Investigation of the relationship between statistical characteristics of substrate surface roughness and the strength of adhesive joints

The relationship between statistical characteristics of butadiene styrene rubber (BSR) surface roughness and shear strength of adhesive joints has been investigated. The assumption of stationary normal distribution of coordinates of surface points was made to determine the statistical characteristics of surface roughness. The profile length above the selected level l ₁ (u) was introduced as a new surface roughness parameter to characterize adhesive penetration depth. The validity of simulated l ₁ (u) value was verified experimentally. A good correlation between experimental and calculated results was found. A relationship between adhesive penetration depth and the bonding pressure during adhesive joint preparation was also obtained. The dependences among lap shear joint strength, bonding pressure and roughness characteristic l ₁ (u) were determined.     

Stress analysis and strength evaluation of single-lap band adhesive joints of dissimilar adherends subjected to external bending moments

Single-lap band adhesive joints of dissimilar adherends subjected to external bending moments are analyzed as a four-body contact problem using a two-dimensional theory of elasticity (plane strain state). In the analysis, the upper and lower adherends and the adhesive which are bonded in two regions are replaced by finite strips. In the numerical calculations, the effects of the ratio of Young's moduli of the adherends, the ratio of the adherend thicknesses, and the ratio of the band length to the half lap length on the stress distributions at the interfaces are examined. A method for estimating the joint strength is proposed using the interface stress and strain obtained by the analysis. An elasto-plastic finite element analysis (EP-FEA) was conducted for predicting the joint strength more exactly. Experiments to measure strains and the joint strength were also carried out. The results show that the strength of a single-lap band adhesive joint is almost the same as that of a single-lap adhesive joint in which the two adherends are completely bonded at the interfaces. Thus, the single-lap band adhesive joints are useful in the design of single-lap joints.     

FEM stress analysis and strength of adhesive butt joints of similar hollow cylinders under static and impact tensile loadings

The stress wave propagations in butt adhesive joints of similar hollow cylinders subjected to static and impact tensile loadings are analyzed in elastic and elasto-plastic deformation ranges using the finite-element method (FEM). The impact loading is applied to the joint by dropping a weight. The upper end of the upper adherend is fixed and the lower adherend of which the lower end is connected to a guide bar is subjected to the impact loading. The FEM code employed is DYNA3D. The effects of the adhesive thickness and Young's modulus of the adhesive on the stress wave propagation at the interfaces are examined. In addition, the characteristics of the joints subjected to impact loadings are compared with those of the joints under static loadings and the joint strengths are estimated by using the interface stress distributions. It is found that the maximum value of the maximum principal stress, σ₁ occurs at the outside edge of the interface of the lower adherend to which the impact loading is applied. The maximum value of the maximum principal stress, σ₁ increases as Young's modulus of the adhesive increases when the joints are subjected to impact loadings. It is found that the characteristics of the joints subjected to impact loadings are opposite to those subjected to static loadings. In addition, experiments were carried out to measure the strain response of the butt adhesive joints subjected to impact and static tensile loadings using strain gauges and the joint strengths were also measured. Fairy good agreements are observed between the numerical and the measured results.     

An investigation on the initiation and propagation of damaged zones in adhesively bonded lap joints

In this study, the initiation and propagation of damaged zones in the adhesive layer and adherends of adhesively bonded single and double lap joints were investigated considering the geometrical non-linearity and the non-linear material behaviour of the adhesive and adherends. The modified von Mises criteria for adherends and Raghava and Cadell's failure criteria (J. Mater. Sci. 8, 225 (1973) [1]) including the effects of the hydrostatic stress states for the epoxy adhesive were used to determine the damaged adhesive and adherend zones which exceeded the specified ultimate strains. The stiffness of all finite elements corresponding to these zones was reduced so that they could not contribute to the overall stiffness of the adhesive joint. This approach simplifies to observe the initiation and propagation of the damaged zones in both the adhesive layer and adherends. A tensile load caused first the damaged adhesive zones to appear at the right free end of the adhesive-lower adherend interface and at the left free end of the adhesive-upper adherend interface, and then to propagate through the adhesive regions near the adhesive-adherend interfaces (interfacial failure). In the bending test, the damaged zone initiated at the left free end of the adhesive-upper adherend interface in tension, and similarly propagated through the adhesive regions close to the adhesive-adherend interface (interfacial failure). In the double-lap joint subjected to a tensile load, the damaged adhesive zones initiated first at the right free end of the adhesive-middle adherend interface and then propagated through the adhesive region near the adhesive-adherend interface. After the damaged zone reached a specific length it also grew through the adhesive thickness, and the adhesive joint failed. The SEM micrographs of fracture surfaces around the free edges of the overlap region indicated that the failure was interfacial. An additional damaged zone growth was observed in the side adhesive regions due to lateral straining, called the Poisson effect.     

