# An improved meshless method for finite deformation problem in compressible hyperelastic media

Tóm tắt An improved meshless method for finite deformation problem in compressible hyperelastic media: ...nctions Φ and nodal displacement vector usp in a support domain. The 30 Nha Thanh Nguyen, Minh Ngoc Nguyen, Thien Tich Truong, Tinh Quoc Bui Fig. 1. Hyperelastic model with boundary conditions approximated displacement, its variational and the gradient of test functions are written as u = Φusp;...em in cases F = 16 N/mm2 and F = 24 N/mm2, 20× 20 nodes are used for the simulation. Fig. 4. Vertical displacement of point A with five nodal distributions given by CTM-based RPIM, comparing with VDQ (23× 23 nodes) [19] Fig. 5. First Piola Kirchhoff stress distributions in Cook’s membrane proble...rial is compressible hyperelastic, the 38 Nha Thanh Nguyen, Minh Ngoc Nguyen, Thien Tich Truong, Tinh Quoc Bui properties are the same with the previous examples. Various values of distributed force q = 0.2, 0.3, 0.4 and 0.5 N/mm2 are considered in the simulations. A distribution of 9× 101 nodes...

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φ(Ω) σ : ∇δudV − ∫ φ(Ω) δu · b¯dV − ∫ φ(Γt) δu · t¯dΓ = 0, (6) where σ is the Cauchy stress and δu is the vector of test functions. In this study, the meshless RPIM that is developed basing on the Galerkin weak form and discretization procedure [21,22]. The displacement field u is approximated by the use of RPIM shape functions Φ and nodal displacement vector usp in a support domain. The 30 Nha Thanh Nguyen, Minh Ngoc Nguyen, Thien Tich Truong, Tinh Quoc Bui Fig. 1. Hyperelastic model with boundary conditions approximated displacement, its variational and the gradient of test functions are written as u = Φusp; δu = Φδusp; ∇δu = Bδusp, (7) where B is the B-operator, this matrix contents the derivatives of shape functions with respect to the current configuration. Substituting Eq. (7) into Eq. (6), the weak form can be rewritten as follow∫ φ(Ω) BTσdV − ∫ φ(Ω) ΦTb¯dV − ∫ φ(Γt) ΦT t¯dΓ = 0, (8) or R (u) = Fint − Fext = 0, (9) where R (u) denotes the residual vector that is the function of displacement field. Apply the linearization procedure to Eq. (8) or Eq. (9), obtain the two nonlinear parts for large deformation hyperelastic problem [23, 24] ∆R (u) = ∫ φ(Ω) ∆BTσdV + ∫ φ(Ω) BT∆σdV = Ktan∆u. (10) The first part in Eq. (10) relates to the stress state of the current configuration, so it represents for the geometric nonlinear of the problem. The second one shows the varia- tion of the stress and it stands for the material nonlinear of the model. Finally, the tangent stiffness matrix in Eq. (10) can be written as Ktan = ∫ φ(Ω) GTMGdV + ∫ φ(Ω) BT∆CeBdV, (11) where the matrix M and matrix G at node i are defined as M = σ ⊗ I2×2; GI = ∂φi ∂x I2×2 ∂φi ∂y I2×2 . (12) where I2×2 is the (2× 2) identity matrix. An improved meshless method for finite deformation problem in compressible hyperelastic media 31 3.2. CTM-based meshless RPIM method In this study, the RPIM is used for the definition of meshfree shape functions. The procedure for the construction of RPIM shap functions is described in [21, 22]. Similar to the conventional element free Galerkin (EFG) method, the meshless RPIM approach is also based on the global weak form. The most advantage of RPIM shape function is its satisfaction of Kronecker delta property, this leads to convenience in treatment of boundary conditions. However, if the traditional RPIM method is used, the integrals in Eq. (6) or Eq. (11) are still conducted with the help of background cells. In this proposed approach, the CTM is chosen to alternate the Gaussian integration. Suppose that a 2D domain integral is to be calculated I = ∫ Ω f (x, y) dΩ, (13) where f (x, y) is an arbitrary function. Let ΩA be the auxiliary domain that contains the domain Ω and the new function g (x, y) is defined over the domain ΩA as g(x, y) = { f (x, y), (x, y) ∈ Ω 0, (x, y) /∈ Ω (14) Fig. 2. An auxiliary domain ΩA containing domain Ω In this study, a rectangle is used as an auxiliary domain (see Fig. 2), the domain integral in Eq. (13) can be rewritten as a double boundary integral I = ∫ y2 y1 [∫ b a g (x, y) dx ] dy = ∫ y2 y1 h (y) dy, (15) where h (y) = ∫ b a g (x, y) dx. To calculate the integral in Eq. (15), the boundaries Γ1 and Γ3 are divided into n inte- gration intervals and the Gaussian quadrature method is used. The integral I in Eq. (15) can be computed as I = n ∑ i=1 (∫ yi+1 yi h(y)dy ) = n ∑ i=1 (∫ 1 −1 h(η)Jdη ) = n ∑ i=1 m ∑ j=1 h(ηj)Jwj, (16) 32 Nha Thanh Nguyen, Minh Ngoc Nguyen, Thien Tich Truong, Tinh Quoc Bui where ηj is Gaussian points, wj is weight numbers and J = dy/dη = (yi+1 − yi)/2 de- notes the Jacobian of the 1D transformation. In numerical practice, to perform the integrals, a system of CTM integration points is generated on the rectangular auxiliary domain ΩA and a simple algorithm is applied to detect if an integration point belongs to the domain or not. In CTM-based meshless method, there is no 2D integration cell created. This is the advantage of CTM approach because meshing is a time-consuming task and CTM can make the RPIM method become a truly meshless method. The detail procedure for CTM technique can be found in papers [15–18]. 