Fatigue limit of textile composites: a link to quasi-static damage

“Fatigue limit” represents a load level below which the fatigue damage will never happen, or will happen with a very low probability. The definition does not have a practical value for composite materials, for which the increase of temperature during accelerated fatigue tests is not possible for the polymer matrix and, therefore, the test frequency must be limited. This makes the testing for determination of the fatigue limit very challenging and time consuming. Therefore, the “fatigue limit” is most often redefined as the load level below which the fatigue failure at a given (large) number of cycles does not happen. The number of cycles is selected balancing the requirements of the application and the practical limitations of the test program. The most often used value is 10^6 cycles (“operational definition”), “1M” fatigue limit. The fatigue limit of cross-ply and multidirectional laminates was discussed in [1], where it is stated: “Where is the fatigue limit? … To determine the fatigue limit, if not available from the test data, a good estimate will be given by the strain at which transverse cracking initiates. The reasoning is simply that if no cracks initiate in the transverse plies, no progression of crack multiplication and subsequent damage is possible”. The practical meaning of the statement is extremely valuable. It allows finding the fatigue limit without exhausting and costly test programs, or at least design such a program in an economical way, without wasting the resources in narrowing the search range. The authors adopted this idea for textile reinforced composites through experimental investigations. The fatigue limit of textile composites was restricted to tension-tension cyclic loading with constant amplitude and ratio R of 0.1 (ratio of the minimum to the maximum stress in the cycle). Moreover, the loading conditions were also limited to the yarns direction of the textile composites. The behaviour of textile composites proved to be more complex than the above simple hypothesis. The latter was observed to be reasonable for glass textile reinforced composites. For carbon reinforced materials the 1M fatigue limit is normally higher than the first damage threshold (onset of the transverse cracking) and is rather linked to the second threshold (corresponding to strong transverse cracks and local delamination).