Evaluation of Residual Stresses due to Spherical Impact using LS – DYNA

Jason FayerMANE-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 PlateEntire Plate

BC’s applied at 4 edges

Status- St Venant’s Principle Reduce Run Time by using a submodel

Reduced PlateEntire Plate

Status- R. M. Davies Calculations Experimental calculations used to

validate model and contact algorithm  LS-DYNA

Hand Calculations Source

Duration of Impact 1E-5 s 9E-5 s Davies

Radius of Impact 0.0074" 0.0082" Davies

Impact Force 56 lbs 18 Lbs Jackson Green

Status- Matching Experimental Data

Modified Johnson Cook Material Card used for analysis

Boyce Experimental FEM- Boyce FEM- LS DYNA

σ radial (ksi) -76 -61 -39

σ hoop (ksi) 21.7 39 18

displacement (in) 0.016 0.016 0.015

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

Risks

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.