Effect of external operational damage on the mechanical behavior of GFRP under quasi-static and fatigue loading

Polymer composite materials (PCM) are widely used in various industries and are used in the manufacture of critical parts. Thanks to the use of composite materials (CM), it is possible to reduce the weight of the structure and increase its strength. Since composite structures are subjected to various impacts during service that can induce defects, it is necessary to study the influence of these defects on the mechanical properties of materials.
Various defects, acting as stress concentrators, may arise both during the manufacturing of composite materials and during their service life [1]. The presence of defects lowers the load at which composite structures fail, resulting in a reduction of their overall strength.
Currently, there is a problem of the lack of a unified classification and terminology of defects for reinforced PCM. Problems also include translation inaccuracies, as a result of which different defects can be understood under the same term, and the presence of its own internal classification of defects in each area related to composite materials [2].
In [3], a classification of defects in multilayer adhesive structures made of PCM is presented according to their degree of severity. Defects that may occur during operation include cracks, dents, scratches, and chips [4,5]. The reasons for their formation may be shock or mechanical effects during operation [6]. In aviation technology, such damage can occur as a result of bird strikes during flight or from the impact of foreign objects, such as stones. They can also be caused by fragments of ice or concrete that affect the structure during takeoff and landing, hail strikes, or during ground maintenance.
Defects of the stratification type may occur during operation in areas of stress concentration [7], such as holes, stringer protrusions, and similar features. In addition, they can be caused by temperature variations and local loads, for example, impacts on the surface of the structure.
Movement of the composite structure relative to a rough surface or protrusion can cause scratches or "potholes" (breakdowns) that affect the load-bearing layers [8]. Such damage is more likely to occur when the aircraft is parked between flights, during icing, cargo handling, refueling, or interaction with other ground vehicles.
The influence of scratch geometry on the delamination of a laminated composite under tensile loading is investigated in [8, 9]. An indenter was used to create scratches on the carbon fiber sample. It was found that, in a composite laminate containing a deep scratch that penetrates several load-bearing layers, the dominant failure mechanisms are bending and twisting deformations. It was found that the location of the scratch tip within the layer sequence and the scratch depth were identified as the primary factors affecting tensile strength, whereas the overall size and shape of the scratch have a lesser influence.
In cellular structures, defects that occur during operation include one - and two-sided holes, cracks, delamination of the skin due to impacts, separation (the skin from the frame, cellular aggregate from the skin or frame).) [1, 3, 4, 10, 11].
Carbon fiber sandwich panels are significantly more susceptible to impact damage than fiberglass panels, and the predominant damage mechanisms differ: in carbon fiber sandwich panels, fiber fracture is dominant, whereas in fiberglass panels, core failure prevails [12].
During aircraft operation, impacts and operational loads cause damage. Such damages can reduce the residual strength and durability of the structure, potentially leading to failure and endangering the safety of aircraft operation [13, 14].

