Rods glued in engineered hardwood products part I: Experimental results under quasi-static loading

Glued-in Rods (GiR) represent an adhesively bonded structural connection widely used in timber engineering. Up to now, common practice largely focused on softwood. Most structural adhesives have been, accordingly, specifically formulated to perform on softwood, in particular spruce. The increased use of hardwood, and corresponding engineered wood products (EWP), calls for deeper insights regarding GiR for the connection thereof. This paper, the first of a two part series, presents an overview over extensive research carried with 9 adhesives, 3 EWP, and 4 types of rods. Investigations started at component level, by fully characterising all adhesives, EWP, and rods. They were then extended to characterise the behaviour of interfaces, providing by this a methodology for selecting adhesives. Investigations at full scale followed, involving 5 different adhesives, 3 EWP, and 4 rod types. A total of 180 individual samples were tested. The results allowed to draw conclusions about the relationship between performance of GiR connections, and mechanical properties of their components. This relationship, however, has been found to be relatively weak. The companion paper will present a design methodology based on the material properties determined herein, and explain the ambiguous relationship between performance of the GiR and the mechanical properties of the adhesive, wood, and rods     

Influence of the mechanical behaviour of different adhesives on an interference-fit cylindrical joint

Hybrid adhesive joining techniques are often used in many industrial sectors to design lightweight structures. A hybrid adhesive joint results from the combination of adhesive bonding with other traditional joining methods such as welding and mechanical fastening, with the aim of combining the advantages of the different techniques and overcoming their drawbacks.This study focuses on the interference fitted/adhesive bonded joining technique. In this application, two cylindrical components are coupled together by inserting one into the other, after having placed an adhesive between them. Generally anaerobic acrylic adhesives, also known as “retaining compound” are used for this application. However the effect of the adhesive nature and of its mechanical and adhesive responses on the performance of the hybrid joint is still unclear. The aim of the present research is to improve the understanding of the behaviour of different adhesives, including rigid epoxies and flexible polyurethanes, in the presence of an interference-fit. Static strength of bonded and unbonded interference fit joints have been compared in order to investigate the role of the different adhesives.     

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.     

Withdrawal strength of threaded steel rods glued with epoxy in wood

In this study, the axial strength of joints made with threaded steel rods glued in timber with epoxy is investigated. Although numerous experimental studies have investigated these joints made in glued laminated timber (glulam) from softwood, experimental data concerning tests on a whole range of hardwood species are still lacking. Thus, to evaluate the influence of timber characteristics on the behaviour of the joint, test results from different species are presented and discussed in this paper. The experimental results from samples using softwood as well as high-density hardwood glued laminated timber are compared. Diverse geometries of the joint are studied in both cases. From this experimental analysis, a formula to predict the strength of the glued-in bars is proposed. The prediction of the strength is made from two parameters that are easily quantifiable the density of the timber and the slenderness of the glued joint. This model shows a good accuracy with the test results of joints made on different species both from softwood and hardwood.     

Effect of material on the mechanical behaviour of adhesive joints for the automotive industry

One parameter that influences adhesively bonded joints performance is the adherend material and its effect should be taken into consideration in the design of adhesive joints. In this work, the effect of material on the mechanical behaviour of adhesive joints was investigated experimentally and numerically by single lap joints (SLJs) with different adherend materials (high strength steel, low strength steel and composite). The adhesives selected were two new modern tough structural adhesives used in the automotive industry. It was found that, for relatively short overlaps in SLJs bonded with structural modern tough adhesives, failure is dominated by adhesive global yielding and the influence of material on joint strength is not significant. For larger overlaps, the failure is not anymore due to global yielding and the effect of material becomes more important. Moreover, it was possible to evaluate which adhesive is more suited for each material.     

Three-Dimensional Finite Element Analysis of the Stress Distribution in Bi-Adhesive Bonded Joints

A 3-D elastic finite element model was developed to investigate the stresses distribution of bi-adhesive bonded joints (i.e., the bond line of joints filled with two adhesives of dissimilar toughness). The effects of the loading mode on the stress distribution of joints, including the single-lap joints under tensile loading (i.e., single-lap joints) and the butt joints under cleavage loading (i.e., cleavage joints), were also studied in detail. Results showed that higher stress, distributed at the contact position of the dissimilar adhesives placed along the bond line of bi-adhesive bonded joints. Also, the maximum stress of the adhesive layer decreased when the length ratios and bonding sequence along the bond line, filled with two dissimilar adhesives, was appropriately designed. At the same time, stress convergence in the adhesive layer of bi-adhesive joints was also obviously reduced in contrast to the mono-adhesive joints. The numerical investigation shows that it is necessary to take into account the change of loading modes when optimizing the bi-adhesive joint design, because of the uneven and complex loading modes of the adhesive bonding structure in the engineering applications.     

