Issue 37

M. Mokhtarishirazbad et alii, Frattura ed Integrità Strutturale, 37 (2016) 114-123; DOI: 10.3221/IGF-ESIS37.16 115 DIC technique for calculating displacement fields [13]. While fracture problems can be simplified by considering mode I loading, cracks in structural materials are generally under mixed-mode loading condition [3]. Therefore, estimation of the fracture parameters based on mixed-mode loading condition will be more representative of the material fracture behaviour under the actual working condition. Different optical methods have been used for obtaining full-field information required for mixed-mode loading analysis previously. Sanford and Dally [14] have determined the mixed- mode SIFs by utilizing isochromatic fringes near the crack-tip . They have reported that employing an over-deterministic approach on the data points provided by the full filed fringe patterns led to a highly accurate SIF estimation. Displacement fields derived by DIC technique has been utilized by Yoneyama et al. [15] to evaluate the mixed-mode SIFs of a polymer (polymethylmethactylate). While they used a non-linear least square method for their solutions, Réthoré et al. has develop a method based on the Lagrangian conservation law for mixed-mode SIFs estimations. A good agreement between analytical displacement fields generated based on the Muskhilishvili’s complex function approach and the experimentally measured displacement fields (obtained by DIC) has been also reported by Lopez-Crespo et al [16]. By fitting nominal and experimental data, they have determined mixed-mode SIFs for a crack in a fastener hole. For estimating the crack closure level by DIC, virtual extensometers can be introduced behind the crack-tip for local measurements of crack opening displacement. Nowell et al. [6] have compared Moiré interferometry techniques with DIC method for studying closure levels of propagating fatigue cracks. The effect of overloads on the crack growth has been studied under uniaxial loads with a number of different experimental techniques. These include photo-elasticity [17], pulse reflection microscope [18], electronic speckle pattern interferometry [19] and synchrotron X-ray diffraction [20, 21]. However, the overload effect under biaxial conditions is much less studied. In the present paper, we use a hybrid method for estimating mixed-mode SIF from in-plane crack-tip displacements obtained by DIC. This is used for studying the effect of overload under biaxial loading. The methodology can be used to evaluate crack closure level before and after applying the overload cycle from local crack opening displacement. M ATERIALS AND METHODS rack propagation in a low carbon steel (St-52-3N) was studied [22]. Tab. 1 shows the composition of the alloy. Microstructural examination with the optical microscope has revealed ferrite and pearlite bands as vertical black and white bands respectively [23]. A schematic of the geometry is shown in Fig. 1. C Si Mn P S Cr Ni Mo 0.17 0.22 1.23 0.01 >0.0001 0.07 0.06 0.16 Table1. Chemical composition in weight % of St-52-3N steel. The balance is Fe. Yield stress, σ y 386 MPa Ultimate tensile stress, σ u 639 MPa Young’s modulus, E 206 GPa Shear Modulus 78 GPa Table 2 . Monotonic properties of St-52-3N steel. A MTS 809 servo-hydraulic loading rig coupled by a biaxial extensometer Epsilon 3550 was used to apply biaxial loads under stress control mode in a similar way to previous works [22,24]. Cyclic sinus signal with axial load ratio of 0.1 (R a = 0.1) and torsional load ratio of -1 (R t =-1) was applied while the angle between the axial and the torsional stress, φ, was set to 45°. Fig. 2 shows how φ has been defined. In order to study the effect of the overload on the crack propagation behaviour, a 40% overload was applied when the crack length for the specimen S2 was 0.669 mm. That is, the load range in the overload cycle was 1.4 times larger than load range during the rest of the test. The cyclic loading then continued until the crack length reached 1.4 mm. Tab. 3 shows the loading condition of for samples with and without overload. C