Issue 41

S. Beretta et alii, Frattura ed Integrità Strutturale, 41 (2017) 269-276; DOI: 10.3221/IGF-ESIS.41.36 271 The CCT and SENT geometries are shown in Fig.. 1. The field-of-view dimension is herein indicated as d , while the crack length in indicated as a and the specimen width as W . N UMERICAL ANALYSES ig. 2 illustrates the preliminary FE analysis performed to set the dimension of the finite element in proximity of the crack tip necessary to model the stress singularity determining the SIF. For a crack length of a=0.9mm , and a ratio a/W=0.3 , the FE simulations show convergence of the stress fields for element sizes in proximity of the crack tip of, respectively, 3.6µm and 2.5µm . Figure 2 : Convergence analysis for finite element model according to finite element sizes of 3.6µm and 2.5µm in proximity of the crack tip. The stress plot indicates that the dimension of the elements chosen provides the same stress distribution. Following the FE model definition, a set of FE analyses was implemented. The crack lengths analyzed range from a/W=0.05 to a/W=0.5 . Fig. 3 shows the results of the regression of the FE displacement fields for different number of terms included in the Williams expansion. The regression adopting only the first term (K) yields to precise SIF estimations only for very restricted field-of-view dimension d (less than 0.01mm ). However, the real material behavior shows local plasticity in front of the crack tip which includes the small area where the K regression provides accurate results. In addition, performing DIC measurements with such small field-of-view is a challenging task and it would involve other problems with out-of-plane displacements that affect the DIC quality. It turns out that the regression based only on the first term of the Williams expansion is not pursued, and the additional terms are required to be fitted. The SIF measurement based on the 2-terms regression is also observed to provide generally inaccurate SIF values. In particular, only for field-of-view dimensions lower than d<0.2mm the SIF calculated shows a 10% discrepancy with the FE solution. The 3-terms regression is shown to be the most accurate and stable for field-of-view dimensions raging from d>0.2mm to d=1.4mm giving an accuracy of less than 5% on the SIF estimation. F