Free vibration analyses of multiple delaminated angle-ply composite conical shells – A finite element approach

In this paper, the finite element method is employed to investigate the effects of delamination on free vibration characteristics of graphite–epoxy pretwisted shallow angle-ply composite conical shells. The generalized dynamic equilibrium equation is derived from Lagrange’s equation of motion neglecting Coriolis effect for moderate rotational speeds. The theoretical formulation is based on the Mindlin’s theory and the multi-point constraint algorithm is considered for an eight noded isoparametric plate bending element. The standard eigenvalue problem is solved by applying the QR iteration algorithm. The mode shapes are also depicted for a typical laminate configuration. Non-dimensional natural frequencies obtained are the first known results for the type of analyses carried out here.     

Thermoelastic free vibration and dynamic behaviour of an FGM doubly curved panel via the analytical hybrid Laplace–Fourier transformation

The paper presents an analysis of functionally graded material doubly curved panels with rectangular planform under the action of thermal and mechanical loads. Based on the first-order shear deformation theory of modified Sanders assumptions, five coupled partially differential equations (PDEs) are established as equations of motion. Each thermo-mechanical property of the shell follows the power law distribution across the thickness, except Poisson’s ratio, which is kept constant through the panel. Assuming that four edges of the shell-panel are simply supported, a Navier-based solution is adopted to reduce the PDEs into time-dependent ODEs. Applying the Laplace transformation, the equations of motion are transformed into the Laplace domain. With the aid of analytical Laplace inverse method, solutions of stresses, strains, and displacements are obtained in time domain and expressed in explicit phrases. Dynamic, free vibration, and thermo-mechanical bending analysis of the panel is carried out for various geometries. Obtained results are validated with the well-known available data reported in the literature.     

Active control of large amplitude vibrations of smart magneto–electro–elastic doubly curved shells

This paper deals with the analysis of active constrained layer damping (ACLD) of large amplitude vibrations of smart magneto–electro–elastic (MEE) doubly curved shells. The constraining layer of the ACLD treatment is composed of the vertically/obliquely reinforced 1–3 piezoelectric composite (PZC). The constrained viscoelastic layer of the ACLD treatment is modeled by using the Golla–Hughes–McTavish method in the time domain. A three-dimensional finite element model of the overall smart MEE doubly curved shells has been developed taking into account the effects of electro–elastic and magneto–elastic couplings, while the von Kármán type nonlinear strain displacement relations are used for incorporating the geometric nonlinearity. Influence of the curvature ratio, the curvature aspect ratio, the thickness aspect ratio on the nonlinear frequency ratios of the MEE doubly curved shells has been investigated. Effects of the location of the ACLD patches and the edge boundary conditions on the control of geometrically nonlinear vibrations of paraboloid and hyperboloid MEE shells have been studied. Particular attention has been paid to investigate the performance of the ACLD treatment due to the variation of the piezoelectric fiber orientation angle in the 1–3 PZC constraining layer of the ACLD treatment.     

Smart control of nonlinear vibrations of doubly curved functionally graded laminated composite shells under a thermal environment using 1–3 piezoelectric composites

This paper addresses the active control of geometrically nonlinear vibrations of doubly curved functionally graded (FG) laminated composite shells integrated with a patch of active constrained layer damping (ACLD) treatment under the thermal environment. Vertically/obliquely reinforced 1-3 piezoelectric composite (PZC) and active fiber composite (AFC) are used as the materials of the constraining layer of the ACLD treatment. Each layer of the substrate FG laminated composite shell is made of fiber-reinforced composite material in which the fibers are longitudinally aligned in the plane parallel to the top or bottom surface of the layer and the layer is assumed to be graded in the thickness direction by way of varying the fiber orientation angle across its thickness according to a power law. The novelty of the present work is that, unlike the traditional laminated composite shells, the FG laminated composite shells are constructed in such a way that the continuous variation of material properties and stresses across the thickness of the shell is achieved. The Golla-Hughes-McTavish (GHM) method has been implemented to model the constrained viscoelastic layer of the ACLD treatment in time domain. Based on the first-order shear deformation theory (FSDT), a finite element (FE) model has been developed to model the open-loop and closed-loop nonlinear dynamics of the overall FG laminated composite shell under a thermal environment. Both symmetric and asymmetric FG laminated composite doubly curved shells are considered for presenting the numerical results. The analysis suggests that the ACLD patch significantly improves the damping characteristics of the doubly curved FG laminated composite shells for suppressing their geometrically nonlinear transient vibrations. It is found that the performance of the ACLD patch with its constraining layer being made of the AFC material is significantly higher than that of the ACLD patch with vertically/obliquely reinforced 1-3 PZC constraining layer. The effects of variation of piezoelectric fiber orientation in both the obliquely reinforced 1-3 PZC and the AFC constraining layers on the control authority of the ACLD patch have also been investigated.     

