PSI - Issue 40

S.A. Bochkareva et al. / Procedia Structural Integrity 40 (2022) 61–69 Bochkareva S. A., Panin S. V. / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction Ultrasonic welding (USW) is one of the most common methods for joining thermoplastics. In this case, high frequency low-amplitude mechanical vibrations are used to input frictional heat between contacting surfaces. This causes their local (in the contact areas) melting and subsequent formation of a permanent joint without additional external he ating Sackmann et. el. (2015) and Sánchez - Sánchez (2017). An effective approach to improve USW procedures implemented for thermoplastics is to insert intermediate (consumable) films as adhesion layers (so called ‘Energy Director’ – ED). In the USW process, an ED melts and a dense interlayer is formed connecting the joined plates (similar ones, as a rule). In some cases, materials with lower melting points, comparing to the base ones, are inserted as an ED to improve functional characteristics of the polyetheretherketon (PEEK) USW-joints. In addition, the USW procedures are optimized by varying the ED thickness and porosity, as well as loading with additional reinforcing particles, fibers, etc. Zhao et. el. (2020) and Zhu et. el. (2020). Most researchers emphasize that the adhesive interaction at the interface between reinforcing synthetic fibers and PEEK is relatively weak due to the different polarity of the filler and the matrix, the lack of reactive functional groups in PEEK, as well as smooth surfaces of the fibers Zhu et. el. (2020) and et. el. Cole (2017). As a result of the low interfacial adhesion between the PEEK matrix and fibers, they delaminate under mechanical loads. This fact significantly limits implementation of PEEK-based laminated composites. The use of prepreg (a reinforcing fabric in a thermoplastic binder) enables to fabricate laminated composites by the USW procedures. However, there is a knowledge gap in this field of science currently. To analyze complex deformation mechanisms for the laminated composites and identify key factors in their manufacturing that exert the maximum impact on the mechanical properties of finished products, typically physico mathematical models are developed and implemented based on various numerical methods. For example, the finite element method (FEM) enables to predict a wide range of phenomena through their computer simulation, including mechanical characteristics, initiation and propagation of cracks, the interfacial behavior of matrices and reinforcing particles (fibers, fabrics), viscoelastic and plastic properties, as well as the energy absorption ability. The key feature of such models is their extensibility, that is, capacity to predict mechanical reactions in any events that differ from those applied for their calibration. Most of the authors have used mainly constitutive and structural models for these purposes Guo et. el. (2021), some of which are considered below. In Chen et. el. (2021), a constitutive model has been developed for linear viscoelastic fiber-reinforced composites using a homogenization procedure in a time domain. The authors Andrajuet el. (2020) have proposed a progressive damage model in the ABAQUS FEM-based software package, consisting of the 3D Hashin failure criteria and a cohesion zone model for simulation of both intra- and inter-laminate damages for unidirectionally reinforced composites {0}20, diagonally {0/90}5s and at different angles {45/0/-45/90/0}2s. Computer simulation of the composites’ characteristics has been carried out by the authors Pérezet el. (2013) using the theory of matrix-reinforced mixing, a simplified version of the theory of sequential and parallel mixing, which does not require an iterative procedure or calculating tensor tangent stiffness. In Luccioniet el. (2006), an overall constitutive model has been developed for fiber-reinforced laminate composites as a generalization of the classical mixture theory, taking into account relations between strains and stresses in the composites and their components in the main directions of the material symmetry. A nonorthogonal constitutional model has been developed by the authors Peng et. el. (2005) to characterize the material anisotropic behavior in fabric-reinforced composites at high strains. Also, a state-of-the-art multi-scale model applied to reinforced polymers is presented in LLorca et. el. (2013). In this case, computer simulation is carried out by calculating properties of a single object (for example, a layer) at its dimensional scale. Then, the obtained results are summarized into a constitutive model and transmit this data to the next large-scale level to determine the mechanical behavior of a larger object (a laminate composite, as an instance). So, this approach enables to consider various scales upon simulation. In this research, a similar one is implemented to predict the tensile strength of welded lap joints of PEEK plates. At the microlevel, functional properties are determined for the PEEK-based prepreg reinforced with carbon fibers (CF), taking into account their micron diameters, as well as different adhesion levels between the binder and CF. Next, the obtained properties are used at the macrolevel to simulate the USW process for the PEEK plates, since