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Design Basics

Design of a joint should satisfy the following conditions:

• Allowable shear stress of adhesive not exceeded
• Allowable tensile (peel) stress of adhesive not exceeded
• Allowable in-plane shear stress of adherend not exceeded
• Allowable through-thickness tensile stress of adherend not exceeded

Basic design considerations for maximising the static strength and fatigue performance of adhesively bonded joints include:

• The stress on the bond-line should be kept to a minimum. 
• Design the joint so that the stress is distributed as uniformly as possible across the entire bonded area, and that the stress in the adhesive is predominantly shear.  Peel and cleavage stresses should be minimised.  Avoid shear and peel stress concentrations - shear and peel stress concentrations present at bond-line ends can be reduced by the use of a combination of tapered or bevelled external scarf or radiused adhesive fillets.  Significant increases in the joint strength compared with square-ended bond-lines can be achieved.  It is recommended that the taper ends of lap joints should have a thickness of 0.76 mm and a slope of 1/10.
• Increasing either the adherend stiffness (i.e. elastic modulus) or adherend thickness generally results in a reduction in deformation and an increase in load-bearing capacity of the bonded joint.  The use of stiff or thick adherends will minimise peak stress levels and yield a more uniform adhesive stress distribution.  However, the use of absolutely rigid adherends will not prevent the formation of stress concentrations at the bondline .
• Rigid adhesives perform better in shear, whilst flexible adhesives are better under peel or cleavage loading conditions.
• The joint strength is proportional to the joint width.  An increase in width will always increase joint strength.  Whereas, increasing joint length does not always increase joint strength (see below). 
• The total overlap length must be sufficiently long to ensure that the shear stress in the middle of the overlap is low enough to avoid creep.  Short overlaps can result in failure through creep-rupture.  It is recommended that the overlap length is ~10 times the minimum adherend thickness to ensure a uniform shear distribution.  Increasing the overlap lengths beyond this value does not result in substantial increases in static and fatigue performance.  The low stress region in the middle of a long overlap contributes to joint strength by providing elastic restoring force or reserve.  It is recommended to maximise bond area.  Longer overlap lengths are highly desirable (provided cost and weight penalties are not too high).
• Ensure the joint is loaded in the direction of maximum strength of the adherend.  The bonded joint needs not only to be loaded in the direction of maximum strength, but also loads in the weak directions need to be minimised.
• The adhesive bond-line (within reasonable limits) does not have a strong influence on joint strength.  Factors, such as stress uniformity, surface preparation and porosity (voids – entrapped air and volatiles) will have far more an influential effect.  Poor surface preparation and the presence of voids have a major detrimental effect on joint strength no matter how well the joint is designed.  Stiffness is relatively unaffected.
• Maintain a uniform bond thickness and wherever possible join identical adherends to minimise skewing of the peak and normal stresses, and to minimise thermal residual stresses and bending stresses due to differences in coefficient of thermal expansion (CTE) values of the adhesive and adherends.
• In the case of fibre-reinforced laminates, avoid interlaminar shear or tensile failures of composite adherends.  Also, ensure the laminated adherend is symmetric, thus ensuring the coupling stiffness components of the laminate are zero (i.e. no twisting).

The level of allowable stress in the adhesive layer at the limit load (i.e. the highest load expected to be experienced during the service life of the structure) is generally established from the ultimate load (i.e. load at failure) multiplied by suitable safety factors.

The overall joint geometry is critical to performance.  Loads causing peel stresses will compromise joint performance.  For example, single-lap joint (Figure 3) in which eccentric forces acting on the joint induce a bending moment.  The bending moment causes additional tensile (peel) stresses to be induced in the adhesive layer, concentrated at the ends of the joint.  There are various methods for minimising the negative influence of bending forces caused through eccentric loading.  These include: increasing the bondline thickness; stiffening the adherends (i.e. increase adherend thickness or use of stiffer materials); use of double overlapping, single and double cover plates, and scarf and step configurations (Figure 4); and modifications to the adhesive fillet at the ends of joints.  Appendix 1 describes an analytical procedure for producing satisfactory single-lap joints.

Figure 3:  Relative deformation of a single-lap joint for different substrate materials

(a - lowest and d - highest stiffness (see Table 1))

Table 1:  Elastic Properties of Adherends

Figure 4:  Various lap joint configurations

(joint strength increases from top to bottom)

Next: Safety Factors

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