Adhesives have become an integral technology across a range of industries, including aerospace, automotive, domestic appliances, medical, consumer electronics, construction, marine, sports equipment, and more.
Widely used for a variety of fabricated products, adhesives allow for new and challenging product designs. However, all adhesives need to be applied correctly, including the relevant surface preparations and post-bond treatments, while selection must take account of thermal expansion and other use-case factors.
Our experts have been advising many of the biggest names in industry on the use of adhesive bonding solutions for a wide range of applications, ensuring the best results for bonded materials, whether metal-to-metal, metal-to-plastic, or any other material combinations.
As part of our ongoing work supporting industry, we have been involved in many adhesive bonding-related projects over recent decades, providing research and testing as well as creating novel new solutions for a wealth of different use cases.
- A Review of Mechanical Test Procedures for Adhesively Bonded Joints
Dating back to 1993, we undertook ‘A Review of Mechanical Test Procedures for Adhesively Bonded Joints,’ a piece of core research designed to assess the suitability of adhesively bonded joints for different designs, with a strong focus on their mechanical performance and longevity. The research investigated factors including creep rupture, fatigue and thermal degradation due to fatigue, in order to provide increased confidence in the use of adhesives by industry.
- Non-Destructive Testing Methods for Adhesively Bonded Joint Inspection - A Review
By 1994, the focus had moved to assessing non-destructive testing methods for adhesively bonded joints. When the project launched, NDT was capable of detecting gross discontinuities such as disbands and voids, but was deemed unreliable for detecting weaknesses at the bond interface. The project led to the creation of a report acting as an industrial and literature review to allow for the increased exploitation of adhesively bonded joints across industry.
More specifically, in 1994, our experts also undertook fatigue testing of fibre-reinforced laminates that had been adhesively bonded…
- Fatigue Testing Adhesively Bonded Fibre-Reinforced Laminates
This project investigated the fatigue performance of adhesively bonded of an Aramid Reinforced Aluminium Laminate (ARALL-3), a widely-available fibre reinforced metal laminate (FRML) that was developed specifically for fatigue-sensitive applications. Plain ARALL has very high fatigue strength, indicating that the strength of any structure made of ARALL and subjected to fluctuating loading would almost certainly be governed by the fatigue strength of the joint, where experimental data was still fairly limited. Our experts tested single-lap shear specimens in two different thicknesses and three different overlap lengths were tested in air under constant amplitude sinusoidal loading. The mode of failure, including the most likely site for fatigue crack initiation, was also investigated.
- Adhesive Durability and Environmental Attack Mechanisms
This 1996 core research project aimed to ascertain which adhesives were most suited to different service conditions. Although adhesive joints may seem reliable when first made, exposure to conditions such as damp, solvents or loading cycles can weaken them, reducing joint strength. This makes it particularly important to understand the in-service behaviour of different adhesives used for critical or structural engineering applications. In addition, this work investigated the adherend and surface preparations to help preserve the joint interface.
A 1998 project continued our investigations into the testing of adhesively bonded joints, this time with ultrasonic surface, plate and interface waves:
- Surface, Plate and Interface Waves in Adhesive Bond Joints
Adhesive bonding of structurally critical components requires high quality processes and reliable non-destructive testing (NDT) techniques. However, many NDT methods only deliver limited information about a joint, so a study was conducted to assess the suitability of ultrasonic surface, plate and interface waves that showed the potential to provide more information about the joint condition than conventional techniques. As well as assessing the suitability of these testing methods, the project team worked to develop a practical means of generating ultrasonic surface waves in thin sheet material
One year later, in 1999, our experts turned their attention to the matter of fracture mechanics for adhesively bonded joints with another core research project…
- A Fracture Mechanics Approach to Adhesively Bonded Joints
A study conducted by TWI showed that difficulties in predicting the long-term performance of adhesively bonded joints in service had restricted their use by industry. In order to increase the confidence in these joints, we acquired fatigue endurance and crack growth data from a galvanneal steel adherend/epoxy adhesive system, against which we compared fatigue life predictions based on fracture mechanics concepts. This allowed us to correlate the structural performance of the joints with key constituents and fracture appearance.
