Issue 51

A. A. Lakhdari et alii, Frattura ed Integrità Strutturale, 51 (2020) 236-253; DOI: 10.3221/IGF-ESIS.51.19 241 divided into a determined number of segments. For the axial, radial and angular directions, the number of segments is given as parameters n_axis, n_tang and n_thickness. By modifying the values of these parameters, it is possible to easily enlarge or refine the mesh of finite elements in the whole of the structural element. When applying the mesh of finite elements, it is important to have on several thicknesses of the wall of the cylinder (not less than 10), because as a result of the influence of the hydrogenated medium will have changes in the mechanical properties, including the thickness of the cylinder. In this case, follow the form EF. Elements that are too thin and elongated may give a greater error. In case of gross violation of the geometry of the finite element, ANSYS will issue a warning. That is why, by increasing the number of elements according to the thickness, it is desirable to increase it and according to other directions. To solve the problem, it took three types of finite element from the ANSYS library. For the diffusion problem, was chosen element SOLID239, for the structural problem - SOLID95, and for the mesh of finite elements - MESH200. All element types have a modification of 20 nodes (hexahedra), so changing the element type to solve the corresponding problem does not add any difficulties - all the nodes of the finite element model remain in place. The finite element SOLID95 was chosen because it supports the material, for which a deformation diagram can be given. At present, this type of element is considered obsolete, but it is suitable for the resolution of some problems. The number of points on the deformation diagram must not exceed 100. Step 3: Load application and problem resolution At this point, the initial conditions and limits are defined and the problem is solved. To construct the initial deformation diagram (before the effect of hydrogen, θ (C, S) = 1), we need the values of stress strain intensities, which will calculate the stress intensities according to the equation (4). This is why, first of all, the problem of Blade is solved for a hollow cylinder in linearly elastic material for the pressures (external and internal), which we will use later in calculations under the action of hydrogen. The initial strain diagram is shown in Fig. 1. Figure 1 : Initial diagram of material deformation The solution of the problem occurs according to the following algorithm: 1. Define the boundary conditions and determine the initial stress-strain state of the structural element. Determine the value of the S parameter for each finite element. 2. Define the boundary conditions for the diffusion equation and determine the initial value of the hydrogen C concentration for each finite element. 3. Perform a new calculation: - the diffusion coefficient D for each finite element according to relation (1); - points of the deformation diagram for each finite element according to relation (4). 4. Definite the boundary conditions corresponding to a given time step for the diffusion equation and its resolution with new values of the diffusion coefficient for each finite element. 5. Define the boundary conditions corresponding to a given time step and determine the new stress-strain state and the new values of the S parameter for each finite element.