Browsing by Author "Zhang, X."

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    Carbon-coated fluorinated graphite for high energy and high power densities primary lithium batteries
    (Elsevier Science B.V., Amsterdam., 2010-05-01T00:00:00Z) Zhang, Qi; D'Astorg, S.; Xiao, P.; Zhang, X.; Lu, L.
    The electrochemical performances of fluorinated graphite have been improved by coating a uniform carbon layer on commercial CFx (x = 1) powder used as cathode material in lithium battery. In comparison with the cell using un-coated CFx as cathode, the cell using carbon coated CFx cathode has a higher energy density and higher power density, particularly at higher discharge current rates (1C above). This is because the conductive carbon coating provides the exterior connectivity between particles for facile electron conduciton, resulting in high rate performance.
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    Cutout shape and reinforcement design for composite C-section beams under shear load
    (Elsevier Science B.V., Amsterdam., 2009-04-30T00:00:00Z) Guo, Shijun J.; Morishima, R.; Zhang, X.; Mills, A.
    This paper reports a study of the performance of two forms of cutout and various edge reinforcements in a composite C-section beam under static shear load. Firstly, cutout shape effect on stress concentration was studied. This was followed by a comparative study of a range of reinforcement doublers, which were 20 mm wide rings made of a steel alloy or composite laminate, or by a novel fibre tow placement technique. The comparisons are made in terms of the stress and strain reductions at a hot spot at the cutout edge. Good agreement between the numerical and test results has been achieved for different cutout shapes and reinforcements, and this study has demonstrated that the cutout induced stress concentration can be reduced significantly by appropriate cutout shape and edge reinforcements. The stress reduction magnitude is found to be strongly related to the stiffness of the reinforcement rings. The diamond-shaped cutout and the fibre tows reinforcement show clear advantages over the widely adapted circular cutout and laminated ring reinforcement. These findings should contribute to future design improvement of composite aircraft structures in similar shape and loading conditions.
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    Fatigue life enhancement of aircraft structures through bonded crack retarders (BCR)
    (Cranfield University, 2015-09) Doucet, Jeremy; Irving, Phil E.; Zhang, X.
    The trend in aircraft design is to produce greener airplanes through lighter structures and/or structures with extended life and reduced maintenance. Bonded crack retarders (BCR) are one of the solutions towards that objective. BCR are reinforcing straps bonded to the structure in order to improve the fatigue and damage tolerance properties of the assembly. The aim of this study was to demonstrate that the BCR hybrid technology – beneficial for upper wing cover – could also be applied to lower wing covers. The project also focused on evaluating BCR most important parameters. The fatigue life improvement obtained from BCR was evaluated through a series of coupons and skin-stringer assemblies tested under constant and variable amplitude loading. While the coupon tests demonstrated a life improvement of only 17% under constant amplitude loading, the variable amplitude load tests performed on the skin-stringer assembly demonstrated increased fatigue lives with a factor of 5 and reduced crack growth rates with a factor of 5 to 6. A finite element calculation tool was developed in order to conduct a parametric analysis of BCR geometry through the evaluation of the substrate stress intensity factor in the case of fatigue loading. The main difficulty was to include the interacting mechanism of the substrate lead crack and the disbond of the adhesive layer. The novelty of the approach was to incorporate the fatigue delamination calculation in order to evaluate the fatigue disbond propagation with crack growth. This was embedded in a 3D finite element design tool ReSLIC (Reinforced Structures Life Improvement Calculation). A necessary step to the development of ReSLIC was the analysis of fatigue properties of the adhesive system in order to provide input data for fatigue delamination calculations. To that end, a series of fatigue tests were performed in pure Mode I, pure Mode II and mixed mode with ratios of 25%, 50% and 75% of mode II ... [cont.].
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    An Integrated Approach to the Determination and Consequences of Residual Stress on the Fatigue Performance of Welded Aircraft Structures.
    (2006-02-01T00:00:00Z) Edwards, L.; Fitzpatrick, M. E.; Irving, Phil E.; Sinclair, I.; Zhang, X.; Yapp, David
    Although residual stress in welded structures and components has long been known to have an effect on their fatigue performance, access to reliable, spatially accurate residual stress field data has been limited. Recent advances in neutron and synchrotron X-ray diffraction allow a far more detailed picture of weld residual stress fields to be obtained that permits the development and use of predictive models that can be used for accurate design against fatigue in aircraft structures. This paper describes a fully integrated study of the three- dimensional residual stress distribution accompanying state-of-the-art fusion welds in 2024-T4 aluminum alloy, and how it is affected by subsequent machining and service loading. A particular feature of this work has been the development of techniques allowing the nondestructive evaluation of the residual stress field in the full range of specimens used to provide the design data required for welded aircraft structures and the integration of this information into all aspects of damage tolerant design.
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    A modelling technique for calculating stress intensity factors for structures reinforced by bonded straps. Part I: Mechanisms and formulation
    (Elsevier Science B.V., Amsterdam., 2010-04-01T00:00:00Z) Boscolo, M.; Zhang, X.
