Some Aspects of Bond Formation and Failure in Adhesive Wood Joints

An investigation has been made of the effect of varying glue-spread on the bond strength of holly (Ilex aquifolium) using three adhesives and three different wood sections. The glue-spreads are lower than those normally used, and it has been found with edge-grain joints that 100% cohesive wood failure can occur with a glue-spread as low as 2.7mg/cm². Scanning electron microscopy shows that interlocking between adhesive and adherend does not occur. Factors leading to delamination and joint failure are discussed.

Lignin, without further addition, has been shown to be a useful wood adhesive. It has also been shown that it is possible to make end-grain joints without the use of an adhesive; the lignin present in the wood specimens is considered to be responsible for such joints.     

Evaluation of FRP/wood adhesively bonded epoxy joints on environmental exposures

This article describes the evaluation of the durability of joints composed of wood adherends with a bonded layer of fibre-reinforced polymer (FRP) fabric. Carbon and glass fibres in an epoxy matrix were studied. The main purpose of FRP usage with timber in the construction industry is generally to improve the stiffness/strength of reinforced members without any influence on their service-life or any environmental impact. From the perspective of the timber reinforcement process, optimal dimensional stability during moisture changes in wood should be one of the most important criteria for such joints. Therefore, FRP/wood joints were evaluated with regard to the influence of real external environmental conditions on the bondline over a period of 40?months. During exposure to these conditions, specimen failures and defects were continuously visually evaluated. The decisive factor in this evaluation was bond integrity, verified by the tensile shear strength of the FRP/wood joint. After the experimental study, it was noted that the first 20?months have a significant effect on bondline failure occurrences, which involve decreases in tensile shear strength. In the next 20?months, the FRP/wood bondlines resist other severe hygrothermal stresses without significant strength decreases. An additional observed parameter was the percentage of wood failure in the bonded area of single lap joints, which characterises the mode of failure of the bonded joint. To determine the influence of ageing on adhesive due to ultraviolet radiation and varying temperature, infrared absorption spectroscopy analysis was performed to reveal changes in the macromolecular structure of the epoxy adhesive. Findings showed that UV radiation had a significant influence on the degradation of the adhesive structure.     

Wood bonding by vibrational welding

—Mechanically-induced wood welding, without any adhesive, is shown here to rapidly yield wood joints satisfying the relevent requirements for structural application. The mechanism of mechanically-induced vibrational wood fusion welding is shown to be due mostly to the melting and flowing of some amorphous, cells-interconnecting polymer material in the structure of wood, mainly lignin, but also hemicelluloses. This causes partial detachment, the 'unglueing' of long wood cells, wood fibres, and the formation of a fibre entanglement network in the matrix of molten material which then solidifies. Thus, a wood cells/fibre entanglement network composite having a molten lignin polymer matrix is formed. During the welding period some of the detached wood fibres which are no longer held by the interconnecting material are pushed out of the joint as excess fibres. Crosslinking chemical reactions also have shown to occur. The most likely one of these identified by NMR appears to be a cross-linking reaction of lignin with carbohydrate-derived furfural. The presence of these reactions has been identified by CP-MAS 13C-NMR. These reactions, however, are relatively minor contributors during the very short welding period. Their contribution increases after welding has finished, which explains why long holding times under pressure after the end of welding contribute strongly to obtaining a good bond.     

Analysis and design of adhesive-bonded tee joints

In this study, the stresses in adhesive-bonded tee joints, in which a right-angled plate is bonded to a rigid plate with an adhesive, have been analysed with a finite element method. It was assumed that the adhesive and adherends had linear elastic properties. The tee joint was analysed under three loading conditions, two linear and one bending moment. The stress distributions in the joint area are given by stress contours and XY plots under the three load conditions. It was found from the results that high stress concentrations occur in the inside corner of the angle plate for loading in the x-direction (P_x) and under bending moment (M), this suggesting that failure would not occur in the bonded joint. However, for loading in the y-direction (P_y), the maximum normal stresses are concentrated at the left free end of the adhesive layer in the joint, and the first failure may be expected at this edge. Since the geometry of the joints affects the analysis and design of such joints, the influences on the stress distributions of the overlap length, adhesive thickness and adherend thickness were investigated. Practical experiments were carried out and it was found that experimental results were in good agreement with those of the finite element analysis.     

