On the experimental modelling of delaminations in composite materials
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Transcript of 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
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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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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
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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
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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
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Unwrapping with custom made algorithm.
p: grid pitchh: distance grid-sample
point-to-point differentiation
{}=t/2{}
“Equivalent strains”
Strain extraction
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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)
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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
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Results (1/4)• To compare correctly experiments and FE
» Same processing
Out-of-planedisplacement
differentiation Slopes
differentiation Curvatures
Scaling:Thin plate theory
Equivalentstrains
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Equivalent strain maps (in m/m)for the undamaged sample (2/4)
Longitudinal strains Transverse strains Twist strains
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Equivalent strain maps (in m/m) for the sample witha single, full width, 50mm-long delamination in the midplane (3/4)
Longitudinal strains Transverse strains Twist strains
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Equivalent strain maps (in m/m)for two impacted samples (4/4)
Longitudinal strains Transverse strains Twist strains
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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.
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Thank you for your attention
Do you have any questions??
[email protected]@chalons.ensam.fr [email protected]