| For concrete structures, they are subjected not only to regular design loads, but also some dynamic ones, such as impact, seism, blast and so on. Since these dynamic loads are usually unpredictable and have large destructive effect, they become the key factors in structure design. At present, analysis on dynamic mechanical behavior is usually based on static mechanical properties. However, many researches indicate that concrete is one of the rate-dependent materials and its dynamic mechanical behavior is different far from the static one. The dynamic constitutive law provided by commercial FEM software is not fit for concrete, and some of the expressions are too complicated and the parameters contained are too difficult to be set. Further more, there is significant difficulty on carrying out the structural dynamic calculation with self-defining dynamic constitutive law of concrete by the secondary development of commercial FEM software. Therefore, the research group of numerical simulation in HHU developed by itself a program named RDDP (Rate-Dependent Dynamic Program) to solve the dynamic analysis problems.RDDP program is composed of one main program and twenty five subroutines. It has nearly three thousand statements. After a detailed study on its flowchart and technical course, some incomplete sections in the program are found as follows: 1) RDDP program doesn't describe the stress characters after the materials are cracked or crashed. The type of matrix [D] (matrix [D] is stress-strain matrix) used in each stress state condition is always three-dimensional isotropic. However, when the concrete material is at the higher stress state, it is behaved more like anisotropic material. 2) In the program, the calculation is based on the load incremental method, and the softened part of the load-deflection curve can't be obtained. 3) Owing to the complexity of the program, functions of some processes are not distinct and too much hand-intervention is needed when preparing the input and output documents.In view of the above problems, this paper makes an improvement from three aspects which are constitutive model, the methods of solving non-linear equations and the design frame of the program. The main contents are as follows: 1) Based on available information and the constitutive models used in commercial FEM software, this paper selects a non-linear elastic model which can describe the main mechanical characters of the conctrete and the expression is relative simple. The model is modified with the strain rate factor just like in the original program. And an equivalent uniaxial dynamic constitutive law under the multi-axis stress condition is established by introducing a strength modifying factor. 2) A smeared crack model is introduced in this paper. By modifing the constitutive matrix of the materials, ten types of [D] matrix suitable for corresponding stress states are summarized, and the mechanical characters can be described after the concrete is cracked or crashed. 3) The Arc-Length method is added to simulate the descent part of the load-displacement curve. According to the need, the program can carry on the iterative computation by using the Newton method or Arc-Length method, or both of them. 4) Redesign and compile the framework of the program. The new program is named RDDP-V2 (Rate-Dependent Dynamic Program: Version 2). As a result, the calculation process is more modularized, preprocessor and postprocessor are more optimized, and each connection is much clearer.Finally, the improved program is used to simulate a concrete prism specimen under various conditions, such as uniaxial static linear condition, static nonlinear condition, rate-dependent nonlinear condition, dynamic linear condition and dynamic nonlinear condition. Compared with the solution of the theory or ADINA, each module in the program is analyzed and verified. |
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