E.A.S. Marques

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.

Adhesives and adhesive joints under impact loadings: An overview

ABSTRACT The study of the behaviour of adhesive joints under impact loadings is a very active field of research, driven by significant industrial interest. Many industries, such as the automotive industry, are currently employing adhesive joints extensively, making use of the inherent properties of adhesive joints to improve the mechanical behaviour, reduce weight, and simplify manufacturing. Reduced structural weight is achieved by combining multiple lightweight materials, which is made possible by using adhesive joints. Impact strength is also a major factor, as vehicles must be able to provide adequate safety levels for their occupants during collisions. Another example of industrial application is the defence industry, which uses bonded structures to withstand ballistic impacts, with extremely high impact velocities. Understanding the behaviour of adhesive joints under impact is, therefore, crucial for designing stronger and safer structures. This document aims to review the research that has been previously undertaken in this field. Discussed research topics include high strain rate property determination, adhesive joint testing, effects of coupling environmental conditions with impact loads, and sections on numerical and constitutive modelling procedures. The final sections describe some practical applications of adhesive joints under large strain rates and relate them to the fundamental concepts previously discussed.

Overview of different strength prediction techniques for single-lap bonded joints

Adhesive joints have been used in several fields of engineering, and their applications are vast. Due to their easy and quick fabrication process, single-lap joints are a common configuration. The increase of strength, weight reduction and resistance to corrosion are some of the advantages of this kind of joint over traditional joining methods. However, stress concentrations at the overlap edges are one of the main disadvantages. There are very few accurate design techniques for the diversity of bonded joints that can be found in real applications, which constitutes an obstacle to the use of this bonding method in structural applications. This work aims at comparing different analytical and numerical methods in the strength prediction of single-lap joints with different overlap lengths (LO). The main objective is to evaluate which predictive method is the best. Adhesive joints were produced between aluminium adherends using a brittle epoxy adhesive (Araldite® AV138), a moderately ductile epoxy adhesive (Araldite® 2015) and a ductile polyurethane adhesive (Sikaforce® 7888). Different analytical methods were considered, together with two numerical techniques: cohesive zone models (CZM) and the extended finite element method (XFEM), allowing the comparative analysis. The analytical methods showed that they only give relatively accurate results in very specific conditions. The CZM analysis with the triangular law revealed to be a very accurate method, with the exception of joints with very ductile adhesives. On the other hand, the XFEM analysis was not adequate, especially for crack growth in mixed mode.

Mode I fracture toughness of CFRP as a function of temperature and strain rate

Composite structures currently used in the automotive industry must meet strict requirements for safety reasons. They need to maintain strength under varied temperatures and strain rates, including impact. It is therefore critical to fully understand the impact behaviour of composites. This work presents experimental results regarding the influence of a range of temperature and strain rates on the fracture energy in mode I, GIC, of carbon fibre reinforced plastic plates. To determine GIC as a function of temperature and strain rate, double cantilever beam specimens were tested at 20, 80 and −30℃, with strain rates of 0.2 and 11 s−1. A complementary numerical study was performed with the aim of predicting strength using the measured values. This work has demonstrated a significant influence of the strain rate and temperature on GIC of the composite materials, with higher strain rates and lower temperatures causing a decrease in the GIC values.

Experimental estimation of the mechanical and fracture properties of a new epoxy adhesive

The automotive industry is currently increasing its use of high performance structural adhesives in order to reduce vehicle weight and increase the crash resistance of automotive structures. To achieve these goals, the high performance adhesives employed in the automotive industry must not only have high mechanical strength but also large ductility, enabling them to sustain severe dynamic loads. Due to this complex behaviour, the design process necessary to engineering structures with these materials requires a complete knowledge of their mechanical properties. In this work, the mechanical properties of a structural epoxy, Sikapower® 4720, were determined. Tensile tests were performed to determine the Young’s modulus (E) and tensile strength (σf). Shear tests were performed to determine the shear modulus (G) and the shear strength (τf). Tests were also performed to assess the toughness of the adhesive. For mode I toughness determination (GIc), the double-cantilever beam (DCB) test was employed. For determination of toughness under mode II (GIIc), the end-notched flexure (ENF) test was performed. The data obtained from the DCB and ENF tests was analysed with the compliance calibration method (CCM), corrected beam theory (CBT) and compliance-based beam method (CBBM) techniques. The test results were able to fully mechanically characterize the adhesive and demonstrate that the adhesive has not only high mechanical strength but combines this with a high degree of ductility, which makes it adequate for use in the automotive industry.

