JEC COMPOSITES MAGAZINE - Issue #129 - July/August 2019 - 58

TECHNOLOGY ageing

Fig. 2: Immersion of an E-glass/epoxy
composite material in seawater for experimental tests

Fig. 1: Failure in pipe joints due to fluid absorption/ageing
process (redrawn from A. Toulitsis et al. [32])

or the plasticisation when water diffuses into the layers. Additionally,
various other salt ions present in seawater (NaCl, MgCl2) are likely to
disrupt the bond between hydrogen and carbon in the epoxy resin and
will attempt to join the carbon. This also produces strain in the matrix
owing to size variations of the atoms. The influence of seawater absorption on composites using various types of matrices was examined by
Dell'Annoand Lees [30].
As shown in Figure 1, the fluid absorption/ageing process starts at the
pipe's inner surface. In the first phase, physical changes due to water
diffusion occur within the pipeline boundary. In the second phase, the
diffusion continues through the thickness of the pipeline and the inner surface of the pipeline structure may start to age chemically. Fluid
diffusion and chemical ageing continue during the third phase, but the
development of cracks on the inner surface of the pipe results in faster
fluid diffusion into the inner layers of the material. In the fourth phase,
diffusion, mechanical damage and chemical ageing damage continue
and the mechanical damage on the inner surface is sufficient to remove
the pipe's piles. Finally, during the fifth phase, complete damage occurs
followed by leaking. In the oil and gas industry, the definition of failure generally depends on the type of equipment and the consequences
of failure, but the equipment should be designed to work until phase
three. However, the analysis conducted shows that the ideal target for
designers to minimize the risk of sudden disastrous failure is somewhere between phases two and three [31].
The breakdown process starts when the fluid enters into the piles of the
composite pipe. As the process progresses, the failure occurs ultimately,
as shown in Figure 1.
There are three types of common failures in composite pipes:
1) weeping, a passage of fluid through cracks; 2) burst, failure of the
fibres over a prolonged period of use; and 3) strain corrosion, a combination of resin cracks and fibre rupture, and of bending strains in
corrosive media. Other less common failure modes result from leakage
in pipe connections or impact damage. According to several studies,
weeping failure is controlled only by the resin and occurs in pipes under
high strains. The creation and growth of cracks allow the passage of liquid [32, 33]. Burst failure is controlled only by the fibres and measures
the ability of glass fibres to resist chemical attack by liquids [34].

jec composites magazine / N°129 July - August 2019

Fig. 3: Causes of failures in composite pipes due to the ageing process in seawater

Finally, strain corrosion failure is controlled both by the resin and fibres

[35]. To investigate the ageing period of composite laminates/speci-

mens, researchers usually prepare artificial seawater as per the ASTM
D1141 standard technique. Likewise, some try to use natural seawater
to make calculations according to environmental conditions such as
temperature, humidity, etc.
Analysis of composite pipe failure due to seawater ageing
Various factors affecting the mechanical and structural properties of
glass/epoxy composite pipes may cause failure in seawater due to ageing.

Moisture absorption analysis
Previous works [36-41] showed that the passage of water in glass/epoxy
composite pipes involves three different stages. The first stage is the circulation of water molecules in the minor gaps between the composite fibre chains. The second stage involves the transport of water into
the openings and cracks that combine the fibre and the matrix, due to
half-finished saturation or impregnation.
The passage in the matrix of micro-cracks created during the compounding process is the third mechanism [42, 43]. Swelling of the matrix is also due to moisture absorption. Swelling of composite pipe piles
is self-possessed in the fibre direction and major residual stresses are

JEC COMPOSITES MAGAZINE - Issue #129 - July/August 2019

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