62 results found
Yuan J, Scarpa F, Allegri G, et al., 2016, Efficient computational techniques for mistuning analysis of bladed discs: A review, MECHANICAL SYSTEMS AND SIGNAL PROCESSING, Vol: 87, Pages: 71-90, ISSN: 0888-3270
Zhang B, Allegri G, Hallett SR, 2016, An experimental investigation into multi-functional Z-pinned composite laminates, Materials and Design, Vol: 108, Pages: 679-688, ISSN: 0264-1275
This paper investigates the feasibility of monitoring progressive delamination growth in Z-pinned composite laminates via the measurement of through-thickness electrical resistance. This novel health monitoring technique is based on connecting Z-pins both in series and in parallel by means of arrays of electrodes arranged on the laminate surfaces. This creates a multi-functional (through-thickness reinforcing and sensing) laminated structure. Experimental results on double-cantilever beam coupons demonstrate that the entire Mode I bridging response of Z-pins can be monitored, from the arrival of the delamination front to the complete pull-out of the through-thickness reinforcement. Hence the extent of delamination can be inferred from the through-thickness resistance. This is proved for both conductive (carbon fibre-reinforced) and non-conductive (glass fibre-reinforced) laminates. The premature Z-pin failure during progressive pull-out corresponds to an abrupt increase of through-thickness electrical resistance. The delamination sensing/suppression method presented in this paper can be readily applied to Z-pinned composites at structural level.
Osmiani C, Mohamed G, Treiber JWG, et al., 2016, Exploring the influence of micro-structure on the mechanical properties and crack bridging mechanisms of fibrous tufts, COMPOSITES PART A-APPLIED SCIENCE AND MANUFACTURING, Vol: 91, Pages: 409-419, ISSN: 1359-835X
Gan KW, Allegri G, Hallett SR, 2016, A simplified layered beam approach for predicting ply drop delamination in thick composite laminates, Materials and Design, Vol: 108, Pages: 570-580, ISSN: 0264-1275
The prediction of delamination onset is a challenging task in the design of thick tapered composite laminates, where multiple ply terminations (“drop-offs”) are present. This paper addresses the development of a global-local finite element-based design approach for tapered laminates, whereby layered Timoshenko beam models are employed to predict delamination initiation from individual drop-offs. This modelling strategy provides a fast and conservative method for evaluating the strength of tapered composite laminates. Parametric test cases are presented in order to validate the methodology and understand its limitations. Finally, the application of the tool to a relatively thick tapered composite test specimen comprising multiple ply-drops is demonstrated.
Warzok F, Allegri G, Gude M, et al., 2016, Experimental characterisation of fatigue damage in single Z-pins, Composites Part A: Applied Science and Manufacturing, Vol: 91, Pages: 461-471, ISSN: 1359-835X
Z-pins have been shown to significantly improve delamination resistance and impact strength of carbon fibre reinforced (CFRP) composites. In this paper, an experimental investigation of the influence of different fatigue parameters (mean opening/sliding displacement, amplitude, frequency, number of cycles) on the through-thickness reinforcement (TTR) is presented. For mode I, it is shown that the degradation on pin behaviour during fatigue is mostly affected by the applied displacement amplitude. The degradation is primarily caused by surface wear. Due to the brittleness of the Z-pins, mode II fatigue does not have a significant effect for very small sliding displacements. Exceeding a critical displacement causes the pin to rupture within the very first cycles.
