Investigating three-dimensional interlayer stresses in mechanical connections in layered composite materials

Master thesis in mechanical engineering (applied design)

Abstract

Investigation of three-dimensional interlayer stresses in mechanical connections in layered composite materials

The purpose of this thesis is to investigate the interlayer stresses in three-dimensional form in mechanical connections in layered composite materials. For this purpose, finite element method and ABAQUS software were used. The stress field in each layer of the composite sheet has been obtained, and the contact pressure between the pin and the sheet and the radial displacement around the hole have also been calculated. In the following, its separation is examined in the paper. At first, interlayer stresses and contact pressure have been obtained without considering the friction on the contact surface between the screw and sheet for symmetrical layering in three different layers. After that, the effect of friction between the screw and the sheet has been evaluated. Then, the separation between layers has been studied for two layers, and the effect of friction between the screw and sheet on the separation has been evaluated. The results show that neglecting friction on the contact surface leads to 50% error in the amount of stresses and 46% in the applied force to cause separation. rtl;">Most machines and devices are composed of several members that are connected to each other with various connections. Easier manufacturing and servicing of parts is one of the main reasons for making machines or devices with more than one member. Connecting members, like a normal screw, can be used in different cases, and in most of these cases, connecting members are responsible for transferring incoming loads. The idea of ??repeating the use of connecting members becomes important when we know that, for example, a car has more than ten thousand parts, a machine tool has more than twenty thousand parts, and a rolling mill has more than one million parts.

Connections can be permanent or non-permanent. The choice of this topic depends on the purpose of using the connection and economic issues. It is only possible to use permanent connections once, but these connections are cheaper and bear dynamic forces more easily, so this type of connection can be safely used where strength is desired. Non-permanent joints have more reusability, but they are not very suitable for bearing various incoming loads.

Nowadays, composite materials [1] have shown their importance in the industry, and therefore its issues should be examined more carefully. One of the most important issues is connecting multi-layer composite parts [2]. In general, there are three main groups of connections in multilayer composite panels: adhesive connections [3], mechanical connections [4], a combination of adhesive and mechanical connections [5]. Mechanical connections are widely used in the industry, especially in aerial structures. Therefore, designers have paid special attention to it for a long time. Determining the ultimate strength and the type of connection rupture necessitates stress analysis at the location of the pin, which has a high stress concentration.

Considering that conducting simple and basic analyzes cannot be a suitable criterion for determining stress concentration points, stress distribution in connections and determining inter-plane stresses, many researchers have turned to complex analytical methods. Although the aforementioned complex methods examine more parameters such as the issue of rivet contact with the sheet in such connections and deal with their quantitative and qualitative effect on the behavior of the connection, but their practical use is not possible. Accordingly, in recent years, with the expansion and progress in the industry of computer hardware and software production, the use of numerical methods, especially the finite element method and its special software, has become an increasing trend. By using the finite element method, complex geometries, various loadings and various boundary conditions can be modeled and analyzed in these connections.

1-1- Review of past works

The issue of stress concentration in a circular hole on a sheet that is subjected to a load from a pin and is under plane stress [6] or plane strain [7] is one of the classic problems in the theory of linear elasticity.

In the years between 1940 and 1950, relatively good analytical studies were carried out on adhesive joints in metals, and then this research was applied to composite materials in the late 1960s. But the mechanical connections of composite materials were less considered analytically. After the growth and development of composite materials in the industry, especially the aviation industry, the design and analysis of mechanical joints in composite materials received the attention of researchers.

In general, in the last three decades, many articles have been presented in the field of mechanical joints in composite materials, a major part of the articles, by the experimental results of the effects of temperature and humidity, the order of layers, geometric properties, tolerance and slack between pin and hole [1 and 2] Analytical methods of a smaller part He has devoted himself to research in the field of mechanical connections. In these cases, two-dimensional models were used and finally it was obtained by simplifying the problem of stress distribution around the hole [3 and 4]. In another part of the articles, numerical methods were used to predict the stress field in mechanical connections [5-8]. In general, the most important step in numerical methods is stress analysis and obtaining the stress distribution diagram around the hole, because various parameters such as: how to model the connection (two-dimensional or three-dimensional), The way of applying the boundary conditions, laxity and friction between the pin and the hole, whether the connection is symmetrical or asymmetrical, and the order of the layers have a great impact on the stress distribution between the layers.

In terms of finite element modeling, the type of elements used in the geometry of the model in two-dimensional and three-dimensional states, the type of contact elements used and their characteristics such as the contact hardness and the degree of permeability of the elements involved, or the friction between the contact surfaces can be considered as factors affecting the behavior of connections. Non-permanent screws and pins counted. Of course, it should be mentioned that most researchers have used two-dimensional linear models of finite elements.

In two-dimensional models, three methods have been used by researchers to model pins. The first method is that the radial displacement of the nodes around the hole is constrained and the load is applied to the edge of the connection [9]. In another method, the load is applied as a cosine radial stress on the edge of the hole and the edge of the connection is bound [10]. Some researchers have also used rigid or truss elements to model the pin, where the load is applied centrally to the center of the pin [11]. It should be noted that these methods do not always show the exact answer. Therefore, in order to obtain better answers, the applied load of the pin should be modeled more realistically.

In addition to the research that has been done experimentally on the interlayer stresses and the effect of these stresses on the delamination of plates (for example, by Tosaz Daniel [8] in 1974 [18], many analytical methods were presented to determine the stress field around open holes.

But the mechanical behavior of composite plates that have joints is much more difficult. In 1997, Kamenho and Mathew [9] [19] conducted a comprehensive study on damage to plates near holes. In the meantime, many simplified models were presented for the numerical study of the stress field and failure in mechanical joints in composite plates. As an example, Dano [10] [6] investigated a two-dimensional model to predict the response of joints between composite plates under loading, which in In this context, three-dimensional analyzes were also carried out by Chen [11] in 1998 [20]. In his study, Eirman assumed elastic connections and ignored the effect of friction between composite plates. Yang [13] and Yan [14] in 2003. [5] determined a precise numerical method to investigate the mechanical behavior of composite plates with elastic connections. They assumed the joints to be isotropic and considered the contact surface of the joint and friction plates. The plates are symmetrical in their analysis.