PSI - Issue 65

P.B. Severov / Procedia Structural Integrity 65 (2024) 215–224

216

P.B. Severov / Structural Integrity Procedia 00 (2024) 000–000

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of accumulated damages by Zubova and Wil’deman (2019), Severov et al. (2015). The level of accumulated damages, on the one hand, is a relative concept, since it is determined from the moment of continued observation, experiment or operation. But, on the other hand, it has an absolute measurement in the form of the number of carbon fibers destroyed, the density of interlayer fractures, the length of the main crack, and other factors. Accumulated damages change the mechanical properties of a material on a macroscopic level. Such a change in the properties is usually called degradation of material properties. The fundamental mechanical property of a material is the stress strain diagram, which is an experimentally determined dependence between stress and strain. This article discusses the issues of identifying and developing the non-linearity of the stress-strain dependence by Lagas (1986) against the background of the accumulation of mechanical damages during repeated uni-axial quasi static tension test until fracture of unidirectional CFRP in the direction of highest stiffness. The continuity of inelastic deformation and destruction at the macro level is ensured by discrete acts of relative displacement of material particles that are in contact with one another (breaking bonds) at deeper scale levels (meso-, micro-), accumulation of damages of different scales and their transfer to a higher level by Makarov end Yeremin (2013). In the future, two other parts of the same experimental work will be presented: the registration of mechanical damages using acoustic emission equipment and a joint analysis of the development of the non-linearity of the stress-strain dependence and the acoustic emission registration of the damages accumulation process. Other results from experimental studies on the mechanical behavior of CFRP with simultaneous recording of acoustic emission pulse flow are presented in Severov (2016) and (2019). The test specimen was cut from a unidirectional CFRP plate, which was based on an epoxy resin binder, in the direction of carbon fiber orientation. The geometric dimensions of the specimen are: the total length is approximately 330 millimeters, the width is approximately 40 mm, the dimensions of the minimum cross-section of the working part are 26.15 by 4.06 millimeters, and the length of the working part is approximately 40 millimeters. The radius of curvature of the working part is approximately 120 millimeters. The x-axis coincides with the longitudinal axis of symmetry of the specimen. The specimen was tested in the hydraulic grippers of the INOVA IK-6033 servo-hydraulic testing machine. An external data collection system was used, based on the LabVIEW hardware and programming environment from National Instruments USA. The virtual data collection device made it possible to synchronously measure the time since the start of the experiment, the displacement of the active gripper, the force applied to the specimen, and the elongation of the specimen in the extensometer zone at a frequency of 2 Hz. The limits of the measured values are: displacement of the active gripper is ± 10 mm, force on the specimen is ± 100 kN, elongation of the specimen is ± 1 mm, measured by a 20 mm extensometer. Highly sensitive semiconductor force and elongation sensors were used. The acoustic emission recording of the damages accumulation process was conducted using equipment provided by INTERUNIS-IT RF. When testing the specimen, the loading process was performed in accordance with Fig. 1. The output signal from the cylinder plunger position sensor was used as feedback signal in the control circuit of the testing machine. In the sections of increasing displacement s ↑ , the velocity of the gripper displacement was maintained at a constant 5 microns per second. The distance between the maximum values s max of adjacent cycles is 100 microns. The holding time for both the maximum s max = const and minimum s min = const values of the displacement was 20 seconds. During the same 20-second period, the displacement decreased from s max to s min in sections of decreasing displacement s ↓ . The total stress-strain diagram is shown in Fig. 2. It contains the entire array of experimental data on strains and stresses, which were synchronously measured during loading in nine sections of increasing displacements, eight sections of decreasing displacements and sixteen sections of constant displacements. In the horizontal sections, where s max and s min are constant, the stress-strain dependencies are highly specific and differ for the upper and lower sections. It should be noted that, in each section where s max is constant, the destruction of the material continued with a decrease in the number of acoustic emission pulses. Certain acoustic-emission activity was 2. Material and testing equipment 3. Test method