Single Lap Joint Specimens Research Articles

Stress, damage, and fatigue performance analysis of CFRP/Al double-sided countersunk riveted joints with variable rivet-hole clearance

The CFRP/Al countersunk rivet joints are one of the most fatigued dangerous points of the aircraft fuselage. To deeply investigate the failure mechanisms of the riveted joint structures, this paper utilizes electromagnetic riveting technology to prepare single-lap joint specimens and investigates the quasi-static tensile and fatigue failure behaviours of the joints under different rivet-hole clearances and stress levels. In addition, to assist this investigation, a three-dimensional finite element model, combining riveting and fatigue loading, is developed. This model takes into account the strain rate effects during riveting process and progressive failure behavior of various typical damage modes in CFRP laminates. The results indicate that Aluminum alloy sheets fracture dominates the fatigue failure of the joints. Increasing the clearance can reduce the stress amplitude on the shear plane and further enhance the fatigue life of the joints. Furthermore, all additional CFRP damage during the loading process is the growth of the initial riveting damage, and the damage-induced fibre bearing capacity decrease is considered the reason for the increase in stress amplitude of the Al sheet. Last, excessive clearance enlargement does not appreciably diminish the initial riveting damage, instead, it could result in a joint strength reduction.

Enhancing shear strength of single lap composite adhesive joint with graphene nano-particles and Z-pins reinforcement through co-curing technique

Glass fiber-reinforced polymer (GFRP) co-cured (CC) single lap joint (SLJ) adhesive specimens with varying weight percentages (wt%) of graphene nanoparticles (GNP) and different volume percentages (vol%) of carbon fiber-reinforced polymer (CFRP)Z-Pin went through a shear test, and the results showed that 0.75 wt% GNP and 2.5 vol% 90°/Z-Pin reinforced specimens exhibited higher shear strength. Optimised GNP% and vol% of Z-Pin applied to SLJ showed high shear strength. The study was further compared with SLJ incorporating 0.75 wt% GNP with a 45°/Z-Pin specimen, and the results showed that the optimised 0.75 GNP% and 2.5 vol% 90°/Z-Pin SLJ specimen showed higher shear strength. After testing, microscopic images of the fractured specimen surface were taken to examine the failures. Fractography shows that adding GNP and Z pin to SLJ roughens surfaces, improving adhesive-adherent interaction.

Extended Finite Element Method (XFEM) Model for the Damage Mechanisms Present in Joints Bonded Using Adhesives Doped with Inorganic Fillers

The use of adhesive bonding in diverse industries such as the automotive and aerospace sectors has grown considerably. In structural construction, adhesive joints provide a unique combination of low structural weight, high strength and stiffness, combined with a relatively simple and easily automated manufacturing method, characteristics that are ideal for the development of modern and highly efficient vehicles. In these applications, ensuring that the failure mode of a bonded joint is cohesive rather than adhesive is important since this failure mode is more controlled and easier to model and to predict. This work presents a numerical technique that enables the precise prediction of the bonded joint’s behavior regarding not only its failure mode, but also the joint’s strength, when inorganic fillers are added to the adhesive. To that end, hollow glass particles were introduced into an epoxy adhesive in different amounts, and a numerical study was carried out to simulate their influence on single lap joint specimens. The numerical results were compared against experimental ones, not only in terms of joint strength, but also their failure pattern. The neat adhesive, which showed 9% and 20% variations in terms of failure load and displacement, respectively. However, looking at the doped configurations, these presented smaller variations of about 2% and 10% for each respective variable. In all cases, by adding glass beads, crack initiation tended to change from adhesive to cohesive but with lower strength and ductility, correctly modeling the general experimental behavior as intended.

