PSI - Issue 13

Tatyana Tretyakova et al. / Procedia Structural Integrity 13 (2018) 1774–1779 Author name / Structural Integrity Procedia 00 (2018) 000–000

1775

2

Prediction of the failure process and analysis of survivability and safety of structures require the taking into account an initiation and propagation of cracks and crack-like defects during loading. It is necessary to study the conditions of stable crack growth [Lomakin et al (2016), Tretyakov et al (2017)]. Here, the behavior of bodies with stress concentrators under decreasing loading with increasing displacements (crack opening and crack length) is considered by analogy with the regularities of postcritical deformation stage in tests. Towards this, experimental investigation was carried out on the flat specimens of aluminum alloy in the form of bands with a central cut and a preliminary fatigue crack. The DIC-technique and IR-analysis were applied to observe the evolution of the inhomogeneous strain and temperature fields [Tretyakova et al (2017)], and to study the kinetics of stable crack growth in Al-Cu-Mg alloy under various stiffness during uniaxial tension tests.

2. Material and experiment procedure 2.1. Material and geometry of specimens

The material used in the study was an Al-Cu-Mg alloy. Chemical composition is shown in Table 1. The specimens in the form of plates with a central cut (see Fig. 1, a) were machined by hydro-abrasive cutting from rolled sheets to obtain four groups of specimens differing in gauge length l 0 . The thickness h of the specimen is 2.0 mm, the width b is 50.0 mm, and the gauge lengths l 0 are 50.0, 100.0, 150.0 and 200.0 mm.

Table 1. Chemical composition of aluminum material (in weight percent).

Si

Fe

Cu

Mn

Mg

Cr

Ni

Zn

Ti

Pb

Zr

0.29

0.28

4.28 0.75

1.48 0.017 0.009 0.12 0.06

0.05

0.001

2.2. Test procedure and equipment Flat specimens with a central notch and initial fatigue crack were extended with a biaxial (tension/torsion) servo hydraulic testing system Instron 8850 (100 kN/1000 N∙m, 30 Hz) at room temperature and at the strain rate of 2.67 %/min. The mechanical tests were complemented by in situ technique, such as a digital image correlation (DIC) and an infrared (IR) thermography. The experimental setup is shown in Fig. 1 (b). The displacement and strain fields were analysed by the 3D DIC-measurement system Vic-3D (by Correlated Solutions).

a b Fig. 1. Experimental set-up: the Instron 8850 biaxial servo-hydraulic testing system, the DIC measurement system Vic-3D and the infrared system Flir SC7700M (a); two surfaces of specimen for DIC and IR analysis (b). The images were captured with two CCD cameras (Prosilica, 16 Mp resolution) at a frequency of 3 Hz, using a commercial software package called ‘Vic-Snap’. By using of the ‘Vic-3D’ software module ‘virtual extensometer’, it was possible to evaluate the crack opening value and to estimate inhomogeneity due to the initiation and evolution of macroscopic cracks as well.