Analysis of adhesive bonded composite lap joints with transverse stitching

The effect of transverse stitching on the stresses in the adhesive is investigated using an adhesive sandwich model with nonlinear adhesive properties and a transverse stitching model for adhesive bonded composite single-lap and double-lap joints. Numerical results indicate that, among all stitching parameters, thread pretension and stitch density have significant effect on the peel stresses in the adhesive; increase in the thread pretension and the stitch density leads to a decrease in peel stress in the adhesive, while an increase in other parameters generally results in a negligible reduction in peel stress. The effect of stitching was found to be negligible on the shear stresses in the adhesive. Thus it is concluded that stitching is effective for the joints where peel stresses are critical and ineffective for those where shear stresses are critical.     

Influence of adhesive spew in bonded single-lap joints

Adhesively bonded lap joints involve dissimilar material junctions and sharp changes in geometry, possibly leading to premature failure. Although the finite element method is well suited to model the bonded lap joints, traditional finite elements are incapable of correctly resolving the stress state at junctions of dissimilar materials because of the unbounded nature of the stresses. In order to facilitate the use of bonded lap joints in future structures, this study presents a finite element technique utilizing a global (special) element coupled with traditional elements. The global element includes the singular behavior at the junction of dissimilar materials with or without traction-free surfaces.     

Environmental durability of aluminium adhesive joints protected with hydration inhibitors

The effect of adsorbed inhibitor films on the durability of aluminium adherends prepared by the Forest Products Laboratory process was determined. Dilute aqueous solutions of phosphoric acid (PA) and nitrilotris (methylene) phosphonic acid (NTMP) were used. The presence of a monolayer of NTMP resulted in a four-fold increase in bond durability over that of untreated adherends. Dilute solutions of PA were ineffective in improving bond durability. Correlated wedge tests (coupled with a fracture energy analysis), surface composition determinations using X-ray photoelectron spectroscopy, and surface examination using high resolution scanning electron microscopy suggested that a compound's effectiveness in improving bond durability depends on its ability to inhibit the conversion of aluminium oxide to hydroxide and form chemical bonds with the adhesive.     

Enhancement of mechanical properties of aluminium/epoxy composites with silane functionalization of aluminium powder

In this study, the effect of surface modification of aluminum powders on the tensile, fracture, and tribological behaviors of aluminum/epoxy composites was investigated. Aluminum powders were surface-modified with 3-aminopropyltriethoxysilane. Aluminium/epoxy composites were fabricated by cast molding method using 10 wt.% untreated and silane-treated aluminium powders. Tensile, mode I fracture, and tribological tests were performed on both composites. The results showed that the tensile modulus and strength of silane-treated aluminum/epoxy composites were ～9% and ～12% greater, respectively than those of untreated aluminum/epoxy composites. The results also showed that the fracture toughness and wear resistance of silane-treated aluminum/epoxy composites were ～32% and ～56% greater than that of untreated aluminum/epoxy composites. Scanning electron microscope (SEM) examination showed the improvement of tensile and fracture properties of silane-treated aluminum/epoxy composites was attributed to the improved dispersion and bonding of aluminum particles in the epoxy, due to the silanization of aluminium powders.     

Fatigue characteristics of plasma treated aluminium repaired by graphite/epoxy composite patch

Abstract

In the present study, the fatigue behaviour of plasma treated aluminium patched by a graphite/epoxy composite (carbon fibre reinforced plastic, CFRP) has been investigated. The aluminium was surface treated using a dc plasma containing acetylene gas and nitrogen gas at a volume ratio of 5 : 5 for 30 s. The effect of plasma treatment on the fatigue behaviour of the aluminium/CFRP specimen was determined from fatigue testing using two different single edge notched (SEN) specimens of cracked aluminium repaired with a CFRP patch and plasma treated aluminium also repaired with a CFRP patch. The load ratio and the frequency applied in the fatigue tests were 0.1 and 10 Hz, respectively. The surfaces of the aluminium specimens were examined using atomic force microscopy (AFM) to investigate the effect of plasma treatment on the surface morphology. The results showed that plasma treated specimens exhibited almost 12% more fatigue life than untreated specimens. The surface roughness of aluminium was increased ~1.5 times by plasma treatment. The increased surface roughness improved the bonding strength between aluminium and the CFRP patch, increasing the fatigue life of aluminium patched by CFRP.     