Methodology for the mechanical characterisation of hyperelastic adhesives. Experimental validation on joints of different thicknesses

This work focuses on the mechanical characterisation of adhesives with hyperelastic behaviour and on the determination of the behavioural laws that best represent it, in order to introduce them in simulation models. First, a test plan is carried out on simple specimens: uniaxial and planar configuration. These are designed to measure the non-linear behaviour of adhesives in both tensile and pure shear. Unlike the uniaxial specimen, which is governed by a test standard (UNE-ISO 37) that defines its geometry, the planar specimen does not have a standard that defines its dimensions. Therefore, in this research it is proposed to carry out tests with specimens of different width-length sizes to evaluate how these dimensions affect the stress-strain curves.For mechanical characterisation, finite element programs provide the tool to evaluate the predicted behaviour of a hyperelastic material from the experimental results, displaying in the same graph the degree of approximation obtained for the results of each test (Dumbbell and planar) with different hyperelastic models, allowing us to select the hyperelastic model that best fits the experimental data.The Mooney-Rivlin model was found to be the best fitting model and therefore the most appropriate to describe the behaviour of hyperelastic adhesives used in this study. To conclude this study, the obtained law was validated by comparing the results of tests carried out on single lap joint (SLJ) specimens of different thickness.

An experimental investigation of static and fatigue behavior of various adhesive single lap joints under bending loads subjected to hygrothermal and thermal conditions

ABSTRACT The objective of this study is to investigate the impact of hygrothermal and thermal aging on adhesive bonding joints of similar and dissimilar specimens. The research focuses on the single lap joint (SLJ) configuration using Aluminum/Aluminum and CFRP/CFRP for similar bonding and Aluminum/CFRP for dissimilar bonding. Understanding the behavior of such joints over time and eventual failure can enhance their durability, reliability, and safety in structures. The study submerged SLJ specimens in tap water at a temperature of 53°C for 20 days to assess hygrothermal aging and kept another group of joints in an oven set at the same temperature for thermal aging. The research evaluated the strength of samples that underwent no aging, hygrothermal and thermal aging, tested through three-point bending tests. Various load levels were tested utilizing a recently designed fixture, including 70%, 60%, and 50% of the static load level. Results showed the highest joint strength in Aluminum/Aluminum SLJs and reduced strength following thermal and hygrothermal aging. The failure patterns and number of cycles leading to failure also varied with load percent and aging. Ultimately, the study aimed to evaluate and contrast the strength of samples that underwent different aging processes.

Analysis of adhesively bonded joints of laser surface treated composite primary components of aircraft structures

The performance of the adhesively bonded aerospace structures highly depends on the adhesion strength between the adhesive and adherents, which is affected by, in particular, the condition of the bonding surface. Among the various surface treatment methods, as state of the art, laser surface treatment is a suitable option for the CFRP composite structures to enhance the adhesion performance, adjusting the roughness and surface free energy with relatively minimizing the damage to the fibers. The aim of this study is the validation and evaluation of the adhesive bonding behavior of the laser surface-treated CFRP composite structures, using the finite element technique to perform a conservative prediction of the failure load and damage growth. Such objectives were achieved by executing both experimental and numerical analyses of the secondary bonded CFRP parts using a structural adhesive. In this regard, to complement physical experiments by means of numerical simulation, macro-scale 3D FEA of adhesively bonded Single Lap Joint and Skin-Spar Joint specimens has been developed employing the Cohesive Zone Model (CZM) technique in order to simulate bonding behavior in composite structures especially skin-spar relation in the aircraft wing-box.

Numerical Design of a Thread-Optimized Gripping System for Lap Joint Testing in a Split Hopkinson Apparatus.

Currently, few experimental methods exist that enable the mechanical characterization of adhesives under high strain rates. One such method is the Split Hopkinson Bar (SHB) test. The mechanical characterization of adhesives is performed using different specimen configurations, such as Single Lap Joint (SLJ) specimens. A gripping system, attached to the bars through threading, was conceived to enable the testing of SLJs. An optimization study for selecting the best thread was performed, analyzing the thread type, the nominal diameter, and the thread pitch. Afterwards, the gripping system geometry was numerically evaluated. The optimal threaded connection for the specimen consists of a trapezoidal thread with a 14 mm diameter and a 2 mm thread pitch. To validate the gripping system, the load-displacement (P-δ) curve of an SLJ, which was simulated as if it were tested on the SHB apparatus, was compared with an analogous curve from a validated drop-weight test numerical model.

