“In today’s era of rapid national development with a high demand for deep diving and high load capacity, carbon fibre materials have been widely used in the pressure-resistant structures of manned and unmanned submersibles due to their excellent properties, such as low density, high strength, high modulus, fatigue resistance, corrosion resistance and design flexibility,” wrote Guo and his collaborators from Harbin Institute of Technology, a university sanctioned by the US.

In the West, carbon fibre has been considered unsuitable for use in the submarine industry.

Many industry insiders attributed this tragedy to the weakness of carbon fibre underwater: under immense pressure, water can penetrate gaps between the fibres, and repeated ascents and descents potentially crack and deform the hull.

The carbon fibre pressure hull made in China has a wall about 3cm thick – one-quarter the thickness of Titan’s walls. According to Guo’s team, however, during laboratory tests it easily withstood water pressure of 77 megapascals (MPa), more than double the pressure Titan’s hull could withstand in lab tests at the University of Washington.

Guo and his colleagues estimate a cabin made of this carbon fibre material could withstand 90MPa pressure, equivalent to an ocean depth of nearly 9,000 metres. However, to ensure safety, they have set its operational depth at 6,000 metres, corresponding to a pressure of 60 MPa.

The external layer of the carbon fibre pressure hull is a slim 1mm waterproof coating. The researchers said the coating provided a certain degree of protection and did not allow leaks, damage, interfacial debonding, cracking or other issues during normal use.

It could withstand a degree of impact, scraping and other challenges without failure or damage, according to Guo’s team.

“After experiencing high pressure for a long time, the appearance of the pressure test sealing system outside the pressure hull remained intact, with neither leakage nor weight increase in the pressure hull,” Guo’s team wrote.

The front and rear ends of the carbon fibre pressure hull use T4 titanium alloy material. Because it must withstand alternating seawater pressure from zero to 6,000 metres, the designers have considered fatigue issues, such as pressure and temperature variations in the carbon fibre hull and the joints between carbon fibre and titanium alloy.

“It can tolerate alternating cycles more than 2,000 times,” they said in the paper.

Guo’s team has also developed a carbon fibre unmanned submersible hull suitable to operate at less than 200 metres deep. With a thickness of just 3mm but a diameter of 1 metre, it could be used for low-cost underwater drones operating in the shallows.

“Underwater submersibles are an indispensable means to safeguard national marine rights, interests and resource security. Their technological level represents, to some extent, national defence capabilities and technological prowess,” Guo’s team wrote in a separate paper published in the same journal in May.

03:25

North Korea claims to test nuclear-capable underwater attack drone

North Korea claims to test nuclear-capable underwater attack drone

Both China and the United States plan to deploy a large number of unmanned weapons in key waters, such as the Taiwan Strait and the South China Sea. The side that wins in a potential conflict will largely depend on the performance and quantity of their drones.

According to a report released last month by the Washington-based think tank Centre for a New American Security, a drone battle in the Taiwan Strait differs significantly from the situation on the Ukrainian battlefield because underwater drones are more difficult to destroy or interfere with than aerial drones.

Chinese factories produced nearly half the world’s carbon fibre last year. With many new production lines under construction, some industrial experts estimate China’s production capacity may increase significantly in the coming years, further reducing the carbon fibre price.

Carbon fibre produced in China is mainly used in sport and leisure products. In the Chinese market, the price of a carbon fibre bicycle has dropped to just over 3,000 yuan, or about US$410 – one-tenth of the price several years ago.