One of the keys to achieve and comply with lower consumption standards for automobiles thus higher mileage MPG, is to lower the weight of the vehicle. As increasingly plastic substrates are used for constructing a vehicle, including composites materials such as GFRP and CFRP, welding suddenly becomes less of an operational task as opposed to more adhesive bonding.
Adhesive suppliers develop better products to allow for structural bonding while also strategically link up with surface pre-treatment solution providers such as Plasmatreat, to allow for an effective but low cost bonding solution.
Advantages and drawbacks of adhesive bonding for composites*
Compared to mechanical fasteners, adhesive bonding provides many advantages:
- Mechanical fasteners require drilling holes in the parts, and this weakens the composites because it cuts through the reinforcing fibers and also creates weak points. Bonding improves tensile resistance.
- Bonded joints exhibit lower stresses concentrations than mechanical joints when holes are needed, and thus provides increased static strength,
- Risks of cracks propagation are reduced,
- Bonded joints provide always 10 to 25 % weight savings in primary and secondary structures,
- Bonded joints enable the design of smooth external structures,
- For large surfaces bonding costs less than mechanical assembly, because it needs less manpower
- Adhesive may join together all kinds of materials: metals, composites, plastics, wood etc…
- Adhesives can join very thin materials which could not be riveted or bolted,
- Adhesives can join dissimilar materials without the risk of galvanic corrosion,
- Adhesives may be flexible or rigid according to their formulation,
- Adhesives have an excellent resistance to fatigue.
However, adhesives bonding has also some drawbacks:
- Elevated temperature creep resistance is fair or even poor for some structural adhesives.
- Adhesives cannot be used in or near to the motors of automotive.
- In general adhesives do not resist to peel stresses, and this is a drawback compared to welding for instance,
- Bonded parts cannot be dismantled easily,
- Bonding requires specific design so that the parts will be stressed only in shear mode,
- Bonding requires an excellent and specific surface preparation of the materials immediately before bonding,
- Bonded joints are difficult to inspect in a non destructive manner, although there are several NDT such as X rays, ultrasonic inspection, shearography, and others,
- Structural bonding requires an accurate mating of the parts because adhesives do not give high performances in thick joints,
- Water resistance of adhesives are often only fair,
- Durability of bonded joints must be assessed by difficult laboratory accelerated aging tests.
Pre-treatment with plasma (atmospheric in-line or vacuum low-pressure) can help overcome some of the drawbacks associated with bonding. Many car manufacturers are catching on. Check out this new article in Automotive News:
Till next time,