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## Publications

Publications (94)

A time domain deconvolution method is developed to allow for continuous and differentiable impulse response function (IRF) constructions and applied force reconstructions. This is a two stage method where (1) the IRF is constructed using measured outputs and applied dynamic loads and (2) a dynamic applied load is reconstructed using measured output...

The delamination resistance of Z-pinned laminates is directly dependent on the strength of the pin–laminate bonding at the interface. In this paper, we investigate novel approaches to the Z-pinning technology in order to increase delamination strength via enhancing mechanical interlocking of the pins. Toward this end, we study the effect of pin ins...

A new method for input signal reconstruction is presented. This approach utilizes the convolution relationship of inputs and outputs of linear systems. A linear discretization of sampled points was assumed in formulating the discrete convolution integral. Subsequently, the resulting equation was modified via a linear constraint to facilitate soluti...

This paper attempts at developing a computational-analytical model to represent the behavior of Z-pin reinforced X-Cor composite sandwich panels under out-of-plane compression and shear loading. Parameters important in representing the behavior of the individual components of the sandwich are identified. The softening of Z-pins under compression fr...

Pin-reinforced sandwich composites have recently attracted the attention in lightweight structural applications where it substantially improves out-of-plane and shear properties for sandwich composites. However, there is not a great deal of understanding in regards to how shaping these composites affects their mechanical performance when the orient...

This work focuses on the development of a hybrid force balance technology that uses piezoelectric sensors in series with conventional strain gauge balance technology to enable broad frequency range force measurements in hypersonic wind tunnel test facilities. Here, sensitivity analyses are carried out to examine the effects of design parameters, in...

Pin reinforced foam cores with composite facesheets offer a viable means of structural construction owing to their low weight and high stiffness and strength. In this paper, various models are introduced to estimate the stiffness and strengths of such a sandwich structure, called K-Cor™ , under out-of-plane compressive loading conditions. Since the...

This research is focused on developing appropriate macro-mechanical models that account for the microstructural details unique to X- and K-cor composite sandwich panels. Digital Image Correlation (DIC) is used to elucidate on the details of the deformation fields within the sandwich structure. These details are used to enhance the models by providi...

A micromechanical model is developed to characterize the constitutive behavior of stretch broken carbon fiber (SBCF) materials. The model takes into account the characteristics of the viscous resin materials and the effect of randomly distributed discontinuous fibers. The predicted force-to-stretch (FTS) values for uniaxial stretching of SBCF prepr...

A combined thermal-structural analysis methodology based on finite element modeling is developed for the analysis of composite structures exposed to high temperatures due to fire. A simplified heat transfer model proposed by Lattimer is adopted for the thermal response model to determine temperature distributions in composite structures with and wi...

The objective of this study is to investigate the applicability of assumed-strain solid-shell finite elements for analysis of gossamer space and near-space structures such as solar sails and scientific balloons. The solid-shell element formulation that alleviates the element locking via the assumed-strain approach has been successfully used for ana...

This article presents a study on composites exposed to elevated temperatures associated with fire. A thermo-viscoplastic model is proposed for description of the rate-dependent and temperature-dependent behavior of composites at elevated temperatures. AS4/PEEK and E-glass/vinylester composites are chosen to examine the proposed model. Thermo-viscop...

The effect of progressive damages in composite materials is incorporated into a geometrically nonlinear solid shell element formulation to investigate the dynamic response of composite structures under the blast wave pressure loading. The effect of material damages such as fiber failures, matrix cracking, and fiber-matrix shearing failures on the b...

The objective of this study is to investigate the applicability of assumed strain formulation solid shell finite elements to the analysis of gossamer structures such as solar sails and scientific balloons. The solid shell element formulation that alleviate the element locking via incorporating the assumed strain approach has been successfully used...

Recently, most of fatigue cracks in ship structures are reported within a few years after delivery. This type of fatigue characteristics cannot be explained adequately by the S-N curve based on high cycle fatigue. Calculation results under critical loading conditions reveal that stress magnitude higher than three times the yield stress occurs at so...

