Airy Stress Function Research Articles

Porosity effects on post-buckling behavior of geometrically imperfect metal foam doubly-curved shells with stiffeners

This papers studies nonlinear stability and post-buckling behaviors of geometrically imperfect metal foam doubly-curved shells with eccentrically stiffeners resting on elastic foundation. Metal foam is considered as porous material with uniform and non-uniform models. The doubly-curved porous shell is subjected to in-plane compressive loads as well as a transverse pressure leading to post-critical stability in nonlinear regime. The nonlinear governing equations are analytically solved with the help of Airy stress function to obtain the post-buckling load-deflection curves of the geometrically imperfect metal foam doubly-curved shell. Obtained results indicate the significance of porosity distribution, geometrical imperfection, foundation factors, stiffeners and geometrical parameters on post-buckling characteristics of porous doubly-curved shells.

Stress Analysis by Two Cuboid Isoparametric Elements

The finite element method is a powerful tool for solving most of the structural problems. This technique has been used extensively, since the complexity of the elastic field equations does not allow the specialist to find analytical solutions, especially for the three-dimensional structures. It is well-known that the finite element formulation yields the approximate stress responses. To remedy this defect, the Airy stress function is utilized in this study. The stress function formulation leads to a valid solution since it satisfies equilibrium and compatibility equations simultaneously. Two cuboid isoparametric elements are formulated for solving three-dimensional elastic structures. To demonstrate the performance of the proposed technique, various benchmark problems are analyzed. The errors between the exact, displacement-based finite element and recommended scheme solution are also calculated. All the obtained outcomes show the good merit of the presented new elements.

Hybrid stress analysis of a near-surface circular hole in finite structures

The ability to stress-analyze complicated structures from recorded load-induced temperatures is demonstrated. The considered structures have a near-surface hole and subjected to a concentrated load. The complexity of the structure is simplified by conformal mapping, the traction-free condition on the boundary of the hole is analytically satisfied by analytic continuation, and the equilibrium and compatibility conditions are satisfied by means of Airy stress function in complex-variable formulation. For isotropic member that is cyclically loaded within its elastic range, the produced in-phase temperature variations are linearly proportional to the local changes in the normal stresses. Even though no recorded thermal data were used at or near to the edges, the present hybrid method simultaneously separates the load-induced temperatures into the individual stress components, determines reliably the boundary stress and hence the stress concentration, and smooths the measured input data. Unlike prior capabilities of using geometrical symmetry to simply the stress function representation, the present analysis retains all the terms in the stress functions. Therefore, the considered hybrid stress analysis approach of such complex structures extends significantly the applicability of thermoelastic stress analysis compared to prior capabilities and is considered to be the most complicated formulation of the hybrid complex-variable method to date. To support the reliability of the present hybrid method, the results were compared with finite element predictions and previous results based on Mitchell solution.

Flamant solution of a half-plane with surface flexural resistibility and its applications to nanocontact mechanics

This article presents a semianalytical solution to a half-plane contact problem subjected to an arbitrarily distributed surface traction. The half-plane boundary is treated as a material surface of the Steigmann–Ogden type. Under the assumption of plane strain condition, the problem is formulated by coupling the methods of an Airy stress function and Fourier integral transforms. Stresses and displacements in the form of semi-infinite integrals are derived. A non-classical Flamant solution that is able to simultaneously account for the surface tension, membrane stiffness, and bending rigidity of the half-plane boundary is derived through limit analysis on the half-plane contact problem owing to a uniform surface traction. The fundamental Flamant solution is further integrated for tackling two half-plane contact problems owing to classical contact pressures corresponding to a rigid cylindrical roller and a rigid flat-ended punch. The resultant semi-infinite integrals are integrated by the joint use of the Gauss–Legendre numerical quadrature and the Euler transformation algorithm. Extensive parametric studies are conducted for comparing and contrasting the effects of Gurtin–Murdoch and Steigmann–Ogden surface mechanical models. The major observations and conclusions are two-fold. First, the introduction of either surface mechanical model results in size-dependent elastic fields. Second, the incorporation of the curvature-dependent nature of the half-plane boundary leads to bounded stresses and displacements in the fundamental Flamant solution. This is in contrast to the otherwise singular classical and Gurtin–Murdoch solutions. For all four case studies, the Steigmann–Ogden surface model also results in much smoother displacement and stress variations, indicating the significance of surface bending rigidity in nanoscale contact problems.

