BEM PCE Part B Mechanical key terms

Internal force per unit area within a material, calculated as load divided by cross-sectional area. It is expressed in pascals or megapascals and determines whether a component will yield or fracture.

The fractional change in length or shape of a material caused by applied stress. It is dimensionless and is calculated as change in length divided by original length.

The ratio of tensile stress to tensile strain in the linear elastic region of a stress-strain curve. It measures a material's stiffness and is expressed in gigapascals.

The stress level at which a material begins to deform permanently, marking the end of elastic behaviour. It is a key design limit for ductile materials such as mild steel.

The maximum stress a material can withstand before necking and eventual fracture. It is determined from a tensile test and is higher than yield strength for ductile metals.

The ratio of a material's failure strength to the actual working stress applied. Values above one provide margin against uncertainty in loads, material properties, and analysis assumptions.

Progressive damage and cracking of a material under repeated or fluctuating stress cycles below its yield strength. It causes most service failures in rotating and cyclically loaded machinery.

Localised increase in stress near abrupt changes in geometry such as holes, notches, or fillets. The concentration factor multiplies nominal stress and drives crack initiation.

The internal moment that resists rotation of a beam section under transverse loads. It is plotted on a bending moment diagram and used to calculate bending stress.

The internal transverse force acting along a beam cross-section at a given point. Its variation along the span is plotted as a shear force diagram used in beam design.

A geometric property describing how a cross-section's area is distributed about an axis, governing its resistance to bending. Also called moment of inertia in beam theory.

The ratio of a section's second moment of area to its distance from the neutral axis. It allows direct calculation of maximum bending stress from a bending moment.

Twisting of a shaft or member caused by an applied torque. Torsional shear stress depends on torque, shaft radius, and polar second moment of area.

A geometric property of a circular cross-section that quantifies resistance to twisting about its longitudinal axis. It is used with torque to compute torsional shear stress.

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

The effective length of a compression member divided by its least radius of gyration. High values indicate buckling-prone columns and guide the choice of design method.

Internal stress that tends to shorten or crush a material, produced by loads pushing toward each other. It is treated as negative in many sign conventions.

Stress acting parallel to a material surface or cross-section, tending to cause sliding between adjacent layers. It arises in bolts, rivets, and shaft torsion.

Change in length or volume of a material caused by temperature change, proportional to the coefficient of thermal expansion. It can generate large restrained thermal stresses.

A material's resistance to localised plastic deformation such as indentation or scratching. Common scales include Brinell, Vickers, and Rockwell, often correlated with tensile strength.

The ability of a material to absorb energy and deform plastically before fracturing. It is represented by the area under the stress-strain curve up to fracture.

Time-dependent permanent deformation of a material under sustained load, significant at elevated temperatures. It limits design stresses in boilers, turbines, and high-temperature piping.

A closed container designed to hold fluids at pressure significantly different from ambient. Design considers hoop and longitudinal stresses using thin- or thick-cylinder theory.

Circumferential tensile stress in the wall of a thin cylindrical pressure vessel, calculated as internal pressure times diameter divided by twice wall thickness. It is twice the longitudinal stress.

A graph plotting stress against strain from a material test, revealing elastic modulus, yield point, ultimate strength, and ductility. It is fundamental to material selection and design.

The negative ratio of lateral strain to axial strain in a material under uniaxial loading. Typical values for metals are around 0.3 and it links elastic constants.

The line within a beam cross-section where bending stress is zero and which passes through the centroid. Fibres farther from it experience higher bending stress.

The distance from an axis at which a section's entire area could be concentrated to give the same second moment of area. It is used in column buckling calculations.

Locked-in stress remaining in a component after manufacturing or loading, such as from welding or cold working. It can add to or subtract from service stresses and promote cracking.

Contact pressure between two surfaces transmitting load, such as a bolt pressing against a hole. It is calculated as load divided by projected bearing area and checked against allowable limits.