Adhesive Thickness and Ageing Effects on the Mechanical Behaviour of Similar and Dissimilar Single Lap Joints Used in the Automotive Industry

The effects of the adhesive thickness and overlap of a polyurethane adhesive have been studied by using different substrate configurations. Single lap joint (SLJ) specimens have been tested with homologous substrates, carbon fibre-reinforced plastics and painted metal substrates. Furthermore, a configuration with dissimilar substrates has been included in the experimental campaign. Both types of these adhesive and substrates are used in the automotive industry. The bonding procedure has been carried out without a surface treatment in order to quantify the shear strength and stiffness when surface treatments are not used on the substrates, reproducing typical mass production conditions. Three different ageing cycles have been used to evaluate the effects on SLJ specimens. A finite element model that uses cohesive modelling has been built and optimised to assess the differences between the different adopted SLJ configurations.

Experimental investigation of the loading speed effects on the performances of ultrasonically welded PA66/glass fabric composites

These past decades, the ability of thermoplastic composites to be welded has been of great interest, with numerous studies investigating the process and mechanical strength of joints for quasi-static loadings. The present study aims to determine the loading speed influence on the behaviour of these structures, as crashworthiness is of tremendous interest in the transport industry for safety issues. The polyamide-based composite used in this work is sensitive to strain rate, which may influence the interface and the entire structure's behaviour. Tensile tests were performed on single lap joint specimens for loading speeds from 1.2 mm min−1 to 7 m s−1, considering several substrate orientations. Loading speed influences the lap shear strength for the different configurations: increasing loading speed induces a performance enhancement of 11.2%–22.9%. This phenomenon is caused by the matrix strain rate sensitivity, which stiffens and strengthens the substrate shear behaviour and the welded joint behaviour. Finally, the fractography analysis confirms that fracture mechanisms observed are similar for each loading speed tested with primarily fibre/matrix debonding. Therefore, improvement in the welded structures' performance is due to the mechanical performance of its constituents and their influence on the interfacial strength.

Testing, numerical and analytical modelling of self-drilling screw connections between thin steel sheets in shear

The shear behaviour of self-drilling screw connections between thin steel sheets is investigated via experimental testing, numerical and analytical modelling in this paper. A total of nineteen sets of single-lap joint specimens with self-drilling screw connections were tested to failure under shear. The full force versus deformation curves were recorded, and three typical failure modes were obtained and were shown to be closely related to the ratio of the screw diameter to the sheet thickness. Prior to testing, constitutive responses of the employed thin steel sheets were acquired from tensile coupon tests, and were further represented by a three-stage model including an elastic branch, a plastic branch described by the Ludwik equations, and a damage branch defined by the Johnson–Cook model and the linear accumulation damage evolution law. Advanced finite element (FE) models of the screw connections were developed and validated against the obtained shear test results, and a parametric study on the influences of the sheet thickness and the screw diameter was carried out afterwards. Based on the obtained test and FE results, a new prediction model for the shear deformation behaviour of self-drilling screw connections was proposed and calibrated. The newly proposed prediction model was further introduced into the numerical models of the cold-formed box girders fabricated by self-drilling screws, the applicability of which was verified by the close agreement with the available experimental evidence.

Co-cured carbon fibre/epoxy composite joints by advanced thermoplastic films with excellent structural integrity and thermal resistance

Carbon fibre/epoxy composite joints were assembled with Poly-etherether-ketone (PEEK) and Poly-ethylenimine (PEI) films using a co-curing process to prepare single-lap joint specimens. The joints were tested under quasi-static loading conditions at 22 °C and 130 °C and a fatigue loading condition. The experimental results demonstrated better or comparable structure integrity of the composite joints co-cured by PEEK and PEI films than the reference joints bonded by aerospace FM300 adhesives. In particular, the PEEK co-cured joints exhibited extraordinary mechanical performance at 130 °C and excellent fatigue resistance. For instance, the lap-shear strength at 130 °C and the fatigue life of the composite joints co-cured by 200μm PEEK films was 2.1 times and 2.7 times higher than that of the aerospace adhesive joints, respectively. Overall, the results of this work proved that advanced thermoplastic films are promising alternatives to epoxy adhesives for the co-cure joining of thermoset composites with significantly enhanced structural integrity and thermal stability.

