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RETRACTED ARTICLE: A limit analysis of Mindlin plates using the cellbased smoothed triangular element CSMIN3 and secondorder cone programming (SOCP)
Asia Pacific Journal on Computational Engineering volume 1, Article number: 6 (2014)
The Retraction Note to this article has been published in Asia Pacific Journal on Computational Engineering 2015 2:1
Abstract
Background
The paper presents a numerical procedure for kinematic limit analysis of Mindlin plate governed by von Mises criterion.
Methods
The cellbased smoothed threenode Mindlin plate element (CSMIN3) is combined with a secondorder cone optimization programming (SOCP) to determine the upper bound limit load of the Mindlin plates. In the CSMIN3, each triangular element will be divided into three subtriangles, and in each subtriangle, the gradient matrices of MIN3 is used to compute the strain rates. Then the gradient smoothing technique on whole the triangular element is used to smooth the strain rates on these three subtriangles. The limit analysis problem of Mindlin plates is formulated by minimizing the dissipation power subjected to a set of constraints of boundary conditions and unitary external work. For Mindlin plates, the dissipation power is computed on both the middle plane and thickness of the plate. This minimization problem then can be transformed into a form suitable for the optimum solution using the SOCP.
Results and Conclusions
The numerical results of some benchmark problems show that the proposal procedure can provide the reliable upper bound collapse multipliers for both thick and thin plates.
Background
Limit analysis is a branch of plasticity analysis and plays an important role in determining the limit loads of a structure. The fundamental theorems of limit analysis ignore the evolutive elastoplastic computations but focus to determine the upper or lower bound loads which cause the plastic collapse of structures.
Using analytical methods and different yield criteria such as the maximum principal stress criterion, Tresca criterion, and von Mises criterion, many scholars derived the analytical solutions for the limit loads of plates. Some systematic and comprehensive summaries can be found in the monographs of Hodge [1], Save and Massonnet [2], Zyczkowski [3], Xu and Liu [4], Lubliner [5], Yu et al. [6], etc. Using numerical methods, some early works for the limit loads of plates can be mentioned such as those by Hodge and Belytschko [7] and Nguyen [8]. However, due to the lack of efficient optimization algorithms and the limit of the computing power, the numerical limit analysis of plates seems to be ignored for a certain times.
Recently, the interest of scientists in numerical limit analysis [9–15] has been resurged, principally thanks to the rapid development of efficient optimization algorithms and the continuous improvement in computer facilities. Current research is focusing on developing numerical limit analysis tools which are efficient and robust for the practice usage of engineers. In the numerical limit analysis, once the stress or displacement/velocity fields are approximated and the bound theorems are applied, the limit analysis becomes a problem of optimization involving either linear programing (LP) or nonlinear programming (NLP) which can be solved respectively by the available LP or NLP algorithms [16–23].
For the LP algorithms, some significant contributions have been published such as the active set LP algorithm by Sloan [16], the bespoke interiorpoint algorithm for LP by Andersen and Christiansen [17], and the commercial LP code XA by Pastor et al. [18]. For the NLP algorithms, some recently important contributions can be mentioned such as the algorithm based on feasible directions by Zouain et al. [10] or by Lyamin and Sloan [19], the algorithm based on the interiorpoint method by Andersen et al. [20] or by Krabbenhoft and Damkilde [21], and the generalpurpose NLP codes CONOPT and MINOS by TinLoi and Ngo [22]. Recently, one of the most efficient NLP algorithms based on the primaldual interiorpoint method was proposed by Andersen et al. [23]. The algorithm can be applied to von Misestype yield functions and can handle problems with any nonlinear yield functions. The algorithm is implemented in secondorder cone programming (SOCP) [24] of the commercial software MOSEK [25] and has been applied for the limit loads of some limit analysis problems [26, 27].
Using such LP and NLP algorithms for the numerical limit analyses of plate structures, many significant researches have been published. For the Kirchhoff plates, we can list the works by Christiansen and Larsen [28], Turco and Caracciolo [29], Corradi and Vena [30], Corradi and Panzeri [31], Tran et al. [32], Le et al. [33–35], and Zhou et al. [36]. For the Mindlin plates, we can list the works by Capsoni and Corradi [37] and Capsoni and Vicente da Silva [38]. In comparison, it is seen that many studies in the literature are concerned with the limit analysis of Kirchhoff plates, while the literature related to those of Mindlin plates is somehow still limited. This paper hence aims to further contribute a numerical limit analysis of Mindlin plates by using a Mindlin plate element proposed recently together with the SOCP.