Zhang B, Allegri G, Yasaee M, et al., 2016, On the delamination self-sensing function of Z-pinned composite laminates, Composites Science and Technology, Vol: 128, Pages: 138-146, ISSN: 0266-3538
Yuan J, Allegri G, Scarpa F, et al., 2015, A novel hybrid Neumann expansion method for stochastic analysis of mistuned bladed discs, Mechanical Systems and Signal Processing, Vol: 72-73, Pages: 241-253, ISSN: 0888-3270
The paper presents a novel hybrid method to enhance the computational efficiency of matrix inversions during the stochastic analysis of mistuned bladed disc systems. The method is based on the use of stochastic Neumann expansion in the frequency domain, coupled with a matrix factorization in the neighbourhood of the resonant frequencies. The number of the expansion terms is used as an indicator to select the matrix inversion technique to be used, without introducing any additional computational cost. The proposed method is validated using two case studies, where the dynamics an aero-engine bladed disc is modelled first using a lumped parameter approach and then with high-fidelity finite element analysis. The frequency responses of the blades are evaluated according to different mistuning patterns via stiffness or mass perturbations under the excitation provided by the engine orders. Results from standard matrix factorization methods are used to benchmark the responses obtained from the proposed hybrid method. Unlike classic Neumann expansion methods, the new technique can effectively update the inversion of an uncertain matrix with no convergence problems during Monte Carlo simulations. The novel hybrid method is more computationally efficient than standard techniques, with no accuracy loss.
Ma X, Scarpa F, Peng H-X, et al., 2015, Design of a hybrid carbon fibre/carbon nanotube composite for enhanced lightning strike resistance, AEROSPACE SCIENCE AND TECHNOLOGY, Vol: 47, Pages: 367-377, ISSN: 1270-9638
Allegri G, Mohamed G, Hallett SR, 2015, Multi-scale modelling for predicting fracture behaviour in through-thickness reinforced laminates, Numerical Modelling of Failure in Advanced Composite Materials, Pages: 457-478, ISBN: 9780081003329
Modelling approaches for the analysis of through-thickness reinforced laminates are presented in this chapter, with emphasis on Z-pinned composites. We consider two characteristic scales, namely meso 1 (single Z-pin) and meso 2 (multiple Z-pins), bridging delaminated interfaces. We introduce a semi-analytical model to predict the mixed-mode response of individual Z-pins (meso 1) inserted in composite laminates. The model is based on representing the Z-pin as a Euler-Bernoulli beam embedded in an elastic foundation. The semi-analytical model is implemented in a cohesive zone formulation to predict the quasi-static fracture behaviour of Z-pinned laminates. The cohesive zone formulation is based on a 'smeared' bridging force approach. The modelling framework is validated against experimental data.
Yuan J, Allegri G, Scarpa F, et al., 2015, Probabilistic dynamics of mistuned bladed disc systems using subset simulation, JOURNAL OF SOUND AND VIBRATION, Vol: 350, Pages: 185-198, ISSN: 0022-460X
Zhang B, Allegri G, Yasaee M, et al., 2015, Micro-mechanical finite element analysis of Z-pins under mixed-mode loading, Composites Part A - Applied Science and Manufacturing, Vol: 78, Pages: 424-435, ISSN: 1359-835X
This paper presents a three-dimensional micro-mechanical finite element (FE) modelling strategy for predicting the mixed-mode response of single Z-pins inserted in a composite laminate. The modelling approach is based upon a versatile ply-level mesh, which takes into account the significant micro-mechanical features of Z-pinned laminates. The effect of post-cure cool down is also considered in the approach. The Z-pin/laminate interface is modelled by cohesive elements and frictional contact. The progressive failure of the Z-pin is simulated considering shear-driven internal splitting, accounted for using cohesive elements, and tensile fibre failure, modelled using the Weibull's criterion. The simulation strategy is calibrated and validated via experimental tests performed on single carbon/BMI Z-pins inserted in quasi-isotropic laminate. The effects of the bonding and friction at the Z-pin/laminate interface and the internal Z-pin splitting are discussed. The primary aim is to develop a robust numerical tool and guidelines for designing Z-pins with optimal bridging behaviour.