An analytical solution for analysis of toppling-slumping failure in rock slopes

When a slope consists of a horizontal weak rock or soil overlaid by some vertical strong slender rock columns, the slope is prone to a secondary toppling failure which is known as “toppling-slumping” failure. In the slope, rock columns impose pressure on the lower continuous weak rock or soil mass which leads to a differential settlement at the base of each rock column or a circular shear failure in the continuous mass. If the differential settlements reach a special threshold, rock column overturn and toppling-slumping will happen. However, the circular shear failure of the soil or weak rock mass which is a well-recognized failure mechanism will occur when shear stress in the slope exceeds its shear strength. In this paper, an attempt has been made to better understanding of the “toppling-slumping” failure mechanism through developing an analytical solution. For this purpose, first an Airy stress function is acquired for the lower continuous soil mass and its stress distribution and displacement are accordingly determined analytically due to the overlaid rock columns loads. Then, differential settlements at the base of all rock columns and shear stresses in the soil or weak rock mass are computed and compared with the toppling and shear failure criteria to assess slope failure. Finally, several design-charts are developed that determine the stability of a slope against toppling-slumping failure mechanism based on geometric characteristics of the slope and shear strength of its constituent material. Also, a Mathematica package code was developed to evaluate the stress distribution within the slope, and the safety factor. The results show that the vertical stress distribution plays a significant role in the settlement of the rock columns. Results also manifested that at a certain distance from the slope crest, the set of rock columns will act like a single column on a half-plane. The shear or toppling failure of the slope not only depends on the material's strength but also on the geometry and the normalized column distance.

Harmonic resonances of graphene-reinforced nonlinear cylindrical shells: effects of spinning motion and thermal environment

This work investigates nonlinear harmonic resonance behaviors of graded graphene-reinforced composite spinning thin cylindrical shells subjected to a thermal load and an external excitation. The volume fraction of graphene platelets varies continuously in the shell’s thickness direction, which generates position-dependent useful material properties. Natural frequencies of shell traveling waves are derived by considering influences of the initial hoop tension, centrifugal and Coriolis forces, thermal expansion deformation, and thermal conductivity. A new Airy stress function is introduced. Harmonic resonance behaviors and their stable solutions for the spinning cylindrical shell are analyzed based on an equation of motion which is established by adopting Donnell’s nonlinear shell theory. The necessary and sufficient conditions for the existence of the subharmonic resonance of the spinning composite cylindrical shell are given. Besides the shell’s intrinsic structural damping, the Coriolis effect due to the spinning motion has a contribution to the damping terms of the system as well. Comparisons between the present analytical results and those in other papers are made to validate the existing solutions. Influences of main factors on vibration characteristics, primary resonance, and subharmonic resonance behaviors of the novel composite cylindrical shell are discussed. Furthermore, the mechanism of how the spinning motion affects the amplitude–frequency curves of harmonic resonances of the cylindrical shell is analyzed.

Experimental and numerical investigation of stiffener effects on buckling strength of composite laminates with circular cutout

In the notched structures, to achieve maximum buckling resistance in comparison with structural weight, the optimal design of a stiffener is very important. In this research, after a review of the existing literature, nonlinear buckling behavior of composite plates containing the cutout with three different designs of stringer was investigated. The considered stiffeners are planer, longitudinal, and ring types. The buckling experiments were carried out on the stiffened plates containing a circular notch. Moreover, to achieve an efficient prediction of the buckling in the stiffened laminate with the hole, a finite strip method is developed based on the Airy stress function and von Karman’s large deformation equations. Studies show that there is a good agreement between the postbuckling behaviors derived from developed finite strip method with experimental results. Fast convergence of the considered finite strip method compared with the finite element results shows its efficiency for prediction of buckling behavior in laminated composites. The results show that the buckling load-bearing capacities of perforated plates with a longitudinal and planer stiffener are higher compared with the other stiffener, respectively. The detailed parametric study on the effects of thickness of the plate and stiffener and opening diameter on buckling behavior was performed using experiments and modeling.

Nonlinear vibration of FGM moderately thick toroidal shell segment within the framework of Reddy’s third order-shear deformation shell theory

Nonlinear vibration and dynamic response of functionally graded moderately thick toroidal shell segments resting on Pasternak type elastic foundation are investigated in this paper. Functionally graded materials are made from ceramic and metal, and the volume fraction of constituents are assumed to vary through the thickness direction according to a power law function. Reddy’s third order shear deformation, von Karman nonlinearity, Airy stress function method and analytical solutions are used to derive the governing equations. Galerkin method is used to convert the governing equation into nonlinear differential equation, then the explicit expressions of natural frequencies and nonlinear frequency–amplitude relations are obtained. Using Runge–Kutta method, the nonlinear differential equation of motion is solved, and then nonlinear vibration and dynamic response of shells are analyzed. The effects of temperature, material and geometrical properties, and foundation parameters on nonlinear vibration and dynamic characteristics are investigated and discussed in detail.

