PSI - Issue 41

Yu.G Matvienko et al. / Procedia Structural Integrity 41 (2022) 192–198 Author name / Structural Integrity Procedia 00 (2019) 000–000

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Fig. 1. (a) Scheme of the specimen; (b) measurement parameters.

The loading program has the following parameters: stress range 350 MPa; stress ratio –0.52; maximal remote stress ��� = 230 MPa; minimal remote stress ��� = –120 MPa. Thus, cyclic uniaxial tension-compression is performed in the remote stress interval –120 � � � 230 MPa with a zero starting point and a positive zero half cycle. The parameters of loading cycles are most typical for high loaded components of pin/rivet joints of aircraft structures. Complete failure of the specimen occurred after 1480 cycles. The experimental technique includes the following main steps. The specimen is mounted into grips of universal servo-hydraulic testing machine. Quantitative description of local strain distributions inherent in different stages of low-cycle fatigue is the main goal of this study. That is why step-by-step loading of the specimen and simultaneous recording of reflection holographic interferograms must be performed to obtain the fringe pattern sets, which are essential to completely describe local deformation process at each chosen loading cycle. This is a consequence of the high sensitivity of the holographic interferometry method. Quantitative characteristics of local deformation process have been obtained for the 1st, 2nd, 3rd, 14th, 218th, 521st, 1017th and 1418th loading cycles. Each loading step, at which specific reflection holographic interferogram is recorded, is defined by remote stress increment ∆ � � � � ��� � � � . The measurement procedure involved provides uncertainties 0.3 μm for in-plane displacement components u and v and 0.15 μm for displacement components w , which are normal to the surface of the specimen. Typical interference fringe patterns obtained at the 521st and 1418th cycles are shown in Fig. 2. Initial experimental information represents the values of all three displacement components, which are determined at 16 uniformly separated points along the hole edge. The distribution of the circumferential strain � along the circular hole boundary of � radius, when the input data are expressed through the displacement components u and v in the Cartesian coordinate system, has the following form: � � � � � � � � � � ��� � � � � � � ��� �� , (1) where u , v are in-plane displacement components along the hole edge; polar angle φ is counted from tension compression direction ( x -axis) anti-clockwise (see Fig. 1).