If a joint is to be subjected to an alternating stress then the fatigue properties of the adhesive system may be important in determining the life of the structure. Fatigue loading within the adhesive can be caused by either i) direct mechanical loading of the joint or ii) thermally induced loading - caused by the restriction of movement of the adherends during bondline contraction (during a cooling cycle).
Fatigue failure is caused by the initiation and propagation of cracks as a result of the loading cycles, to the point where the remaining bonded area can no longer withstand the applied load.
To asses the performance of different adhesive systems to fatigue loading, or perhaps compare them to the fatigue performance of joints formed by other joining methods, testing can be performed using simple joint geometries, such as the lap-shear or T-peel samples. Testing is performed at different stress levels from the single cycle ‘static’ strength down to the endurance limit, the stress corresponding to no failures at 107 cycles. This will generate an S-N curve, similar to the one shown in Figure 1 below - cycles to failure are typically plotted on a logarithmic scale..
The fatigue resistance of the adhesive joint is reduced at elevated temperature.
The joint should be designed so that fatigue loads are below the endurance limit with an appropriate factor of safety for the application. It is important that the joint geometry used to determine the endurance limit conservatively represents the joint design of interest, in particular that peel loads are adequately accounted for. The limitation of the S-N approach is that it is difficult to apply data from one joint geometry to another for the same adhesive system.
A more fundamental approach is to relate fatigue life to a local criterion such as the maximum stress in the adhesive. This approach is not used because a suitable local failure criteria have not be found.
An alternative approach is to use fracture mechanics, where the criterion is the strain energy release rate (SERR) of the fatigue crack which can be related to the rate of crack growth in a model for the adhesive system. More information is given on this in design criteria.
Standard tests that determine fatigue resistance
ASTM D3166-99 Standard Test Method for Fatigue Properties of Adhesives in Shear by Tension Loading (Metal/Metal)
CEN TC 193/WG2 Test Methods for Assessing Long Term Durability of Bonded Metallic Structures (Draft Standard)
ENISO 9664:1995 Adhesives. Test methods for fatigue properties of structural adhesives in tensile shear,
