|Composites Design and Manufacture (Plymouth University teaching support materials)
Adhesives and bonded structures.
Adhesives Product Locator (Huntsman Araldite)
Adhesives selection and stress analysis software (Permabond PAL and P-STRESS)
This teaching support material complements that in lecture C10: Machining, bonding and repair.
Adhesives come in a variety of types, usually distinguished by the chemistry used [1, 2]:
The above is not an exhaustive list. For example other variants include plastisols (based on PVC dispersions) and rubber solutions (where solvent evaporation effect bonding). In particular, toughened adhesives may be any of the above families of adhesive with the incorporation of low molecular weight rubbers (either chemically incorporated in the polymer backbone or as physical particles).
Adhesives can bond most materials in common engineering use and are especially useful where the substrates are different materials. For optimum bonding, avoid:
The advantages of adhesives (compared to welding, brazing, soldering or mechanical fasteners) are:
Good joint design is essential for highly-stressed applications. Bonded joints are best loaded in compression and give acceptable performance in shear. Tension, especially peel (where at least one component is flexible) and cleavage (where rigid components are involved) should be avoided . See pages 15-16 of reference  or page 50 of reference  for illustrations of acceptable and unacceptable bad designs for adhesively bonded joints (or the PowerPoint presentation for this lecture: slides 12-15).
Surface preparation is crucial to the achievement of a good bond and for composites normally includes a degrease-abrade-degrease-dry sequence. The creation of a good bond depends critically on the adhesive wetting the substrate surface. The wetting of a solid by a liquid depends on the relationship between the interfacial tensions for the three phases: solid/liquid (SL), liquid/vapour (LV) and solid/vapour (SV). The contact angle for a liquid on a given smooth surface is described by Young's equation . A contact angle of <90° will result in wetting (the fluid is hydrophilic when the liquid is water). A contact angle >90° will not result in wetting (the fluid is hydrophobic when the liquid is water). Wenzel  recognized that the quality of the surface affects the surface wetting properties and modified Young's equation to include roughness as an additional factor. The Young (left) and Wenzel (right) equations are:
where θγ is Young’s contact angle, θW is Wenzel’s contact angle, γSL, γLV and γSV are the interfacial energies per unit area for the solid-liquid (SL), liquid-vapor (LV), solid-vapor (SV) interfaces respectively and r is the the roughness factor (the ratio of the actual area of a rough surface to the geometric projected area). Marmur  has presented a useful summary of the issues arising during the measurement and interpretation of contact angles.
The use of shot-blasting to abrade the surface is inappropriate: it tends to remove too much substrate. Plastic bead blasting (or similar blast media) permits greater control of material removal. For adhesively bonded composite components, co-curing is often adopted: the substrate and the adhesive joint are cured simultaneously.
Failure of an adhesive joint is called "debonding". In this context, debonding differs from the composite failure mode where the fibre/matrix interface breaks. debonding of an adhesive joint is separation of the bulk adhesive from the substrate.
The National Physical Laboratory has an on-line Adhesive Design Toolkit.
The Engineering Sciences Data Unit (ESDU) report 92041 , together with ESDUpac A9241, "presents a Fortran computer program for the analysis of the distribution of the stresses and strains in the adhesive of a bonded single lap joint under an applied load, together with the nominal stresses in the adherends". "Section 1 describes the theory and application of two, alternative, analyses of a single-step lap joint that are included in the program described in Section 2". "The first analysis, the simple shear stress analysis, may be used to analyse the shear stress distribution in the adhesive of a bonded lap joint loaded in either tension or shear. This analysis may be applied to either single or double lap joints, provided that the latter are symmetric, and can allow for the inelastic shear stress-strain properties of the adhesive. The second analysis, the flexible analysis, is applicable to single lap joints loaded in tension and takes account of bending of the joint in deriving the peel stresses resulting from that bending. This analysis also takes account of thermal stresses arising as a result of differences between joint working temperature and curing temperature but it does not allow for the inelastic shear stress-strain properties of the adhesive".
The Araldite/Huntsman Advanced Materials website offers the following documents:
The Loctite website offers the following documents:
The Permabond Engineering Adhesives website offers pages on:
This link lists some major suppliers of adhesives.
Videos (no longer available in PU CSH Library):