Polymer composites are vulnerable to impacts even at low speeds, and low-energy impacts can cause complex matrix cracks and delaminations within the composite. The mechanisms of matrix cracking and delamination typically render composite structures unserviceable during operation, necessitating their replacement. The danger of such damage is that, in most cases, it is not visible on the surface and cannot be detected by visual inspection of the structure. This is called barely visible impact damage. With a stronger impact, a chip can be observed on the reverse side of the CM, while no visible traces remain on the front side [12, 13].
Delamination caused by barely noticeable impact damage may not significantly affect the tensile strength, but it can significantly reduce the compressive strength. Delaminating layers exhibit significantly lower compressive resistance than the same layers when firmly bonded together. For this reason, considerable attention is devoted to testing the compressive strength after impact of composite structures [12, 15, 16].
The compression test was performed for undamaged and damaged aircraft panels with stiffeners made of PCM. The decrease in residual strength due to the cutout under the stiffener and due to the impact applied at a low speed was studied.
It was found that the decrease in the residual strength during testing is about 30 kN [15].
During axial compression, the composite fails due to buckling. This can be avoided by choosing the length of the part depending on its bending stiffness so that no buckling occurs under the specified boundary conditions and operating load levels [12].
During the manufacturing of composite parts, various defects may occur, negatively affecting the operational and strength characteristics of the final product. In [17, 18], static tests of CFRP samples with embedded technological defects (folds, dry spots) for tension and compression were considered using such systems as acoustic emission and digital image correlation, which were used to determine the location of defects and their influence on the mechanical characteristics of CFRP [19].
According to [20], which reviewed the types of defects in composite materials, the following types of cracks that occur during operation are distinguished: trans-layer (rare), interlayer (most common during service), and trans-fiber (fiber break). In addition to the above-mentioned types of operational defects, they also include fatigue damage, fibers fracture or damage, erosion, matrix cracking, moisture ingress, temperature effects, and damage caused by ultraviolet radiation, chemical exposure, and lightning strikes [20].
In [21], carbon fiber samples were subjected to simulated lightning strike damage at various current levels, after which the samples were tested for residual strength and modulus of elasticity under tension and compression. The results show that the residual tensile strength increases after impact, while the residual compressive strength decreases.
Papers [12, 22] are devoted to the study of the mechanical behavior of layered composites under fatigue loading, specifically under cyclic tensile conditions. The failure mechanisms are described, and the following stages of damage progression are identified: matrix cracking, crack coalescence and interfacial delamination, delamination, fiber fracture, and ultimate composite failure.
Thus, the damage modes that can occur in polymer composites are considered. PCMs are particularly susceptible to operational defects caused by low-speed impacts, static, and cyclic loads. The presence of such operational defects significantly reduces the service life and structural integrity of the components. The relevance of the work is due to the increasing use of composite materials in critical structures and the need to assess the impact of external operational defects on their residual mechanical properties. This work aims to evaluate the impact of operational defects on the fatigue characteristics and failure processes of CFRP.

This work was carried out in Perm National Research Polytechnic University using Unique Scientific Equipment "A complex of testing and diagnostic equipment to study the properties of structural and functional materials under complex thermomechanical loading conditions http://ckp-rf.ru/usu/501309.
An experimental program of the influence of external operational defects on the fatigue life of polymer-laminated fiberglass was developed and implemented. The study was carried out on samples in the form of strips cut along the weft, with dimensions of $150\times 20\times 6\mathrm{mm}$$150\times 20\times 6\mathrm{mm}$150 xx20 xx6mm150 \times 20 \times 6 \mathrm{~mm}$150\times 20\times 6\mathrm{mm}$ (Fig. 1). Before testing, the samples were damaged to simulate the following operational defects: scratch and dent.

