title: Structural assessment of composite bolted joints under bearing-bypass load interaction using analytical methods
Creators: name: Nguyen-Hoang, Minh
Publisher: name: UNSPECIFIED
Year: 2022

description: Bolted joints are a common means to connect thin parts due to benefits such as inexpensive manufacturing and their ability to be disassembled. These parts can be plates made of composite laminates, which is common practice for many lightweight structures such as air- and spacecraft. However, holes need to be introduced and stress concentrations arise. In structural analysis, special focus should lie on preventing fatal tension failure. This is the most critical failure mode leading to instantaneous destruction of the connection and must not occur in safety-relevant structures. Hence, precise structural assessment means are crucial to create safe and lightweight optimal designs. These means can be based on analytical methods, which are advantageous in terms of computational effort. Hence, the purpose of the present thesis is to develop an efficient and comprehensive framework for tension failure assessment of composite bolted joints using analytical means. Usually, rows of fasteners are placed in a plate. Then, one part of the load is introduced into a bolt while the remaining load stays in the plate. This problem setting is also referred to as bolted joint under combined bearing-bypass load and is treated analytically in this work using linear two-dimensional models. In doing so, the bolted joint under bearing-bypass load is idealised as a superposition of the open- and pin-loaded hole in a plate with finite dimensions. Regarding the latter setting, the bolt contact is idealised by sinusoidal radial tractions along half of the hole edge. First, the stress field of the joint’s mechanical model is determined. In doing so, use is made of the complex potential method in the Lekhnitskii formalism for stress state representation of orthotropic composite laminates. One objective is to render finite plate dimensions since narrow connections are prone for tension failure. This is achieved by an iterative calculation scheme. Therein, an important part is the development of a novel periodic arrangement technique that enables the efficient and robust implementation of stress-free edges of symmetric finite-domain problems. The results are validated against Finite Element analyses showing excellent agreement for common geometries and layups. Further, the impact of finite dimensions, material orthotropy and ratio between bearing and bypass loads on the characteristic stresses and the corresponding stress concentration factors is extensively investigated. Based on the stress results, a failure analysis is conducted, which aims to precisely predict the critical and minimal loads that lead to tension failure. This part is dedicated to quasi-isotropic laminates only. The following nonlocal concepts capable of capturing the hole size effect are employed: first, the Theory of Critical Distances (TCD), which is frequently used in industry contexts and second, the recent state-of-the-art concept of Finite Fracture Mechanics serving as reference to assess the limits of the TCD. The predictions for the special cases of open and filled holes are validated against test data and good agreement is found. Then, the failure load reduction with increasing hole and bolt diameter in the context of the hole size effect is analysed and effects by finite dimensions as well as the ratio between bearing and bypass load are investigated. Furthermore, the sustained bolt loads are discussed with focus on the nonlinear load interaction. Moreover, failure envelopes that enable the engineer to graphically obtain the critical bearing and bypass stresses are provided.