The Koiter’s asymptotic method is combined with the assumed strain solid shell element formulation for postbuckling analysis of composite and sandwich structures. The assumed strain solid shell element is free of locking and the small angle assumption, and it allows multiple plies through the element thickness. While laminated composite structures...

A postprocessing scheme is developed to accurately determine transverse stresses in composite and sandwich panels undergoing geometrically nonlinear deformations. Transverse stresses are recovered at a point of interest via a one-dimensional, equilibrium-based least-square finite element method that utilizes the in-plane stresses and shear forces o...

A geometrically nonlinear assumed strain formulation is introduced in conjunction with bubble function displacements to improve the performance of a nine-node solid shell element. The assumed strain field has been selected to avoid both element locking and undesirable spurious kinematic modes. The results of numerical tests demonstrate that the pre...

A formulation for 36-DOF assumed strain triangular solid shell element is developed for efficient analysis of plates and shells undergoing finite rotations. Higher order deformation modes described by the bubble function displacements are added to the assumed displacement field. The assumed strain field is carefully selected to alleviate locking ef...

Bubble function displacements are used in conjunction with the assumed strain formulation to construct efficient triangular solid shell elements tailored for shell analysis. Two versions of 36-DOF triangular elements are presented with different bubble function displacements and the corresponding assumed strain fields. In the first version the bubb...

A geometrically nonlinear assumed strain formulation in conjunction with bubble function displacements is introduced to improve the performance of a nine-node solid shell element. The assumed strain field has been carefully selected to avoid both element locking and undesirable spurious kinematic modes. The results of numerical tests demonstrate th...

A finite element code is developed for the two-dimensional cure simulation of thick composite structures. In contrast to one or two-dimensional codes based on the finite difference method, the present finite element method can easily model composite structures with arbitrary shapes including the mandrel used for the cure set-up. For the two-dimensi...

Deployable space structures such as sunshields and solar sails are extremely thin and highly flexible with limited bending rigidity. For analytical investigation of their responses during deployment and operation in space, these structures can be modeled as thin shells. The present work examines the applicability of the solid shell element formulat...

Finite element analysis is conducted to investigate the effect of coupling between the stiff facesheets and the flexible
core on the stability of sandwich plates. The facesheets and the core are treated separately as three-dimensional solids.
Initially, linear buckling analysis was carried out to determine the buckling loads for a wide range of the...

An efficient assumed strain triangular solid element is developed for the analysis of plate and shell structures. The finite element formulation is based on the two-field assumed strain formulation with two independent fields of assumed displacement and assumed strain. The assumed strain field is carefully selected to alleviate the shear locking ef...

An efficient assumed strain triangular solid element is developed for the analysis of plate and shell structures. The finite element formulation is based on the two-field assumed strain formulation with two independent fields of assumed displacement and assumed strain. The assumed strain field is carefully selected to alleviate the shear locking ef...

An element-base superconvergent stress recovery technique is developed for accurate boundary stress extraction. In the present method, higher-order stress fields are assumed for all stress components and higher order elements are used for the construction of necessary matrices. Unknown coefficients for the assumed stress field are obtained by minim...

A set of four-node shell element models based on the assumed strain formulation is considered here. The formulation allows for changes in the shell thickness. As a result, the kinematics of deformation are described by purely vectorial variables, without using rotational angles. The present study investigates the use of bubble function displacement...

A set of four-node shell element models based on the assumed strain formulation is considered here. The formulation allows for changes in the shell thickness. As a result, the kinematics of deformation are described by purely vectorial variables, without using rotational angles. The present study investigates the use of bubble function displacement...

A solid shell element model with six degrees of freedom per node is applied to buckling and postbuckling analysis of geometrically nonlinear shell structures. The present model allows changes in the thickness direction and does not require rotational angles or parameters for the description of the kinematics of deformation. The finite element model...