The scratch defect was applied using a 10 mm wide steel blade inserted into the gripper of the test machine. When applying the defect, the indentation load $(1\mathrm{kN})$$(1\mathrm{kN})$(1kN)(1 \mathrm{kN})$(1\mathrm{kN})$ on the sample and the movement of the active gripper $(1\mathrm{mm})$$(1\mathrm{mm})$(1mm)(1 \mathrm{~mm})$(1\mathrm{mm})$ were controlled. The appearance of the sample with the scratch defect is shown in Fig. 1, $b$$b$bb$b$.
To apply the "dent" defect under controlled loading parameters (force, displacement), a method was developed [23], applying a load to a fiberglass sample in the transverse direction through a steel shaft with a hemispherical tip with a diameter of 10 mm . In preliminary tests, this type of defect was applied with loads of $10\mathrm{kN},12\mathrm{kN},15\mathrm{kN},17\mathrm{kN},20\mathrm{kN}$$10\mathrm{kN},12\mathrm{kN},15\mathrm{kN},17\mathrm{kN},20\mathrm{kN}$10kN,12kN,15kN,17kN,20kN10 \mathrm{kN}, 12 \mathrm{kN}, 15 \mathrm{kN}, 17 \mathrm{kN}, 20 \mathrm{kN}$10\mathrm{kN},12\mathrm{kN},15\mathrm{kN},17\mathrm{kN},20\mathrm{kN}$, and 22 kN (Fig. 1, c). The process of applying the dent defect to the sample is shown in Fig. 2, a.
As a result of preliminary tests of a dent defect, ranges of indentation loads were established, which either did not lead to damage to the sample at all or did not affect the load-bearing capacity of the GFRP. We also determined the load ranges that caused visible macroscopic defects (such as through-thickness cracks, surface layer rupture, etc.) during indentation. It was found that a load of 22 kN is critical, leading to the destruction of the sample. Thus, the indentation defect was applied at three load levels $(10,12$$(10,12$(10,12(10,12$(10,12$, and 15 kN$)$$)$))$)$ of the indenter into the sample. These loads caused moderate damage to the material. To determine the cyclic loading, a series of quasi-static tensile tests was performed. Samples with dent defects ( 10 kN , 12 kN , and 15 kN ) and a 10 mm wide transverse scratch were tested, as well as samples without defects (initial ones). Three samples were tested in each batch. Tensile tests were performed on an Instron 5882 electromechanical testing machine (Fig. 2, b). The loading rate during tensile tests for all sample groups was $2\mathrm{mm}/\min$$2\mathrm{mm}/\min$2mm//min2 \mathrm{~mm} / \mathrm{min}$2\mathrm{mm}/\min$, and the sample was loaded to failure. Deformations were measured using a VIC-3D system based on the digital image correlation method. Shooting was done with cameras at a resolution of 16.0 Mp with a shooting frequency of up to 3 Hz .

Based on the obtained results of static tests, the types of simulated defects and loading parameters for cyclic testing were determined. Tests to assess the effect of external operational defects on the fatigue life of polymer fiberglass were performed for groups of samples without defects, with a dent defect of 10 kN , with a dent defect of 15 kN , and a scratch defect. Tensile fatigue tests were performed on the Instron 8802 servo-hydraulic test system ( 100 kN ). Parameters of cyclic loading were as follows: frequency of 10 Hz , load ratio $\mathrm{R}=0.1$$\mathrm{R}=0.1$R=0.1\mathrm{R}=0.1$\mathrm{R}=0.1$, and a maximum stress-to-ultimate strength ratio $\sigma /\sigma \mathrm{b}=0.3-0.7$$\sigma /\sigma \mathrm{b}=0.3-0.7$sigma//sigmab=0.3-0.7\sigma / \sigma \mathrm{b}=0.3-0.7$\sigma /\sigma \mathrm{b}=0.3-0.7$. The shape of the cycle is chosen as a sine. The average values of maximum stresses for each sample series were taken as the ultimate strength. One sample were tested in each batch. The following conditions were used as the failure criterion: reduction of the maximum load from cycle to cycle by $50\mathrm{\%}$$50\mathrm{\%}$50%50 \%$50\mathrm{\%}$ or specimen failure into parts.

Based on experimental static test data obtained from the Vic-3D video system, deformation diagrams were presented in Fig. 3. The deformation diagrams of the initial samples and samples with defects coincide in the initial linear section, which may indicate that the presence of dent and scratch defects does not significantly affect the material's tensile stiffness. Based on the tensile test results, mechanical characteristics were determined for samples without defects (Tab. 1) and the following characteristics of GFRP samples with defects: load-bearing capacity, maximum fracture stresses, and stiffness (Tab. 2). The stiffness of a sample of a material with a defect is an equivalent characteristic of the elastic modulus. The maximum stresses during failure of samples with defects were determined similarly to the ultimate strength, i.e, as the ratio of the maximum load to the initial cross-sectional area of the sample without a defect. All samples exhibit similar initial behavior, and as the dent application force increases, the maximum stresses at fracture decrease significantly, indicating substantial material degradation. The maximum stresses at fracture for specimens with a dent defect decrease relative to defect-free specimens as follows: by $17\mathrm{\%}$$17\mathrm{\%}$17%17 \%$17\mathrm{\%}$ for samples with a dent applied with a force of 10 kN ; by $25\mathrm{\%}$$25\mathrm{\%}$25%25 \%$25\mathrm{\%}$ for samples with a dent applied with a force of 12 kN ; by $30\mathrm{\%}$$30\mathrm{\%}$30%30 \%$30\mathrm{\%}$ for samples with a dent applied with a force of 15 kN . Applying a defect scratch does not affect the conditional tensile strength. The presence of operational defects such as dents and scratches does not reduce the stiffness of the defective material sample.

