An attempt has been made to characterize high‐cycle fatigue behaviour of high‐strength spring steel wire by means of an ultrasonic fatigue test and analytical techniques. Two kinds of induction‐tempered ultra‐high‐strength spring steel wire of 6.5 mm in diameter with a tensile strength of 1800 MPa were used in this investigation. The fatigue strength of the steel wires between 106 and 109 cycles was determined at a load ratio R = −1. The experimental results show that fatigue rupture can occur beyond 107 cycles. For Cr–V spring wire, the stress–life (S–N ) curve becomes horizontal at a maximum stress of 800 MPa after 106 cycles, but the S–N curve of the Cr–Si steel continues to drop at a high number of cycles (>106 cycles) and does not exhibit a fatigue limit, which is more correctly described by a fatigue strength at a given number of cycles. By using scanning electron microscopy (SEM), the crack initiation and propagation behaviour have been examined. Experimental and analytical techniques were developed to better understand and predict high‐cycle fatigue life in terms of crack initiation and propagation. The results show that the portion of fatigue life attributed to crack initiation is more than 90% in the high‐cycle regime for the steels studied in this investigation.

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.

The objective of this paper is to determine the very long fatigue life of ferrous alloys up to 1 × 1010 cycles at an ultrasonic frequency of 20 kHz. A good agreement is found with the results from conventional tests at a frequency of 25 Hz by Renault between 105 and 107 cycles for a spheroidal graphite cast iron. The experimental results show that fatigue failure can occur over 107 cycles, and the fatigue endurance stress Smax continues to decrease with increasing number of cycles to failure between 106 and 109 cycles. The evolution of the temperature of the specimen caused by the absorption of ultrasonic energy is studied. The temperature increases rapidly with increasing stress amplitudes. There is a maximum temperature between 106 and 107 cycles which may be related to the crack nucleation phase. Observations of fracture surfaces were also made by scanning electron microscopy (SEM). Subsurface cracking has been established as the initiation mechanism in ultra‐high‐cycle fatigue (>107 cycles). A surface–subsurface transition in crack initiation location is described for the four low‐alloy high‐strength steels and a SG cast iron.

Low-cycle fatigue tension-torsion tests were performed on a low-carbon steel. The formation of microcracks as a function of orientation and state of stress was studied. Quantitative measurements of microcrack density and length showed that microcracking occurred in the maximum shear direction for various stress states. Transgranular microcracks were mostly observed. An increasing amount of intergranular microcracks was observed for out-of-phase tension-torsion loading. A shear-based microcrack propagation approach incorporating a normal stress effect was shown to provide a better correlation of the damage distribution of a pure torsion test.