Femap and LS-DYNA: Rocket Science Made Easy

Transcription

1 George Laird, Ph.D., P.E., Principal Mechanical Engineer Adrian Jensen, EIT, Sr. Staff Mechanical Engineer Femap and LS-DYNA: Rocket Science Made Easy Femap Symposium 2014 May 14-16, Atlanta, GA, USA Unrestricted Siemens AG 2014 FEMAP SYMPOSIUM 2014 Discover New Insights

2 Agenda LS-DYNA: Unified Numerical Architecture for FEA Focus of Presentation (Abstract) Why Explicit Implicit versus Explicit Element Quality is Paramount Building LS-DYNA Models Best Practices LS-DYNA Unified Solver Page 2

3 Femap + LS-DYNA: Rocket Science Made Easy ABSTRACT LS-DYNA is well-known in the world of transient, dynamic nonlinear analyses but is often perceived as a difficult code to use on a daily basis for general implicit-type work that more standard implicit codes have dominated for years. To address this perception gap, this paper demonstrates how easily Femap can be used to drive LS-DYNA in the solution of simple linear static analyses all the way to the most complex simulations involving aviation seats and satellites. Along the way, the capabilities of Femap and LS-DYNA will be showcased to show how one analysis package can solve problems from rigid-body dynamics to normal modes analysis (PSD) to complete system level models. Case studies will be used to highlight the effectiveness of this combination. Page 3

4 LS-DYNA: Unified Numerical Architecture Why Explicit? Page 4

5 Explicit Solves Where Implicit Can t 16g Seat Crash Test TSO-C127a / SAE AS8049A / 14 CFR Part Femap FEA Model FEA + LS-DYNA Model Validation is Gold Page 5

6 Explicit Solves Where Implicit Can t Page 6

7 Explicit Solves Where Implicit Can t Page 7

8 Explicit Solves Where Implicit Can t Cargo Net Development Impact Analysis Impact of Plastic Foams Plastic Thread Design Modal Analysis Digger Tooth Failure Simulation Page 8

9 Explicit Solves Where Implicit Can t Page 9

10 LS-DYNA: Unified Numerical Architecture Implicit versus Explicit? Page 10

11 Implicit versus Explicit Explicit only works when there is acceleration (dynamic) whereas an implicit approach can solve the dynamic and the static problem. For dynamic problems, this means that we are solving the following equation: ma n + cv n + kd n = f n where n=time step. A common terminology is to call the kd n part the internal force in the structure. The basic problem is to determine the displacement at some future time or d n+1, at time t n+1. In conceptual terms, the difference between Explicit and Implicit dynamic solutions can be written as: Explicit: d n+1 = f d n, v n, a n, d n 1, v n 1, All these terms are known at time state n and thus can be solved directly. For implicit, the solution depends on nodal velocities and accelerations at state n+1, quantities which are unknown: Implicit: d n+1 = f v n+1, a n+1, d n, v n,. Given these unknowns, an iterative solution at each time step is required. Explicit is fast since it is a direct linear algebra calculation to arrive at all quantities for the future time. It also means that you can t jump very far ahead. Implicit, you have to calculate future entities and requires a decomposition of the stiffness and mass matrices pain in the ass and slow. But one can use large steps. Page 11

12 Implicit versus Explicit Explicit: Direct No Iteration Implicit: Matrix Algebra & Iteration Page 12

13 Implicit versus Explicit: Explicit Time Step Explict: F = ma + cv + ku and solve for u i+1 = f u i, v i, a i, u i 1, v i 1, (no F = Ku solution with u = k f since there is no mass) C AccousticWaveSpeed = E Material ρ Material C Aluminum = 10e6 (1 2 ) = 11,400 mph 2.75x10 4 ExplicitTimestep = Length Element C Wavespeed = 1" 11,400 MPH = 5.0x10 6 second Page 13

14 Implicit versus Explicit: Explicit Time Step Standard Mesh: Solution Controlled by Misc. Small Elements Better Hex Mesh & 3x Faster Page 14

15 Implicit versus Explicit: Explicit Time Step Mesh Controls but there are tricks: Mass Scaling & CPU Scaling Page 15

16 Implicit versus Explicit Don t Fear the Explicit Standard Linear Implicit Explicit / Implicit Explicit Page 16

17 LS-DYNA: Unified Numerical Architecture The Importance of Mesh Page 17

18 The Importance of Mesh Quality Classic Explicit Clean, Mapped Mesh Classic Implicit Random Element Shapes Explicit propagates at the speed of sound o Mapped mesh provides greater confidence o Typically worth the effort to spend time on mastering the quad and hex meshing techniques Page 18