Charalambous G, Allegri G, Hallett SR, 2015, Temperature effects on mixed mode I/II delamination under quasi-static and fatigue loading of a carbon/epoxy composite, Composites Part A: Applied Science and Manufacturing, Vol: 77, Pages: 75-86, ISSN: 1359-835X
Abstract This paper addresses the effect of temperature on the mixed-mode interlaminar fracture toughness and fatigue delamination growth rate of a carbon-fibre/epoxy material, namely IM7/8552. Quasi-static and fatigue characterisation tests were carried out at -50 °C, 20 °C, 50 °C and 80 °C, using asymmetric cut-ply coupons. The experimental results show that temperature may have an accelerating or delaying effect on delamination growth, depending on the loading regime, i.e. either quasi-static or fatigue. Fractographic examinations were also carried out in order to assist the interpretation of the experimental data. A semi-empirical equation is introduced to describe the experimentally observed fatigue delamination growth rates at elevated temperatures.
Yuan J, Allegri G, Scarpa F, et al., 2015, Novel parametric reduced order model for aeroengine blade dynamics, Mechanical Systems and Signal Processing, Vol: 62-63, Pages: 235-253, ISSN: 1096-1216
The work introduces a novel reduced order model (ROM) technique to describe the dynamic behavior of turbofan aeroengine blades. We introduce an equivalent 3D frame model to describe the coupled flexural/torsional mode shapes, with their relevant natural frequencies and associated modal masses. The frame configurations are identified through a structural identification approach based on a simulated annealing algorithm with stochastic tunneling. The cost functions are constituted by linear combinations of relative errors associated to the resonance frequencies, the individual modal assurance criteria (MAC), and on either overall static or modal masses. When static masses are considered the optimized 3D frame can represent the blade dynamic behavior with an 8% error on the MAC, a 1% error on the associated modal frequencies and a 1% error on the overall static mass. When using modal masses in the cost function the performance of the ROM is similar, but the overall error increases to 7%. The approach proposed in this paper is considerably more accurate than state-of-the-art blade ROMs based on traditional Timoshenko beams, and provides excellent accuracy at reduced computational time when compared against high fidelity FE models. A sensitivity analysis shows that the proposed model can adequately predict the global trends of the variations of the natural frequencies when lumped masses are used for mistuning analysis. The proposed ROM also follows extremely closely the sensitivity of the high fidelity finite element models when the material parameters are used in the sensitivity.
Charalambous G, Allegri G, Lander JK, et al., 2015, A cut-ply specimen for the mixed-mode fracture toughness and fatigue characterisation of FRPs, Composites Part A: Applied Science and Manufacturing, Vol: 74, Pages: 77-87, ISSN: 1359-835X
The characterisation of mixed-mode fracture toughness and fatigue delamination growth in fibre-reinforced composites is crucial for assessing the integrity of structural elements in service. An asymmetric cut-ply coupon (ACP) loaded in four-point bending is here proposed to carry out the aforementioned characterisations. Analytical expressions of the energy release rate and mode-mixity for the ACP are derived and validated by means of finite element analysis. A fracture toughness and fatigue characterisation of the carbon/epoxy material IM7/8552 is carried out via ACP specimens. It is proved that the material data obtained from ACP specimens match those generated using ASTM standard mixed-mode bending (MMB) coupons. The main reason for the introduction of the ACP test resides in its applicability to characterisation scenarios where measuring the delamination length with optical means, as required for MMB coupons, is difficult. Such scenarios include the investigation of static and fatigue delamination growth at low and high temperatures, which requires the usage of environmental chambers. This poses significant constraints in terms of volume available for the test rigs, and, most importantly, limitations on visual access to observe delamination propagation. However, the manufacturing of ACP coupons is more complex than for MMB specimens and the testing requires several additional precautions that are here discussed in detail.
