IESL Part II Mechanical key terms

Internal force per unit area within a material, expressed as force divided by cross-sectional area. Normal stress acts perpendicular to the surface; shear stress acts parallel to it. Units are pascals or psi.

The deformation of a material relative to its original length, calculated as change in length divided by original length. It is dimensionless, though often quoted in microstrain or percent.

The ratio of normal stress to normal strain in the linear elastic region of a stress-strain curve. It measures a material's stiffness; a steeper slope means a stiffer material. Also called the elastic modulus.

The stress at which a material begins to deform permanently, marking the end of elastic behavior. Often defined by the 0.2 percent offset method for materials without a distinct yield point.

The maximum engineering stress a material can sustain before necking and fracture in a tension test. It is not the stress at failure; the fracture stress is usually lower because of area reduction.

The ability of a material to undergo plastic deformation before fracture, commonly measured as percent elongation or percent reduction in area in a tension test. Ductile materials warn before failing; brittle ones do not.

The amount of energy a material absorbs before fracturing, represented by the total area under the stress-strain curve. It combines strength and ductility and differs from hardness, which resists surface indentation.

The negative ratio of lateral strain to axial strain in a material under uniaxial loading. Typical metals range from about 0.25 to 0.35; a value near 0.5 indicates near-incompressible behavior.

The ratio of a material's failure strength to the allowable working stress, or of failure load to design load. It accounts for uncertainty in loads, material properties, and analysis methods.

The maximum stress a designer permits in a component under service loads, obtained by dividing a failure strength by a factor of safety. It is the basis for sizing members in codes and standards.

A localized increase in stress near geometric discontinuities such as holes, notches, fillets, or sharp corners. The stress concentration factor Kt is the ratio of peak local stress to nominal stress.

Progressive damage and eventual fracture of a material under repeated or fluctuating stresses below its yield strength. Failure typically initiates at stress concentrations and proceeds by crack growth.

The stress amplitude below which a material, typically steel, can endure an essentially infinite number of cycles without fatigue failure. Nonferrous metals often lack a true limit and are rated at a set cycle count.

A plot of stress amplitude against number of cycles to failure for a material under cyclic loading, usually on semilog axes. It defines the fatigue life at a given stress level for a specified mean stress and surface condition.

Time-dependent permanent deformation of a material under sustained load, significant at elevated temperatures relative to its melting point. Creep stages are primary, secondary, and tertiary, ending in rupture.

The gradual decrease of stress in a material held at constant strain over time, common in bolted joints and springs at elevated temperature. It differs from creep, which is strain change at constant stress.

Sudden lateral deflection of a slender compression member when the applied load reaches a critical value. Euler buckling load depends on end conditions, length, and the minimum second moment of area.

The effective length of a compression member divided by its least radius of gyration. It indicates whether a column fails by yielding or by elastic or inelastic buckling.

A geometric property of a cross-section describing how its area is distributed about an axis, governing bending stiffness and buckling resistance. Often called moment of inertia in engineering practice, though distinct from mass moment of inertia.

Normal stress in a beam caused by bending moment, calculated as moment times distance from the neutral axis divided by the second moment of area. It is tensile on one side and compressive on the other.

Twisting of a shaft or member by an applied torque, producing shear stress that increases with radial distance from the axis. Angle of twist depends on torque, length, shear modulus, and polar moment of area.

Stress acting parallel to a material surface, arising from transverse forces, torsion, or direct shear. Average shear stress is force divided by area; transverse shear in beams peaks at the neutral axis.

A graphical method for transforming stress or strain components between coordinate axes, showing principal stresses and maximum shear stress. Its center lies at the average normal stress and its radius equals maximum shear.

The normal stress acting on a plane where shear stress is zero, found as the eigenvalues of the stress tensor. Maximum and minimum principal stresses define the critical planes for many failure theories.

An equivalent stress combining all stress components, used to predict yielding of ductile materials under multiaxial loading. Yielding is predicted when it reaches the uniaxial yield strength.

The change in length of a material caused by temperature change, calculated as the coefficient of thermal expansion times original length times temperature difference. Restrained expansion generates thermal stress.

A test, such as Charpy or Izod, in which a notched specimen is struck by a swinging mass to measure absorbed energy and assess toughness. Results are sensitive to temperature and reveal ductile-to-brittle transition behavior.

The study of crack behavior using stress intensity factors and fracture toughness to predict whether a crack will grow. It underlies damage-tolerant design and inspection intervals for critical components.

The initial tension deliberately induced in a bolt or fastener when tightened, which keeps a joint clamped and improves fatigue resistance. It is set by torque, turn-of-nut, or stretch measurement methods.

An equation giving the critical buckling load of a slender column as pi squared times modulus times second moment of area divided by effective length squared. Valid only when buckling stress stays below the proportional limit.