Progressive damage of multiple bolt connections in thick composite plates
Authors:   , Elsisi Alaaeldin Abdelkader 
Published: April 2021

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] Bonded connections are typically preferred for joining composite member. However, when supporting relatively large forces, the thickness and/or size of a composite member can increase, which leads to large forces being transferred through the connection. In such scenarios, bonded connections could become inefficient. Thus, it may become necessary to use bolted joints. Previous research on bolted connections in composite plates depended on empirical and physical approaches. No efficient models exist to represent the 3D progressive damage in bolted thick composite laminates. Therefore, the objective of this dissertation is to develop an efficient model for simulating the progressive damage in bolted composite plates based on simple theories and continuum damage modeling. Published constitutive models were qualified and a group of new 3D constitute models were developed, implemented, and verified to find the most efficient model for the damage behavior of bolted thick composite laminates. In addition, an experimental program was developed to validate the numerical models and to provide a clear understanding of the failure response of thick composite bolted joints. Various parameters were investigated, such as stiffness, strength, clamping force, friction coefficient, and number and arrangement of blots. The experimental program was performed to investigate the behavior of multiple bolts lapped joints. Different off-the-shelf, hand-fabricated, and hybrid laminates were used in conjunction with steel plates to test the connections. Both woven fabrics and unidirectional fibers were used in the laminated plates tested in this dissertation. Hand lay-up was performed to reinforce the transverse direction of the unidirectional ultra-high modulus carbon fiber plies. Inspection of samples during testing showed that the first crack always occurred at the first bolt from the softer plate side (Composite plate), and final crack occurred at the location of the end bolt due to the lack of edge distance. It was found that due to the lateral constrain condition and the lack of secondary stresses, the double lapped joint can carry about 160% of load of the single lapped joint. Although bolted joints with woven fabric laminates exhibited lower failure stresses than the unidirectional fiber laminates, their toughness was larger. To develop an effective tool to calculate the mechanical characteristics of the composite materials, a graphical user interface (GUI) application was developed. It was found that the software is able to predict the experimental results within 5%. A nonlinear Matlab finite element code was developed to simulate the damage of laminated composite plates. A softening model based on the theory of damage mechanics was used. The GUI code was used to calculate the strength of the laminates utilized in this study, which predicted the strength of these laminates within 8%. To develop a 3D global model for bolted joints, which includes the bolts, the washers, the nuts, and all contact surfaces between all these parts, ANSYS software was used as a modeling tool. A group of phenomenal and physical material constitutive models were developed and implemented to enhance the capabilities of ANSYS material models. The developed model results were compared with existing models and existing experimental data. Continuum damage mechanics models (CDMM) provided the best comparison of all developed models. The staggered bolt arrangements in composite-steel plates were studied and compared with various bolting patterns. It was found that the staggered bolts patterns produced horizontal forces that could cause cracks in the composite material. Single and double lapped bolted joints were modeled using the developed CDMM, and the effect of clamping force was studied. It was found that using the composite plate between two steel plates enhanced the favorable effect of the clamping force significantly and decreased the compression damage in the direction of the composite thickness. The strength of the staggered lapped joints was studied in this research. A two dimensional (2D) parametric study was performed to study the effect of the number of bolts. The first bolt was found to have the maximum force, where the bearing damage was localized and initiated. Staggered bolt arrangements were found to cause significant in-plane bending, which should be accounted for during design. The maximum clamping force for a bolted joint was studied and analyzed. According to this study, it is recommended to use a washer hole diameter of less than or equal to the composite plate bolt hole diameter.