Progressive damage analysis in pin joints of composite materials

Master thesis in the field of mechanical engineering (applied design):

Abstract

Analysis of progressive damage in pin connections of composite materials

The purpose of this research is to analyze the progressive damage in pin connections of composite materials. To do this, finite element software ABAQUS and FORTRAN programming have been used. The contact between the composite plate with the symmetrical porcelain layer and the elastic pin is considered. First, the stress distribution in each of the layers of the composite plate has been investigated, and then the investigation and development of different failure modes, including the compression and tensile failure modes of the matrix, fibers, lamination, and the fiber-matrix shear failure mode, have been investigated. After that, according to the zoya of the fibers through a formulation, the effective friction coefficient has been calculated in each of the sliding paths of the pin and the plate (Z, ?) and finally, the effect of the presence of friction on the load and the path of failure in each of the frictionless states, the presence of friction in the Z direction, the presence of friction in the ? direction, and the presence of friction in the ? and Z directions have been investigated. The greatest effect of friction in the presence of friction in the Z direction is related to the stretching mode of lamination and in the presence of friction in the ? direction, it is related to the fiber failure mode, which respectively increases the breaking load by 12% and 32% compared to the frictionless state.

Introduction

The devices and in general the structures that we see around us are made up of various components and parts, each of which is made in this way according to necessity. In some cases, due to construction limitations such as the dimensions and size or different types of materials used, or due to early wear of some parts and the need to replace them and various other factors, the structure is not built uniformly and the need for connections is an undeniable part. Therefore, the topic of connections is one of the important fields in the industry.

Generally, connections include permanent and non-permanent connections according to their application. Permanent joints are used when there is no need to replace the limb. This type of connection is cheaper and has the ability to withstand dynamic forces. On the other hand, in non-permanent connections, there is the ability to open and close and control the connection method, and because of this characteristic, they are much more widely used than permanent connections. Due to the high strength-to-weight ratio of composite materials [1], these materials are widely used in the industry. One of the issues that has been taken into consideration in the topic of composite materials is the issue of connecting composite materials, which is complicated to predict their distribution due to their non-isotropic nature [2]. Connections in composite materials include adhesive connections [3] and mechanical connections [4] and sometimes a combination of them [5] is used. Due to its ability to open and close, mechanical joints are more useful in the industry than adhesive joints, and therefore it has received a lot of attention from designers.

In the past years, due to the limited available parameters and the simplicity of the topics, analytical methods were used, but today, taking into account the existing complexities, the existence of various effective factors in the analysis and practical use in the industry, numerical methods, especially the finite element method [6] and its special software, have been of great interest to researchers. Using the finite element method, it is possible to investigate the types of porcelain layers, boundary conditions, friction, laxity and other factors.   

1-1-Research background

Many researches have been done in the field of stress distribution and how to break in a circular hole on a composite plate that is subjected to the load from the pin and is under the conditions of plane stress[7] and plane strain[8]. In the past decades, a large part of past articles and researches about composite material joints have been formed as experimental results, which with the expansion of composite materials in the industry, the need for analytical work has been more and more noticed by researchers.In general, the research in the field of connections consists of two parts, which include stress distribution and analysis of the desired connection, and the other part is the examination of failure and how the failure modes are created and the growth of cracks in the connection. In the field of stress distribution and connection analysis, many works have been done from the past until now, which include investigating the effects of geometry, friction, laxity, and the arrangement of layers on the stress distribution [1-6]. In the field of damage growth and established failure modes, the research has not been as extensive as before, one of the first researches conducted in the field of damage growth in composite layers was conducted by Lizard and his colleagues in 1991 [7]. This research has been done on the composite layer including a hole under compressive load. They used the progressive damage model [9] to obtain the internal and external failure modes and their expansion as a function of the applied initial load to the final load. This model includes stress and fracture analysis in which the internal stresses and strains of the layers are obtained as a non-linear finite element analysis, this analysis is based on the finite deformation theory [10] and taking into account the non-linear properties of materials and geometry. In this analysis, which includes several failure criteria and models that include the gradual change of material properties, the types and how to expand the damage caused in the material have been investigated.

In 1999, Xiao and his colleagues investigated the friction coefficient on the contact surface [8]. They showed that the coefficient of friction is dependent on the angles of the fibers in the contact surface and with the help of laboratory tests, they obtained the graph of the coefficient of friction according to the angles of the fibers with the contact surface and finally obtained the effective coefficient of friction by using the formulation.  

In 2000, a research was done by Dano on stress analysis and fracture of composite layers under load on the pin [9]. In this research, a finite element model has been used to analyze the composite layer. In the mentioned model, the contact between the surface of the hole and the pin, gradual damage and non-linear stress-strain relationships are considered. In order to investigate the gradual failure of the layers, they have used a combination of two failure criteria: maximum stress [11] and Heshin [12]. In this study, they investigated the effect of failure criteria, nonlinear shear behavior on the prediction of resistance and pin-bar displacement curve. Kermandis and his colleagues also used a model in 2000 to obtain the gradual damage caused in a screw connection under tensile load [10]. In this model, the radial resistance and radial hardness of layers with different geometries and screw locations are calculated. In this study, they analyzed hardness, fracture analysis and the gradual reduction of material properties. They have used the ANSYS finite element program for stress analysis and have also considered the desired criterion.

Kadivar and Shahi [11] in 2002 calculated the contact surface of the pin and the hole using a three-dimensional finite element model and with the help of ANSYS software along the thickness of several layers. They assumed the pin to be rigid and by applying appropriate boundary conditions, they modeled the effects of the pin on the hole and investigated the effect of the geometric shape on the contact area and stress distribution.

In 2002, Okatan, in the form of a doctoral thesis, investigated the strength of the connection by considering a two-dimensional pin connection [12]. In this analysis, the properties of the material are considered non-linear and the failure is up to the final state. It has been investigated by reducing properties. In this research, the focus is more on the effects of geometry on failure, and less attention has been paid to investigating failure modes.

In 2002, Tizerps and his colleagues investigated the progressive damage in a bolted connection under uniform tensile load [13]. They have predicted the damage and ultimate strength of the last failure mode according to the reduction of material properties according to the occurred failure mode. In this model, the connection along with the friction on the contact surface is modeled by considering the constant friction coefficient on the contact surface and is analyzed based on the Heshin fracture theory. In 2005, McCarthy and his colleagues investigated the progressive damage in the connection of several screws with different laxity [14]. They have considered the connection in three dimensions and have investigated the effect of the laxity on the forward damage by considering the laxity of the screws differently. In this research, the properties of the material are considered as non-linear and variable according to the failure modes.