Workbench120力学非线性_procedure幻灯片.ppt

Workbench - Mechanical Structural Nonlinearities,Chapter 2 General Procedures,In this chapter, general tools and procedures, not specific to a particular source of nonlinearity, but useful for achieving convergence and post processing results are introduce: Building a Nonlinear Model Analysis Settings Postprocessing Nonlinear Results Workshops,Chapter Overview,A. Building a nonlinear model,What is different about building a nonlinear model vs. a linear model? In some cases, there will be no difference! A model undergoing mildly nonlinear behavior due to large deflection and stress stiffening effects might need no modification with regards to geometry set up and meshing. In other cases, you must include special features: Elements with special properties (such as contact elements) Covered in Chapter 3 & 4 Nonlinear Material data (such as plastic stress-strain data) Covered in Chapter 5 & 6 Include geometric features to overcome singularities that cause convergence trouble. (i.e. add radius to sharp corner for example) You might also need to give special attention to: Load and boundary condition limitations under large deflection Mesh control considerations under large deflection Element technology options under large deflection with nonlinear materials,It is important to note the orientation of loads and its effect on the structure in large-deflection analyses:,Load,Direction Before Deflection,Direction After Deflection,,Acceleration (constant direction),Pressure (always normal to surface),,,,,,,,,,,,,,,. Building a nonlinear model,Force, Moment, Bolt Load (constant direction),,,,,,,,,,,,,,,,,,,,,Meshing considerations are usually the same in nonlinear analyses. However, if large strains are expected, the shape checking option may be changed to “Aggressive” For large-deflection analyses, if elements may undergo some change in shape, this may reduce the fidelity of the solution By using “Aggressive” shape checking, WB-Mechanical will ensure that the element quality is much better prior to solution in order to anticipate distortion of the element in the course of a large-strain analysis. The quality of the “Standard” shape checking is suitable for linear analyses, so it does not need to be changed in linear analyses With “aggressive” shape checking set, some mesh failures may be more likely. See WB-Mechanical - Intro for ways to detect and remedy mesh failures.,,. Building a nonlinear model,For any structural element, DOF solution Du is solved at nodes Stresses and strains are calculated at integration points. They are derived from DOF. For example, we can determine strains from displacements via: Where B is called the strain-displacement matrix The image on the right shows a 4-node quad element with 2x2 integration, integration points shown in red. When we post-process results, stress/strain values at integration points are extrapolated or copied to nodal locations linear results are extrapolated Nonlinear results are copied,. Building a nonlinear model,With Element Control set to Manual, users can manually toggle between Full and Reduced Integration Schemes This option influences the number of integration points within an element. This switch only applies to higher order elements. It is sometimes helpful to force full integration when only one element exists across the thickness of a part for improved accuracy.,,,. Building a nonlinear model,By default, workbench mechanical element technology will mesh geometry with higher order elements (with midside nodes). Users have the option to drop midside nodes In challenging large deflection, bending dominated problems with nearly or fully compressible nonlinear materials, it can sometimes be advantageous to drop the midside nodes and allow the code to implement enhanced strain formulations automatically Refer to Appendix B for a more detailed discussion of element technology.,,. Building a nonlinear model,Kept midside nodes (Quadratic shape function),Dropped midside nodes (Linear shape function),,,,,,,,,,,,What is different about obtaining a nonlinear solution? Multiple matrix solutions: Linear static requires only one pass through the matrix equation solver (Figure on left) Nonlinear performs a new solution with every iteration (Figure on right).,,B. Obtaining a nonlinear solution,,,,,,K,F,u,F,u,,,,,,,,,,,,,1,2,3,4,F = Ku,Fi = Kiui,Ki,…What is different about obtaining a nonlinear solution? Analysis settings has many options that need to be considered for a nonlinear run. Step Control - Load steps and substeps Solver Control - Solver types Nonlinear Controls - N-R convergence criteria Output Controls - controlling what data is saved thru the load history In the following slides, we consider each of these tools,,. Obtaining a nonlinear solution,Step Controls “Auto Time Stepping” under Step Controls, enables user to define an initial, minimum and maximum number of substeps per loadstep. If Mechanical has trouble converging, it will use these auto time stepping spe。