13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- understanding of mechanisms leading to crack initiation in metals and alloys in VHCF. Our strategy is first to analyze the response of metals and alloys having “simple” microstructure and deformation mechanisms but also controlled initial states, with special attention paid to the surfaces. From the existing knowledge of cyclic deformation mechanisms (High Cycle Fatigue range), we choose to study face-centered (fcc) and body centered (bcc) cubic metals and the role of two intrinsic properties of solid crystals: the tendency to wavy or planar slip and the lattice friction resistance. Second, the VHCF range is reached using ultrasonic fatigue machines. The working frequency is 20 kHz. The use of higher loading frequencies than conventional frequencies (~ 30 Hz) brings advantages (1) to reach the VHCF regime within a reasonable time and (2) to increase the dissipated power (energy per unit of time) and thus to generate temperature variations which can be detect with the current thermal measurement devices. Thus, the aim is to correlate surface dissipation field, surface relief changes due to the appearance of slip bands and plastic deformation mechanisms. The project brings together four academic partners with complementary fields of expertise and equipments (PIMM – Arts et Métiers ParisTech, LEME - Université Paris X, LMGC - Université Montpellier II, LSPM – UPR CNRS). A technical center (CETIM Senlis) is also associated to transfer the results of this fundamental work to industrial applications. Most of participating researchers have worked in the HCF fatigue field and Professors Bathias and Mughrabi who contributed to the project provided pioneering and well-known works in the VHCF range. In this paper, we focus on results got for pure copper polycrystals. First, we explain our experimental procedure. Then, we introduce the calorimetric analysis. Finally, some results are presented in order to illustrate our methodology. 2. Experimental procedure 2.1. Materials and specimens From the existing knowledge of cyclic deformation mechanisms [2-3], we choose to study the role of two intrinsic properties of solid crystals: the tendency to wavy or planar slip and the lattice friction resistance. In this project, we propose to study two classes of ductile single-phase metals with fcc and bcc structures. In both classes, we used a quasi-pure metal and alloys in order to change the ability to cross slip. Concerning bcc metals, low Fe-C, such as Armco iron, were used. Concerning fcc metals, pure copper and Cu-Zn (α-brass) alloys are good candidates. While pure Cu is known to deform with wavy slip, α-brass Cu-Zn displays planar slip [4-5]. Here, we only present results obtained on commercial polycrystalline copper CuOF 99.95% and α-brass Cu-15wt%Zn supplied by Griset Company. In order to facilitate surface observation, a new hourglass shaped specimen with flat faces was designed (Fig. 1). The specimen dimensions were determined so that all the parts, such as transmission and amplification pieces, vibrate at a resonance frequency of 20 kHz in tension-compression [6-7]. After mechanical and electrolytic polishing, the specimen surfaces were mirror finished without any residual stresses. The stress distribution along the specimen axis
RkJQdWJsaXNoZXIy MjM0NDE=