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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. 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

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  1. On the experimental modelling of delaminations in composite materials C. Devivier, F. Pierron and M. R. Wisnom

  2. 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. 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. 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)

  5. Samples Undamaged 50mm delamination Single layer of PTFE (~25µm thick)

  6. Experimental set-up • 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

  7. Strain extraction 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

  8. Strain extraction Unwrapping with custom made algorithm. point-to-point differentiation “Equivalent strains” {e}=t/2{k} p: grid pitch h: distance grid-sample

  9. Resolutions and noise level 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)

  10. 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. Results (1/4) • To compare correctly experiments and FE » Same processing Out-of-plane displacement Slopes differentiation Curvatures differentiation Equivalent strains Scaling: Thin plate theory

  12. Equivalent strain maps (in mm/m)for the undamaged sample (2/4) Transverse strains Longitudinal strains Twist strains

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

  14. Equivalent strain maps (in mm/m)for two impacted samples (4/4) Transverse strains Longitudinal strains Twist strains

  15. 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

  16. 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. Thank you for your attention Do you have any questions?? devivier@ensam.fr fabrice.pierron@chalons.ensam.frm.wisnom@bristol.ac.uk

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