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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their sizes are in centimeters being incommensurable with the CF micron diameters. Due to the FEM features, structural elements that differ in several orders of magnitude cannot be simultaneously considered explicitly at a single scale level when forming a finite-element mesh. 2. Formulation of the problem In the developed model, a welded lap joint of two PEEK plates with an intermediate prepreg layer was considered. Figure 1 shows a scheme of a computational domain for simulation of the strain behavior of the welded joint under tension. The scheme corresponded to the requirements of both ASTM D5868 and Russian state standard (GOST) R 57066.

Fig. 1 – The scheme of the computational domain for simulation of the strain behavior of the welded lap joint under tension

When simulating tension of the welded joint, the interlayer was taken as a homogeneous material with the mechanical properties obtained at the microlevel in Kosmachev et. el. (2021). Calculation of the interlayer functional characteristics was carried out for a representative volume containing explicitly beams of CF impregnated with PEEK. The cases of tension along and across CF, as well as their shear, were considered. Also, the adhesion level and discontinuities between CF and the polymer were taken into account Kosmachev et. el. (2021). According to the CF tape specification, the CF portion along one direction was 80%. Accordingly, the ratio of polymer and fibers in the unidirectional fabric was 20/80 vol.%, where 20% was free space between CF filled with PEEK (taken into account when calculating the functional characteristics of a layer). However, due to CF overlapping and touching, the matrix layer was not uniform around the fibers. As a result, discontinuities formed, whose presence caused a decrease in the strength properties of the composite. In the model under consideration, discontinuities were determined by the number of contact points. It was shown in Kosmachev et. el. (2021) that the number of contact points affected the transverse and shear moduli, as well as the tensile strength. With a decrease in the number of contact points, the elastic modulus and the strength characteristics of the composites decreased. Similarly, ultimate stresses also reduced with lowering the adhesion level. These obtained quantitative results were used as the functional properties of layers at the macro level. The left boundary of the computational domain (Figure 1) was rigidly fixed under tension. Displacements along the X(u) and Y(v) axes were equal to zero. At the right boundary, stepwise displacements along the X(u) axis were set, while they were zero along the Y(v) one. Both top and bottom boundaries were free. The imperfect contact between the prepreg interlayer and the PEEK plates was taken. In this case, the materials did not contact over all surfaces, but partially in several locations. Respectively, the number of contacting nodes was considered between a prepreg and the plates, as well as delamination stresses between these nodes. In real USW joints, these parameters determined the adhesion level. The contacting nodes were distributed evenly along the layer length. To ensure the connection of a prepreg and the plates, the equality conditions were set for displacements in the contacting nodes. In addition, the corresponding changes were made to the stiffness matrix Bochkareva et. el. (2021). Computer simulations were carried out using an author's FEM-based software package. At each step, the fracture criteria were checked, which was taken as the achievement of one of the stress tensor components of the limiting values equal to the material yield point (maximum stress criterion). The maximum strain criterion was also assessed that was considered as the strain intensities corresponding to the fracture limit. If the fracture criterion was fulfilled in an element, then the elastic modulus in it decreased by 100 times relative to the initial level and stresses were equal to zero. At the contact boundary between the interlayer and the plates, delamination stresses (along both X and Y axes, as well as shear ones) were considered to be at the adhesion level,