Yuan J, Scarpa F, Allegri G, et al., 2015, NUMERICAL ASSESSMENT OF USING SHERMAN-MORRISON, NEUMANN EXPANSION TECHNIQUES FOR STOCHASTIC ANALYSIS OF MISTUNED BLADED DISC SYSTEMS, ASME Turbo Expo: Turbine Technical Conference and Exposition, Publisher: AMER SOC MECHANICAL ENGINEERS
Allegri G, Yasaee M, Partridge IK, et al., 2014, A novel model of delamination bridging via Z-pins in composite laminates, International Journal of Solids and Structures, Vol: 51, Pages: 3314-3332, ISSN: 1879-2146
A new micro-mechanical model is proposed for describing the bridging actions exerted by through-thickness reinforcement on delaminations in prepreg based composite materials, subjected to a mixed-mode (I-II) loading regime. The model applies to micro-fasteners in the form of brittle fibrous rods (Z-pins) inserted in the through-thickness direction of composite laminates. These are described as Euler-Bernoulli beams inserted in an elastic foundation that represents the embedding composite laminate. Equilibrium equations that relate the delamination opening/sliding displacements to the bridging forces exerted by the Z-pins on the interlaminar crack edges are derived. The Z-pin failure meso-mechanics is explained in terms of the laminate architecture and the delamination mode. The apparent fracture toughness of Z-pinned laminates is obtained from as energy dissipated by the pull out of the through-thickness reinforcement, normalised with respect to a reference area. The model is validated by means of experimental data obtained for single carbon/BMI Z-pins inserted in a quasi-isotropic laminate. © 2014 Elsevier Ltd. All rights reserved.
Ajaj RM, Friswell MI, I Saavedra Flores E, et al., 2014, An integrated conceptual design study using span morphing technology, Journal of Intelligent Material Systems and Structures, Vol: 25, Pages: 989-1008, ISSN: 1045-389X
A comprehensive conceptual design study is performed to assess the potential benefits of span morphing technology and to determine its feasibility when incorporated on medium altitude long endurance unmanned air vehicles. A representative medium altitude long endurance unmanned air vehicle based on the BAE Systems Herti unmanned air vehicle was selected. Stability and control benefits are investigated by operating the morphing span asymmetrically to replace conventional ailerons. The Tornado vortex lattice method was incorporated for aerodynamic predictions. The sensitivity of rolling moment generated by span morphing for different flight parameters (instantaneous vehicular weight and angle of attack) is studied. The variation of roll rate (steady and transient response) with span morphing (for constant rolling moment) for different rolling strategies (extension and retraction) is investigated. It turns out that the optimum rolling strategy is to extend one side of the wing by 22% while retract the other by 22%. Operational performance benefits are investigated by operating the morphing span symmetrically to reduce drag, increase endurance and reduce take-off and landing distances. Twenty-two per cent symmetric span morphing reduces the total drag by 13%, enhances the endurance capability by 6.5% and reduces the take-off field length and landing distance by 28% and 10%, respectively. © 2013 The Author(s).
Yasaee M, Lander JK, Allegri G, et al., 2014, Experimental characterisation of mixed mode traction-displacement relationships for a single carbon composite Z-pin, Composites Science and Technology, Vol: 94, Pages: 123-131, ISSN: 0266-3538
This paper presents an experimental characterisation of the mechanical performance and behaviour of through-thickness reinforced composite laminates. To achieve this, composite blocks with individual reinforcing pins were manufactured, quality assessed and tested. Individual specimens were inspected using X-ray Computed Tomography and only the specimens with acceptable quality pin insertions were tested experimentally under a range of mode mixities. Two stacking sequences, uni-directional (UD) and quasi-isotropic (QI) were investigated. It was found that the pins inside the UD samples experienced significantly larger pin/matrix bond strength than those in the QI laminates. The resulting experimental data indicates that a non-UD laminate type may experience pin pull-out and thus increased energy absorption for a wider range of mode mixities than a UD laminate type. Energy plots show a clear transition from a pull-out to a pin rupture region for both laminate types. Specimens that experienced pin rupture during low mode mixity tests exhibited similar failure energies to those loaded in pure mode II. © 2014 Elsevier Ltd.