In the other front of the development of numerical methods, Liu and Nguyen Thoi [39] have integrated the strain smoothing technique [40] into the finite element method (FEM) to create a series of smoothed FEMs (SFEMs) such as cell/elementbased smoothed FEM (CSFEM) [41–43], nodebased smoothed FEM (NSFEM) [44–46], edgebased smoothed FEM (ESFEM) [47, 48], facebased smoothed FEM (FSFEM) [49], and a group of alphaFEM [50–53]. Each of these smoothed FEMs has different properties and has been used to produce desired solutions for a wide class of benchmark and practical mechanics problems. Several theoretical aspects of the SFEM models have been provided in [54, 55]. The SFEM models have also been further investigated and applied to various problems such as plates and shells [56–68], piezoelectricity [69, 70], fracture mechanics [71], viscoelastoplasticity [72–74], limit and shakedown analysis for solids [75–77], and some other applications [78, 79].
Among these SFEM models, the CSFEM [39, 41] shows some interesting properties in solid mechanics problems. Extending the idea of the CSFEM to plate structures, NguyenThoi et al. [80] have recently formulated a cellbased smoothed threenode Mindlin plate element (CSMIN3) for static and free vibration analyses of isotropic Mindlin plates by incorporating the CSFEM with the original MIN3 element [81]. In the CSMIN3, each triangular element will be divided into three subtriangles, and in each subtriangle, the MIN3 is used to compute the strains. Then, the strain smoothing technique on whole the triangular element is used to smooth the strains on these three subtriangles. The numerical results showed that the CSMIN3 is free of shear locking and achieves high accuracy compared to the exact solutions and other existing elements in the literature.
In this paper, the CSMIN3 is further extended to the kinematic limit analysis of Mindlin plates governed by the von Mises criterion. The CSMIN3 is combined with a SOCP to determine the limit load of the plates. The limit analysis problem of Mindlin plates is formulated by minimizing the dissipation power subjected to a set of constraints of boundary conditions and unitary external work. For Mindlin plates, the dissipation power is computed on both the middle plane and the thickness of the plate. This minimization problem can then be transformed into a form suitable for the optimum solution using the SOCP. The accuracy and reliability of the proposed method are verified by comparing its numerical solutions with those of other available numerical results.
Methods
Limit analysis of Mindlin plateskinematic formulation
We now consider a rigidperfectly plastic plate identified by its middle plane Ω, the thickness h, and the boundary Γ = Γ_{ u } ∪ Γ_{ t } where Γ_{ t } is the boundary subjected to a surface traction and Γ_{ u } is the constrained boundary. Let w be the transverse displacement (deflection) and be the vector of rotations, in which β _{ x } and β _{ y } are the rotations of the middle plane around the yaxis and xaxis, respectively, with the positive directions defined as shown in Figure 1.
The unknown vector of three independent field variables at any point in the problem domain of the Mindlin plates can be written as
The curvature of the deflected plate κ and the shear strains γ are defined, respectively, as
where and L _{ d } is a differential operator matrix defined by
Because the material in the limit analysis is assumed to be rigidperfectly plastic, the bending stress σ and transverse shear τ are confined within the convex domain Φ(σ, τ) ≤ 0, where Φ(σ, τ) is the yield function. If von Mises's criterion is adopted, we have
where σ _{0} is the yield stress and
Deformations cannot occur as long as Φ(σ, τ) < 0, while plastic flow may develop when Φ(σ, τ) = 0. In this case, strain rates and obey the normality flow rule as
Equation 6 might impose restrictions on strain rates, by confining them within a convex domain , the subspace spanned by the outward normals to the yield surface.
