Adhesive bonding, like most techniques, has restrictions that must be considered while designing bonded parts and planning their production. With sufficient forethought, many of the constraints can be alleviated during the design and planning stage.
Temperature Resistance
Adhesives are made from polymers, plastics, or synthetic resins, which have some inherent restrictions. They are not as strong as metals, however, this is compensated for by the increased surface contact area provided by the bonded joints.
Bond strength reduces with rising temperature, and the adhesive’s strain properties transition from elastic to plastic. This transition typically occurs between the temperature range of 20 – 220 °C (68 – 428 °F), with transition temperature depending on the type of adhesive used.
Chemical Resistance
The resistance of bonded joints to the in-service environment depends on the polymer’s properties from which the adhesive is produced.
The possibility of the bonded structure being exposed to oxidizing agents, solvents, and so on must be considered when selecting the type of adhesive.
Curing Time
Unlike mechanical fastening or welding, most adhesives do not achieve maximum bond strength immediately. The bonded assembly must be supported as the bond strength develops.
Hybrid joining, in which mechanical fasteners (such as screws or rivets) are used alongside the adhesive, can eliminate the need for support during bonding.
Surface Preparation
The bond quality may suffer if the adhesive does not readily wet the surfaces during the bonding process.
Process Controls
To ensure consistently good results, unfamiliar process controls may need to be implemented. A poorly executed bonding is often impossible to fix.
In Service Repair
Bonded assemblies are usually challenging to disassemble and repair.
Health and Safety
High-performance structural adhesives are typically made from chemical products that pose some environmental and health risks until they are completely cured. Appropriate precautions must therefore be taken during mixing, application, and bonding. The safety data sheet (SDS) contains information for each product.
Quality Management
Adhesive bonding is classified as a "special process" under the ISO 9000 quality standard because bond strength and durability cannot be completely verified using non-destructive testing (NDT). This means that additional quality standards, such as DIN 2304-1 and DIN 6701, may need to be applied to meet quality management requirements.
Designing A Bonded Joint - Loading Conditions
An assembly that will eventually be bonded must be designed with bonding in mind, rather than merely bonding a design meant for welding or mechanical fastening. When designing bonded joints, the following aspects must be considered:
- Joint geometry
- Adhesive choice
- Adhesive performance characteristics
- Service conditions
- Stress in the joint
- Manufacturing process
Bonded assemblies may be subjected to tensile, compressive, shear, or peel stresses, or a combination thereof (Figure 1). Adhesives are more resistant to shear, compression, and tension stresses. They perform less well under peel and cleavage loads.
A bonded joint must be designed so that the loading stresses are channeled along the adhesive’s greatest strengths.
The Huntsman Advanced Materials technical datasheet usually reports shear strength and peel strength measured on a variety of standard substrates to show typical adhesive performance properties.
Image Credit: Huntsman Advanced Materials
For instance, the standard shear test method (ISO4587) employs a simple lap joint made of a metal sheet, typically an aluminum alloy 25 mm wide with 12.5 mm of bonded overlap. The mean breaking stress at room temperature will range from 5 to 45 N/mm2, depending on the adhesive.
At the top end of this breaking stress range, bonded assemblies made of aluminum alloy up to 1.5 mm thickness will frequently cause the substrate to yield or break (the lap joint is only one of several different types of bonded assemblies).
The breaking load of a lap joint corresponds to its width, but not to the overlap length. While the breaking load increases with increased overlap length, the mean breaking stress decreases. The joint strength is a complex function of the stress concentrations generated by the load.
A simple lap joint created from a thin metal sheet experiences two types of stress: shear and peel. Both the shear and peel stresses vary along the joint’s length, with concentrations at each end.
Alternative joint designs are shown below, with these stresses distributed more evenly. The increased efficiency leads to stronger joints.
Figure 1. Loading Conditions. A bonded joint can be loaded in five basic ways (as shown in the adjoining diagrams). Cleavage and peel loading are the most severe as they
concentrate the applied force into a single line of high stress. In practice, a bonded structure has to sustain a combination of forces. For optimum strength, the bonded assembly should be designed in such a way as to avoid cleavage and peel stresses. Image Credit: Huntsman Advanced Materials
A bonded joint can be loaded in five different ways (as demonstrated in Figure 1). Cleavage and peel loading are the most severe as they concentrate the applied force into a single, high-stress line.
In practice, a bonded structure must withstand a combination of forces. To achieve optimum strength, the bonded assembly should be designed to avoid cleavage and peel stresses.
This information has been sourced, reviewed and adapted from materials provided by Huntsman Advanced Materials.
For more information on this source, please visit Huntsman Advanced Materials.