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

Cutting temperature and heat generated at the tool-chip interface during high speed machining operations have been recognized as major factors that influence tool performance and workpiece geometry or properties. This paper presents an experimental setup able to determine the temperature field in the cutting zone, during an orthogonal machining operation with 42 CrMo 4 steel. The machining was performed with a gas gun, using standard carbide tools TiCN coated and for cutting speeds up to 50 ms -1 . The technique of temperature measurement was developed on the principle of pyrometry in the visible spectral range by using an intensified CCD camera with very short exposure time and interference filter at 0.8 µm. Temperature gradients were obtained in an area close to the cutting edge of the tool, along the secondary shear zone. Effects of the cutting speed and the chip thickness on the temperature profile in the chip were determined. Maximum chip temperature of about 825 °C was found, for cutting speed close to 20 ms -1 , located at a distance of 300 µm of the tool tip. It was established that this experimental arrangement is quite efficient and can provide fundamental data on the temperature field in materials during orthogonal high speed machining.

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. a b s t r a c tThe surfaces of commercially pure polycrystalline copper specimens subjected to interrupted 20 kHz fatigue tests in the very high cycle fatigue regime were investigated. The stress amplitude needed to form the early slip markings was found twice lower than the stress amplitude required to fracture which confirmed the results obtained by Stanzl-Tschegg et al. (2007). Three types of slip markings were classified according to their morphology and their location in the polycrystalline material. They are compared to slip markings observed during fatigue tests at frequencies lower than 100 Hz and numbers of cycles lower than 10 7 . For 20 kHz fatigue tests, stress amplitudes ranging from 45 MPa to 65 MPa produce straight and long early persistent slip markings located along twin boundaries. Stress amplitudes lower than 45 MPa produce clusters of fine early persistent slip markings mainly located at triple junctions.

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.

An experimental method is presented in this paper to measure flash temperatures of sliding surfaces. High sliding velocities are reached by using a ballistic setup equipped with a high speed camera. The temperature field on the friction surface was recorded during the process. Tests were conducted under dry sliding conditions by using an identical material for the rubbing bodies, which are of middle hard steel (C22). Experiments showed that the temperature distribution generated by frictional heating is made up of small hot spots that correspond to the friction of asperities located on the sliding surface during very short time. Deduced from observations, maximum local surface temperatures can exceed about 1100 • C around an area less than 100 m in diameter.

The dynamic strain aging phenomenon which occurs in some materials under certain temperature and strain rate conditions can cause a localization of the plastic strain in the form of Portevin-Le Châtelier (PLC) bands. According to the strain rate and the temperature, the spatiotemporal pattern of these bands changes and three main types of band are identifiable (types A-C). The increment of plastic strain associated with each band causes an increment in temperature which can be measured by infrared pyrometry. This experimental technique was used in this paper to study the PLC phenomenon in an aluminium-copper alloy at ambient temperature and to visualize the strain localization patterns. Some characteristics of these bands are measured: bandwidth, slope angle, increment of plastic strain in the band, time between two band formations, time of band formation, propagation velocity of the bands, band spacing... The evolution of these characteristics according to the strain enables us to suggest an explanation of the type A-type B transition.

During the cutting process, the temperature field in the chip is measured by using the principle of pyrometry in the visible spectral range. The mechanical device developed to reproduce orthogonal cutting conditions and to reach very high cutting speed (up to 120 m/s) is used for a range of velocities from 10 to 70 m/s. The presented experimental results concern two materials chosen following the form of chip generated: a low carbon steel (C15) and a low alloyed medium carbon steel (42CrMo4). The performances of the measurement setup are completed by the possibility of recording real time photographs of the chip formation. These records make the analysis of temperature maps easier and allow specific parameters as the contact length at the tool-chip interface or the shear angle to be determined. The non-uniform heating in the chip is emphasized by the presence of a maximal temperature area. The temperature fields measured for a cutting speed around 20 m/s present maximums of 870 1C for 42CrMo4 and 630 1C for C15 located near the tool-chip interface. The effects of cutting velocity on the maximum temperature value in the chip and the location of this heat zone are presented. This maximum increases with the cutting velocity contrary to its location which presents few variations. The experimental results are compared with an analytical approach. r