Experimental failure strengths of composite single-lap joints reinforced with I-Fiber stitching process

Composite lap joints have been extensively used due to their excellent properties and the demand for light structures. However, composites exhibit weak mechanical properties in their thickness, which may cause failure in composite lap joints. Various reinforcement methods that delay fracture by dispersing stress concentration have been discussed to overcome this problem, such as z-pinning and conventional stitching. After several studies and publications, the I-Fiber stitching process is a promising technology that combines the advantages of both z-pinning and the conventional stitching process. In this paper, the reinforcing effect of the I-Fiber stitching process was verified by fabricating composite single-lap joint specimens with different thicknesses, number of I-Fibers, and stitching angles to evaluate the lap shear strengths. Experimental results showed an improvement of up to 52.1% using the I-Fiber stitched process compared with the unstitched specimens. In addition, a better ratio of improvement in the reinforcement was observed with thinner composite adherends, and a higher failure load was achieved using a 45° stitching angle compared with 0°. Finally, through finite element analysis, it was possible to study the behavior and predict the interface failure of the single-lap joint. The results obtained showed good agreement with the experimental data and supported the failure mechanism’s understanding using the I-Fiber stitching process.

Experimental study on aluminium to tungsten carbide/polycrystalline diamond (WC/PCD) adhesive bonding for milling tools

Nowadays the use of adhesives is deeply embedded in the modern automotive and aeronautical industries. However, in other areas, such as the milling tool sector, it is still a quite unexplored technology. This work studies the adhesive bonding process between aluminium and WC/PCD, both commonly used in woodworking milling tools. To allow for electric discharge machining (EDM) as a sharpening process, an electrically conductive adhesive combined with conductive microspheres was used. Bulk tensile tests, thick adherend shear tests (TAST), double cantilever beam (DCB) tests and end notched fracture (ENF) tests were used to mechanically characterise the adhesive. The joining process was studied using small-scale single lap joint (SLJ) specimens manufactured with three v/v% of spheres - 0%, 5% and 10%. The need for surface treatments was evaluated by measuring the surface energy of the substrates, applying then the necessary procedures. And the electrical resistance of the studied joints was measured with the four-probe method. It was found that with the increase in v/v% of spheres both the electrical resistance and strength decreased. However, when looking for a good compromise between both mechanical and electrical properties, the results point towards the 5% spheres content. To finalise this study, the previously detailed adhesive characterisation was performed once again for the chosen v/v% of spheres, showing a reduction in fracture toughness, as expected.

Analysis of the effect of the process parameters on the mechanical strength of 3D printed and adhesively bonded PETG single lap joint

The current study deals with the effect of 3D printing parameters on the strength of 3d printed and adhesively bonded PETG single lap joints. Polyethylene terephthalate glycol (PETG) is used to print standard specimens with different printing parameters (raster angle, raster width, and layer thickness) using fused deposition modelling (FDM) process. By conducting number of tensile tests on the single lap joint specimens, the effect of abovementioned 3D printing parameters on the mechanical behaviour of PETG joints is determined. Furthermore, a computational investigation was carried out to confirm the experimental results and gain a better understanding of the fracture behaviour of these types of joints. The effect of above mentioned 3D printing parameters in stress distribution and failure behaviour of the joints are examined and reported. Taking into Consideration of a growing interest in FDM 3D printing technology in manufacturing and many other applications of adhesively bonded joints, obtained results are useful for future studies on new designs of 3D printed adhesively bonded products with a greater strength and improved structural performance.

