The introduction, originally in 2009, by the FAA of a ‘slow growth’ approach to the certification of polymer-matrix fibre composites has focused attention on the experimental data and the analytical tools needed to assess the growth of delaminations under cyclic-fatigue loads. Of direct relevance is the fact that fatigue tests on aircraft composite components and structures reveal that no, or only little, retardation of the fatigue crack growth (FCG) rate occurs as delamination/impact damage grows. Therefore, of course, the FCG data that are ascertained in laboratory tests, and then employed as a material-allowable property to design and life the structure, as well as for the development, characterisation and comparison of composite materials, must also exhibit no, or only minimal, retardation. Now, in laboratory tests the double-cantilever beam (DCB) test, using a typical carbon-fibre reinforced-plastic (CFRP) aerospace composite, is usually employed to obtain fracture-mechanics data under cyclic-fatigue Mode I loading. However, it is extremely difficult to perform such DCB fatigue tests without extensive fibre-bridging developing across the crack faces. This fibre-bridging leads to significant retardation of the FCG rate. Such fibre-bridging, and hence retardation of the FCG, is seen to arise even for the smallest values of the pre-crack extension length, ap − a0, that are typically employed. The results from the DCB tests also invariably exhibit a relatively large degree of inherent scatter. Thus, a methodology is proposed for predicting an ‘upper-bound’ FCG curve from the laboratory test data which is representative of a composite laminate exhibiting no, or only very little, retardation of the FCG rate under fatigue loading and which takes into account the inherent scatter. To achieve this we have employed a novel methodology, based on using a variant of the Hartman-Schijve equation, to access this ‘upper-bound’ FCG rate curve, which may be thought of as a material-allowable property and which is obtained using an ‘A basis’ statistical approach. Therefore, a conservative ‘upper-bound’ FCG curve may now be calculated from the DCB laboratory test data for material development, characterisation and comparative studies, and for design and lifing studies.@en

Temperature can significantly affect fatigue delamination growth (FDG) behavior in composites, while fiber bridging has been frequently reported during FDG. The focus of this study was therefore on investigating temperature effects on FDG behavior with fiber bridging. Mode I fatigue delamination experiments were conducted on a thermoset composite laminates M30SC/DT120 at different temperatures. The Paris relation and fatigue resistance curve (i.e. fatigue R-curve) were used to interpret bridging effects on FDG behavior and to explore temperature effects on fiber bridging development. A modified Paris relation was employed to determine the effects of temperature on the intrinsic FDG behavior at the crack front excluding fiber bridging. The Paris interpretations clearly demonstrate that fiber bridging can significantly retard FDG behavior at different temperatures. Temperature can have different effects on fiber bridging development and the intrinsic FDG behavior. Particularly, elevated temperature can promote more bridging fibers, whereas decreased temperature has negligible influence on fiber bridging. When looking at the intrinsic delamination resistance, mode I FDG can accelerate at elevated temperature but decrease at freezing temperature. Fractographic examinations indicate that fiber/matrix interface debonding is the dominant damage mechanism in mode I FDG at different temperatures. Elevated temperature can lead to the weakening of interface adhesion, contributing to faster intrinsic mode I FDG behavior and more fiber bridging development. And a semi-empirical fatigue model based on normalization was finally proposed to determine mode I intrinsic FDG behavior at different temperatures for engineering applications.@en

This paper aims to investigate stress ratio effect on fibre bridging significance in mode I fatigue delamination growth of composite materials. Fatigue resistance curves (R-curves) of different stress ratios are determined and compared with the quasi-static R-curve. The fatigue R-curve of a high stress ratio is similar to the quasi-static results. However, fatigue resistance of a low stress ratio is smaller than quasi-static resistance. These indicate that fibre bridging significance is stress ratio dependent. More bridging fibres can be generated in delamination of a high stress ratio, as compared to that of a low stress ratio. This can lead to fatigue bridging laws are stress ratio dependent and fatigue delamination is block load sequence dependent.@en

This paper provides an investigation on thickness effects on fibre-bridged fatigue delamination growth (FDG) in composite laminates. A modified Paris relation was employed to interpret experimental fatigue data. The results clearly demonstrated that both thickness and fibre bridging had negligible effects on FDG behaviors. Both energy principles and fractography analysis were subsequently performed to explore the physical reasons of this independence. It was found that the amount of energy release of a given crack growth was not only independent of fibre bridging, but also thickness. Fibre print was the dominant microscopic feature located on fracture surfaces, physically making the same energy dissipation during FDG. Furthermore, the present study provides extra evidence on the importance of using an appropriate similitude parameter in FDG studies. Particularly, the strain energy release rate (SERR) range applied around crack front was demonstrated as an appropriate similitude parameter for fibre-bridged FDG study.@en