4. NUMERICAL EXAMPLES 4.1. Cook’s membrane problem The first numerical example for finite deformation analysis deals with a popular benchmark problem, Cook’s membrane, which aims to show the accuracy of the devel- oped CTM-based RPIM. As shown in Fig. 3(a), the model has a trapezoidal shape that is clamped at the left edge and loaded by a traction force along the right edge. The com- pressible neo-Hookean model is used for the hyperelastic material of the problem, the shear modulus is µ = 80.194 N/mm2 and bulk modulus is κ = 120.291 N/mm2. Various values of traction force are chosen for the test (i.e., F = 8, 12, 16, 24 N/mm2). There are five nodal distributions (10× 10, 15× 15, 20× 20, 25× 25 and 30× 30) selected to inves- tigate the accuracy of the proposed method in the nonlinear analysis. Fig. 3(b) shows the distribution of 10× 10 scattered nodes and a system of CTM integration points is also displayed (5196 points). Fig. 3. Cook’s membrane problem: geometry and scattered model The obtained results are verified with the solutions given by variational differential quadrature (VDQ) method [19] that used 23× 23 nodes for the simulation. The present CTM-based RPIM offers acceptable results as the computed vertical displacements at point A (see Fig. 3) are in good agreement with reference solutions [19], which are clearly shown in Fig. 4. The larger the traction force is, the higher value of vertical displacement is obtained. Fig. 5 displays the first Piola Kirchhoff stresses (P = JσF−T) distributions in An improved meshless method for finite deformation problem in compressible hyperelastic media 33 Cook’s membrane problem in cases F = 16 N/mm2 and F = 24 N/mm2, 20× 20 nodes are used for the simulation. Fig. 4. Vertical displacement of point A with five nodal distributions given by CTM-based RPIM, comparing with VDQ (23× 23 nodes) [19] Fig. 5. First Piola Kirchhoff stress distributions in Cook’s membrane problem The computational time estimated for this example by both methods, the conven- tional Gaussian and CTM techniques, whose computed results are given in Fig. 6, show- ing the advantage of our development. In this comparison, several nodal densities are used for the Cook’s membrane problem with shear force F = 8 N/mm2 that is divided in 34 Nha Thanh Nguyen, Minh Ngoc Nguyen, Thien Tich Truong, Tinh Quoc Bui to 5 load steps. Because Gaussian-based RPIM method uses a system of background cells for integration, so the number of integration points increases significantly when a large number of nodes are used for the interpolation and this essentially requires more time in the computation. As a consequence, the implementation for large deformation problems is effective with the developed CTM-based RPIM. In other words, the computational cost for the present model can be reduced by using the CTM technique. Fig. 6. Computational time comparison between Gaussian-based and CTM-based RPIM methods Tab. 1 shows the comparison of the vertical displacement of point A between Gaussian-based and CTM-based RPIM methods. A model of 25 × 25 nodes are used for both methods and several cases of number of integration points are investigated for CTM approach. The data shows the stable of CTM method and the obtained results from CTM fit well with those from conventional RPIM method although CTM-based RPIM uses less integration points than Gaussian-based method. Table 1. Vertical displacement of point A given by Gaussian-based and CTM-based RPIM (25× 25 nodes are used for both methods) q (N/mm2) Vertical displacement of point A (mm) Gaussian-based CTM-based 9216 points 5196 points 7112 points 8604 points 8 10.52 10.49 10.47 10.46 12 13.80 13.74 13.70 13.72 16 16.37 16.29 16.26 16.28 24 20.29 20.21 20.25 20.26 An improved meshless method for finite deformation problem in compressible hyperelastic media 35 4.2. Inhomogenous compression problem The second numerical example is devoted to numerical simulation for a rectangular plate under compression using our CTM-based RPIM. This is to further show the ap- plicability and effectiveness of the developed approach in modeling large deformation problem. The material of plate is compressible hyperelastic, the neo-Hookean model is used and the material parameters are the same with the previous problem. It is assumed that the horizontal displacement of the top edge and vertical displacement of the bottom edge are set zero. A constant distributed load applies on a part of the top edge. The geometry, dimensions and boundary conditions are presented in Fig. 7(a). Because of the symmetry, only a half of the plate is considered in the scattered model, as shown in Fig. 7(b). Fig. 7. Inhomogenous compression problem: geometry and