Issue 36

M. Ouarabi et alii, Frattura ed Integrità Strutturale, 36 (2016) 112-118; DOI: 10.3221/IGF-ESIS.36.11 113 complex phase ferritic-martensitic steel of 1.2 mm thickness were received for carrying out endurance tests in the very high cycle regime and crack growth tests at ultrasonic frequency (20 kHz). The method for designing the thin specimens and both the procedures for testing them at high frequency and the results are presented hereafter. M ATERIAL AND TESTING METHODS he investigated material is a complex phase ferritic-martensitic steel produced in sheet form by rolling and used for manufacturing automotive parts. The chemical composition this material is given in Tab. 1, its mechanical characteristics are in Tab. 2. These steel sheets are covered by zinc coated (Fig. 1b). The specimens were machined without removing this coating and with their longitudinal axis parallel to the rolling direction. The specimens were tested at ultrasonic loading frequency (20 kHz) with a loading ratio R= –1. All the specimens were tested as received condition. C Mn Si P 0.1674 2.004 0.225 0.017 Table 1 : Chemical composition of the ferritic-martensitic CP1000 steel (w %). Dynamics modulus (GPa) Volumetric mass (kg·m -3 ) UTS (MPa) 211 7850 1000 Table 2 : Mechanical characteristics of the ferritic-martensitic CP1000 steel. Figure 1 : a) Microstructure of the studied steel CP1000. b) Galvanized coat. The ultrasonic fatigue testing machine is composed of the following components. An ultrasonic generator transforms the electrical signal of 50 (or 60) Hz to 20 kHz. The piezoelectric converter transforms the 20 kHz electrical signal in a mechanical vibration at 20 kHz. A horn amplifies the displacement amplitude in order to obtain the required strain amplitude in the middle section of the specimen and a computer control system is used to control the different parameters of the test, such as the amplitude and to maintain the resonance frequency. Indeed, the specimen works in resonance vibration state and its center is a displacement node experiencing the maximum stress amplitude. For the calibration of the test, an optical fiber displacement sensor was used. If the resonance frequency drops outside the 19.5 – 20.5 kHz range, the system shuts down automatically. This is characteristic of crack initiation or specimen failure. Two different types of tests were carried out in this work. The first one is for determining the fatigue strength and the second one is crack growth test. The geometry of the flat specimen for the fatigue strength assessment is illustrated in Fig. 2a. The geometry for the crack propagation test is illustrated in Fig. 2b. T