The Impact Resistance of Structural Adhesive Joints

The mechanical behaviour of structural adhesives and adhesive joints under impact loading is of growing interest as adhesives are used increasingly in the construction of vehicles ranging from the family motor car to large trucks and buses. The present paper describes some initial work on the development of an instrumented impact test to study the impact behaviour of epoxy adhesives and the use of a linear-elastic fracture-mechanics approach to characterising the fracture properties.     

Confined compression of elastic adhesives at high rates of strain

Elastic adhesives are used in composite armours to bond the ceramic front face and the metallic backing plate. The mechanical behaviour of different elastic adhesives under impact loads have been studied by means of dynamic compression tests performed in a split Hopkinson pressure bar. In this experiments, the stress-strain curve of confined materials at high strain rates and the capability of transmitting and reflecting the impact energy have been determined. The influence of thickness and ageing on the response of the adhesive layer have been also considered.     

Toughening epoxy adhesives with multi-walled carbon nanotubes

In this paper, the effect of adding multi-walled carbon nanotubes (MWCNTs) with an outer diameter of less than 8 nm to an epoxy adhesive was studied on the adhesive fracture resistance and damage behaviour. The fracture energies of the neat and toughened adhesives were measured by testing double-cantilever beam specimens. Moreover, a cohesive zone model (CZM) was used to numerically study the effect of MWCNTs on the damage behaviour of the toughened adhesives. The maximum improvement of 58.4% in the adhesive fracture energy was obtained when the adhesive was toughened with 0.3 wt% of MWCNTs. The fracture surfaces were analysed using the scanning electron microscopy (SEM) technique. It was found that the presence of MWCNTs in the toughened adhesives caused rougher fracture surfaces. Moreover, some fracture mechanisms including nanotube pull-out and de-bonding were observed in the fracture surfaces. The numerical analyses showed that the damage process zone length was also influenced by MWCNTs. The longest damage process zone was obtained for the toughened adhesive with 0.3 wt% of MWCNTs.     

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     

Testing different cohesive law shapes to predict damage growth in bonded joints loaded in pure tension

ABSTRACT

Adhesive bonding is a widely used joining method because of specific advantages compared to the traditional fastening methods. Cohesive zone modelling (CZM) is currently the most widely used technique for strength prediction. CZM supposes the characterization of the CZM laws in tension and shear. This work evaluated the tensile fracture toughness (G_IC) and CZM laws of bonded joints with three adhesives by the double-cantilever beam (DCB) test. The experimental work consisted of the adhesives’ tensile fracture characterization by the J-integral technique. As the main novelty of this work, the precise shape of the cohesive law of adhesives ranging from brittle to highly ductile was defined by the direct method, using a digital image correlation method to evaluate the tensile relative displacement (δ_n) of the adhesive layer at the crack tip and adherends’ rotation at the crack tip (?_o). Moreover, finite element (FE) simulations permitted assessing the accuracy of triangular, trapezoidal and linear-exponential CZM laws in predicting the experimental behaviour of the DCB bonded joints with markedly distinct behaviours. As output of this work, fracture data and information regarding the applicability of these CZM laws to each type of adhesive is provided, allowing the subsequent strength prediction of bonded joints.     

Comparison of the Shear Stress-strain Behaviour of some Structural Adhesives

The shear stress-strain behaviour of structural adhesives provides important data for the designer. Shear modulus, strength, and elastic and plastic strain to failure have been determined using a torsional butt joint technique which is relatively quick to perform and is believed to be very accurate. A range of structural adhesives have been compared, which has highlighted some important differences in their behaviour. Increasing the bond line thickness of an adhesive lowers the plastic strain to failure.     

Characterising bonded joints with a thick and flexible adhesive layer–Part 1: Fracture testing and behaviour

Adhesively bonded structural joints have increasingly found applications in automotive primary structures, joining dissimilar lighter-weight materials. Low-modulus rubbery adhesives are attracting rising interest as an alternative to conventional rigid structural adhesives due to benefits such as the excellent impact resistance they provide. This paper is the first of two parts that investigate, both experimentally and numerically, the mechanical behaviour of a rubbery adhesive and the bonded joints to be used in a lightweight automobile structure. This part 1 paper characterises the fracture behaviour of the flexible adhesive layer with thick bondlines and presents a way to reliably determine the fracture mechanics parameters under a range of loading modes. Assessment of the various fracture tests indicated that DCB and SLB should provide mode I and mixed mode fracture energies but that the conventional ENF for mode II would not be practical for such compliant adhesive layers. Instead a cracked thick adherend shear specimen was developed and used. Reliable fracture energies were obtained from these specimens and a mixed mode fracture criterion developed for application in the part 2 paper.     

Fatigue Performance of Two Structural Adhesives

The fatigue performance of two toughened epoxy adhesives suitable for use in heavy structural engineering is assessed using a purpose-built fatigue rig. One adhesive is of the single-part, hot-cure type and the other is a two-part, cold-cure system. It is found that the single-part adhesive performs better than the two-part adhesive in fatigue even though the latter has the higher fracture toughness. Crack growth rates in both adhesives are found to satisfy the Paris Law as closely as any other crack growth model, and no dependence is found on the frequency of load cycling in the range studied (0.5Hz-5Hz). The fatigue performance of both adhesives is very promising for their likely uses in large-scale structures.     

