Issue 41

P. Lopez-Crespo et alii, Frattura ed Integrità Strutturale, 41 (2017) 203-210; DOI: 10.3221/IGF-ESIS.41.28 204 complete behaviour [7]. However, for many engineering components, the surface may not be at all representative of the majority of the bulk of the material [8,9] and not describe accurately the complete component behaviour [10,11]. Intense hard X-ray beams at synchrotron facilities now give us the possibility of probing the bulk of engineering materials both in terms of the geometry and in terms of elastic strain. Geometrical features of the crack can be studied through synchrotron X-ray micro-tomography [12,13]. Elastic strains can be investigated through synchrotron X-ray diffraction [14,15]. Synchrotron X-ray diffraction can also be used to obtain the J-integral [16]. In this work synchrotron X-ray diffraction is used to estimate the SIF deep within the bulk of a cracked specimen. The methodology combines a mathematical model describing the elastic crack tip field and elastic strain data obtained experimentally by synchrotron X- ray diffraction. First, the material and specimen employed are described. Then, details about the experimental setup for strain measurement are given. This is followed by an explanation of how experimental data and the analytical model are combined to extract the SIF. Finally, the influence of key parameters affecting the estimation of the SIF is studied. M ATERIALS AND METHODS he tests were undertaken on a bainitic steel similar to Q1N having the chemical composition shown in Tab. 1. The material exhibited a Yield Stress σ y =690 MPa, Ultimate Tensile Stress σ uts =858 MPa and very fine grain that allowed high quality high spatial resolution strain data to be collected [17]. The bainitic steel also has a good combination of fatigue resistance and low environmental impact for applications where no energy is consumed during the use phase of the component [18]. The fatigue tests were conducted on a Compact Tension (CT) specimen of 60 mm width and 3.3 mm thickness (Fig. 1). Alloy C Si Mn P S Cr Ni Mo Cu Q1N 0.16 0.25 0.31 0.010 0.008 1.42 2.71 0.41 0.10 Table 1 : Chemical composition in weight % of Q1N steel. The balance is Fe. 3000 cycles were applied in the pre-cracking stage of the experiment at a frequency of 10 Hz. During the pre-cracking stage, the stress intensity range, ΔK, was approximately 35 MPa√m and the load ratio, P min /P max , 0.03. Plane stress conditions dominated through the thickness for all loads applied during the experiment, since each XRD measurement included information over 1.4 mm through the thickness [19]. The crack length was measured perpendicular to the loading direction from the centre of the loading holes [20]. Figure 1 : Illustration of beam and Compact Tension specimen configuration. T