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.     

Effects of different metals on adhesive wear behaviour of alloy surface produced by TIG process

Abstract

Low carbon steel surfaces were alloyed with composite powders using the tungsten inert gas welding method. After the alloying process, the effects of cladding surface on the microstructural characteristics and adhesive wear of the alloyed samples were examined. The sliding wear behavior of samples was investigated in a block on ring apparatus under the loads of 20, 40, 60 and 80 N respectively. In the experimental investigation, a low carbon steel surface was alloyed with austenitic stainless steel powder and austenitic stainless steel powder mixed with 4·5% Co, Mo and Ti particles respectively. Following surface alloying, conventional characterisation techniques, such as optical microscopy, scanning electron microscopy, energy dispersive spectrograph and X-ray diffraction, were used to study the microstructure of the alloyed zone. Examination of the microstructure revealed the presence of M₂₃C₆ carbides, solid melt phases and intermetallic phases, such as Ni₃Ti, depending on the alloying element in the composite. As the amount of the reinforcing material increased, the saturation rates for the samples decreased, while their hardness increased. The adhesive abrasion tests conducted revealed that temperature input plays a significant role on the microstructure characteristics, which positively affected the adhesive abrasion values of the samples. Consequently, the tungsten inert gas welding method was successfully used for the surface alloying of low carbon steels.     

Non-destructive characterization of bondlines in composite adhesive joints

Aerospace structures use polymeric composite materials extensively. These composite materials are normally bonded together by adhesives to form structural parts. The existence of any kind of defects or discontinuities in the bonds is completely undesirable for such applications. Ultrasonic imaging (UI) is a widely used technique for non-destructive evaluation (NDE) and can be adopted to evaluate the integrity of such adhesively-bonded joints. However, characterization of adhesive bonds in composite materials using UI has deficiencies due to problems such as high acoustic attenuation and high signal-to-noise ratio. These problems can be attributed to the inhomogeneity in composite structures. The present study addresses the problems of detection of disbonds and porosity in adhesively bonded carbon fiber reinforced composite panels. Five sets of adhesively-joined carbon/epoxy composites with different adherend surface preparations were fabricated and subjected to UI. The panels contained known defects in the bondline of the samples. UI results are interpreted to identify various existing defects such as voids, cracks and disbonds in the joints. Attenuation coefficient values for all types of composites are utilized to ascertain the validity of the image analysis.     

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.     

Stress analysis and strength evaluation of bonded shrink fitted joints subjected to torsional loads

This paper deals with the stress analysis and strength evaluation of bonded shrink fitted joints subjected to torsion. The stress distributions in the adhesive layer of bonded shrink fitted joints are analyzed by the axisymmetric theory of elasticity when an external torsion is applied to the upper end of the shaft. The effects of the outer diameter and the stiffness of rings on the interface stress distributions are clarified by numerical calculations. Using the interface stress distributions, the joint strength is predicted. In addition, the joint strength was measured experimentally. It is seen that rupture of the adhesive layer is initiated from the upper edge of the interface when torsion is applied to the upper end of the shaft. The numerical results are in fairly good agreement with the experimental results. It was found that the joint strength of bonded shrink fitted joints is greater than that of shrink fitted joints.     

A novel mathematical procedure to evaluate the effects of surface topography on the interfacial state of stress. Part I: Verification of the method for flat surfaces

A mathematical procedure is developed to utilize the complementary energy method, by minimization, in order to obtain an approximate analytical solution to the 3D stress distributions in bonded interfaces of dissimilar materials. The stress solutions obtained predict the stress jumps at the interfaces, which cannot be captured by current FEA methods. As a novel method, the penalty function is used to enforce the displacement boundary conditions at the interfaces. Furthermore, the mathematical procedure developed enables the integration of different interfacial topographies into the solution procedure. In order to incorporate the effects of surface topography, the interface is expressed as a general surface in Cartesian coordinates, i.e. F (x, y, z) = 0. In this paper, the flat interface problem, i.e. y = 0 surface is considered for verification of the method by comparison with the FEA method. A comparison of the results reveals our new mathematical procedure to be a promising and efficient method for optimizing interface topographies.     