This was followed by a 2001 project, which aimed to increase confidence in and thereby the uptake of composite materials…
- Adhesive Bonded GFRP Joints and Interlaminar Flaw Tolerance
This project aimed to improve the quality of data available around the design and reliability of composite joints. Impact damage resulting in interlaminar delamination is a major concern for structural integrity, leading to over-conservative safety factors being applied to designs. A better understanding of flaw tolerance, damage propagation and failure modes in these materials allowed for the safety factors to be reduced, leading to the development of flaw assessment methodologies for the design and monitoring of composite joints, facilitating increased efficiency and extended joint lifetimes. To achieve this, our experts experimented with the effect of adherend flaw size, position and joint geometry parameters on the performance of adhesively bonded composites containing interlaminar flaws under loading.
- Silane Pre-Treatment of Al Alloys for Adhesive Bonding
Adhesive bonding has been successfully used within the aerospace industry for decades with highly controlled, rigorous surface pre-treatments being used to address the key issues of high strength and durability prior to the bonding process. However, tightening environmental regulations made the use of toxic materials such as those associated with acid etching pre-treatments increasingly difficult. As a result, there was need to identify and develop 'green' solutions to these important processes.
A 2002 core research project investigated the practicalities of using organo-silanes as a more environmentally-friendly alternative. The aim of the project was to define the preparation and application parameters of a commercial silane coupling agent to maximise the durability performance of a bonded joint. This included assessing chemical reactions associated with the silane and the alloy surface, as well as the effect of the adherend position during silane application on joint durability and the effect of hydrolysis (via exposure to boiling water) on the adherend surface prior to silanation on the durability of the joint.
- Samples and Techniques for Kissing Bonds in Adhesive Joints
This 2009 core research project continued our work to improve confidence in adhesive bonding throughout industry by investigating the issue of ‘kissing bonds’ that could not be reliably detected by conventional NDT techniques. Kissing bonds in adhesive bonding refer to situations where there is intimate contact between the materials to be joined but without adhesion of molecular bonding between them. These joints are not totally disbanded, but has an adhesion strength 80-90% lower than a reliable join. As a critical defect, kissing bonds are difficult to simulate, yet were a limiting factor against the widespread use of adhesive bonding in industries such as aerospace. Research into previous studies determined that a technique using ultrasonic shear waves could be a good candidate for the detection of kissing bonds, which led to a feasibility study focusing on two techniques involving ultrasonic shear waves; a through-transmission and a pitch-catch technique. The resulting project work sought to develop a consistent and reproducible methodology for producing satisfactory sets of samples to meet the kissing bond requirements as defined in the literature before identifying ultrasonic NDT techniques suitable for finding adhesive kissing bond flaws.
- The ‘SWAK’ Project
Research into kissing bonds continued with a collaborative project alongside GMI Aero SAS and Brunel University. The SealedwithoUTaKiss (SWAK) project investigated NDT methods for use in the aerospace industry to detect these types of defects. Good results were achieved with phased array (PA) immersion inspection, thermography, tomography as well as guided wave and laser shock. The overall aim of this project was to deliver a solution for the detection of kissing bonds for the aerospace industry so it could fully realise the light-weighting potential of adhesively bonded parts without fear of kissing bond defects.
- Gamma Sterilisation and Adhesive Bonded Dissimilar Materials
As interest grew in the use of bio-compatible polymers within the medical industry, so our experts were approached to ensure these materials maintained a good adhesive bond following sterilisation.
The resulting project looked into the effect of gamma sterilisation on the strength of PEEK-OPTIMA to titanium and PEEK-OPTIMA to cobalt chrome alloy joints bonded with Loctite® 4061 adhesive. This work followed a previous TWI study that investigated the influence of various surface treatments on the mechanical properties of dissimilar joints adhesively bonded with different types of medical grade adhesives. This resulted in Loctite® 4061 being selected as the adhesive that gave the best lap shear strength (LSS) of the joints.
- Dissimilar Joining of Steel to Aluminium for Automotive Applications - Detailed Review of Mechanical Fastening and Adhesive Bonding
Moving from the medical to the automotive sector, a 2020 research project investigated both mechanical joining and adhesive bonding of steel to aluminium, including an experimental study, technical process details, information on how a given process can be applied across a range of materials, mechanical properties and physical characteristics of the joints produced, and an economic study.
The ensuing report provided a highly detailed set of data to be considered when selecting a suitable joining method, including a consideration of economic and performance-related constraints. The research findings determined that there was a large difference in speed, cost, robot payload, joint size and mechanical performance between the various joining technologies available, meaning that users must strike a balance of performance, economics and quality when selecting a process.