Wood bonding by mechanically‐induced in situ welding of polymeric structural wood constituents

Mechanically induced wood fusion welding, without any adhesive, is shown here to yield rapidly bonding wood joints satisfying the relevant requirements for structural application. The mechanism of mechanically induced vibrational wood fusion welding is shown to be due mostly to the melting and flowing of amorphous cells–interconnecting polymer material in the structure of wood, mainly lignin, but also some hemicelluloses. This causes a partial detachment, the “ungluing,” of long wood cells and wood fibers and the formation of an entanglement network drowned in a matrix of melted material which then solidifies, thus forming a wood cell/fiber entanglement network composite with a molten lignin polymer matrix. During the welding period some of the detached wood fibers which are no longer being held by the interconnecting material are pushed out of the joint as excess fiber. Crosslinking chemical reactions of lignin and carbohydrate‐derived furfural also occur. Their presence has been identified by CP‐MAS ¹³C‐NMR. These reactions, however, are relatively minor contributors during the very short welding period. Their contribution increases after welding has finished, which explains why relatively longer holding times under pressure after the end of welding contribute strongly to obtaining a good bond. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 243–251, 2004     

Wood Welding: Chemical and Physical Changes According to the Welding Time

Wood welding using linear friction is a technique that has been developed in the past five years. The goal of this study was to analyze the microstructure development in the interphase enabling the wood-to-wood adhesion without any adhesive. Chemical and physical analyses have been carried out using infrared thermography, mechanical shear tests, transmitted light microscopy and X-ray densitometry. They have been considered as efficient to qualify the characteristics of the welded joints. The aim of this paper is to present a study using these analysis methods to observe the physical modifications of the wood in the interphase according to the welding time. The welding process of beech wood (Fagus sylvatia) with a welding time between 0 and 11 s could be divided into three different phases. The first phase describes changes in the physical and chemical characteristics of the wood. Densification and anatomical modifications occur in this phase. The second phase represents stabilization of the welded joint. The last phase of the cycle is a conditioning phase. All phases are controlled by the heat spread in the interphase and the time of heat exposure. Various parameters such as welding time, shear strength, temperature and width of the welded joint have been correlated and a hypothesis on the chemical reactions occurring in the interphase has been put forth. This study allowed discovering a window of parameters in which the quality of the welded joint is quite stable. Improving the quality of manufactured welded wood products without adhesive can now be done more easily due to this method.     

Quantitative Analysis of Gross Adhesive Penetration in Wood Using Fluorescence Microscopy

An optimum amount of adhesive penetration is desirable for economy of production and development of bond strength in wood composites. A general method that allows quantitative measurement of gross adhesive penetration in wood is described. Staining techniques have been developed that can provide sharp contrast between a cured adhesive and the wood substrate using fluorescence microscopy. An image analysis system utilizes this contrast to quantify gross adhesive penetration in wood. An example of this technique is provided, whereby the effect of molecular weight distribution of phenol formaldehyde prepolymers on gross adhesive penetration into yellow poplar (Liriodendron tulipifera) flakes is observed and quantified. Adhesive penetration into wood flakes was shown to be correlated with the molecular weight distribution of the prepolymer, decreasing with higher weight average molecular weight. Gross adhesive penetration into hardwoods is likely to be dominated by flow into vessel elements, as demonstrated by the wood species studied here.     

Improving the mechanical behavior of the adhesively bonded joints using RGO additive

In this research, Araldite 2011 has been reinforced using different weight fractions of Reduced Graphene Oxide (RGO). Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) analyses were conducted and it has been shown that introduction of the RGO greatly changes the film morphology of the neat adhesive. Uni-axial tests were carried out to obtain the mechanical characteristics of the adhesive-RGO composites. It has been observed that introducing 0.5 wt% RGO enhances the ultimate tensile strength of the composites by 30%. In addition, single lap joints using neat adhesive and adhesive-RGO composites were fabricated to investigate the effect of the added RGO on the lap shear strength of the joints. Results show that the joints with added 0.5 wt RGO exhibited 27% higher lap shear strength compared to the joints bonded with neat adhesive. Finally, Finite Element (FE) numerical solutions using Cohesive Zone Modeling (CZM) have been carried out to simulate the failure behavior of the joints, and it has been shown that the FE models can predict the joint’s failure load.     

