(1)Safety factors used in the design of adhesive joints must take into account the uncertainties associated with [1]:
These include [1]:
The safety factors presented in Table 2 are for fibre-reinforced laminates, but may be used as a guide for adhesives in the absence of other guidance.
|
Safety Factor ( γm) |
Value |
|
Adhesive Properties, γm1 |
1.5 |
|
Adhesive thickness, γm2 |
1.5 |
|
Long-term loading, γm3 |
1.5 |
|
Environmental conditions, γm4 |
2.0 |
|
Fatigue (non-fail safe joints), γm5 |
|
|
Periodic inspection, good access |
2.0 |
|
Periodic inspection, poor access |
2.5 |
|
No inspection/maintenance |
3.0 |
Table 2: Recommended Values for Partial Safety Factors [1]
In designing a joint, the partial safety factor γm by which the adhesive properties should be divided to give design values is shown below (refer to [2]):
(1)
Note: For long-term testing, the overall partial safety factor γm should be no less than 4.0
For example, if the joint fails at an ultimate load of 100 kN, then the allowable load limit should be 25 kN (or less) for a safety factor of 4.0. As joints will deform, limits may need to be placed on deflections to ensure that at the limit load the structure does not become non-functional. It is recommended that proof tests should be conducted on the structure in order to determine the critical design load.
In design of adhesively bonded joints, consideration should be given to the adherends (geometry and material properties) and adhesive, actual and potential failure modes, thermal properties, magnitude and nature of loading involved, and environmental conditions. Stress analyses of adhesive joints require a database of basic engineering properties of the adhesive, adherend and joint geometry. Basic property requirements for the design of bonded structures are listed below, although not all of these properties would necessarily be required for any given joint configuration.
Cyclic fatigue, creep or high-rate (impact) data may be required, depending on the loading conditions. Fatigue or creep modelling of joint behaviour would require S-N data or time-dependent properties (i.e. creep moduli) in addition to the static material properties of the adhesive-adherend system.
Appendix 2 provides a list of test methods and associated standards for generating design data for bonded and bolted metal and composite structures.
Next: Cyclic Fatigue
