Researchers have come up with a new method of internal stress assessment for composite materials, which are regularly used in aerospace.

Scientists from the NUST MISIS Center of Composite Materials, led by Professor Sergey Kaloshkin, have proposed a new, non-contact method of internal voltage control in polymer composites.

They say their technology significantly simplifies, speeds up and reduces the cost of assessing the state of composite materials.

A “huge problem”

Andrey Stepashkin, Candidate of Technical Sciences and Senior Research Associate at the NUST MISIS Center of Composite Materials, explains: “There are composite materials in which the internal stress reaches 95% of the tensile strength after fabrication. It will crack if we add even a little more pressure. For example, a number of composite materials, processing excellent heat and thermal resistance and created for the ‘Buran’ spacecraft, had a high level of internal stresses due to the characteristics of their manufacturing. This has become a huge problem: to get one [functioning] piece of ‘Gravina’ material (which was used for the ship’s black plating), 50 had to be thrown out.”

There is no such level of internal stress in carbon plastics, fibreglass, or hybrid composite materials after manufacturing. Instead, stresses arise and accumulate under the influence of operating loads, the external environment and weather, which can lead to damage in the material and reduce its load-bearing capacity. Such changes affect operational safety and must be identified in a timely manner.

There are methods to control stresses in composite structures but they are often inconvenient and lack forecast accuracy. For example, non-contact methods (ultrasonic, acoustic flaw detection, shearography) allow the detection of defects that have already occurred, and do not provide information about the stress within the material or about their distribution throughout the structure.

The accepted methods for assessing the stress state in a framework all require contact and connection to the material using the sticker of various film sensors. To date, detecting something before a defect appears has been impossible with non-contact methods.

Measuring stress

The research uses amorphous soft magnetic circuits of 10-60 microns in diameter to assess the stress state in composite materials. Wires, during the manufacturing stage, are laid between the carbon fibre layers, forming a stress-sensitive grid.

The stress state, surrounding the micro-wire, affects how the substance reacts to external magnetic fields. Accordingly, these measurements can be carried out without contact, a connection to the sensing element, or the sticker – which is embedded inside the material during the manufacturing stage.

The researchers only have to use one sensor, unlike some methods of flaw detection where equipment needs to be exposed on the equipment on both sides.

The researchers say their technology significantly simplifies, speeds up, and reduces the cost of assessing the state of composite materials by making repairs easier and predictions about future defects more accurate, all in a contactless manner.

The team has worked out how to introduce soft magnetic wires into the composite material and how to make sure that the properties of the composite material from this do not deteriorate. Likewise, they have also mastered different measurement modes.

The findings have already received positive feedback from representatives in the aviation and space industries, as well as developers of composite materials, the researchers report.

Get out of the lab

Andrey Stepashkin says that now researchers have to “get out of the laboratory” to develop a ‘field’ prototype and measure systems based on lab devices.

Stepashkin comments: “We have made the first step of a long journey. But we already see a practical application of our development. In addition, it has more features: the micro-wire mesh introduced into the material can provide an additional drain of the static charge that occurs in fibreglass structures. Our wires are quite capable of replacing metal grids which are now inserted into these materials.”

The research work is published in the Journal of Alloys and Compounds.

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