Voltage without contact

    Composite materials have penetrated almost all spheres of our life, but especially strongly - into all types of transport. And from the very beginning of their implementation, there is the problem of estimating the internal stresses in a structure made of composite material, both at the stage of its manufacture and during use.



    Scientists from the Center for Composite Materials NITU "MISiS" under the guidance of Professor Sergey Kaloshkin have proposed a non-contact method for monitoring internal stresses in polymer composites. Using the new method it is much easier and cheaper to assess the state of stress and the degree of internal damage during the operation of structures made of composite materials, such as parts of aircraft, high pressure vessels, ship hulls.



    Why the problem of evaluation is critically important, says one of the authors of the work, a senior researcher at the Center for Composite Materials at NITU “MISiS”, Ph.D. Andrey Stepashkin: “There are composite materials, where the internal stress after manufacturing reaches 95% of the ultimate strength. That is, he has to add a little more stress - and he will crack. For example, a number of composite materials created for the reusable Buran spacecraft, possessing excellent heat resistance and thermal strength, due to the characteristics of their manufacturing technology, had a high level of internal stresses. This has become a huge problem: in order to get one product from the Gravimol material (which was used for the black skin of the ship), it took fifty to make a marriage. ”



    In CFRP, fiberglass, hybrid composite materials after manufacturing there is no such level of internal stresses. But they arise and accumulate under the action of operational loads, the external environment and weather factors, which can lead to the appearance of damage in the material and a decrease in its carrying capacity. Such changes affect the safety of operation, and they must be detected in a timely manner.

    Methods of stress control in composite structures exist, but they are often inconvenient and sometimes completely inapplicable because of the accuracy of the prediction. For example, non-contact methods (ultrasonic, acoustic flaw detection, sherography) can only detect defects that have already appeared and do not provide information about the stresses in the material or their distribution over the structure of the body. And the accepted methods for estimating the stress state in the structure are contact and require connecting to the material with the help of a sticker of various film sensors - says Andrey Stepashkin. So, the “before the appearance of the defect” method is practically not covered by contactless methods. Therefore, we are developing such a method. ”



    The idea behind the work is to use amorphous magnetically soft microwires with a diameter of 10 - 60 microns to assess the stress state in the composite material. The wires at the manufacturing stage are laid between the CFRP layers, forming a grid sensitive to stresses.

    The stress state in the material surrounding the microwire affects how the substance in the wire reacts to an external magnetic field. Accordingly, these measurements can be carried out without contact, no connection is required to the sensitive element, its label is not required, since it is embedded inside the material to the required depth at the manufacturing stage. It is also important that you can use only one sensor, in contrast to some of the applied methods of flaw detection, for which you need to put the equipment on both sides of the part under study. In fact, this technology significantly simplifies, speeds up and cheapens the process of assessing the state of the composite, allowing not only to fix, but also to predict the appearance of defects in a non-contact way.
    At the moment, the researchers have worked out the method of introducing soft magnetic wires into the composite material, made sure that the properties of the composite material do not deteriorate due to this, and also have worked out various measurement modes.



    The methodology of scientists at this stage was appreciated by several representatives of the aerospace and aviation industries, as well as developers of composite materials. According to Andrei Stepashkin, now the researchers will have to “leave the laboratory”: on the basis of the laboratory instrument, develop a “field” prototype of the sensor and the measuring system.
    “We have taken only the first step of a long journey,” said the scientist. “But we are already seeing the concrete practical application of our development.” In addition, it has additional features: the wire mesh introduced from the microwires can additionally provide a drain of static charge arising in fiberglass structures. Our wires may well replace the metal mesh that is being inserted into these materials now. ”

    The work of researchers published in the Journal of Alloys and Compounds .

    Photo author: Sergey Gnuskov

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