Luận án: Phát triển phương pháp Mesh-free mới cho phân tích giới hạn và thích nghi kết cấu

Declaration of Authorship I declare that this is my own research. The data and results stated in the thesis are honest and have not been published by anyone in any other works. Ho Chi Minh city, October 2020 PhD candidate HO LE HUY PHUC i Acknowledgements The research presented in this thesis has been carried out in the framework of a doctorate at Faculty of Civil Engineering, Ho Chi Minh city University of Technology and Education, Vietnam. This work would have never been possible without the support and help of many people to whom I feel deeply grateful.

First and foremost, I would like to express my most sincere thanks to my su- pervisors, Assoc. Le Van Canh and Assoc. Phan Duc Hung, for their guidance, valuable academic advice, mental support and constant encouragement during the course of this work. I am deeply indebted to my major supervisor, As- soc.

Le Van Canh. He is one of most influential people in my life, both profes- sionally and personally. His guidance is precious, helping me develop the personal skills needed to succeed in future work. I would like to thank the co-author of my papers - Prof.

Tran Cong Thanh for his encouragement, support and guidance. I would also like to express my admiration for his unsurpassed knowledge of mathematics and numerical methods. I really appreciate the financial support received from the Institute for Com- putational Science and Technology (ICST) - HCMC, the Science and Technology Incubator Youth Program - HCMC, and International University - VNU-HCMC throughout the research projects. I take this opportunity to thank my colleagues in International University - VNU-HCMC, HCMC University of Technology and Education, and HUTECH Uni- versity, especially Dr.

Tran Trung Dung, PhD candidate Nguyen Hoang Phuong, PhD candidate Do Van Hien, Dr. Khong Trong Toan and Dr. Vo Minh Thien, for fruitful discussions about a range of topics and their mental support. I sincerely thank my parents and my younger sisters for their unconditional love and support.

I am also definitely indebted to my wife, Nguyen My Lam, for her love, understanding and encouraging me whenever I needed motivation. ii Acknowledgements Finally, I would like to dedicate this thesis to my little son - Ho Nguyen Nhat Duy. No word can describe my love for him. Ho Chi Minh city, October 2020 PhD candidate HO LE HUY PHUC iii Abstract The proposed research is essentially concerning on the development of powerful numerical methods to deal with practical engineering problems.

The direct methods requiring the use of a strong mathematical tool and a proper numerical discretiza- tion are considered. The current work primarily focuses on the study of limit and shakedown analysis allowing the rapid access to the requested information of structural design with- out the knowledge of whole loading history. For the mathematical treatment, the problems are formulated in form of minimizing a sum of Euclidean norms which are then cast as suitable conic programming depending on the yield criterion, e. second order cone programming (SOCP).

In addition, a robust numerical tool also requires an excellent discretization strat- egy which is capable of providing stable and accurate solutions. In this study, the so-called integrated radial basis functions-based mesh-free method (iRBF) is em- ployed to approximate the computational fields. To eliminate numerical instability problems, the stabilized conforming nodal integration (SCNI) scheme is also intro- duced. Consequently, all constrains in resulting problems are directly enforced at scattered nodes using collocation method.

That not only keeps size of the optimiza- tion problem small but also ensures the numerical procedure truly mesh-free. One more advantage of iRBF method, which is absent in almost meshless ones, is that the shape function satisfies Kronecker delta property leading the essential boundary conditions to be imposed easily. In summary, the iRBF-based mesh-free method is developed in combination with second order cone programming to provide solutions for direct analysis of structures and materials. The most advantage of proposed approach is that the highly accu- rate solutions can be obtained with low computational efforts.

