R.J.C. Carbas

Adhesive Joints for Low- and High-Temperature Use: An Overview

This work presents a review of several investigations on the topic of adhesive bonding at high and low temperatures. Durability and strength at extreme temperatures have always been a major limitation of adhesives that, given their polymeric nature, exhibit substantial degradation at temperatures where other structural materials (such as metals for example) have minute changes in mechanical properties. However, due to the inherent advantages of bonding, there is a large and continued effort aiming to improve the temperature resistance of adhesive joints, and this effort has been spread among the various topics that are discussed in this review. These topics include adhesive shrinkage and thermal expansion, adhesive properties, joint geometry optimization, and design techniques, among others. The findings of these research efforts have all found use in practical applications, helping to solve complex problems in a variety of high-tech industries where there is a constant need to produce light and strong components that can withstand large temperature gradients. Therefore, the final sections of this work include a discussion on two specific application areas that demonstrate the strict demands that extreme temperature use imposes on adhesive joints and the methods used to improve their performance.

Effect of Cure Temperature on the Glass Transition Temperature and Mechanical Properties of Epoxy Adhesives

This paper describes the influence of the curing temperature on the physical and mechanical properties of three structural adhesives. This work was undertaken to improve the understanding of the effect of curing temperature in the glass transition temperature, T g , and stiffness of epoxy adhesives. The mechanical properties (Youngs modulus and yield strength) of the adhesives were measured in bulk specimens. T g was measured by a dynamic mechanical analysis using an in-house developed apparatus. The curing process was the same for all tests, consisting of a curing stage followed by a post cure stage. The initial stage was performed at different temperatures. T g and the mechanical properties was found to vary as a function of the cure temperature of the adhesive. When cured below the cure temperature, T cure , at which the T g of the fully cured network, T g ∞, is achieved, the strength and stiffness of the adhesive increase as the cure temperature increases and the T g is higher than the cure temperature. When cured above the T cure at which the T g ∞ is achieved, the strength and stiffness decrease as the cure temperature increases and the T g is higher than the cure temperature.

Effect of post-cure on the glass transition temperature and mechanical properties of epoxy adhesives

The effects of post-curing and cure temperature on the glass transition temperature, T g, and the mechanical properties of epoxy adhesives were studied. T g was measured by a dynamic mechanical analysis apparatus developed in-house and the mechanical properties of the adhesives (yield strength, Young’s modulus and failure strain) were measured by a tensile machine. The relationships between T g and mechanical performance under various post-cure conditions were investigated. The curing process was the same for all tests, consisting of an initial stage performed at different temperatures followed by cooling at room temperature. Three sets of specimens were considered, sharing the same initial cure process, but with a different post-curing procedure. In the first set, the specimens were only subjected to a curing process; in the second set, the specimens were subjected to a curing process followed by a post-cure performed at a temperature below the T g of the fully cured network, T g∞; and in the third set, the specimens were subjected to a curing process followed by a post-cure performed at a temperature above the T g∞. When post-cured at a temperature above T g∞, the mechanical and physical properties tend to have a constant value for any cure temperature.

Modelling of Functionally Graded Adhesive Joints

Nowadays, there is a strong trend towards the use of functionally graded materials, with particular importance for the functionally graded joints. The main objective of this work was to study a functionally modified adhesive in order to have mechanical properties that vary gradually along the overlap of a joint, allowing a uniform stress distribution along the overlap. This allows for a stronger and more efficient adhesive joint and would permit to work with much smaller areas, reducing considerably the weight of the structure which is a key factor in the transport industry. In the proposed joint, the adhesive stiffness varies along the overlap, being maximum in the middle and minimum at the ends of the overlap. The functionally graded joint was found to have a higher joint strength compared to the cases where the adhesive has homogeneous properties along the overlap. A simple analytical model to study the performance of the functionally graded joints was developed. The differential equation of this model was solved by a power series. Numerical modelling by finite element analysis was performed to validate the analytical model developed.

Functionally Graded Adhesive Patch Repairs of Wood Beams in Civil Applications

Several investigations have been made concerning the fracture behavior of scaled specimens of wood beams repaired with adhesively bonded carbon fiber reinforced polymer (CFRP). However, one of the problems associated to these joints is the fact that the stress distribution (shear and peel) is concentrated at the ends of the overlap, leading to premature failure of the joint. The stress concentration can be reduced with use of a functionally graded adhesive, in which the mechanical properties vary along the bond length. This can be achieved with a graded cure, in which the temperature varies along the bond length. In this study, the repair of wood structures with CFRP was made using a homogeneous cure and a graded cure. The graded cure was performed by induction heating. This technique has already been successfully tested in single lap joints to increase the strength of the joint. Two common types of defects on beams under bending loads were analyzed (compression and cross grain tension damage). Scaled specimens of damaged wood beams were repaired and tested under four point bending. The results show that the cross grain tension beams repaired with a graded bondline were able to withstand higher loads than the beams repaired with a homogeneous bondline. This failure load of the graded repaired beam increases with an increase of the overlap length.

