On the experimental modelling of delaminations in composite materials
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On the experimental modelling of delaminations in composite materials. C. Devivier, F. Pierron and M. R. Wisnom. Introduction. Dramatic effects from very small impacts Barely visible impact damage (BVID) caused by tool drops BVID involves delaminations Resulting damage pattern is complex.

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On the experimental modelling of delaminations in composite materials

C. Devivier, F. Pierron and M. R. Wisnom


Introduction
Introduction materials

  • Dramatic effects from very small impacts

  • Barely visible impact damage (BVID)caused by tool drops

  • BVID involves delaminations

  • Resulting damage pattern is complex


Introduction1
Introduction materials

  • More simple model

     samples with single delaminations

  • Test in cantilever bending using grid method by deflectometry

  • Issues with artificial delaminations

    • in literature  PTFE film works for mode I

    • in this paper  mode II studied

  • Objective:

    Create a non-destructive evaluation for delaminations


Dimensions and material
Dimensions and material materials

  • Simple cantilever beam with a point load

  • Samples: 250mm long, 50mm wide and 4mm thick

  • 32 plies in a quasi-isotropic layup : ([0 45 -45 90]4s)

  • Carbon fibre composite (IM7-8552)


Samples
Samples materials

Undamaged

50mm delamination

Single layer of PTFE

(~25µm thick)


Experimental set up
Experimental set-up materials

  • Distance grid-sample=1.66 m

  • Load=5 N

  • Grid pitch = 1.5 mm

  • 7 pixels per grid pitch

  • 5 Mpix camera with a 28-200mm nikon zoom

  • Coated with an opaque resin

Point load

Camera

Sample

Top view

Grid


Strain extraction
Strain extraction materials

Spatial phase shifting (windowed discrete Fourier transform)

Unloaded state

» 1 picture

unloaded state subtracted to loaded state

Loaded state

» 1 picture

-Unloaded state:

» Longitudinal

» Transverse

-Loaded state:

» Longitudinal

» Transverse


Strain extraction1
Strain extraction materials

Unwrapping with custom made algorithm.

point-to-point differentiation

“Equivalent strains”

{e}=t/2{k}

p: grid pitch

h: distance grid-sample


Resolutions and noise level
Resolutions and noise level materials

Out of plane:

In plane:

Resolution in strains:

=2µm/m

=6800 µm/m

x10-3

h: distance grid-sample (1600 mm)

sf: phase standard deviation (0.01 rad)

dx: pixel size

(1.5 mm inplane

0.7 mm out of plane)

t: specimen thickness (4 mm)

p: grid pitch (1.5 mm)


Fe model
FE model materials

  • Elements

    • Type: 8-nodes linear elastic brick

    • Dimensions: 1 mm x 1 mm x 0.125 mm

    • Properties: UD material properties + orientation

  • Delaminations:

    • Coincident nodes disconnected

    • Surface contact introducedto prevent penetrations


Results 1 4
Results (1/4) materials

  • To compare correctly experiments and FE

    » Same processing

Out-of-plane

displacement

Slopes

differentiation

Curvatures

differentiation

Equivalent

strains

Scaling:

Thin plate theory


Equivalent strain maps in m m m for the undamaged sample 2 4
Equivalent strain maps (in materialsmm/m)for the undamaged sample (2/4)

Transverse strains

Longitudinal strains

Twist strains


Equivalent strain maps (in materialsmm/m) for the sample witha single, full width, 50mm-long delamination in the midplane (3/4)

Transverse strains

Longitudinal strains

Twist strains


Equivalent strain maps in m m m for two impacted samples 4 4
Equivalent strain maps (in materialsmm/m)for two impacted samples (4/4)

Transverse strains

Longitudinal strains

Twist strains


Conclusion
Conclusion materials

  • Behaviour of artificial delaminations characterized by experimental method.

  • Created delamination:

    • Good agreement with FE for longitudinal strains,

    • Not so good for twist strains,

    • Inconclusive for transverse strains because of loading.

  • Real impact:

    • Indication on damage severity by measurement system


Future work
Future work materials

  • Test on samples with different types of inserts:

    • Double layer,

    • Different insert material,

    • Release agent.

  • Find a way to avoid the issue of deforming in plasticity the outer edges.

  • Link delamination behaviour with porosity.

  • Compare artificially introduced and real impacts in plates.


Thank you for your attention
Thank you for your attention materials

Do you have any questions??

[email protected]

[email protected]@bristol.ac.uk


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