Repetitive fatigue Load

Fatigue is defined as the failure or decay of mechanical properties after repeated applications of stress. Fatigue tests give information on the ability of a material to resist the development of cracks which eventually bring about failure as a result of a large number of cycles. For polymeric composites that have been designed primarily for loading in the fibre reinforced direction with minimal out-of plane loading, fatigue is generally not considered a design limiting property. However, the presence of out-of-plane loads are not always immediately apparent and can cause unexpected fatigue failures. For components with high cycle fatigue applications, such as vibration components or rotorcraft components, fatigue must be considered as part of the design and development process. Because of the out-of-plane loading caused by defects such as delaminations, disbonds, fibre breaks, etc. it is important to evaluate their effect in fatigue to identify if they will grow significantly during the life of the component or before they are detected by inspection.

Fatigue data is generally generated under three different R-ratios (or minimum load/maximum load). The three most common R-ratios are: R=0.1 (tension-tension), R=10.0 (compression-compression), and R=-1.0 (tension-compression). With these three R-ratios a Goodman diagram can be constructed to aid in predicting fatigue at any given R-ratio. Most applications experience a range or spectrum of loads with various amplitudes. Spectrum loading introduces another complexity that has proved difficult to account for in life prediction. Cumulative damage approaches used for metal structures are less effective for composite structures. Hence, there is an increased reliance on empirical data from component testing.

The test frequencies are either chosen to be the application frequency or as high a frequency as possible without causing a temperature rise in the specimen. Generally, test frequencies of 5-10 hertz are used. Temperature measurements should be made during the test to ensure specimen heating is insignificant during all fatigue testing. Failure mode evaluation of fatigue test specimens is just as important as for static test specimens.

The data from fatigue testing of composites is plotted on a semi-log fatigue stress vs. cycles plot (S/N curve). The number of data points required per curve varies depending on its desired use. Mil-Hdbk-17 identified 8 data points as sufficient to establish the fatigue performance. However, for design allowables, it recommends 15 data points over three batches. Regression analysis on these data may take different approaches and should allow confidence fits to be made.

The fatigue data may be used to identify a maximum stress value for a specific number of cycles or to establish a load/life enhancement factor when component testing.