Strength Prediction Of Joints Research Articles

EXtended Finite Element Method applied to the strength prediction of adhesively-bonded joints

High-strength composites are widely used in several industries, such as aeronautical, automotive and naval, and they can be combined with metals to provide significant advantages in structural design. The application of adhesive bonding to these assemblies supposes the existence of reliable design tools to accurately analyse the joints’ behaviour. In this context, the eXtended Finite Element Method (XFEM) is a recent possibility to predict bonded’ joints fracture behaviour. This work aims to study by XFEM single-L adhesive joints between aluminium components and carbon-epoxy composites under peeling loads, considering the variation of the L-shaped adherend’s thickness (tP2) and adhesives of distinct ductility. The XFEM analysis was either based on stress or strain criteria for assessment of damage initiation, and different damage law types for crack propagation. Validation was undertaken with experimental data. The XFEM analysis revealed that this method is very accurate when using the stress-based quadratic initiation criterion and the triangular propagation law. It was shown that the L-shaped adherend’s geometry and the adhesive type are the most important parameters affecting the joints’ strength.

Detailed investigation of the analysis conditions in the evaluation of bonded joints by cohesive zone models

Cohesive Zone Models (CZM) are widely used for the strength prediction of adhesive joints. This work studies the influence of different conditions used in CZM simulations to model a thin adhesive layer in single-lap joints (SLJ) under a tensile loading, for an estimation of their influence on the strength prediction under diverse geometrical and material conditions. Adhesives ranging from brittle to highly ductile and overlap lengths (LO) between 12.5 and 50 mm were considered. Several damage initiation and growth criteria were tested. The analysis carried out in this work allowed to conclude that CZM is a powerful technique for strength prediction of bonded joints, provided that the modelling conditions are properly defined.

Investigation of headed bar joints between precast concrete panels

The paper addresses the design and behaviour of narrow cast in-situ joints between precast concrete elements in which continuity of reinforcement is achieved through overlapping headed bars. Using headed bars minimises the lap length required within the cast-in-situ joint region. Confining reinforcement in the form of transverse bars and vertical shear studs is also installed in the joint. The paper describes a series of tensile tests which were carried out to simulate the tensile zone of a joint loaded in pure flexure. The headed bars used in the tests were 25mm in diameter with 70mm square heads and yield strength of 530MPa. The tests studied the influences of concrete strength, headed bar spacing, splice length, transverse reinforcement and confining shear studs on joint strength. A lap length of 100mm in concrete with 28MPa cylinder strength was found to be sufficient to develop the full strength of the headed bars. A strut-and-tie model (STM) is presented for determining joint strength. Analysis shows that the STM gives safe results even though it does not fully capture the observed joint behaviour. An upper bound plasticity model is found to give relatively good predictions of joint strength in most cases, although it also does not always capture the correct failure mechanism. The tests provide insights into joint behaviour which, in conjunction with numerical modelling, will facilitate the development of an improved design method. Widespread use of this system would lead to improvements in buildability, sustainability and health and safety in the construction of concrete structures.

Failure strength prediction of aluminum spot-welded joints using kernel ridge regression

The current paper presents an alternative method for failure strength prediction of spot-welded joints in aluminum, based on nonlinear regression analysis, namely, the kernel ridge regression metho ...

Ultrasonic Welding of Magnesium–Titanium Dissimilar Metals: A Study on Influences of Welding Parameters on Mechanical Property by Experimentation and Artificial Neural Network

The advancement in the application of light alloys such as magnesium and titanium is closely related to the state of the art of joining them. As a new type of solid-phase welding, ultrasonic spot welding is an effective way to achieve joints of high strength. In this paper, ultrasonic welding was carried out on magnesium–titanium dissimilar alloys to investigate the influences of welding parameters on joint strength. The analysis of variance was adopted to study the weight of each welding parameter and their interactions. The artificial neural network (ANN) was used to predict joint strength. Results show that in ultrasonic welding of magnesium and titanium alloys, clamping force is the most significant factor, followed by vibration time and vibration amplitude; the interactions between vibration time and vibration amplitude, and between vibration amplitude and clamping force also significantly impact the strength. By using the artificial neural network, test data were trained to obtain a high precision network, which was used to predict the variations of joint strength under different parameters. The analytical model predicts that with the increase in vibration time, the increase in optimal joint strength is limited, but the range of welding parameters to obtain a higher joint strength increases significantly; the minimum joint strength increases as well; and the optimal vibration amplitude expands gradually and reaches the maximum when the vibration time is 1000 ms, then shifts toward the low end gradually.

