This method allows researchers to easily and efficiently evaluate the level of the damage to composite structures, such as aircraft parts and ship hulls, for example. The article on this research was published in the Journal of Alloys and Compounds.
Composites are used in many transport vehicles today. To evaluate the quality and durability of a material, the value of the internal tensile stresses of the structure — during both manufacturing and in use must be identified. In some composites, the value of internal tensile stresses after manufacturing can reach up to 95 percent of the ultimate tensile strength, which means, a slight increase in pressure will result in failure.
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Carbon fiber composites, fiberglass and hybrid composites do not have this level of internal tensile stresses after manufacturing, but structural stress can build up due to working loads, the environment and the weather. This results in the deformation of the material and a decrease in load-bearing capacity.
"Other methods of stress monitoring in composite structures are often inconvenient," explained Andrei Stepashkin, senior researcher at the NUST MISiS Center for Composite Materials.
"Non-contact methods (such as ultrasonic and acoustic testing), for example, allow us to detect existing defects only. They provide no indication of internal stresses in a material or stress distribution throughout a structure. Traditional methods for stress monitoring are contact-based, requiring physical tag attachment to a material. It turns out there are no non-contact methods for testing a material before a defect occurs, which is why we saw a need for this application."
The method developed by researchers from NUST MISiS includes monitoring the stresses using amorphous ferromagnetic microwires which are embedded between layers of carbon fiber composite to form a mesh that will be sensitive to applied stresses.
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The tensile stresses in the composite material surrounding the microwires affect the way the wire material reacts to an external magnetic field. This means stress levels can be measured without direct contact, without a sensitive element: it requires no physical contact because it was embedded in the material at the required depth during manufacturing.
The authors also point out that the new method requires a single sensor — unlike other popular stress testing methods that require placing sensors on both sides of a part to be monitored. Thus, this technology makes the process of stress monitoring of composite materials much easier, faster and more efficient, allowing it to not only detect, but also predict the emergence of defects without direct contact.
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Researchers have tested this technology in various modes, as well as the process for embedding the wires into a composite material and have found no negative effect on the properties of the material.
This new approach has received recognition from several sources in the aerospace sector, and composite material developers as well. Stepashkin says the team has a new goal to "get out of the laboratory" to develop prototypes of a sensor and a measuring system based on the device created in the laboratory.
"What we have so far is the first step, but we can see how our development can actually be applied," the scientist noted. "This manufacturing technique has other potential applications, too: the embedded microwires can serve as a static charge "run-off" in fiberglass plastics. This means, our wires can easily replace the metallic mesh, which is used today."