Fig. 4 shows the fields of longitudinal strains on the surface of a sample without defects and on the surface of samples with scratch and dent defects. The load level at point 1 corresponds to $0.2{\sigma }_{\text{max}}$$0.2{\sigma }_{\text{max }}$0.2sigma_("max ")0.2 \sigma_{\text {max }}$0.2{\sigma }_{\text{max }}$, at point 2 corresponds to $0.5{\sigma }_{\sigma max}$$0.5{\sigma }_{\sigma max}$0.5sigma_(sigma max)0.5 \sigma_{\sigma \max }$0.5{\sigma }_{\sigma max}$, and at point 3 corresponds to ${0.99}_{{\sigma }_{max}}$${0.99}_{{\sigma }_{max}}$0.99_(sigma_(max))0.99_{\sigma_{\max }}${0.99}_{{\sigma }_{max}}$. Between the fracture $\left({0.99}_{{\sigma }_{max}}\right)$$\left({0.99}_{{\sigma }_{max}}\right)$(0.99_(sigma_(max)))\left(0.99_{\sigma_{\max }}\right)$\left({0.99}_{{\sigma }_{max}}\right)$ and the ultimate strains, the maximum concentration of longitudinal strains in the defect region before fracture is estimated.

where $K_{\epsilon }$$K_{\epsilon }$K_(epsi)K_{\varepsilon}$K_{\epsilon }$ is the maximum concentration of longitudinal strains, ${\epsilon }_c$${\epsilon }_c$epsi_(c)\varepsilon_{c}${\epsilon }_c$ is the value of the maximum longitudinal strain in the defect area before failure, and ${\epsilon }_v$${\epsilon }_v$epsi_(v)\varepsilon_{v}${\epsilon }_v$ is the maximum value of the longitudinal strain from virtual extensometers installed outside the defect area.
For the scratch defect, the maximum concentration of longitudinal strains is 2.45 , and for the dent defects applied with forces of 10,12 , and 15 kN : $3.27,3.19$$3.27,3.19$3.27,3.193.27,3.19$3.27,3.19$, and 3.61 , respectively. Thus, it can be noted that as the intensity of exposure to the sample increases, the strain concentration in the defect region increases.
The assessment of the risk of operational defects (dent and scratch) on the fatigue life of GFRP was carried out. The test results are shown in Tab. 3. Samples that were not destroyed during the tests are marked with (*).

Based on the results of fatigue tests, the tensile fatigue curves of fiberglass for various types of external damage are plotted in logarithmic coordinates (Fig. 5). The most dangerous type of external damage is determined: a dent of 15 kN . For this series of samples, a decrease in the fatigue life of GFRP is observed with an increase in the level of loading relative to
samples without a defect. For example, at a loading level of 0.6 , the fatigue life of samples with a dent of 15 kN relative to samples without defects decreases by $93\mathrm{\%}$$93\mathrm{\%}$93%93 \%$93\mathrm{\%}$.

The Basquin function is essential in material fatigue analysis. This well-known approach, with its different variations, continues to be frequently used by researchers in their studies. For instance, see studies [22, 24, 25]. The Basquin function can be expressed as follows:

For numerical comparison of the results, the coefficients of the Basquin equations for GFRP with and without external defects were determined. The obtained coefficients are shown in Tab. 3.

