Cyclic fatigue fracture of composites: What has testing revealed about the physics of the processes so far?

Abstract

Where for metallic materials fatigue fracture testing has contributed significantly to understanding macroscopic and microscopic fatigue failure, the understanding of fatigue fracture in fibre reinforced polymers (FRP) still seems limited. It appears that the research on fatigue in FRPs raises more questions, rather than providing a framework for understanding the underlying mechanisms. Open questions include for example the correct formulation of the driving force and fatigue damage resistance, how to account for mixed mode loading and fibre bridging? But also the question to what extent averaging and homogenising over time-and length scales hinders the understanding in relation to the microscopic mechanisms?To contribute to the development of understanding, this paper discusses several of these open questions. The selected questions were discussed at a recent workshop on 'Physics of fatigue damage growth' at TU Delft with participants from several laboratories covering expertise on fatigue fracture testing of both metals and polymer composites. The discussion focuses on potential experimental and simulation approaches that may lead to a better understanding of the physics of the fatigue fracture process.

Scrutinizing interlaminar fatigue loading cycle in composites using acoustic emission technique: Stress ratio influence on damage formation

“…In addition, these models also assume that delamination growth is continuous within a single loading cycle. These approximations lead to a misinterpretation of damage development during fatigue, because damage development discontinuity is hidden, as also observed in the work of Alderliesten et al [9]. Table 2 shows the average values of the number of hits detected over the first 500 cycles of each fatigue test and the number of cycles with no hit detection.…”

“…This behavior is expected because the maximum energy of the subsequent cycles will not increase due to the displacement-controlled conditions. On the other hand, according to the results presented in Table 2 and the work of Alderliesten et al [9], the AE results indicated that the delamination could propagate in fatigue even after a sequence of cycles without the detection of any damage mechanism.…”

“…In order to guarantee the reliability of composite structures, it is essential to comprehend the physics underlying their fatigue behavior, to enable the development of FDG models capable of estimating reliable component lifetimes and maintenance periods. A large number of FDG models has been developed so far, but the understanding of the fracture process' underlying physics is still lacking [8,9]. The knowledge of the micro-mechanisms involved in the delamination propagation increases the understanding of the phenomenon and enables physical explanations.…”

“…The creation of new micromechanical models relies on knowledge of how the damage is created over a single loading cycle and the correct quantification of this damage [9,10]. There is a consensus on using the macroscopic crack length to quantify the damage developed during the fatigue process, which is not capable of representing the physical damage created within a single loading cycle.…”

Fatigue delamination of composite materials – approach to exclude large scale fibre bridging

“…In that respect, one has to acknowledge the contribution to scatter through fibre bridging in the definition of the pre-crack length [20]. Defining the initial pre-crack length with a tolerance of ±1 mm, could impose significant scatter in the obtained crack resistance curve.…”