1. Failure of composites
John Summerscales

2. Outline of lecture
Strength
Failure mechanisms
Fractography
Failure criteria
Fracture mechanics

3. 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

4. Strength Kelly-Tyson equation for UD composites:
σc = σfVf + σm*(1-Vf) at high Vf, or
σc < σm#(1-Vf) 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:
σc = σfVf / cos2θ = σfVfsec2θ

5. Failure mechanisms matrix cracking fibre fracture
debonding = interface failure
delamination = interlayer failure
fibre pullout
micro-buckling
kink bands
cone of fracture

6. 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 %

7. Matrix cracking polyester resin ε’ = 0.9-4.0 %
max min
polyester resin ε’ = %
vinyl ester ε’ = %
epoxy resin ε’ = %
phenolic resin ε’ = %
data from NL Hancox, Fibre Composite Hybrid Materials, Elsevier, 1981.

8. Fibre fracture S/R-glass ε’ = 4.6-5.2 % …. E-glass ε’ = 3.37 % ……….…
Kevlar 49 ε’ = 2.5 % ……………….
HS-carbon ε’ = 1.12 % …………………
UHM-carbon ε’ = 0.38 % …………………..
data from NL Hancox, Fibre Composite Hybrid Materials, Elsevier, 1981.

9. Fibre-matrix debondingc a b
Crack can run through (not shown), or around the fibre
NB: ~12000 carbon or 1600 glass UD fibres / 1mm2

10. Fibre-matrix debonding:

11. 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

12. 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)

13. 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

14. Micro-buckling In bending tests, failure occurs due to:
poor fibre/matrix adhesion in combination with
the stress concentration at the loading roller

15. 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
aem/people/samit/ nanoclay.htm

16. 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

17. Fractography
use of optical or electron microscopes to image the fracture surface:

18. 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),
crack

19. 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)

20. Failure criteria
failure occurs when local stress reaches a critical value:
σi ≥ σi' or τij ≥ τij' (' 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

21. 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 x).