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.

Geometrical study of mixed adhesive joints for high-temperature applications

Abstract The use of adhesives for high-performance structural applications has significantly increased in the last decades. However, the use of adhesive joints in adverse environmental conditions is still limited due to the reduced capability of adhesives to withstand large thermal gradients. Dual adhesive joints, which contain two adhesives with remarkably different mechanical behaviours, are a technique suitable for being used in extreme temperatures. The object of this study is a ceramic–metal joint, representative of the thermal protection systems of some aerospace vehicles. In this paper, several joint-mixed joint geometries are presented, studied with recourse to finite element analysis. In a first phase, the three-dimensional finite element models and the material properties are validated against experimental data. In a second phase, the model geometry is modified, with the aim of understanding the effect of several changes in the joints’ mechanical behaviour and comparing the merits of each geometry. The models’ presented good agreement was found between experimental and numerical data and the alternative geometries allowed the introduction of additional flexibility on the joint but at the cost of lower failure load.

Micro Cork Particles as Adhesive Reinforcement Material for Brittle Resins

Structural adhesives are progressively replacing conventional bonding methods, being constantly adopted for new applications. The most commonly used structural adhesives are epoxies due to their good mechanical, thermal and chemical properties, having a wide range of application. Epoxies are recognized for their high stiffness and strength, induced by their high degree of crosslinking. While the densely cross-linked molecular structure is responsible for the excellent properties of these materials, it also makes them inherently brittle, resulting in low ductility and toughness. Several researchers have, in the past decades, found necessary to mitigate this effect and developed new methods to increase the toughness of structural adhesives. There are many processes depicted in the literature on how to increase the toughness of adhesives. For example, the inclusion of particles (of nano or micro scale) is a successful technique to improve the toughness of structural adhesives. In this chapter, natural micro particles of cork are used with the objective of increasing the toughness of a brittle epoxy adhesive. The fundamental basis of this concept is for the cork particles to act like crack stoppers, leading to more energy absorption. An overview of how the micro cork particles can be used as reinforcement material for brittle resins is described. The main parameters that affect the mechanical properties of composite resin/cork, kinetic and chemical reactions between resin and cork and how this new material behaves in hygrothermal degradation, were analysed. It is concluded that the cork can be used as reinforcing material, promoting increased toughness of the adhesive without any chemical changes in the molecular structure or premature degradation of the adhesive.

A strategy to reduce delamination of adhesive joints with composite substrates

The use of bonding for joining composite materials in high-performance structures has increased significantly, as this joining method offers improved stress distributions and capability of joining dissimilar materials. However, the use of adhesive bonding for this purpose might lead to delamination failure, caused by peel stresses acting on the generally weaker transverse direction of the composite adherends. This work focused on improving the resistance to delamination of composite adhesive joints by using a novel composite with a reinforced high toughness resin on the surfaces. Single-lap joints using the novel composite material as adherends, were found to have 22% higher failure loads when compared with the specimens using carbon fiber reinforced polymer only adherends, with the failure mode changing from delamination of the adherends to cohesive failure in the adhesive. The lap shear strength was also close to that attained when using high strength steel adherends. A finite element analysis, using cohesive elements, was performed with the objective of reproducing the experimental results and better understanding the failure mechanism. Using this model, it has been determined that the change of failure mode and the plasticity on the surface layers are the two key factors underlying the increase in strength obtained with the novel adherends.

Mechanical behaviour of adhesively bonded composite single lap joints under quasi-static and impact conditions with variation of temperature and overlap:

The use of adhesively bonded joints in structural components for the automotive industry has significantly increased over the last years, supported by the widespread integration of composite materials. This synergy allows vehicle manufacturers to offer a significant weight reduction of the vehicle allowing for fuel and emissions reduction and, at the same time, providing high mechanical strength. However, to ensure vehicle safety, the crashworthiness of these adhesive joints must be assessed, to evaluate if the structures can sustain large impact loads, transmitting the load and absorbing the energy, without damaging the joint. The novelty of this work is the study of the strain rate dependent behaviour of unidirectional composite adhesive joints bonded with a ductile epoxy crash resistant adhesive, subjected to low and high testing temperatures and using different overlap lengths. It was demonstrated that joints manufactured with this type of adhesive and composite substrates can exhibit excellent quasi-static and impact performance for the full range of temperatures tested. Increasing the overlap length, and independently of the testing temperature, it was observed an increase of energy absorbed for both quasi-static and impact loads, this is of considerable importance for the automotive industry, demonstrating that composite joints exhibit higher performance under impact.