Mode I fracture modeling of elastomer toughened epoxy adhesive joints

An analytical model is developed to correlate the mode I fracture energy of toughened epoxy adhesive joints with microdamage mechanisms generated around a crack tip. The analytical expression for the mode I fracture energy is derived on the basis of total energy dissipation during crack extension. Three dominant damage modes, plastic shear band formation, plastic void growth, and plastic deformation of the entire matrix resin in a crack-tip region, are identified in the proposed model as the primary energy dissipation mechanisms. Numerical results show that the models can predict the effects of adhesive thickness on the mode I fracture energy of toughened adhesive joints. The analytical model involving material constants and microstructural variables should provide some guidelines toward achieving optimum fracture toughness for these types of adhesive joints. This revised version was published online in July 2006 with corrections to the Cover Date.     

Environmental durability of phosphoric acid anodized aluminium adhesive joints protected with hydration inhibitors

An adsorbed monolayer of the organic inhibitor nitrilotris methylene phosphonic acid (NTMP) improves the bond durability of 2024 aluminium adherends prepared by phosphoric acid anodization (PAA). As had previously been determined for Forest Products Laboratories (FPL)-prepared adherends, maximum improvements occurred when a monolayer of NTMP was adsorbed onto the surface. Examination of the wedge test failure surfaces of PAA adherends treated in NTMP revealed that although crack propagation had initially involved oxide to hydroxide conversion of the original PAA oxide, the locus of failure transfers to the adhesive near the surface quite early in the test. This means that the failure of NTMP-treated PAA adherends was predominantly cohesive through the adhesive.     

Effect of heat treatment on tensile properties of friction stir welded joints of 2219-T6 aluminium alloy

Abstract

The effect of post-weld heat treatment (PWHT) on the tensile properties of friction stir welded (FSW) joints of 2219-T6 aluminium alloy was investigated. The PWHT was carried out at aging temperature of 165°C for 18 h. The mechanical properties of the joints were evaluated using tensile tests. The experimental results indicate that the PWHT significantly influences the tensile properties of the FSW joints. After the heat treatment, the tensile strength of the joints increases and the elongation at fracture of the joints decreases. The maximum tensile strength of the joints is equivalent to 89% of that of the base material. The fracture location characteristics of the heat treated joints are similar to those of the as welded joints. The defect free joints fracture in the heat affected zone on the retreating side and the joints with a void defect fracture in the weld zone on the advancing side. All of the experimental results can be explained by the hardness profiles and welding defects in the joints.     

Strength of carbon/epoxy composite single-lap bonded joints in various environmental conditions

Composite single-lap bonded joints were tested to study the combined effect of the environmental condition and the manufacturing method as they pertain to joint strength and failure conditions. Three different environmental conditions and four different manufacturing methods were considered. In terms of the manufacturing method, cocured joints without an adhesive (CCN) showed the highest strength in all environments. Comparing the effect of the environmental conditions, the elevated temperature and wet condition (ETW) gave the best results for all joints except co-bonded joints. In co-bonded joints, the highest strength was found in the cold temperature condition (CTD).     

A simple methodology for the design of metallic lap joints bonded with epoxy/ceramic composites

In the present study it is proposed a shape factor that allows correlating the static strength of two single lap joints with different geometries. The formula is valid for epoxy/ceramic composites used as adhesive in special applications in oil and gas industry and highly resistant metallic adherends. Therefore, only a few tests performed in particular joint taken as reference are necessary for assessing the structural integrity of any similar joint with arbitrary geometry. Results from tensile tests performed in joints with different bonding areas are compared with model prediction showing a good agreement.     

