Fatigue Properties of Adhesive Joints

Under normal service conditions, stresses due to cyclic loading are small and deformations can therefore be considered to be elastic. However, cyclic loading may cause fatigue of the adhesive. A conventional fatigue testing technique involves determination of so-called S-N curves, where S is the stress amplitude and N is the number of cycles to failure. At this point, it would be useful to define some important parameters that are used in the phenomenological description of the fatigue phenomenon. Based on Fig. 33.5, the stress amplitude can be defined as in Eq. (3) we use t since samples were loaded in shear. 

S-N curves were measured at 23 °C, 5 Hz and a stress ratio of R=0.2 for single lap joints the results are given in Table 33.5. The S-N curves appear as a more or less straight line in a double-logarithmic plot (Fig. 33.6) and can therefore be represented by the power law of Eq. (6). 

The slope of the S-N curves is set as -1/k, such that a high value of k indicates a long fatigue life. N is the number of cycles to failure, o is the stress amplitude at N = 1 and can be interpreted as the static strength only if In fact, the measured static strength values fall on the S-N curve at R=-l in most cases. 

Combining Eqs. (3-5), it follows that the ratio between amplitude and mean stress is given by Eq. (7). 

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