Experimental Investigation on the Significance of the Conventional Endurance Limit of a Spheroidal Graphite Cast Iron

Abstract

is an open access repository that collects the work of Arts et Métiers ParisTech researchers and makes it freely available over the web where possible. Abstract-Fatigue tests were performed on a spheroidal graphite cast iron in four point plane bending under constant stress amplitude and block loading conditions. The microstructure of this material has a 'bull's eyes' appearance, i.e. the spheroids of graphite are surrounded by ferrite and these nodules and ferrite zones are included in a pearlitic matrix. Scanning electronic microscope observations were carried out at different fractions of life for constant stress amplitude loadings above and below the conventional endurance limit. Non-propagating micro-cracks were observed at a stress level equal to the conventional endurance limit. These observations showed that another limit can be defined below the conventional endurance one, i.e. one below which micro-cracks were not observed to initiate in the matrix. These cracks were found to arrest at the ferrite/pearlite interface when the material was tested below this new limit. This concept was used to rationalize fatigue results from tests with loading in blocks above and below the conventional endurance limit.

Fatigue Life of a SG Cast Iron under Real Loading Spectra: Effect of the Correlation Factor Between Bending and Torsion

“…Nevertheless, a synchronism shift seems to improve fatigue strength (around a median life factor of five between tests where C = 0.04, and C = 0.94). This observation also has been done on cast iron by Grubrisic [14] and Palin-Luc [10] under sinusoidal loading conditions. For the studied material, under constant amplitude multiaxial fatigue tests (bending and torsion), the phase shift effect on the fatigue limit improves the fatigue strength, but only 7 % at 10 6 cycles [17].…”

“…After the treatment of heating and holding at 920°C for 1-2 h, slow cooling at 870°C for 3-4 h, then air cooling, the main mechanical characteristics of this cast iron are as follows: Young's modulus = 164.9 GPa, Poisson's ratio = 0.275, Proof (0.02 %) stress = 320 MPa, Proof stress (0.2 %) = 462 MPa, maximum tensile strength = 795 MPa, elongation after failure = 9 %, endurance limit in fully reversed plane bending = 294 MPa, endurance limit in fully reversed torsion = 220 MPa. This is the same material as used by PalinLuc et al [10] (see this reference for other details).…”

How to reduce the duration of multiaxial fatigue tests under proportional service loadings

“…4, are the same than those used in [4,13]. Details and mechanical characteristics of the material are given in Refs [4,13].…”

“…Based on fully reversed plane bending fatigue tests (with stress amplitude varying in blocks) on smooth specimens made of a spheroidal graphite cast iron, Palin-Luc et al [4] proves that blocks with stress amplitude below the conventional endurance limit s D , and above a threshold stress amplitude called s*, participate in damage. If the stress amplitude is lower than s*, the fatigue life is not reduced significantly.…”

“…This paper presents a technique to transform a real proportional multiaxial stress strain time sequence (recorded in-service from strain gauges stuck on a component) in a simplified one which is shorter in time than the original one and equivalent in term of fatigue life: the number of simplified sequences to fatigue crack initiation is the same as the number of original sequences. The proposal is based on the non-damaging threshold stress proposed by Palin-Luc et al [4][5][6][7] and its equivalent strain work density given to an elementary volume of material between a peak and a valley of the stress history. These concepts are summarized first hereafter; secondly, the technique to simplify an original sequence is presented.…”