using ncode designlife for fatigue of composite structures · using ncode designlife for fatigue of...

HBM Prenscia: Public

© 2016 HBM

Using nCode DesignLife for Fatigue of Composite Structures

Jeff MentleyHBM Prenscia

2016 nCode User Group Meeting

© 2016 HBM October 5-6, 2016 www.ncode.com


© 2016 HBM

3

1. Introduction2. Short Fiber Injection Molded Composites

Material CharacterizationStrain Energy Damage Parameter

3. Continuous Fiber Composite Failure Criteria

Agenda

3




© 2016 HBM

4

• What is a fibre‐reinforced plastic composite material?

Ideal material when high stiffness‐to‐weight and strength‐to‐weight ratios are required –For example:

Introduction

“A structural composite is a material system consisting of two or more phases on a macroscopic scale, whose mechanical performance and properties are designed to be

superior to those of the constituent materials acting independently.”(Daniel and Ishai, 2006)

BMW i3(fireballbase.com)

Boeing 787 Dreamliner(1001crash.com)

(Bloomberg, 2013)




© 2016 HBM

5

• Categories of composite materials

Composite Materials

Injection MoldedMolding

Compound

Continuousfibers

Discontinuous fibers

Particulate filler Other(e.g. sandwiches)




© 2016 HBM

6

• Categories of composite materials

Composite Materials

Unidirectional Woven fabrics Laminates

ContinuousFibers

Discontinuous Fibers

Particulate filler Other(e.g. sandwiches)

Unidirectional composite lamina(Jones, 1999)

Longitudinal direction

Transverse direction Fill

direction

Warp direction

Woven composite lamina(Jones, 1999)

Unbounded view of a laminate(Jones, 1999)




© 2016 HBM

Short Fiber Injection Molded Composites

7




© 2016 HBM

8Flow of material into mold controls microstructure

Courtesy of

Resulting material is:• Inhomogeneous• Anisotropic• Temperature sensitive• Environment sensitive (polyamides)• Viscous component to behaviour

2-D random

fibers alignedwith flow

fibers aligned transverse to flow

FLOW

Skin-core effect




© 2016 HBM

9Process Modelling for Fatigue

Damage,LIFE

Manufacturing simulation

Microstructureorientation, aij

Fatigue material model

Basic material information and fatigue test

data

Structural FE calculation

Structural material model

Basic material information and physical test

data

Fatigue simulation Loading




© 2016 HBM

10

• Test coupons are:• Inhomogeneous

• Anisotropic

• Leading to non‐uniform stress distribution spatially and through thickness

• We are not testing a material, but rather a complex structure• Calibration of material models requires a measure of “reverse engineering” based

on detailed FE analysis of coupons• If we are creating SN curves

• What is stress?

Fatigue characterisation challenges




© 2016 HBM

11

• Nylon (PA66) with 30% glass fibers• Injection molded plaques• Determine microstructure • Specimens cut from plaques

• 0, 45 & 90 degree orientations

• Environmental chamber• Reduce environmental variations

• Room temperature (23 C)

• 50% humidity

• R = ‐1 loading• Reduce viscoelastic behaviour

• Anti‐buckling guides

• Calibrate 3 fatigue damage models• Simple interpolation method

• Digimat high cycle fatigue model

• Elastic strain energy approach

Material characterisation and validation program




© 2016 HBM

12Experimental Measurement of Microstructure

0%W_40%L_Avg z/h A11 A12 A22 A33 A13 A23

‐0.9231 0.7858 0.0069 0.0618 0.1523 0.005 0.0594 ‐0.7692 0.8993 ‐0.0136 0.0096 0.0911 ‐0.0219 0.0181 ‐0.6154 0.8365 ‐0.0072 0.0289 0.1345 ‐0.015 0.0285 ‐0.4615 0.8083 7.75E‐04 0.0464 0.1453 0.0082 0.0479 ‐0.3077 0.8063 0.0127 0.0216 0.1722 0.0139 0.0362 ‐0.1538 0.7036 0.05 0.0243 0.2721 0.0638 0.045

