Composite Bolted Joints Research Articles

АНАЛІЗ РОЗПОДІЛУ КОНТАКТНИХ НАПРУЖЕНЬ У ДВОЗРІЗНОМУ БОЛТОВОМУ З'ЄДНАННІ КОМПОЗИТНИХ ТА МЕТАЛЕВИХ ЕЛЕМЕНТІВ

Bolted joining technique is the dominant joining method for CFRP load-carrying structures in aircraft. However, the weak point of such structures are the areas located in the vicinity of the fastener holes, which are the areas of increased stress concentration. In addition, drilling the holes results in structural discontinuities due to cutting material fibers. Among all failure modes of composite bolted joint, bearing failure is one of the most common type of damage. This failure is caused by the local compressive stresses acting normal to the contact surface, that leads to delamination of material due to insufficient strength of matrix. Therefore, it is extremely important to optimize the design of composite bolted joints in order to enhance the entire composite structure. To increase the load-carrying capacity of the composite joint structural parts and protect the hole walls from damage, a method for installing titanium bushing with glue into the holes of composite plate is proposed. The paper presents the results of numerical analysis of the impact of stacking sequence on the distribution of contact stresses in a double-shear bolted joint of composite plate to steel cover plates. Using the finite element method and ANSYS Workbench 2019 R3 software, a three-dimensional finite element model of a double-shear bolted joint was created, which enables to analyze the impact of the stacking sequence on the distribution of contact stresses at the bolts to titanium bushings interface in the case of uniaxial tension of the middle composite plate at the level of tensile stresses in the gross section of 100 MPa. The obtained result explains the fundamental difference in the behavior of composite and metal materials: due to the change in the orientation and sequence stacking, the stiffness of the composite element of the joint changes, which affects the ovalization of the holes and the level of local deformations of the bushings, which in turn leads to a change in the maximum contact stresses and their concentration. By optimizing stacking sequence, it was possible to reduce the level of maximum contact stresses by 1.1 times. At the same time, the degree of unevenness of the distribution of contact stresses decreased by 1.21 times. The optimal stacking sequence was one in which 37.5% of the fibers oriented at an angle of 0°, 50% of the fibers oriented at angles of ±45°, and 12.5% of the fibers oriented at an angle of 90°

Progressive bearing failure analysis and strength prediction method for the initial assembly and tensile process of composite bolted joints

Composite bolted joints have extensive application in aerospace and marine equipment. Failure analysis and strength prediction are crucial steps in the design of composite bolted joints. This study proposes a method that can accurately analyze the progressive bearing failure and static strength of composite bolted joints under different initial assembly parameters. This method first obtains an accurate initial assembly state based on the high-fidelity finite element model with detailed thread structure. Then it uses this as an input to analyze the progressive bearing failure process and the bearing strength value. Using the above method, the formation and evolution mechanism of the tightening yielding phenomenon during the assembly process are analyzed. And the influence of the two key assembly parameters, washer type and initial bolt tension, on the bearing strength of composite bolted joints is analyzed. The experimental results verify the effectiveness and accuracy of the simulation methods and results.

Prediction method for re-damage behavior of the repaired CFRP bolted joint

Composite materials are widely used in various aircraft due to their high specific strength, stiffness, corrosion resistance, and robust design flexibility. However, when subjected to complex loading conditions, composite structures are susceptible to damage, necessitating prompt repair upon aircraft return. Analyzing the re-damage process of repaired structures is crucial for assessing aircraft structural reliability, lifespan, and determining whether a repeat flight is warranted. In this study, a prediction method was proposed for re-damage behavior through meso-modeling and energy analysis of the repaired composite bolted joint. First, this article established a meso-mechanical model for composite laminates, analyzed force transfer characteristics at the fiber − matrix interface, and derived equations for stress field. Additionally, a strain energy distribution model was developed for composite bolted repaired joint near fibers and repaired area, proposing a damage prediction method based on the strain energy criterion. This method could anticipate the location, type, sequence, and magnitude of damage. Finally, simulation calculations yielded mechanical characteristics of the repaired area under typical load conditions, while loading tests verified the feasibility of the prediction method. This study offered an effective means of predicting performance degradation in composite structures and provided a vital direction for optimal spacecraft structure repairing methods and parameters.