The Shear Stress-Strain Behaviour of Low-modulus Structural Adhesives

The shear stress-strain behaviour of two low-modulus structural adhesives has been measured using the butt-torsion test. The Nadai correction for non-linear shear behaviour is explained as it is necessary to understand how this correction can be applied to butt joints. The results for one adhesive were accurately used to predict the strength of a lap joint, and it was shown that the strength of such a joint can approach that of a conventional, modern, structural epoxy. Structural adhesives are usually reckoned to be those with a high strength (50 MPa and upwards) and (these days), a strain to failure of at least 10% in tension, and which usually have a tensile modulus of 2 GPa or so. However, adhesives which are significantly less stiff, less strong, but much more ductile are entering the 'structural' arena. In order to evaluate their effectiveness and use in design, it is necessary to be able to measure accurately their stress-strain behaviour. Two such materials are 3M 9245 Structural Bonding Tape (SBT) and 3M 7838 B/A.     

Dynamic compressive creep behaviour of structural silicone-to-aluminum and silicone-based sealant-to-aluminum joints

This paper outlines an experimental study on the static and dynamic compressive creep behaviour of structural silicone adhesively bonded joints. The silicone adhesives are subjected to dynamic compressive loading, which is a common case for structural façade and hybrid glazing system. Typical crack propagation of adhesives, relations between compressive load (stress) and displacement (creep strain) are examined experimentally. It is shown that the test specimen with adhesives featured by lower hardness and higher elongation at break exhibit notable crack distribution concentrated in the middle of the crack surface. The compressive behaviour consists of three regions as initial elastic, nonlinear transition and post linear, in which the latter has notable strength increase with the increase of compressive deformation. The secant compressive modulus are measured based on compressive stress and creep strain relations. It is demonstrated that the joint has higher secant compressive modulus due to less crack propagation. All test joints exhibit significant degradation of strength and energy absorption, which can be well fitted in similar exponential forms with normalized cycle numbers for test joints with different adhesives.     

Accuracy of cohesive laws with different shape for the shear behaviour prediction of bonded joints

The use of adhesive bonding as a joining technique is increasingly being used in many industries because of its convenience and high efficiency. Cohesive Zone Models (CZM) are a powerful tool for the strength prediction of bonded joints, but they require an accurate estimation of the tensile and shear cohesive laws of the adhesive layer. This work evaluated the shear fracture toughness (J_IIC) and CZM laws of bonded joints for three adhesives with distinct ductility. The End-Notched Flexure (ENF) test geometry was used. The experimental work consisted of the shear fracture characterization of the bond by the J-integral. Additionally, by this technique, the precise shape of the cohesive law was defined. For the J-integral, digital image correlation was used for the evaluation of the adhesive layer shear displacement at the crack tip during the test, coupled to a Matlab sub-routine for extraction of this parameter automatically. Finite Element Method (FEM) simulations were carried out in Abaqus® to assess the accuracy of triangular, trapezoidal and linear-exponential CZM laws in predicting the experimental behaviour of the ENF tests. As output of this work, fracture data is provided in shear for the selected adhesives, allowing the subsequent strength prediction of bonded joints.     

Shear creep behaviour of elastomeric adhesives

The shear creep behaviour of elastomeric adhesives has been investigated at various temperatures, loading stresses and adhesive thicknesses. Three adhesive types were included in the study: two polysulphides, one silicone and one polyurethane elastomer. The creep compliance of the two polysulphide adhesives could be described by an Arrhenius-type relationship incorporating time, temperature and stress. The silicone and polyurethane adhesives, on the other hand, showed an initial creep response followed by a long period of zero creep over the ranges of temperature and load studied.     

The Shear Stress–Strain Behaviour of Low-modulus Structural Adhesives

The shear stress–strain behaviour of two low-modulus structural adhesives has been measured using the butt-torsion test. The Nadai correction for non-linear shear behaviour is explained as it is necessary to understand how this correction can be applied to butt joints. The results for one adhesive were accurately used to predict the strength of a lap joint, and it was shown that the strength of such a joint can approach that of a conventional, modern, structural epoxy. Structural adhesives are usually reckoned to be those with a high strength (50 MPa and upwards) and (these days), a strain to failure of at least 10% in tension, and which usually have a tensile modulus of 2 GPa or so. However, adhesives which are significantly less stiff, less strong, but much more ductile are entering the ‘structural’ arena. In order to evaluate their effectiveness and use in design, it is necessary to be able to measure accurately their stress–strain behaviour. Two such materials are 3M 9245 Structural Bonding Tape (SBT) and 3M 7838 B/A.     

