Browsing by Author "Bianchi, Francesco"

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    A cohesive zone model for predicting delamination suppression in z-pin reinforced laminates
    (Elsevier Science B.V., Amsterdam., 2011-11-14T00:00:00Z) Bianchi, Francesco; Zhang, Xiang
    This paper presents a cohesive zone model based finite element analysis of delamination resistance of z-pin reinforced double cantilever beam (DCB). The main difference between this and existing cohesive zone models is that each z- pin bridging force is governed by a traction-separation law derived from a meso- mechanical model of the pin pullout process, which is independent of the fracture toughness of unreinforced laminate. Therefore, two different traction- separation laws are used: one representing the toughness of unreinforced laminate and the other the enhanced delamination toughness owing to the pin bridging action. This approach can account for the large scale bridging effect and avoid using concentrated pin forces, thus removing the mesh dependency and permitting more accurate analysis solution. Computations were performed using a simplified unit strip model. Predicted delamination growth and load vs. displacement relation are in excellent agreement with the prediction by a complete model, and both models are in good agreement with test measured load vs. displacement relation. For a pinned DCB specimen, the unit strip model can reduce the computing time by 85%.
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    A finite element model for predicting the static strength of a composite hybrid joint with reinforcement pins
    (MDPI, 2023-04-22) Bianchi, Francesco; Liu, Yiding; Joesbury, Adam M.; Ayre, David; Zhang, Xiang
    This paper presents a finite element model for predicting the performance and failure behaviour of a hybrid joint assembling fibrous composites to a metal part with reinforcement micro pins for enhancing the damage tolerance performance. A unit-strip model using the cohesive elements at the bond interface is employed to simulate the onset and propagation of debonding cracks. Two different traction–separation laws for the interface cohesive elements are employed, representing the fracture toughness properties of the plain adhesive bond and a pin-reinforced interface, respectively. This approach can account for the large-scale crack-bridging effect of the pins. It avoids using concentrated pin forces in the numerical model, thus removing mesh-size dependency, and permitting more accurate and robust computational analysis. Lap joints reinforced with various pin arrays were tested under quasi-static load. Predicted load versus applied displacement relations are in good agreement with the test results, especially for the debonding onset and early stage of crack propagation.
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    Numerical modelling of through-thickness reinforced structural joints
    (Cranfield University, 2012-06) Bianchi, Francesco; Zhang, Xiang
    The main objective of this research study was to develop numerical models to analyse the mechanical and fracture properties of through-thickness reinforced (TTR) structural joints. The development of numerical tools was mainly based on the finite element (FE) method. A multi-scale approach was used: the bridging characteristics of a single reinforcement was studied at micromechanical level by simulating the single-pin response loaded either in mode-I or in mode-II. The force-displacement curve (bridging law) of the pin was used to define the constitutive law of cohesive elements to be used in a FE analysis of the global structure. This thesis is divided into three main parts: (I) Background, context and methodology, (II) Development for composite joints, and (III) Development for hybrid metal-composite joints. In the first part the objectives of the thesis are identified and a comprehensive literature review of state-of-art throughthickness reinforcement methods and relative modelling techniques has been undertaken to provide a solid background to the reader. The second part of the thesis deals with TTR composite/composite joints. The multi-scale modelling technique was firstly applied to predict delamination behaviour of mode-I and in mode-II test coupons. The bridging mechanisms of reinforcements and the way these increase the delamination resistance of bonded interfaces was deeply analysed, showing how the bridging characteristics of the reinforcement features affected the delamination behaviour. The modelling technique was then applied to a z-pin reinforced composite T-joint structure. The joint presented a complicated failure mode which involved multiple crack path and mixed-mode delamination, demonstrating the capability of the model of predicting delamination propagation under complex loading states. The third part of the thesis is focused on hybrid metal/composite joints. Mode- I and mode-II single-pin tests of metal pin reinforcements embedded into a carbon/epoxy laminate were simulated. The model was validated by comparing with experimental tests. Then the effects of the pin geometry on the pin bridging characteristics were analysed. The model revealed that both in mode-I and mode-II small pins perform better than large pins and also that the pin shape plays an important role in the pin failure behaviour. The modelling technique was then applied to simulate a metal-composite double-lap joint loaded in traction. The model showed that to obtain the best performance of the joint an accurate selection of pin geometry, pin arrangement and thickness of the two adherends should be done.
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    Predicting mode II delamination suppression in z-pinned laminates
    (Elsevier Science B.V., Amsterdam., 2012-05-02T00:00:00Z) Bianchi, Francesco; Zhang, Xiang
    A finite element model for predicting delamination resistance of z-pin reinforced laminates under the mode-II load condition is presented. End notched flexure specimen is simulated using a cohesive zone model. The main difference of this approach to previously published cohesive zone models is that the individual bridging force exerted by z-pin is governed by a specific traction- separation law derived from a unit-cell model of single pin failure process, which is independent of the fracture toughness of the unreinforced laminate. Therefore, two separate traction-separation laws are employed; one represents unreinforced laminate properties and the other for the enhanced delamination toughness owing to the pin bridging action. This approach can account for the so-called large scale bridging effect and avoid using concentrated pin forces in numerical models, thus removing the mesh-size dependency and permitting more accurate and reliable computational solutions.