19 The Importance of Mesh: Looks Ain t Everything Explicit Elements Use 1-Point Guass Integration (default) Explicit can be very sensitive to mesh density due to element formulation and nonlinear effects Page 19

20 The Importance of Mesh: A Couple of Bad Elements Can Kill You Bad Elements Can Generate Interesting results Solution stability can be affected by bad elements Femap provides Explicit Time Step checking Time spent up-front typically pays dividends Explicit contains more physics and it helps to understand them in debugging Page 20

21 LS-DYNA: Unified Numerical Architecture Building LS-DYNA Models in Femap Page 21

22 Building LS-DYNA Models Render Unto Caesar. Airplane Seat 16g Sled Test with ATD Build model in Femap o Check Explicit Time Step o Add Contacts o Material Laws o Perform FEA checks (component masses) Add anthropomorphic test device o Leverage LS-PrePost Dummy Positioning o Create *Include file and add to Femap Post Process in LSPP o Fast o Extensive support of results files and cross-plotting o Tracking ability for objects that move Page 22

23 Building LS-DYNA Models Glue the Un-Hexable and Tet the Rest. Viscoelastic Plastics and Seal Design Hex mesh is more robust o o Gluing between hex regions Easy within Femap Four-Node Tetrahedrals o Tet Growth Ratio = 1.0 o ELFORM=13 Full Contact / Full Nonlinear o Material Stress/Strain Curves o Femap base material support o Robust model setup for quick runs Femap Fully Supports Axisymmetric (super sweet for seal analysis) Page 23

24 Building LS-DYNA Models Be Economical..and Play Chess Hex Blade-Out Containment Turbine Blade Hexed o Complex Hexing in Femap Model Organization o o LS-DYNA likes Properties Contact based on Properties Implicit Spin Up o Femap verification - Static Implicit / Explicit Switching o o Femap base material support Robust model setup for quick runs Page 24

25 Building LS-DYNA Models Leverage the Complete Toolbox Bird-Strike (FEA + SPH) Build in Femap o o o RBE2 to CNRB Composite to *PART_COMPOSITE Model Checkout LSPP for SPH Page 25

26 LS-DYNA: Unified Numerical Architecture Best Practices Page 26

27 Best Practices No Substitute for Organic Learning Get On-Line and Ramp Up Page 27

28 Best Practices No Substitute for Organic Learning Our LS-DYNA Class Notes Are Open to Femap Clients Page 28

29 Best Practices No Substitute for Organic Learning Page 29

30 Best Practices Attend Classes and Conferences Networking is Awesome Visit Predictive Engineering at Booth 300 We will be presenting papers on DEM and SPH Methods Page 30

31 LS-DYNA: Unified Numerical Architecture LS-DYNA Implicit and Multi-Physics Page 31

32 LS-DYNA: Unified Numerical Architecture for Everything LS-DYNA s Objective is MPP for Every Discipline Page 32

33 LS-DYNA: One Code Strategy Unified Numerical Architecture for Everything Combine the Multi-Physics Capabilities into One Scalable Code for Solving Highly Nonlinear Transient Problems to Enable the Solution of Coupled Multi-Physics and Multi-Stage Problems Implicit Capabilities (partial list) Static and Dynamic Linear and Nonlinear Analysis Normal Modes Analysis and Flexible Body Analysis Frequency Domain Analysis, Response Spectrum, PSD, NVH, Fatigue Acoustic Analysis (BEM Method and FEM) Explicit Capabilities (partial list) o Dynamic Nonlinear Analysis and Rigid Body Dynamics o FSI with ALE or CFD (compressible / incompressible solvers) o DEM, SPH, EFG, Electromagnetism, Optimization, Topology Page 33

34 LS-DYNA: Unified Solver Capabilities LS-DYNA s Objective is MPP for Every Discipline: Implicit Mechanics CPU Scaling Normal Modes / PSD Page 34

35 LS-DYNA: Unified Solver Capabilities Strong Coupling Between DEM and FEA Page 35

36 LS-DYNA: Unified Solver Capabilities FSI with ALE (short duration / explicit) and CFD (long duration / implicit) Page 36

37 LS-DYNA: Unified Solver Capabilities FSI with ALE (short duration / explicit) and CFD (long duration / implicit) Page 37

38 Predictive Engineering, Inc. Located in Portland, OR George Laird, Ph.D., P.E. Principal Mechanical Engineer Adrian Jensen, BSME, EIT Sr. Mechanical Engineer Predictive Engineering, Inc SE 11 th, Suite 310 Portland, OR Phone: +01 (503) Mobile: +01 (503) Page 38