Joints In Composite Structures Research Articles

Credal identification of damage patterns in ultra-thin-ply composite bonded/bolted interference-fit joints

Investigating the mechanical behavior and damage patterns of ultra-thin-ply composite joints is essential for ensuring their widespread application in the aerospace field. It is currently an advanced method of exploring the damage patterns of composite joint structures via a combination of acoustic emission and machine learning algorithms. However, the existing research lacks the evaluation indexes for the determination of the number of damage categories, and on the other hand, the overlapping area and noise of the monitored damage data cannot be accurately identified. In this paper, load-bearing performance of ultra-thin-ply composite hybrid bonded/bolted interference-fit joints is studied, then the SDI(Structural Damage Index) evaluation index is proposed to quantitatively analyze the number of damage categories of the complex joints, and finally the adaptive optimization algorithm based on the credal partition for the complex joints of composite is studied. The experimental results demonstrate the accuracy and generalization of the SDI formulae, and the introduction of the credal partition that submits data where overlapping regions cannot be accurately identified to ensembles of special single classes (meta-clusters) obtains deeper insights into the damage data and improves identification and isolation of noise.

Modeling study on the mechanical performance of CFRP/Al single-lap rivet joints under hygrothermal environmental conditions

Carbon Fiber Reinforced Polymers (CFRP) and aluminum alloys, owing to their exceptional mechanical properties and lightweight attributes, are extensively used in aircraft manufacturing. However, prolonged exposure to a hygrothermal environment can compromise the mechanical integrity of composite joint structures. In this paper, accelerated aging experiments were designed to test the mechanical properties of CFRP after hygrothermal aging, a novel mechanical property prediction model tailored for the hygrothermal coupled environment is presented. The model accurately predicts the modulus and strength of CFRP after hydrothermal aging. A three-dimensional finite element model for the CFRP interference riveted structure, considering a tri-coupled state of moisture, temperature, and force was established by the application of subroutine and field superposition. The precision of this finite element model has been affirmed through accelerated aging tests. By integrating finite element analysis with experimental methods, this research delves into the failure modes and mechanisms of CFRP and its joints under hygrothermal conditions. It was discerned that the hygrothermal environment undermines the bond between fibers and the matrix, resulting in pronounced interlaminar delamination and shear failure in CFRP. The failure forms gradually change from “flaky” at lower levels of aging to “filamentary” at higher levels of aging. For CFRP riveted structures, the hygrothermal conditions influence their loadbearing capability and shift the primary positions of failure.

Dynamic properties and plastic dissipative mechanisms of polymeric adhesive under dynamic compression at various strain rates

This work investigated the dynamic compression behavior of structural vinylester adhesive under dynamic compression tests using the Split Hopkinson Pressure Bars technique. The obtained results showed that the dynamic parameters were strain rate sensitive. No macroscopic damage was observed in the specimen at the different applied strain rates. However, the presence of a characteristic second peak mainly at high strain rates on the strain rate profiles was attributed to the plasticity occurrence in the polymer under impact. The non-linear response that appeared on the stress-strain curves of the adhesive had been attributed to the combined material softening and thermal softening. These results demonstrated that the studied adhesive could be considered a potential material for high-rate applications due to the combination of significant toughness with high strength and extensive plasticity. An energetic study determined the energy balance and quantified energy absorption. It was observed that used vinylester adhesive was suitable for dynamic applications because it could absorb a large amount of energy and undergo large compressive deformations without damage. This work provided a complete range of experimental data for structural adhesive, which could help the designer of an energy-absorbing adhesively bonded composite joint structure for high-strain rate applications.

Influence of bolt dynamic installation on topography characteristics and mechanical behaviors of CFRP interference-fit bolted joints

As the controlled research of Dynamic Installation (DI) and Static Installation (SI), a new interference installation method was developed based on electromagnetic loading to enhance the mechanical properties of composite structures. Four different interference-fit sizes were considered, ranging from a net fit to 2.0%. The experiments were conducted to evaluate the installation resistance and the mechanical behavior of the joint under external loads. Meanwhile, an FEA model to model the stress distribution and damage behavior of the bolt-hole contact interface was established. The load–displacement curve and damage modes of experiments were used to verify the FEA results. The results show that the installation resistance during DI process was remarkably lower than that of SI process corresponding to all interference-fit sizes, and the stress amplitudes induced by interference were larger and widely distributed. The damage of the hole wall was positively correlated with interference fit size, but DI can significantly reduce the damage compared to SI. In performance tests, DI enhanced the static bearing capacity and extended longer fatigue life of the joints than SI. DI methods can be an effective way to achieve highly reliable interference joints in composite structures.

