The Preloading Behavior And Strength Of Bolted CFRP Laminate Joints With Interference-fit

Composite materials are already widely used in the aerospace industries where their high specific strength and stiffness make them ideally suited to reduce weight. But a major weakness of composite structures is that mechanically fastened joints are often weak parts of the structure. It is therefore important that they are properly designed from static, fatigue and damage tolerance points of view. In the past, a significant amount of research, based on the numerical and experimental study, has shown the beneficial effects of interference-fit or bolt clamping on the composite structures. For composite structures, it has been shown that the ultimate bearing strength is almost two times with increasing clamping force,and the samples with 1% of interference-fit have approximately 6 times longer average fatigue life than the samples with 0% transition-fit. However, when the combination of the two techniques(interference-fit and bolt clamping) is used in composite joints, it will probably lead to some negative influence on structural strength instead of the enhancement. The stress state and strength of bolted joints with interference-fit in composite structures exhibit complex non-linear because of the combined effect of interference-fit and bolt clamping.The aim of this paper is to investigate the combined effect of interference fit and clamping force on the mechanical behavior and strength of composite joint. To do so, analytical, experimental and numerical studies have been conducted. The key research works covered in the dissertation are as follows.(1)A compact analytical model is developed to predict the bolt-to-laminate friction force for the CFRP interference-fit joint. Based on the complex functions of stress distribution, Lekhnitksii’s model is extended into the friction calculation by using Coulomb friction criteria. An experiment of interference-fit bolt inserting is conducted to investigate the mechanical behavior, as well as the finite element analysis(FEA). Good correlation is observed between these results of friction force, when the value of interference-fit is lower than 1%.(2)A method for analyzing the clamping force of composite joints with interference-fit is presented. The experiment of clamping force measurement is conducted using interference-fit bolt applied with different tightening torques and interference-fit percentages. The values of clamping force are obtained from two interfaces, one is between nut and lower plate, and another is between bolt head and upper plate. Based on the experimental data, the torque coefficient related to tightening torque and clamping force of the nut side has been calibrated, and the function of relationship between them has been established. A three-dimensional FEA simulation of the clamping force state is carried out, and the results show a good agreement with the experimental data. The results obtained from experiment and FEA show that the clamping force on the two interfaces is not the same, as there is a bolt-to-laminate friction force induced by the inference-fit. The differences of the clamping forces is dominated by the friction force, the maximum of the differences is equal to the friction force.(3)An analytical solution is presented for determining the stress state in bolted composite joints with interference-fit. The clamping pressure in the conical clamping region is formulated in terms of the clamp load by using elastic semi-inverse method Based on the anisotropic property of composite, an analytical model to predict the stresses distribution around a bolted hole is proposed. The model combines an equivalent spring stiffness model and a potential transform, and it takes into account the combined effect of interference-fit and bolt-clamping. The accuracy of the analytical model is validated through comparison with FEA results. The analytical and FEA methods have been used in a parametric study to illustrate the influence of interference-fit percentage and clamping force on the stress distribution.(4)The experimental and FEA investigations are conducted in order to study the strength and failure of bolted composite joints with interference-fit. A progressive damage model(PDM) for simulating the failure and response characteristics of bolted composite joints under tensile loading conditions is developed. The model was implemented using the ABAQUS subroutine USDFLD. To predict the progressive ply failure, the analysis combines the three-dimensional Chang–Lessard stress failure criteria and álvaro material properties degraded criteria. Different combinations of interference-fit percentage and tightening torque are selected to point out the influence on the damage propagation and failure load under tensile loading experimental conditions. The accuracy of PDM is validated through comparison with experimental results. The load versus displacement curves have proven the ability of the implemented progressive damage approach to follow the trend of experimental results until final failure especially for joints with a bolted composite joint with interference-fit. The results obtained from experiment and FEA show that the average failure load has been improved when clamping force is applied. Two opposite tendencies for the change of interference-fit have been found, depending on whether clamping force is applied.