Issue 41

S. Seitl et alii, Frattura ed Integrità Strutturale, 41 (2017) 323-331; DOI: 10.3221/IGF-ESIS.41.43 324 I NTRODUCTION haracterization of crack tip stresses has been an area of active research for many decades; see short state-of-art in [1, 2] presenting an over-deterministic least squares technique to evaluate the mixed-mode multi-parameter stress field by photo- elasticity underlining the fact that the use of a multi-parametric representation is not just for academic curiosity but a necessity in some cases of engineering interest. This fact is also underlined by Ayatollahi et al. in [3] who used displacement fields obtained from finite element analysis and provided a specific algorithm for fast determination of the unknown parameters. In [4] the Digital Image Correlation technique (DIC) was applied to study the fracture properties of a commercial NiTi pseudoelastic alloy. The near crack tip displacement field of a single edge specimen was measured and the stress intensity factor (SIF) was estimated by using a proper fitting procedure based on the Williams series expansion. The effects of higher-order terms in the Williams expansion ( T -stress) were analysed for different sizes of the crack tip fitting region and the results were compared with analytical predictions. The aim of the contribution is to verify the stress fields near the crack tip reconstructed based on the displacement data obtained experimentally via comparison with the stress field approximations derived for the normalized compact tension specimen from hybrid elements [5]. The displacement field in the vicinity of crack tip, which is necessary for the subsequent analysis, is measured in a compact tension (CT) specimen made from Al 2024-T351. Crack tip stress tensor components are expressed using the linear elastic fracture mechanics (LEFM) theory in this work, more precisely via its multi-parameter formulation, i.e. by Williams power series (WPS). Determination of coefficients of terms of this series is performed using a least squares-based regression technique known as over deterministic method (ODM) for which displacements data obtained experimentally via optical measurements are taken as inputs. Note that this work is only a part of the extensive ongoing research of the authors on the application of this multi-parameter approach in more advanced fracture mechanics tasks. E XPERIMENTAL STUDY Material Al 2024 and method of measurement xperiments were conducted on a CT specimens which was extracted and machined in T-L direction (crack propagation along rolling direction) from a Al 2024-T351 according to ASTM E-647 [6]. Fig. 1 illustrates the specimen geometry and dimensions. The mechanical properties of the material are summarized in Tab. 1. Cyclic loading was applied then with a 100 kN Instron servo hydraulic testing machine. The specimen was pre-cracked under mode I load for 120,000 cycles at a frequency of 10 Hz, a load ratio R = 0.1. Small scale yielding conditions were met in all tests. Figure 1 : Geometry of the compact tension (CT) specimen in accordance with ASTM standard [6]. W =48 mm, B =24 mm. Young’s modulus Yield stress UTS Elongation at break 73 GPa 325 MPa 470 MPa 20 % Table 1 : Mechanical properties of Al 2024-T351 aluminium alloy. C E