numero25

B. Moreno et alii, Frattura ed Integrità Strutturale, 25 (2013) 145-152; DOI: 10.3221/IGF-ESIS.25.21 145 Special Issue: Characterization of Crack Tip Stress Field High magnification crack-tip field characterisation under biaxial conditions B. Moreno, P. Lopez-Crespo, J. Zapatero Department of Civil and Materials Engineering, University of Malaga, Malaga (Spain) A BSTRACT . This work presents a novel methodology for characterising fatigue cracks under biaxial conditions. The methodology uses high magnification Digital Image Correlation (DIC) technique for measuring displacement and strain crack-tip fields. By applying micro-speckle pattern on the metal surface it is possible to achieve high magnification for DIC technique. The speckles were created by electro-spray technique. The validity of this novel technique is demonstrated by direct comparison with standard extensometer measurements, under tension-compression and torsion conditions. In order to image the correct region, the notch effect on the fatigue life was also evaluated. K EYWORDS . Biaxial fatigue; Digital image correlation; Crack-tip strain field; Electro-spray. I NTRODUCTION umerous engineering components and structures are subjected to biaxial loading conditions. In these cases two principal stresses and strains change in time, both in magnitude and directions. However, due to biaxial experimental tests being difficult, expensive and time-consuming, the number of experimental data available in the literature is much limited than that for uniaxial fatigue. The aim of this work is to develop a methodology capable of characterising crack-tip fields under biaxial conditions. A procedure to enforce the crack initiation at a specific location without changing the fatigue life is explained. Then a new method for measuring displacement and strain crack-tip fields on fatigue cracks grown following the previous procedure is shown. This method is based on high-magnification Digital Image Correlation (DIC) technique. Surface finish required by DIC was achieved by electro-spray technique. M ATERIALS AND METHODS he material investigates was St-52-3N steel with the chemical composition shown in Tab. 1. This is a low carbon steel often employed in structural applications. Alloy C Si Mn P S Cr Ni Mo St-52-3N 0.17 0.225 1.235 0.010 0.0006 0.072 0.058 0.16 Table 1 : Chemical composition in weight % of St-52-3N steel. The balance is Fe. Tubular hollow specimens with the dimensions described in Fig. 1 (RHS) were used in this work. All experiments were conducted with MTS 809 servo-hydraulic loading rig, allowing applying tension-compression-torsion load. Biaxial N T

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