Validation of advanced numerical techniques for the strength prediction of adhesively-bonded joints

Cohesive Zone Models (CZM) are widely used for the strength prediction of adhesive joints. This work studies the influence of different conditions used in CZM simulations to model a thin adhesive layer in single-lap joints (SLJ) submitted to a tensile loading, for an estimation of their influence on the strength prediction under diverse geometrical and material conditions. Adhesives ranging from brittle to highly ductile and overlap lengths (LO) between 12.5 and 50 mm were considered. Several damage initiation and growth criteria were tested. The analysis carried out in this work allowed to conclude that CZM is a powerful technique for strength prediction of bonded joints, provided that the modelling conditions are properly defined.

Shear fracture toughness and cohesive laws of adhesively-bonded joints

Adhesive bonding is a viable technique to reduce weight and complexity in structures. Additionally, this joining technique is also a common repair method for metal and composite structures. However, a generalized lack of confidence in the fatigue and long-term behaviour of bonded joints hinder their wider application. Suitable strength prediction techniques must be available for the application of adhesive bonding, and these can be based on mechanics of materials, conventional fracture mechanics or damage mechanics. These two last methodologies require the knowledge of the fracture toughness (GC) of materials. Being damage mechanics-based, Cohesive Zone Modelling (CZM) analyses coupled with Finite Elements (FE) are under investigation. In this work, CZM laws were estimated in shear for a brittle adhesive (Araldite® AV138) and high-strength aluminium adherends, considering the End-Notched Flexure (ENF) test geometry. The CZM laws were obtained by an inverse methodology based on curve fitting, which made possible the precise estimation of the adhesive joints’ behaviour. It was concluded that a unique set of shear fracture toughness (GIIC) and shear cohesive strength (ts0) exists for each specimen that accurately reproduces the adhesive layer behaviour. With this information, the accurate strength prediction of adhesive joints in shear is made possible by CZM.

A Comparative Study of Ductile Damage Models Approaches for Joint Strength Prediction in Hot Shear Joining Process

Abstract Ductile fracture prediction is often considered as a challenging task in complex stress state applications, such as metal joining process. A systematic method for predicting the joint strength in the hot shear joining process is presented in this paper by considering two different ductile damage models, namely Johnson-Cook and modified Lemaitre damage models. Since the crack formation in metalworking is considered to be a combination of both load conditions and metallurgical properties, the microstructure evolution is taken into account by characterizing the considered material, AISI-1045, at different temperatures and for different strain rates and by testing different specimen shapes, namely round bar, joint bar, flat and notched specimens. The derived material properties have been inversely calibrated in order to be used in the developed numerical model. A shear joining process simulation has been implemented in DEFORM and have been validate with shear joining experiments carried out at 950°C, allowing to validate the accuracy of the proposed model in simulating the hot shear joining, both in terms of joining strength and identification of the damage locations. Thanks to a precise material characterization, and by utilizing the proposed numerical model, both the joining tool design and the process condition can be studied and improved.

Strength prediction of T-peel joints by a hybrid spot-welding/adhesive bonding technique

ABSTRACT The need of joining methods that best meet the design requirements has led to the increased use of adhesive joints at the expense of welding, fastening and riveting. Hybrid weld-bonded joints are obtained by combining adhesive bonding with a welded joint, providing superior strength and stiffness, and higher resistance to peeling and fatigue. In the present work, an experimental and numerical study of welded, adhesive and hybrid (weld-bonded) T-peel joints under peeling loads is presented. The brittle Araldite® AV138, the moderately ductile Araldite® 2015 and the ductile Sikaforce® 7752 were the considered adhesives. An analysis of the experimental values and a comparison of these values with Finite Element Method (FEM) results in Abaqus® were carried out, which included a stress analysis in the adhesive and strength prediction by Cohesive Zone Models (CZM) considering failure simulation of both the adhesive layer and weld-nugget. It was found that the Sikaforce® 7752 performs best in the bonded and hybrid configurations. The good agreement between the experimental and numerical results enabled the validation of CZM to predict the strength of adhesive and hybrid T-peel joints, giving a basis for reducing the design time and enabling the optimization of these joints.

