This paper studies the thermal effects associated with the propagation of a fatigue crack in a gigacycle fatigue regime. Ultrasonic fatigue tests were carried out on a high-strength steel. The temperature fields measured by infrared thermography show a significant and very local increase in the temperature just before fracture. In order to better understand these thermal effects and to make a connection with the initiation and the propagation of the fatigue crack, a thermomechanical model is developed. The fatigue crack is modeled by a circular ring heat source whose radius increases with time. The numerical resolution of the thermal problem allows determination of the time evolution of the temperature fields in specimens and shows a good correlation with experiment. These results provide experimental proof that in a very high cycle regime, the propagation stage of the crack constitutes a small part of the lifetime of the specimen.

A B S T R A C T In this paper, the study of the temperature variation during fatigue tests was carried out on different materials (steels and aluminium alloys). Tests were performed at ambient temperature using a piezoelectric fatigue system (20 kHz). The temperature field was measured on the surface of the specimen, by means of an infrared camera. Just at the beginning of the test, it was observed that the temperature increased, followed by a stabilization which corresponds to the balance between dissipated energy associated with microplasticity and the energy lost by convection and radiation at the specimen surface and by conduction inside the specimen. At the crack initiation, the surface temperature suddenly increases (whatever the localization of the initiation), which allows the determination of the number of cycles at the crack initiation and the number of cycles devoted to the fatigue crack propagation. In the gigacycle fatigue domain, more than 92% of the total life is devoted to the initiation of the crack.So, the study of the thermal dissipation during the test appears a promising method to improve the understanding of the damage and failure mechanism in fatigue and to determine the number of cycles at initiation.Keywords infrared pyrometry; number of cycles at crack initiation; temperature recording; very high cycle fatigue. N O M E N C L A T U R Eb = burger vector E = Young's modulus K eff = effective stress intensity factor K = stress intensity factor σ = stress amplitude σ y = yield stress σ y = cyclic yield stress

In the very high cycle regime (N f > 10 7 cycles) cracks can nucleate on inclusions, "supergrains" and pores, which leads to fish-eye propagation around the defect. The initiation from an inclusion or other defect is almost equal to the total crack growth lifetime, perhaps much more than 99% of this lifetime in many cases. Integration of the Paris law allows one to predict the number of cycles to crack initiation. A cyclic plastic zone around the crack exists, and recording the surface temperature of the sample during the test may allow one to follow crack propagation and determine the number of cycles to crack initiation. A thermo-mechanical model has been developed. In this study several fish-eyes from various materials have been observed by scanning electron microscopy, and the fractographic results analyzed as they related to the mechanical and thermo-mechanical models.

The deformation heterogeneity related to the Portevin-Le Chatelier effect has been studied for aluminium-copper alloy during a tensile test at ambient temperature. Plastic deformation is accompanied by dissipation of mechanical energy into heat. To observe the localization of deformation in bands, we used a pyrometer coupled to an infrared camera. This device serves to visualize the formation of the deformation bands and to quantify apparent velocity. From these thermal data, we can also determine the geometrical characteristics of the bands: orientation, bandwidth, and space between two bands. The aperture time (1500 s) and the acquisition frequency of the camera (60 Hz) allow the bandformation time and the middle-band time to be estimated. The measurement of the variation in temperature during the appearance of a band is used to quantify the plastic deformation in the band.