Generally, the shape of the S-N curve beyond 10 7 cycles is unknown except in some statistical approaches, and this is also true for the fatigue limit. In the case of a statistical approach, the standard deviation applied to the average fatigue limit is certainly not the best way to reduce the risk of rupture in fatigue. Only the exploration of the life range between 10 6 and 10 10 cycles will create a safer basis for modelling. Today, some piezoelectric fatigue machines are very reliable, capable of producing 10 10 cycles in less than 1 week. We based our research on accelerated fatigue tests which were performed at 20 kHz in the gigacyclic fatigue regime in order to study several typical alloys from the aeronautical and space industries.

There is no infinite fatigue life in metallic materials

Roles of microstructure in fatigue crack initiation

Subsurface crack initiation and propagation mechanism in high-strength steel in a very high cycle fatigue regime

Effect of inclusion on subsurface crack initiation and gigacycle fatigue strength

The S–N curve obtained from cantilever-type rotary bending fatigue tests using hour-glass-shaped specimens of high carbon-chromium bearing steel clearly distinguished the fracture modes into two groups each having a different crack origin. One was governed by crystal slip on the specimen surface, which occurred in the region of short fatigue life and a high stress amplitude level. The other was governed by a non-metallic inclusion at a subsurface level which occurred in the region of long fatigue life and low stress amplitude. The inclusion developed a fish-eye fracture mode that was distributed over a wide range of stress amplitude not only below the fatigue limit defined as the threshold for fracture due to the surface slip mode but also above the fatigue limit. This remarkable shape of the S–N curve was different from the step-wise one reported in previous literature and is characterized as a duplex S–N curve composed of two different S–N curves corresponding to the respective fracture modes. From detailed observations of the fracture surface and the fatigue crack origin, the mechanisms for the internal fracture mode and the characteristics of the S–N curve are discussed.

S–N curve characteristics and subsurface crack initiation behaviour in ultra‐long life fatigue of a high carbon‐chromium bearing steel

The objective of this paper is to determine the very long fatigue life of ferrous alloys up to 1 x 10 10 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 10 5 and 10 7 cycles for a spheroidal graphite cast iron. The experimental results show that fatigue failure can occur over 10 7 cycles, and the fatigue endurance stress S max continues to decrease with increasing number of cycles to failure between 10 6 and 10 9 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 10 6 and 10 7 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 (> 10 7 cycles). A surface-subsurface transition in crack initiation location is described for the four low-alloy high-strength steels and a SG cast iron.

Gigacycle fatigue of ferrous alloys

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.

Technical note High-cycle fatigue crack initiation and propagation behaviour of high-strength sprin steel wires

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 10 6 and 10 9 cycles was determined at a load ratio R = -1. The experimental results show that fatigue rupture can occur beyond 10 7 cycles. For Cr-V spring wire, the stress-life (S-N) curve becomes horizontal at a maximum stress of 800 MPa after 10 6 cycles, but the S-N curve of the Cr-Si steel continues to drop at a high number of cycles (> 10 6 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.

High-cycle fatigue crack initiation and propagation behaviour of high-strength spring steel wires

L’objectif de cette etude est d’adapter la fatigue vibratoire (f = 20 kHz) a la determination de la courbe d’endurance d’une fonte GS aux tres grandes durees de vie, jusqu’a 5.109 cycles. Des essais sont realises a temperature ambiante pour les rapports de charge R = -1 et R = 0. Les resultats d’endurance a 20 kHz sont en accord avec ceux obtenus a 25 Hz par Renault entre 5 × 105 et 107 cycles ; ils montrent qu’entre 106 et 109 cycles, l’asymptote presumee de la courbe de Wohler est faiblement inclinee et non pas horizontale. Dans cette etude, on a egalement mesure l’evolution de la temperature de l’eprouvette liee a l’absorption d’energie ultrasonique au cours des essais.

J. Y. Berard

Papers

Gigacycle fatigue of ferrous alloys

Technical note High-cycle fatigue crack initiation and propagation behaviour of high-strength sprin steel wires

High-cycle fatigue crack initiation and propagation behaviour of high-strength spring steel wires

Comportement en fatigue gigacyclique d’une fonte GS