A digital twin-based assembly model for multi-source variation fusion on vision transformer

For the manufacture and assembly of the mechanical products, quality management is the key feature to improve the service performance, reduce the overall costs and enhance the sustainability of the manufacturing systems. While, with the emergence of advanced data-driven and digital twin technologies, the Zero-Defect Manufacturing (ZDM), which corrects, predicts, and prevents product defects based on multi-sources of data, is of great significance in improving assembly quality. As the specific application for the utilization of ZDM in product assembly, the critical performance index of the assembly results is predicted by taking into account the multi-source factors such as geometric deviation of the parts, material properties, assembly sequences and process boundary conditions during assembly. In this paper, we address the high computational cost and low computational efficiency of numerical simulation methods under multi-source factors, and propose a data-driven approach named DTA-VIT based on the fusion of heterogeneous variables for digital twin assembly modeling of products. Firstly, the geometric and performance variables of the assembly process are analyzed and modelled. Secondly, a multi-source assembly data fusion network under the Vision Transformer framework is developed. This network takes the parameter space, which fuses multi-source variables from the assembly process as input and the assembly result as output. Finally, a case study of the assembly process of composite bolted joint structures in aircraft assembly is conducted to verify the effectiveness and feasibility of the proposed method. The methodology provides a solid foundation for subsequent assembly quality control and prevent by predicting assembly performance efficiently, ultimately enabling the production of high-quality products.

Experimental investigation on the effect of forced assembly on fatigue behavior of single-lap, countersunk composite bolted joints

Forced assembly is a common countermeasure for eliminating the interface gap during composite airframe assembly, which modifies the bolt-hole bearing behavior and brings failure risks. In this study, an experimental investigation on the effect of forced assembly on the fatigue behavior of single-lap, countersunk composite bolted joints is presented. The hole elongation and dynamic stiffness are obtained based on ASTM D6873. The S-N curves are fitted according to ASTM E739. The fatigue damage is observed through microscopy, and the residual strength is acquired by static tensile tests. The results indicate that forced assembly significantly shortens the fatigue life of joints. After forced assembly, the hole elongation evolves faster and reaches the fatigue failure threshold earlier. In addition, the initial dynamic stiffness is reduced, and the loss of stiffness is accelerated. Apart from the designed hole bearing failure, another failure mode, i.e., bolt head cracking, is observed under low fatigue load level (70% of 2% offset bearing static strength), leading to faster degradation of fatigue property. Under equal hole elongation (4% of the hole diameter), the joints with forced assembly display severe hole bearing damage, and visible shearing cracks are observed, lowering the residual strength.

SR-M−GAN: A generative model for high-fidelity stress fields prediction of the composite bolted joints

Predicting the stress fields induced by composite bolted joints constitutes a valuable research endeavor aimed at enhancing the structural integrity of equipment in the aerospace and maritime fields. The stress field characteristics of composite bolted joints may undergo changes due to potential mechanical behaviors, such as damage, constitutive relationships and so on, which exhibit complexity and pose challenges for prediction. Nevertheless, modeling the intricate stress field distribution directly from experimental data poses a formidable challenge, necessitating a substantial investment in materials and resources. Traditional finite element numerical methods have the capability to model the distribution of the stress fields. However, to obtain a high-fidelity solution of this distribution, it is imperative to generate a fine-grid mesh and allocate the significant computational resources. The incorporation of computational techniques in coarse −grid would enhance the efficiency of computations, but these approaches compromise the accuracy of the calculations. In this paper, an innovative deep learning framework is introduced. The network named as the SR-M−GAN is drew inspiration from the conditional generative adversarial network (cGAN) used in the image super-resolution. The primary aim of SR-M−GAN is to mitigate the intrinsic trade-off between result fidelity and computational complexity. The model proposed in this paper effectively reconstructs high-fidelity stress field data from calculations on coarse grids and exhibits excellent reconstruction metrics. Simultaneously, the model only incurs additional computational costs during the deployment of the network model and training. When compared to experimental data, the model continues to demonstrate superior performance in both accuracy and computational efficiency. Therefore, this method offers a high-fidelity data augmentation approach for stress analysis in composite bolted joints, laying the groundwork for large-scale analysis of stress field characteristics in composite bolted joints.

Fatigue life prediction of composite bolted joints based on finite element model and machine learning

AbstractThis study proposes a fatigue life prediction method for composite bolted joints, which combines algorithm optimization‐based hybrid neural networks with finite element modeling. First, based on the Hashin failure criterion of physical mechanism, a finite element model for fatigue life prediction of composite bolted joints is established, and the simulation calculations have been conducted using various initial conditions. Then, by integrating the simulation and experiment data, we have established a fatigue life database that serves machine learning training and prediction. Finally, the data undergo a comprehensive process of deep feature extraction through the utilization of a convolutional neural network (CNN). The resulting deep features are utilized as inputs for training the backpropagation neural network (BPNN) to predict fatigue life. The results indicate this synergistic combination of CNN and BPNN results in a substantial improvement in prediction accuracy and has remarkable superiority in predicting the fatigue life of composite bolted joints.

