Case Study: Strain Rate Effect in Falling Dart Test Study: Strain Rate Effect in Falling Dart ......

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Transcript of Case Study: Strain Rate Effect in Falling Dart Test Study: Strain Rate Effect in Falling Dart ......

  • Case Study: Strain Rate Effect in Falling Rate Effect in Falling Dart Test Simulation

    Steven Hale CAE AssociatesAccurate FEA of EngineeringAccurate FEA of Engineering

    Plastics SeminarOctober 14, 2014

    2014 CAE Associates

  • Goal

    A model of a falling dart test was developed in ANSYS Workbench/LS- A model of a falling dart test was developed in ANSYS Workbench/LS-Dyna to demonstrate procedures for including strain-rate sensitivity in plastics and to show the importance of including this sensitivity when plastic parts are subjected to dynamic loads.plastic parts are subjected to dynamic loads.

    As described earlier, yield and flow in plastics varies with strain rate. Failure can also vary with strain rate.

    LS-Dyna includes a number of material models that include strain-rate LS-Dyna includes a number of material models that include strain-rate sensitivity.

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  • Falling Dart Test

    Falling Dart Impact Test Falling Dart Impact Test Simulation of ASTM Test D3763 High-speed Puncture Properties of Plastics Provides information about the energy absorbing behavior of plastics under

    impact conditionsimpact conditions. Measure the peak force at impact Measure the impact velocity and the output velocity after puncture.

    Determines energy absorptionDetermines energy absorption.

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  • Falling Dart Model

    Model: Created in ANSYS Workbench/LS-Dyna Model: Created in ANSYS Workbench/LS-Dyna Powerful and intuitive interface for preparing LS-Dyna models.

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  • Falling Dart Model

    Model: Model: Polycarbonate plate: 3.175 mm thick, 76 mm diameter

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  • Falling Dart Model

    Model:ode Impact velocity = 3350 mm/s Fully-fixed on the OD faces to simulate clamped conditions

    Eroding surface to surface contact Eroding surface-to-surface contact Sphere modeled as rigid

    Density adjusted to equal the total falling mass of the dart

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  • Falling Dart Model

    Other model settings: Other model settings: Transient time = 0.015 sec. Parallel processing (number of cpus) = 6

    H l t l B l t hk Bi d (T 6) ith HG ff 0 5 Hourlass control: Belytschko-Bindeman (Type 6) with HG coeff. = 0.5 Output points

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  • Material

    Material:ate a Impact modified Polycarbonate

    High energy absorbtion and elongation to break

    E = 2 000 MPa E = 2,000 MPa Plastic strain to failure = 85% Plastics exhibit rate-dependent behavior. This can be modeled as an

    increase in yield stress with increasing strain rateincrease in yield stress with increasing strain rate.

    IncreasingIncreasing strain rate

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  • Material Model

    Material models: Material models: Want strain-rate dependency to follow the Eyring model Yield stress vs. log strain rate is linear

    70

    80

    40

    50

    60

    Stress(M

    Pa)

    EPS=0

    10

    20

    30

    YieldS EPS=0.038

    EPS=0.45

    01 10 100 1000

    StrainRate(/s)

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  • Material Model

    Material models: Material models: Some material models can also include failure strain or elastic modulus as a

    function of strain rate.LS Dyna material models used: LS-Dyna material models used:

    Material model 24: *MAT_PIECEWISE_LINEAR_PLASTICITY Elasto-plastic material with a user-defined stress-strain curve AND strain rate scaling curve Can include a viscoplastic formulationCan include a viscoplastic formulation

    Material model 89: *MAT_PLASTICITY_POLYMER Elasto-plastic material with a user-defined stress-strain curve AND strain rate scaling curve Can also include a strain-rate dependent failure strain

    Can enter these in Workbench as a command block and include file

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  • Material Model

    Include file contains the required curves used by the material model: Include file contains the required curves used by the material model:

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  • Results

    Impact response Impact response

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  • Results

    Plastic strain Plastic strain

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  • Results

    Results: Material Model 24 Results: Material Model 24 Maximum force with and without strain rate effects included

    Excluding strain rate effects significantly underpredicts the peak force! (by 33%)

    6

    7

    4

    5

    e(kN)

    Test

    2

    3Forc

    Analysis

    Analysis NoSR

    0

    1

    0 2 4 6 8

    Time (ms)

    14

    Time(ms)

  • Results

    Results: Material Model 24 Results: Material Model 24 Impactor velocity with and without strain rate effects included

    Velocity +7% higher with strain-rate effectsVelocity +23% higher without strain rate effects Velocity +23% higher without strain-rate effects

    3.5

    2.5

    3

    m/s)

    2Velocity(m

    Test

    Analysis SR

    Analysis NoSR

    1

    1.5

    0 2 4 6 8

    15

    0 2 4 6 8Time(ms)

  • Results

    Results: Material Model 89 Results: Material Model 89 Maximum force and Impactor velocity

    Strain rate effects were reduced to improve the fit and are thus not as significant.However this model does a better job of predicting the velocity/energy losses However, this model does a better job of predicting the velocity/energy losses.

    8 3.5

    5

    6

    7

    N) 2.5

    3

    m/s)

    2

    3

    4

    Force(k

    Test

    Analysis

    AnalysisNoSR

    1.5

    2

    Velocity(m

    Test

    Analysis SR

    Analysis NoSR

    0

    1

    0 2 4 6 8

    Time(ms)

    10 2 4 6 8

    Time(ms)

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  • Conclusions

    Material models exist in LS-Dyna that can accurately predict the complex response of a plastics under impact conditions, including general failure behavior.

    It is important to include strain rate sensitivity. Material models can also account for strain rate sensitive failure.

    ANSYS Workbench/LS-Dyna provides an efficient and intuitive interface y pfor preparing LS-Dyna models, including models with plastic parts requiring complex material models.

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