To more comprehensively assess the influence of external defects on fatigue life over the entire range of cycles tested, we introduce an analogy with the effective stress concentration factor. This factor is calculated using the fatigue limits of smooth (unnotched) samples and samples containing a stress concentrator. In our analysis, the coefficient $Kf$$Kf$KfK f$Kf$ is defined as the ratio of the stress amplitude of an undamaged sample to the stress amplitude of a sample with an external defect, for the same fatigue life (number of cycles to failure). The $Kf(\mathrm{N})$$Kf(\mathrm{N})$Kf(N)K f(\mathrm{~N})$Kf(\mathrm{N})$ relationship was obtained based on the fitted Basquin equations. Fig. 6 presents the dependence of the calculated effective stress concentration coefficient $Kf$$Kf$KfK f$Kf$ on the fatigue life N for all types of external defects.
For samples with a scratch, the maximum effect of about $5\mathrm{\%}$$5\mathrm{\%}$5%5 \%$5\mathrm{\%}$ is observed closer to the region of low-cycle fatigue. In the region of high-cycle fatigue, the effect is almost negligible. For samples with a dent of 10 kN , the maximum effect, on the contrary, occurs in the high-cycle fatigue region and reaches $9\mathrm{\%}$$9\mathrm{\%}$9%9 \%$9\mathrm{\%}$ whereas in the low-cycle region it is about $6\mathrm{\%}$$6\mathrm{\%}$6%6 \%$6\mathrm{\%}$. For a 15 kN dent, the maximum effect was found in the low-cycle fatigue region and reaches around $20\mathrm{\%}$$20\mathrm{\%}$20%20 \%$20\mathrm{\%}$, decreasing to about $13\mathrm{\%}$$13\mathrm{\%}$13%13 \%$13\mathrm{\%}$ in the high-cycle region. The different behavior at dents of 10 and 15 kN shows that at 10 kN the fibers do not experience
significant damage compared to the matrix. And with a dent of 15 kN , the fibers at the dent site receive significant damage, which leads to a decrease in fatigue life in the low-cycle region.

The failure sites of GFRP samples with operational defects were analyzed after quasi-static tension and fatigue testing. Photos of the samples after static testing are shown in Fig. 7. For samples with a scratch, failure occurred both in the grip region due to fiber rupture and in the defected area due to interlayer delamination, followed by rupture of the loaded layers (Fig. 7a). In samples with a dent, failure was caused by fiber rupture at the defect site, accompanied by local delamination and transverse separation of layers (Fig. 7b).

Representative images of failed samples after fatigue tests are shown in Fig.8. Analysis of the fracture surfaces indicates that failure occurs in the area of the applied operational defect due to fiber rupture.

Based on the photo of the fractures from the side, it was noticed that the crack (initiation of failure) begins from the surface of the defect, then grows under the defect and is accompanied by normal separation of layers (yellow areas), and the breakage of the samples occurs due to inertia from the bending of the sample with delamination (red zone).

We developed methods to introduce external defects that simulate operational damage, such as scratches and dents. A series of experiments was conducted to investigate the effect of these defects on the elastic and strength properties of the composite using GFRP samples. New data were obtained on how operational defects, specifically scratches and dents, influence the fatigue life of the GFRP samples.
The fracture surfaces of samples with operational defects were examined after tensile and fatigue tests. It was found that during cyclic loading, failure occurs predominantly at the sites of the applied defects, whereas under quasi-static tension, samples may fail either in the working section or in the grip region.

This work was carried out with the support of the Russian Science Foundation (Project No 25-29-00188, https://rscf.ru/project/25-29-00188/).