Overview of different strength prediction techniques for single-lap bonded joints

Adhesive joints have been used in several fields of engineering, and their applications are vast. Due to their easy and quick fabrication process, single-lap joints are a common configuration. The increase of strength, weight reduction and resistance to corrosion are some of the advantages of this kind of joint over traditional joining methods. However, stress concentrations at the overlap edges are one of the main disadvantages. There are very few accurate design techniques for the diversity of bonded joints that can be found in real applications, which constitutes an obstacle to the use of this bonding method in structural applications. This work aims at comparing different analytical and numerical methods in the strength prediction of single-lap joints with different overlap lengths ( LO). The main objective is to evaluate which predictive method is the best. Adhesive joints were produced between aluminium adherends using a brittle epoxy adhesive (Araldite® AV138), a moderately ductile epoxy adhesive (Araldite® 2015) and a ductile polyurethane adhesive (Sikaforce® 7888). Different analytical methods were considered, together with two numerical techniques: cohesive zone models (CZM) and the extended finite element method (XFEM), allowing the comparative analysis. The analytical methods showed that they only give relatively accurate results in very specific conditions. The CZM analysis with the triangular law revealed to be a very accurate method, with the exception of joints with very ductile adhesives. On the other hand, the XFEM analysis was not adequate, especially for crack growth in mixed mode.

Application of the direct method for cohesive law estimation applied to the strength prediction of double-lap joints

Currently, significant efforts are being made in the design of aircraft and aeronautical applications to reduce weight and improve reliability. Thus, adhesive bonding techniques have been largely employed, which also enables the combined use of steel with lighter materials such as aluminium or high strength composites. Cohesive Zone Models (CZM) are a powerful tool for the design of bonded structures, but they require careful estimation of the cohesive laws for reliable results. This work experimentally evaluates by the J-integral/direct method the tensile and shear CZM laws of three adhesives with distinct ductility. The Double-Cantilever Beam (DCB) and End-Notched Flexure (ENF) specimens were considered to obtain the tensile and shear CZM laws of the adhesives, respectively. After obtaining the tensile and shear CZM laws, triangular, exponential and trapezoidal CZM laws were built to reproduce their behaviour. Validation of these CZM laws was undertaken with a mixed-mode geometry (double-lap joint) considering the same three adhesives and varying overlap lengths (LO). The strength prediction by this technique revealed accurate predictions for a given CZM law shape, depending on the adhesive ductility, although all CZM law shapes were moderately accurate.

Joint strength reduction factor of internally ring-stiffened tubular joints at elevated temperatures

This article presents the results of a numerical parametric study on the ultimate strength of internally crown- and saddle-stiffened circular hollow section tubular DT-, T-, and Y-joints subjected to brace axial compression or tension at elevated temperatures. Using well-validated finite element models, an extensive study of 640 ring-stiffened tubular joints consisting of 36 DT-joints, 36 T-joints, and 8 Y-joints at different temperatures ranging from 20°C to 800°C was conducted. The strength of the stiffened joints was obtained from finite element analysis. The joint strength reduction was compared with the reduction factors of steel yield stress and elastic modulus at elevated temperatures. The parametric study shows that the effects of joint geometric parameters and stiffening position on the strength reduction of the stiffened DT-, T-, and Y-joints at elevated temperatures fall in a narrow range. An equation for predicting the strength reduction of the stiffened DT-, T-, and Y-joints at elevated temperatures was proposed by introducing a temperature factor. The statistical analysis shows that the proposed equation could provide reasonably accurate joint strength predictions.