The performance of proposed method is justified via the comparison of obtained results and available ones in the literature. iv Tóm tắt Luận án này hướng đến việc phát triển một phương pháp số mạnh để giải quyết các bài toán kỹ thuật, và phương pháp phân tích trực tiếp được sử dụng. Phương pháp này yêu cầu một thuật toán tối ưu hiệu quả và một công cụ rời rạc thích hợp. Trước tiên, nghiên cứu này tập trung vào lý thuyết phân tích giới hạn và thích nghi, phương pháp được biết đến như một công cụ hữu hiệu để xác định trực tiếp những thông tin cần thiết cho việc thiết kế kết cấu mà không cần phải thông qua toàn bộ quá trình gia tải.

Về mặt toán học, các bài toán được phát biểu dưới dạng cực tiểu một chuẩn của tổng bình phương các biến trong không gian Euclide, sau đó được đưa về dạng chương trình hình nón phù hợp với tiêu chuẩn dẻo, ví dụ chương trình hình hón bậc hai (SOCP). Hơn nữa, một công cụ số mạnh còn đòi hỏi phải có kỹ thuật rời rạc tốt để đạt được kết quả tính toán chính xác với tính ổn định cao. Nghiên cứu này sử dụng phương pháp không lưới dựa trên phép tích phân hàm cơ sở hướng tâm (iRBF) để xấp xỉ các trường biến. Kỹ thuật tích phân nút ổn định (SCNI) được đề xuất nhằm loại bỏ sự thiếu ổn định của kết quả số.

Nhờ đó, tất cả các ràng buộc trong bài toán được áp đặt trực tiếp tại các nút bằng phương pháp tụ điểm. Điều này không những giúp kích thước bài toán được giữ ở mức tối thiểu mà còn đảm bảo phương pháp là không lưới thực sự. Một ưu điểm nữa mà hầu hết các phương pháp không lưới khác không đáp ứng được, đó là hàm dạng iRBF thỏa mãn đặc trưng Kronecker delta. Nhờ vậy, các điều kiện biên chính có thể được áp đặt dễ dàng mà không cần đến các kỹ thuật đặc biệt.

Tóm lại, nghiên cứu này phát triển phương pháp không lưới iRBF kết hợp với thuật toán tối ưu hình nón bậc hai cho bài toán phân tích trực tiếp kết cấu và vật liệu. Thế mạnh lớn nhất của phương pháp đề xuất là kết quả số với độ chính xác cao có thể thu được với chi phí tính toán thấp. Hiệu quả của phương pháp được đánh giá thông qua việc so sánh kết quả số với những phương pháp khác. v Contents Declaration of Authorship i Acknowledgements iii Abstract v Contents ix List of Tables xi List of Figures xvi List of Abbreviations xvii List of Notations xix Chapter 1: Introduction 1 1.1 Limit and shakedown analysis .4 The direct analysis for microstructures .5 Mesh-free methods - state of the art .4 The objectives and scope of thesis .5 Original contributions of the thesis.

25 vi Contents Chapter 2: Fundamentals 27 2.1 Plasticity relations in direct analysis .1 Upper bound theorem of shakedown analysis .2 The lower bound theorem of shakedown analysis .3 Separated and unified methods .1 Upper bound formulation of limit analysis .2 Lower bound formulation of limit analysis .4 Conic optimization programming .6 The iRBF-based mesh-free method .1 iRBF shape function .2 The integrating constants in iRBF approximation .3 The influence domain and integration technique. 49 Chapter 3: Displacement and equilibrium mesh-free formulation based on integrated radial basis functions for dual yield design 53 3.2 Kinematic and static iRBF discretizations .1 iRBF discretization for kinematic formulation .2 iRBF discretization for static formulation .2 Square plates with cutouts subjected to tension load .3 Notched tensile specimen. 67 Chapter 4: Limit state analysis of reinforced concrete slabs using an integrated radial basis function based mesh-free method 68 4.2 Kinematic formulation using the iRBF method for reinforced con- crete slab .2 Regular polygonal slabs .3 Arbitrary geometric slab with a rectangular hole. 81 Chapter 5: A stabilized iRBF mesh-free method for quasi-lower bound shakedown analysis of structures 82 5.2 iRBF discretization for static shakedown formulation .1 Punch problem under proportional load .2 Thin plate with a central hole subjected to variable tension loads .3 Grooved plate subjected to tension and in-plane bending loads 95 5.4 A symmetric continuous beam .5 A simple frame with different boundary conditions.