Thermal characteristics of tubular single lap adhesive joints under axial loads

When an adhesive joint is exposed to high environmental temperature, the tensile load capability of the adhesive joint decreases because both the elastic modulus and failure strength of the adhesive decrease. The thermo-mechanical properties of a structural adhesive can be improved by addition of fillers to the adhesive. In this paper, the elastic modulus and failure strength of adhesives as well as the tensile load capability of tubular single lap adhesive joints were experimentally and theoretically investigated with respect to the volume fraction of filler (alumina) and the environmental temperature. Also the tensile modulus of the filler containing epoxy adhesive was predicted using a new equation which considers filler shape, filler content, and environmental temperature. The tensile load capability of the adhesive joint was predicted by using the effective strain obtained from the finite element analysis and a new failure model, from which the relation between the bond length and the crack length was developed with respect to the volume fraction of filler.     

The role of interfacial interactions and loss function of model adhesives on their adhesion to glass

The measurement of adhesion and the evaluation of influencing factors are of great scientific and technological importance. There are two distinct viewpoints on adhesion: (i) surface chemistry, and (ii) fracture mechanics. For elucidation of the relative importance of mechanical properties in the bonding of adhesives, the strength of adhesion between model adhesives and glass plates was measured by the wedge cleavage (WC) test method. Copolymers of methyl methacrylate (MMA) with n-butyl acrylate (nBA) and methyl methacrylate with styrene (S) were prepared as model adhesives. The results show that in MMA-nBA copolymers, by increasing the amount of nBA, both the loss function and the adhesion energy of the adhesives increase. However, by increasing the amount of nBA above a certain level, the adhesion strength begins to decrease. In this situation, the cohesive strength of the adhesive dominates the failure mechanism. On the other hand, a decrease in adhesion was expected upon increasing the amount of styrene in the poly(styrene-co-methyl methacrylate) adhesive, because methyl methacrylate, an interactive monomer with glass, is replaced by a non-interacting styrene monomer, while the loss function of the adhesive is almost constant. But our practical adhesion measurement technique was not sensitive enough to detect this adhesion loss.     

Theoretical and experimental studies for the failure criterion of adhesively bonded joints

The objective of this work was to develop a criterion for predicting the failure strength of joints bonded by ductile adhesives. To obtain the criterion, first, fracture tests were carried out on T-peel joints and single-lap joints with various joint geometries, adhesives, and adherend materials. Then using the fracture loads obtained in the tests, a finite element analysis was performed by which the stresses in the adhesive joints were calculated. It is concluded that the failure of an adhesively bonded joint occurs when the maximum of the ratio of the mean to effective stresses exceeds a certain value, which can be considered a new material constant of a ductile adhesive.     

Selective use of rubber toughening to optimize lap-joint strength

This paper introduces a novel approach to increasing the lap joint strength, different from the traditional methods of either increasing the lap joint area or altering the joint geometry. This is accomplished by the selective use of rubber toughening in epoxy to optimize lap joint strength. This was accomplished in three stages. In the first stage an adduct was prepared, this was used to make bulk tensile specimens to calculate the bulk properties for various concentrations of rubber, i.e. 0, 10, and 20 parts per hundred parts of resin (epoxy). In the second stage finite element models were developed using the bulk properties previously obtained. Interfacial stresses were used to access the trends obtained by the selective use of rubber toughening at different locations of the overlap in different configurations. The modeling of adhesive joints was done using ALGOR 2-D, linear and nonlinear finite element analyses (FEA). In the third stage, tensile shear tests conducted on the lap joints validated the trends from the finite element models. Finite element modeling and meshing of the lap joints having 25.4 and 50.8 mm adhesive overlap lengths were completed. Different configurations of rubber toughened and untoughened adhesive were tried in these two overlaps. The validation was done by lap joint tests conducted on an Instron mechanical tester coupled with an extensometer. Comparable strengths were obtained for completely toughened overlap and the configuration where only the edges of the adhesive overlap were toughened and the region in-between was untoughened. Also, the nonlinear FEA was shown to represent the experimental results more closely than the linear approach.