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.     

A Preliminary Study of the Use of Kynar® Piezoelectric Film to Measure Peel Stresses in Adhesive Joints

A variety of test techniques have been developed to test the performance of adhesives bonded in situ within joints. Most of these techniques measure strength, fracture toughness, or adhesive modulus of the bonded joint. Techniques to measure actual stress or strain values within a bonded joint are quite few in number. The Krieger gage¹ is able to measure the average shear displacement along a 12.5 mm. gage length of a thick adherend joint. It has been used primarily to measure in situ shear moduli of adhesives. Brinson and his colleagues² proposed bonding strain gages within adhesive joints to measure strains within the adhesive. Unfortunately, these gages are only sensitive to the lateral strains and not shear or peel strains. Because the lateral strains are dominated by the behavior of the adherends rather than the adhesive, the information which can be gained is incomplete.     

Effect of hole drilling at the overlap on the strength of single-lap joints

Bonded unions are gaining importance in many fields of manufacturing owing to a significant number of advantages to the traditional fastening, riveting, bolting and welding techniques. Between the available bonding configurations, the single-lap joint is the most commonly used and studied by the scientific community due to its simplicity, although it endures significant bending due to the non-collinear load path, which negatively affects its load bearing capabilities. The use of material or geometric changes in single-lap joints is widely documented in the literature to reduce this handicap, acting by reduction of peel and shear peak stresses at the damage initiation sites in structures or alterations of the failure mechanism emerging from local modifications. In this work, the effect of hole drilling at the overlap on the strength of single-lap joints was analyzed experimentally with two main purposes: (1) to check whether or not the anchorage effect of the adhesive within the holes is more preponderant than the stress concentrations near the holes, arising from the sharp edges, and modification of the joints straining behaviour (strength improvement or reduction, respectively) and (2) picturing a real scenario on which the components to be bonded are modified by some external factor (e.g. retrofitting of decaying/old-fashioned fastened unions). Tests were made with two adhesives (a brittle and a ductile one) varying the adherend thickness and the number, layout and diameter of the holes. Experimental testing showed that the joints strength never increases from the un-modified condition, showing a varying degree of weakening, depending on the selected adhesive and hole drilling configuration.     

Effect of temperature on the mechanical performance of glued-in rods in timber structures

Structural bonding technology has proven to be an economically and attractive connection process in timber engineering. Within old or historical wooden buildings, local reinforcement of weak zones is often performed with glued-in rods. This kind of connection typically allows the transfer of loads within wooden elements by means of threaded steel rods glued with a structural adhesive. This paper relates to experimental and numerical investigations on small sized specimens, with the aim of providing a better knowledge about the elastic behavior according to temperature. Experimental results reveal that stiffness of bonded-in rods significantly decreases once the glass transition temperature of the adhesive is reached. However, the ultimate shear strength is constant and sudden failures occur in the wood close to the adhesive whatever the temperature is. Then, an elastic finite element model allows the evolution of the Young modulus of the adhesive with temperature changes and also reveals the stress distribution along the glued-in depth during the elastic regime.     

胶接接头在静态负载条件下的蠕变和松弛行为

Modern high performance adhesives are designed to offer an optimized balance of elasticity,toughness and plastic deformation capacity for the individual fields of application in e.g. the building and construction or transportation and vehicle industry. The long-term life prediction for adhesive joints based on laboratory tests requiring only days,weeks,or months is still a demanding challenge. Testing in practice is carried out with the intention of accelerating time dependent aging effects that may occur in a bonded joint during its service time. Initial strength values of bonded joints,such as shear or peel properties can often be obtained from the adhesive manufacturers or retrieved from literature. They are useful to compare different adhesives and to demonstrate the effect of parameters such as bond line thickness,overlap length or curing conditions,and,in some cases,the surface state. On the other hand only few data are available describing the mechanical long-term properties of adhesives related to creep and relaxation under static load conditions. Due to the nature of the polymer network of organic adhesives their viscoelastic-plastic deformation behavior is strongly time-and temperature dependent. The objective of this paper is to illustrate effective methods for investigating and predicting the creep and relaxation properties of adhesively bonded joints in the long-term region and for creating basic data for the design and engineering with adhesives.     