The description of finite rotations of beam-like structures using rotational parameters is not the most efficient, from a computational standpoint, because of the non-vectorial nature of finite rotations in three-dimensional space. In the present study, the classical rigid cross-section assumption is abandoned and the motions of beam directors in t...

A set of four-node solid shell element models based on the assumed strain formulation is considered here. The present study investigates the use of bubble function displacements and the assumed strain field. Careful selection of the assumed strain terms generates an element whose order of numerical integration does not increase even when the bubble...

A stress recovery technique is developed to extract more accurate nodal stress values from the raw stress values obtained directly from the finite element analysis. In the present method a stress field is assumed over a patch of elements, and a least-squares functional is formed using the discrete stress errors at the superconvergent stress points...

Assumed strain formulation for geometrically nonlinear analysis is reexamined with particular attention to recovery of incremental strain and compatibility mismatch. Two approaches considered for strain recovery lead to alternate expressions for compatibility mismatch but the same incremental load vector due to equilibrium imbalance and compatibili...

Delamination suppression by altering the sequence of ply drops is evaluated for tapered glass/epoxy laminates. Two different stacking sequences with thin section layups of [04/±452]s and [±452/04]s containing drops of three sets of ±45-deg plies are investigated. A finite element model using three-dimensional solid elements is constructed to evalua...

The present paper describes an assumed strain finite element model with six degrees of freedom per node designed for geometrically non-linear shell analysis. An important feature of the present paper is the discussion on the spurious kinematic modes and the assumed strain field in the geometrically non-linear setting. The kinematics of deformation...

This study examines the feasibility of using artificial neural network in conjunction with system identification techniques to detect the existence and to identify the characteristics of damage in composite structures. The methodology proposed here includes a training phase and a recognition phase. In the training phase, candidate models for struct...

The stress state and the resulting failure mechanisms of composite tapered components are investigated. Finite-element modeling and experimental evidence summarized from previous and present efforts are used to determine the location of the damage initiation, the interaction between the free edge and the taper discontinuities, the effect of realist...

A new local coordinate system is introduced for the assumed strain formulation so that the resulting shell element can pass both the patch test and the locking test. The coordinate system is tested by implementing it in two nine-node assumed strain shell elements. The elements adopting the new local coordinate system not only pass various patch tes...

A new local coordinate system is introduced for the assumed strain formulation so that the resulting shell element can pass both the patch test and the locking test. The coordinate system is tested by implementing it in two nine-node assumed strain shell elements. The elements adopting the new local coordinate system not only pass various patch tes...

A nonlinear viscoelastic solid model, comprising a combination of linear and nonlinear springs and dashpots, is developed to represent an elastomeric damper. The nonlinear constitutive differential equation obtained from the model completely characterizes the damper behavior. A method is presented to determine the spring-dashpot parameters (coeffic...

The vibration suppression problem is an important aspect of the overall control problem of flexible structures such as rotary and fixed-wing aircraft, as well as spacecraft. Collocated control appears to be suitable for structures with distributed sensors and actuators. However, collocated velocity feedback and collocated positive position feedback...

A finite element model that can accurately represent the warping of composite beams is presented. A p-version formulation for the assumed warping displacement is used, with warping displacements, and higher-order derivatives as nodal degrees of freedom over the cross-section. The arbitrary warping is then superposed over the deformed cross-section....

An assumed strain finite element formulation with a stabilization matrix is developed for analysis of geometrically non-linear problems of isotropic and laminated composite shells. The present formulation utilizes the degenerate solid shell concept and assumes an independent strain as well as displacement. The assumed independent strain field is di...

Two different methods of assuming independent strain fields are examined for the nine node degenerate solid shell element. In the first case, the assumed strain field is chosen for the local orthogonal co-ordinate systems defined at the Gaussian integration points. In the second case, the independent strain is assumed for a local orthogonal co-ordi...