Phenomenological failure criteria analysis of a composite bolted joint under multi-axial loading using the finite element method

Carbon Fiber Reinforced Polymer Composites (CFRP) are commonly used in various sectors due to their excellent properties. However, they present complex failure modes, particularly in bolted joints, which are widely used in the aeronautical industry due to their ease of assembly and disassembly. Phenomenological failure criteria can be used to evaluate failure modes analytically and reduce the number of experimental tests. It is important to determine the criterion that best reflects reality. The Hashin, Puck, and LaRC04 failure criteria were evaluated using a finite element method software - FEMAP 2021.2 educational version - through simulation. Numerical simulations were conducted on a 2D model of a carbon/epoxy composite bolted joint under multiaxial loads, following ASTM D5661 dimensions. The failure criteria were compared to determine the most appropriate one for this type of component. Among the evaluated criteria, the Hashin criterion showed an intermediate level, while the Puck criterion was the most conservative and the LaRC04 criterion was the least conservative. It is recommended to use the Hashin criterion for general analyses. For analyses that require a detailed examination of compression failure modes, it is recommended to use the LaRC04 criterion. The Puck criterion is suggested for more conservative analyses.

Bearing failure and strengthening mechanism of countersunk thin-ply laminated joints using an interference-fit bolt

By experimentation, this study investigated interference-fit effects on the bearing failure of thin-ply composite countersunk single-lap joints. A comparative study of the mechanical behaviour and strengthening of the bolt-hole wall during the assembly of thin-ply and thick-ply specimens with varying interference sizes has been carried out. The effect of the interference-fit on the strength of countersunk composite bolted joints with thin-ply has been studied in detail under quasi-static tensile loading. The interface strengthening and failure mechanisms of thin-ply interference joints were characterized by means of SEM micrographs. Thin-ply laminates exhibited lower installation forces during assembly. The use of thin-ply in the interference joints' area could still demonstrate suppression of the damage's generation and propagation. More importantly, thin-ply laminates could accommodate larger interference sizes than thick-ply laminates and have higher load-bearing capacities, approximately 10%–20 % higher. Thin prepregs could be used to facilitate the design process and improve composites' structural properties, providing new perspectives for improvements and innovative applications of interference joining technology. The results of this study have some guidance for the aerospace application of thin prepregs in thin-walled structures.

Damage evolution monitoring and failure level evaluation of composite bolted joints by MXene/CNTs sensors

AbstractThe composite bolted joints are the weak points of overall structure, lacking an effective and accurate monitoring method. In this paper, a novel MXene/CNTs film sensor with high sensitivity and stability was synthesized for the damage evolution monitoring of composite bolted joints. The correlation between the response signals of the sensors and the mechanical behaviors of the composite structures is established. Based on this research, the damage evolution monitoring and failure level evaluation of composite bolted joints were studied. The damage properties of the composite bolted joints can be reflected in real‐time by the resistance change rates of the sensors. The experimental results show that the failure level of composite bolted joints can be divided into four levels (I‐II‐III‐IV) according to the resistance change rates of sensors. Especially when the damage extended to the III stage, the surged from 0.3 to 0.75. The time point of this step signal occurred about 1 min earlier than the time point of max bearing strength, which can effectively warn the occurrence of structural failure.Highlights Research used high‐sensitivity MXene/CNTs sensor to monitor bolt structure Sensor array can be integrated with the composite bolted structure Structural damage of composite bolted joints was accurately monitored Failure level evaluation of composite bolted structure was achieved

Seismic behavior of composite bolted T-shaped exterior joints in modular steel construction

This paper presents a new joint that is connected by common friction high-strength bolts and blind bolts. To study the seismic behavior of the proposed joint, four full-scale joints were tested by quasi-static tests and simulated by the finite element method. According to the stiffness, resistance and ductility of the joints, the influences of column-to-column blind bolt connection, beam-to-beam common bolted connection, and arrangement of inner diaphragm are analyzed. The results show that the beam-to-beam bolted connection, column-to-column blind bolted connection, and the arrangement of inner diaphragms can obviously improve the bearing capacity of the joints, which are 24.9%, 39.2% and 44.6%, respectively. The ductility of each joint is between 1.86 and 2.74, with an average value of 2.15. The joints with cover plate and inner diaphragm plate can meet the requirements of special moment frames (SMF). Therefore, the modular steel construction (MSC) in the seismic zone should adopt cover plate and inner diaphragm plate, and the joint should meet the requirements of strong column-to-weak beam to obtain better ductility.