Ageing of adhesively bonded joints—fracture and failure analysis using video imaging techniques

An investigation into the durability of adhesively bonded joints has been undertaken to help improve the prediction of joint lifetimes. Polymethylmethacrylate (PMMA) substrates have been bonded with a two-part acrylic adhesive to make single lap-shear joints. Joints have been aged in a hot/wet environment (40°C and 95% humidity) with no applied stress for up to 4000 h and were tested in tension. The novel aspect of the research has been the development of a video imaging analysis technique which allows damage initiation and propagation within the joint to be detected as load is applied to the joint. Images of fracture initiation and damage propagation have been correlated with stress/displacement data for joints under tensile loading. The data from aged samples is compared with data from un-aged samples. Both the stresses at which damage is seen to initiate and the final failure stress of the joints decrease as the ageing time increases. The failure mode changes from cohesive failure within the PMMA substrate to failure within the adhesive, near the PMMA/acrylic adhesive interface.     

Strength prediction of bonded single lap joints by non-linear finite element methods

A non-linear finite element technique has been used to predict the mode of failure and failure load of single lap joints made from three aluminium alloys and four epoxy adhesives, and the results compared with those obtained from experiment and closed-form analyses. The finite element program used was able to account for the large displacement rotations that occur in a single lap joint under load, and allowed the effects of elasto-plasticity in both the adhesive and adherends to be modelled. A failure criterion based on the uniaxial tensile properties of the adhesive was used: for two untoughened adhesives a maximum stress criterion was found to be appropriate while for two toughened adhesives a maximum strain criterion was employed.     

新一代Vinail(R)硬木用水基胶粘剂——远超出木材破坏

在非承载类硬木结构中使用水性热塑性胶粘剂,要求最终胶接接头具有较高的机械性能(高于木材本身)和抗蠕变性.当胶接接头受到静负荷作用时,其机械性能会受到胶粘剂本身性能和木材类型的影响,即与胶粘剂和基材之间密切相关.选取化学成分相同、合成路径不同的两类聚合物,并参照国际标准[12]对胶粘剂的相关性能进行光谱分析、量热分析、形态结构分析和测试.有趣的是,两例胶在剪切试验中表现出显著差异:一类导致被粘基材的内聚破坏,另一类导致被粘基材表面的胶粘剂破坏.由此可知,所测试聚合物所表现出的不同机械性能与粘接机理有关.因此,本文合成了新一代Vinavil水基聚合物胶粘剂,由于采用新颖的前瞻性技术,其特殊性能显而易见,且远超过传统标准.     

An investigation of fatigue failure prediction of adhesively bonded metal/metal joints

A fracture mechanics-based model for fatigue failure prediction of adhesive joints has been applied in this work. The model is based on the integration of the kinetic law of evolution of defects originated at stress concentrations within the joint. Final failure can be either brittle (fracture toughness-driven) or ductile (tensile/shear strength-driven) depending on the adhesive. The model has been validated against experiments conducted on single-lap shear joints bonded with a structural adhesive. Three different kinds of adhesives, namely a modified methacrylate, a one-part epoxy and a two-part epoxy supplied by Henkel, have been considered and three different overlap lengths have been tested. Fracture toughness and fatigue crack growth properties of the adhesives have been determined with mode I tests. The number of cycles to failure has been successfully predicted in several cases. It is interesting to notice that in the case of joints loaded at the same average shear stress, the shorter the joint, the longer the duration. This fact is also captured by the model.     

Application of Numerical Modelling to Dowel-Welded Wood Joints

Numerical models using a 3-D finite element analysis method and the behaviour of dowel-welded wood joints are presented. Simulation results for step butt wood joints with two welded wood dowels under shear are analyzed and a good agreement with the experimental results is shown. Anisotropic elasto-plastic constitutive law with hardening associated with material densification, without distinction between radial and tangential properties, was used for the compressive behaviour of wood. The good coherence of the results obtained demonstrates clearly the capability of the model developed to simulate accurately the non-linear behaviour of dowel-welded wood joints to failure.     

Failure criterion for the fracture of structural adhesive joints

The long-term strength of stressed, structural adhesive joints, consisting of aluminium alloy substrates bonded with an epoxide adhesive, has been investigated. The applied adhesive fracture energy, G_Ic, is shown to be linearly dependent upon the logarithmic time-to-failure; the failure time decreases as the value of G_Ic is increased. The fracture of these joints over eight decades of time is uniquely described by the hypothesis that there is a critical plastic-zone size developed at the crack tip at failure.     

