Mechanical and thermal parameters for high‐cycle fatigue characterization in commercial steels

Abstract

A B S T R A C T In the present paper, the fatigue behaviour of two commercial steels has been investigated by means of thermal and mechanical measurements on specimens during fatigue testing.In particular, the investigated parameters are the surface thermal increment of the specimen and the hysteresis loop area along with the corresponding integrals of the two parameters for a number of cycles. An analytical description was firstly presented, focusing on the relations between surface temperature and hysteresis area. To both verify the theoretical relationships and emphasize the fatigue damage behaviour, an experimental procedure has been set up for mechanical and thermal parameters monitoring. Results have been finally compared in terms of fatigue limit estimation and fatigue parameters evolution.A h = subtended area by the hysteresis loops evolution to a specified number of cycles, MPa A shl = hysteresis loop area when loops evolution stabilizes, MPa A ΔT = area subtended by the thermal increment profile to a specified number of cycles for each loading block, K s or°C c = steel specimen specific heat, J (K kg) À1 dE = differential value of macroscopic alternate strain d 1 = intrinsic dissipation heat source, J (s m 3 ) À1 e d 1 = mean intrinsic dissipation heat source during a load cycle, J (s m 3 ) À1 E = Young's modulus, MPa E 0 = macroscopic alternate strain component f r = fatigue test frequency, Hz N b = number of cycles per loading block N f = number of cycles to failure for a fatigue tested specimen R m = ultimate tensile stress, MPa R p02 = elastic limit, MPa s = fatigue limit scatter, MPa t = time, s T l = oscillation period during fatigue test, s α = hysteresis cycle area, J m À3 ΔT stab = superficial thermal increment in the stabilization zone, respectively K or°C Δσ = stress amplitude increment in step loading fatigue tests, MPa Δ(∑ 0 ) = global dissipated energy density in a cycle due to microplasticization activation, J m À3 ε(t) = instant strain value ρ = steel density, kg m À3 σ aCAL = stress amplitude in constant amplitude loading fatigue tests, MPa σ ai = first loading block stress amplitude in step loading fatigue tests, MPa