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.     

Performance of Dowel-Welded T-Joints for Wood Furniture

Comparison of two types of furniture joints namely step butt T-joints and mortise and tennon T-joints held together either by one or two welded or glued dowels showed that the shear strength results of welded dowel and glued dowel joints were comparable. For mortise and tennon T-joints there is, in general, no difference if the dowel is inserted at a 45° or 90° angle. Also there is no significant difference between welded and glued dowel joints stiffness values in both step butt and mortise and tennon T-joints of the same geometry. Also, there is no significant difference between dowels inserted at 45° or 90° for mortise and tennon T-joints. Glued dowel and welded dowel step butt T-joints behave quite differently from linear joints. Thus, in step butt T-joints a higher shear strength is obtained if a single dowel is inserted at 45° for both glued and welded dowels. Both shear strength and stiffness increase as the number of dowels increases, namely from one to two. The application of the welded joint technique to joints where the number of dowels is limited by the limited space in which they can be inserted, such as in furniture, can give shear strength results comparable to those obtained by gluing the same dowels. This is particularly the case for mortise and tennon T-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.     

Global Fault Detection in Adhesively Bonded Joints Using Artificial Intelligence

In general, non-destructive evaluation is applied to detect and localize structural faults using a signal with a wavelength smaller than the detected fault. But the method requires analyzing the object in numerous small sections to detect the damage. Non-invasive diagnosis methods for fault detection are used in different industrial sectors. In this work, the main focus is on global fault detection for structural mechanical components such as a bonded beam using artificial intelligence, i.e., neural nets. Therefore, the fault detection procedure requires only a global measurement in the structural component in operational conditions. An experimental setup using two aluminum beams bonded with an adhesive was used to simulate a bonded joint. Different sizes of adhesive surface simulate faults in the original adhesive joint. Thereafter, resonance frequency shifts in the Frequency Response Functions (FRFs) were used to detect structural faults. Damage in structures causes small changes in the structural resonances. Then, the FRFs were used as an input into an artificial supervised neural network. This work considers global non-destructive tests focused only on the soundness estimation of the system. The neural network involved is a supervised feed-forward network with Levenberg–Marquardt backpropagation algorithm, which classifies the beams in four clusters. The classification consists in beam damaged or not damaged. If the beam is damaged the intensity of the fault is established.     

Strength and failure of bulky adhesive joints with adhesively-bonded columns

Strength and failure properties of bulky (i.e. with thick substrate) adhesive joints with adhesively-bonded columns (ABCs) subjected to external loads were investigated experimentally and analytically. From the experimental results, it was found that the strengths of the bulky adhesive joints with ABCs increased considerably when they were subjected to external tensile loads or lateral bending loads. And the joint strengths increased with increasing depths of the blind holes. The failure process of the joints with ABCs was simulated by the technique of element birth and death developed in the finite element method (FEM). The conclusions obtained from FEM coincide with those obtained from the experiments.     

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.     

Durability of metal joints bonded with aluminium powder filled epoxy adhesive

Abstract

Durability of the metal joints bonded with aluminium powder filled epoxy adhesive was investigated by measuring the joint strength by the single lap shear test before and after exposure to distilled water and the hot and humid Arabian Gulf atmosphere. Fractured specimens were examined by photography. The epoxy adhesive retained its strength with as much as 50 wt-% addition of aluminium filler. Moreover, varying the Al filler content in the adhesive did not have a significant effect on adhesive behaviour in either of the two environments studied. Exposure to atmosphere for as long as 6 months did not cause a deterioration of strength for metal joints bonded with aluminium powder filled epoxy. They failed almost completely within the adhesive, similar to the cohesive mode of unexposed specimens. However, a significant strength decrease was observed in adhesive joints after exposure to distilled water for 6 months. The joints failed in more than a single mode. The interior part of the adhesive lap area failed in cohesive mode while an adhesion failure mode was observed near the edges of the adhesive lap area, which is believed to be a result of moisture diffusion through the edges.     

Evaluation of high-performance pressure-sensitive adhesives and VHB™ acrylic foam tapes bonded aluminum joints subjected to environmental aging

A range of 3M^? high performance pressure-sensitive adhesives (PSAs) and 3M^? VHBTM^? acrylic foam tapes were used to bond 25.4 mm × 3.175 mm aluminum 2024 T-4 adherends in both single-lap joint (SLJ) and three-point end-notch flexure (ENF) configurations. The samples were subjected to two types of aggressive environments to simulate extreme service conditions: freeze–thaw and heat–cool cycling, both for 21 days. Impulse-frequency response vibration and electrochemical impedance spectroscopy (EIS) were used for monitoring bond quality non-destructively. Data were first obtained on a set of specimens at room conditions (i.e., before being subjected to freeze–thaw, or heat–cool cycling), referred to as "baseline" in this paper. After obtaining baseline data, several batch sets were subjected to quasi-static lap-shear and dynamic impact loadings to compare the mechanical and electrochemical properties before and after environmental cycling. EIS results show that moisture absorption caused a reduction in low-frequency impedance, whereas decrease in adhesive thickness caused a reduction of impedance over the entire frequency range. The impulse-frequency response vibration NDE technique was able to detect the changes in loss factor (damping) of adhesive joints after environmental aging. Quasi-static lap-shear loading of SLJs showed that acrylic foams took less failure load compared to PSA tapes, and SLJs subjected to dynamic impact showed all PSAs and acrylic foams taking about the same impact load to failure, except the softer acrylic foam.     

Effect of Substrate Material on Fatigue Crack Propagation Rate of Adhesively Bonded DCB Joints

The effect of substrate material on the fatigue crack propagation rate was investigated using adhesively bonded DCB specimens with CFRP and aluminum substrates. The experimental results show that the increase in thickness of the adherend lowers the fatigue threshold, ΔG _th, and raises the crack growth parameter, n, irrespective of the substrate material, and that the crack growth parameter, n, for the aluminum joints is less than that for the CFRP joints. To elucidate the fatigue crack propagation behavior, fracture surface observation and finite element analysis have been conducted. Besides, Gurson's model is applied to the adhesive layer. SEM images show that numerous voids are formed in the fracture surface for the joints with aluminum substrate, but the growth of voids is suppressed for the joints with CFRP substrate. FEM results also show that the void area fraction for the joint with aluminum substrate is greater than that with CFRP substrate. Thus, the above experimental and numerical trends of voids correspond to the trends of the fatigue crack propagation behavior.