The application of the iosipescu shear test to structural adhesives

The Iosipescu shear test method, adapted for adhesives, was used to evaluate shear stress–strain properties of five epoxy-based film adhesives. The shear strength of some adhesives, determined by this technique, was found to differ greatly from that determined by either lap shear and/or thick adherend tests. Accurate measurements of the adhesive bond-line deformation enabled evaluation of the adhesive elastic and plastic properties as well as the calculation of shear modulus. A high-speed video camera was employed to augment observation on the joint deformation.     

Comparative Study of the Results of Various Experimental Tests Used for the Analysis of the Mechanical Behaviour of an Adhesive in a Bonded Joint

The use of adhesively bonded joints is often limited by a lack of reliable models able to accurately predict their behaviour in industrial applications, in which the stress distribution is often complex. The mechanical behaviour of an adhesive in a bonded joint is often heavily dependent on its stress state (i.e., the tensile–shear combinations). Thus, a large experimental database is required to accurately represent the complex behaviour of an adhesive in a bonded joint. On the one hand, the initial yield surface (initial elastic limit) often has to be described taking into account the two stress invariants, hydrostatic stress and von Mises equivalent stress, and on the other hand the non-linear behaviour of the adhesive is also quite complex to model. However, the mechanical response of adhesively bonded joints often presents quite large stress concentrations; thus, the analysis of experimental tests is made particularly difficult. Obtaining reliable experimental results makes it possible to contribute to optimization of an adhesive in a bonded joint. This paper presents comparisons between results of different experimental tests (with bulk and bonded joints), some of them are designed to greatly limit the edge effects. Results are presented for two adhesives under proportional monotonic loadings. The two adhesives have very different behaviours (a ductile adhesive and a brittle adhesive) and two different surface preparations of aluminium substrates (a mechanical preparation and a chemical preparation recommended by the adhesive manufacturer) were studied.     

Surface Energetics and Adhesion

The relationships between surface energetics and adhesion are critically reviewed. New data that confirm such relationships, for peel tests as well as lap shear tests, are presented. The effect of hydrothermal aging of aluminum surfaces on surface energetics can be used to predict degradation in bond strength. The mechanism of failure for elastic adhesives (such as Scotch ® tape) in peel tests may be essentially the same as for more brittle adhesives (such as epoxies) in lap shear tests. This mechanism may involve brittle fracture that forms a critical flaw at the adherend-adhesive interface (on a microscopic level), followed by crack propagation which then may include considerable elastic and plastic deformation. The locus of propagation (fractography) is generally not (but may be) relevant to the problem of how to remedy mechanical weakness in an adhesive joint, since the local region of critical flaw formation rather than the general surface area determines the joint strength.     

Shear properties of isotropic conductive adhesive joints under different loading rates

Epoxy-based conductive adhesives have been widely used in the electronic field given the lead-free development of electronic packaging. The conductive adhesive joints must be subjected to shear loads during the service of electronic products considering the mismatch in mechanical properties between packaged chip and substrate. In this study, INSTRON 5544 universal material testing machine was used for tensile–shear tests of isotropic conductive adhesive joint specimens, which were prepared using pure copper plate adherend in the form of single-lap joints. Four loading rates, that is, 0.05, 0.5, 5, and 10 mm/min, were adopted. The relationship between shear load and displacement of two overlapping copper plates is deduced from a mechanical perspective. A mechanical model of the conductive adhesive shear specimen was developed by introducing dimensionless parameters, which are obtained from interfacial fracture energy and shear strength, to interpret the effect of loading rate on the shear properties of the conductive adhesive specimen considering the loading rate. Results show that this model can effectively reflect the relationship between shear load and displacement in the range of 0.05–10 mm/min.     

Visco-elasto-plastic constitutive model of adhesives under uniaxial compression in a range of strain rates

Determining the constitutive model of adhesives enables the prediction of the mechanical response of hybrid structures in which the adhesive is used. In this study, the stress–strain behavior of a two-component structural adhesive was first measured in uniaxial compression experiments at the strain rates ranging from 0.001 to 2600 s⁻¹ for developing a constitutive model. It was found that the compressive response, including elastic modulus, yield strength, and plastic flow stress, is substantially influenced by the strain rate. In plastic deformation, the strain rate sensitivity is not constant but varies with the rate; moreover, strain softening and hardening dominate the plastic deformation at low and high strains, respectively. A visco-elasto-plastic constitutive model was then proposed for the adhesive, integrating the strain rate sensitivity that was quantified by empirical equations. The model which was validated can reliably represent the strain rate dependent compressive behavior of the adhesive. © 2020 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48962.     