Moisture and temperature degradation of double cantilever beam adhesive joints

In this work, the double cantilever beam (DCB) test is analysed in order to evaluate the combined effect of temperature and moisture on the mode I fracture toughness of adhesives used in the automotive industry. Very few studies focus on the combined effect of temperature and moisture on the mechanical behaviour of adhesive joints. To the authors’ knowledge, the simultaneous effect of these conditions on the fracture toughness of adhesive joints has never been determined. Specimens using two different adhesives for the automotive industry were subjected to two different ageing environments (immersion in distilled water and under 75% of relative humidity). Once they were fully degraded, they were tested at three different temperatures (?40, 23 and 80 °C), which covers the range of temperature an adhesive for the automotive industry is required to withstand. The aim is to improve the long term mechanical behaviour prediction of adhesive joints. The DCB substrates were made of a high strength aluminium alloy to avoid plastic deformation during test. The substrates received a phosphoric acid anodisation to improve their long term adhesion to the adhesive. Results show that even though a phosphoric acid anodization was applied to the adherends, when the aged specimens were tested at room temperature and at 80 °C, they suffered interfacial rupture. At ?40 °C, however, cohesive rupture was observed and the fracture toughness of the aged specimens was higher.     

An experimental method dedicated to the dynamic characterization of structural adhesives under drop weight conditions

Adhesive bonding is a commonly used method in multi-materials assemblies dedicated to the transport fields. In order to ensure structures integrity and users safety, the knowledge of the mechanical behaviour of structural adhesives used in these assemblies under impact conditions appears to be an essential prerequisite. To date, numerous tests combining usual specimens geometry e.g. single lap joint, butt joint, etc. and high velocity testing rigs exist and are used. Among these, most allow comparative studies and a few provide a partial identification of the material properties of the investigated adhesive.In this study, an experimental method dedicated to the dynamic characterization of structural adhesives under drop weight condition is proposed. On the basis of existing works, a modified Arcan specimen and a dynamic tensile testing mean were developed and are presented. The Arcan geometry allows to test the adhesive under various loading directions and so to obtain its mechanical response envelope. Design strategies are also implemented in order to obtain time stable and quasi-homogeneous stress distributions in the adhesive during the tests.At last, the dynamic characterization of a Dow^® Betamate^TM 1496 V adhesive is proposed. Results are repeatable and show a strain rate dependent behaviour validating the appropriateness of the experimental approach.     

Durability of adhesives based on different epoxy/aliphatic amine networks

The mechanical and adhesives properties of epoxy formulations based on diglycidyl ether of bisphenol A cured with various aliphatic amines were evaluated in the glass state. Impact tests were used to determine the impact energy. The adhesive properties have been evaluated in terms single lap shear using steel adherends. Its durability in water at ambient temperature (24 °C) and at 80 °C has also been analyzed. The fracture mechanisms were determined by optical microscopy. It was observed a strong participation of the cohesive fracture mechanisms in all epoxy system studied. The 1-(2-aminoethyl)piperazine epoxy adhesive and piperidine epoxy adhesive presents the best adhesive strength and the largest impact energy. The durability in water causes less damage to piperidine epoxy networks. This behavior appears to be associated with the lower water uptake tendency of homopolymerised resins due to its lower hydroxyl group concentration.     

Experimental investigation and numerical prediction of static strength and fracture behaviour of notched epoxy-based structural adhesives

The influence of stress concentrations on the tensile static strength and fracture behaviour of notched bulk specimens was investigated by comparing the response of two epoxy-based structural adhesives (a rubber-toughened and a polyurethane-toughened). Numerical predictions of failure stress were carried out using a 3D-FEA model with a hydrostatic dependent elastoplastic material behaviour and the equivalent plastic strain for failure assessment. The ductile adhesive, which plastically deforms more under high stresses, provided experimental evidence of a notch strengthening effect. Conversely, the less ductile adhesive has shown a reduction in tensile strength compared to un-notched samples. For both adhesives fracture surface analysis showed the presence of stress whitening and voids close to the notch regions. These regions could be correlated to higher values of stress triaxiality using numerical simulation. The more ductile adhesive underwent widespread stress whitening prior to failure, whereas the response of the less ductile adhesive was more localised. Numerically based predictions showed excellent agreement with experimental results with average error of 5.1% for different notch types in both adhesives.     

Intrinsic mechanical characterization of structural adhesives

Methods and specimens are described for obtaining the mechanical characteristics of structural adhesives needed by engineers for the design and analysis of adhesive-bonded structural joints. These mechanical characteristics can be obtained on bulk adhesives (in tension and compression loading) or on adhesive joints in shear loading. The first experimental results confirm those of other authors that the mechanical behaviour of the bulk adhesive material is similar to that in bonded joints.     

XD1系列石质文物修复胶粘剂研制及应用

XD1系列石质文物修复胶粘剂包括3种不同用途的液体聚硫橡胶改性环氧类胶粘剂。对此3种胶粘剂经剪切、拉伸、不均匀扯离等强度测试以及耐高低温交变、耐水、耐化学介质、抗振动、抗冲击、户外放置10年自然老化等性能试验,证明3种改性环氧胶粘剂粘接强度较高,抗振动、抗冲击、耐水、耐化学介质、耐老化性能好,室温固化,操作简便,适用于各类石质文物粘接修复。