Issue 36

M. Ouarabi et alii, Frattura ed Integrità Strutturale, 36 (2016) 112-118; DOI: 10.3221/IGF-ESIS.36.11 112 Focussed on Crack Paths Very high cycle fatigue strength and crack growth of thin steel sheets Mohand Ouarabi, Ruben Perez Mora, Claude Bathias Université Paris Ouest Nanterre La Defense, LEME, 50 rue de Sèvres, Ville d’Avray, France mohand.ouarabi@yahoo.fr, ruben.perez-mora@hotmail.fr, claude@bathias.com Thierry Palin-Luc Arts et Métiers Paris Tech, I2M, CNRS, Université de Bordeaux, Esplanade des Arts et Métiers, Talence, France thierry.palin-luc@ensam.eu A BSTRACT. For basic observations or for industrial applications it is of interest to use flat specimens at very high frequency in the gigacycle regime. In this work, thin flat sheet, with 1.2 mm thickness of a complex phase ferrite-martensitic steels were considered for carrying out fatigue tests at high frequency (20 kHz) up to the gigacycle regime (>10 9 cycles). The crack initiation tests were carried out with water cooling, while the crack growth test were carried out in laboratory air at room temperature. All the tests were carried out under loading ratio R=-1. To do that, special designs of specimens were made and computed using FEM for defining the stress amplitude for endurance tests. Special attachments for specimens to the ultrasonic system’s horn were enhanced. A particular FEM computing of the stress intensity range on crack growth specimens was carried out for determining the specimen dimensions and an equation that defines the stress intensity range as a function of the harmonic displacement amplitude, dynamic Young’s modulus, material density and crack length. Detailed procedures and fatigue results are presented in this paper. K EYWORDS . Ultrasonic fatigue; Plate steel fatigue; Fatigue resistance; Fatigue crack growth. I NTRODUCTION atigue in high and very high cycle regime has been studied by Bathias et al. [1] during the last 30 years. Advances in piezoelectric fatigue testing machines have allowed researchers to carry out accelerated fatigue tests in the very high cycle fatigue (VHCF) regime [2] by using adapted specimens and testing methods for obtaining suitable results at high frequency. Actually, such machines, specimens and methods have been optimized and practically standardized by means of cylindrical specimens (dog bone, hourglass) for endurance tests and block notched specimens for crack growth tests. It is even possible to tests such specimens under controlled environment or temperature [3, 4]. As actually known, the vibration system on a piezoelectric fatigue machine is composed by a generator, a converter, an amplifier (horn), and the specimen, all working in resonance allowing the application of maximum stress amplitude in the smallest cross section of the specimen located at its central part. If the raw material or the component to study is sufficiently plenty for machining cylindrical round specimens the work becomes simple. It is the same for thick hyperbolic profile crack growth specimens [4]. Conversely, for thin raw material, such as steel sheets, fatigue specimens are not common and easy to test at high frequency; there is not a lot of VHCF data in literature. In this work, thin flat sheets of a F