PRC Mechanical Engineer key terms

The ratio of the highest stress at a notch, hole, or abrupt change in section to the nominal stress calculated for the same load on the plain section. It quantifies how sharply geometry amplifies local stress.

The stress amplitude below which a ferrous material can withstand an essentially unlimited number of load cycles without fatigue failure. It appears as the horizontal portion of an S-N curve.

The stress level a material can endure for a specified finite number of cycles before failure under repeated or fluctuating loading. Unlike endurance limit, it applies to lives such as ten thousand to one million cycles.

The ratio of a material's strength or allowable stress to the working stress expected in service. Values above one indicate margin against failure and cover uncertainty in loads, properties, and analysis.

An assembly in which the shaft is slightly larger than the hole, so elastic compression and expansion create a holding pressure after the parts are forced or shrunk together. It transmits torque without a key.

The ratio of mean coil diameter to wire diameter in a helical spring. It indicates how sharply curved the wire is; small values mean high curvature and higher stress concentration.

The shortest distance from the root of a fillet weld to its face, measured perpendicular to the weld axis. It is the effective dimension used to compute weld shear area and strength.

The rotational speed at which a shaft's natural frequency matches the frequency of its rotating unbalance, causing large whirling vibration. Operation near it must be avoided or the shaft stiffened.

The tensile force deliberately locked into a tightened bolt so that the clamped parts stay in compression under service loads. It improves fatigue life and prevents joint separation or loosening.

A heat treatment that heats steel above or near its critical temperature, then cools it very slowly, usually in the furnace. It softens the metal, relieves internal stresses, and improves machinability and ductility.

Heating steel above its upper critical temperature and cooling it in still air. It produces a finer, more uniform grain structure than annealing and gives moderate strength with improved machinability.

Rapid cooling of heated steel by immersion in water, oil, or polymer, or by forced air. The fast cooling keeps carbon in solution and forms hard martensite, which is usually tempered afterward.

Reheating quenched steel to a temperature below its critical range, holding, then cooling. It reduces brittleness and relieves stresses while trading some hardness for toughness.

A hardness value obtained by pressing a hardened ball of known diameter into a surface under a standard load and dividing that load by the resulting indentation area. It suits coarse or cast structures.

An empirical relation stating that cutting speed multiplied by tool life raised to an exponent equals a constant for a given tool and workpiece pair. The exponent measures how sensitive life is to speed.

The rate at which the workpiece surface passes the cutting edge, usually expressed as surface meters per minute. It is the main machining variable controlling tool wear and surface finish.

The ease with which a molten metal fills a mold and solidifies into sound parts free of shrinkage cavities, hot tears, and gas porosity. High fluidity and low solidification shrinkage improve it.

Accelerated attack of one metal when two dissimilar metals are joined and exposed to an electrolyte. The less noble metal becomes the anode and dissolves while the more noble metal is protected.

The ideal thermodynamic cycle used to model steam power plants, with isentropic pumping, constant-pressure heat addition in a boiler, isentropic expansion in a turbine, and constant-pressure heat rejection in a condenser.

The ratio of heat absorbed by the steam or water to the heat released by burning fuel, expressed as a percentage. It accounts for stack losses, radiation, and unburned fuel.

The mass of fuel an engine or plant burns per unit of useful output, commonly kilograms per kilowatt-hour for plants or grams per kilowatt-hour for engines. Lower values mean better fuel economy.

For a reciprocating engine, the ratio of brake power delivered at the shaft to the heat energy supplied by the fuel. It reflects both combustion quality and mechanical friction losses.

The difference between the temperature of the water leaving a cooling tower and the ambient wet-bulb temperature of the air. A small approach means better tower performance.

The average load over a period divided by the peak load during that same period. It indicates how steadily a system is used and strongly affects the cost of supplying power.

The maximum demand of a system divided by its total connected load. It shows how much of the installed equipment is likely to operate at the same time.

The mass fraction of vapor in a saturated liquid-vapor mixture. A quality of one is dry saturated steam; values below one indicate moisture that can erode turbine blades.

An integral transform that converts a function of time into a function of a complex variable, turning linear differential equations into algebraic equations that are easier to solve.

A scalar for which a nonzero vector is mapped onto a scalar multiple of itself by a given square matrix. Eigenvalues govern natural frequencies and stability of linear systems.

A function whose integral over an interval gives the probability that a continuous random variable falls within that interval. Its total area over all possible values equals one.

An iterative root-finding technique that updates an estimate using the function value divided by its derivative at the current point. It converges quickly when the starting guess is close to the root.

A technique that rewrites the integral of a product using the rule that the integral of u dv equals uv minus the integral of v du, chosen so the remaining integral is simpler.

The equivalent value today of a stream of future cash flows, found by discounting each amount at a chosen interest rate. Alternatives are compared by bringing all costs and revenues to time zero.

The factor that converts a present sum into an equivalent series of equal end-of-period payments, covering both interest and principal repayment over the stated life.

A method that allocates the depreciable cost of an asset, first cost minus salvage value, equally over its useful life. The annual charge is constant.

The production volume or sales level at which total revenue equals total cost, so profit is zero. It separates operating losses from profits and helps set minimum acceptable output.

The discount rate at which the present worth of a project's cash inflows equals that of its outflows, making net present value zero. Projects are accepted when it exceeds the required rate.

The statement that stress is directly proportional to strain within the elastic range of a material, with the constant of proportionality being the modulus of elasticity.

The principle that energy cannot be created or destroyed in a closed system: heat added minus work done equals the change in internal energy.