Influence of Design Parameters on Mechanical Behavior of Multi-Bolt, Countersunk C/SiC Composite Joint Structure.

Aerospace vehicle connection constructions are in urgent need of joint structures with excellent aerodynamic profiles and environmental adaptability. To address issues such as poor aerodynamic profile, material thermal expansion coefficient mismatch, and limited joint structure evaluation indexes, a multi-bolt, countersunk C/SiC composite joint structure is presented in this study. The development of a 3D Hashin progressive damage model and its dedicated solver code is presented. The validity of the model is confirmed by comparing simulation results with experimental data. Three evaluation indexes are proposed, peak load, weight increment efficiency, and bolt load distribution, to thoroughly evaluate the mechanical performance of multi-bolt, countersunk C/SiC composite joint structures. Using the proposed model and evaluation indices, we evaluate sixteen different designs of multi-bolt, countersunk C/SiC composite joint structures and analyze how design parameters affect their mechanical properties and damage patterns. The results show that the best mechanical properties of the joint structure are achieved when the ratio of bolt pitch to through hole diameter is 3, the ratio of bolt spacing between columns to through hole diameter is 4, the ratio of the distance between the free edge of the substrate to through hole diameter is 1.5, the ratio of through hole diameter to specimen thickness is 1.7, and the ratio of the distance between the edge of the substrate to through hole diameter is 1.5.

Parametric investigation of bonded composite joints under Mode II using a new methodology based on design of experiments

ABSTRACT In the pursuit of a sustainable future with limited resources and growing environmental concerns, adhesive joining stands out as a pivotal technology for the advancement of lightweight structures. This study aims to investigate the influence of various variables on the critical strain energy release rate in mode II (GIIc) for End-Notched Flexure (ENF) bonded composite joints, employing a new methodology based on Design of Experiments (DoE) approach. Two-dimensional finite element models of the ENF bonded composite joints are developed using commercial software, with all numerical models generated via Python™ scripts linked with AbaqusⓇ. Eleven design parameters related to the specimen geometry and material properties are systematically evaluated to determine the influence of each variable on GIIc. Numerical force–displacement curves are obtained, followed by the application of the Compliance-Based Beam Method (CBBM) to estimate the critical fracture energy in mode II, represented by a numerical envelope. The influence of each parameter is assessed using the main effect (ME) metric. The top five most influential variables affecting GIIc and CBBM are identified as the adhesive’s critical strain energy release rate in mode II (GIIc), effective laminate longitudinal elastic modulus (), adhesive thickness (tA), adherent thickness (h), and pre-crack length (). Furthermore, the study highlights the epistemic uncertainty associated with the geometric variables. Despite the limitations inherent in the computational model, the Plackett-Burman method consistently proves effective in conducting sensitivity analysis of the variables in the ENF test. These findings demonstrate promising prospects for the application of this procedure in the design of composite joint structures.

Investigation of Design Parameters in Tensile Loads of the Diamond Joints in Composite Structures with a Finite Element Approach

Investigation of Design Parameters in Tensile Loads of the Diamond Joints in Composite Structures with a Finite Element Approach

Damage Analysis of CFRP Hybrid Bonded-Bolted Joint during Insertion of Interference-Fit Bolt.

In this study, experiments and finite element analysis (FEA) were used to evaluate the impact of interference-fit sizes on CFRP hybrid bonded-bolted (HBB) joint damage during bolt insertion. The specimens were designed in accordance with the ASTM D5961 standard and bolt insertion tests were performed at selected interference-fit sizes (0.4%, 0.6%, 0.8%, and 1%). Damage to composite laminates was predicted using the Shokrieh-Hashin criterion and Tan's degradation rule via the user subroutine USDFLD, while damage to the adhesive layer was simulated by the Cohesive Zone Model (CZM). The corresponding bolt insertion tests were performed. The variation of insertion force with interference-fit size was discussed. The results showed that matrix compressive failure was the main failure mode. With the growth of the interference-fit size, more failure modes appeared, and the failure region expanded. Regarding the adhesive layer, it did not completely fail at the four interference-fit sizes. This paper will be helpful in designing composite joint structures and especially for understanding CFRP HBB joint damage and failure mechanisms.