Let represent the admissible stresses contained within the convex yield surface and represent the stress point on the limit surface associated to any given strain rate through the plasticity condition, then the plastic dissipation power (per unit volume) is defined by Hill's maximum principle as
The plastic dissipation power is a uniquely defined function of strain rates, and its explicit expression is available for a number of yield criteria [5]. If von Mises's criterion is adopted, one has [37]
with
Using the relation ϵ = z κ between the membrane strain ϵ with the curvature κ, Equation 8 can be rewritten as
The internal dissipation power for the twodimensional plate domain Ω with the thickness h is now expressed as
The limit analysis of the Mindlin plate considers a rigidperfectly plastic plate subjected to body forces on its middle plane Ω and to surface tractions on the boundary Γ_{ t }. The constrained boundary Γ_{ u } is fixed. Loads now are defined as basic values b and , affected by a load multiplier λ. Then, the kinematic theorem of limit analysis states that the limit value λ ^{+} (collapse multiplier) of λ is the optimal value of the minimization problem [37]
subject to
Equations 13(a) and 13(b) express the compatibility of the constrained boundary and the strain rate with a velocity field , respectively, and Equation 13(c) denotes the power of basic loads, which is normalized to unity.
Note that in Equation 11, the dissipation power is a positively homogeneous function of degree 1 in the strain rates and not differentiable at strain rate zero. Equations 13(a) to 13(c) hence bring the computation of the collapse multiplier to the search of the minimum of a convex but not everywhere differentiable functional. The functional minimized in Equation 12 is only differentiable in the region Ω_{ p } where plastic flow develops, but not so in the remaining portion Ω_{ r } of the plate, which keeps rigid in the mechanism, and hence, the minimum does not correspond to a stationary point.
Brief on kinematic formulation of CSMIN3 for Mindlin plates
Kinematic formulation of the MIN3 for Mindlin plates
In the original MIN3 [81], the rotations are assumed to be linear through the rotational degrees of freedom (DOFs) at three nodes of the elements, and the deflection is initially assumed to be quadratic through the deflection DOFs at six nodes (three nodes of the elements and three midedge points). Then, by enforcing continuous shear constraints at every element edge, the deflection DOFs at three midedge points can be removed and the deflection is now approximated only by vertex DOFs at three nodes of the elements. The MIN3 element can hence overcome shear locking and produces convergent solutions. In this paper, we just brief on the kinematic formulation of the MIN3 which is necessary for the kinematic formulation of the CSMIN3.
Using a mesh of threenode triangular elements, the approximation of displacement flow for an element Ω _{ e } shown in Figure 2 can be written as
where , I = 1, 2, 3, is the flow vector of the nodal degrees of freedom of associated to node I and N _{ I }(x), I = 1, 2, 3, are linear shape functions at node I.
The curvature rates of the deflection flow in an element are then defined by
where is the vector of nodal displacement flow of the element and B contains the constants which are derived from the derivatives of the shape functions as
in which N _{,x } and N _{,y } are the matrices of derivatives of the shape functions in the xdirection and ydirection, respectively.
The shear strain rates of the deflection flow in an element are then defined by
where
in which L _{,x }, L _{,y }, H _{,x }, and H _{,y } are the matrices of derivatives of the shape functions in the xdirection and ydirection, respectively, and and are the vectors of shape functions, with L _{ I } and H _{ I }, I = 1, 2, 3, given by
in which a _{ i } and b _{ i } (i = 1 ÷ 3) are the geometric distances as shown in Figure 2.
Kinematic formulation of CSMIN3
In the CSMIN3 [80], the domain discretization is the same as that of the MIN3 using N _{ n } nodes and N _{ e } triangular elements. However, in the formulation of the CSMIN3, each triangular element Ω_{ e } is further divided into three subtriangles Δ_{1}, Δ_{2}, and Δ_{3} by connecting the central point O of the element to three field nodes as shown in Figure 3.
In the CSMIN3, we assume that the vector of displacement flow at the central point O is the simple average of three vectors of displacement flow , , and of three field nodes as
On the first subtriangle Δ_{1} (triangle O12), the linear approximation is constructed by
where is the vector of displacement flow of nodal degrees of freedom of the subtriangle Δ_{1} and is the vector containing the linear shape functions at nodes O, 1, 2 of the subtriangle Δ_{1}.
The curvature rates of the deflection flow and the altered shear strain rates in the subtriangle Δ_{1} are then obtained by
where and are respectively computed similarly as the matrices B and S of the MIN3 in Equations 16 and 18 but with two following changes: (1) the coordinates of threenode , i = 1, 2, 3, are replaced by x _{ O }, x _{1}, and x _{2}, respectively, and (2) the area A _{ e } is replaced by the area of subtriangle Δ_{1}.