A 32-node three-dimensional solid element and a 16-node degenerate solid element that are applicable to analysis of thin-shell structural problems are presented in this paper. Both elements are formulated by using the Hellinger-Reissner principle with independent strain. The assumed independent strain field is divided into higher-and lower-order po...

The 20node three-dimensional solid element based on the assumed displacement model was compared with the 18node solid element based on the assumed strain model. The kinematic stability associated with different integration rules was investigated for the 20node element.

A mixed finite element formulation with stabilization matrix is presented for geometrically nonlinear thin shells. This formulation is based on the degenerate solid shell element concept and the Hellinger-Reissner principle with independent strain field. The independent strain field is divided into a lower order part and a higher order part. Using...

The delamination of tapered composite laminates with multiple internal ply drop steps is investigated. Both experimental testing of glass-epoxy coupon specimens and finite element modeling of the tapered region are conducted. The average stress concept is applied to out-of-plane stresses from a three-dimensional finite element model. Delamination f...

A finite element model that can be applied to helicopter rotor blades has been developed with a particular emphasis on the proper representation of out-of-plane warping of arbitrary cross-sections. The model can describe accurately coupled bending, torsion and extensional behavior of beams undergoing small strain, moderately large deflection. The m...

A finite element formulation has been developed to take into account the warping effect of composite beams undergoing large deflection or finite rotation. This formulation is to be used to model combined bending, torsional and extensional behaviour of composite helicopter rotor blades. The new approach can model thin to moderately thick walled comp...

An 18-node solid element has been developed to model the behavior of laminated composite shells undergoing large deflection. The element formulation utilizes independently assumed strain in addition to assumed displacement. The strain and the determinant of the Jacobian matrix are assumed to be linear in the thickness direction. This allows analyti...

A nine node shell element based on the assumed displacement formulation and an eight node shell element based on a modified version of the Hellinger-Reissner principle with assumed linear transverse shear strain are combined in a ‘checkerboard’ pattern in order to eliminate locking while suppressing adverse kinematic modes. Simple 2 × 2 × 2 point i...

A finite-element formulation has been developed to take into account the warping effect of composite beams. This formulation is to be used to model combined bending, torsional, and extensional behavior of composite beams. The new approach can model thin-walled beams with complicated cross sections, tapers, and arbitrary planforms. The strain is ass...

A nine node finite element is presented for the analysis of thin shell structures undergoing large deflection. The finite element formulation is based on the concept of degenerate solid shell element and the Hellinger-Reissner principle with independent strain. Three versions of assumed independent strain are selected to suppress spurious kinematic...

An eighteen-node, three-dimensional, solid element with 54 degrees of freedom is presented for the finite element analysis of thin plates and shells. The element is based on the Hellinger-Reissner principle with independent strain. The assumed independent strain is divided into higher and lower terms. The stiffness matrix associated with the higher...

A nine node finite element model has been developed for analysis of geometrically non-linear laminated composite shells. The formulation is based on the degenerate solid shell concept and utilizes a set of assumed strain fields as well as assumed displacement Two different local orthogonal co-ordinate systems were used to maintain invariance of the...

A sixteen node shell element is developed using a matrix stabilization scheme based on the Hellinger-Reissner principle with independent strain. Initially the assumed independent strain is divided into a lower order part and a higher order part. The stiffness matrix corresponding to the lower order assumed strain is equivalent to the stiffness matr...

A finite element approach has been developed to accurately model coupled bending, torsion and extensional behavior of beams undergoing large deflection or finite rotation. The finite element model can handle beams with arbitrary cross-sections, taper, pretwist and planform.

A nine node finite element is developed for the analysis of thin shell structures undergoing large deflection. The finite element formulation is based on the concept of degenerate solid shell element and the Hellinger—Reissner principle with independent strain. Two versions of assumed independent strain are selected to suppress spurious kinematic m...