New Developments in Structural Adhesives for the Automotive Industry

Recently developed epoxy paste adhesives, reactive hot melts, adhesive film tape and polyurethane adhesives are presented for structural bonding in the automotive industry. Paste adhesives usually require a precure stage to obtain handling strength of the joints and to guarantee wash-out resistance of the adhesive in the paint baths. This step can be omitted with reactive hot melts and adhesive film tape, which are solid before and after their application. In addition they allow an improved working hygiene. Some mechanical properties of the adhesives are shown such as lap shear strength and peel strength as well as lap shear strength as a function of the bondline thickness. Results of the excellent durability of epoxy one-component pastes, reactive hot melts and adhesive film tape are given from cyclic environmental and salt spray tests.     

An improved method for testing the shear stress/strain behaviour of adhesives

The thick-adherend short-overlap specimen together with an appropriate extensometer is a system that is often used to test the shear stress/strain behaviour of adhesives. With the aim of developing a constitutive model of the Cyanamid adhesive FM73 that could adequately account for time-dependent deformation, a strain-rate-controlled test was attempted. It was found that tests at a set crosshead speed have a strain rate that varies by two orders of magnitude over the duration of the test. The authors have devised a method that achieves true strain rate control. In view of the time-dependent nature of most adhesives, this method should be generally useful whenever accurate material parameters for the mechanical behaviour of an adhesive are required for design purposes.     

Analytical modelling of the automated dispensing of adhesive materials

The integrity of joints is a critical issue in structural bonding and the shape of adhesive beads needs to be consistent. The right amount of adhesive must be dispensed and to ensure this, the dispensing process must be automated. To have a fully automated dispensing cell, the use of a knowledge-based expert system for set-up and diagnosis is appropriate. To achieve this, the modelling of dispensing flow of adhesive material is essential to ensure that the correct sized bead is dispensed at a variety of robot speeds. For a given adhesive, it is often necessary to perform many ad hoc adjustments and tests to produce optimal adhesive beads at different operational and environmental conditions. To avoid this, the rheological behaviour of the adhesives is analysed, to find a constitutive equation for representing an adhesive model which best describes the way that different types of adhesives flow under a variety of conditions. Modelling the material properties provides an initial approximation of the flow rate for dispensing the adhesive. The automated dispensing cell can then adapt to the operational conditions where the model also provides for its initial set-up and diagnosis. This modelling method can also be applied to similar adhesives with known rheological data and nozzle flow rate.     

Design of experiments for optimization a biodegrable adhesive based on ramon starch (Brosimum alicastrum Sw.)

In this work, a Central Composite Design (CCD) and Response Surface Methodology (RSM) were used to study the effect of starch content, hydrolyzing agent (NaOH) content, temperature and cooking period on peel strength and shear strength of biodegradable adhesives based on Ramon (Brosimum alicastrum Sw.) and Corn (Zea mays L.) starch. Scribe® paper was used as substrate or adherent. The CCD consisted of 36 experiments (including 12 central points). The second-order regression models of the response surface method, used to predict the response variables, exhibited a high correlation between the data obtained and the predicted data, and were thus considered reliable to optimize the mechanical properties for peel strength and shear strength of the Ramon starch adhesives. Starch content, hydrolyzing agent content and the cooking temperature of the adhesives proved to be the most significant factors affecting peel strength and shear strength of the adhesives of both the Ramon and corn starch. Moreover, the interactions of Starch-NaOH and Starch-Temperature were found to be the most significant in the adhesive properties in both adhesives. The mechanical properties (peel strength and shear strength) of both adhesives increased until reaching approximately their temperatures of gelatinization (T _{RAMON GEL} = 83 °C, T _{GEL CORN} = 72 °C). At higher temperatures, the mechanical properties of the adhesives diminished. The results of this study show that the adhesive prepared with the Ramon starch presents adhesive properties similar to those of an adhesive prepared with corn starch. This would imply that the Ramon starch is a viable alternative to substitute corn starch in industrial applications not relating to food production.     

Reworkable adhesives composed of photoresponsive azobenzene polymer for glass substrates

ABSTRACT

A reversible isothermal phase transition between the liquid and solid states in response to light irradiation was achieved in side chain-type azobenzene polymers. These materials can be used as adhesives that are detachable without applying any mechanical and thermal stress but also repeatedly reworkable because of their photoinduced liquefaction and solidification properties. The adhesive strength to glass plates was more than 3 MPa in single lap shear tests. This value is three times higher than previously reported and is sufficiently strong for glass substrates.     