Experimental Study of the Impact of Glass Beads on Adhesive Joint Strength and Its Failure Mechanism.

The use of modern structural adhesives provides a lightweight, practical, and high strength joining methodology, which is increasingly being adopted in the automotive and aeronautical sectors, among many others. However, the strict mechanical performance standards that must be met in these applications require a constant search for ways of improving the adhesives’ behavior, which has led to the growing use of reinforcements as a way of improving the capabilities of bonded joints. The aim of this work was, thus, to analyze how the addition of inorganic fillers to the adhesive layer affects a joint’s strength and its failure mechanism. To this end, single lap joint specimens with mild steel and high strength steel substrates were tested, at quasi-static speeds, and with different amounts of glass microspheres reinforcing two different structural adhesives. The experimental results indicated that the addition of glass particles reduced the joint performance for both substrates under study. Furthermore, the failure pattern was found to evolve from adhesive failure to a cohesive type of failure as the amount of glass particles present in the adhesive was increased.

The Mechanical Performance of Re-Bonded and Healed Adhesive Joints Activable through Induction Heating Systems

This work aims to study the healing potential properties of a reversible thermoplastic adhesive. The adhesive is activable by using induction heating systems that can induce thermal heat in the particles throughout the electromagnetic field so they can melt the adhesive for bonding or separation procedures. The healing procedure consists of damaging single lap joint (SLJ) specimens with quasi-static and fatigue tests and then using an inductor to generate an electromagnetic field able to heat the adhesive to its melting point in order to heal the damaged SLJ specimens. SLJ tests were performed on damaged and healed specimens to assess, respectively, the residual mechanical properties of the damaged specimens and the mechanical properties after healing. SLJ tests showed that the healing procedure can completely recover the joint stiffness of the damaged adhesive joints, a huge part of the maximum shear strength and the SLJ absorbed energy. This work shows also the possibility of re-bonding completely failed or separated SLJs by using the same procedure. The mechanical properties of SLJs after healing and re-bonding are compared to the SLJ compared on virgin specimens to assess the recovered mechanical properties.

Low-speed bending impact behaviour of adhesively bonded dissimilar single-lap joints

This study investigates the low-speed bending impact behaviour of adhesively bonded dissimilar single-lap joints and the effects of both strength and plastic deformation capability of adherend material on adhesive failure. Dissimilar adhesive single-lap joint specimens, such as Al 2024-T3 (top adherend)-Al 5754-0 (bottom) and Al 5754-0 (top)-Al 2024-T3 (bottom), were tested at two impact energy levels (3 and 11 J) for two overlap lengths (25 and 40 mm). The progressive failure analysis of the adhesive layer was also conducted by the non-linear explicit finite element method. The adhesive layer was modelled with a 3D cohesive layer along with the upper and lower adhesive interfaces and a non-linear continuum adhesive region between two cohesive layers. The continuum adhesive region had elasto-plastic adhesive properties whilst the cohesive layers obeyed 3D cohesive rules. The experimental and predicted contact force-time, contact force-displacement diagrams, axial separation lengths of the failed adhesive region, permanent deflection of the bonded region, fracture surfaces were in good agreement. The strength and plastic deformation capability of adherend materials and impact energy levels affected the progressive adhesive failure behaviour. The proposed finite element model was successful reasonably in predicting the initiation and propagation of the adhesive failure.

The effects of nanostructure additive on fracture strength in adhesively bonded joints subjected to fully reversed four-point bending fatigue load