Substituting in Equation 21 into Equations 23 and 24, and then rearranging, we obtain
Similarly, by using cyclic permutation, we easily obtain the curvature rates of the deflection flow , the shear strains , and matrices and , j = 2, 3, for the second subtriangle Δ _{2} (triangle O23) and third subtriangle Δ _{3} (triangle O31), respectively.
Now, by applying the cellbased strain smoothing operation in the CSFEM [39, 41], the bending and shear strain rates and , j = 1, 2, 3, are used to create the smoothed bending and smoothed shear strain rates and , respectively, on the triangular element Ω _{ e }, such as
where Φ_{ e }(x) is a given smoothing function that satisfies the unity property Using the following constant smoothing function
where A _{ e } is the area of the triangular element, the smoothed bending strain rate and the smoothed shear strain rate in Equations 27 and 28 become
Substituting and , j = 1, 2, 3, into Equation 30, the smoothed bending strain rate and the smoothed shear strain rate are expressed by
where and are the smoothed bending and shear strain gradient matrices given by
Discretization of kinematic formulation by CSMIN3
In Equation 11, when c _{ s } becomes small at the thin plate limit, the last term is very nearly singular and numerical integration is preferable. To avoid inaccuracies associated with the point z = 0, twice the integral over half thickness is considered, and Equation 11 can be rewritten as [37]
where ζ = 4z/h − 1 and ζ _{ g } and W _{ g } are the usual Gauss integration point coordinates and weights, respectively; n _{ G } is the number of Gauss integration points; m _{0} = σ _{0} h ^{2}/4 is the plastic moment of resistance per unit width of the plate of thickness h.
By discretizing the domain Ω into n _{ e } triangular plate elements such that and Ω _{ i } ∩ Ω _{ j } = ∅, i ≠ j, and using the kinematic formulation of the CSMIN3 as presented in the ‘Brief on kinematic formulation of CSMIN3 for Mindlin plates’ section, the plastic dissipation in Equation 33 is expressed as
where and in which and are computed at the Gauss points by Equation 31.
Combining Equations 9 and 31, and are now expressed explicitly as
The plastic dissipation in Equation 34 is hence expressed as
where
Equation 37 can be rewritten in the matrix form as
where
and
Note that using Equations 31 and 32, the vector y _{ i } in Equation 40 can be rewritten in the form of the discrete element displacement flow vector
where
and and are the components extracted, respectively, from the matrices and in Equation 32.
Similarly, the external energy in Equation 13(c) and the boundary condition of displacement flow Equation 13(a) can be combined and rewritten in the matrix form of the discrete system displacement flow vector as [33]
Combining Equations 36, 38, 42, and 44, the minimization problem (12) associated with the CSMIN3 now becomes the problem of finding the optimal value λ ^{+} such that
subjected to the constraints
The minimization problem (45) is a convex programming problem in which the objective function is a positively homogeneous function of degree 1 in the variables z _{ i } (or in the strain rates) and is not differentiable at any points in the rigid domain which do not undergo plastic flow (z _{ i } = 0). The minimization problem (45) is also categorized into the group of the problems of minimizing a sum of Euclidean norms which has a natural dual maximization formulation [23].
For solving this group of problem, one of the wellknown approaches used is to replace the terms z _{ i } in the objective by the differentiable quantity , where μ is a fixed positive number. This method is robust but converges slowly as μ → 0 because some of the norms in the objective function have zero as their optimal value [23].
Recently, Andersen et al. [24] recently employed the aspect of duality of the problem to propose a primaldual interiorpoint method for solving a homogeneous selfdual model of conic quadratic programming. In this method, the terms z _{ i } are also replaced by , but the quantity μ is treated as an extra variable, whose value is determined by duality estimates. Using this method, the minimization problem (45) is now solved rapidly and accurately even if there are a large number of variables and many norms z _{ i } are zero at a solution point. Also, the primaldual interiorpoint method is recently integrated in an available general software (e.g., MOSEK [25]) which specializes secondorder cone programming (SOCP) problems [24]. The limit analysis problem can hence be solved efficiently using such software.
The minimization problem (45) is hence rewritten in the form of a standard SOCP problem by introducing auxiliary variables t _{ i }, i = 1, 2, …, n _{ e } × n _{ G }, such that
subjected to the constraints
where Equation 48(c) represents quadratic cone constraints. With the form of standard SOCP problem, the minimization problem (47) for finding the collapse multipliers of the Mindlin plates can now be solved efficiently by using the software MOSEK.
