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
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