Failure of composites

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Failure of composites

• John Summerscales

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Outline of lecture

• Strength
• Failure mechanisms
• Fractography
• Failure criteria
• Fracture mechanics

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Strength

• strength stress at failure
• failure may be
• yielding in metals
• non-recoverable loss of elastic response
• first-ply failure
• ultimate failure
• one material can have several different
  strengths

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Strength

• Kelly-Tyson equation for UD composites
• sc sfVf sm(1-Vf) at high Vf, or
• sc lt sm(1-Vf) at low Vf
• where sm is the tensile stress in the matrixat
  the failure strain of the fibre, and
• sm is the maximum tensile strength of the matrix
• For small mis-alignments
• sc sfVf / cos2? sfVfsec2?

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Failure mechanisms

• matrix cracking
• fibre fracture
• debonding interface failure
• delamination interlayer failure
• fibre pullout
• micro-buckling
• kink bands
• cone of fracture

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Failure strain of composites

• The key criterion for composite failure is the
  local strain to failure e a.k.a. elongation
  at breakand not stress
• Note that e for the fibre/matrix interfacei.e.
  transverse fibres 0.25

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Matrix cracking
max min

• polyester resin e 0.9-4.0
• vinyl ester e 1.0-4.0
• epoxy resin e 1.0-3.5
• phenolic resin e 0.5-1.0
• data from NL Hancox, Fibre Composite Hybrid
  Materials, Elsevier, 1981.

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Fibre fracture

• S/R-glass e 4.6-5.2 .
• E-glass e 3.37 .
• Kevlar 49 e 2.5 .
• HS-carbon e 1.12
• UHM-carbon e 0.38 ..
• data from NL Hancox, Fibre Composite Hybrid
  Materials, Elsevier, 1981.

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Fibre-matrix debonding
c
b
a

• Crack can run through (not shown), or around
  the fibre
• NB 12000 carbon or 1600 glass UD fibres / 1mm2

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Fibre-matrix debonding
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Delamination of layers

• one layer is a lamina (plural laminae)
• several layers in a composite is a laminate
• separation of the layers is delamination
• to avoid delamination
• 3-D reinforcement (often woven or stitched)
• Z-pinning

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Stress whitening of GFRP

• both debonding (fibre/matrix separation) and
  delamination (layer separation)create internal
  defects which scatter light
• the consequence is that the transparency of the
  laminate becomes more opaque,referred to as
  stress whitening
• similar effects may be seen in other composites
  (e.g. at stitches in NCF CFRP)

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Fibre pullout

• as parts of a fractured composite separate,
• the fibres which have debonded can fracture
  remote from principal fracture plane.
• energy is absorbed by frictional forcesas the
  fibre is pulled from the opposite face
• debonding and pullout absorbs high energies and
  results in a tough material

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Micro-buckling

• In bending tests, failure occurs due to
• poor fibre/matrix adhesion in combination
  with
• the stress concentration at the loading roller

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Kink bands (HM fibre composites)

• Compressive load causes buckling followed
  byco-operative failure of a group of fibres to
  produce short lengths of parallel mis-oriented
  fibre
• Image from
• http//coeweb.eng.ua.edu/aem/people/samit/nanocl
  ay.htm

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Cone of fracture (CFRP)

• the impacted face shows no sign of damage
• delamination occurs in a cone
• fibre spalling from the back face
• known as BVID
• barely visible impact damage
• difficult to detect unless reported

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Fractography

• use of optical or electron microscopesto image
  the fracture surface

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• Fracture mechanics
• stress intensity factor (Pa.m1/2 )
• fracture toughness(critical stress intensity
  factor, Pa.m1/2 )
• separate parameters in each plane
• mode I (x) II (y) III (z)
• JG Williams,Fracture mechanics of composite
  failure, Proc IMechE Part C Journal of
  Mechanical Engineering Science, 1990, 204(4),
  209-218.

crack
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Design to avoid failure

• Beware first ply failuredependent on laminate
  stacking sequence
• failure index (FI) of gt1 failure dependent
  on the failure criteria selected
• reserve factor (RF) lt1 failure for Tsai-Hill
  failure criteria, RF 1/v(FI)

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Failure criteria

• failure occurs when local stress reaches a
  critical value
• si si' or tij tij' (' indicates failure
  condition)
• von Mises yield criterion
• critical distortional strain energy
• Tresca yield criterion
• maximum shear stress
• Tsai-Hill criterion
• an envelope in stress space
• . and many others

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• Failure criteria
• those above plus many other criteria
• no agreement !(see MJ Hinton, AS Kaddour and PD
  Soden, Failure criteria in fibre reinforced
  polymer composites the world-wide failure
  exercise,Elsevier, Amsterdam, 2004.  ISBN
  0-08-044475-x).