Browsing by Author "Allegri, Giuliano"
Now showing 1 - 12 of 12
Results Per Page
Sort Options
Item Open Access Bridging mechanisms of through-thickness reinforcement in dynamic mode I&II delamination(Elsevier, 2017-04-13) Cui, Hao; Yasaee, Mehdi; Kalwak, Gordon; Pellegrino, Antonio; Partridge, Ivana K.; Hallett, Stephen R.; Allegri, Giuliano; Petrinic, NikZ-pin through-thickness reinforcement is used to improve the impact resistance of composite structures; however, the effect of loading rate on Z-pin behaviour is not well understood. The dynamic response of Z-pins in mode I and II delamination of quasi-isotropic IM7/8552 laminates was characterized experimentally in this work. Z-pinned samples were loaded at both quasi-static and dynamic rates, up to a separation velocity of 12 m/s. The efficiency of Z-pins in mode I delamination decreased with loading rate, which was mainly due to the change in the pin misalignment, the failure surface morphology and to inertia. The Z-pins failed at small displacements in the mode II loading experiments, resulting in much lower energy dissipation in comparison with the mode I case. The total energy dissipation decreased with increasing loading rate, while enhanced interfacial friction due to failed pins may be largely responsible for the higher energy dissipation in quasi-static experiments.Item Open Access Cohesive element formulation for z-pin delamination bridging in fibre reinforced laminates(Elsevier, 2017-03-26) Mohamed, Galal; Allegri, Giuliano; Yasaee, Mehdi; Hallett, Stephen R.Z-pins are an effective method of reinforcing laminated composite materials for resisting the propagation of delamination. In this paper, a novel numerical method combines the classical cohesive finite element (FE) method with a semi-analytical z-pin crack bridging model. Special purpose cohesive elements, in which the generalized traction-displacement characteristics are provided by the semi-analytical model z-pin bridging map, are implemented in macro-scale FE models. This cohesive element offers the flexibility to employ two cohesive laws concurrently for prediction of delamination propagation, for both the pinned and unpinned behaviour. Its efficacy is evaluated by the simulation of double cantilever beam (DCB), mixed-mode bend (MMB), and pure mode II End-Loaded Split (ELS) fracture tests at 2% z-pin areal density. The numerical results in terms of load-deflection predictions agree well with experiments. The different simulations were all performed using a single set of input parameters derived from single z-pin tests with no fitting factors.Item Open Access Coupon scale Z-pinned IM7/8552 delamination tests under dynamic loading(Elsevier, 2019-08-01) Cui, Hao; Mahadik, Yusuf; Hallett, Stephen R.; Partridge, Ivana K.; Allegri, Giuliano; Ponnusami, Sathiskumar A.; Petrinic, NikDynamic impact onto laminated composite structures can lead to large-scale delamination. This can be mitigated by the introduction of through-thickness reinforcement, such as z-pins. Here, mode I & II and mixed-mode delamination tests have been designed and conducted at high loading rate, for both unpinned and Z-pinned coupons to study the effect of rate of loading. It was found that the Z-pins were not effective in delaying the dynamic crack initiation or resisting the dynamic propagation of delaminations shorter than 5 mm. However, the further growth of cracks was substantially delayed by Z-pinning, especially for the pure mode I and mode I dominated failure modes. On the other hand, the effectiveness of Z-pins in shear tests was relatively modest. The mode I dominated delamination resistance of Z-pinned laminates was found to be sensitive to the loading rate.Item Open Access Dataset for the publication: "Coupon scale Z-pinned IM7/8552 delamination tests under dynamic loading"(Cranfield University, 2019-08-27 15:00) Cui, Hao; R. Hallett, Stephen; Mahadik, Yusuf; K. Patridge, Ivana; Allegri, Giuliano; Anusuya Ponnusami, Sathiskumar; Petrinic, NikDatasets for a paper published in Composites Part A: Applied Science and Manufacturinghttps://doi.org/10.1016/j.compositesa.2019.105565The quasi-static tests were done at velocity of around 0.01mm/sDynamic tests were conducted at norminal velocity of around 4m/s, some of the WDCB tests were done at 7m/s.Item Open Access Design and Modeling of Selective Reinforcements for Integral Aircraft Structures(American Inst of Aeronautics and Astronautics, 2008-09-01T00:00:00Z) Boscolo, M.; Allegri, Giuliano; Zhang, XiangA numerical simulation is presented in this paper on the performance of crack retarders bonded to integral metallic structures. The work is described in two main parts. First, a novel modeling approach employing the finite element method has been developed for simulating the various failure mechanisms of a