Analytical method for assembly-induced deformation prediction of composite bolted joints with assembly gap

Shimming position and amount have a significant influence on the weight and mechanical performance of the thin-walled product that having assembly gap during aerospace components assembly process. An analytical method for assembly-induced deformation prediction of the bolted composite joints with assembly gap was developed. The assembly-induced deformation under the preloading force was computed as bending deformation and bolt head compression deformation by evaluating whether assembly gap can be eliminated and whether the calculating point is within the conical and spherical envelope of the fastener. The proposed method was validated by the preloading experiment of three-bolt composite joints with relative error less than 10%. Then it was applied to analyse the assembly-induced deformation of a composite box structure including composite spars and skins. Under the guide of the analytical result, shimming assembly of the composite box structure was carried out. The proposed analytical method has a good application prospect to predict the assembly-induced deformation during preloading process for composite/composite joints or composite/metal joints.

Effect of forced assembly on bearing performance of single-lap, countersunk composite bolted joints – Part Ⅰ: Experimental investigation

This paper presented an experimental study on the effect of forced assembly on bearing performance of single-lap, countersunk composite bolted joints. The forced assembly deformation amounts in bearing tests were chosen according to the acoustic emission (AE) damage detection results of the bending test. 3D digital image correlation (DIC) technology was used to measure out-of-plane deformation and surface strain field. Bearing damage was observed through microscopy after bearing tests. It is found that forced assembly causes pre-tension as well as higher loading eccentricity, thus obviously weakening the effective bearing strength of single-lap, countersunk composite joints. Specimens with forced assembly display S-shape out-of-plane bending under effective tensile loading. The damage degree of the non-countersunk hole is close to that of the countersunk hole, which indicates that forced assembly uniformizes bearing load distribution at countersunk hole while aggravates the non-uniformity of bearing load distribution at the non-countersunk hole.

Study on failure of single-bolt double-lap composite joints based on test and numerical emulation

In order to study the mechanical properties of ZT7G/LT-03A carbon fiber reinforced composite bolted joints, the single-bolt double-lap joints tests were carried out with 3 kinds of ply and 3 kinds of width, and the effects of the different ply ratios, width to diameter ratios on the failure load and failure mode were analyzed. Then, the failure process of the specimens was simulated by using the progressive damage and failure analysis method. The failure mode and failure load of the model are consistent with that of the specimen, which verified the accuracy of the model. On this basis, the stress and damage status of each ply during initial load shedding were studied, and the influence of the ply ratios and width to diameter ratios on the performance of laminates was further studied. The results show that when the width to diameter ratio of laminates increases, the failure mode changes from tensile failure to bearing failure, and to change the ply ratios does not affect the failure mode in a certain range.

Effect of forced assembly on bearing performance of single-lap, countersunk composite bolted joints – Part Ⅱ: Numerical investigation

A progressive damage model with the combination of 3D Hashin criterion and exponential material degradation was developed and validated according to the experimental results of the Part Ⅰ. The effect of forced assembly on stress distribution and bearing damage of single-lap, countersunk composite bolted joints were revealed by utilizing the present model. It is found that the joints with forced assembly show higher contact stress levels. The stress distribution of the countersunk hole tends to be uniform, while that of the non-countersunk hole is more uneven, and the maximum contact stress occurs at the non-countersunk hole. Furthermore, the forced assembly made a bolt sustain additional axial tensile load, introducing high failure risks to the bolts. With forced assembly, the fiber compressive damage and in-plane matrix compressive damage are more serious, and the most serious damage is transferred from the countersunk hole to the non-countersunk hole.

High-fidelity Modelling Approach for Investigating the Tightening Torque Effect in Thread Connection

Accurate replication of the torque-tightening effect is important when simulating the mechanical behavior of composite joints using the finite element (FE) approach. In this study, a parametric mesh construction strategy was developed to automatically build a high-fidelity 3D FE model with lift angle to simulate bolt thread interactions. An experimental approach was proposed to calibrate the FE model. The relation between tightening torque and the axial force was studied by the FE method, experiment, and engineering algorithm. Results showed that traditional empirical formula overestimate the clamping force. By using the proposed model to calculate the axial force of the bolt, the accuracy of the axial force can be significantly improved when analyzing the composite bolted joints. Finally, parametric simulations were performed to investigate the influence of coefficients of friction at thread interfaces and the nut/washer interfaces. The pre-tightening force was significantly influenced by the frictional coefficients at these interfaces. Results indicated that increasing the coefficient of friction (CoF) at the nut/washer interface from 0.1 to 0.2 can lead up to 28% reduction in the clamping force, while this increase was 23% for the threads interfaces.