[1] Golovan, V.I., Dudarkov, Y.I., Levchenko, E.A., Limonin M.V. (2020). Load bearing capacity of composite panels with in-service damages, Trudy MAI, 110, pp. 5. DOI: https://doi.org/10.34759/trd-2020-110-5 (In Russia).
[2] Dikov, I.A., Yakovleva, S.I., Boychuk, A.S., Chertishchev, V.Yu. (2023). Defect classification for 2D fiber-reinforced composites (review), Trudy VIAM, 3 (121), pp. 67-83. DOI: https://doi.org/10.18577/2307-6046-2023-0-3-67-83.
[3] Murashov, V.V., Rumyantsev, A.F. (2007). Defects of monolithic parts and multilayer structures of polymer composite materials and methods for their detection. Part 1. Defects of monolithic parts and multilayer structures of polymer composite materials. Kontrol'. Diagnostika, (4), pp. 23-32. (In Russia).
[4] Wang, H., Zhang, J., Li, B., Xuan F. (2025). Machine learning-based fatigue life prediction of laser powder bed fusion additively manufactured Hastelloy X via nondestructively detected defects, International Journal of Structural Integrity, 16 (1), pp. 104-126. DOI: https://doi.org/10.1108/IJSI-09-2024-0161.
[5] Dang, L., He, X., Tang, D., Xin, H., Wu, B. (2025). A fatigue life prediction framework of laser-directed energy deposition Ti-6Al-4V based on physics-informed neural network, International Journal of Structural Integrity,16 (2), pp. 327-354. DOI: https://doi.org/10.1108/IJSI-10-2024-0170.
[6] Meng, D., Nie, P., Yang, S., Su, X., Liao, C. (2025). Reliability analysis of wind turbine gearboxes: past, progress and future prospects, International Journal of Structural Integrity, 16 (1), pp. 4-38.
DOI: https://doi.org/10.1108/IJSI-08-2024-0129.
[7] Lobanov, D., Yankin, A., Mullahmetov, M., Chebotareva, E., Melnikova, V. (2023). The Analysis of Stress Raisers Affecting the GFRP Strength at Quasi-Static and Cyclic Loads by the Theory of Critical Distances, Digital Image Correlation, and Acoustic Emission, Polymers, 15(9), 2087. DOI: https://doi.org/10.3390/polym1509208.
[8] Shams, S.S., El-Hajjar, R.F. (2013). Effects of scratch damage on progressive failure of laminated carbon fiber/epoxy composites, International journal of mechanical sciences, 67, pp. 70-77. DOI: 10.1016/j.ijmecsci.2012.12.008.
[9] Masmoudi, A., Khechai, A., Bouaziz, A., Belhi, G., Zeroual, A., & Adimi, Y. (2025). Experimental investigation on mechanical behavior of sandwich structures using Digital Image Correlation (DIC), Fracture and Structural Integrity, 19(73), pp. 41-58. DOI: 10.3221/IGF-ESIS.73.04.
[10] Anoshkin, A.N., Vildeman, V.E., Lobanov, D.S., Chikhachev, A.I. (2014). Evaluation of repair efficiency in structures made of fibrous polymer composite materials, Mechanics of Composite Materials, 50(3), pp. 311-316.
DOI: https://doi.org/10.1007/s11029-014-9416-0.
[11] Pokrovskii, A.M., Chermoshentseva, A.S. (2017). Experimental study of nano-additives effect on properties of composite materials with interlayer defects, Vestnik Moskovskogo aviatsionnogo instituta, 24(3), pp. 212-221. (In Russia).
[12] Bossi, R.H., Giurgiutiu, V. (2015). Nondestructive testing of damage in aerospace composites Polymer composites in the aerospace industry, Woodhead Publishing, pp. 413-448. https://doi.org/10.1016/B978-0-85709-523-7.00015-3.
[13] Katunin, A., Dragan K., Dziendzikowski, M. (2015). Damage identification in aircraft composite structures: A case study using various non-destructive testing techniques, Composite structures, 127, pp. 1-9.
