Technical report ADENG: a programme for the transfer of

adhesives bonding design technology to UK industry

J Hill*

Abstract - Although a considerable amount of good theoretical work has been done on the mechanical performance of bonded joints, an accepted design procedure, suitable for use by design engineers, has not been available. A recent project by the UK's Pera International has tried to redress the balance. This report summarizes the end results.

The benefits of adhesive bonding are diverse. They incude the abil i ty to join d iss imi lar materials, automatic sealing, reduction of noise and vibration transmission, creation of smooth aesthetic finishes, and high load transfer efficiency due to low joint stresses. These benefits, however, can only be real ized if the design of the joint is suited to the bonding process. It is not usually sufficient to replace another joining method with adhesive bonding without at least some modifi- cation to the joint details.

Unfortunately, whi le a considerable amount of good theor- etical work has been done on the mechanical performance of bonded joints, an accepted design procedure, suitable for use by design engineers, has not been available. In an effort to redress the balance Pera International, a mult i-discipl ined technology centre with over 15 years exper ience of poly- meric and adhesives research and technology development, has recently completed a four-year, £1 mil l ion adhesive research programme called ADENG. The objectives of the ADENG project were to overcome the diff iculties exper- ienced by designers, by developing a design procedure, determining which adhesive propert ies were required by the designer and their best method of acquisit ion. The most suitable preparat ions to ensure repeatable and long life with methods of performance prediction were also required.

The project provides many tools for the designer resulting from theoretical and practical research. These include over 50 reference reports covering areas such as:

• Joint configurat ions • Analytical techniques to predict joint performance • Equations and d iagrams used to determine supportable

loads • Yield characterist ics • Adhesives selection • Adherend pre-treatments • Environmental effects on adhesion • Adhesives application techniques • Component assembly techniques • Non-destructive testing techniques

In addit ion to these reference reports are databases deve- loped by CETIM, one of France's leading research organiza- tions. These databases, writ ten in French, cover:

• Adhesive selection • Suppl ier selection • Adherend pre-treatment selection

Research & Technology Division, Pera Internat ional Melton Mowbray, Leicestershire LE13 OPB, UK. Tel: +44 (0)664 501501. Fax: +44 (0)664 501264

• Relevant European standards

Design software, developed by Pera International to predict adhesive performance for a number of the more common joint configurations, is also included. At the centre of the research outputs is a design procedure for adhesively bonded joints, which interacts with the other tools to produce an iterative package for the designer.

A focused procedure exists for a number of joint configu- rations. However, the theme to which each is constructed is examined in the fol lowing sections.

The basis of each design procedure The choice of adhesive is, of course, fundamental to the design. It must, first, adhere well to the surfaces which are to be bonded. The surfaces must be condit ioned prior to bond- ing; the adhesive must be appl ied in a control led quantity and distribution, and must adhere and retain sufficient co- hesive strength at the operat ing temperatures and environ- ments expected.

At the initial stage in joint design, assembly of the compo- nents should also be considered. Joint faces should be assembled with a preferred movement perpendicular to the joint without any transverse (wiping) component of assembly which might clear adhesive from the joint. The adhesive thickness should be control led to the recommended value by fixturing, separators or self-f ixturing lands.

A bonded joint has two mechanical elements, and once adhesion is assured by choice of adhesive and joint surface condit ioning, then the strength of each of those elements, adhesive and adherends must be balanced by a des;'gn pro- cedure. Usually it is possible to extend the joint dimensions until fai lure occurs init ial ly in the adherends (Fig. 1). Since these are usually of a structural material, their fai lure is gradual , more control led and more predictable so that the joint can usually fail-safe. Somet imes the strength of the adhesive bond is below that of the adherends, however great the adhesive area. In this case the bond must be designed as the weaker link in the chain, capable of withstanding any expected joint load.

In the calculation of the strength of the adhesive, adhesion strength is assumed to be greater than the cohesive strength of the adhesive in all areas of the joint. This means that surface preparat ion and matching of adhesive to adherends must be good enough to ensure this. When considering the cohesive strength, most joints are found to have local areas of high shear stress where fai lure wil l initiate. These areas are usually at an edge or at two symmetr ical edges of a joint (Fig. 2). The cri terion of fai lure adopted for the adhesive is when the predicted shear stress in the highest stressed area

MATERIALS & DESIGN Vol. 13 No. 4 1992 241

Technical report

Fig 1 Testing the mechanical strength of a bonded jo int using an extensometer

Fig 2 Bonded jo int stress analysis using a polaroscope

of the joint exceeds the ' l imit of reversibi l i ty of shear stress' for the adhesive.

This l imit of reversibi l i ty is the highest shear stress at which continuous cyclic appl icat ion and release of the shear stress does not cause permanent set of adhesive. This l imit occurs at a level below yield shear stress of the adhesive and lies on the elastic portion of the adhesive stress/strain characteristic. If the l imit is not known, a value of 1/2 × yield of the adhesive might be approximated.