Effect of adhesive thickness and surface roughness on the shear strength of aluminium one-component polyurethane adhesive single-lap joints for automotive applications

Adhesively bonded technology is now widely accepted as a valuable tool in mechanical design, allowing the production of connections with a very good strength‐to‐weight ratio. The bonding may be made between metal–metal, metal–composite or composite–composite. In the automotive industry, elastomeric adhesives such as polyurethanes are used in structural applications such as windshield bonding because they present important advantages in terms of damping, impact, fatigue and safety, which are critical factors. For efficient designs of adhesively bonded structures, the knowledge of the relationship between substrates and the adhesive layer is essential. The aim of this work, via an experimental study, is to carry out and quantify the various variables affecting the strength of single-lap joints (SLJs), especially the effect of the surface preparation and adhesive thickness. Aluminium SLJs were fabricated and tested to assess the adhesive performance in a joint. The effect of the bondline thickness on the lap-shear strength of the adhesives was studied. A decrease in surface roughness was found to increase the shear strength of the SLJs. Experimental results showed that rougher surfaces have less wettability which is coherent with shear strength tests. However, increasing the adhesive thickness decreased the shear strength of SLJs. Indeed, a numerical model was developed to search the impact of increasing adhesive thickness on the interface of the adherend.     

Mechanical and thermal behavior of modified epoxy–novolak film adhesives

Modified epoxy-based film adhesives were developed for bonding structural joints. Film adhesives with different compositions were prepared by hot pressing the molten resins. Peel and shear tests were carried out to evaluate the adhesion properties. Dynamic mechanical thermal analyses were conducted to follow the changes in the adhesive structure and also the trend of impact strength. Incorporation of thermoplastic poly(vinyl butyral) (PVB) into an epoxy- novolak combination resulted in higher cohesive strength, better film-forming ability, enhanced adhesive shear and peel strengths, but decreased thermostability. However, due to the lower chemical functionality of PVB, a lower crosslink density was achieved. Incorporation of a small amount of ethylene glycol dimethacrylate (EGDM) as a flexibilizer led to improved mechanical properties, easy handling and facile application. Finally, good shear strength retention up to 200 °C for 1 h was observed in the case of EGDM-modified adhesives.     

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.     

The performance of different adhesives for archaeological ceramics under mechanical stress

The influence of environmental conditions, in particular temperature, on the performance of adhesives joining different archaeological ceramic fabrics is assessed in laboratory conditions. Different types of model ceramic fabrics were manufactured and joined with commercially available adhesives, which are commonly employed in the conservation of archaeological ceramics, and a series of systematic tests on the various adhesive–ceramic systems was conducted. The response of joins to static mechanical stress was assessed in controlled loading tests. Results of those tests as well as failure mechanisms observed are discussed.     

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.     

Shear Refinement of Formaldehyde-Free Corn Starch and Mimosa Tannin (Acacia mearnsii) Wood Adhesives

The aim of this work was to reduce the viscosity of formaldehyde-free corn starch–mimosa tannin wood adhesives, without adversely affecting the mechanical properties of the product. The reduction of viscosity was achieved using shear refinement. The study focused on the physical phenomena before cross-linking of the wood adhesive. The physical (rheological characterization) and mechanical (bond strength) properties of formaldehyde-free corn starch and mimosa tannin wood adhesives were measured. The results showed that the shear refinement (290 rpm and 5 min, optimal conditions) reduced the viscosity of the corn starch–mimosa tannin wood adhesives (from 100 000 to 458 Pa s) with the advantage of being stable over time. Mechanical tests showed that the shear refinement did not influence the mechanical properties of corn starch–mimosa tannin wood adhesives.     

Comparison of fracture behavior between acrylic and epoxy adhesives

An experimental study was conducted on the strength of adhesively bonded steel joints, prepared epoxy and acrylic adhesives. At first, to obtain strength characteristics of these adhesives under uniform stress distributions in the adhesive layer, tensile tests for butt, scarf and torsional test for butt joints with thin-wall tube were conducted. Based on the above strength data, the fracture envelope in the normal stress-shear stress plane for the acrylic adhesive was compared with that for the epoxy adhesive. Furthermore, for the epoxy and acrylic adhesives, the effect of stress triaxiality parameter on the failure stress was also investigated. From those comparison, it was found that the effect of stress tri-axiality in the adhesive layer on the joint strength with the epoxy adhesive differed from that with the acrylic adhesive. Fracture toughness tests were then conducted under mode l loading using double cantilever beam (DCB) specimens with the epoxy and acrylic adhesives. The results of the fracture toughness tests revealed continuous crack propagation for the acrylic adhesive, whereas stick-slip type propagation for the epoxy one. Finally, lap shear tests were conducted using lap joints bonded by the epoxy and acrylic adhesives with several lap lengths. The results of the lap shear tests indicated that the shear strength with the epoxy adhesive rapidly decreases with increasing lap length, whereas the shear strength with the acrylic adhesive decreases gently with increasing the lap length.     