Development of micron-sized Cu–Ag composite paste for oxidation-free bare Cu bonding in air condition and its deterioration mechanism during aging and power cycling tests

The development of next-generation power electronics requires the semiconductor power devices with higher voltage endurance, higher electrical current density under high-temperature operations above 250 °C. Herein, we propose the uses of combined micron-sized Cu particles with Ag–amino composite to achieve a robust direct Cu–Cu bonding under a low sintering pressure and temperature (300 °C) in ambient atmosphere. The strong bonding strength of the Cu–Cu joint structure was evaluated as high as 45.3 MPa under 300 °C sintering temperature with low pressure (1 MPa). For the high temperature reliability evaluation, the sintered Cu–Ag composite Cu–Cu joints were stored at 250 °C in air. The shear strength of the joints remains above 30 MPa after 500 h ageing, suggesting an excellent mechanical stability during high temperature storage. Additionally, a SiC Schottky barrier diode with the sintered Cu–Ag composite joint structure was evaluated during a power cycle test at a junction temperature of 200 °C for 10,000 cycles. The thermal resistance of the joint structure was almost unaffected, and no large delamination was observed after the power cycle test, indicating that the Cu–Ag composite material exhibit remarkable potential in SiC power devices packaging.

Improving the damage tolerance of composite T-joints using shape memory alloy tufts

This paper presents a novel approach to improve the structural properties and damage tolerance of fibre-reinforced polymer composite joints via tufting using shape memory alloy (SMA) filaments. T-shaped joints made of carbon-epoxy composite material were reinforced with thin SMA (Ni-Ti nitinol) tufts. Experimental testing and finite element (FE) modelling reveal that the ultimate load, ultimate displacement and absorbed energy capacity of the T-joint increase with the areal density of SMA tufts. Improvements of over 110% in strength and 450% in absorbed energy capacity were achieved by SMA tufting. Under certain conditions, the shape memory effect of the tufts can be activated via electrical resistance heating to partially or completely close cracks in the T-joint caused by over-loading. The novel use of SMA tufts both to improve the resistance against cracking and to close cracks when they occur is a unique approach to improving the damage tolerance of composite structural joints.

Structure health monitoring of composites joint reinforced by Acoustic Emission based Smart Composite Fasteners

This paper proposed an Acoustic Emission (AE) based Smart Composite Fastener (SCF) concept for health monitoring of bonded/bolted composite single lap joints. The SCF was made of 3D-printed continuous carbon fibre reinforced thermoplastic materials with an embedded piezoelectric sensor. The SCF detected signals were found to be successfully associated with AE damage sources during the loading period. It was discovered that the adhesive crack/delamination AE sources resulted in burst-type signals with identifiable onset and end, whereas AE sources of frictional sliding between the SCF and fastener holes resulted in continuous-type signals producing broad frequency content. Furthermore, the amplitudes of the burst-type signal measured from the network of SCFs were successfully correlated with the locations of the damages. In the direction away from the damage, the amplitudes of the burst-type voltages measured from the SCF showed a decreasing trend, with 10195mv, 9,995mv, and 7,426mv respectively. Generally, the research in this paper explores the correlation between the voltage signal from a damaged AE source and the SCF, providing the feasibility of using a novel SCF for health monitoring in composite joint structures.

Development and validation of a simulation methodology for composite joints with delamination arresting features

This paper presents the development of a modelling methodology in a finite element framework for the simulation of novel composite joints with delamination arresting features. The investigated composite joints employ the co-consolidation technique for the assembly of composite skins and stiffeners in a structural joint; in order to arrest and retard delamination damage evolution that may appear during service life, crack stopping features are included in the joint. Progressive damage modelling and cohesive zone modelling methodologies are employed for the analysis of intralaminar and interlaminar failure of laminate composite materials components. The modelling and simulation of experimental test cases available in the open literature are used for validation of the developed progressive damage and cohesive zone models through comparison of numerical results to respective experimental results for all test cases. The mechanical response of the butt-joint structural element is experimentally investigated and linear/nonlinear finite element models are developed to simulate its mechanical behaviour. The comparison between the experimental and numerical results demonstrates the applicability of the developed methodology in the simulation of novel composite structural joints with crack-stopping features.