A new hybrid smoothed FEM for static and free vibration analyses of Reissner–Mindlin Plates

A new smoothed finite element method (S-FEM) is proposed using hybrid smoothing operations based on nodes and edges of the mesh for static and free vibration analyses of plates governed by the Reissner–Mindlin plate theory. In the present approach, both the node-based smoothed finite element method (NS-FEM) and edge-based smoothed finite element method (ES-FEM) are utilized in a careful designed manner to overcome the shear locking. The formulations use 3-node triangular elements for easy automatic mesh creation, and linear interpolation functions are used for simplicity and robustness. The bending strains field are smoothed by the means of gradient smoothing technique over smoothing domains constructed by element edges, while the shear strains filed is smoothed based on the combination of NS-FEM and ES-FEM with a proper weightage controlled by a coefficient. A simple formula is developed for automatic selection of the coefficient by considering mesh size and thickness of the plate. For easy reference, the present technique is termed as NS+ES-FEM. The numerical examples demonstrate that the proposed method passes the shear-locking test and improves accuracy of the solution.     

Battle damage repair of a helicopter composite frame-to-skin junction – A sole external repair approach

Research was conducted to examine the effectiveness of a rapid repair to a helicopter composite frame-to-skin junction subjected to battlefield damage. The repair design consists of a laminate patch and aluminium angle bracket adhesively bonded and riveted, respectively, to the helicopter external surface. The assessment involved a relative comparison of three models, representing pristine, damaged and repaired configurations. Computational analyses were conducted to examine the stiffness and buckling onset load of the overall structure and the strengths of individual components (laminates, adhesive bondlines and rivets) under three typical load conditions, namely in-plane shear, axial compression and transverse compression. The results showed that the damage would cause significant stiffness and strength reduction. The repair could sufficiently restore the stiffness and static strength for the load cases considered. However, for the specimen without support from its adjacent helicopter structure, it is predicted that the failure mode under the transverse compression loading would be via buckling under a relatively low load. A compression test was conducted to further validate the repair design. The result agreed well with the prediction. It showed that compared with an un-repaired damaged specimen, the external repair increased the strength by 83%. The equivalent far field failure strain exceeded 3300 με which is considered satisfactory for a rapid field battle damage repair (BDR).     

Vibroacoustic analysis of thermo-elastic laminated composite sandwich curved panel: a higher-order FEM–BEM approach

International Journal of Mechanics and Materials in Design - The present paper deals with the vibration induced acoustic responses of baffled sandwich curved shell panels constituted of laminated...     

Structural health monitoring of bonded composite repairs – A critical comparison between ultrasonic Lamb wave approach and surface mounted crack sensor approach

The effectiveness of the Lamb waves Acoustic-Ultrasound Structural Health Monitoring (SHM) technique for the determination of the adhesive joint failure was evaluated and compared to that of another SHM technique: Surface Mountable Crack Sensor (SMCS). A commercially available Acoustic-Ultrasound system was used. The SMCS consists of a “hot spot” monitoring system based on a three-layer electrically insulating and conductive paint. An electrical signal is used as a boolean operator for the characterization of the passage of damage through the sensor and the underlying material. The coupons consisted of carbon fiber substrates with boron patches bonded to them. The advantages and disadvantages of both techniques are discussed. It has been demonstrated that the Lamb waves Acoustic-Ultrasound technique possesses the capability to identify and quantify damage in a bonded repair. The SMCS proved to be a successful SHM technique for the monitoring of the formation of a disbond at the edges of the patch. All results were validated using the C-Scan ultrasound method.     