Tensile and fatigue investigations of timber joints with glued-in FRP rods

Glued-in rods are an effective method to form timber connections that are increasingly in the focus of research. Compared to steel rods, fibre-reinforced polymer (FRP) bars provide higher resistance against corrosion, reduced weight, and lower heat conductivity. Despite excellent mechanical performance, high fire resistance, and improved aesthetics, they are, however, not yet widely used due to lack of design regulation. This is particularly true for cyclic loads, where determination of fatigue characteristics depends upon time-intensive experimental procedures. In this research, 50 glued-in FRP rod specimens with different embedment lengths were manufactured and tested in uniaxial tension: a first set under quasi-static load and a second set under cyclic load. For the fatigue tests, a new approach based on a stepwise load increase was used to estimate fatigue strength, a method that aims at reducing the experimental effort usually associated with Woehler curves. Experimental results indicated that quasi-static and fatigue strength increased with the overlap length up to an apparent maximum. The estimated fatigue strength from the load increase tests (LITs) was approx. 41% of the quasi-static strength. Additionally, constant-amplitude tests were performed and the obtained data points fit well into established Woehler curves for timber.     

Hardwood rods glued into softwood using environmentally sustainable adhesives

ABSTRACT

Over the past years, a large number of studies about the implementation of adhesive bonding in timber engineering were published. Current practice is to use two-component (2K) epoxies and polyurethanes in conjunction with threaded steel bars, reinforced concrete bars, and rods made of fibre reinforced polymer. All these material might be considered questionable in terms of sustainable ecology. The current article investigates the possible substitution of aforementioned metallic bars by hardwood dowels, and the polymer-based adhesives by glutine- and casein-based ones. It summarises experimental investigations carried out with different geometric parameters, in which beech (a hardwood) rods were glued into spruce (a softwood) blocks. Joint capacities achieved using glutine- and casein-based adhesives were compared with results using 2K epoxies and 2K polyurethane adhesives under dry conditions, and under the influence of moisture. Results indicate that current glutine and casein adhesives, if used under dry conditions, achieve similar strength; however, the influence of moisture is still a limiting factor that severely restricts their usage under adverse conditions. The presentation of the results is completed by a numerical modelling of the considered joints, including a methodology to predict their capacity with a reasonable degree of accuracy.     

A Preliminary Study of the Use of Kynar® Piezoelectric Film to Measure Peel Stresses in Adhesive Joints

A variety of test techniques have been developed to test the performance of adhesives bonded in situ within joints. Most of these techniques measure strength, fracture toughness, or adhesive modulus of the bonded joint. Techniques to measure actual stress or strain values within a bonded joint are quite few in number. The Krieger gage¹ is able to measure the average shear displacement along a 12.5 mm. gage length of a thick adherend joint. It has been used primarily to measure in situ shear moduli of adhesives. Brinson and his colleagues² proposed bonding strain gages within adhesive joints to measure strains within the adhesive. Unfortunately, these gages are only sensitive to the lateral strains and not shear or peel strains. Because the lateral strains are dominated by the behavior of the adherends rather than the adhesive, the information which can be gained is incomplete.     

Single Lap Joints with Rounded Adherend Corners: Experimental Results and Strength Prediction

The objective of the present study was to better understand the effect of the change in the geometry of the adherend corners on the stress distribution in single lap joints and, therefore, on the joint strength. Various degrees of rounding were studied and two different types of adhesives were used: one very brittle and another which had a large plastic deformation. Experimental results on the strength of joints with different degrees of rounding are presented. For joints bonded with brittle adhesives, the effect of the rounded adherend corners is larger than that with ductile adhesives. The strength of joints with brittle adhesives with a large radius adherend corner increases by about 40% compared to that with a sharp adherend corner. It is shown that for joints bonded with brittle adhesives, crack propagation occurs for a short period before it grows into catastrophic failure. However, for ductile adhesives, there is large adhesive yielding and small crack propagation before final failure. Another important feature of joints bonded with ductile adhesives is that there may be more than one crack in the adhesive layer before failure. This makes strength predictions more difficult. The second part of the paper presents an approximate method for predicting the strength of joints bonded with brittle and ductile adhesives, with and without adherend corner rounding. The predictions, based on an average value around the singularity, compare well with the experimental results, especially for joints bonded with ductile adhesives.     

