Compression after impact (CAI) tests on stitched materials

Compression after impact (CAI) tests on stitched materials

MATERIALS

Basic composite: E glass-epoxy laminate (Vf.= 60 %) 10 layers pre-preg
Stitching fibres: Kevlar 49

Three series of specimens:

  • without stitching (standard S)
  • stitched with grid size of 5 mm (stitched St 5)
  • stitched with grid size of 10 mm (stitched St10)
  • TESTING PROCEDURE
    Impact test apparatus ( CEAST Fractovis) with 0.5 inch-ball bearing (ASTM D3029)
    Three impact energies: 10 J, 18 J, 22 J
    After impact, image analysis (false colours: pixels 0-255)
    Compression test: loading velocity 0.04 kN/s

    NON-DESTRUCTIVE MONITORING
    Acoustic emission monitoring (LOCAN-AT): 2 acquisition sensors at two sides of impact zone & 2 guard sensors on compression frame + Image analysis of impacted area (pixel counting and colour discrimination).

    RESULTS

  • Image analysis discerns the different levels of damage and allows us to evaluate if the post-impact situation can be allowed. If not, the material should be redesigned.
  • The growth of damage area with impact energy was easily discernible.
  • The introduction of more reinforcement wires throughout the structure produces preferential ways for delamination, due to the non constant volume distribution of the wires themselves.
  • Acoustic emission monitoring during compression tests shows a decreased ultimate load due to increased stitching density and to presence of relevant mechanical problems even in the basic material.
    Post-impact damage is exalted, as the dimensions and inaccuracies in grid size of the stitching proved not able to stop progressing of delamination.
  • COLLABORATION : SISTEMA COMPOSITI

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