Effect of Low Temperature on Tensile Strength and Mode I Fracture Energy of a Room Temperature Vulcanizing Silicone Adhesive

Aerospace applications have an increasing demand for strong and reliable adhesives, able to withstand large temperature gradients. The variation of the adhesive’s mechanical properties with temperature is therefore one of the factors that must be well understood before safe and reliable adhesive joints can be designed for these applications. The stress–strain curve and the toughness properties of an adhesive show strong dependency with temperature for most adhesives, especially near the glass transition temperature (Tg). In this work, an experimental procedure is undertaken to evaluate the effect of low temperatures on the adhesive strength and mode I fracture toughness of a room temperature vulcanizing silicone (RTV) adhesive. Firstly, the temperature at which the glass transition of the RTV occurs was obtained by means of an in-house developed measurement apparatus. Bulk specimens were manufactured and tested at temperatures above and below the Tg in order to obtain a strength envelope of the adhesive over this large temperature range. Single lap joints were also manufactured with this adhesive to assess the behaviour of the adhesive when assembled in a complete joint. For the determination of pure mode I fracture toughness, double cantilever beam specimens were also tested at negative temperatures near Tg. The results showed that the failure loads of all the tests performed have strong temperature dependence and this must be taken into account during adhesive joint design using this type of adhesives.

Effect of post-cure on adhesively bonded functionally graded joints by induction heating:

Functionally graded joints with an adhesive functionally modified by induction heating confer a more uniform stress distribution along the overlap and reduce the stress concentrations located at the ends of the overlap. The adhesive stiffness varies gradually along the overlap, being maximum in the middle and minimum at the ends of the overlap. This work studies the effect of post-curing on functionally graded joints obtained by induction heating. The performance of functionally graded joints, when submitted to different post-cure temperatures, was experimentally tested. Three different post-curing conditions were considered, with temperatures above and below the glass transition temperature of the fully cured network, Tg∞. The functionally graded joints (with and without post-cure) were compared with joints cured isothermally (with and without post-cure). The cure temperature values applied to the ends and to the middle of the graded joint are the same temperatures used to cure the isothermally cured joints. Analytical modelling was used to predict the failure load and to assess the effectiveness of the graded joint concept. The functionally graded joints subjected to post-cure at low temperatures (below Tg∞) show a slight decrease of the strength and the joints cured isothermally show a slight increase of the strength. With increase of the post-cure temperature (above Tg∞) the functionally graded joints exhibit strength similar to that of the joints cured isothermally. However, even for the highest post-cure temperatures, the functionally graded joints have a slightly higher strength.

Effect of carbon black nanoparticles concentration on the mechanical properties of a structural epoxy adhesive

The present work describes the influence of different carbon black nanoparticles with different concentrations on the mechanical properties of a structural epoxy adhesive cured by dielectric and thermal heating. This work was undertaken to improve the understanding of the effect of carbon black nanoparticles concentration on the stiffness (Young’s modulus), strength (yield strength) and deformation of the adhesive. Two kinds of spherical carbon black nanoparticles with different dielectric properties and sizes were used. Specimens with different amounts of carbon black were manufactured for each nanoparticle. The mechanical properties of the adhesive were measured in bulk specimens. The mechanical properties were found to vary as a function of the carbon black amount. For the dielectric cure, the strength and stiffness of the adhesive decrease as the amount of carbon black nanoparticles increases. On the other hand, the adhesive showed an increase of the deformation with an increase of the carbon black concentration. The thermal cure showed a mechanical behaviour similar as the dielectric cure, but the curing time increases substantially. A scanning electron microscopy analysis was performed to analyse the surface fracture of the adhesive. The fracture surfaces of specimens cured by dielectric and thermal heating and without nanoparticles are similar, typical of brittle adhesive. For high carbon black amount, the fracture surfaces are typical of ductile adhesive.

Functionally graded adhesive joints by using thermally expandable particles

ABSTRACT The main objective of this work was to use thermally expandable particles (TEPs) in order to create a graded adhesive along the overlap by local mixing of the particles. Different amounts of TEPs were used along the overlap with two different adhesives used in automotive industry. Tensile and four-point bend tests were performed on single lap joints with hard steel adherends in order to investigate the behaviour of TEPs-modified graded joints. It was found that the strength of the joints under tension decrease with increasing TEPs content even for small amount of particles (i.e. 1 and 5%). However, a slight increase in strength was found for the graded joints compared to homogeneous joints modified with the same %wt TEPs. In the four-point bending test, the graded joints presented slightly higher strength, if compared with the homogeneous joints modified with the same wt% TEPs. 5%wt TEPs-modified joints presented the highest strength when submitted to bending loading. The experimental results are compared with a finite element model and generally a reasonably good agreement was found.