Engineer General Machinery key terms

Ratio of a material's failure strength to the working stress a part is designed to carry. It accounts for uncertainty in loads, material properties, and analysis, and must exceed one for a safe design.

The stress at which a material begins permanent plastic deformation, typically identified by the 0.2 percent offset method on a stress-strain curve. It is the usual limiting stress for ductile machine parts.

The maximum engineering stress a material sustains before necking and fracture in a tension test. It is not a design limit for ductile parts but is used for brittle materials and comparative ranking.

The stress amplitude below which a material survives an essentially infinite number of load cycles without fatigue failure. Steels often show a distinct limit, while most nonferrous alloys do not.

A plot of cyclic stress amplitude against the number of cycles to failure for a material or component. It quantifies fatigue behavior and is generated by rotating-bending or axial fatigue tests.

A multiplier greater than one relating local peak stress at a geometric discontinuity, such as a hole, fillet, or keyway, to the nominal stress in the section. It drives fatigue crack initiation.

A factor between zero and one describing how much a material's fatigue strength is reduced by a theoretical stress concentration. Ductile materials are less notch sensitive than brittle or high-strength ones.

A plot of stress versus strain from a mechanical test, revealing elastic modulus, yield point, ultimate strength, ductility, and toughness. Its shape identifies whether a material is ductile or brittle.

The linear relationship stating that stress is proportional to strain within the elastic range, with the constant of proportionality being the elastic modulus. It underpins most elastic deformation analysis.

The slope of the linear portion of a stress-strain curve, expressing stiffness as stress per unit strain. It is largely independent of heat treatment for a given alloy family.

The negative ratio of lateral strain to axial strain in a uniaxially loaded elastic body. Typical metals fall near 0.3, and the value is needed for multiaxial stress and thermal stress calculations.

The extent to which a material plastically deforms before fracture, commonly measured as elongation at break or reduction of area. It signals tolerance of overload and forming operations.

Sudden failure with little or no plastic deformation, often driven by flaws and low temperature. It can occur below the ductile-to-brittle transition temperature in ferritic steels.

A pendulum test measuring energy absorbed when a notched specimen is broken in one blow. It assesses toughness and the ductile-to-brittle transition behavior of materials.

Time-dependent permanent deformation under sustained stress, significant at elevated temperature relative to a metal's melting point. Design against creep uses rupture and deformation data at service temperature.

The gradual loss of stress in a bolted or clamped assembly held at constant strain, especially at elevated temperature. It reduces preload and can loosen joints over time.

The tensile force deliberately induced in a fastener during tightening. Correct preload resists joint separation and fatigue, while insufficient or excessive preload causes loosening or bolt failure.

Environmentally assisted cracking that occurs when a susceptible alloy is held under tensile stress in a specific corrosive medium. Cracks propagate with little visible deformation, making inspection difficult.

Accelerated corrosion of a less noble metal when it is electrically coupled to a more noble one in an electrolyte. Susceptibility is predicted from the galvanic series of the metals involved.

A corrosion control method that makes a metal the cathode of an electrochemical cell, either with sacrificial anodes or an impressed direct current. It is standard for buried pipelines and tanks.

A hardenability test in which one end of a heated cylindrical bar is water quenched while hardness is measured along its length. The resulting curve shows how deep a steel hardens.

A steel's ability to form martensite and achieve hardness at depth during quenching, distinct from maximum attainable hardness. Alloying elements improve it by slowing transformation rates.

Controlled heating and cooling of a metal to alter its microstructure and mechanical properties. Common operations include annealing, normalizing, hardening, quenching, and tempering.

The fractional change in length per unit temperature change for a material. Mismatched coefficients between joined parts generate thermal stress and distortion under temperature swings.

The rotational speed at which a shaft's natural bending frequency coincides with an excitation frequency, causing large vibration. Operation well above or below this speed is required.

The number of revolutions or hours a rolling bearing can endure before fatigue spalling, usually stated as the L10 rating at which 90 percent survive. Load capacity scales inversely with roughly the cube of load.

The ratio of pitch diameter to number of teeth in a metric gear, defining tooth size. Two gears mesh only if their modules and pressure angles match.

The angle between the tooth load line and the common tangent at the pitch point in a gear pair. Standard values are 20 and 25 degrees, affecting tooth strength and bearing reactions.

