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Cyclic stresses can occur in the form of a stress varying between positive and negative values of equal amplitude, or as a stress varying between zero and a certain maximum value. Cyclic loading of the latter kind can occur only in compression or only in tension. Depending on the manner of loading, fatigue strength can be considered as bending fatigue strength, tension-compression fatigue strength or torsional fatigue strength. Structural elements which have to withstand only a limited number of load reversals can be subjected to correspondingly higher loads.

With stresses below the proportionality limit σE elongation increases in direct proportion to the stress σ (Hooke’s law). Stress σ σ The ratio ———————— = —–E = E is termed the elasticity modulus. εE Elongation ε E is an imagined stress serving as a measure of the resistance of a material to deformation due to tensile or compressive stresses; it is valid only for the elastic region. e. 01 % of the measured length of the test specimen. 36 1 If the stresses exceed the yield point σs, materials such as steel undergo permanent elongation.

Referred to plotted axis. 1 Area of polygons Regular polygons (n angles) The area A, length of sides S and radii of the outer and inner circles can be taken from Table 1- 23 below. 885419 · 10 –11 F/m relative permittivity electric capacitance electric current electric current density specific electric conductivity specific electric resistance electric conductance electric resistance electromotive force C V/m C/m2 V V F/m F/m 1 F A A/m2 S/m ϕ ε εo εr C I J x, γ, σ ρ G R θ Ωm S Ω A Table 2-2 Mathematical symbols for magnetic quantities (general) Symbol Quantity - Sl unit Φ magnetic flux magnetic induction magnetic field strength magnetomotive force magnetic potential permeability absolute permeability, µo = 4 π · 10–7 · H/m relative permeability inductance mutual inductance Wb T A/m A A H/m H/m 1 H H B H V ϕ µ µo µr L Lmn 51 Table 2-3 Mathematical symbols for alternating-current quantities and network quantities Symbol Quantity Sl unit S P Q D apparent power active power reactive power distortion power phase displacement load angle power factor, λ = P/S, λ cos ϕ 1) loss angle loss factor, d = tan δ impedance admittance resistance conductance reactance susceptance impedance angle, γ = arctan X/R W, VA W W, Var W rad rad 1 rad 1 ϕ ϑ λ δ d Z Y R G X B γ Ω S Ω S Ω S rad Table 2-4 Numerical and proportional relationships Symbol Quantity Sl unit η efficiency slip number of pole-pairs number of turns transformation ratio number of phases and conductors amplitude factor overvoltage factor ordinal number of a periodic component wave content fundamental wave content harmonic content, distortion factor increase in resistance due to skin effect, ζ = R~ / R — 1 1 1 1 1 1 1 1 1 1 1 1 1 s p w, N ü m γ k ν s g k ζ 1) Valid only for sinusoidal voltage and current.