Air voids reduction phenomena of asphalt concrete – A continuum approach

Asphalt concrete used in flexible highway pavement construction has 5–8 percent air voids immediately after laying of the roadway. Constitutive laws for asphalt concrete developed till now have modelled the mix as a linear elastic or viscoelastic material and have not taken into account the effect of void concentration on the mechanical behaviour of the material. In this paper, the theory of linear elastic material with voids is used to model asphalt concrete under isothermal conditions. Two cases of void reduction behaviour are studied, one in which the void volume reduces asymptotically under a constant load and the other in which it reaches the refusal air void content. The model is used to predict the creep behaviour under constant compressive stress as well as to obtain the hysteretic stress-strain behaviour. Solutions for the case of uniaxial deformation are derived and the strains are simulated for a constant compressive stress. Use of the air voids reduction measure as a possible damage parameter is also examined. This revised version was published online in July 2006 with corrections to the Cover Date.     

Methionine-montmorillonite composite – A novel material for efficient adsorption of lead ions

A novel and platelike nanomaterial of methionine-montmorillonite composite was successfully prepared and applied to the adsorption of lead ions in aqueous. The application of a series of characterization methods, such as XRD, FT-IR, BET, TG/DTG, SEM revealed the morphology, structure and physicochemical properties of the nanocomposites and confirmed the anchoring effect of methionine on Na-Mt. Furthermore, it has been demonstrated that the filling arrangement of methionine in the interlayer channel can be oriented along the y-axis or the z-axis or inclined to the silicate surface to form a single layer structure. Under optimized experimental conditions, Met-Mt-0.8 composites were verified to be the most effective adsorbent for Pb²⁺ ion along with a maximum adsorption capacity of 117.79 mg/g, significantly higher than that of Na-Mt (101.11 mg/g). Gaussian program and Multiwfn program were united to take a sight on the microscopic adsorption mechanism of Met-Mt and Pb²⁺, concluding that methionine formed a complex with lead ions and facilitated them to lock in the montmorillonite layer. Moreover, methionine is ligated with Pb²⁺ by oxygen atom on C-OH in the carboxyl group.     

Assessment of patentability by means of semantic patent analysis – A mathematical-logical approach

To obtain patent protection, a patent must fulfill statutory patentability requirements examined by a patent office. Such examinations are mostly performed manually and are quite time-consuming. Therefore, we suggest a computer-based process for the assessment of patentability by means of a mathematical-logical approach comparing patents with semantic structures. In order to make such an assessment, we compare the feature combinations of patent claims with the pertinent prior art. For proof of concept, the process has been tested successfully on an US-application claiming a method for raising a crane boom which can be categorized as non-patentable with regard to the requirement of non-obviousness. The result is consistent with that of a USPTO patent examiner, which underpins that at least under certain conditions not only patent examiners but also applicants and third parties can assess the chance and scope of protection for claimed inventions and patent applications with regard to patentability by our process.     

Modelling and comparative study of viscoelastic laminated composite beam – an operator based finite element approach

Mechanics of Time-Dependent Materials - Finite element formulation of damped laminated composite beam considering both Euler-Bernoulli and Timoshenko beam theory is proposed based on Equivalent...     

Mechanics of mechanically fastened joints in polymer–matrix composite structures – A review

As the applications of advanced composite structural materials continue to increase, so does the need to understand the mechanical behavior of mechanically fastened joints in such structures. The most recent and relevant review article on this subject was published more than a decade ago, but it was restricted to stress analysis and strength prediction of mechanically fastened joints in fiber-reinforced plastics. The present article attempts a more comprehensive review of recent literature in the broader area of mechanics of mechanically fastened joints in polymer–matrix composite structures. Since experimental characterization has traditionally played such a fundamental role in such studies, the article begins with a review of relevant mechanical test methods and standards. This is followed by a discussion of the mechanics aspects of design, including joint design methodologies, considerations of the influence of geometric effects, and fastener preload selection. The remaining sections are devoted to failure modes such as bearing failure, failure prediction for both statically and dynamically loaded joints, time-dependent joint preload relaxation, the effects of temperature and moisture on joint strength and failure, and non-destructive evaluation techniques for monitoring the joints. Finally, comments are offered regarding the most important remaining problems in this area, and recommendations for future work.     