The maximum theoretical efficiency of a heat engine operating between two constant-temperature reservoirs, equal to one minus the cold-to-hot absolute temperature ratio. It sets an upper bound no real cycle can exceed.

A thermodynamic property measuring energy dispersal and irreversibility. It increases in any real process, and its generation quantifies the lost work potential in a system.

An ideal adiabatic process with no entropy generation, used as a reference in compressor and turbine analysis. Real machines are compared against it via isentropic efficiency.

A compression or expansion path following pressure times specific volume raised to a constant exponent. The exponent indicates whether the process is isothermal, adiabatic, or intermediate.

The ratio of ideal work to actual work for a compressor, or actual to ideal for a turbine, under the same inlet and outlet pressures. It measures how closely a machine approaches ideal behavior.

A sequence of processes that returns a working fluid to its initial state, such as Rankine, Brayton, or Otto cycles. Net work is the enclosed area on a pressure-volume diagram.

A dimensionless ratio of inertial to viscous forces that predicts whether flow is laminar or turbulent. It also governs friction factor and heat transfer correlations.

A dimensionless coefficient in the Darcy-Weisbach equation relating head loss to velocity head in a pipe. It depends on Reynolds number and relative roughness.

Net positive suction head, the absolute pressure margin above a liquid's vapor pressure at the pump suction. Insufficient NPSH available versus required causes cavitation and impeller damage.

Formation and collapse of vapor bubbles in a liquid where local pressure falls below vapor pressure. It causes noise, erosion, and loss of pump or turbine performance.

Scaling relationships showing that flow varies with speed, head with speed squared, and power with speed cubed for geometrically similar pumps. They guide impeller trimming and variable-speed control.

A plot of the head a piping system requires versus flow rate, combining static lift and friction losses. The pump operating point is where it intersects the pump performance curve.

A dimensionless ratio of convective to conductive heat transfer at a surface, used to compute convection coefficients from correlations. It typically depends on Reynolds and Prandtl numbers.

A combined conductance describing total heat flow through a wall or heat exchanger per unit area and temperature difference. It includes convection on both sides and conduction through the material.

A groove melted into the base metal along the weld toe that remains unfilled, reducing throat thickness and creating a stress raiser. It is a common weld discontinuity rejected by visual inspection.

A written document defining the variables, such as current, voltage, travel speed, and filler, for producing a weld to a qualified standard. It is supported by a procedure qualification record.

Inspection methods that reveal defects without damaging the part, including visual, liquid penetrant, magnetic particle, ultrasonic, and radiographic testing. Method choice depends on material and defect type.

A volumetric inspection technique that transmits high-frequency sound into a part and analyzes echoes from internal flaws or back walls. It detects planar defects well but needs trained interpreters.

A safety procedure isolating hazardous energy sources and applying locks and tags before maintenance. It prevents unexpected startup or release of stored energy, which is a leading cause of injury.

Physical barriers or devices that prevent access to hazardous moving parts such as belts, gears, and shafts. Guards must be secured and not create new pinch points.

A structured process identifying hazards, estimating severity and likelihood, and prioritizing controls. It follows a hierarchy that begins with eliminating the hazard before relying on protective equipment.

A formal written authorization controlling high-risk tasks such as hot work, confined-space entry, or line breaking. It verifies precautions are in place before work starts.

A calibrated tool that applies a measured twisting moment to a fastener, used to achieve specified bolt preload. Its accuracy depends on calibration and correct tightening technique.

The equivalent value today of a stream of future cash flows discounted at a chosen interest rate. It allows alternatives with different timing to be compared on a common basis.

The discount rate at which a project's net present worth equals zero. It is compared against a minimum attractive rate to accept or reject an investment.

The present worth of an infinite series of recurring costs, computed as annual cost divided by interest rate. It is used to compare assets with differing lives and maintenance burdens.

A situation where personal, financial, or other interests could improperly influence professional judgment. It must be disclosed and managed, often by recusal from the affected decision.

A set of principles governing professional conduct, typically covering public safety, competence, honesty, and confidentiality. Licensed engineers are bound by the code of their registering body.

Failure to exercise the care and skill a reasonable engineer would provide in similar circumstances, causing harm. It is judged against the standard of the profession, not against perfection.