Structural adhesive joints used in aerospace are usually subjected to fatigue loading rather than static loading. This study experimentally and numerically investigated the static four-point bending loads of adhesively bonded joints after fully reversed four-point bend fatigue loading where nanoadhesives – obtained by adding carbon nanostructures into aerospace grade structural adhesive – were used to bond the joints. Single lap joint specimens were produced using a nanocomposite adhesive obtained by adding 1 wt % graphene, 1 wt % carbon nanotubes-COOH and 1 wt % fullerene C60 nanostructures to a DP460 structural adhesive. AA2024-T3 aluminum alloy and carbon fiber-reinforced composites (CFRCs) with a plain weave fabric (0/90°) were used as adherend materials. First, static four-point bending tests were applied to these joints to obtain their strengths and then fully reversed sinusoidal fatigue tests were applied under a constant load amplitude, a frequency of 20 Hz and a load ratio of R = −1. Fatigue tests were performed over 1,000,000 cycles – accepted as an infinite life – at four different load levels by considering the strengths obtained from the static tensile tests. After obtaining the static four-point bending strengths of the joints to which fatigue loading was applied, the changes in the strengths of joints with and without fatigue testing were examined. The static strengths of aluminum joints bonded with nanoadhesive and subjected to fully reversed fatigue loading significant increased, depending on the nanostructure type added to the adhesive. Moreover, such increases in the strengths of joints are highly dependent on the adherend type (composite with [0/90]6 stacking sequence or AA2024-T3 aluminum).

Fracture and structural performance of adhesively bonded 3D-printed PETG single lap joints under different printing parameters

The present study, deals with fracture behavior of adhesively bonded 3D-printed joints. Polyethylene terephthalate glycol (PETG) is used to print standard specimens with different printing parameters (raster angle, raster width, and layer thickness) using fused deposition modeling (FDM) process. Moreover, adhesive layer with different thicknesses is employed to determine effects of adhesive layer thickness on the structural integrity of 3D-printed joints. By a series of tensile tests on the single lap joint specimens, influence of above-mentioned 3D printing parameters on the mechanical behavior of PETG joints is determined. Moreover, numerical study was conducted to verify experimental results and provide enhanced knowledge on fracture behavior of these types of joints. The nonlinear 3D finite element model of the joint supports mechanical characteristics of both 3D-printed adherends and adhesive materials. The effect of above mentioned 3D printing parameters in stress distribution and failure behavior of the joints are examined and reported. Considering a growing interest in FDM 3D printing technology in manufacturing and numerous applications of adhesively bonded joints, reported results are beneficial for future developments of new designs of 3D printed adhesively bonded products with a higher strength and better structural performance.

A study on the effects of nanostructure reinforcement on the failure load in adhesively bonded joints after the subjected to fully reversed fatigue load

ABSTRACT Today, adhesively bonded joints are frequently used in the space and aviation industries. Joints used in these sectors are generally subject to dynamic loads due to environmental factors. This study experimentally and numerically investigated the static tensile loads of adhesively bonded joints after fully reversed (combination of tensile and compressive) fatigue loading where nanoadhesives – obtained by adding carbon nanostructures into aerospace grade structural adhesive – were used to bond the joints. Single lap joint specimens were produced using a nanocomposite adhesive obtained by adding 1 wt. % graphene, 1 wt. % carbon nanotubes-COOH and 1 wt. % fullerene C60 nanostructures to a DP460 structural adhesive. AA2024-T3 aluminum alloy and carbon fiber-reinforced composites (CFRCs) with a plain weave fabric (0/90°) were used as adherend materials. First, static tensile tests were applied to these joints to obtain their failure loads and then fully reversed sinusoidal fatigue tests were applied under a constant load amplitude, a frequency of 20 Hz and a load ratio of R = −1. Considering the failure loads obtained from the static tensile tests, 106 fully reversed fatigue loading cycles were applied – which was accepted as an infinite life – at 400 N, 800 N and 1200 N load levels. The static tensile failure loads and energy values absorbed of these joints were obtained and the change in the failure loads and energy values absorbed of the joints subjected to fatigue was investigated. The static failure loads of aluminum joints bonded with nanoadhesive and subjected to fully reversed fatigue loading increased by approximately 5% to 17%, depending on the nanostructure type added to the adhesive. Moreover, it was observed that there was an increase in loading of approximately 3% to 20% for the nanostructure reinforced joints obtained by using CFRCs with [0/90]6 stacking sequence.