on the experimental modelling of delaminations in composite materials

ON THE EXPERIMENTAL MODELLING OF DELAMINATIONS IN COMPOSITE

MATERIALS

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

2/17

5th CompTest conference14-16 February 2011

Introduction

• Dramatic effects from very small impacts

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

• BVID involves delaminations

• Resulting damage pattern is complex

3/17


Introduction• 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

4/17


Dimensions and material

•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)

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Undamaged

50mm delamination

Samples

Single layer of PTFE(~25µm thick)

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•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

Top view

Grid

Camera

Sample

Point load

Experimental set-up

7/17


Unloaded state » 1 picture

Loaded state » 1 picture

Spatial phase shifting (windowed discrete Fourier transform)

-Unloaded state: » Longitudinal » Transverse

-Loaded state: » Longitudinal » Transverse

unloaded state subtracted to loaded state

Strain extraction

8/17


Unwrapping with custom made algorithm.

p: grid pitchh: distance grid-sample

point-to-point differentiation

{}=t/2{}

“Equivalent strains”

Strain extraction

9/17


Resolutions and noise level

In plane: Out of plane:

Resolution in strains:

=6800 µm/m

=2µm/m

x10-3

p: grid pitch (1.5 mm)dx: pixel size(1.5 mm inplane0.7 mm out of plane)

t: specimen thickness (4 mm)

h: distance grid-sample (1600 mm): phase standard deviation (0.01 rad)

10/17


FE model

• 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

11/17


Results (1/4)• To compare correctly experiments and FE

» Same processing

Out-of-planedisplacement

differentiation Slopes

differentiation Curvatures

Scaling:Thin plate theory

Equivalentstrains

12/17


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

Longitudinal strains Transverse strains Twist strains

13/17


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


14/17


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


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Conclusion

• 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

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Future work

• 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.

17/17


Thank you for your attention

Do you have any questions??

[email protected]@chalons.ensam.fr [email protected]

on the experimental modelling of delaminations in composite materials

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