The production of advanced composites from recycled carbon fibres (rCFs) is critical for the sustainable development of carbon fibre industry. Herein, non-woven mats consisting of commingled rCFs and Polyphenylene-sulfide (PPS) fibres were compression moulded to manufacture rCF/PPS composites, with the fibre/matrix adhesion being tailored by UV-irradiating the non-woven mats. The intralaminar and interlaminar fracture resistance and mechanical performance of the rCF/PPS composites were characterised. The experimental results had demonstrated that improving the PPS/rCF adhesion of the composites significantly increased the intralaminar fracture energies and mechanical properties under tensile and shear loading conditions. However, it also negatively affected the interlaminar fracture resistance. The main fracture mechanism was observed to be fibre evulsion for the intralaminar fracture mode, while crack bridging by the rCFs was the primary fracture mechanism for the interlaminar fracture condition. That led to the contrary influences of the improved fibre/matrix adhesion on the intralaminar and interlaminar fracture resistance of the rCF/PPS composites. In summary, this study had shedded lights on tailoring the crack resistance and mechanical performance of rCFRPs by adjusting the fibre/matrix adhesion using the UV-treatment technique.@en

Fatigue delamination with fibre bridging in composite laminates with different thicknesses was investigated. The experimental results clearly demonstrated fibre bridging had significant retardation effects on fatigue delamination behavior, making it insufficient to use a single Paris resistance curve to determine fatigue crack growth. To address this problem, the coefficients of the Paris relation were correlated to the normalized crack extension (a − a0)/Lpz. It was found that the exponent n was independent on the normalized crack extension and specimen thickness, whereas the parameter log(c) bi-linearly decreased with the normalized crack extension and kept constant once fibre bridging became saturation. And the magnitude of log(c) was independent on specimen thickness. Thus, it was concluded that fatigue delamination behavior and fibre bridging significance were independent on specimen thickness at a given normalized crack extension (a − a0)/Lpz. With substitutions of these correlations into the Paris relation, an empirical power law relation was developed to characterize fatigue delamination behavior. And its validation was verified by a comparison between predictions and experiments.@en

Fatigue delamination with fibre bridging in composite laminates with different thicknesses was investigated. The experimental results clearly demonstrated fibre bridging had significant retardation effects on fatigue delamination behavior, making it insufficient to use a single Paris resistance curve to determine fatigue crack growth. To address this problem, the coefficients of the Paris relation were correlated to the normalized crack extension (a − a0)/Lpz. It was found that the exponent n was independent on the normalized crack extension and specimen thickness, whereas the parameter log(c) bi-linearly decreased with the normalized crack extension and kept constant once fibre bridging became saturation. And the magnitude of log(c) was independent on specimen thickness. Thus, it was concluded that fatigue delamination behavior and fibre bridging significance were independent on specimen thickness at a given normalized crack extension (a − a0)/Lpz. With substitutions of these correlations into the Paris relation, an empirical power law relation was developed to characterize fatigue delamination behavior. And its validation was verified by a comparison between predictions and experiments.@en

The aim of present research is to determine fatigue delamination with fibre bridging in composite laminates. Both the Paris relation and the Hartman-Schijve equation were employed to explore fatigue delamination behavior. The use of the Paris relation can result in fatigue delamination growth being crack scale dependent. This dependence was significantly reduced in case of using the Hartman-Schijve relation in data reduction. This difference can lead to controversies on fatigue delamination behavior in composite laminates. To address this dispute, a new parameter, which was consistent with the hypothesis of similitude as well as damage mechanisms, was introduced to represent the similitude in fatigue delamination growth. A modified Paris relation based on this parameter was proposed and used to determine fatigue delamination. And a master resistance curve was obtained to determine fatigue delamination growth with different amounts of fibre bridging. Thus, fatigue delamination is crack scale independent, if the similitude is well characterized. And in the Paris region, the modified Paris relation can provide good predictions in fatigue delamination growth with fibre bridging.@en

The aim of present research is to determine fatigue delamination with fibre bridging in composite laminates. Both the Paris relation and the Hartman-Schijve equation were employed to explore fatigue delamination behavior. The use of the Paris relation can result in fatigue delamination growth being crack scale dependent. This dependence was significantly reduced in case of using the Hartman-Schijve relation in data reduction. This difference can lead to controversies on fatigue delamination behavior in composite laminates. To address this dispute, a new parameter, which was consistent with the hypothesis of similitude as well as damage mechanisms, was introduced to represent the similitude in fatigue delamination growth. A modified Paris relation based on this parameter was proposed and used to determine fatigue delamination. And a master resistance curve was obtained to determine fatigue delamination growth with different amounts of fibre bridging. Thus, fatigue delamination is crack scale independent, if the similitude is well characterized. And in the Paris region, the modified Paris relation can provide good predictions in fatigue delamination growth with fibre bridging.@en