104 Chapter 6: Kinematic yield design computational homogenization of micro-structures using the stabilized iRBF mesh-free method 106 6.2 Limit analysis based on homogenization theory .3 Discrete formulation using iRBF method .2 Metal with cavities .3 Perforated material with different arrangement of holes. 121 Chapter 7: Discussions, conclusions and future work 123 7.2 The convergence and reliability of obtained solutions .1 The advantages of present method .2 The disadvantages of present method .4 Suggestions for future work. 129 List of publications 131 Bibliography 154 ix List of Tables 3.1 Prandtl problem: upper and lower bound of collapse multiplier .2 Prandtl problem: comparison with previous solutions .3 Collapse multipliers for the square plate with a central square cutout 65 3.4 Collapse multipliers for the square plate with a central thin crack .5 Plates with cutouts problem: comparison with previous solutions .6 The double notched specimen: comparison with previous solutions .1 Rectangular slabs with various ratios b/a: limit load factors .2 Results of simply supported and clamped square slabs .3 Square slabs: limit load multipliers in comparison with other methods 77 4.4 Clamped regular polygonal slabs: limit load factors in comparison with other solutions (mp /qR2 ) .5 Collapse load of an arbitrary shape slab (×m− p) .1 Computational results of iRBF and RPIM methods .2 Plate with hole: comparison of limit load multipliers .3 Plate with hole: comparison of shakedown load multipliers .4 Grooved plate: present solutions in comparison with other results .5 Symmetric continuous beam: limit load factors .6 Symmetric continuous beam: shakedown load factors .7 A simple frame (model A): limit and shakedown load multipliers .8 A simple frame (model B): limit and shakedown load multipliers. 102 x List of Tables 6.1 Perforated materials: the given data .2 Rectangular hole RVE (L1 × L2 = 0.

113 xi List of Figures 1.1 Direct analysis: numerical procedures.2 The discretization of FEM and MF method .3 The computational domain in mesh-free method .4 Numerical procedures: Mesh-free method vesus FEM .2 Stable and unstable material models .3 The normality rule .4 The equilibrium body .5 The different behaviors of structures under the cycle load .6 Loading cycles in shakedown analysis .7 Homogenization technique: correlation between macro- and micro- scales .8 The iRBF shape function and its derivatives .9 The influence domain and representative domain of nodes .10 The SCNI technique in a representative domain .2 Prandtl problem: approximation displacement and stress boundary conditions .3 Bounds on the collapse multiplier versus the number of nodes and variables. 62 xii List of Figures 3.4 Thin square plates .5 The upper-right quater of plates .6 Uniform nodal discretization .7 Convergence of limit load factor for the plates .8 Double notch specimen .9 Convergence study for the double notched specimen problem .1 Slab element subjected to pure bending in the reinforcement direction 71 4.2 Rectangular slab: geometry, loading, boundary conditions and nodal discretization .3 Simply supported square slab: normalized limit load factor λ+ versus the parameter αs .4 Limit load factors λ+ (mp /qab) of rectangular slabs (b/a = 2) with different boundary conditions: CCCC (56.5 Rectangular slabs (b = 2a) with various boundary conditions: plastic dissipation distribution .6 Nodal distribution and computational domains of polygonal slabs: (a) triangle; (b) square; (c) pentagon; (d) hexagon; (e) circle .7 Plastic dissipation distribution and collapse load multipliers (mp /qR2 ) of polygonal slabs: (a, b, c, d, e)-clamped; (f, g, h, i, j)-simply supported 79 4.8 Arbitrary shape slabs: geometry (all dimensions are in meter) and discretization .