Strength Analysis of Adhesively-Bonded Tubular Single Lap Steel-Steel Joints Under Axial Loads Considering Residual Thermal Stresses

The static tensile load bearing capability of adhesively-bonded tubular single lap joints calculated using linear mechanical adhesive properties is usually far less than the experimentally-determined one because the majority of the load transfer of adhesively-bonded joints is accomplished by the nonlinear behavior of the rubber-toughened epoxy adhesive

In this paper, both the nonlinear mechanical properties and the residual thermal stresses in the adhesive resulting from joint fabrication were included in the stress calculation of adhesively-bonded joints. The nonlinear tensile properties of the adhesive were approximated by an exponential equation which was represented by the initial tensile modulus and ultimate tensile strength of the adhesive.

From the tensile tests and the stress analyses of adhesively-bonded joints, a failure model for adhesively-bonded tubular single lap joints under axial loads was proposed.     

Influence of Fabrication Residual Thermal Stresses on Rubber-toughened Adhesive Tubular Single Lap Steel-Steel Joints under Tensile Load

The tensile load bearing capability of adhesively-bonded tubular single lap joints which is calculated under the assumption of linear mechanical adhesive properties is usually much less than the experimentally-determined because the majority of the load transfer of adhesively-bonded joints is accomplished by the nonlinear behavior of rubber-toughened epoxy adhesives. Also, as the adhesive thickness increases, the calculated tensile load bearing capability with the linear mechanical adhesive properties increases, while, on the contrary, the experimentally-determined tensile load bearing capability decreases.

In this paper, the stress analysis of adhesively-bonded tubular single lap steel-steel joints under tensile load was performed taking into account the nonlinear mechanical properties and fabrication residual thermal stresses of the adhesive. The nonlinear tensile properties of the adhesive were approximated by an exponential equation which was represented by the initial tensile modulus and ultimate tensile strength of the adhesive.

Using the results of stress analysis, the failure criterion for the adhesively-bonded tubular single lap steel-steel joints under tensile load was developed, which can be used to predict the load-bearing capability of the joint. From the failure criterion, it was found that the fracture of the adhesively-bonded joint was much influenced by the fabrication residual thermal stresses.     

胶接接头的声技术无损检测

介绍了胶接接头无损检测技术近年来的发展情况,对超声、声-超声、声等无损检测技术的发展作了概要的阐述。同时对胶接接头的强度检测进行了分析,最后指出了制约胶接接头强度无损检测的主要障碍。     

Experimental studies on adhesively-bonded scarf joints: influence of natural ageing on progressive damage

This work characterizes the effects of natural ageing on the micro-mechanical behaviour of two adhesively-bonded scarf joints. The samples studied are made of XC18-type steel with different scarf angles (33 and 6°) and the adhesive is an epoxy resin. Contrary to most experimental studies which determine the strength of bonded joints in terms of their failure load, in this study sensitive strain gauges have been used to measure progressive damage of the adhesive joints. The results show that the damage is closely linked to the mechanical and geometrical properties of the test joints and that ageing increases the load thresholds of the first microcracks initiation and the ultimate failure of both adhesively bonded scarf joints.     

Application of bi-adhesive in double-strap joints subjected to bending moment

Generally, all failures in adhesively-bonded joints begin at the overlap ends because of the stress concentration occurring at the ends. The approach which reduces stress concentration at the overlap ends increases the load capacity and delays the failure. The lower the stiffness of the adhesive used, the lower the stress concentration, and the lower stress concentration gives rise to higher joint strength. In this work, the results of the application of two adhesives, one stiff and one flexible, with very different mechanical behaviors along the overlap length in double strap joints subjected to bending moment, were analyzed. A stiff adhesive was applied in the middle portion of overlap, while a flexible adhesive was applied towards the edges. The results show that the bi-adhesively-bonded joints carry more loads and have higher strength when compared with single-adhesively-bonded joints.     

Experimental analysis of the behavior of adhesively bonded joints under tensile/compression–shear cyclic loadings

Adhesive bonding is an interesting structural assembling technique which requires accurate numerical models in order to optimize the design of high-tech applications. In this paper, following the strategy previously developed for monotonic loadings, crack initiation in adhesively-bonded joints, under various tensile/compression–shear cyclic loadings is analyzed using a modified Arcan device with a single bonded joint designed to strongly limit stress concentrations. Moreover, such a system is associated with the maximum stress state in the center of the adhesive, and thus, allows the analysis of some influences of the stress triaxiality. Experimental results, for a ductile adhesive, under cyclic loadings are presented for different load amplitudes and mean loads; they underline that the evolutions of viscous deformations and of damage depend on the loading type.     