scattered model Similar to the previous example, to investigate the accuracy and nodal independence of the proposed method, five nodal distributions are selected. The percent of compres- sion is computed at point I with four cases of distributed force F = 50, 100, 150 and 200 N/mm2. Graphs in Fig. 8 (a) show the compression percentages at point I that is obtained by the proposed method and compared with those of VDQ method [19]. The higher values of compression force give larger percents of compression. It is clearly to see that the CTM-based RPIM can give accurate results with less degrees of freedom than the reference method. Fig. 9 depicts the non-linear relationship between the vertical dis- placement of point I and pressure force F, three cases of load steps are also investigated to verify the accuracy of the proposed approach. The charts in Fig. 10 display the comparison of computational time between the con- ventional Gaussian and CTM techniques. The distributed force F = 200 N/mm2 with five load steps is chosen and five cases of nodal distribution are used for the comparison. Once again, the CTM-based RPIM shows its effectiveness in implementation for non- linear problem. Finally, Fig. 11 represents the deformations in which the color indicates the distribution of norm of the first Piola Kirchhoff stress ( ‖P‖ = √ PijPij ) on the models with different values of pressure force. 36 Nha Thanh Nguyen, Minh Ngoc Nguyen, Thien Tich Truong, Tinh Quoc Bui Fig. 8. Percent of compression at point I with five nodal distributions given by CTM-based RPIM, comparing with VDQ (45× 45 nodes) [19] Fig. 9. Force-Displacement diagrams for vertical displacement of point I with different numbers of load steps An improved meshless method for finite deformation problem in compressible hyperelastic media 37 Fig. 10. Computational time comparison between Gaussian-based and CTM-based RPIM methods for inhomogenous compression problem Fig. 11. Norm of first Piola Kirchhoff stress distributions for inhomogenous compression problem (grey grid indicates the initial undeformed configuration of the model) 4.3. Curved beam under bending In the last example, a model of curved beam under bending load is considered. The geometry and boundary conditions are given in Fig. 12. The dimensions are Rin = 9 mm Rout = 10 mm. The beam is fixed at the lowest edge while its right edge is subject to a distributed force q with angle α = 45◦. The material is compressible hyperelastic, the 38 Nha Thanh Nguyen, Minh Ngoc Nguyen, Thien Tich Truong, Tinh Quoc Bui properties are the same with the previous examples. Various values of distributed force q = 0.2, 0.3, 0.4 and 0.5 N/mm2 are considered in the simulations. A distribution of 9× 101 nodes and twenty load steps are used for the problem. Fig. 12. Curved beam under bending problem: geometry and scattered model Fig. 13. Force - total displacement diagrams given by CTM-based RPIM, Gaussian-based RPIM and FEM The charts in Fig. 13 show the non-linear relationship between the external force and total displacement at point A. The obtained results from both CTM-based and Gaussian- based RPIM methods match well with FEM solutions given by ANSYS program. Finally, the distributions of total displacement in the curved beam in four cases of distributed force q are displayed in Fig. 14. An improved meshless method for finite deformation problem in compressible hyperelastic media 39 Fig. 14. Distributions of total displacement in the curved beam 5. CONCLUSIONS In this paper, we have developed an enhanced and effective CTM-based RPIM for finite deformation analysis of compressible hyperelastic materials. The CTM integra- tion technique is used as alternative to traditional Gaussian quadrature method and this makes the meshless RPIM method become independent from the 2D integration cells. There is no 2D meshing algorithm is required to generate the background cells. Three numerical examples are performed and the obtained results are verified with reference solution. The proposed method has shown good performance in both accuracy and com- putation cost for large deformation problem. It is promising for further investigation on behavior of hyperelastic materials such as dynamic problems and fracture analysis. However, in this study, only regular nodal distributions are considered so the variations of nodal density would be taken in account in the future works. Moreover, because CTM method is being developed, there is still not any general algorithm to generate CTM inte- gration points for every type of geometry. These drawbacks should be improved in next studies. ACKNOWLEDGMENT This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 107.02-2019.327. 40 Nha Thanh Nguyen, Minh Ngoc Nguyen, Thien Tich Truong, Tinh Quoc Bui REFERENCES [1] A. M. Maniatty, Y. Liu, O. Klaas, and M. S. Shephard. Higher order stabilized finite element method for hyperelastic finite deformation. 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