Structural behavior of double-lap shear adhesive joints with metal substrates under humid conditions

Structural adhesive bonding is very often used joining method in aerospace and automotive industry, but in civil engineering, especially in façade applications, semi-flexible or semi-rigid adhesives are still rarely used. The article is focused on experimental analyses of structural adhesive joints intended for façade applications (e.g. bonding of façade cladding elements to the supporting substructure). The experimental study contains a comparison of the structural behavior of two different adhesives in joints with aluminum or zinc-electroplated steel substrates with various surface pre-treatments. The main goal of the study is a comparison of the mechanical properties of joints exposed and unexposed to laboratory ageing conditions; immersion on demineralized water according to ETAG 002 (Guideline for European Technical Approval for Structural Sealant Glazing Kits). Water content in adhesive layer can change significantly its mechanical properties and adhesion of glue to the substrate. Ageing resistance of joint can be improved by durability increasing of the substrate. For this reason, two different substrate materials with various surface treatments (mechanical roughening, smooth surface, anodizing) were tested. Different adhesive resistance against humid conditions was observed depending on the substrate material and pre-treatment. STP polymer joints showed strength reduction by 30% after immersion for almost all substrates, while acrylate adhesive proved 20% strength reduction for roughened aluminum substrate and 60% strength reduction for zinc-electroplated steel substrate with a roughened surface. The zinc-electroplated steel substrate showed problematic adhesion in case of the acrylate adhesive both reference set of specimens and specimens exposed to laboratory ageing. The positive effect of roughening on adhesion and ageing resistance was clearly observed in the specimens bonded by the acrylate adhesive.

     

Crack path selection in the fracture of fresh and degraded epoxy adhesive joints

The crack paths and fracture surfaces of aluminum–epoxy adhesive joints were characterized as a function of the mode ratio of loading and the amount of degradation that had been generated using the open-faced aging technique. A finite element (FE) model was used to predict the extent of the plastic zone at different crack growth lengths and mode ratios, and a close relationship was found between the evolution of the plastic zone and the previously reported R-curve behavior of these joints. The micro-topography of the fracture surfaces, measured using an optical profilometer, showed that a ductile–brittle transition occurred in the fracture behavior of the joints as degradation progressed. The crack path in the (brittle) degraded specimens was normal to the first principal stress, but could not be predicted in the undegraded joints because of its highly three-dimensional nature. Based on the distribution of the maximum von Mises stress in the adhesive layer ahead of the crack tip, a crack growth mechanism was proposed that is consistent with these experimental observations and explains the highly three-dimensional nature of fracture in these highly constrained joints.     

Effects of post-weld heat treatment on microstructure and mechanical properties of friction stir welded joints of 2219-O aluminium alloy

Abstract

Post-weld heat treatment (PWHT) of 2219-O aluminium alloy friction stir welding joints was carried out at solution temperatures of 480, 500 and 540°C for 32 min followed by aging at 130°C for 9 h. The effects of PWHT on the microstructure and mechanical properties of the joints were investigated. Experimental results show that PWHT causes coarsening of the grains in the weld, and the coarsening degree increases with increasing solution temperature. The tensile strength of the heat treated joints increases with increasing solution temperature. The maximum tensile strength can reach 260% that of the base material at the solution temperature of 540°C. PWHT has a significant effect on the fracture locations of the joints. When the solution temperature is lower than 500°C, the joints fracture in the base material; when the temperature is higher than 500°C, the joints fracture in the weld. The change of the fracture locations of joints is attributed to the presence of precipitate free zones beside the grain boundaries and coarsening equiaxed grain structures in the weld.     