Fatigue Failure Criterion of Adhesively-Bonded Joints Under Combined Stress Conditions

A study was conducted to investigate fatigue failure criteria for adhesively-bonded joints under combined stress conditions. Two types of adhesively-bonded joint specimens were used: the scarf joint and the butterfly-type butt joint. Both types of joints have considerably uniform combined stress distributions in the adhesive layer. Furthermore, the stress distributions of these joints were analyzed by a finite element method. The results showed that the maximum principal, the von Mises equivalent and the maximum shear stresses in the uniform stress region of the adhesive layer at the endurance limit are correlated with the principal stress ratio.     

预偏角对单搭接接头强度的影响

研究了在被粘物搭接区顸偏转角度对钢单搭接拉伸接头剪切强度的影响,并用弹性有限元法分析了预偏角变化时单搭接接头上胶层中的应力分布情况。数值分析的结果表明：当预偏角从0°增加到12°时,结构钢单搭接接头胶层中的所有应力峰值分量均显著下降。而在所采取实验条件下,接头的剪切强度最高值出现在预偏角为6°时。因而在进一步研究预偏角对单搭接接头承载能力的作用时,应将外载作用下接头的本征偏转情况考虑在内。     

Environmental Ageing of Adhesively-Bonded Joints. II. Mechanical Studies

The effects of exposure to moisture on the mechanical properties of a series of adhesively-bonded structures are reported. Changes observed in the maximum load, shear modulus, strain at maximum load, fracture energy, fracture toughness and stress are discussed and correlated with variation of the dielectric parameters. An initial increase in fracture toughness observed in the joints correlates well with the uptake of moisture having led to a lowering of the glass-rubber transition temperature. Differences in the ultimate strength and energy to failure for different surface pretreatments are observed. Loss in mechanical properties observed over the period of the study are paralleled by changes in the dielectric properties of the joints.     

Torque Transmission Capabilities of Bonded Polygonal Lap Joints for Carbon Fiber Epoxy Composites

Although carbon fiber epoxy composite materials have excellent properties for structures, the joint in composite materials often reduces the efficiency of the composite structure because the joint is often the weakest area in the composite structure.

In this paper, the effects of the adhesive thickness and the adherend surface roughness on the static and fatigue strengths of adhesively-bonded tubular polygonal lap joints have been investigated by experimental methods. The dependencies of the static and fatigue strengths on the stacking sequences of the composite adherends were observed.

From the experimental investigations, it was found that the fatigue strength of the circular adhesively-bounded joints was quite dependent on the surface roughness of the adherends and that polygonal adhesively-bonded joints had better fatigue strength characteristics than circular adhesively-bonded joints.     

Controlled reduced-strength epoxy-aluminium joints validated by ultrasonic and mechanical measurements

In this study a series of joint systems, consisting of aluminium substrates bonded using an epoxy adhesive, were produced. Several levels of adhesion were achieved by altering the substrate surface treatment and the curing cycle of the adhesive. The goal of this study was to produce reduced-strength epoxy-aluminium joints that could be used as reference samples for ultrasonic non-destructive testing (NDT) studies. There is clearly a continuing challenge to improve the quality of the adhesively-bonded joint inspection to ensure the durability of the bonds, to monitor repairs, and to evaluate the strength of the bonds. However, developing and qualifying innovative or advanced non-destructive testing requires an essential preliminary step: a method for repeatedly producing reduced-strength bonded test specimens must be developed. In this study, in addition to a rigorous protocol to produce bonded joints, complementary ultrasonic CSCAN were realised to validate the homogeneity of the joints and to ensure that samples met all requirements so as to be considered as reference samples. Mechanical tests were performed to evaluate the mechanical strength of each joint and Acoustic Emission (AE) was used during the tests in order to confirm the expected fracture mechanisms.     

Tensile Strength of Single Lap Joint and Scarf Joint between CFRP and Carbon Steel

The strength of single lap joints and scarf joints between carbon cloth laminated plastics (CFRP) and carbon steel bonded with epoxy resin was investigated both analytically and experimentally. The stress and strain distributions under tensile loads of the joints were analyzed by applying the elastic finite element method.

