The evaluation of adhesive bonding safety: a techno-economical approach
Adhesives have been assuming a leading role in the world of joining technology. They were successfully established as a viable option amongst alternative bonding methods such as screwing, welding, riveting and soldering. Especially in the aircraft, vehicle and building industries, high safety standards are necessary. By the end of 1970, a lot of mechanical testing principles were introduced for characterising the adhesive bonding strength from a technical point of view.
The most common are:
- the lap-shear test according to DIN EN 1465, ISO 4587, ASTM D3163, ASTM D1002,
- the tensile strength test via ASTM D897, ISO 527, ASTM D638 and
- the peel test by ASTM D1876 and ISO 8510.
However, at a time when safety and longevity are becoming increasingly important, such assessment approaches are no longer up to date. This is because such testing procedures merely produce single-stress-based metrics such as tensile strength, ignoring the characterisation of the damage and softening behaviour of an interface during the cracking process. In other words, this is not sufficient criteria to produce reliable and significant safety statements for adhesive-bonded joints. Hence, alternative ways to enable a thorough safety assessment of adhesive-bonded composites are required as stakeholders need reliable benchmarks for their decision-support.
Regarding the economic analysis of adhesive bonds, only a few studies are known in literature with vital information missing. That is why I conducted a recent scientific study with the techno-economic assessment of the bonding safety of structural adhesives. Here I briefly report about its principles, results and benefits.
A techno-economical approach for evaluating bonding safety
In many industrial sectors, increasing demands are placed on products’ construction and design. To meet these requirements, adhesive bonding is an ideal supplementary joining technique. However, there are still numerous objections and scepticism regarding the use of bonding technology, especially in the construction industry. The reason for this is that traditional joining processes, such as welding or screwing, still occupy a strong position regarding operational reliability and longevity. As reliable data and comparison parameters are still scarce, establishing confidence in adhesive bonding technology is still an open road. To strengthen trust in this key technology, scientific safety parameters and benchmarks are necessary. With such, a deep evaluation and classification of safety, reliability and structural integrity of adhesively bonded composites and joints is possible.
The introduction of the adhesive safety factor
For the purpose of decision-support for purchasing and product engineers, a scientific safety factor was created, called adhesive safety factor SF. It is a function of several parameters derived by fracture analytical testing procedures. By applying such analysis, parameters derived are independent of state and type of loading, thus serving as pure material laws. As they cannot be obtained from technical datasheets (TDS) directly, they must be deduced experimentally.
The quantification of the price for bonding safety
Once the technical safety factor has been defined, it is also necessary to incorporate economic aspects in order to justify the decision-maker’s choice for bonding adhesively. For this purpose, an approach from the insurance industry was adopted, namely in the form of security premiums. This scientific parameter created thus is called adhesive safety premium SP. With all those science-based safety parameters, an independent and safety-adjusted selection of adhesives beyond the standards provided by TDS is possible.
The latest scientific study indicates interesting insights
To demonstrate the safety behaviour of adhesive-bonded structures in practice, three high-performance structural adhesives used in automotive and construction industries were selected. The adhesive safety factor SF and the adhesive safety premium SP were derived. Except for lap-shear strength 𝜏ls, which was derived from TDS directly, all values were measured separately.
As can be observed from Figure 1, low-strength industrial adhesives used for bonding wood (type 1–6) were compared with high-strength structural adhesives used for bonding aluminium (type A–C). Results obtained show that, for instance, adhesive 4 reveals the same safety premium as A and C. This means that a premium level of around 50 euros must be spent in order to guarantee the same safety level. However, since the safety premium consists of both the price and the damage tolerance, additional information must be provided.
Therefore, the adhesive safety factor in the form of coloured balloons was included and visualised. As can be seen, the larger the balloon, the higher the safety factor of the adhesive compound and the better the damage tolerance behaviour against crack propagation in the case of rupture. From the decision-maker’s point of view, adhesive C should be chosen, since it has the highest safety level at the same high strength level as the epoxy alternative at a balanced safety premium. The alternative is the hybrid adhesive B. It displays only a slightly lower safety level than C, but offers the highest strength and lowest safety expenses. Finally, it can be said that from an economic point of view, this option should be chosen.
Figure 2 contrasts the adhesive price with its safety premium. It can be well observed that adhesive A (epoxy) requires about 20% of its purchase price to achieve the same safety level as the hybrid candidates B and C. The safety premium of adhesive B is equal to those of adhesive C. With a safety premium of around 5–6 % from the unit price, these options are relatively cost-efficient.
Summary and outlook
The results obtained from my recent study indicate that the safety-adjusted evaluation of high-strength structural adhesives is a necessary task for the adhesive selection process. In order to enable a thorough analysis, the structural integrity of an adhesive-bonded joint was expressed in terms of its damage tolerance behaviour by applying fracture analysis. Then, a scientific safety benchmark was created both technologically and economically, respectively. Using such parameters and metrics can excellently support stakeholders in selecting the safest adhesive regarding cost and performance.
From a technical and economic point of view, there are indeed alternatives to high-strength epoxy adhesives. The relatively low purchase price and the high strength seem to be convincing at first glance. However, a closer look reveals that these factors do not allow any conclusions to be drawn about the safety classification of the adhesive in practice. Contrary to options, it can be concluded that a high strength is no guarantee for fail-safe ability in the case of damage and a low price is no indicator for an efficient purchase decision. Thus, in the future, safety considerations, as introduced in this study, should be part of any selection processes of high-tech structural adhesives.
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