Evaluation of reverse referral partnership in a tiered hospital system – A queuing-based approach

Reverse referral implies referring patients from a comprehensive hospital to a community healthcare centre. In recent years, it has become popular for post-acute care in the tiered Chinese hospital system as it helps achieve more balanced utilisation between facilities at different levels. Nevertheless, it remains unclear how to implement the reverse referral partnership given the likely conflicting interests between the involved hospitals. In this paper, we evaluate a reversal referral partnership involving one hospital of each type, for which we develop queuing models for the two hospitals to capture multiple patient waits, services and transitions, with reverse referral in place. We use queuing-based performance outcomes to evaluate the profitability of each hospital and its willingness to join the partnership. We combine the above evaluations to further assess the partnership effectiveness. Through this work, we expect to provide policy insights to the government for forming hospital partnerships effectively.     

Numerical simulation of delamination in laminated composite components – A combination of a strength criterion and fracture mechanics

An approach for the numerical treatment of delamination in laminated composite components is presented. A first ply failure criterion is employed to predict delamination initiation, while delamination propagation is analyzed using linear elastic fracture mechanics. The combination of initiation and propagation criteria yields a conservative estimation of the load earring capacity of a structure. Furthermore, the growth stability, the sensitivity of the results with respect to a change in the interface properties, and the non-linear structural response caused by the delamination growth process are determined. Two structures are investigated, which show the capability of the proposed approach, a curved laminate and a double lap shear test specimen.     

The Master SN curve approach – A hybrid multi-scale fatigue simulation of short fiber reinforced composites

Typical short fiber reinforced composites (SFRCs) components have a different statistical distribution of orientation of fibers at different points leading to different static and fatigue behavior at different locations across the component. To link component-scale calculations with this variability of fiber orientations, each element in the FE model is modeled as a Representative Volume Element (RVE); the static and fatigue properties must be calculated for each of these elements. While there are established methods to estimate the static properties, there are none for the fatigue properties. A hybrid (combination of micromechanics and tests) and multi-scale (damage in micro-scale linked to macroscale fatigue properties) method of predicting the SN curve for every point in a short fiber composite has been developed. This proposed method is based not only on tests but on a combination of manufacturing simulation, tests and multi-scale mechanics. An extensive test program was undertaken to study the fatigue behavior of short fiber composites and validate the concept of the Master SN curve (MSNC) approach. The MSNC approach is compared with two prevalent approaches – strength based scaling and test based interpolation. The MSNC approach was found to be in a good agreement with the experimental results and was confirmed to be more accurate than the prevalent methods.     

Mechanisms of cyclic softening in precipitation-hardening alloys—A ball model approach and tests at 78 K

Tests of random, alternating cuts on a ball-model of in Al–Cu alloy have been run to check the kinetics at which disordering occurs in an initially ordered precipitate subject to cyclic strain. In addition, fatigue tests at low temperature (78 K) and various microscopical observations have been made on Al–Ag alloy containing GP zones to check whether a structure containing an initially disordered precipitate will cyclically soften or not. It was thus found that the ball model disorders as a function of accumulated plastic strain consistently with the kinetics of cyclic softening in actual material. Further, Al–Ag alloy was found not to soften at 78 K. Both of these results supoort the disordering hypothesis of cyclic softening over the dissolution or shearing-off mechanisms, although other systems may be subject to these mechanisms. In spite of the low temperature at which the Al–Ag alloy was tested, small / precipitates were found to have formed in the longest lived test; however, this result did not interfere with the validity of the experiment as it did previously at room temperature. The fatigue lives at 78 K were much longer than those previously measured at room temperature although the failure mechanisms were not affected at high strain and only somewhat affected at low strain (there was increased incidence of transgranular crack nucleation and propagation).