    This paper describes a 2D FE modelling technique for predicting fatigue crack growth life of integral structures reinforced by bonded straps. This kind of design offers a solution to the intrinsic lack of damage tolerance of integral structures. Due to the multiple and complex failure mechanisms of bonded structures, a comprehensive modelling technique is needed to evaluate important design parameters. In this Part I of a two-part paper, the actions and mechanisms involved in a bonded structure are discussed first, followed by presenting the modelling approaches to simulate each mechanism. Delamination or disbond of the strap from the substrate is modelled by computing the strain energy release rate on the disbond front and applying a fracture mechanics criterion. Thermal residual stresses arising from the adhesive curing process and their redistribution with the substrate crack growth are calculated and taken into account in the crack growth analysis. Secondary bending effect caused by the un-symmetric geometry of one-sided strap is also modelled. In the classic linear elastic fracture mechanics, a non-dimensional stress intensity factor, i.e. the geometry factor β, depends only on the sample’s geometry. This β factor cannot be found for this kind of bonded structures, since the magnitude of disbond is related to the applied stress and the disbond size modifies the geometry of the structure. Moreover, secondary bending effect is geometric nonlinear thus the stress intensity factor cannot be normalised by the applied stress. For these reasons an alternative technique has been developed, which requires calculating the stress intensity factors at both the maximum and minimum applied stresses for each crack length. This analysis technique is implemented in a computer program that interfaces with the NASTRAN commercial code to compute the fatigue crack growth life of strap reinforced structu
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    A modelling technique for calculating stress intensity factors for structures reinforced by bonded straps. Part II: Validation
    (Elsevier Science B.V., Amsterdam., 2010-04-01T00:00:00Z) Boscolo, M.; Zhang, X.
    In this second part of the two-part paper validation of the 2D FE modelling technique described in the first part is presented for a range of test configurations. Each mechanism that influences crack growth behaviour of strap reinforced structures is modelled for different substrate geometries, strap materials and dimensions in order to test the accuracy and robustness of the methodology. First, calculated through-thickness strain energy release rate distribution is compared with the result of a 3D FE model to validate this 2D model. Second, calculated disbond areas, thermal residual stresses and their redistribution with crack propagation are validated against experimental measurements. Third, influence of geometric nonlinearity and the need to use the alternate analysis method described in part I are demonstrated by examples, and errors generated by not following this analysis rule are given. Finally, using the 2D model calculated stress intensity factors, fatigue crack growth rates and lives are predicted for different specimens, strap materials and applied stress levels and are compared with the experimental tests. Good or acceptable agreement has been achieved for each case.
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    Predicting fatigue crack growth rate in a welded butt joint: The role of effective R ratio in accounting for residual stress effect
    (Elsevier Science B.V., Amsterdam., 2009-07-01T00:00:00Z) Servetti, Guido; Zhang, X.
    A simple and efficient method is presented in this paper for predicting fatigue crack growth rate in welded butt joints. Three well-known empirical crack growth laws are employed using the material constants that were obtained from the base material coupon tests. Based on the superposition rule of the linear elastic fracture mechanics, welding residual stress effect is accounted for by replacing the nominal stress ratio (R) in the empirical laws by the effective stress intensity factor ratio (Reff). The key part of the analysis process is to calculate the stress intensity factor due to the initial residual stress field and also the stress relaxation and redistribution due to crack growth. The finite element method in conjunction with the modified virtual crack closure technique was used for this analysis. Fatigue crack growth rates were then calculated by the empirical laws and comparisons were made among these predictions as well as against published experimental tests, which were conducted under either constant amplitude load or constant stress intensity factor range. Test samples were M(T) geometry made of aluminium alloy 2024-T351 with a longitudinal weld by the variable polarity plasma arc welding process. Good agreement was achieved.
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    Weld residual stress effects on fatigue crack growth behaviour of aluminium alloy 2024-T351
    (Elsevier Science B.V., Amsterdam., 2009-06-30T00:00:00Z) Liljedahl, C. D. M.; Brouard, J.; Zanellato, O.; Lin, J.; Tan, M. L.; Ganguly, Supriyo; Irving, Phil E.; Fitzpatrick, M. E.; Zhang, X.; Edwards, L.
    The interaction between residual stress and fatigue crack growth rate has been investigated in middle tension and compact tension specimens machined from a variable polarity plasma arc welded aluminium alloy 2024-T351 plate. The specimens were tested at three levels of applied constant stress intensity factor range. Crack closure was continuously monitored using an eddy current transducer and the residual stresses were measured with neutron diffraction. The effect of the residual stresses on the fatigue crack behaviour was modelled for both specimen geometries using two approaches: a crack closure approach where the effective stress intensity factor was computed; and a residual stress approach where the effect of the residual stresses on the stress ratio was considered. Good correlation between the experimental results and the predictions were found for the effective stress intensity factor approach at a high stress intensity factor range whereas the residual stress approach yielded good predictions at low and moderate stress intensity factor ranges. In particular, the residual stresses accelerated the fatigue crack growth rate in the middle tension specimen whereas they decelerated the growth rate in the compact tension sample, demonstrating the importance of accurately evaluating the residual stresses in welded specimens which will be used to produce damage tolerance design data.