The applicability of plate wave techniques for the inspection of adhesive and diffusion bonded joints

The role of plate waves in the inspection of adhesive and diffusion bonded joints is examined. This involves a review of the modal techniques which have been proposed for the measurement of the adhesion and cohesion properties of adhesive joints and the presentation of some of our own studies on the detection and characterization of an unwanted layer of brittle alpha case in diffusion bonded titanium. It is concluded that Lamb waves, which occupy the whole joint, are viable in principle but are limited in both applications by their strong sensitivity to the material properties and the thicknesses of the adherends and their relative insensitivity to those of the bondline layer. On the other hand, embedded modes, which propagate along an embedded layer, are largely insensitive to the adherends, the dispersion curves showing a major improvement in sensitivity to the properties of the layer and to the boundary conditions between the layer and the adherends. The drawback is that their exploitation is limited in practice because it is difficult to excite and detect them. True modes offer good potential but require access to the ends of the joints. In attempting to excite leaky modes, minima of the reflection coefficient, commonly used to measure Lamb wave dispersion curves in immersion coupled plates, do not correspond to the dispersion curves because the acoustic impedance of the adherends is too large. Therefore, although measurement of the minima offers good potential for inspection, this is a response technique and cannot be associated directly with the plate modes. In neither of the examples studied could an interface wave exist at a single interface between the bondline layer and an adherend. However, in general such modes could be rather attractive for inspection, provided that their wavelengths are much smaller than the layer thickness, because they are sensitive to the interface region but not to the thicknesses of the layers, and they are relatively simple to measure.     

Accurate evaluation of 3D stress fields in adhesive bonded joints via higher-order FE models

Abstract

The accurate representation of the 3D stress fields at the bonded areas of adhesive joints is essential for their design and strength evaluation. In the present study, higher-order beam models developed in the framework of the Carrera Unified Formulation are employed to reduce the complexity and computational cost of numerical simulations on adhesive joints. The different components of the adhesive joint, i.e. adherends and adhesive, are modeled as beams with independent kinematics based on the Hierarchical Legendre Expansion (HLE). HLE models make use of a hierarchical polynomial expansion over the cross-section of the beam, thus allowing for the control of the accuracy of the stress solutions via the polynomial expansion. Recalling the Finite Element method, the beam axis is discretized by means of 1D elements. In this manner, generic geometries of the adhesive bonded joints can be studied. The proposed model is assessed through comparison against numerical and analytical references from the literature for single lap and double lap joints. Finally, a detailed 3D analysis is performed on the single lap joint problem, showing that the stress gradients along the adhesive are correctly and efficiently described if the proposed methodology is employed.     

Microstructure and strength of TiAl/steel joint induction brazed with Ag-Cu-Ti filler metal

Abstract

Intermetallic TiAl was induction brazed to steel in an induction furnace with Ag-Cu-Ti filler metal at 1143 K for 0·2–2·4 ks. Microstructural analysis indicates that Ti, Al, and C atoms in base metal diffuse to the interface and react strongly with the filler metal during brazing. The interface structure of the joint can be divided into three distinct zones: the reaction layer near TiAl, composed of Cu-Al-Ti compounds and Ag based solid solutions; the central zone of the interface, consisting of Ag based solid solutions in which Ag-Cu eutectic phases are dispersed; a TiC reaction layer adjacent to the steel. The relationship between brazing parameters and tensile strength of the joints is discussed, and the optimum induction brazing parameters obtained. When brazed at 1143 K for 0.9 ks, the tensile strength of the joint is 298 MPa.     

Impact response of Kevlar composites with filled epoxy matrix

Kevlar fibres have been widely used as impact-resistant reinforcement in composite materials. The paper studies the impact behaviour as well as damage tolerance of Kevlar/filled epoxy matrix. Two different fillers, cork powder and nanoclays Cloisite 30B, were used in order to improve the impact response of these laminates. For better dispersion and interface adhesion matrix/clay nanoclays were previously subjected to a silane treatment appropriate to the epoxy resin. The fillers adding increases the maximum impact load but the opposite tendency was observed for the displacement. Nanoclays promote higher maximum impact loads, lower displacements, the best performance in terms of elastic recuperation and the maximum residual tensile strength.