Multidirectional (MD) composite laminates are extensively employed in structural applications owing to their superior mechanical characteristics. Nevertheless, the evaluation of the fracture toughness of composite laminates primarily relies on tests using unidirectional (UD) specimens. This study evaluates the reliability of characterizing mode II delamination behaviour in MD laminates by using UD specimens. The quantification of delamination area through Digital Image Correlation (DIC) analysis is integrated with a physical Energy Release Rate (ERR) method to ascertain the fracture resistance, which is compared with the ERR derived via a modified J-integral method and the standardized compliance methods. Fractographic analysis reveals similar fracture mechanisms in specimens with identical interfaces. The physical ERR increases notably due to large-scale fibre bridging induced by fibre nesting at 0^∘//0^∘ interfaces. Conversely, in 0^∘//90^∘ interfaces, large-area matrix cracking enhances the intrinsic fracture resistance, excluding the extrinsic toughening provided by fibre bridging.

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Dynamic vibration is believed to be a basic property of the impacted composite laminates; however, its effect on delamination formation requires further investigation. This study proposes a numerical model in collaborating with ABAQUS, which was calibrated using experimental results, to investigate the effect of plate vibration on delamination formation in composite laminates subjected to two consecutive identical ice or steel projectile impacts with a fixed loading distance. The only variable parameter for the different simulations was the time interval between the two impacts. The loading condition considered in this study is an extreme case where the composite laminate was still vibrating after the first impact when the second impact occurred. The results showed that the delaminations that formed later were significantly affected by the time intervals of the two identical successive ice or steel projectiles. As the vibrated impact points travel from the minimum peak to the adjacent maximum peak during the first vibration period, the newly formed delamination areas monotonically increase with time and vice versa. The change in the maximum contact forces of two identical impacts induced by dynamic vibration is suggested to be a major reason for the discrepancy between the newly formed delamination and previous ones.

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For the past few decades, research into fatigue delamination behaviour of Carbon Fibre Reinforced Polymer (CFRP) composites has predominantly relied on standard test methods. These methods typically utilize a uniaxial delamination length to quantify the fatigue delamination process. However, this approach is inadequate to describe the nature of delamination growth in a planar manner. In this work, an experimental study was conducted to gain insights into the planar delamination behaviour of CFRP composites under mode II fatigue loading. Delamination growth of two specimen configurations, each containing an embedded initial defect, was monitored through ultrasound scanning (C-scan). During load-control fatigue testing, the growth rate exhibited an initial rise, followed by a subsequent decrease as loading cycles increased. Despite the development of a larger delamination area under fatigue loading, a consistent overall stiffness was observed. Fractography revealed the presence of various fracture mechanisms ocurring at different locations near the initial delamination front. Micro matrix cracking and fibre-matrix debonding emerged as dominant mechanisms in 2D fatigue delamination growth following the fibre direction. Matrix cracking within the laminate ply occurred at the locations where the growth direction deviated from the fibre direction, consequently triggering delamination migration. @en

In this study, a novel experimental approach was devised to investigate shear dominant and combined opening-shear planar delamination behaviours in composite laminates subjected to quasi-static out-of-plane loading. The patterns of planar delamination growth were depicted through different inspection techniques, including digital image correlation (DIC), C-scan, and microscopic observation. The artificially embedded delamination propagated in the direction parallel to the fibre orientation of the layer above the mid interface, but migrated to an upper interface in the direction transverse to the directing ply. A continuous stiffening process was recognized with increasing delamination area. Furthermore, a numerical analysis based on virtual crack closure technique (VCCT) indicated that the local mode II was dominant for delamination growth, while the local mode III triggered delamination migration.

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Delamination growth is a key damage mode threatening the structural integrity of fibre reinforced polymer composite structures. To guide design and damage management of composite structures, research efforts have been made to understand delamination behaviours and establish standardized evaluation methods based mainly on one-dimensional delamination tests. However, as most delamination growth in real structures will be planar, the question arises whether these approaches are adequate to evaluate planar delamination behaviour. In this study, a novel experimental method was developed to investigate the planar delamination behaviour under quasi-static out-of-plane loading. The planar central loaded split (PCLS) specimen was designed to investigate the planar delamination behaviour under mode II loading condition. By analysing digital image correlation (DIC) and C-scan data, the delamination progress was monitored. An acoustic emission (AE) system was used to capture the initiation of damage and to identify different damage types. The planar delamination growth was found to be dependent on the stacking sequence and interface properties. Additionally, it was found that positioning a rubber mat between the indenter and the specimen prevented the occurrence of delaminations at undesired interfaces. The artificially embedded delamination propagated in the direction to which the fibre orientation of the layer above the crack interface was parallel, but migrated initially to an upper interface at the place where the fibre was perpendicular. A constant increase in the load was observed even though the delamination propagated. The significant drop of loading seen at the end of the test was attributed to the substantial surface cracking. The research results provide a clearer understanding of the mechanisms of planar delamination under out-of-plane loading. Furthermore, combining with the experimental results, numerical simulation will be conducted to characterize planar delamination behaviour qualitatively and quantitatively, thus to establish a more reliable assessment method for planar delamination growth@en

Fibre reinforced polymer composites have found increasing use in aircraft structures. This means that fleet managers need damage assessment tools for such materials, in order to decide on appropriate sustainment strategies. Developing such tools is hindered by the difficulty of generalising from lab tests to predict the behaviour of full-scale structures. A more focussed research effort is needed to address the knowledge gaps that prevent such generalisations. This paper highlights the limitations of current non-destructive inspection techniques, and how this can lead to misinterpretation of experimental results. Furthermore, it discusses differences between coupons and full-scale structures caused by 1D vs 2D delamination growth and the effects of lay-up. New test methods to deal with these issues are discussed and some preliminary results are presented. Finally, the paper discusses the prediction of residual strength and the difficulties of determining a suitable end-point for damage propagation analyses. While the residual strength in the presence of a given damage can be accurately modelled for small components, if sufficiently computational resources are available, further development is needed to develop tools that can be implemented for sustainment of operational aircraft.@en