Influence of the filler material on the thermal stability of one‐component moisture‐curing polyurethane adhesives

Filler materials are part and parcel for the adjustment of adhesives, in particular, their rheological and mechanical properties. Furthermore, the thermal stability of adhesives can be positively influenced by the addition of an expedient filler, with inorganic types common practice in most cases. In this study, one‐component moisture‐curing polyurethane adhesives for engineered wood products based on isocyanate prepolymers with different polymer‐filled polyether polyols were investigated with regard to the filler's potential to increase the thermal stability of bonded wood joints. The property changes due to the addition of fillers were determined by means of mechanical tests on bonded wood joints and on pure adhesive films at different temperatures up to 200°C. Additional analyses by atomic force and environmental scanning electron microscopy advanced the understanding of the effects of the filler. The tested organic fillers, styrene acrylonitrile, a polyurea dispersion, and polyamide, caused increases in the cohesive strength and stiffness over the whole temperature range. However, the selected filler type was hardly important with regard to the tensile shear strength of the bonded wood joints at high temperatures, although the tensile strength and Young's modulus of the adhesive films differed over a wide range. Prepolymers with a lower initial strength and stiffness resulted in worse cohesion, in particular, at high temperatures. This disadvantage, however, could be compensated by means of the filler material. Ultimately, the addition of filler material resulted in optimized adhesive properties only in a well‐balanced combination with the prepolymer used. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Comparative study of zein- and gluten-based wood adhesives containing cellulose nanofibers and crosslinking agent for improved bond strength

Many of the currently used wood adhesives contain chemicals that are harmful to human health and the environment. Increasing environmental and human health concerns have made the development of safe biobased adhesives a priority. In this study, two plant proteins, i.e., zein and wheat gluten, were used to develop wood adhesives and their performance was compared through simple lap shear tests and plywood flexural/internal bond tests in dry and wet conditions. To increase their bond strength, cellulose nanofibers were added to create nanocomposite adhesives and glutaraldehyde was also used to crosslink the proteins. Single-lap shear test was performed to measure the bond strength of different adhesive formulations and determine the optimal formulations and processing conditions. Fractured bond surfaces were studied using optical observation and scanning electron microscopy to determine bond failure mechanisms. Thermal and chemical properties of the adhesives were evaluated using thermogravimetric analysis and Fourier transform infrared spectroscopy, respectively. The bond strength of both zein and gluten adhesives was significantly increased by the addition of the cellulose nanofibers and/or glutaraldehyde, although the two adhesives responded differently to the two reinforcement materials due to the different solvents used to prepare the adhesives. The bond failure mode changed from cohesive failure of the adhesive to structural failure of the adherent for the gluten adhesive containing CNFs and glutaraldehyde. Potential zein and gluten adhesive formulations were used to produce plywood samples and their performance was assessed under different conditions. The formulations with industrial potential were discovered through this study.     

Adhesive joints in glass structures: effects of various materials in the connection,thickness of the adhesive layer,and ageing

This paper reports on an investigation of glued joints in glass load-bearing structures, with reference to the effect of various substrates (glass, steel, stainless steel, aluminium) and their surface treatment (sandblasting for the glass surface) on the adhesion of selected adhesives. The thickness of the adhesive layer and the effect of artificial ageing – a simulation of 5 years of ageing in outdoor central-European conditions – are also discussed. Tensile and shear tests were carried out on three series of specimens with various adhesives and substrates – two series for tensile and shear tests, and one series for shear tests on specimens exposed to ageing. Our results show that sandblasting the glass surface can improve the adhesion, and thus the strength values, of an adhesive joint in cases where, with a smooth glass surface, cohesive failure is not reached. The thickness of the adhesive layer had a significant effect for a semi-rigid acrylate adhesive, where the joint achieved higher strength values with less thickness of the glue. The effect of ageing varied according to the adhesive. The most visible changes were observed for a two-component acrylate adhesive and for methacrylate UV-adhesives. One of the selected glues was marked as unsuitable for load-bearing connections due to significant worsening of its mechanical properties after ageing.