bonded structure and for predicting fatigue crack growth life. Crack growth in the substrate and the substrate/strap interface disbond failure are modeled in the framework of linear elastic fracture mechanics. A computer code interfacing with the commercial package MSC NASTRAN has been developed and validated by experimental tests. Second, the effectiveness of different strap configurations on crack growth retardation has been modeled; these include different strap materials, strap dimensions, and their locations on the substrate. The research has included two substrate materials and four strap materials, and at this stage the specimens were cured at room temperature. Strap stiffness and adhesive toughness are found to be the most influential parameters in designing crack retarders. A design tool has been developed based on the numerical simulation to achieve optimal crack retarder design in terms of prescribed fatigue life target and minimum structural weight added by the bonded reinforcement.Item Open Access Dynamic bridging mechanisms of through-thickness reinforced composite laminates in mixed mode delamination(Elsevier, 2017-11-23) Cui, Hao; Yasaee, Mehdi; Hallett, Stephen R.; Partridge, Ivana K.; Allegri, Giuliano; Petrinic, NikDelamination resistance of composite laminates can be improved with through-thickness reinforcement such as Z-pinning. This paper characterises the bridging response of individual carbon fibre/BMI Z-pins in mixed mode delamination at high loading rate using a split Hopkinson bar system. The unstable failure process in quasi-static tests, was also captured with high sampling rate instruments to obtain the complete bridging response. The energy dissipation of the Z-pins were analysed, and it was found that the efficacy of Z-pinning in resisting delamination growth decreased with an increase in mixed mode ratio, with a transition from pull-out to pin rupture occurring. The Z-pin efficacy decreased with loading rate for all mode mix ratios, due to the changing in failure surface with loading rate and rate-dependent frictional sliding.Item Open Access Dynamic bridging response of through-thickness reinforcement in composite laminates(International Committee on Composite Materials, 2017-12-31) Cui, Hao; Melro, António R.; Mahadik, Yusuf; Yasaee, Mehdi; Allegri, Giuliano; Partridge, Ivana K.; Hallett, Stephen R.; Petrinic, NikThe present experimental study aims to extend the understanding of delamination crack bridging mechanisms in Z-pinned laminates subjected to highly dynamic loading conditions. The bridging response of single Z-pins was characterized with both quasi-static and high loading rate. Standard delamination tests of Z-pinned laminates were carried out at varying velocity. The experimental results at both length scales showed that Z-pin efficiency in improving delamination resistance decreases with increasing loading rate.Item Open Access Fail-Safe Design of Integral Metallic Aircraft Structures Reinforced by Bonded Crack Retarders(Elsevier Science B.V., Amsterdam., 2009-01-01T00:00:00Z) Zhang, Xiang; Boscolo, M.; Figueroa-Gordon, Douglas J.; Allegri, Giuliano; Irving, Phil E.This paper presents an investigation on the effectiveness of crack growth retarders bonded to integral metallic structures. The study was performed by both numerical modelling and experimental tests. It focuses on aluminium alloy panels reinforced by bonded straps made of carbon-epoxy, glass-epoxy composite materials or a titanium alloy. The goal was to develop a fail-safe design for integrally stiffened skin-stringer panels applicable to aircraft wing structures. The modelling strategy and finite element models are presented and discussed. The requirements that the models should meet are also discussed. The study has focused on establishing the extent of crack retarder benefits, in terms of fatigue crack growth life improvement, by numerical simulation and experimental tests of various crack retarders. The results of predicted fatigue crack growth retardation have been validated by tests of laboratory samples. This study concludes that by bonding discrete straps to an integral structure, the fatigue crack growth life can be significantly improved.Item Open Access Novel frame model for mistuning analysis of bladed disc systems(American Society of Mechanical Engineers (ASME), 2017-03-06) Yuan, Jie; Scarpa, F.; Titurus, B.; Allegri, Giuliano; Patsias, S.; Rajasekaran, R.The work investigates the application of a novel frame model to reduce the computational cost of the mistuning analysis of bladed disc systems. A full-scale finite element (FE) model of the bladed disc is considered as benchmark. The single blade frame configuration is identified via an optimization process. The individual blades are then assembled by 3D springs, whose