The effect of the fatigue damage accumulation process on the damping and stiffness properties of adhesively bonded composite structures

Wind turbine blade spar cap to shear web adhesively bonded connections can suffer from damage at the bond-line which can propagate through the structure compromising blade integrity. This study investigates changes in the stiffness and damping of a thick adhesive joint test specimen during a fatigue test. The stiffness is calculated using an extensometer and damping is determined using vibration and thermography. Fatigue tests showed three distinct phases of damage. Firstly, transverse cracks and delamination initiate and grow with little change in stiffness observed while damping increases by 18.6% and 17.8% inferred from the vibration and thermographic methods, respectively. In the second phase, the number of transverse cracks increases reaching saturation and the stiffness reduces by 4.7%. The increase in the loss factor is 65.7% and 95.6% from the vibration and thermographic methods, respectively. In the final phase, the crack density remains constant whilst de-bonding of the joint begins and grows until failure. The loss factor increases by 111.4% and 116.9% from the vibration and thermographic methods, respectively though the stiffness shows a cumulative reduction of only 8.6%. The results show the potential for monitoring changes in damping to infer incipient damage in an adhesively bonded composite joint structure.

Hygrothermal Damage Monitoring of Composite Adhesive Joint Using the Full Spectral Response of Fiber Bragg Grating Sensors.

Adhesive joints in composite structures are subject to degradation by elevated temperature and moisture. Moisture absorption leads to swelling, plasticization, weakening of the interface, interfacial defects/cracking and reduction in strength. Moisture and material degradation before the formation of defects are not readily revealed by conventional non-destructive examination techniques. Embedded fiber Bragg grating (FBG) sensors can reflect the swelling strain in adhesive joints and offer an economical alternative for on-line monitoring of moisture absorption under hygrothermal aging. Most of the available works relied on the peak shifting phenomenon for sensing. Degradation of adhesive and interfacial defects will lead to non-uniform strain that may chirp the FBG spectrum, causing complications in the peak shifting measurement. It is reasoned that the full spectral responses may be more revealing regarding the joint’s integrity. Studies on this aspect are still lacking. In this work, single-lap joint composite specimens with embedded FBGs are soaked in 60 °C water for 30 days. Spectrum evolution during this period and subsequent tensile and fatigue failure has been studied to shed some light on the possible use of the full spectral response to monitor the development of hygrothermal degradation.

Experimental and numerical investigation of the traction-separation law of mode II fracture in single-edge ultrasonic welding in polypropylene composite reinforced by glass fibers

The use of welded joints in composite structures with reinforced fibers has rapidly increased owing to their advantages in terms of strength-to-weight ratio, design flexibility and ease of production. Herein, the generalized progressive damage model is numerically and experimentally studied to predict the degradation process in end notch flexure (ENF) composite specimens welded by the ultrasonic method. In numerical modeling, a trapezoidal traction-separation model that expresses the embedded process zone (EPZ) is developed using three data reduction methods of the compliance calibration method (CCM), classical beam theory (CBT) and compliance-based beam method (CBBM), and formulated by combining failure and damage mechanics. Finally, the force-displacement diagrams obtained from experimental investigations and the force-displacement diagrams extracted from numerical modeling are compared. The results demonstrate that the models extracted using CBBM and CBT methods make accurate predictions compared to the CCM method.

Testing of mechanical butt joints in composite structures

Abstract As the main contribution of the present study, an I-shaped joining element, which allows the plates used in large composite structures to be joined butt-to-butt, has been improved in terms of load capabilities. It is desirable that the joining zones of composite plates not be visible according to the design of composite structures and the requirements for the desired use in particular. In other words, it is not desirable to create any surfaces that would cause protrusions in the joining zones. The only joining technique that fulfils this condition is the butt joint. Generally, butt joints are performed by bonding. With this technique, it is possible to make a more durable mechanical butt joint using an I-shaped joining element. In this way, instead of bonding butt joints, stronger non-bonding or bonding mechanical butt joints can also be achieved. In this study, the geometric changes in the shape of an I-shaped joining element used in mechanical butt joints and the changes in load-carrying capabilities have been studied experimentally and numerically. Experiments were carried out with tensile, three-point bending and four-point bending tests. The experimental specimens and I-shaped joining elements were cut using a water jet machine. Abaqus finite element analysis software was used for numerical analysis. The numerical data obtained in the study were found to be consistent with the experimental data. The load-carrying capabilities of the joining elements of different geometric shapes were studied numerically and experimentally, and it was found that the ideal element was the joining element with a 60° angle.

