Machine Design Fundamentals

Summary of Machine Design and Shop Practice

• Offset: Usually 0.2% (0.002 in./in. of strain) for steel, aluminum, and magnesium alloys.
• Stress (s): Measured in pounds or kips per square inch (psi or ksi). A kip equals 1000 lb.
• Strain: Represents the deformation per unit gage length (in. per in).
• Ultimate Stress/Tensile Strength: The maximum load divided by the original area before straining. Located at the highest point on the stress-strain curve.
• Elastic Limit: Maximum stress a specimen can withstand without permanent deformation.
• Proportional Limit: Where the stress-strain curve deviates from a straight line.
• Modulus of Elasticity (E): A proportionality constant in tension, representing the slope of the straight part of the stress-strain curve; a measure of stiffness.
• Yield Point (YP): Stress at which a low or medium carbon steel undergoes a marked elongation without an increase in load.
• Yield Strength: Stress for a specified deviation from the straight part of the stress-strain curve.
• Factor of Safety (N): A number dividing the criterion of strength, to determine a design criterion (sometimes called 'factor of ignorance').
• Computed/Nominal Stress: Stress calculated from a stress equation.

Summary of Age Hardening/Precipitation Hardening

• Age Hardening: Occurs in some metals (stainless steel, aluminum, copper); after solution heat treatment at ambient temperature. The process involves constituent precipitation from a solid solution.
• Consequences: Increased strength and hardness, decreased ductility. Accelerated at moderately elevated temperatures. Artificially induced.
• Alloy: Substance with metallic properties from at least one metal.
• Alloying Elements: Metallic elements added to steel to adjust properties.

Summary of Charpy Test

• Charpy Test: Specimen, supported at both ends, is broken by a falling pendulum.
• Measurement: Energy absorbed in breaking specimen
• Significance: Measure of material's impact strength.

Summary of Cold Shortness

• Cold Shortness: Brittleness of metals at ordinary or low temperatures.
• Metal Deformation Temperatures: Cold working occurs below the recrystallization temperature.

Other Material Properties

• Elasticity: Ability to return to original shape after deformation. Stress is proportional to strain during elastic deformation.
• Embrittlement: Loss of ductility due to physical or chemical change
• Ductility: Property that permits permanent deformation before fraction; percentage elongation and percentage reduction of area are indices.
• Brittleness: Tendency to fracture without appreciable deformation; opposite of ductility.
• Homogeneity: Material having the same structure throughout; steel is composed of randomly oriented iron crystals of different sizes with other matter.
• Isotropy: Material having the same properties in all directions; wood has grain, rolled steel isn't isotropic.

Summary of Other Important Concepts

• Decarburization: Loss of carbon from steel surface during heat treatments.
• Case Hardening: Significantly hardens the surface (case) of an iron alloy, while the core remains softer.
• Carburizing: Process to add carbon to the surface of steel. Typically with hot carbonaceous solids, liquids or gasses above the transformation temperature.
• Pack/Box Carburizing and Gas Carburizing: Common Carburizing methods.
• Liquid Carburizing: Part is immersed in molten salt bath
• Relaxation Stresses: Stresses not directly from loads or temperature gradients; due to factors like unequal cooling rates.
• Hardening: Heating of steels beyond the transformation range, then quenching, to increase hardness
• Toughness: Ability to withstand shock load without breaking
• Stiffness: Resistance to deformation; Measured by modulus of elasticity in elastic range. Higher modulus, higher stiffness.

Summary of Mechanical Testing

• Transverse Strength: Results of a transverse bend test. The specimen is mounted as a simple beam. Also referred to as rupture modulus.
• Critical Range/Transformation Range: Temperature interval where austenite is formed or disappears during heating or cooling.
• Hardening of a Material: Measured by resistance of a material to indentation.
• Brinell Hardness Number (BHN): Load in kilograms divided by the area of the surface of the indentation in square millimetres.
• Hardenability: Capacity of steel to become hard after being cooled from above its transformation range.
• Stress Relieving: Heating to suitable temperature just below transformation range, holding for a period and slowly cooling to reduce internal residual stress.
• Tempering: Reheating hardened or normalized steel to a temperature below the transformation range.
• Normalizing: Heating to about 100°F above transformation range and slowly cooling to below that range in still air. Used to produce a uniform structure..
• Spheroidizing: Prolonged heating at a temperature slightly below transformation range, often followed by slow cooling. Produces a rounded shape of carbides.