JEC COMPOSITES MAGAZINE - Issue #81 - May/June 2013 - 72

NDT

Fig. 4: Mode I and Mode II matrix damage, (a) rivers (b) cusps

This type of damage mainly appears
in the final stages of ruin, in areas
where fibre orientation more or less
coincides with the stress axis. This is
usually the case in high stress-applying regions of parts.

c
Fig. 3: Different types of failure, (a) interlaminar
failure, (b) intralaminar failure, (c) translaminar
failure

In the case of a laminated composite, an interlaminar damage called
delamination can occur in addition
to intralaminar damage. When cracks
develop in a ply, their propagation is
stopped by the adjacent plies. At the
intralaminar crack tip, the singularities of stress make the cracks propagate at the interface between two
adjacent ply layers. In the case of a
laminated composite with different
ply directions, delamination can also
be initiated by the stiffness difference
of the various plies that form the laminate. This damage can also result from
causes like manufacturing defects or
impact events.
Finally, when the volume ratio of
matrix damage reaches a certain level,
the final stage of damage is a failure of
the fibres, called translaminar failure.

The different stages of the damage
process occur earlier or later depending on the type and direction of the
reinforcement and on the type of
mechanical stresses applied.
Matrix damage (intra and interlaminar
failure) in Mode I (opening mode)
grows by forming characteristic events
like rivers (see below) or other events.
In Mode II (shearing mode), matrix
damage grows forming cusps and
scallops caused by the failure coalescence and evenly spaced at an angle
of 45° relative to the fibres. The shape
of cusps gives information about the
quality of the matrix/fibre interface.
Damage to the reinforcement (translaminar failure) is much more harmful
to the residual strength of a composite than matrix damage. Fibre static
failures under tension have a characteristic failure surface on each individual fibre, with an initiation point and
propagation lines.
The fibres are subjected to compres-

sion break due to geometric instability. The localized damage grows with
shear failure and step region of microbuckling (see below). The individual
fibres then fail through bending (tension on one side, compression on the
other). With thermoplastic matrices,
the fibres may fragment in the failure
zone due to the lack of stiffness of the
matrix. This unstable damage mechanism makes the composite significantly less strong in compression than
under tension.
Although composites have excellent
performance levels under fatigue
stress, this type of stress sometimes
leads to the catastrophic in-service
failure of structures. The distribution
of damage is then different to that
observed under static stress. Translaminar failures develop in the zones
of stress concentration or around
manufacturing defects. The pictures
below illustrate some characteristic
events observed on fatigue failure in
TPCs and TSCs.
Understanding damage mechanisms
is also useful for going back to the
failure scenario when composite parts
fail in service. The example below
shows the reconstruction carried out
following the in-flight failure of a helicopter tail. Non-destructive testing

Fig. 5: Compression failure: (a) shear bands, (b) TSC fibre failure, (c) TPC fibre failure

72 jec composites magazine / No81 May - June 2013

JEC COMPOSITES MAGAZINE - Issue #81 - May/June 2013

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