Investigation of the Effects of Drilling on Mechanical Joints in Composite Structures

The use of a variety of composites due to their superior properties, which include low weight, high strength, and high corrosion resistance, which are generally many applications in the aerospace, automotive, and marine structures have increased significantly. Fiberglass reinforced composites have several industrial applications due to their suitable mechanical and physical properties. Drilling is one of the common methods for connecting structures made of reinforced materials with fibers. Composite structures usually result from the joining of members to transmit and tolerate incoming forces. One of the methods of connecting the members, due to the ease of maintenance and repair, is the mechanical connection through screws or rivets, which inevitably must create holes in these members. Composites as materials that are difficult to machining, drilling these materials can lead to delamination and, in some cases, degradation. In this paper, the effects of drilling on mechanical joints in composite structures and solutions to prevent damage to them due to drilling are investigated.

The effect of delamination in free vibration responses of adhesively bonded spar wingskin joints

In this article, the effects of damage (delamination) on the structural vibration characteristics of spar wingskin joints (SWJs) made with curved laminated composites are investigated. Three dimensional finite element analysis is employed to evaluate the natural frequencies and corresponding mode shapes of the joint structure considered. Virtual elements are implemented in the delamination region to prevent inter element penetration. The effectiveness of the finite element analysis technique used is validated with available results. Additionally, the effects of various parameters such as delamination size, curvature effect (shallowness angle), laminate stacking sequence, overlap length, and material anisotropy (modular ratio) on the free vibration responses of the curved SWJ are analyzed in details. The present delamination modeling and simulated results could serve as guidelines in designing curved laminated composite joint structures with/without delaminations.

The structural response of the thermoplastic composite joint subjected to out-of-plane loading

This paper presents a study on thermoplastic single-lap double rivet joints (SLDJ) with and without adhesive subjected to out-of-plane projectile impact and quasi-static concentrated loading. The three-dimensional finite element (FE) models considering strain-rate effects and damage evolution were developed and implemented in the commercial code Abaqus/Explicit through a user-defined subroutine VUMAT. The FE models were validated against the experimental results, in terms of the peak load, the maximum displacement and the dissipated energy. It was found that there were three stages in the load-displacement trace up to the failure, i.e. rivet slip, hole failure and laminate break for the rivet joint, and debonding, rivet slip and laminate break for the rivet joint with adhesive. The rivet joint with adhesive has a higher stiffness before debonding occurs; however, its out-of-plane load carrying capacity is slightly lower than the rivet only counterpart. The validated models were used to study the failure mechanisms of SLDJ through the progressive failure simulation, and further to investigate the effects of boundary conditions, ply angle and impact energy on the impact response of the composite joints. The laminates with 45° ply could be used to enhance the impact resistance of composite joint structures with the reduced damaged area.

Mechanism of bolt pretightening and preload relaxation in composite interference-fit joints under thermal effects

The present research investigates the mechanism of bolt pretightening and preload relaxation in composite interference-fit joint structures under thermal effects. In view of the dissimilar material properties and the interfacial friction in such joints, the mechanical behavior of bolt and composites during assembly can be regarded as immediate preload response which is described by a linear elastic model, whereas the long-term behavior of composites during preload relaxation in service is considered as delayed response which is characterized by a viscoelastic model. The clamping forces on both sides of joints were captured to evaluate the preload balanced by the frictional behavior. The preload relaxation of joints with various interference-fit sizes and tightening torques under thermal effects were monitored for 240 hours to calibrate and validate the proposed model. The research revealed that the interference-fit size determined the level of frictional force which resulted in a difference between clamping forces at two sides of the fasteners. The preload first increases slowly with the growth of temperature, then decreases sharply when it approaches to glass transition temperature of matrix. The interference-fit joint behaves better in terms of maintaining the stability of preload than clearance-fit joints.