An eighteen-node, three-dimensional, solid element is presented for the finite element analysis of thin shell structures. The element is based on the Hellinger-Reissner principle with assumed independent strain divided into higher and lower order terms. The stiffness matrix associated with the higher order independent strain plays the role of a sta...

A new finite element for modeling fiber-reinforced composite plates and shells is developed and its performance for static linear problems is evaluated. The element is a nine-node degenerate solid shell element based on a modified Hellinger-Reissner principle with independent inplane and transverse shear strains. Several numerical examples are solv...

As a first step toward developing a finite element formulation that can model coupling among extensional, bending and torsional behaviour of beams, a new method is proposed to properly represent the warping of arbitrary cross-sections. The basic approach is to introduce a small warping displacement superimposed over flat cross-sections of a shear-f...

A finite element modeling and solution technique capable of determining the time response of unrestrained flexible structures which are undergoing large elastic deformations coupled with gross nonsteady translational and rotational motions with respect to an inertial reference frame has been developed. The governing equations of motion are derived...

A new mixed finite element formulation is developed based on the Hellinger-Reissner principle with independent strain. By dividing the assumed strain into its lower order and higher order parts, the new formulation can be made much more efficient than the conventional mixed formulation. In addition the present new approach provides an alternative w...

A nine-node element, designated as SHEL9, has been developed for analysis of thin plates and shells. The element formulation is based on the degenerate solid shell concept and a modified Hellinger-Reissner principle with independent in-plane and transverse shear strains. Numerical tests indicate that the present SHEL9 element with uniform 3 × 3 poi...

A finite element modelling technique which utilizes a triangular element with 45 degrees-of-freedom and seven-point integration has been tested for analysis of thin plate and shell structures. The element is based on the degenerate solid shell concept and the mixed formulation with assumed independent inplane and transverse shear strains. Numerical...

A six-node plate bending element has been developed by employing mixed formulation based on a modified Hellinger–Reissner principle and the Reissner–Mindlin plate bending theory. The numerical result indicates that, among the types of assumed independent transverse shear strains considered, a combination of 2 version with either 5 version or 6 vers...

A finite element modeling technique which utilizes different elements for the boundary region and the interior region is tested for plate bending analysis. Numerical results indicate that model types designated as TYPE 1, TYPE 2 and TYPE 3 do not exhibit detrimental locking effect even for very thin plate situations. For extremely weakly constraine...

The analytical study of the nature of interlaminar stresses near stress free edges has been the subject of substantial research interest. Such studies are significant because the high interlaminar stresses or stress singularities near stress-free edges may cause delamination failure as shown by experimental investigations. Also the accurate predict...

An eight node curved thin shell slement was tested. The element is based on the degenerate solid concept and the mixed formulation with the independent inplane and transverse shear strains. The number of unknown parameters in the assumed strains is chosen to alleviate the spurious constaining or locking effect. It is indicated that for a pinched cy...

Two new finite elements are developed for the Mindlin theory plate bending problem. The formulation is based on the modified Hellinger-Reissner principle with independent transverse shear strains. Numerical examples indicate that, with properly assumed transverse shear strains, these new elements designated as PLAT8 and PLAT8H do not exhibit lockin...

Assumed stress hybrid finite element methods are developed for linear fracture analysis of three dimensional solids and for fracture of metals under high temperature creep conditions. For the 3-D fracture analysis a series of special elements with hexagonal shape have been derived. These elements include the asymptotic singular behavior at the crac...

Thesis. 1978. Ph.D.--Massachusetts Institute of Technology. Dept. of Aeronautics and Astronautics. MICROFICHE COPY AVAILABLE IN ARCHIVES AND AERONAUTICS. Vita. Includes bibliographical references. Ph.D.

Mixed formulations are introduced as a means for reducing severe constraints in finite-element derivations. For plate bending elements to include transverse shear effect and to be applicable also to thin plates, the method can reduce the conditions of constraints of zero transverse shear strain energy. For shell elements, the constraints of rigid-b...