Strength prediction of self-pierce riveted joint in cross-tension and lap-shear

This paper describes a parametric study of the mechanical behaviour of self-pierced riveted (SPR) joints of steel sheets in two loading conditions (lap-shear and cross-tension). Higher strength was always observed in lap-shear testing than in cross-tension. In both loading conditions, the strength of a joint was greatly influenced by the hardness and thickness of sheet materials and die depth. An empirical model was developed to predict the joint strength in cross-tension loading using characteristic joint data determined directly from the SPR process (force-displacement) curve. All predictions of joint strength fell within 10% of the measured joint strength. Finally, a relationship was established between the joint strength in lap shear and cross-tension with <8% error. The developed relationship provides a useful tool for further studies especially for different rivet and die geometry.

Investigation of shear strength models for exterior RC beam-column joint

Various models have been proposed by several researchers for predicting the exterior RC beam-column joint shear strength. Most of these models were calibrated and verified with some limited experimental database. From the models it has been identified that the joint shear strength majorly depends on ten governing parameters. In the present paper, detailed investigation of twelve analytical models for predicting shear strength of exterior beam-column joint has been carried out. The study shows the effect of each governing parameter on joint shear strength predicted by various models. It has been observed that the consensus on effect of few of the governing parameters amongst the considered analytical models has not been attained. Moreover, the predicted joint strength by different models varies significantly. Further, the prediction of joint shear strength by these analytical models has also been compared with a set of 200 experimental results from the literature. It has been observed that none of the twelve models are capable of predicting joint shear strength with sufficient accuracy for the complete range of experimental results. The research community has to reconsider the effect of each parameters based on larger set of test results and new improved analytical models should be proposed.

Calculation of the joint strength of hot shear joined bars based on a modified lemaitre model

To join the head and tail ends of two steel bars together, a solid-state joining technology named “shear joining” has been developed. In the process, no external heating is required since the bar temperature is around 1000 °C and sufficiently high for joining. Compared with the laser welding and fusion welding, the head and tail ends are joined together within a short time by pressing and shearing. So the process is very easy, quick and reliable without special heating and forming processes. As an innovative process, there is no appropriate method to evaluate the strength of joint up to now. In this research, a modified Lemaitre model was put forward to predict the strength of joint. This model could be also used to improve the design of joining tools and process conditions. Through comparison of load-displacement curves of tensile test obtained from FE simulation and experiment, the reliability of numerical model was verified. Compared with experimental results, the predicted joint strength showed a good quantitative agreement.

Comparative evaluation of the Double-Cantilever Beam and Tapered Double-Cantilever Beam tests for estimation of the tensile fracture toughness of adhesive joints

The continuous development observed in bonded joints, along with the improvements of the adhesives’ properties, are resulting in an increase of the bonded joint applications, as well as the variety of applications. Regarding the strength prediction of adhesive joints, two highly relevant methods are Fracture Mechanics and Cohesive Zone Models (CZM). By Fracture Mechanics, this is usually carried out by an energetic analysis. CZM enable the simulation of damage initiation and propagation. The tensile critical strain energy release rate (GIc) of adhesives is one of the most important parameters for predicting the joint strength. Two of the most commonly used tests are the Double-Cantilever Beam (DCB) and the Tapered Double-Cantilever Beam (TDCB). This work aims to assess the capability of the DCB and TDCB test to estimate the value of GIc of adhesive joints. Three types of adhesives with different levels of ductility are used, to study the accuracy of the typical data reduction methods under conditions that are not always consistent with Linear Elastic Fracture Mechanics (LEFM) principles. For both test protocols, methods that do not require measurement of the crack length (a) during the test are evaluated. In the DCB test, these are the Compliance Calibration Method (CCM), Corrected Beam Theory (CBT) and Compliance-Based Beam Method (CBBM). The methods used in the TDCB test are the Simple Beam Theory (SBT), CCM and CBT. With few exceptions, the results were consistent between the different methods considered for each test. The discrepancy of results is higher when comparing the two types of tests, except for the brittle adhesive. It was concluded that the data reduction methods for the TDCB test are too conservative to measure GIc of ductile adhesives.

