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An investigation into the face sheet (skins) debonding of glass balsa sandwich composites

An investigation into the face sheet (skins) debonding of glass balsa sandwich composites. Comptest Lausanne 02/2011 Dr M. Colin de Verdiere ( mcdv1u07@soton.ac.uk ) Professor J.M Dulieu - Barton, Professor R.A Shenoi and Dr J.I.R Blake. Content. Introduction. Manufacture of specimens.

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An investigation into the face sheet (skins) debonding of glass balsa sandwich composites

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  1. An investigation into the face sheet (skins) debonding of glass balsa sandwich composites Comptest Lausanne 02/2011 Dr M. Colin de Verdiere (mcdv1u07@soton.ac.uk) Professor J.M Dulieu - Barton, Professor R.A Shenoi and Dr J.I.R Blake

  2. Content • Introduction. • Manufacture of specimens. • Testing for debonding characterisation. • Material and crack characteristics. • Experimental results. • Digital image correlation for added material information. • Parameters estimation. • Conclusion.

  3. Introduction • Advantages • Requirement Weather (hail) Mine countermeasure vessels using glass-balsa sandwiched structures Mine blast Tool falling on deck Rough seas

  4. Variability of balsa wood End grain balsa sheets are made of many different blocks Defects (gaps in between blocks) Balsa core coated in resin and drying in oven to avoid excessive resin absorption Each blocks has different mechanical properties

  5. Specimen manufacture Specimen 10 Specimen 2 Specimen 1 Specimen 7 Specimen 6 Specimen 12 Specimen 11 Specimen 9 Specimen 4 Specimen 5 Specimen 3 Specimen 8 Specimen 13 Specimen 10 Specimen 2 Specimen 1 Specimen 9 Specimen 7 Specimen 6 Specimen 11 Specimen 4 Specimen 3 Specimen 5 Specimen 8 Crack film Crack film Reduced panel Specimen 23 Specimen 26 Specimen 15 Specimen 14 Specimen 19 Specimen 20 Specimen 22 Specimen 24 Specimen 25 Specimen 17 Specimen 18 Specimen 16 Specimen 21 Large panel

  6. Specimen manufacture Baslsa core Crack film • Vacuum bag • Flow media • Peel ply • DBL 800 • CSM Mat • CSM Mat • DBL 800 • Peel ply • Flow media • Mould

  7. Specimen characteristics Face sheet (skin) Core Pre crack area Face sheet (skin) Width: 35 mm Crack length: 50 mm Length: 200mm Core thickness: 13-40mm Face sheet (skins) thickness: 4 mm

  8. Specimen characteristics • During debonding of the face sheet (skins) the following effects are looked at: • Core thickness • Crack film thickness • CSM mat or no CSM mat • Epoxy or vinylester resin 40 mm 13 mm 60 μm 120 μm 14 μm CSM Mat No CSM Mat Epoxy resin Vinylester resin

  9. Debond testing Mode I

  10. Debonding of epoxy specimen Mode I (14μm crack film, core thickness 13 mm, Mat CSM)

  11. Debonding of epoxy specimen Mode I (14μm crack film, core thickness 13 mm, Mat CSM)

  12. Debonding of epoxy specimen Effect of core thickness (14μm crack film, core thickness 40 mm, Mat CSM) Two different modes of failure depending on the specimen position in the panel: interface or wood crack propagation (just below interface) Interface crack propagation Balsa cracks propagation

  13. Debonding of epoxy specimen Effect of crack film thickness (60-120 μm crack films, core thickness 13 mm, Mat CSM) Crack film thickness 120 μm Crack film thickness 60 μm

  14. Debond testing Mode II

  15. Debonding of epoxy specimen Mode II (14μm crack film, core thickness 13 mm) Mat CSM No mat CSM

  16. Debond testing Mixed Mode

  17. Debonding of epoxy specimen Mixed Mode (14μm crack film, core thickness 13 mm, Mat CSM)

  18. Vinylester specimen Mixed Mode (14 μm crack film, core thickness 13 mm, Mat CSM)

  19. Epoxy – vinylester

  20. Digital image correlation Strain versus crack path through time Strain extraction along the crack path. The strain is extracted at different time during crack loading and propagation: Pixels 10% 5% 0% -5% -10% Strain εyy 0 20 40 60 80 100

  21. Parameter estimation • Face sheet and core stiffness • Stress at which the crack propagate in Mode I and II • G strain release energy rate in Mode I and II F (N) σ : Stress to propagate the crack G d (mm)

  22. Conclusion • Variability of balsa wood is important • Thicker core specimens provided less reproducible results and lower GIC. • Thick crack film led to unsteady crack tip initiation and propagation and • should be avoided. • The presence of mat layer is beneficial. • Vinylester resin was weaker than epoxy resin in mix mode loading. • GIC computed by recording of the crack tip location during loading. • GMMB calculation scatter due to the difficulty to locate the crack. • Mode II crack location was not detectable to the naked eyes. • To improve this reading usage of digital image correlation may help to refine the crack location.

  23. Future work • Validation of the optical crack location method in Mode I and usage in Mode II for GIIC calculation. • Refinement of materials parameters • Numerical validation in Mode I, II and mix mode and comparison to experimental results.

  24. Thank you

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