Investigation on the Progressive Damage and Bearing Failure Behavior of Composite Laminated Bolted Joints under Tension

Composite laminated bolted joints are increasingly used in the aerospace industry, and most researchers are involved in the study of the failure behavior of composite bolted joints’ structures. Because of the complexity and stability of the structure, precisely predicting the damage evolution and failure behavior of the composite laminated bolted joint becomes rather difficult. In this paper, an asymptotic damage model is proposed to predict the failure behavior of the composite bolted joint structure. The model is based on the frame of mainstream criteria and some improvements are made to adapt to the particularity of composite laminated bolted joints. Combining the damage model with the finite element method, the failure behavior of single-lap and double-lap bolted joint structures are predicted and analyzed. In order to guarantee the reliability of the model, the corresponding experimental study is conducted, and the results show that the simulation curve and the experimental data are in good agreement. This damage model can further predict the failure behavior of various types of complex composite laminated bolted joints effectively.

Mesoscale modelling of progressive damage and failure in single-lap and double-lap thin-ply laminated composite bolted joints

Composite bolted joints are being used extensively in primary load-bearing structures of modern aircrafts. However, simulating progressive damage and failure of the bolted composite structures under high bearing loading is still challenging. In this paper, numerical simulation and experimental verification on the bearing failure of single-lap and double-lap thin-ply laminated composite bolted joints were carried out for the entire loading process. 3D explicit finite element models have been developed using Abaqus/Explicit together with a 3D physically-based intralaminar damage model implemented in a VUMAT subroutine. An interface cohesive-zone model based on the Benzeggagh-Kenane (B-K) law was used to simulate delamination initiation and evolution. For the intralaminar damage, the initiation was determined based on Pinho’s failure criteria while the evolution was regularized with the crack-band approach to alleviate mesh-size dependence. Element deletion and in-situ effect as practical numerical strategy are discussed and analyzed in detail. Element deletion is used to prevent excessive element distortion and capture corresponding post-peak bearing failure behavior. In-situ effect has been proved to have a key influence on the damage initiation and progression of thin-ply composite matrix. The final simulation results are shown to agree well with experimental data and reproduce the mechanical response curves. More importantly, the model can accurately capture the local crushing of bolt hole in the later loading stage. This study shows that the numerical model can be helpful for the design and optimization of composite bolted structures.

An improved 3D model of composite bolted joints with detailed thread structure and progressive damage analysis of realistic tightening process

Bolted joint is a common connection method in composite structure assembling. The initial assembly state has a significant influence on the subsequent performance. To realize the simulation of the assembly process of composite bolted joints, an improved 3D model is proposed. This model considers the detailed thread structure and can be used to simulate the realistic tightening process. The relationship between the tightening torque and bolt tension is analyzed and the effect of the friction coefficient on the value of the tightening coefficient is analyzed using this model. A new fitting model for the nonlinear pressure distribution state on the outer surface of the composite laminates during the assembly process is proposed. And a quantitative analysis and prediction method of the nonlinear pressure distribution state is established. Finally, the effects of bolt tension and washer type on initial assembly damage are analyzed and experimentally verified.

Tension failure assessment of composite bolted joints under bearing-bypass load interaction using analytical means

Bolted joints are often used in thin parts such as plates or shell-like components in lightweight structures, e.g. air- and spacecraft. This is also motivated by an inexpensive manufacturing and the ability to disassemble. To make these joints, holes need to be drilled and stress concentrations arise. Leading to the instantaneous destruction of the connection, attention should be drawn to fatal tension failure. The efficient and precise prediction of the corresponding failure stresses using analytical methods is the focus of this paper. Usually, rows of bolts are placed. Then, the load is partly introduced into one bolt while the rest stays in the plate. This setting is also referred to as bolted joint under combined bearing-bypass load, which shall be idealised as a linear 2D plate problem. The corresponding characteristic stresses are discussed for orthotropic laminates showing good agreement to Finite Element values. Then, failure analysis is conducted using Finite Fracture Mechanics. This part is dedicated to quasi-isotropic laminates. The size effect and in this context the failure stress reduction with increasing bolt diameter is analysed. Primary physical effects due to finite dimensions and the ratio of bearing and total load are identified. The higher this ratio and the larger the bolt diameter, the lower the sustained failure stress. Failure envelopes for the graphical determination of the critical bearing and bypass stresses are provided. All in all, a comprehensive and efficient framework for tension failure assessment of composite bolted joints under bearing-bypass load interaction is developed.