Fibre bridging can significantly enhance delamination resistance making the use of a single Paris resistance curve to determine fatigue crack growth insufficient. An empirical Paris-type relation has been developed in a previous study to take fibre bridging into account in fatigue delamination growth. This relation was developed by correlating the Paris constants C and n to the amount of fibre bridging. This paper provides a further investigation on the interface configuration effect on fatigue delamination growth, illustrating the significance of fibre bridging. The results demonstrated that more bridging fibres can be generated in a multidirectional interface, making both log(C) and n significantly depend on fibre bridging. Thus, the method proposed in the previous study is further extended to take into account of the interface configuration effect.@en

Delamination is one of the most important damage in composite materials. It can propagate under fatigue loading and cause failures of composite structures. With the application of damage tolerance design philosophy in engineering and requirement of light weight structures in advanced aircrafts, how to characterize fatigue delamination growth behavior in composite materials and develop reliable prediction models become critical issues for the application of these materials. A large amount of studies have been conducted on the characterization of fatigue delamination growth under constant amplitude loading in composite materials. However, little attention has ever been put into the block loading sequence effect on delamination growth. Referring to fatigue crack growth in metals, shielding mechanism of plasticity deformation around the crack front plays a dominant role in the loading sequence effect. Fibre bridging, acting as the shielding mechanism, therefore can contribute to the loading sequence effect on fatigue delamination growth in composite materials. Double Cantilever Beam (DCB) specimens were designed and manufactured for the mode I fatigue delamination tests with HI-HI and LO-HI block loading sequences. Paris relation was subsequently used to interpret the experimental results. It demonstrates there is an obvious loading sequence effect on fatigue delamination growth in composite materials. The fatigue crack growth in the HI-HI block loading is slower than it in the LO-HI block loading.@en

Multidirectional DCB specimens with different thicknesses were manufactured and tested to have in-depth understanding of delamination behavior in composite laminates. The initiation crack growth is demonstrated to be ply orientation and thickness independent. However, interlaminar resistance and damage mechanisms in delamination growth are significantly related to the interface configuration as well as thickness. Fractography analysis demonstrated that resistance difference between unidirectional and multidirectional DCB specimens is related to damage mechanisms. Optical microscope observation revealed that crack path in the multidirectional specimens is zigzag. This phenomenon becomes vague with thickness increase. The appearance of zigzag crack indicates both interlaminar and intralaminar damage can occur in the delamination growth. However, interlaminar damage becomes the dominant failure mode in the thick specimens. SEM (Scanning Electron Microscope) observation demonstrated fibre prints and cusps are two typical morphologies located on the fracture surface. These features are even more significant in the multidirectional interface with thickness decrease. This paper will conclude that interface configuration and specimen thickness have significant effects on the damage mechanisms and interlaminar resistance during delamination growth in composite laminates. It is, therefore, insufficient to apply the unidirectional DCB specimen with a given thickness to determine delamination growth and damage mechanisms of a composite material.@en

Multidirectional DCB specimens with different thicknesses were manufactured and tested to have in-depth understanding of delamination behavior in composite laminates. The initiation crack growth is demonstrated to be ply orientation and thickness independent. However, interlaminar resistance and damage mechanisms in delamination growth are significantly related to the interface configuration as well as thickness. Fractography analysis demonstrated that resistance difference between unidirectional and multidirectional DCB specimens is related to damage mechanisms. Optical microscope observation revealed that crack path in the multidirectional specimens is zigzag. This phenomenon becomes vague with thickness increase. The appearance of zigzag crack indicates both interlaminar and intralaminar damage can occur in the delamination growth. However, interlaminar damage becomes the dominant failure mode in the thick specimens. SEM (Scanning Electron Microscope) observation demonstrated fibre prints and cusps are two typical morphologies located on the fracture surface. These features are even more significant in the multidirectional interface with thickness decrease. This paper will conclude that interface configuration and specimen thickness have significant effects on the damage mechanisms and interlaminar resistance during delamination growth in composite laminates. It is, therefore, insufficient to apply the unidirectional DCB specimen with a given thickness to determine delamination growth and damage mechanisms of a composite material.@en