An analytical solution for nonlinear dynamic response and vibration of FG‐CNT reinforced nanocomposite elliptical cylindrical shells resting on elastic foundations

AbstractThe present study deals with the nonlinear dynamic response and vibration of functionally graded carbon nanotubes (FG‐CNT) reinforced composite elliptical cylindrical shells resting on elastic foundations in thermal environments using an analytical solution. The nanocomposite elliptical cylindrical shells are reinforced by single‐walled carbon nanotubes. Based on the classical shell theory with the geometrical nonlinearity in von Kármán and the nonlinear dynamic response and vibration results are obtained by using the Airy stress function, Galerkin method, and Runge‐Kutta method. In numerical results, the effect of geometrical parameters, initial geometric imperfection, Winkler and Pasternak elastic foundations and volume fractions and types of distribution of CNTs on the nonlinear dynamic response and vibration of FG‐CNT reinforced composite elliptical cylindrical shells are investigated.

A novel boundary element formulation for anisotropic fracture mechanics

A novel boundary element formulation for two-dimensional fracture mechanics is presented in this work. The formulation is based on the derivation of a supplementary boundary integral equation to be used in combination with the classic displacement boundary integral equation to solve anisotropic fracture mechanics problems via a single-region approach. The formulation is built starting from the observation that the displacement field for an anisotropic domain can be represented as the superposition of a vector field, whose components satisfy a suitably defined anisotropic Laplace equation, and the gradient of the Airy stress function. The supplementary boundary integral equation is then obtained using such representation into the integral expression of the aforementioned Laplace equation and employing the relationship between the stress function gradient and the boundary tractions. The supplementary equation neither requires the computation of hyper-singular integrals nor does it introduce additional variables for the problem, as it involves boundary displacements and tractions only. Numerical results are obtained for both uncracked and cracked bodies and show the accuracy and potential of the proposed approach.

Efficient post-buckling analysis of variable-stiffness plates using a perturbation approach

The present work discusses the development of a formulation for efficiently analyzing variable-stiffness plates operating in the post-buckling regime. The approach relies upon the combined use of a single-mode Koiter's perturbation strategy along with a mixed variational formulation expressed in terms of Airy stress function and out-of-plane displacement, where the unknowns are approximated using global trial functions based on Legendre polynomials. A highly efficient numerical tool is achieved, which allows the initial post-buckling field to be analyzed with the ease of closed-form solutions, but a much wider field of employ in terms of elastic couplings, boundary and loading conditions. The quality of the predictions is illustrated by means of a comprehensive set of comparisons against results from the literature. Possible applications of the approach are shown, where the exploration of the design space offered by curvilinear fibers requires thousands of non-linear post-buckling analyses to be run.

Three stress-based triangular elements

To obtain proper stresses , three new triangular elements are formulated in this study. First, a complementary energy functional is used within an element for the analysis of plane problems. In this energy manifestation, the Airy stress function will be applied as a functional variable. Then, some basic analytical solutions are assigned for the stress functions. These trial functions are matched with each element number of degrees of freedom. The result is a number of equations with anonymous constants. Subsequently, according to the principle of minimum complementary energy, the unknown constants can be expressed in terms of displacements. Finally, this system can be rewritten in terms of the nodal displacement. In this way, three new triangular elements are formulated. To validate the results, extensive numerical studies are performed. The findings clearly demonstrate accuracies of structural displacements as well as stresses.