DOI: https://doi.org/10.1016/j.compstruct.2015.02.080.
[14] Staroverov, O., Mugatarov, A., Sivtseva, A., Strungar, E., Wildemann, V., Elkin, A., Sergeichev, I. (2024). Fatigue behavior of pultruded fiberglass tubes under tension, compression and torsion, Fracture and Structural Integrity, 18(69). DOI: https://doi.org/10.3221/IGF-ESIS.69.09.
[15] Sepe., R., De Luca, A., Lamanna, G., Caputo, F. (2016). Numerical and experimental investigation of residual strength of a LVI damaged CFRP omega stiffened panel with a cut-out, Composites Part B: Engineering, 102, pp. 38-56.
DOI: https://doi.org/10.1016/j.compositesb.2016.07.009.
[16] Staroverov, O.A., Strungar, E.M., Wildemann, V.E. (2021). Evaluation of the survivability of CFRP honeycomb-cored panels in compression after impact tests, Frattura ed Integrità Strutturale, 15(56), pp. 1-15.
DOI: https://doi.org/10.3221/IGF-ESIS.56.01 .
[17] Lobanov, D.S., Slovikov, S.V., Lunegova, E.M. (2023). Influence of Internal Technological Defects on the Mechanical Properties of Structural CFRP, Frattura ed Integrità Strutturale., 17(65). pp. 74-87.
DOI: https://doi.org/10.3221/IGF-ESIS.65.06.
[18] Slovikov, S.V., Lobanov, D.S., Chebotareva, E.A., Melnikova, V.A. (2024). The influence of technological defects on the mechanical behavior of CFRP during buckling under compression based on DIC data and acoustic emission, Frattura ed Integrità Strutturale., 18(69). pp. 60-70. DOI: https://doi.org/10.3221/IGF-ESIS.69.05.
[19] Barile, C., Casavola, C., Pappalettera, G. (2019). Damage characterization in composite materials using acoustic emission signal-based and parameter-based data, Composite Structures, 208, pp. 388-396.
DOI: https://doi.org/10.1016/j.compositesb.2019.107469.
[20] Adams, R. D., Cawley P. (1988). A review of defect types and nondestructive testing techniques for composites and bonded joints, NDT international. 21(4), pp. 208-222. DOI: https://doi.org/10.1016/0308-9126(88)90333-1.
[21] Feraboli, P., Miller, M. (2009). Damage resistance and tolerance of carbon/epoxy composite coupons subjected to simulated lightning strike. Composites Part A: Applied Science and Manufacturing, 40(6-7), pp. 954-967.
DOI: https://doi.org/10.1016/j.compositesa.2009.04.025 .
[22] Lobanov, D.S., Lykova, A.V., Pankov, A.M., Ugolnikov, M.V. (2025). Effect of Internal Technological Defects and Loading Waveform on Structural Composite Fatigue Life, Fracture and Structural Integrity, 19(71), pp. 1-10.
DOI: https://doi.org/10.3221/IGF-ESIS.71.01.
[23] Chebotareva, E.A., Lobanov, D.S. (2025). Assessment of the influence of operational defects and external damage on the mechanical behaviour of polymer laminated composite material, Problems of Strength and Plasticity, 87(4), pp. 539551. DOI: https://doi.org/10.32326/1814-9146-2025-87-4-539-551.
[24] Yankin, A., Wildemann, V., Belonogov, N., Staroverov, O. (2019). Influence of static mean stresses on the fatigue behavior of 2024 aluminum alloy under multiaxial loading, Frattura Ed Integrità Strutturale, 14(51), pp. 151-163. DOI: https://doi.org/10.3221/IGF-ESIS.51.12.
[25] Firdaus, S.M., Arifin, A., Abdullah, S., Singh, S.S.K., Nor, N.M. (2023). Fatigue Life Assessment of API Steel Grade X65 Pipeline Using a Modified Basquin Parameter of the Magnetic Flux Leakage Signal. Materials, 16, 464. DOI: https://doi.org/10.3390/ma16020464.