Note that the formulation of the minimization problem of the plastic dissipation power in the form of standard SOCP problem was also presented in [33, 35]; however, the form of standard SOCP problem in these references is only for Kirchhoff plates.
Also, note that in the kinematic limit analysis of plates, the ability to obtain the strict upper bound depends not only on the efficient solution of the arising optimization problem but also on the effectiveness of the elements employed. It is required that the flow rule needs hold throughout each element. For the C1continuous elements, this requirement can be satisfied naturally. However, for the C0continuous elements, it can be violated due to the appearance of plastic hinge lines on boundaries of elements. In order to overcome this violation, the internal work dissipated in resulting hinge lines on boundaries of elements should be taken into account as done by Hodge and Belytschko [7] and Makrodimopoulos and Martin [82]. In this paper, the CSMIN3 uses only threenode triangular plate elements and hence belongs to the C0continuous elements. However, for the sake of simplicity of using the CSMIN3 in the limit analysis of plates, we can ignore considering the internal work dissipated in resulting hinge lines on boundaries of elements. It is therefore no longer possible to guarantee that the solution obtained from the minimization problem (47) is a strict upper bound on the collapse multiplier. However, using the smoothed strain rates which are constant over elements, the flow rule only needs to be enforced at any point in each element, and it is guaranteed to be satisfied almost everywhere in the problem domain. Therefore, the computed collapse load obtained using the proposed method can still be reasonably considered as an upper bound on the actual value.
Results and discussion
The number performance of the proposed limit analysis will now be tested by examining a number of benchmark uniformly loaded or pointloaded plate problems for which numerical solutions have been published in the literature. For all the examples considered, the following were assumed: yield stress σ _{ p } = 250 MPa and yield moment M _{ p } = σ _{ p } t ^{2}/4.
Square plates
We now consider a square plate subjected to a uniform outofplane pressure loading (with basic load p = M _{ p }/L ^{2}) with two different boundary conditions: (1) clamped supports on all edges as shown in Figure 4a and (2) simply supported supports on all edges as shown in Figure 4b. For this problem, the full plate is considered and the upper bound reference solution using quadrilateral elements with 867 degrees of freedom (DOFs) can be found in [37, 38]. Figure 4c illustrates four forms of discretization of the plate using triangular elements.
In this example, firstly, the thickness of the plate is chosen such that the ratio L/t = 10 and the plate is discretized by the mesh 12 × 12 with 507 DOFs. We first consider the effect of the collapse multipliers when the number of Gauss points along the half of the thickness of the plate is changed from 1 point to 7 points. Figures 5 and 6 show the convergence of the collapse multipliers versus the different numbers of Gauss points for both cases of boundary conditions by MIN3 and CSMIN3. The results show that both solutions of the CSMIN3 and MIN3 converge to the upper bound reference solution [38] when the number of Gauss points increases, but those of the CSMIN3 are more accurate than those of the MIN3. This hence implies that the CSMIN3 can provide the reliable upper bound collapse multipliers for the Mindlin plates when a suitable number of Gauss points is used along the half of the thickness of the plate. In these analyses, it is seen that the usage of 6 Gauss points is the most suitable for both cases of boundary conditions and hence will be recommended for default employing in the CSMIN3 (and also in the MIN3) for all numerical examples in this paper.
Next, the convergence of the collapse multipliers versus various degrees of freedom of the system is considered. The numbers of degrees of freedom of the system are now changed from 75 (corresponding to the mesh 4 × 4) to 507 (corresponding to the mesh 12 × 12). The results for both cases of boundary conditions by the CSMIN3 and MIN3 are listed in Table 1 and plotted in Figures 7 and 8, respectively. The results show that both solutions of the CSMIN3 and MIN3 converge to the reference solutions when the number of degrees of freedom increases. In addition, Figures 9 and 10 show the patterns of displacement and plastic energy dissipation at collapse for both cases of boundary conditions by using the CSMIN3. It can be observed that the forms of the yield lines are clearly identified reasonably from these dissipation patterns. These results hence imply that the CSMIN3 can provide the reliable upper bound collapse multipliers for the Mindlin plates when a suitable number of degrees of freedom is used. Note that in the above analyses, the results of the CSMIN3 are more accurate than those of the MIN3, especially in the coarse meshes. This hence implies that the cellbased strain smoothing technique in the CSMIN3 is very necessary to improve the accuracy of the MIN3 in the limit analysis of Mindlin plates.
