Evaluation of Residual Stresses due to Spherical Impact using LS – DYNA Jason Fayer MANE-6980 ENGINEERING PROJECT Spring 2010 Status Update
Introduction/Background • Objective of Project • Create an alternate solution for predicting residual stresses in dents. The solution will be using LS-DYNA to provide a cost effective and repeatable solution. • Background Information • In the aerospace industry, components are often damaged due to dents during the assembly process and during the components life cycle due to FOD. When the fatigue life is evaluated in thin components, the ultra conservative assumption that the dent can be modeled as a through crack is used to calculate crack propagation. If realistic stresses can be simulated with LS-DYNA, a more realistic cycle evaluation can be provided, thus providing significant cost savings. • Expected Project Outcome • Obtain residual stresses in a component due to a dent from a spherical impact simulated in LS – DYNA. The residual stresses can then be used to evaluate crack growth and fatigue life
Status • Model Details: Reduced Plate Entire Plate BC’s applied at 4 edges
Status- St Venant’s Principle • Reduce Run Time by using a submodel Entire Plate Reduced Plate
Status- R. M. Davies Calculations • Experimental calculations used to validate model and contact algorithm
Status- Matching Experimental Data • Modified Johnson Cook Material Card used for analysis
Problem Description • Dials to Turn • Mesh Density • Material Properties • Strain Rates • Failure strains (too possibly match high velocity impacts) • Simulation Formulations • Explicit analysis • Implicit Seamless springback simulation • Implicit dynain springback simulation • Contact Parameters • Contact Type • Contact stiffness • Dampening • Element types • Time Steps
Status • Progress • Complete review of reference materials • Calculated strain rates for TI64 • Created Plate and Sphere model • Explicit model running in LS-DYNA • Could not get contact to behave correctly when the sphere was set as a rigid material. Material set as elastic, which differs from the analyses trying to be matched • Currently debugging model • Deformation not consistent with expected results. Need to investigate: • Contact Parameters • Investigated automatic surface to surface, automatic node to surface, automatic single surface, automatic general • Mesh density • Investigated this at beginning of model creation. Global sizing of .02” to 0.01” had negligible effect on von mises stress.
Status • Progress continued • Currently debugging model • Discussed results with industry experts • Determined that the mesh was too coarse for accurate results • Mesh density increased significantly (.0001-.0002” global mesh size) • For debugging purposes, plate reduced to 0.007”x0.007”x0.005” • Run time still ~10 hrs
Methodology/Approach • Review Background Material • Perform Analysis to match experimental data observed in “The Residual Stress State Due to a Spherical Hard-Body Impact” by B.L. Boyce • Perform analytical calculations • Create Ball and Plate model • Apply Material Properties, Boundary conditions, contact parameters, etc. • Perform explicit simulation in LS-DYNA by applying velocity to ball • Debug Accordingly • Perform explicit / implicit Analysis in LS-DYNA by applying springback simulation to analysis • Debug Accordingly to match experimental results • Compare experimental results, analytical results, and numerical results • Record residual stress, strain, displacement, and run-time (cost) • Make conclusions • If time permits, compare crack growth prediction of plate with: • Through crack • Residual Stress
References • B. L. Boyce, X. Chen, J. W. Hutchinson, and R. O. Ritchie, "The residual stress state due to a spherical hard-body impact", Mechanics of Materials, 33(8), 2001. • Input Parameters for Springback Simulation using LS-DYNA. Bradley N. Maker. Xinhai Zhu. Livermore Software Technology Corporation. June, 2001 • Office of the Aviation Research Washington, D.C. 20591 DOT/FAA/AR-00/25: Experimental Investigations of Material Models for Ti-6Al-4V Titanium and 2024-T3 Aluminum, by U.S. Department of Transportation Federal Aviation Administration. Final Report September 2000.