COREN Mechanical key terms

The ratio of a material's or component's failure strength to the maximum stress it is expected to experience in service. Values above one indicate a margin against yielding, fracture, buckling, or other failure modes.

The maximum stress a design is permitted to carry, obtained by dividing a limiting strength by a factor of safety. It is the value compared against computed working stresses during design checks.

The stress at which a material begins to deform permanently, typically reported as the 0.2 percent offset value for metals without a distinct yield point. It marks the practical limit for most static design.

The maximum engineering stress a material sustains before necking and fracture in a tension test. It is a common reference for brittle materials and for setting design limits in some codes.

The slope of the linear portion of a stress-strain curve, expressing stiffness as stress per unit strain. Also called Young's modulus; it governs deflection in the elastic range.

The negative ratio of lateral strain to axial strain under uniaxial loading, typically about 0.3 for metals. It links longitudinal stretching to transverse contraction in elastic deformation.

A multiplier greater than one applied to nominal stress to account for local intensification at holes, notches, fillets, or sudden section changes. It is used in fatigue and static design of discontinuities.

A plot of applied stress against resulting strain from a material test, revealing elastic slope, yield point, ultimate strength, and ductility. It is the primary source of mechanical property data for design.

The extent to which a material plastically deforms before fracture, often expressed as elongation at break or reduction in area. Ductile behavior provides warning and redistribution of load before failure.

The energy a material absorbs before fracture, represented by the area under the stress-strain curve. It combines strength and ductility and is important for impact and shock loading.

Failure with little or no plastic deformation, occurring suddenly when local stress exceeds cohesive strength. It is promoted by low temperature, sharp flaws, and high constraint, and is analyzed with fracture mechanics.

Progressive damage and eventual fracture under repeated or fluctuating stress below the static strength. Cracks usually initiate at stress concentrations and grow with each cycle until the remaining section fails.

The stress amplitude below which a material, notably some steels, survives an effectively infinite number of cycles without fatigue failure. Nonferrous metals often lack a true endurance limit.

A plot of cyclic stress amplitude against the number of cycles to failure, usually on semilog axes. It summarizes fatigue test data and is used to estimate life at a given stress level.

Time-dependent permanent deformation under sustained stress, significant at elevated temperatures relative to a material's melting point. It is characterized in stages and limited by design rules for hot components.

Sudden lateral instability of a slender member under compressive load, occurring at a critical load that depends on length, end conditions, and bending stiffness. It often governs design before material yielding.

The theoretical critical compressive load for a perfectly straight elastic column, proportional to stiffness and inversely proportional to effective length squared. It provides the basis for column design formulas.

The effective length of a compression member divided by its least radius of gyration. It indicates whether buckling or yielding controls and separates short, intermediate, and long column behavior.

A geometric property equal to the square root of the second moment of area divided by cross-sectional area. It measures how far material is distributed from the centroid and appears in buckling calculations.

A section property describing resistance to bending about an axis, computed by integrating area times distance squared from that axis. Larger values reduce bending stress and deflection.

Normal stress produced by a bending moment, varying linearly from zero at the neutral axis to a maximum at the extreme fiber. It is calculated from moment, distance from neutral axis, and second moment of area.

The line in a beam cross-section where bending strain and stress are zero, passing through the centroid for elastic bending of homogeneous material. Distances from it set the stress distribution.

The displacement of a structural member from its unloaded position under load, governed by stiffness and support conditions. Serviceability limits on deflection protect finishes, alignment, and function.

Twisting of a member caused by a moment applied about its longitudinal axis, producing shear stress that increases with distance from the center. Circular shafts are analyzed with the torsion formula.

A section property measuring resistance to twisting about the longitudinal axis, equal to the sum of two perpendicular second moments of area. It appears in the torsion formula for circular shafts.

The normal stress on a plane where shear stress is zero, found by rotating stress components to their extreme values. Principal stresses are used with failure theories for combined loading.

An equivalent stress combining normal and shear components to predict yielding of ductile materials. Yielding is expected when it reaches the material's yield strength in uniaxial tension.

A graphical construction that transforms stress or strain components between coordinate orientations, showing principal values and maximum shear. It is a standard aid for combined-loading analysis.

Simultaneous action of axial, bending, torsional, and transverse shear loads on a member. Resultant stresses are superposed at critical points and evaluated against an appropriate failure theory.

Strain produced when a material's temperature changes, proportional to temperature change and the coefficient of thermal expansion. Restrained expansion generates stress that must be included in design.

Gradual loss of stress in a member held at constant strain, such as a bolt or spring at elevated temperature. It reduces clamping force over time and is related to creep behavior.

Initial tension deliberately induced in a bolt, spring, or bearing to improve joint stiffness, sealing, or fatigue resistance. It must exceed applied service loads to prevent separation or loosening.

The permissible total variation in a dimension between its maximum and minimum limits. It defines manufacturing acceptance and interacts with fits and functional clearances.

The relationship between mating part sizes before assembly, classified as clearance, interference, or transition. Fit selection depends on function, tolerance grade, and assembly method.

An assembly condition in which the shaft is larger than the hole, creating contact pressure that transmits load or torque by friction. It may be achieved by press or shrink fitting.

A symbolic system for specifying form, orientation, location, and runout limits on drawings, often with material condition modifiers. It communicates functional requirements unambiguously to manufacturing and inspection.

A theoretically exact reference feature or axis from which location and orientation tolerances are measured. Datums establish the coordinate framework used during inspection and assembly.

A measure of the small-scale topography of a manufactured surface, commonly expressed as average roughness. It affects friction, wear, sealing, fatigue, and coating adhesion.

A material's resistance to localized plastic deformation such as indentation or scratching, measured by Brinell, Rockwell, or Vickers tests. Hardness correlates empirically with strength and wear resistance.

Controlled heating and cooling of a metal to alter microstructure and properties such as hardness, toughness, and machinability. Common processes include annealing, normalizing, quenching, and tempering.