Failure analysis of AA8011-pultruded GFRP adhesively bonded similar and dissimilar joints

In this work, a comparative failure analysis of aluminum (AA8011/AA8011) and glass fiber reinforced polyester (GFRP/GFRP) based similar and dissimilar joints is presented. The GFRP is prepared using pultrusion technique. Single lap joints are prepared by using Araldite R2011 epoxy as an adhesive. The lap joints are then tested under tension to estimate the average shear strength of the assembly. It is observed that the average bond strength of AA8011/AA8011 is lesser than that of the GFRP/GFRP joint. The failure of similar joints occurred by fracture within the adhesive. The dissimilar joint is failed predominantly by interface debonding. Further, a detailed three dimensional stress analysis of the joints is carried out using finite element method (FEM). The damage analysis of adhesive layer is carried out by coupling FEM with cohesive zone model (CZM). The stress, damage distributions and failure mechanisms are compared for similar joints in detail. A failure mechanism is proposed for AA8011/AA8011 type joint that favours a rapid crack growth in the adhesive after crack initiation, which is responsible for lesser bond strength. The increase in overlap length has positive effect that the peak load increases proportionally with overlap length.     

Scarf repairs to highly strained graphite/epoxy structure

Scarf joints representative of repairs to graphite/epoxy (gr/ep) honeycomb structure, typical of that used in the F/A-18 horizontal stabilator, have been investigated using mechanical tests and finite-element (FE) modelling. The load capacity of such scarf repairs is marginal when tested under hot/wet conditions compared to the required design ultimate strain of 5200 με. An analysis of the scarf joint using FE methods predicts that high stresses will occur at ply drop-offs and at the top of the scarf and good correlation is observed between the FE models and experimental results. Detailed predictions of the shear stress distribution within the joint have also been obtained from the FE models. These show that the shear stress in the adhesive is not uniform over the length of the scarf, as would be the case for isotropic adherends, but rather varies with position by up to 250%, due to the varying longitudinal compliances of the plies within the gr/ep adherends. Experimental evidence indicating the occurrence of stress relaxation or creep within the scarf joints is also reported. An F/A-18 stabilator containing a scarf repair has been loaded satisfactorily to design limit load without failure. External patches have also been shown to be effective as temporary repairs.     

Fracture energy in structural adhesive joints of composite-aluminum under adverse environments conditions

This work analyzes the degradation of composite-aluminum adhesive joints when they are exposed to the weathering and environmental pollution in Madrid for a long period of time. Two adhesives (epoxy and polyurethane) and several surface treatments for adherends have been considered. End-notched flexure bending tests have been performed to evaluate the loss of mechanical properties (failure stress and fracture energy) of adhesive joints that were exposed to the weathering and environmental pollution. Tests results have shown that the environmental degradation of the adhesive leads to a loss of mechanical properties in the adhesive joints. Considering the relative percentage, the reduction of failure stress in the polyurethane is higher than in the epoxy (31.9% for the polyurethane and 21.1% for the epoxy). Similarly and considering relative percentage, fracture energy reduction is 42.0% for polyurethane and 41.5% for epoxy. Likewise, tests have shown that the loss of mechanical properties does not decrease linearly with the time when the samples have been exposed to the weathering. This reduction occurs during the first few weeks. In summary, tests results have allowed to conclude that adhesive joints with epoxy resist the environmental pollution better than the adhesive joints with polyurethane.     

Environmental effect on the fatigue degradation of adhesive joints: A review

ABSTRACT

Environmental factors, such as temperature and moisture, are known to have a degrading effect on the mechanical properties and performance of adhesive joints, which may be perceived as a non-problem because various works have shown that the static response of an adhesive is normally unaffected by slight moisture and temperature variations that occur in real-world applications. While this may be true, performance under purely static conditions is rarely found in commercial uses and most adhesive joints are subjected to cyclic loadings throughout their life. Interestingly, not much work has been done on the effects of the environment on cyclically loaded adhesive joints, but the consensus is that the fatigue response is much more affected by environmental changes than the static response, which is arguably the most important analysis. The general trend is that hygrothermal ageing decreases the number of cycles the joint can withstand and also decreases the threshold fracture toughness value, which translates to cracks initiating sooner, but exceptions to these behaviours also exist.