Review on adhesive joints and their application in hybrid composite structures

Composites have been used extensively in various engineering applications including automotive, aerospace, and building industries. Hybrid composites made from two or more different reinforcements show enhanced mechanical properties required for advanced engineering applications. Several issues in composites were resolved during the last few years through the development of new materials, new methods and models for hybrid joints. Many components in automobile are joined together either by permanent or temporary fastener such as rivets, welding joint and adhesively bonded joints. Increasing use of bonded structures is envisaged for reducing fastener count and riveted joints and there by drastically reducing assembly cost. Adhesive bonding has been applied successfully in many technologies. In this paper, scientific work on adhesively bonded composites and hybrid composites are reviewed and discussed. Several parameters such as surface treatment, joint configuration, material properties, geometric parameters, failure modes, etc. that affect the performance of adhesive bonded joints are discussed. Environmental factors like pre-bond moisture and temperature, method of adhesive application are also cited in detail. A specific case of adhesive joints in hybrid bonded-bolted joints is elaborated. As new applications are expanding in the field of composites joining and adhesive joints, it is imperative to use information on multiple adhesives and their behaviour in different environmental conditions to develop improved adhesive joint structure in mechanical applications.     

混合胶接劈裂接头胶层中应力分布的数值分析

采用ANSYS有限元分析软件,对由两种弹性模量相差悬殊的丙烯酸酯和环氧树脂胶粘剂连接的劈裂接头胶层中部的应力分布进行数值分析和讨论。结果表明:对于采用单一胶连接的劈裂接头来说,采用弹性模量较低的胶粘剂更为有利;混合胶劈裂接头因其存在应力集中程度较高,会导致接头的强度降低。     

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.     

Synthesis and properties of allyloxyethyl 2-cyanoacrylate adhesive

Allyloxyethyl 2-cyanoacrylate monomer was synthesized and characterized for the first time. It was found that this monomer retains the typical properties of cyanoacrylate adhesives such as fast setting time at room temperature, adhesion to most materials, and high strength of bonded joints. Because of its long ester group and the reactive allyl group, this cyanoacrylate monomer produces adhesive bonds which have improved elasticity and heat resistance. IR and DSC studies showed crosslinking of the adhesive layer when subjected to elevated temperature, which explains the increased tensile shear strength of steel bonded joints. It was found that allyloxyethyl 2-cyanoacrylate can also be used as a crosslinking component for cyanoacrylate adhesives, based on ethyl 2-cyanoacrylate. Less than 10% of allyloxyethyl 2-cyanoacrylate in the mixture is needed for increasing, over three times, the tensile shear strength of the adhesive joint after ageing at 100°C.     

GFRP-to-timber bonded joints: Adhesive selection

Recent research at the University of Queensland (UQ) has led to the development of a new type of structures called “Hybrid Fiber Reinforced Polymer (FRP)-Timber structures” (“HFT”). In HFT structures, FRP is combined with timber veneers to create high-performance, lightweight, easy-to-construct structural members. These HFT members take advantage (i) of the orthotropic properties of both, timber and FRP to orientate the fiber direction to produce optimal composite properties, and (ii) of the geometry of the cross sections to maximize the load bearing capacity for a given amount of material. While preliminary experimental work has revealed as such the effectiveness of HFT structural members, no work has been carried out so far to investigate the behavior of these HFT structures. Performance of these new HFT members relies significantly on the bond between FRP and timber. This paper presents the results of an experimental study aimed at selecting a suitable commercially available adhesive for glass fiber reinforced polymer (GFRP)-to-timber bonded joints. The experimental program included 393 single lap joint tests covering four different commercially available adhesives, two different curing temperatures, and two test methods (dry and moisture cycle tests). The test results revealed that both, polyurethane (PUR) and cross-linking polyvinyl acetate emulsion (PVAx) performed as the best under dry conditions, while PUR was shown to be superior to all other adhesives when subjected to moisture cycles. Epoxy and phenol resorcinol formaldehyde adhesive (PRF) commonly used in FRP structures and laminated timber structures, resp., were found to be less performing structural adhesives for HFT structures.