MATERIALS | COMPOSITES
Above: The oil and gas sector is increasingly turning to
composite pipe
Also, pipes in existing studies are usually subjected to uniform radial load – and not sus- tained compression, they said. A key finding of the research was that the pipe layers and the bonding interlayer both exhibit viscoelastic properties. Equations for the viscoelas- tic composite pipe combine the exact elasticity theory with the viscoelastic theory. Solutions are derived using a Fourier series expansion followed by a Laplace transform. “Results indicate that this solution has a higher
computational efficiency than the finite element solution,” said the researchers. In addition, they found that the modulus and thickness of each layer had a significant influence on the stresses and displacements. These can be optimised by adjusting the modulus and thickness of each layer in the viscoelastic composite pipe.
Pipe bending At the same time, researchers at Wuhan University of Technology in China have investigated the circumferential bending of glass fibre-reinforced plastic (GFRP) pipes with varying fibre content across a range of temperatures. “Up to now, most studies on the stiffness of GFRP pipes have been conducted mainly under ambient conditions,” said the researchers, in a paper published in the journal Polymers. “Tempera- ture is an important parameter affecting the mechanical properties of GFRP.” The pipes were tested at 30, 50 and 70°C. They
were also classified into three types, depending on their formulation. Type I was a near-equal mix of resin, fibres and sand; Type II contained more fibres and less sand; and Type III was around one-third resin and two-thirds fibres (with no sand). Results showed that bending performance tended to decrease as temperature rose. Retention of circumferential stiffness was 80-85% and retention of bending strength and damage
14 PIPE & PROFILE EXTRUSION | January/February 2023
displacement was 25-40% from 30 to 70°C. The rate of decay of ring stiffness, bending
strength and damage displacement was much higher at 30-50°C than it was at 50-70°C. The damage pattern was affected by both temperature and continuous fibre content. At 30°C, delamination occurred at the top and
bottom of the Type I GFRP pipe before fracture damage happened at the left and right ends. Fracture damage occurred at both the left and right ends of the Type II and Type III GFRP pipes. Delamination happened at the upper and lower ends of the GFRP pipes at 50°C and 70°C. “The results provide a solid support for the study
of temperature on the bending performance of GFRP pipes,” said the researchers.
Environmental stress Composite pipes are used widely in infrastructure applications, transporting everything from drinking water to sewage. This means that the composite material is tailored to each different application. Now, researchers at Gheorghe Asachi Technical University in Romania have studied the change in the structure of glass-reinforced plastic (GRP) composite materials caused by the characteristics of the surrounding soil. Starting with three types of soil – basic, acidic and
neutral – which affect composite materials, the researchers calculated the pipe damage index and Pearson correlation coefficients for axial tension. The results showed the significance of soil pH on the long-term behaviour of the buried composite pipe. They analysed three pipe samples: one from a
petroleum field (acidic); one from a salt field (basic); and a third from neutral soil. Samples were analysed at the OSPA laboratory in Romania. Sample pipes made of GRP composite material – used to make buried water transport pipes – were also buried in the three experimental locations. After 24 months, the samples were recovered.
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IMAGE: SHUTTERSTOCK
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