0 0.2445 ‐0.0054 0.0319 0.7235 ‐0.05 0.0826 0.1538 0.7124 ‐0.0442 0.0216 0.2659 ‐0.114 0.0486 0.3077 0.7953 ‐0.0048 0.0195 0.1853 ‐0.0105 0.0375 0.4615 0.8341 0.0043 0.0184 0.1475 ‐0.0122 0.0327 0.6154 0.8502 0.0063 0.0116 0.1382 ‐0.0123 0.0272 0.7692 0.8542 ‐0.0016 0.025 0.1208 0.0062 0.0285 0.9231 0.7907 0.0292 0.0236 0.1857 0.0157 0.0405

6005004003002001000

50

40

30

20

10

0

Shape 2.108Scale 272.8N 669

length (um)

Freq

uenc

y

Histogram of length (um)Weibull

• Orientation tensors evaluated at 9 points on plaque

• At 13 points through thickness• Averaged over 6 plaques• Fiber length distribution determined

at plaque centroid




© 2016 HBM

13Simulation Based Prediction of Microstructure

y

z

Courtesy of




© 2016 HBM

14Specimen preparation

Specimens are:

• Machined from plaques in 3 orientations

• Single specimen per plaque• Gage region at plaque center




© 2016 HBM

15

• ANSYS Workbench• 40,000 linear hex elements• 13 layers through thickness• Each layer assigned anisotropic properties based on measured microstructure• Enforced displacements at gripped end tabs

Stress analysis results of specimens




© 2016 HBM

16

0°

Stress analysis results of specimens

45°

90°




© 2016 HBM

17

• Mean effective modulus from tensile tests compared with FE solution

• Based on:• Load cell and clip gauge

Tensile tests and correlation

Effective Modulus (MPa)

0 Degree 45 degree 90 degree

FE 6448 3188 2702Tensile test (mean) 6391 3230 2559




© 2016 HBM

18

• Tests carried out in load control with strain monitoring using a clip gauge

• Elliptical hysteresis loops –viscoelastic behaviour

• Cyclic softening throughout test (modulus change)

• Highly distorted hysteresis loop immediately prior to failure

Fatigue behaviour under test




© 2016 HBM

19Net section stress SN curves

Net se

ction stress ra

nge




© 2016 HBM

20

Damage equation

W = (Nf)

Strain energy history

W(t) = ij(t) ij(t)/2

stiff not stiff

=

Strain Energy Density Damage Parameter

flow

direction




© 2016 HBM

21WN curve based on local stresses and strains

Strain Ene

rgy Den

sity Parameter W

90 degree results conservative wrt W – but based on very thin hot‐spot!

Net section stress‐life curve converted to local strain energy‐life curve based on the FE simulation results for each orientation




© 2016 HBM

22With simple averaging domain approach

Mod

ified

Strain En

ergy D

ensity Parameter W

Compensation for stress gradient effect by averaging strain energy density over a small volume (several elements)




© 2016 HBM

23

• “Simple” test coupons are actually complex structures.• A proper understanding of material behaviour involves reverse engineering from

physical tests, based on manufacturing simulations and detailed stress analysis of the coupons

• Strain energy density parameter shows some promise in correlating fatigue behaviour with material in different orientations

• Reduces fatigue testing by a factor of 3

• Currently doing physical testing to develop and validate enhanced SFC methods

Summary




© 2016 HBM

Continuous Fiber Composite Failure Criteria

24




© 2016 HBM

25

• Use of composite materials is still expanding• Aerospace and Wind turbines largest users carbon fiber

• 43,500 tons used in 2012

• Aerospace 18%

• Wind turbines 23%

• Automotive use of carbon fiber to be >$6 billion by 2020 overtaking aerospace

• Known technology already used within most FE solvers• Utilises DesignLife's standard features to give the user an distinct advantage over

what they currently have• Combined with measured loads allows all load combinations to be analysed• Current FE implementations require each load combination to be determined and

solved separately

Composite Static Failure Criteria




© 2016 HBM

26

• Composite laminates fail in a variety of mechanisms (Herakovich, 1998)