Last, the analysis for the solutions of the thin plate by the CSMIN3 and MIN3 is performed for the clamped square plate by changing the slenderness ratios (L/t) from 5 to 6,250 with the mesh 12 × 12. Convergence of the collapse multipliers of the clamped square plate versus various slenderness ratios (L/t) by the CSMIN3 and MIN3 is plotted in Figure 11. The Kirchhoff reference results can be found in [33, 37]. As expected, the solutions of the CSMIN3 converge to the reference Kirchhoff solution [33] when the slenderness ratio is increased to the limit of the thin plate. This hence shows that the CSMIN3 is free of shear locking in the limit analysis of thin plates. Also, note that the convergence of solutions of MIN3 is much higher than the expected value when the slenderness ratio is increased to the limit of the thin plate. This hence implies that the cellbased strain smoothing technique in the CSMIN3 is very necessary to improve the instable behavior of the MIN3 in the limit analysis of thin plates.
Rectangular plate
We now consider a rectangular plate with simply supported supports on all edges and subjected to a uniform outofplane pressure loading (with basic load p = M _{ p }/(L. H)) as shown in Figure 12a. For this problem, the full plate is considered and the upper bound reference solution using a meshfree method with 1,350 DOFs can be found in [33]. Figure 12b illustrates four forms of discretization using uniform meshes of triangular elements.
The convergence of collapse multipliers with respect to the number of degrees of freedom is considered by choosing the thickness of the plate t = 0.01 m, the width L = 2 m, and the ratio L/H = 2. The results by the CSMIN3 and MIN3 are listed in Table 2 and plotted in Figure 13. In addition, Figure 14 shows the patterns of displacement and plastic energy dissipation at collapse by the CSMIN3. It is seen that the obtained comments from the square plates related to the convergence and accuracy of the CSMIN3 in the limit analysis of Mindlin plates are confirmed for the rectangular plates.
Rhombic plate
We now consider a clamped rhombic plate with the radius R = 0.5 m and the thickness t = 0.02 m subjected to a uniform outofplane pressure loading (with basic load p = M _{ p }/R ^{2}) as shown in Figure 15a. For this problem, the full plate is considered and the upper bound reference solutions using quadrilateral elements with 867 degrees of freedom can be found in [38]. Figure 15b illustrates four forms of discretization using uniform meshes of triangular elements.
The convergence of collapse multipliers of the clamped rhombic plate with respect to the number of degrees of freedom and various skewness angles α is listed in Table 3 and plotted in Figure 16 for two cases of α = 30° and α = 60°. In addition, the patterns of the plastic energy dissipation at collapse by the CSMIN3 for two cases of α = 30° and α = 60° are shown in Figure 17. Again, it is seen that the comments obtained from two previous examples related to the convergence and accuracy of the CSMIN3 in the limit analysis of Mindlin plates are confirmed for the rhombic plates.
In addition, an analysis of collapse multipliers of the clamped rhombic plate with respect to various skewness angles α by the CSMIN3 and MIN3 is illustrated in Figure 18. It is observed that the results of the CSMIN3 are very close to those of reference solutions, especially for small skewness angles α. These results hence imply that the CSMIN3 can provide the reliable solutions in the limit analysis of skew Mindlin plates.
Circular plate
We now consider a clamped circular plate (radius R = 1m and thickness of plate t = 0.01 m) subjected to a uniform outofplane pressure loading (with basic load p = M _{ p }/R ^{2}). Due to its symmetry, only the upper right quadrant of the plate is discretized by 294 triangular elements (507 DOFs) as shown in Figure 19. The upper bound reference solutions using quadrilateral elements with 1,041 DOFs can be found in [38].
The convergence of collapse multipliers with respect to the number of degrees of freedom is plotted in Figure 20, and the patterns of displacement and the plastic energy dissipation at collapse by the CSMIN3 are shown Figure 21. Again, it is seen that the comments obtained from three previous examples related to the convergence and accuracy of the CSMIN3 in the limit analysis of Mindlin plates are confirmed for the circular plates.
