Critical discussions on mode I fatigue delamination with large-scale fibre bridging in composite laminates

Advanced carbon fibre reinforced polymer composites have been widely used in aerospace industry，be- cause of their excellent mechanical properties and great weight-saving potential. Fatigue delamination has been dem- onstrated the main reason for the failure of composite structures in their long-term operations. Fiber bridging，as an im- portant and unique shielding mechanism，has significant effects on fatigue delamination behavior. And it has been demonstrated that composite laminates have even lower mode I fatigue delamination resistance. As a result，it is re- ally important to have thorough investigations on mode I fatigue delamination with large-scale fibre bridging in compos- ite laminates，to guarantee the integrity of composite structures. This paper therefore provides critical reviews and dis- cussions on the research work carried out by the author’s research group in this field in the past several years. And the main contents of this paper can be organized as following aspects：the effects of fibre bridging on fatigue delamina- tion behavior，and bridging mechanism in fatigue delamination；the similarity for fatigue delamination with large-scale fibre bridging，and Paris type correlations for fibre-bridged mode I fatigue delamination characterization；the application and verification of the proposed Paris type correlations in fibre-bridged fatigue delamination interpretations. The results clearly demonstrated that the presence of fiber bridging has significant retardation effects on mode I fatigue delamina- tion behavior. Particularly，fatigue delamination growth can decrease significantly with the development of fibre bridg- ing，using the basic Paris law in fatigue data reduction. The mechanism of fiber bridging in fatigue delamination is dif- ferent from that in static delamination in the perspective of energy dissipation. Particularly，fibre bridging has negligible contribution to permanent energy dissipation in fatigue delamination，unless there is failure in them. Bridging fibres in most cases only periodically store and release strain energy under cyclic loading，making the most energy dissipation still concentrate around the crack front（regardless of fibre bridging）in fatigue delamination. And the fractographic ex- aminations on fatigue fracture surfaces indicate that the presence of fibre bridging indeed has little influence on the damage mechanism in mode I fatigue delamination. According to the energy dissipation and fractographic examina- tions，it is therefore reasonable to use the strain energy release rate that applied around the crack front as the simili- tude parameter to appropriately represent fatigue delamination behavior with significant fibre bridging at a given stress ratio，which can well obey the similitude principles. A two-parameter Paris type correlation was proposed to account for stress ratio. And the results clearly indicated that the use of this two-parameter model can appropriately determine fibre-bridged fatigue delamination behavior at different stress ratios and temperatures. © 2024 Chinese Society of Astronautics. All rights reserved.