parameters are determined via calibration process. The dynamics of the novel beam frame assembly is also compared to those obtained from three state-of-the-art FE-based reduced order models (ROMs): a lumped parameter approach; a Timoshenko beam assembly, and component mode synthesis (CMS) based techniques with free and fixed interfaces. The development of these classical ROMs to represent the bladed disc is also addressed in detail. A methodology to perform the mistuning analysis is then proposed and implemented. A comparison of the modal properties and forced response dynamics between the aforementioned ROMs and the full-scale FE model is presented. The case study demonstrates that the beam frame assembly can predict the variations of the blade amplitude factors with results being in agreement with the full-scale FE model. The CMS based ROMs underestimate the maximum amplitude factor, while the results obtained from beam frame assembly are generally conservative. The beam frame assembly is 4 times more computationally efficient than the CMS fixed-interface approach. This study proves that the beam frame assembly can efficiently predict the mistuning behavior of bladed discs when low order modes are of interest.Item Open Access On the delamination self-sensing function of Z-pinned composite laminates(Elsevier, 2016-03-19) Zhang, B.; Allegri, Giuliano; Yasaee, Mehdi; Hallett, Stephen R.; Partridge, Ivana K.This paper investigates for the first time the usage of through-thickness reinforcement for delamination detection in self-sensing composite laminates. Electrically conductive T300/BMI Z-pins are considered in this study. The through-thickness electrical resistance is measured as the delamination self-sensing variable, both for conductive and non-conductive laminates. The Z-pin ends are connected to a resistance measurement circuit via electrodes arranged on the surface of the laminate. The delamination self-sensing function enabled by conductive Z-pins is characterised for Mode I/II delamination bridging, using single Z-pin coupons. Experiment results show that, if the through-thickness reinforced laminate is electrically conductive, the whole Z-pin pull-out process associated with delamination bridging can be monitored. However, for a non-conductive laminate, delamination bridging may not be sensed after the Z-pin is pulled out from one of the surface electrodes. Regardless of the electrical properties of the reinforced laminate, the through-thickness electrical resistance is capable of detecting Mode II bridging, albeit there exists an initial “blind spot” at relatively small lateral deformation. However, the Z-pin rupture can be clearly detected as an abrupt resistance increase. This study paves the way for exploring multi-functional applications of through-thickness reinforcement.Item Open Access On the delamination suppression in structural joints by Z-fibre pinning(Elsevier Science B.V., Amsterdam., 2007-04-01T00:00:00Z) Allegri, Giuliano; Zhang, XiangThe main objective of this paper is to investigate the benefits of Z-fibre pinning to improve the bonding strength of composite joints. The problem is addressed from a design point of view in order to develop a simulation methodology that can be employed to predict the strength of Z-fibre pinned joints. Firstly, an efficient and accurate computational approach is presented using the well established finite element method in conjunction with a constitutive model of Z-fibre response behaviour under mixed mode loading condition. The Z-fibre bridging model previously developed by the authors is summarised in the paper. Secondly, the computational approach is demonstrated via the analysis of two structural joints, namely a conventional T-joint and a novel cruciform joint. Comparison with test data confirms the model’s predictive capabilitItem Open Access On the inverse power laws for accelerated random fatigue testing(Elsevier Science B.V., Amsterdam., 2008-06-01T00:00:00Z) Allegri, Giuliano; Zhang, XiangThis paper addresses the usage of inverse power laws in accelerated fatigue testing under wide-band Gaussian random loading. The aim is not at predicting an absolute value of fatigue life but assessing the fatigue damage relative accumulation. The widely accepted inverse power scaling laws in fatigue damage assessment is discussed, reviewing the engineering standards and pointing out their inherent limitations. A physically consistent general scaling law is obtained by rigorous mathematical analysis in the framework of random vibration theory and the rules of safe-life fatigue analysis. Simplifications of the general scaling rule are presented, highlighting conditions under which the current standard practice could provide a correct an acceptable estimation of the relative fatigue damage accumulation.