Cohesive law estimation of adhesive joints in mode II condition

The adhesive bonding technique enables both weight and complexity reduction in structures that require some joining technique to be used on account of fabrication/component shape issues. Because of this, adhesive bonding is also one of the main repair methods for metal and composite structures by the strap and scarf configurations. The availability of strength prediction techniques for adhesive joints is essential for their generalized application and it can rely on different approaches, such as mechanics of materials, conventional fracture mechanics or damage mechanics. These two last techniques depend on the measurement of the fracture toughness (GC) of materials. Within the framework of damage mechanics, a valid option is the use of Cohesive Zone Modelling (CZM) coupled with Finite Element (FE) analyses. In this work, CZM laws for adhesive joints considering three adhesives with varying ductility were estimated. The End-Notched Flexure (ENF) test geometry was selected based on overall test simplicity and results accuracy. The adhesives Araldite® AV138, Araldite® 2015 and Sikaforce® 7752 were studied between high-strength aluminium adherends. Estimation of the CZM laws was carried out by an inverse methodology based on a curve fitting procedure, which enabled a precise estimation of the adhesive joints’ behaviour. The work allowed to conclude that a unique set of shear fracture toughness (GIIC) and shear cohesive strength (ts0) exists for each specimen that accurately reproduces the adhesive layer’ behaviour. With this information, the accurate strength prediction of adhesive joints in shear is made possible by CZM.

ABOUT MECHANICAL JOINTS DESIGN IN METAL-COMPOSITE STRUCTURE

Riveting is still one of the main joining methods of thin-walled aircraft structures. Such features as: simplicity of implementation, possibility of two different material connection (e.g. metallic with non-metallic ones) and the fact that is it a well-known (reliable) method causes popularity of riveting. The never-ending attempt to obtain as low mass as possible (mainly to reduce fuel consumption) is the reason for using material of high specific strength in the aerospace industry. High strength titanium or aluminium alloys (e.g. 2024T3) and composite laminates (e.g. CFRP or Glare) are examples of such materials. The article deals with methods of connecting various materials. The paper presents advantages and disadvantages of different/selected connection types. Strength prediction and failure modes of mechanical joints are described for metallic as well as for composite components. Composites are complex materials having an anisotropic structure (and anisotropic mechanical properties) leading to various failure mechanisms. Main principles for appropriate joint design of composite laminate panels (laminate configuration and typical/specific geometrical dimensions) are indicated/specified. The bearing failure mechanism is accepted to be a safe progressive one. Mechanism of bearing (generally compressive) load transfer into composite laminates by shear of the matrix is analysed. Some examples of improvement bearing strength of laminates are presented/shown according to literature. On the base of presented examples and bearing load transfer analysis, some conclusions for an appropriate solution of this problem are drawn.

Cohesive zone finite element analysis of crack initiation from a butt joint’s interface corner

Cohesive zone (CZ) fracture analysis techniques are used to predict the initiation of crack growth from the interface corner of an adhesively bonded butt joint. In this plane strain analysis, a thin linear elastic adhesive layer is sandwiched between rigid adherends. There is no preexisting crack in the problem analyzed, and the focus is on how the shape of the traction–separation (T–U) relationship affects the predicted joint strength. Unlike the case of a preexisting interfacial crack, the calculated results clearly indicate that the predicted joint strength depends on the shape of the T–U relationship. Most of the calculations used a rectangular T–U relationship whose shape (aspect ratio) is defined by two parameters: the interfacial strength σ∗ and the work of separation/unit area Γ. The principal finding of this study is that for a specified adhesive layer thickness, there is any number of σ∗, Γ combinations that generate the same predicted joint strength. Each combination corresponds to a different CZ length. An approximate CZ-like elasticity solution was developed to show how such combinations arise and their connection with the CZ length.

The strength prediction of adhesive single lap joints exposed to long term loading in a hostile environment

This work is concerned with investigating the residual static strength of adhesively bonded joints after long-term exposure to a combined mechanical-hygro-thermal environment. Associated experimental data are also reported. The degradation process of the joints was modelled using a fully-coupled approach, with the moisture concentration affecting the stress distribution and the stress state affecting the moisture diffusion analyses simultaneously. A bilinear cohesive zone model was then used to implement the progressive damage FE analysis of the quasi-statically loaded joints following the ageing phase. This model is degraded using the damage factors (creep strain and moisture uptake) accumulated over the ageing process and calibrated against the experimental results from static tests on the bulk adhesive. Predicted and experimentally-measured quasi-static responses for the aged adhesive joints were found to be in good agreement.