ANSYS Structural Finite Element Analysis Process

ANSYS Structural Finite Element Analysis Process

ANSYS Structural Finite Element Analysis Process

The basic idea of the finite element method is to discretize a continuous structure into a finite number of elements, with a finite number of nodes defined within each element. The continuous body is treated as an assembly of elements connected only at nodes. Meanwhile, node values of the field function are selected as basic unknowns, and an approximate interpolation function is assumed within each element to describe the distribution of the field function in the element. Then, using variational principles in mechanics, finite element equations for solving node unknowns are established, converting an infinite-degree-of-freedom problem in a continuous domain into a finite-degree-of-freedom problem in a discrete domain. After solving, the field function in elements and the entire assembly can be determined using known node values and interpolation functions.

ANSYS Structural Finite Element Analysis Process

1. Preprocessing

The goal of preprocessing is to establish a structural finite element model that conforms to actual conditions, performed in the Preprocessor module. It includes:

• Setting up the analysis environment (specifying the analysis job name and title).
• Defining element types.
• Defining real constants.
• Defining material properties (e.g., elastic modulus, Poisson's ratio, and density for linear elastic materials).
• Building the geometric model (typically using bottom-up modeling: first defining key points, connecting points into lines, lines into surfaces, and surfaces into volumes).
• Meshing the geometric model (in three steps: assigning element properties, specifying mesh density, and performing meshing).

2. Applying Loads, Setting Solution Options, and Solving

These tasks are implemented through the SOLUTION module:

• Specifying the analysis type (static analysis, modal analysis, harmonic response analysis, transient dynamic analysis, spectrum analysis, etc.).
• Setting analysis options (different options for different analysis types, including nonlinear options, linear settings, and solver settings).
• Configuring load step options (including time, sub-steps, load steps, equilibrium iteration counts, and output controls).
• Applying loads (loads in ANSYS structural analysis include displacement constraints, concentrated forces, surface loads, body loads, inertial forces, and coupled-field loads, applied to key points, lines, surfaces, or volumes of the geometric model).
• Running the solution.

3. Postprocessing

After completing the calculation, results are reviewed through postprocessing modules. ANSYS provides two postprocessing modules: the general postprocessor (POST1) and the time-history postprocessor (POST26). These modules enable easy access to solution results, including displacement, temperature, strain, and heat flux. Results can also be processed through mathematical operations and output in graphical or tabular form. Key outputs include structural deformation diagrams, internal force diagrams (axial force, bending moment, and shear force diagrams), node displacements, stresses, strains, and contour plots of displacement, stress, and strain—providing critical data for problem analysis.

ANSYS offers over 100 element types to simulate various materials and structures in engineering. Combinations of different elements form abstract models for specific physical problems. For example:

• In tunnel engineering, linings are simulated using BEAM3 beam elements, and the interaction between surrounding rock and structures is modeled with COMBIN14 spring elements.
• In slope engineering, slope soil is simulated using plane elements.
• In hydraulic engineering, 3D dam analysis uses solid elements, while 2D analysis uses plane elements; reservoir gates are simulated with shell elements.
• In bridge structure simulation, beam elements model steel and concrete beams with different cross-sections, shell elements simulate thin-walled structures like bridge decks and box girders, and link elements model prestressed steel bars and trusses.
• In building structures, beam elements simulate frame columns, shell elements model roof slabs, solid elements represent mass concrete, and link elements simulate prestressed steel bars.

Static analysis is generally required for structures, and results must meet design requirements. When dynamic loads are negligible compared to static loads, static analysis alone suffices. However, practical engineering structures may be subject to significant dynamic loads (e.g., buildings under earthquakes, ships under wave action, bridges under vehicle loads), requiring dynamic analysis. ANSYS dynamic analysis includes four types: modal analysis, harmonic response analysis, transient dynamic analysis, and spectrum analysis, addressing various engineering dynamic problems.

In ANSYS, modal analysis refers to the study of a structure’s natural vibration characteristics, focusing on natural frequencies and mode shapes. Its results serve as the basis for other dynamic analyses. Harmonic response analysis determines the steady-state response of linear structures under sinusoidally time-varying loads, revealing how structural response varies with frequency. Transient dynamic analysis calculates structural dynamic responses under arbitrarily time-varying loads, obtaining time-dependent displacement, strain, stress, and force under combined steady, transient, and harmonic loads. Spectrum analysis links modal analysis results with known spectra to calculate structural responses, used to determine dynamic responses to random or time-varying loads.

ANSYS also supports nonlinear analysis. Structural nonlinear problems fall into three categories: geometric nonlinearity, material nonlinearity, and state nonlinearity. Geometric nonlinearity arises from nonlinear strain-displacement relationships, leading to nonlinearity in the total stiffness equation of finite element analysis. Material nonlinearity stems from nonlinear material constitutive relationships, causing structural stiffness nonlinearity. State nonlinearity is associated with structural state changes, with contact problems being the most common example. For nonlinear analysis, in addition to general analysis options and load step settings, nonlinear options (particularly convergence criteria and equilibrium iteration counts) must be configured, which is critical for accurate results.

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