stress-strain curve. Toughness can be measured with an impact test (Izod or Charpy) TOUGHNESS

Toughness can be measured with an impact test (Izod or Charpy) TOUGHNESS

Resistance to permanently indenting the surface. Large hardness means: --resistance to plastic deformation or cracking in compression. --better wear properties. Adapted from Fig. 6.18, Callister 6e. (Fig is adapted from G.F. Kinney, Engineering Properties and Applications of Plastics, p. 202, John Wiley and Sons, 1957.) HARDNESS

Occurs at elevated temperatures, normally T > 0.4 T melt Deformation changes with time Examples: turbine engine blades CREEP Deformation with time

Fatigue = failure under cyclic stress. Stress varies with time. Key points: Fatigue... --can cause part failure, even though  <  y. --causes ~ 90% of mechanical engineering failures. FATIGUE Hip implant-cyclic loading from walking. Adapted from Fig (b), Callister 6e. Ship-cyclic loading from waves. Adapted from Fig. 8.0, Callister 6e. (Fig. 8.0 is by Neil Boenzi, The New York Times.)

Ductile fracture is desirable for structural applications! Classification: Ductile: warning before fracture, i.e. changes geometry before failure Brittle: No warning Adapted from Fig. 8.1, Callister 6e. DUCTILE VS BRITTLE FAILURE

MECHANICAL PROPERTIES HardStrongBrittle (not Tough) The following associations normally apply: Ductile (Tough)Soft(er ) Weak(er) (not strong)

Material Types Ferrous metals/alloys Nonferrous metals/alloys Polymers Ceramics Glass Diamond, Graphite Wood Composites Iron-based materials, e.g. steels Both made from pure carbon but have different atomic structure Plastics and rubber are prime examples Compounds of metallic & nonmetallic elements, e.g. chinaware Natural and organic material e.g. nickel, silver, etc. A combination of two/more material types Solid with a random atomic structure like a fluid

Atomic Structure of Materials Ferrous metals/alloys Nonferrous metals/alloys Ceramics (metallic & non-metallic elements)

2 Polymer = many mers Covalent chain configurations and strength: Direction of increasing strength Adapted from Fig. 14.2, Callister 6e. Adapted from Fig. 14.7, Callister 6e. POLYMER MICROSTRUCTURE Hydro-carbon based

Based on data provided in Tables 11.1(b), 11.2(b), 11.3, and 11.4, Callister 6e. STEELS High Strength, Low Alloy 10 for plain carbon steels, and 40 for 0.4 wt% C

Based on discussion and data provided in Section 11.3, Callister 6e. NONFERROUS ALLOYS

Material Selection Strength (MPa) Density (Mg/m 3 )

Applications of Material Use Ferrous metals/alloys Iron-based materials, e.g. steels

Applications of Material Use Nonferrous metals/alloys e.g. nickel, silver, etc.

Applications of Material Use Polymers Plastics and rubber are prime examples PVC pipesRubbermaid containers Bakelite (plastic) electric outlet cover

Applications of Material Use Ceramics Compounds of metallic & nonmetallic elements, e.g. chinaware blades oil drill bits

Applications of Material Use Composites A combination of two/more material types Blast resistant panel Fiberglass tube Trailer