The strength of the joints was predicted by applying the strength law of CFRP, metal, adhesive layer and their interfaces to the calculated stress distributions. The predicted strength was compared with the experimental strength of the joints. The critical positions of the joints and the effects of the overlapped length on the joints were examined.     

Torque Transmission Capabilities of Bonded Polygonal Lap Joints for Carbon Fiber Epoxy Composites

Although carbon fiber epoxy composite materials have excellent properties for structures, the joint in composite materials often reduces the efficiency of the composite structure because the joint is often the weakest area in the composite structure.

In this paper, the effects of the adhesive thickness and the adherend surface roughness on the static and fatigue strengths of adhesively-bonded tubular polygonal lap joints have been investigated by experimental methods. The dependencies of the static and fatigue strengths on the stacking sequences of the composite adherends were observed.

From the experimental investigations, it was found that the fatigue strength of the circular adhesively-bounded joints was quite dependent on the surface roughness of the adherends and that polygonal adhesively-bonded joints had better fatigue strength characteristics than circular adhesively-bonded joints.     

Failure behavior of adhesively bonded joints with a rubber modified epoxy adhesive under tensile-shear combined cyclic loading

Rubber-modified epoxy adhesives are used widely as structural adhesive owing to their properties of high fracture toughness. In many cases, these adhesively bonded joints are exposed to cyclic loading. Generally, the rubber modification decreases the static and fatigue strength of bulk adhesive without flaw. Hence, it is necessary to investigate the effect of rubber-modification on the fatigue strength of adhesively bonded joints, where industrial adhesively bonded joints usually have combined stress condition of normal and shear stresses in the adhesive layer. Therefore, it is necessary to investigate the effect of rubber-modification on the fatigue strength under combined cyclic stress conditions. Adhesively bonded butt and scarf joints provide considerably uniform normal and shear stresses in the adhesive layer except in the vicinity of the free end, where normal to shear stress ratio of these joints can cover the stress combination ratio in the adhesive layers of most adhesively bonded joints in industrial applications.

In this study, to investigate the effect of rubber modification on fatigue strength with various combined stress conditions in the adhesive layers, fatigue tests were conducted for adhesively bonded butt and scarf joints bonded with rubber modified and unmodified epoxy adhesives, wherein damage evolution in the adhesive layer was evaluated by monitoring strain the adhesive layer and the stress triaxiality parameter was used for evaluating combined stress conditions in the adhesive layer. The main experimental results are as follows: S–N characteristics of these joints showed that the maximum principal stress at the endurance limit indicated nearly constant values independent of combined stress conditions, furthermore the maximum principal stress at the endurance limit for the unmodified adhesive were nearly equal to that for the rubber modified adhesive. From the damage evolution behavior, it was observed that the initiation of the damage evolution shifted to early stage of the fatigue life with decreasing stress triaxiality in the adhesive layer, and the rubber modification accelerated the damage evolution under low stress triaxiality conditions in the adhesive layer.     

Strength and finite element analyses of single-lap joints with adhesively-bonded columns

This paper introduces a novel approach to increase the loading ability of adhesive joints by incorporating adhesively-bonded columns. Strengths of single-lap adhesive joints with adhesively-bonded columns were measured experimentally. Stress and strain distributions at selective positions in the adhesive layer were analyzed using the Finite Element Method (FEM). Failure mechanisms of the joints were analyzed. It was found that the metal-adhesive columns increased the joint strength and also the joint strength increased with increasing length of the metal-adhesive columns. Therefore, using metal-adhesive columns in adhesive joints is an effective approach for enhancing the strength of bulk adhesive joints.     

Experimental Investigation of Epoxy Bonded Polymethylmethacrylate Joints

This paper presents an experimental investigation into various aspects of epoxy-bonded polymethylmethacrylate (PMMA) and PMMA-to-aluminium joints. The effects of adhesive thickness, overlap area, surface roughness, and environmental exposure on the joint strength were studied. Results indicated that the joint strength was not directly proportional to the overlap area, while sanding had a positive effect on the joint strength. A negative effect was observed when adhesive thickness was increased. The fatigue behaviour of adhesively-bonded joints under dynamic loading was found to be independent of frequency, for the range of values tested; however, it was dependent on the test temperature with greater reduction in fatigue life observed in PMMA-to-aluminium joints at higher temperature. Empirical equations from which the fatigue life of joints can be predicted were obtained by regression analysis. Intermittent fatigue testing of the joints was also performed. The epoxy adhesive tested proved to be a satisfactory choice for outdoor exposure. The rate of degradation of the adhesive was slow with the adherend itself degrading at a faster rate than the adhesive or the bondline.     