Allegri G, Scarpa FL, 2014, On the asymptotic crack-tip stress fields in nonlocal orthotropic elasticity, International Journal of Solids and Structures, Vol: 51, Pages: 504-515, ISSN: 0020-7683
The crack-tip stress fields in orthotropic bodies are derived within the framework of Eringen's nonlocal elasticity via the Green's function method. The modified Bessel function of second kind and order zero is considered as the nonlocal kernel. We demonstrate that if the localisation residuals are neglected, as originally proposed by Eringen, the asymptotic stress tensor and its normal derivative are continuous across the crack. We prove that the stresses attained at the crack tip are finite in nonlocal orthotropic continua for all the three fracture modes (I, II and III). The relative magnitudes of the stress components depend on the material orthotropy. Moreover, non-zero self-balanced tractions exist on the crack edges for both isotropic and orthotropic continua. The special case of a mode I Griffith crack in a nonlocal and orthotropic material is studied, with the inclusion of the T-stress term. © 2013 Elsevier Ltd. All rights reserved.
Yuan J, Allegri G, Scarpa F, et al., 2014, Novel parametric reduced order model for aeroengine blade dynamics, Pages: 413-425, ISSN: 2191-5644
The work proposes a reduced order modelling (ROM) technique for turbofan engine blades. The aim is to develop a simplified structural layout that allows describing the dynamic behaviour associated with the first six modes of fullscale fan blades. This is done by introducing equivalent frame models for the blade, which can be tailored to represent coupled flexural/torsional mode shapes, the relevant natural frequencies and static masses. Both 2D and 3D frame models are considered with initial configurations obtained from structural identification equations. The frame configurations are refined via an optimization process based on simulated annealing with stochastic tunnelling. The cost function comprises a linear combination of relative errors on the vibration frequencies, the individual modal assurance criteria (MAC) and the static mass. We demonstrate that an optimized 3D frame can represent the blade dynamic behaviour with a 6 % error on the MAC and a 1 % error on the associated modal frequencies. The approach proposed in this paper is considerably more accurate than ROMs based on single equivalent beams, either Euler–Bernoulli or Timoshenko, and highly computational efficient. Therefore, this technique is suitable for application to the analysis of mistuned bladed discs, particularly for determining the sensitivity to manufacturing and assembly tolerances in joints.
Yuan J, Allegri G, Scarpa F, et al., 2014, Probabilistic dynamics of mistuned bladed disc systems using Subset Simulation, Pages: 4609-4621
The work describes an assessment of subset simulation (SubSim) techniques to increase the computational efficiency for the predictions of probabilistic dynamic behavior in mistuned bladed disc systems. Subset Simulation (SubSim) is an adaptive stochastic procedure to compute efficiently small failure probabilities, which are expressed as a product of large conditional failures probabilities by introducing intermediate failure events. The original version of SubSim is used in this work to simulate samples related to intermediate failure events. A 2-DOFs model with lumped parameters identified from a finite element model is used to represent a bladed disc. The statistics associated to the maximum forced frequency response amplitudes are evaluated from different levels of the blade mistuning using stiffness perturbation in the blades. The study shows the SubSim method capture efficiently the statistical properties of the mistuned blades with less than 1% of sample size that necessary for the traditional Monte Carlo method.
Blanchfield JP, Allegri G, 2014, Fatigue delamination initiation in L-bend CFRP coupons
Accurate modelling of the delamination of composites due to cyclic loading is important to reduce the costs associated with fatigue testing of structures. This work aims to identify the point of damage initiation in a carbon-fibre laminate subject to pure interlaminar tensile cyclic loading for a range of severities and stress ratios, in order to generate full SN-curves for an aerospace-grade carbon-fibre epoxy composite. Curved beams have been used in 4-point bend tests roughly in line with the ASTM D 6415 test standard to generate the required failure conditions. The tests were monitored in-situ using an Acoustic Emissions system and post-test CT (X-Ray) Scans were employed to identify failure conditions in the specimens.