Non-linear stress and failure analyses of adhesively-bonded joints subjected to a bending moment

In this paper, the mechanical behavior of the Single-Lap Joints (SLJs) bonded with two different adhesives (FM 73 and SBT 9244) under a bending moment was analyzed, both experimentally and numerically. Four-point bending experiments for the joints with different overlap lengths were carried out and fracture surfaces of the SLJs were examined with a Scanning Electron Microscope (SEM). After the stress analysis in the SLJs was performed via a finite element method by considering the material non-linearities of the adhesives and adherend (AA2024-T3), the Finite Element Analysis (FEA) results were compared with experimental results. Finally, the stress analyses and experimental results show that the failure in the SLJs subjected to a bending moment probably initiates from the overlap region on the adhesive–upper adherend interface in tension and propagates towards the centre of the overlap. Also, in the joint subjected to a bending moment, it is seen that the load carried by the SLJ with SBT 9244 adhesive with increasing overlap length is more than that of the SLJ with FM 73 adhesive, although in the bulk form FM 73 adhesive is about three times stronger than SBT 9244 adhesive.     

An End-Loaded Shear Joint (ELSJ) Specimen to Measure the Critical Energy Release Rate in Mode II of Tough,Structural Adhesive Joints

This paper introduces a newly developed specimen type, which is used to measure the critical energy release rate of tough, structural adhesives loaded in shear. This End-Loaded Shear Joint (ELSJ) specimen is loaded until a shear crack propagates through the adhesive layer. When the crack propagation is stopped, by unloading the specimen, the critical energy release rate in mode II, G _IIc, can be obtained by correlating the energy dissipated during the test and the measured crack area on the fracture surface of the specimen. The paper presents the dimensions of the ELSJ specimen, the corresponding test setup and the evaluation method used to obtain G _IIc. An overview of the advantages and the limitations of the new specimen type shows the need for its development and improvement when compared to some state of the art experiments. The first results of ELSJ tests are shown and discussed, using the crash-optimized structural adhesive — Henkel Terokal 5077. The experimental results presented, focus on thin adhesive layers and quasi-static test velocities.     

Analysis of substrate preparation and curing position on mechanical properties of adhesive joints using statistical methods

The main objective of this research was a statistical study of pin-and-collar joints bonded with anaerobic adhesives. Surfaces were prepared in two different ways (by etching and abrading) prior to adhesive bonding. Moreover, samples were cured in two positions (vertical and horizontal) to study its influence. The effects of all these parameters have been analyzed through shear strength measurements. In order to carry out this study, statistical tools based on hypothesis contrast have been used. The results show that, in the studied materials, abrasion of surfaces prior to bonding promotes better mechanical strength than etching. Moreover, curing position has no statistical influence on mechanical properties in samples with a gap of 10 μm between adherends.     

The effects of overlap length and adherend thickness on the strength of adhesively bonded joints subjected to bending moment

In this work, elasto-plastic stress analysis of a Single Lap Joint (SLJ) subjected to bending moment was investigated using 2D non-linear Finite Element Analysis (FEA). The SLJs, consisting of hardened steel as the adherend bonded by two adhesives, one stiff and one flexible, with very different mechanical behaviors were analyzed. In order to determine the effect of geometrical parameters on the performance of the SLJs, four different adherend thicknesses and overlap lengths for each adhesive were used. For verification of the analysis, the FEA results were compared with experimental results. It was observed that there was a significant effect of adherend thickness on the strength of the joint with both adhesives. However, the load carried by the SLJ with the flexible adhesive increased with increasing overlap length.     

Simulation of adhesive joints using the superimposed finite element method and a cohesive zone model

Adhesive joints have been widely used in various fields because they are lighter than mechanical joints and show a more uniform stress distribution if compared with traditional joining techniques. Also they are appropriate to be used with composite materials. Therefore, several studies were performed for the simulation of the bonded joints mechanical behavior. In general for adhesive joints, there is a scale difference between the adhesive and the substrate in geometry. Thus, mesh generation for an analysis is difficult and a manual mesh technique is needed. This task is not efficient and sometimes some errors can be introduced. Also, element quality gets worse.In this paper, the superimposed finite element method is introduced to overcome this problem. The superimposed finite element method is one of the local mesh refinement methods. In this method, a fine mesh is generated by overlaying the patch of the local mesh on the existing mesh called the global mesh. Thus, re-meshing is not required.Elements in the substrate are generated. Then, the local refinement using the superimposed finite element method is performed near the interface between the substrate and the adhesive layer considering the shape of the element, the element size of the adhesive layer and the quality of the generated elements. After performing the local refinement, cohesive elements are generated automatically using the interface nodes. Consequently, a manual meshing process is not required and a fine mesh is generated in the adhesive layer without the need for any re-meshing process. Thus, the total mesh generation time is reduced and the element quality is improved. The proposed method is applied to several examples.     

新肇油田注水压力上升原因及对策浅析

新肇油田在注水开发过程中暴露注水压力上升较快的矛盾,分析新肇油田注水压力上升的原因,提出通过优选粘土稳定剂,井网加密,深度酸化及注活性水等方法缓解注水上升速度。