The operat ing environment, of course, also affects the adhesive layer. High humidity or water saturation have been

found to affect adhesion at the interfaces of adhesives and adherends, and to modify the cohesive propert ies of the adhesive layer. The loss of adhesion at the interface with the adherend is an i rreversible loss of bond strength. Product assembl ies are feasible in which the rate of this loss increases with time, and the loss becomes unstable. Where such instabil i ty occurs, the joint is unusable and a change of surface preparat ion or adhesive is essential to bring the rate of loss of bond strength under control.

Even where the loss rate is under control and decreasing with time, the designer needs to know quantitat ively what wil l be the final loss of bond strength, due to loss of adhe- sion, after a long operating period. Quantitative knowledge in this area is not available, requir ing research to cover the losses of bond strength for the many different combinations of adhesives, adherends, surface treatments and environ- ments. In these circumstances, the designer must note that some loss of adhesion wil l occur and so some al lowance to strengthen the joint must be made, yet the extent of that a l lowance is not known. The most common approach is to make an arbi trary al lowance and set up development tests on the product in which the adhesive joint is used. As this development testing of environmental degradat ion is carried out on more and more products, know-how is acquired relat- ing to the effect and the al lowances necessary to accommo- date it. Design choices are increasingly guided by such know-how as it grows through testing.

The second effect of adverse operat ing conditions, in par- ticular, humidity and water saturation, is the modif ication of the bulk propert ies of the adhesive. Humidity tends to lower shear modulus, Young's modulus and the yield stress of the adhesive. The lowering of the moduli tends to distr ibute stress more uniformly through the adhesive layer and thus reduces the highest adhesive stress found in the joint. Unfor- tunately, humidity also tends to lower yield strength of the adhesive and so the adhesive may yield at this lower applied stress or even below it. These two latter effects, the decrease in moduli and the decrease in yield strength are often reversible, such that when the adhesive dries out in an atmosphere of lower humidity, then the moduli and yield strength again increase. For design, it is necessary to choose those values of moduli and yield strength which apply to the operating condit ions in which the joint must work under load.

If the adherends of the joint are of structural material, i.e. materials, plastics or composites whose strengths are pre- dicted by mechanical engineering formulae, the cri terion of fai lure for s imple stresses is usually the yield stress. If fati- gue condit ions are expected, a stress of around 30% of the yield stress must be chosen.

As an example of the design procedures, the simple lap joint in tension is based on consideration of a load/over lap diagram upon which are plotted design boundaries for both adhesive and adherends. Characteristics are plotted on the diagram representing load/overlap relationship for:

• Fracture of the components outside the joint, correspond- ing to the ult imate tensile stress of the component;

• Local fracture of the components in the joint at the points of highest stress;

• Total yield of the components outside the overlap occur- ring across a complete section of a component;

• Local yield of the components which occur at points of highest stress;

• Local yield of the adhesive at the highest stress in the adhesive;

• Characterist ic of local yield of the adhesive corrected to account for yield of the component at the component/ adhesive interface;

• Fracture of the adhesive at the points of highest stress in the adhesive;

• Characterist ic of adhesive fracture corrected to account

242 MATERIALS & DESIGN Vol. 13 No. 4 1992

for yield in the component at the adhesive/component interface.

The plotted points of each of these characteristics form the boundaries of three regions which the designer must consider within the joint design process.

The elastic region This is the region of use to the designer. The adhesive and components remain elastic and deformations do not remain after removal of the load. Loads within this area cause no permanent set, do not fracture the components and do not break the integrity of the joint. To make sure that loads do not exceed the bounds of this area, the designer must choose factors of safety so that there is a safe margin between the yield boundaries and the expected joint-loads during service.

The elasto-plastic region In this region the designer knows that the load is causing the component to yield, while the adhesive continues to act elas- tically. Joint integrity is complete although there is perma- nent set and irreversible deformation within the joint.

The plastic region In this region both adhesive and components are stressed beyond yield. This is permanent set in the joint, but the joint retains its integrity. This region of load/overlap is not usable by the designer to support service loads. Rather, it is a region where final collapse is resisted although major per- manent deformation has taken place. This region and

Technical report

usually the elasto-plastic region constitute a margin of over- load capacity for the joint. Although the joint itself absorbs some energy, stresses in the components are beyond the yield limit and so major deformation in the components out- side the joint can be expected. These will absorb far greater energy than can be absorbed in the limited area of the joint itself.

Design procedures based on these fundamental consider- ations have been produced through the ADENG programme covering joints such as:

• Single lap joint in tension; • Double lap joint in tension of compression; • Double lap joint in shear (this entails only a minor modifi-

cation of the procedure for the double lap joint in tension of compression);

• Tubular joint in torsion; • Torsion joint in tension or compression.

It is anticipated that the design technology produced within the original ADENG project will greatly enhance the ability to gain competitive advantage in the automotive, aerospace and general engineering industry sectors. Pera's Research & Technology Division is about to launch a new project to further enhance the CAD tools produced within the original project and disseminate the design technology to UK engi- neers and designers. This next project will ensure that the technology is accurately transferred to the companies likely to most benefit quickly. Awareness of the technology among SMEs is also to be addressed within the programme.

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