Next, due to the availability of the thin plate reference solutions [38] (collapse multiplier = 13.231), we hence perform a convergent analysis of the collapse multipliers with respect to various slenderness ratios (2R/t) by the CSMIN3 and MIN3. The results are listed in Table 4 and plotted in Figure 22. As expected, the solutions of the CSMIN3 again converge to the reference solutions when the slenderness ratio is increased to the limit of the thin plate. This hence confirms again that the CSMIN3 is free of shear locking in the limit analysis of thin plates. Also, note that the convergence of solutions of MIN3 is much higher than the expected value when the slenderness ratio is increased to the limit of the thin plate. This hence confirms again that the cellbased strain smoothing technique in the CSMIN3 is very necessary to improve the instable behavior of the MIN3 in the limit analysis of thin plates.
Equilateral triangle plate
We now consider an equilateral triangle plate as shown in Figure 23a with the assigned radius R = 1m and thickness t = 0.04m. The plate is clamped on the boundary and subjected to a uniform outofplane pressure loading (with basic load p = M _{ p }/R ^{2}). For this problem, the full plate is considered and the upper bound reference solutions can be found in [38]. Figure 23b illustrates a discretization using uniform meshes of triangular elements.
The convergence of collapse multipliers with respect to the number of degrees is listed in Table 5 and plotted in Figure 24, and the patterns of displacement and the plastic energy dissipation at collapse by the CSMIN3 are shown in Figure 25. Again, it is seen that the obtained comments from four previous examples related to the convergence and accuracy of the CSMIN3 in the limit analysis of Mindlin plates are confirmed for the equilateral triangle plate.
Conclusions
The paper presents a numerical procedure for the kinematic limit analysis of thick plates governed by the von Mises criterion. The cellbased smoothed threenode Mindlin plate element (CSMIN3) is combined with a secondorder cone optimization programming (SOCP) to determine the upper bound limit load of the Mindlin plates. In the CSMIN3, each triangular element is divided into three subtriangles, and in each subtriangle, the gradient matrices of MIN3 are used to compute the strain rates. Then, the gradient smoothing technique on whole the triangular element is used to smooth the strain rates on these three subtriangles. The limit analysis problem of Mindlin plates is formulated by minimizing the dissipation power subjected to a set of constraints of boundary conditions and unitary external work. For Mindlin plates, the dissipation power is computed on both the middle plane and the thickness of the plate. This minimization problem can then be transformed into a form suitable for optimum solution using the SOCP. Through the formulation and numerical examples, some concluding remarks can be drawn as follows:

1.
The CSMIN3 uses only threenode triangular elements that are much easily generated automatically for arbitrary complex geometrical domains.

2.
The CSMIN3 can provide reliable upper bound collapse multipliers for both thick and thin plates.

3.
The solutions of the CSMIN3 converge from the upper bound, and the CSMIN3 is free of shear locking in the limit analysis of thin plates.

4.
Compared to the MIN3, the CSMIN3 is more accurate in the limit analysis of thick plates and more stable in the limit analysis of thin plates.

5.
The forms of the yield lines by the CSMIN3 are identified reasonably from the dissipation patterns.
In addition, the extension of the present CSMIN3 for the limit analysis of flat shells using triangular elements is very promising.
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Acknowledgements
This work was supported by Vietnam National Foundation for Science & Technology Development (NAFOSTED), Ministry of Science & Technology, under the basic research program (Project No.: 107.022012.05).
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Authors' contributions
NTT proposed the main idea of extending the CSMIN3 to the limit analysis of Mindlin plates, carriout out the theory model, numerical discretization model, and revising the manuscript. PVP carried out the numerical results and wrote the first draft of the manuscript. LVC carried out the source code of the limit analysis for Kirchhoff plates. All authors read and approved the final manuscript.
This article [1] has been retracted by Professor NguyenDang Hung, EditorinChief of Asia Pacific Journal on Computational Engineering.
Upon investigation carried out according to the Committee on Publication Ethics guidelines, it has been found that the authors have duplicated substantial parts from the following article by T. NguyenThoi, P. PhungVan, M.H. NguyenThoi, H. DangTrung [2], published in Journal of Computational Applied Mathematics.
This is a violation of publication ethics which, in accordance with the Springer Policy on Publishing Integrity, warrants a retraction of the article and a notice to this effect to be published in the journal.
A retraction note to this article can be found online at http://dx.doi.org/10.1186/s405400150013z.
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Keywords
 Limit analysis
 Upper bound
 Mindlin plates
 Cellbased smoothed threenode Mindlin plate element (CSMIN3)
 Smoothed finite element methods (SFEM)
 Secondorder cone programming (SOCP)