• They all generally occur in combination:

• At the micro level, e.g.:• Fiber fracture

• Fiber buckling

• Fiber splitting

• Fiber/matrix debonding

• Fiber pullout

• Matrix cracking

• At the laminate (macro) level:• Fiber‐dominated failure

• Matrix‐dominated failure

• Delamination

• Failure at the micro scale does not necessarily lead to immediate failure of a laminate

• Here we focus on failure at the macro scale

Failure mechanisms

Failure modes in UD composites (Reinforced Plastics, 2003)




© 2016 HBM

27

• Metals• Main assumptions:

• Linear elastic behaviour up to material yield strength,

• Isotropic behaviour

Failure criteria for composites

0

Typical ‐ curve

Same curve regardless of the loading direction,

Load cases




© 2016 HBM

28

• Metals• For multiaxial load cases, a yield criteria is used to characterise the end of linear elastic

behaviour

• Examples of yield criteria (Jones, 1999):• Maximum stress

• Tresca

• Von‐Mises





© 2016 HBM

29

• Composites• Main assumptions:

• Brittle material

• Linear elastic behaviour

• Anisotropic behaviour (x‐y coordinate system)


0

• The x‐y coordinate system is not convenient for defining a composite failure criterion because the x‐ and y‐axes are not aligned with the principal material axes

Typical ‐ curvesLoad cases




© 2016 HBM

30

• Composites• Main assumptions:

• Brittle material

• Linear elastic behaviour

• Orthotropic behaviour (1‐2 material coordinate system)


0

Typical ‐ curvesLoad cases

• The 1‐2 coordinate system is convenient for defining a composite failure criterion because the 1‐ and 2‐axes are aligned with the principal material axes




© 2016 HBM

31

• Loading the material with various , combinations gives the following envelope (assuming 0 for now)

• A failure criterion is a mathematical expression, under the form:, 1

giving a curve connecting the experimental points all together


, 1




© 2016 HBM

32

• The previous experimental envelope was utopian

• In reality, there is always a certain amount of scatter in the test results

• Examples of real experimental failure envelopes:


ATJ‐S graphite(Tang, 1979)

Satin‐weave glass/epoxy(Tang, 1989)




© 2016 HBM

33

• Influence of on the shape of the failure envelope, , , 1

• The envelopes are plotted when 0‐100% of the material shear strength,


3D representation( ‐ ‐ space)

2D representation( ‐ space)




© 2016 HBM

34

• Limit or non‐interactive failure criteria• Maximum stress (12.0)

• Maximum strain (12.0)

• Partially interactive failure criteria• Christensen (12.0)

• Hashin‐Rotem (12.0)

• Hashin (12.0)

• Hashin‐Sun (12.1)

• NU (12.1)

• Fully interactive failure criteria• Franklin‐Marin (12.0)

• Hoffman (12.0)

• Norris (12.0)

• Norris‐McKinnon (12.1)

• Tsai‐Hill (12.0)

• Tsai‐Wu (12.0)

• Custom (Python‐based)

Composite Failure Criteria in nCode DesignLife

111

, 1, 1, , 1




© 2016 HBM

35

• Failure criteria currently extended to fatigue

• Approaches developed for counting cycles

• Fatigue property characterization studies currently inderway

• Beta versions currently installed at key OEM accounts

Futures

Fatigue failure envelopeStatic failure envelope




© 2016 HBM

www.hbmprenscia.com

Jeff Mentley

[email protected]

Email: [email protected]

Connect with us on:

linkedin.com/company/hbm‐ncode | youtube.com/hbmncode | @hbmncode | +nCode



using ncode designlife for fatigue of composite structures · using ncode designlife for fatigue of...

Documents