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Résumé Des essais d'application de coups alternés et aléatoires sur un modèle de simulation par des billes de la phase dans les alliages de Cu–Al ont été effectués en vue de vérifier la cinétique suivant laquelle se produisent des désordres dans un précipité initialement ordonné et soumis à des déformations cycliques. En outre, des essais de fatigue à basse température (78°K) ainsi que diverses observations microscopiques ont été effectués sur un alliage Al–Ag comportant des zones GP en vue de vérifier si une structure comportant un précipité initialement en désordre subirait un adoucissement cyclique ou non. On a donc trouvé que le modèle de utilisant des billes fait état de désordres en fonction de l'accumulation de la déformation plastique et ce en rapport avec la cinétique d'adoucissement cyclique dans le matériau réel. En outre, l'alliage Al–Ag s'est révélé ne pas s'adoucir à 78°K. Ces deux résultats soutiennent l'hypothèse suivant laquelle un adoucissement cyclique s'accompagne de désordres plutôt que de mécanismes de disolution ou de séparation par cisaillement, bien que d'autres systèmes puissent être sujets à ces mécanismes. En dépit de la basse température à laquelle l'alliage Al–Ag a été essayé, on a trouvé de petits précipités / qui se sont formés au cours des essais, les plus longs. Cependant, ce résultat ne gène pas la validité de l'expérience comme c'était le cas précédemment à température ambiante. Les vies en fatigue à 78°K ont été trouvées bien plus longues que celles mesurées précédemment à température ambiante, en dépit du fait que le mécanisme de rupture n'ait pas été affecté sous les déformations élevées et ne l'a été que quelque peu sous les déformations faibles, auxquelles apparait une incidence croissante d'une nucléation et d'une propagation d'une fissure transgranulaire.

     

A robust numerical approach for the simulation of skin–stringer debonding growth in stiffened composite panels under compression

In this paper, a numerical study on skin–stringer debonding growth in stiffened composite panels has been carried out. A novel numerical methodology is proposed here to investigate the compressive behaviour of a stiffened composite panel in the presence of skin–stringer partial separation. The novel numerical methodology, able to overcome the mesh size and time increment dependency of the standard Virtual Crack Closure Technique (VCCT), is an evolution of a previously developed and tested numerical approach for the circular delaminations growth. The enhancements, with respect to the previously developed approach, rely mainly in the capability to deal with the different defect shapes characterising a skin–stringer debonding. The proposed novel methodology has been implemented in a commercial finite element platform and tested over single stiffener composite panels. The effectiveness of the suggested numerical methodology, in predicting the compressive behaviour of stiffened panels with skin stringer debondings, has been preliminary confirmed by comparisons, in terms of load versus applied displacement and debonding size at failure, with literature experimental data and numerical results obtained with the standard VCCT approach.     

A nonlinear double-superposition global–local theory for dynamic buckling of imperfect viscoelastic composite/sandwich plates: A hierarchical constitutive model

Nonlinear dynamic thermo-mechanical buckling and postbuckling analyses of imperfect viscoelastic composite laminated/sandwich plates are performed by a proposed theory that takes into account all the interlaminar kinematic and transverse stress continuity conditions, for the first time. Even the dynamic buckling analysis of the multi-layered/sandwich plates employing the hierarchical constitutive model has not been performed before. The proposed theory is a double-superposition high-order global–local theory that is calibrated based on the nonlinear strain–displacement expressions for the thermoelastic loadings taking into account the structural damping. The buckling loads are determined based on a criterion previously published by the author. Various complex sensitivity analyses evaluating effects of the relaxation parameters, rate of the loading, sudden heating, and pre-stress on thermo-mechanical buckling of the viscoelatic multi-layered/sandwich plates are performed. Results show that the viscoelastic behavior may decrease the buckling load. Sudden dynamic buckling loads are higher due to the reflected stress waves.