PHYSICAL PROPERTIES OF MATERIALS

Chapter 3

• Density• Melting point• Specific heat• Thermal conductivity• Thermal expansion• Electrical properties• Magnetic properties• Resistance to oxidation• Resistance to corrosion

PHYSICAL PROPERTIES

DENSITY-MASS PER UNIT VOLUMEAlso called specific gravity which expresses a material’s density with respect to water

TABLE 3.1 PHYSICAL PROPERTIES OF SELECTED MATERIALS AT ROOM TEMPERATURE

TABLE 3.2 PHYSICAL PROPERTIES OF MATERIALS, IN DESCENDING ORDER

• Titanium and aluminum are among the most commonly used metal for aircraft and aerospace applications

FIGURE 3.1 RATIO OF MAXIMUM YIELD STRESS TO DENSITY FOR SELECTED METALS.

FIGURE 3.2 SPECIFIC STRENGTH (TENSILE STRENGTH/DENSITY) AND SPECIFIC STIFFNESS (ELASTIC MODULUS/ DENSITY) FOR VARIOUS MATERIALS AT ROOM TEMPERATURE. (SEE ALSO CHAPTER 9.)

COMPOSITE MATERIALS HAVE BECOME IMPORTANT FOR THEIR HIGH SPECIFIC STRENGTH AND STIFFNESS

Density• High speed

equipment• Textile machines• Printing press• Cameras• High weight

desirable-– Counterweights– Flywheels– Ballasts– Golf clubs

Elevated temperatures-specific strength & stiffness

• Turbines• Automotive• Jet engines• Gas turbines

FIGURE 3.3 SPECIFIC STRENGTH (TENSILE STRENGTH/DENSITY) FOR A VARIETY OF MATERIALS AS A FUNCTION OF TEMPERATURE. NOTE THE USEFUL TEMPERATURE RANGE FOR THESE MATERIALS AND THE HIGH VALUES FOR COMPOSITE MATERIALS. MMC—METAL-MATRIX COMPOSITE; FRP—FIBER-REINFORCED PLASTIC.

• Plastics have lowest useful range

• Graphite and refractory metals have the highest useful range

• Annealing• Heat treating• Hot-working

MELTING POINT-DEPENDS ON THE TEMPERATURE TO SEPARATE ITS ATOMS

• Alloying has a minor effect on specific heat

• Temperature rise in a work piece is a function of the work done and of the specific heat of the work piece material

SPECIFIC HEAT-THE ENERGY TO RAISE THE TEMPERATURE OF A UNIT MASS BY 1 DEGREE

• The rate at which heat flows within and through a material

• Metallically bonded materials (metals) generally have a higher conductivity

• Ionically or covalently bonded materials (ceramics, plastics) have poor conductivity

• Cooling fins• Cutting tools• Die-cast molds to conduct

heat

THERMAL CONDUCTIVITY

• Generally, the coefficient of thermal expansion is inversely proportional to the melting point of the material

• Shrink fits utilize thermal expansion and contraction-heat a part often installed on a shaft, install the part, let the part cool and contract

• Thermal stress• Cracking• Warping• Loosening• Thermal fatigue results from

thermal cycling• Thermal shock-cracks after

just a single thermal cycle• Low expansion alloys-iron-

nickel alloys with low thermal-expansion coefficients

THERMALEXPANSION

ELECTRICAL CONDUCTIVITY

• ELECTRICAL EQUIPMENT• MACHINERY• MANFACTURING

PROCESSSES• UNITS: MHO/M OR

MHO/FT WHERE MHO IS THE REVERSE OF OHM, THE UNIT OF ELECTRICAL RESISTANCE

• ALLOYING OF METALS RAISES THE CONDUCTIVITY

ELECTRICAL CONDUCTIVITY

• INSULATORS OR DIELECTRICS: MATERIALS WITH HIGH ELECTRICAL RESISTIVITY

• ELECTRICAL RESISTIVITY IS THE INVERSE OF ELECTRICAL CONDUCTIVITY

CONDUCTORS: MATERIALS WITH HIGH ELECTRICAL CONDUCTIVITY

• SUPERCONDUCTIVITY IS THE PHENOMENON OF NEAR-ZERO ELECTRICAL RESISTIVITY THAT OCCURS IN SOME MATERIALS OR ALLOYS BELOW A CRITICAL TEMPERATURE (OFTEN ABSOLUTE ZERO)

• HIGH-POWER MAGNETS• MRI-MAGNETIC

RESONANCE IMAGING

SUPERCONDUCTORS

• CORROSION REFERS TO THE DETERIORATION OF METALS AND CERAMICS

• DEGRADATION REFERS TO THE DETERIORATION OF PLASTICS

• FOOD• CHEMICAL• PETROLIUM INDUSTRY• MANUFACTURING

OPERATIONS

CORROSION RESISTANCE

• CORROSION RESISTANT MATERIALS:

• NONFERROUS METALS• STAINLESS STEEL• NONMETALLIC

MATERIALS• STEEL & CAST IRON

MUST BE PROTECTED BY COATINGS AND SURFACE TREATMENTS

PITTING: LOCALIZED CORROSION OF A MATERIAL

SALT…CORROSION?

• TWO ELECTRODES IN AN ELECTROLYTE IN A CORROSIVE ENVIRONMENT THAT INCLUDES MOISTURE CAUSE GALVANIC CORROSION

• STRESS-CORROSION CRACKING

• OXIDATION-REMOVAL OF MATERIAL BY CHEMICAL REACTION

GALVANIC CELL