MATERIALS | COMPOSITES
Right: Composite pipes are commonly used in infrastructure applications
and corrosion resistance. However, defects introduced during manufacturing can affect performance. Closer monitoring of the process can help to overcome this. When a defect is spotted, the process can be stopped and action taken. However, stopping the process is costly – so it is vital to decrease downtime during manufacturing. “Potential solutions are through process optimi-
sation for defect reduction and an in-depth under- standing of the effect of parameters that cause defect formation in the pipe,” say the researchers, in the journal Applied Composite Materials. A well-manufactured TCP will rely on a number
of factors, including both the materials and processes used. For instance, TCPs are made using a melt fusion bonding process involving heating and consolidation – plus other factors such as the consolidation speed and pull force. “Thermal behaviour is essential at this phase, as it determines the curing rate,” said the researchers. “This study indicates that laser heating is the better heat source in efficiency terms.” Defects such as fibre misalignments, voids and
delamination can be induced during manufacturing. There are many sources for these. Voids are the most commonly studied manufacturing defect for composite parts. They are essentially “unfilled pores…occupied with gas rather than solid material”. The study says there is a need to explore the best defect characterisation methods during TCT manufacture. “In-situ characterisation aims to derive high-
quality TCP with reduced defects – or need for repairs – at increased production rate, while maintaining the current manufacturing process,” said the researchers.
Sewage failure Failure analysis is critical in understanding how and why pipes – and other components – fail in service. This allows designers and manufacturers to fix the problem in future designs. Researchers at Sichuan University in China have analysed failures in glass fibre-reinforced polymer (GFRP) pipe in high-pressure sewage transport. The research was carried out on failed GFRP
pipes from sour oilfield gathering and transporta- tion pipelines. The matrix material was an epoxy resin of an aromatic amine system that destructive- ly cracked after 15 years of operation. To find the underlaying cause, the failed part of the pipe was compared to the undamaged area. Then, the pipe’s micro-morphological structure, chemical composition and mechanical properties were examined.
12 PIPE & PROFILE EXTRUSION | January/February 2023
Results revealed cavity defects in the GFRP pipe with insufficient air bubbles and resin filling. In addition, the overall resin content of the pipe was low; the curing degree of the outer layer of the damaged area (DA-OL) was insufficient; and the fibre bonding strength was weak. Fourier transform infrared spectroscopy (FTIR)
and X-ray electron spectroscopy were used to characterise the degradation of the amine curing agent in the outer layer of the pipeline and the oxidative decomposition of the resin. These revealed that an irreversible chemical degradation process had occurred, reducing the performance of the fibre-matrix interface. “Data from nano-indentation and dynamic thermomechanical characteristics confirmed the reduction of nano-hardness and elastic modulus in the outer layer of the GFRP tubes,” said the researchers, in the journal Engineering Failure Analysis. “The failure of the pipeline was caused by the interaction of these factors.”
Under pressure
Although composite pipes have properties such as high strength and strong resistance to external pressure, they are typically subjected to a sustained load – which leads to creep over the long term. Researchers at Nanjing Tech University in China have developed an analytical approach to determine the viscoelastic properties of this type of pipe – leading to a potentially more accurate way of assessing creep behaviour. “In the literature, only the viscoelastic property
of a single-layer pipe— or the bonding interlayer in composite pipes – has been investigated,” said the researchers, in the journal Buildings. “Few studies have addressed viscoelastic composite pipe, in which all of the pipe layers are viscoelastic.”
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