Transcript Title

Failure of composites
John Summerscales
Outline of lecture
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Strength
Failure mechanisms
Fractography
Failure criteria
Fracture mechanics
Strength
• strength = stress at failure
• failure may be
yielding in metals
o non-recoverable loss of elastic response
o first-ply failure
o ultimate failure
o
• one material can have
several different strengths
Strength
• Kelly-Tyson equation for UD composites:
σc = σfVf + σm*(1-Vf)
o σc < σm#(1-Vf)
o
at high Vf, or
at low Vf
where σm* is the tensile stress in the matrix
at the failure strain of the fibre, and
 σm# is the maximum tensile strength of the matrix

• For small mis-alignments:
o
σc = σfVf / cos2θ = σfVfsec2θ
Failure mechanisms
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matrix cracking
fibre fracture
debonding = interface failure
delamination = interlayer failure
fibre pullout
• micro-buckling
• kink bands
• cone of fracture
Failure strain of composites
• The key criterion for composite failure is the
local strain to failure: ε’
a.k.a. elongation at break
and not stress
• Note that ε’ for the fibre/matrix interface
i.e. transverse fibres = ~0.25 %
Matrix cracking
max
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polyester resin
vinyl ester
epoxy resin
phenolic resin
ε’
ε’
ε’
ε’
=
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=
0.9-4.0
1.0-4.0
1.0-3.5
0.5-1.0
%
%
%
%
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data from NL Hancox, Fibre Composite Hybrid Materials, Elsevier, 1981.
min
Fibre fracture
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S/R-glass
E-glass
Kevlar 49
HS-carbon
UHM-carbon
ε’
ε’
ε’
ε’
ε’
=
=
=
=
=
4.6-5.2 % ….
3.37 % ……….…
2.5 % ……………….
1.12 % …………………
0.38 % …………………..
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data from NL Hancox,
Fibre Composite Hybrid Materials, Elsevier, 1981.
Fibre-matrix debonding
a
b
c
• Crack can run
through (not shown), or
around the fibre
• NB: ~12000 carbon or 1600 glass UD fibres / 1mm2
Fibre-matrix debonding:
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)
o Z-pinning
o
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)
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 forces
as the fibre is pulled from the opposite face
• debonding and pullout absorbs high energies
and results in a tough material
Micro-buckling
In bending tests, failure occurs due to:
• poor fibre/matrix adhesion
in combination with
• the stress concentration
at the loading roller
Kink bands (HM fibre composites)
• Compressive load causes buckling followed by
co-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/
nanoclay.htm
Cone of fracture (CFRP)
• the impacted face shows no sign of damage
delamination occurs in a cone
o fibre spalling from the back face
o
• known as BVID
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barely visible impact damage
• difficult to detect unless reported
Fractography
• use of optical or electron microscopes
to image the fracture surface:
Fracture mechanics
• stress intensity factor (Pa.m1/2 )
• fracture toughness
(critical stress intensity factor, Pa.m1/2 )
o
separate parameters in each plane
crack
o
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.
Design to avoid failure
• Beware first ply failure
dependent on laminate stacking sequence
• failure index (FI) of >1 = failure
dependent on the failure criteria selected
• reserve factor (RF) <1 = failure
for Tsai-Hill failure criteria, RF =1/√(FI)
Failure criteria
• failure occurs when
local stress reaches a critical value:
o
σi ≥ σi'
or
τij ≥ τij'
(' indicates failure condition)
• von Mises yield criterion:
o
critical distortional strain energy
• Tresca yield criterion:
o
maximum shear stress
• Tsai-Hill criterion:
o
an envelope in stress space
• …. and many others
Failure criteria
• those above plus many other criteria
o
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).