PSI - Issue 41

Mikhail Bannikov et al. / Procedia Structural Integrity 41 (2022) 518–526 Author name / Structural Integrity Procedia 00 (2019) 000–000

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loading pattern, size and type of a crack-like defect [1-3]. In connection with this problem, the problem arises of transferring the results of the experimental determination of the crack resistance of samples to the crack resistance of the use of machine and structural elements. To adequately describe the stress-strain state in the vicinity of the tip of a crack-like defect and, accordingly, the parameters of fracture mechanics and crack resistance, the following approaches can be used [4]: - Determination of the crack resistance of a non-standard sample, reflecting the stress-strain state of the considered structural element with a crack-like defect. In this case, the task is to justify the geometry and loading scheme of a non-standard sample with a crack, as well as the calculation formulas for determining the parameters of the fracture mechanics of this sample for calculating the crack resistance according to the experimental data [5]. - Creation of a model that allows transforming the crack resistance of a standard specimen with a crack into the crack resistance of a structural element with a crack or a cutout by introducing an additional criterion parameter into the model that reflects the features of the triaxial stress state (or local constraint of deformations) in a small neighborhood of the crack tip. At the same time, basic criterion equations are developed that contain both the characteristics of crack resistance, determined on standard samples with a crack, and the local parameters of the triaxial stress state in the vicinity of the crack tip (notch) of the studied critical element of the system. The stages of the transition from damage to failure can be divided into several stages according to the degree of locality, which can be interpreted on the basis of self-similar solutions that reflect the distribution of stress and damage fields. Damage occurs at the first stages of material deformation, and destruction is associated with the development of cracks. At the first stages of destruction, microcracks appear, and the localization of damage leads to their formation. Material damage processes are concentrated at the crack front, in the so-called “process zone”, which ensures crack propagation. Understanding the physics and mechanics of the transition from damage to failure makes it possible to create models and formulate fracture mechanics criteria that reflect the relationship between the micromechanisms of damage development and the staging of failure In this paper, an analysis of acoustic emission signals and DIC data during cyclic testing of unidirectional composites is envisaged in order to determine the nature and stages of damage accumulation during testing for further model building and determination of PCM failure criteria. This makes it possible to form criterion equations for describing the generalized crack resistance diagram [6-8]. 2. Materials and experimental conditions We studied unidirectional polymer composite materials (PCM) in the form of a strip 2 mm thick, 20 mm wide and 200 mm long. Cyclic loading of PCM specimens was carried out on an electroresonant type testing machine - Testronic-50 with a maximum allowable working load of 50 kN (Fig. 1). During the tests, standard samples - 6 (Fig. 1) were installed in hydraulic grips for flat samples. The grips ensure that the plane of the sample and the vector of application of static and dynamic loads coincide. The tests were carried out using an air-cooled fan, and were accompanied by recording the temperature field of the sample using an infrared camera NEC TH9100 WR - 2 (Fig. 1). The evaluation of dynamic deformation fields was carried out using the digital image correlation (DIC) method implemented in the LaVision - 4 system (Fig. 1). To do this, before testing, a speckle structure was applied to the front surface of the sample. Acoustic emission signals were recorded using piezoceramic microphones - 7 by Vallensysteme, model AE204A with a frequency measurement range of 180-700 kHz, an operating temperature range of -20 to +80 ºС and a capacitance of 46 pF. The overall dimensions of the sensors used are 8x18 mm, the weight of the sensors is 5 grams (Fig. 1, Fig. 2, b). The microphones were connected to the signal processing unit through standard preamplifiers from Vallensysteme, model AEP4, with a gain of 34 dB. The preamplifier supply voltage is +24V DC (25 mA) supplied via the signal cable. Preamplifier bandwidth from 2.5 kHz to 3 MHz.