Effect of protrusion at the ends of bondline in single lap joints under tension and bending

In this work, elasto-plastic stress analysis of single lap joints with and without protrusion in adhesive bondline subjected to tension and bending was carried out using 2D non-linear finite element analysis and confirmed experimentally. AA 2024-T3 aluminum adherends were bonded with SBT 9244 film adhesive. The protrusion was obtained by extending the adhesive film by 2?mm from the overlap length at both overlap ends. Three different adherend thicknesses and overlap lengths for each loading and bondline type were used. The joints with and without protrusion, for comparison, were loaded with the same load for each adherend thickness and overlap length. Finally, it was observed that the protrusion reduces the strength in the joint under tension, while the protrusion increases the strength in the joint under bending.     

Experimental Investigation of Epoxy Bonded Polymethylmethacrylate Joints

This paper presents an experimental investigation into various aspects of epoxy-bonded polymethylmethacrylate (PMMA) and PMMA-to-aluminium joints. The effects of adhesive thickness, overlap area, surface roughness, and environmental exposure on the joint strength were studied. Results indicated that the joint strength was not directly proportional to the overlap area, while sanding had a positive effect on the joint strength. A negative effect was observed when adhesive thickness was increased. The fatigue behaviour of adhesively-bonded joints under dynamic loading was found to be independent of frequency, for the range of values tested; however, it was dependent on the test temperature with greater reduction in fatigue life observed in PMMA-to-aluminium joints at higher temperature. Empirical equations from which the fatigue life of joints can be predicted were obtained by regression analysis. Intermittent fatigue testing of the joints was also performed. The epoxy adhesive tested proved to be a satisfactory choice for outdoor exposure. The rate of degradation of the adhesive was slow with the adherend itself degrading at a faster rate than the adhesive or the bondline.     

Estimation of Fatigue Strength for Adhesively-Bonded Lap Joints Based on Fatigue Failure Criterion Under Multiaxial Stress Conditions

A method for estimation of endurance limits for adhesively-bonded single and single-step double-lap joints was proposed which considers the stress multiaxiality in the adhesive layer. At first, fatigue tests and finite element analysis were conducted for these lap joints. Then, endurance limits of these joints were estimated using their stress distributions and critical regression equations which were obtained from the S-N data of adhesively-bonded scarf- and butterfly-type joints. The results indicate that the endurance limits of these lap joints can be estimated from the regression equation based on the maximum principal stress.     

Influence of the geometry of coaxial adhesive joints on the transmitted load under tensile and compression loads

More and more accurate models have been developed for describing the mechanical behavior of an adhesive in an assembly which allow to numerically optimize the design of bonded assemblies. This paper deals with stress analysis in coaxial bonded joints. The objective was to analyze the effect of various geometries of the different parts of the assembly in order to optimize the maximal transmitted load of such joints. In the case of tensile loads, the stress distributions were analyzed using axisymmetric theory of elasticity. A pressure-dependent elastic limit of the adhesive was used, in order to accurately represent the difference between tensile-shear and compression-shear loads in the mechanical response of the adhesive. In adhesively bonded joints, stress concentrations can contribute to the initiation and propagation of cracks in the adhesive. Therefore, designing adhesively bonded assemblies which strongly limit stress concentrations can significantly increase the load transmitted by the assembly. Moreover, cylindrical joints are associated with high substrate strength in the radial direction, meaning that peel and cleavage forces have different effects compared to single lap joints. A comparison between the mechanical behavior of these two joints is proposed, starting from 2D simulations in the case of tensile and compressive loads. Furthermore, the influence of the angle of conical geometries of the bonded area, which can easily be used for such assemblies, is analyzed with respect to the stress distributions. It is shown that several geometries allow a large reduction of stress concentrations and thus lead to stronger assemblies. Moreover the influences of several geometries which strongly limit stress concentrations are presented.