Russian researchers have proposed a new approach to the mechanical characterization of aerospace composites that involves the combination of miniature specimen testing and computational modeling. The method will allow greater numbers of small-scale tests to be performed at a lower cost. This will help improve the reliability and simultaneously speed up the design of new aircraft and satellites, experts say.

Composites vs. metal alloys

Researchers from Skoltech, MAI and MISIS University have developed a new method for determining the strength of polymer composites reinforced with carbon fibers, which are widely used in the aerospace industry. Tests using the new method can be performed on samples as small as 1 cubic centimeter in volume.

Image. View of the miniature in vacuo test platform designed and fabricated by Skoltech scientists, placed inside the scanning electron microscope chamber. Credit: Skoltech PR

Researchers explain that polymer composites form a class of materials that are stronger and more lightweight compared to aluminum alloys. However, the use of composites in aerospace design presents some challenges: As their fracture can occur by different modes, more sophisticated approaches are required for design against failure and to ensure structural integrity, compared to metal alloys traditionally used in the aerospace industry.

“For metals, knowing the chemical composition and the history of thermomechanical processing is sufficient to predict the key mechanical properties and the material behavior in the finished product. Composites, on the other hand, can have very different properties in standard test specimens and in the full-size final product. To ensure the stability of the properties, we proposed to study them first at the miniature scale, but not only for standard samples, but also in coupons cut from the full-scale parts. We then use computational modeling to extend this data to larger objects. In this way we no longer need to fabricate large expensive samples and load them to destruction time after time to understand how the material works,” says Alexey Salimon, one of the developers and the deputy head of the MISIS Accelerated Particles Laboratory LUCh.

Miniature sample testing

The team designed a miniature test rig that fits inside a scanning electron microscope. The frame-shaped rig contains a force sensor and micromotor actuators controlled remotely through a special port in the microscope vacuum chamber.

Image. Snapshot from the miniature sample bending test conducted inside the scanning electron microscope chamber. Credit: Skoltech PR

To perform the test, the samples are fastened to the frame and deformed in a controlled manner. The maximum force applied to the sample is about 500 newtons, or the weight of 50 kilograms under standard gravity. The test rig can be used for tensile and compression tests, as well as bending and other axially aligned loading modes.

“Additively manufactured materials, such as composites, are highly inhomogeneous and anisotropic — their physical properties vary with position and orientation. Traditionally, tests are done on large samples and require a lot of material and complex test setups. We have shown that the same data can be obtained using small samples, which are easier and cheaper to produce, making it possible to perform many tests and collect large amounts of statistically significant data,” explains Eugene Statnik, a research scientist at Skoltech Engineering.

Image. Miniature loading rig designed and assembled by Skoltech scientists. Credit: Skoltech PR

The next step is to validate the mathematical models by comparing the computational and experimental data, Statnik said. Finding correlations between the two helps achieve better understanding of the material behavior and minimize the error in predicting its performance.

During each test the team uses digital cameras to record the changes in the material and to obtain a high-resolution image at each loading step, Statnik added.

He explained that this helped detect and localize deformation and structural damage in real time down to the individual fiber scale, given that the size of a carbon fiber is about 5 to 7 microns.

“The origin of damage and material vulnerability lies in its microstructure and fine-scale deformation. My team made unprecedented advances in the ability to perform digital image correlation using our own algorithms. This helped us see strain concentration in fine detail and to identify the types of defects responsible for the property degradation based on the detailed micromechanical analysis,” says Professor Alexander Korsunsky, the head of the Hierarchically Structured Materials Laboratory at Skoltech Engineering.

Image. Micrographs obtained with a scanning electron microscope that show carbon fiber-reinforced polymer. Credit: Skoltech PR

Ultimately, this data helps us accurately locate and predict damage initiation and development, he added. As a result, it is possible to calculate the material’s strength reliably and ensure that it meets the performance requirements at the design stage. This, in turn, accelerates the development of new aircraft and engine systems.

Studying microprocesses in a material

“The properties of carbon fiber-reinforced polymers with an epoxy matrix depend on many factors, such as the direction and the compatibility with the binder of the fibers, etc. Safe application of composites in aerospace materials science requires extensive research and repeated testing,” says Evgeny Kurkin, a senior researcher at the Aircraft Structure and Design Department of the Korolev Samara National Research University.

Image. Structural analysis of a composite sample after loading to failure. Credit: Skoltech PR

Kurkin emphasized that small samples will help increase the number of experiments and collect extensive statistical data, allowing engineers to draw more accurate conclusions at a lower cost. Similar studies are being conducted at other research institutions, including Samara University.

“The new method is useful for studying the microscopic processes that occur in composites under load. Such insights are very valuable because ultimately understanding the process evolves into the ability to control it,” says Evgeny Alexandrov, the director of the Digital Materials Science: New Materials and Substances NTI Center at Bauman Moscow State Technical University.

For example, the new method can help find the right balance between surface wettability, adhesion (bonding of surfaces of different solids and/or liquids) at the filler-matrix interface, and roughness, which prevents the fiber from slipping out of the matrix under load, he explained.

Image. Strain localization leading to local failure in a miniature specimen. Credit: Skoltech PR

There are computer systems for modeling such processes, but a good match between the model and the real object has always been a problem. The proposed method can take this match much closer to reality, he concluded.

“High specific strength — the key advantage of aerospace composites — helps reduce the weight of an aircraft or spacecraft. Aerospace engineers could also benefit from local anisotropy, or internal nonuniformity, in the material. For example, fibers could be arranged so that the structure has maximum strength in the direction of the main load,” Artur Gareev, the deputy director for science and innovation at JSC NIIgraphite, commented.

However, he pointed out a potential conflict between a very small sample size and the structure of the material. For example, the fibers could lose contact with the binder, compromising the strength of the composite.

According to Gareev, a large-scale model would require a full set of new tools, which would involve an additional cost. Overall, the new method can be safely used in simple loading cases that do not require validation on large-scale models. In such cases, the calculations, if done properly, will be very close to the real loading scenario, he added.

This text, authored by Andrey Korshunov, originally appeared in Russian on March 19, 2025, on the website of the Russian daily Izvestia. The study by Eugene Statnik, Alexey Salimon, Alexander Korsunsky, and co-authors that is reported in this story was published November 21, 2024, in the journal Fracture and Structural Integrity.