Zhang B, Allegri G, Hallett SR, 2014, High-fidelity fe modelling of Z-pins in quasi-isotropic laminates
This paper presents a three-dimensional FE modelling approach to simulate the mechanical response of individual Z-pins at the micro-scale and the associated bridging mechanism. This modelling strategy accounts for the characteristic features associated with Z-pinning, i.e. the interface between Z-pin and the surrounding laminate block, residual stresses due to the postcure cool down and split within the Z-pin. The Z-pin failure is described using the Weibull's failure criterion. The analysis results are in excellent agreement with the experimental results from tests performed on single Z-pinned quasi-isotropic coupons. The analyses demonstrate that the de-bonding of the Z-pins from the laminate is essentially due to thermal residual stresses. For both Mode I and Mode II loading cases, enhanced friction zones develop along the Z-pin. This is the main cause of the progressive split between fibre strands in Mode II. The split initiates at the Z-pin centre and then propagates along the neutral plane. The modelling strategy presented in this paper can be directly extended to arbitrary stacking sequences as well as asymmetric insertion cases.
Yasaee M, Mohamed G, Allegri G, et al., 2014, Delamination resistance of through thickness reinforced composites
In this study the delamination resistance of arrays of Z-pins are measured in Mode I, Mode II and mixed Mode I/II loading scenarios, using standard fracture toughness test methodologies. Through experimental observations and measured results it is shown that the bridging behaviours predicted in previous single pin tests are analogous to the arrays of pins inside fracture toughness coupons. Although it is not feasible to measure the steady state delamination propagation in a through-thickness reinforced (TTR) region in all Mode mixities, a method is proposed to measure the apparent toughness of these TTR pins at predetermined delamination lengths. This method provides useful, conservative results that could improve the design of composite components which contain TTR elements.
Da Rocha-Schmidt L, Hermanutz A, Baier H, et al., 2014, Progress towards adaptive aircraft engine nacelles
Emissions and noise of aircraft engines have to be significantly further reduced and efficiency further increased in the future. One means is the improvement of airflow though the engine and especially so in its inlet region by proper shapes. Due to changes in the flight conditions, the optimal nacelle shape varies. It would thus be beneficial to be able to change the nacelle shape. Evaluations on system and engine levels including related flow simulations support the identification of proper shaping parameters. Initial concepts of possible morphing technologies are discussed as well.
Ajaj RM, Friswell MI, Dettmer WG, et al., 2013, Dynamic modelling and actuation of the adaptive torsion wing, Journal of Intelligent Material Systems and Structures, Vol: 24, Pages: 2045-2057, ISSN: 1045-389X
This article presents the dynamical modelling of a novel active aeroelastic structure. The adaptive torsion wing concept is a thin-wall, two-spar wingbox whose torsional stiffness can be adjusted by translating the spar webs in the chordwise direction inward and towards each other using internal actuators. The reduction in torsional stiffness allows external aerodynamic loads to induce twist on the structure and maintain its deformed shape. Here, the adaptive torsion wing system is considered as integrated within the wing of a representative unmanned aerial vehicle to replace conventional ailerons and provide roll control. The adaptive torsion wing is modelled as a two-dimensional equivalent aerofoil using bending and torsion shape functions to express the equations of motion in terms of the twist angle and plunge displacement at the wingtip. The full equations of motion for the adaptive torsion wing equivalent aerofoil were derived using Lagrangian mechanics. The aerodynamic lift and moment acting on the aerofoil were modelled using Theodorsen's unsteady aerodynamic theory. A low-dimensional, state-space representation of an empirical Theodorsen's transfer function was adopted to allow time-domain analyses. Four actuation strategies were investigated. Figures of merit, including plunge displacement, twist angle, actuation forces and actuation powers, were quantified and discussed for each of the scenarios. This study allows the conceptual design and sizing of the internal actuators that are required to drive the webs. © The Author(s) 2012.