Robust adaptive fuzzy sliding mode control of nonlinear uncertain MIMO fluttering FGP plate based on feedback linearization

In this study using an adaptive fuzzy sliding mode control (AFSMC) scheme, the robust stabilization of multi-input–multi-output (MIMO) nonlinear aero-elastic fluttering of the Functionally Graded Piezoelectric (FGP) plate in the presence of mismatched time-varying uncertainties have been investigated. It is assumed that the aerodynamic load is modeled by the first order piston theory and the piezoelectric patches are assumed to be bonded to the top and bottom surfaces of the plate in order to produce the controlling bending moment excitations. Using the airy stress function and applying the Hamilton's principle the governing coupled partial differential equations of motion are derived. Then considering the immovable simply supported edges boundary conditions and employing the aero-elastic multi-mode interactions and applying the Galerkin's method, the nonlinear coupled partial differential equations of motion are reduced to nonlinear ordinary differential equations in time. Then using the full state input-output feedback linearization technique, the nonlinear dynamics of the model is linearized and transformed into the multiple decupled single input-single output uncertain subsystems. In order to overcome the chattering phenomenon arises due to the sliding mode control (SMC) discontinuous inputs, a hybrid adaptive fuzzy sliding mode control technique is utilized to approximate the discontinuous synthetic control inputs. It is showed that considering the physical input limitations, the designed AFSMC control system, effectively suppress the fluttering motions in presence of the bounded external inaccuracies and it prevents the unwanted chattering of the subsystems inputs.

A correspondence between compressible and incompressible plane elastostatics

It is shown that a heretofore seemingly unnoticed correspondence (or analogy) exists between the traction boundary value problem for compressible media and the displacement boundary value problem for incompressible media occupying the same domain, in plane, isotropic, linear elastostatics. The Airy stress function, which satisfies equilibrium identically, has to be biharmonic in order to satisfy the compatibility condition in a compressible body. Correspondingly, a displacement potential function, which satisfies the incompressibility condition identically, has to be biharmonic in order to satisfy equilibrium. Since Stokes flow is governed by identical relations as incompressible plane elasticity, if displacement is interpreted as velocity and the shear modulus as dynamic viscosity, the correspondence extends to that between compressible elasticity and Stokes flow for boundary value problems indicated above. This analogy provides a rare example of a constrained system which is equivalent to the same system without the constraint.

Two-dimensional fretting contact analysis considering surface effects

This study reports a two-dimensional size-dependent fretting contact model in which a rigid cylinder oscillatingly slides on an elastic half-plane with both normal and oscillatory tangential loads. The effects of the surface energy density on size-dependent contact behaviors are described by Chen-Yao's surface elastic theory. The Airy stress function is used in conjunction with Fourier transform to derive the general governing equations that describe frictional contact. Finally, an effective numerical iterative method is adopted to solve the fretting problem. The combined effects of the surface energy density and friction coefficient on the fretting contact behavior are highlighted. The numerical results obtained from the present size-dependent model are very different from those predicted by classical fretting models without surface effects. The results of this study imply that the fretting behaviors of nanostructures may result in inaccurate predictions when the surface effects are ignored.

Electromechanical Instability of Dielectric Elastomer Actuators With Active and Inactive Electric Regions

Electrically driven dielectric elastomers (DEs) suffer from an electromechanical instability (EMI) when the applied potential difference reaches a critical value. A majority of the past investigations address the mechanics of this operational instability by restricting the kinematics to homogeneous deformations. However, a DE membrane comprising both active and inactive electric regions undergoes inhomogeneous deformation, thus necessitating the solution of a complex boundary value problem. This paper reports the numerical and experimental investigation of such DE actuators with a particular emphasis on the EMI in quasistatic mode of actuation. The numerical simulations are performed using an in-house finite element framework developed based on the field theory of deformable dielectrics. Experiments are performed on the commercially available acrylic elastomer (VHB 4910) at varying levels of prestretch and proportions of the active to inactive areas. In particular, two salient features associated with the electromechanical response are addressed: the effect of the flexible boundary constraint and the locus of the dielectric breakdown point. To highlight the influence of the flexible boundary constraint, the estimates of the threshold value of potential difference on the onset of electromechanical instability are compared with the experimental observations and with those obtained using the lumped parameter models reported previously. Additionally, a locus of localized thinning, near the boundary of the active electric region, is identified using the numerical simulations and ascertained through the experimental observations. Finally, an approach based on the Airy stress function is suggested to justify the phenomenon of localized thinning leading to the dielectric breakdown.

Modeling of subcritical cracking in acidized carbonate rocks via coupled chemo-elasticity

Injection of acidized water into the crack process area has been known for long to induce subcritical crack propagation. Acid rain or vegetation decomposition can cause activation or reactivation of landslides at subcritical stress, while acid injection is used to enhance oil and gas reservoir stimulation. Equally promising is acid enhancement of crack propagation in geothermal field stimulation. To quantify the above mentioned processes, crack propagation is simulated using a chemo-elasticity model in which a chemically controlled strain is added to linear elastic strain, in analogy to thermal strain. That strain is proportional to accumulated mass removal, controlled by a chemical reaction. The coefficient of chemical shrinkage is either constant, or is postulated to depend on the level of straining of rock using various scalar measures of either volumetric or shear strain to simulate the dependence of mineral dissolution on micro-cracking. With that formulation an Airy stress function, as in classical linear elasticity solution, has been for the first time, to our knowledge, used for solving chemo-elasticity problem, by employing an additional particular potential term. The results suggest that a constant chemically induced shrinkage alone is not sufficient to produce any significant enhancement of the crack tip displacement. However, making the coefficient of chemical shrinkage dependent on a shear strain invariant (and hence making the constitutive law implicit), leads to an enhancement of crack propagation velocity at a desired level. Yet, the obtained crack propagation acceleration appears to be too modest. That suggests that to obtain realistic results, in addition to diffusive transport, the advective–reactive transport process (especially, permeability) should be as well coupled to deformation.

Effects of porosity and thermomechanical loading on free vibration and nonlinear dynamic response of functionally graded sandwich shells with double curvature

Resume The fundamental frequencies and nonlinear dynamic responses of functionally graded sandwich shells with double curvature under the influence of thermomechanical loadings and porosities are investigated in this study. Two material models are considered. The continuity requirement of material properties throughout layers are fulfilled by newly introducing refined effects of two porosity types regarding the average of constituent properties weighted by the porosity volume fraction. The first-order shear deformation theory taking the out-of-plane shear deformation into account is employed to obtain the Lagrange equation of motions. The number of primary variables reduces from five to three after introducing the Airy stress function. The system of dynamic governing equations is obtained by utilizing the Bubnov–Galerkin procedure. The natural frequencies are analytically computed by solving eigenvalue problems, and the fundamental frequencies are acquired by further assumptions about the inertial force caused by the shell rotation variables. The nonlinear dynamic responses of the functionally graded spherical, cylindrical, and hyperbolic paraboloid shells under the influence of different geometry configurations, loading conditions, and porosity types and degrees are obtained by applying the fourth-order Runge–Kutta method. The numerical results are presented and verified with available studies in the literature. Although porosities are usually considered material defects weakening the structure performance, this study has proved clearly that porosities stiffen the shell structures to some extent.

A displacement-based approach to geometric instabilities of a film on a substrate

When a thin film adhered to a compliant substrate is growing, it will eventually buckle in order to release the compressive stresses accumulated within the film due to growth. Such geometric instabilities caused by compressive stresses prevail among all living systems in nature and their outcomes range from highly beneficial to destructive. Therefore, understanding compression induced instabilities is of crucial importance. Note that the origin of the “compression” need not necessarily be differential growth, as it may be due to pre-stretch or thermal expansion. A commonly accepted solution strategy for instabilities in bilayer structures dates back to the seminal work of Allen and employs the Airy stress functions. Owing to its reliance on a stress-based approach, the Allen solution is limited to linear two-dimensional problems and its success depends entirely on choosing an appropriate Airy function. The main objective of this contribution is to circumvent these limitations via a displacement-based approach formally suitable for three-dimensional problems, anisotropic materials, and even applicable to finite deformations. Furthermore, the Allen solution in its original form is valid for the plane-stress condition but often it is mistakenly compared with the numerical simulations corresponding to the plane-strain condition. We analyze the subtle difference between the solutions associated with the plane-strain and plane-stress conditions. Next, the analytical solution is compared against the computational results using the finite element method via eigenvalue analysis. Finally, it is briefly explained how the current approach can be utilized beyond the classical bilayer systems.

Nonlinear Dynamical Responses of Rotary Cylindrical Shells with Internal Resonance

The nonlinear forced vibration response of a thin, elastic, rotary cylindrical shell to a harmonic excitation is investigated in this study. Nonlinearities due to the large-amplitude shell motion are considered by using Donnell’s nonlinear shallow-shell theory, with consideration of the effect of viscous structural damping. Different from the conventional Donnell’s nonlinear shallow-shell equations, an improved nonlinear model without employing the Airy stress function is utilized to study the nonlinear dynamics of thin shells. The system is discretized using the Galerkin method, while a model involving two degrees of freedom and allowing for the traveling wave response of the shell is adopted. The method of harmonic balance is applied to study the nonlinear dynamic responses of the two-degree-of-freedom system. In addition, the stability of steady-state solutions is analyzed in detail. Finally, results are given for exploring the effects of different parameters on the nonlinear dynamic response with internal resonance.