Yuan J, Allegri G, Scarpa F, 2013, Buffeting mitigation using carbon nanotube composites: A feasibility study, Proceedings of the Institution of Mechanical Engineers, Part G: Journal of Aerospace Engineering, Vol: 227, Pages: 1425-1440, ISSN: 0954-4100
The article describes a feasibility study to assess the use of nanotubes-based composites to mitigate tail buffeting. The buffeting of a representative business jet rudder is considered as case study. The baseline rudder configuration consists in a sandwich structure with honeycomb core and carbon/epoxy IM7/8552 skins. The damping characteristics of the baseline rudder configuration are compared to those achieved employing constrained layer Al/3M467 skin patches, and those obtained by dispersing multi-walled carbon nanotubes in the baseline carbon/epoxy material. The loads applied to the rudder during flight are obtained by airworthiness standards. Static and dynamic finite element analyses of the rudder under flight loads are carried out to evaluate the structural response at two different temperatures, -40 °C and +30 °C. IM7/8552/MWNT with 1.5 wt% nanofiller is shown to have the best overall performance for the case study considered here, with the potential of outperforming conventional constrained layer patches for buffeting mitigation. © IMechE 2012.
This paper presents the Zigzag wingbox concept that allows the wing span to be varied by 44% (22% extension and 22% retraction). The Zigzag wingbox consists of a rigid part and a morphing part. The rigid part is a semi-monocoque construction that houses the fuel tank and transfers the aerodynamic loads from the morphing part to the fuselage. The morphing part consists of various morphing partitions where in each partition there are two spars each consisting of two beams hinged together. Each morphing partition is covered by flexible skin and is bounded by two ribs through which the spars are connected. The ribs transfer the loads between the spars of adjacent morphing partitions and serves as the main structure to which the flexible skins are to be attached. The Zigzag wingbox concept is then incorporated in the rectangular wing of a medium altitude long endurance (MALE) UAV to enhance its operational performance and provide roll control. Equivalent modelling and preliminary sizing of the concept are performed to assess its feasibility and quantify its potential benefits. © 2012 Elsevier Masson SAS. All rights reserved.
Allegri G, Wisnom MR, Hallett SR, 2013, A new semi-empirical law for variable stress-ratio and mixed-mode fatigue delamination growth, Composites Part A: Applied Science and Manufacturing, Vol: 48, Pages: 192-200, ISSN: 1359-835X
A new semi-empirical equation that describes the fatigue delamination growth in fibre reinforced toughened epoxies is presented and validated against data available in the literature. The new law accounts for the simultaneous effects of the stress-ratio and mode-mixity on the interlaminar crack propagation. If delamination propagation thresholds are ignored, the proposed semi-empirical equation allows describing interlaminar crack propagation employing only three material dependent parameters, whereas alternative models presented in the literature require four. If reliable threshold data are available from experimental tests, the new semi-empirical law can be extended to a unified description of stress-ratio, mode-mixity and thresholds effects using six material dependent parameters. © 2013 Elsevier Ltd. All rights reserved.
Allegri G, Scarpa F, Chowdhury R, et al., 2013, Wave propagation in periodically supported nanoribbons: A nonlocal elasticity approach, Journal of Vibration and Acoustics, Vol: 135, ISSN: 1048-9002
We develop an analytical formulation describing propagating flexural waves in periodically simply supported nanoribbons by means of Eringen's nonlocal elasticity. The nonlocal length scale is identified via atomistic finite element (FE) models of graphene nanoribbons with Floquet's boundary conditions. The analytical model is calibrated through the atomistic finite element approach. This is done by matching the nondimensional frequencies predicted by the analytical nonlocal model and those obtained by the atomistic FE simulations. We show that a nanoribbon with periodically supported boundary conditions does exhibit artificial pass-stop band characteristics. Moreover, the nonlocal elasticity solution proposed in this paper captures the dispersive behavior of nanoribbons when an increasing number of flexural modes are considered. Copyright © 2013 by ASME.
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