Damage Formation and Performance Degradation of CFRP Bolted Joints Under Thermo- Oxidative Aging Process

doi:10.21203/rs.3.rs-7192605/v1

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Damage Formation and Performance Degradation of CFRP Bolted Joints Under Thermo- Oxidative Aging Process

https://doi.org/10.21203/rs.3.rs-7192605/v1

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published 11 Nov, 2025

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Carbon fiber reinforced polymer (CFRP) is widely used in aerospace due to its excellent mechanical properties, and the corresponding high-efficiency, high-strength joining techniques have become a key research focus. Among various factors influencing the failure mechanisms and residual life of CFRP joints, the actual service environment plays a crucial role. In this study, the damage formation and performance degradation of CFRP bolted joints under thermo-oxidative aging condition were investigated. Experimental results demonstrate that, even in the absence of external loads, a thermo-oxidative environment can lead to material internal damage within the jointed zone. The difference in the coefficients of thermal expansion between the fibers and matrix can also lead to the appearance of matrix damage and delamination at the entrance and exit of the joining holes. In addition, From the experimental results, it can be seen that thermo-oxidative ageing has a significant effect on the tensile load-bearing properties of the joining structure, and with the increase in ageing temperature up to 150℃, the load-bearing stiffness and strength of the joining structure decreases by up to 34.8% and 23.1%, respectively. These findings highlight the impact of thermo-oxidative aging on the integrity and performance of CFRP bolted joints, emphasizing the need to account for environmental degradation in the design and durability assessment of aerospace composite structures.

Carbon fiber reinforced polymer (CFRP)

Interference-fit bolted joints

Thermo-oxidative aging

Mechanical performance

Damage evolution

Carbon-fiber-reinforced polymer (CFRP) has a high specific strength, high specific modulus, good fatigue resistance, and high designability [1], making it the preferred material for advanced aircraft and spacecraft and a revolutionary material in aerospace. To fully exploit CFRP design potential and meet aerospace demands for high-performance structural materials [2], developing high-efficiency, high-strength joining techniques has become a major research area [3]. Due to its simplicity, ease of disassembly, reliability, and cost-effectiveness, bolted joints are widely used [4]. In practice, however, the complex nature of CFRP structures means that when composite joints operate under high-temperature conditions, their damage mechanisms become even more complex [5], posing stringent challenges for joint reliability. Under prolonged thermo-oxidative exposure, composites undergo matrix cracking, interface debonding, and matrix degradation [6], significantly impairing their physical and mechanical properties. Therefore, it is critical to investigate the mechanical behavior of composite joints under thermo-oxidative environments.

To ensure the service life of composite components, research work has been carried out on the influence of thermal and oxygen environments on the mechanical properties of CFRP jointed structures. Khan et al. [7] studied the mechanical behavior of CFRP/aluminum hybrid riveted joints made by fused deposition modeling under elevated temperature. They found that joint load capacity and stiffness decreased exponentially with temperature, and that void location and size in the composite significantly influenced the failure mode. Zain et al. [8] investigated the mechanical behavior and failure mechanisms of CFRP/Al joints at different temperatures. The results showed that as temperature increased, the failure mode of the CFRP plate shifted: at room temperature it was predominantly interlaminar failure, while at high temperature it became a mixed mode of intralaminar and interlaminar failure. High temperature also induced noticeable bending in the inner layers of the CFRP. Zlatan et al. [9, 10] examined the fatigue behavior of CFRP/Al joints under elevated temperature and biaxial loading. They reported that when composite joints are subjected to both mechanical and thermal stresses simultaneously, fatigue life is significantly reduced and the failure mechanisms differ from those under room-temperature or single-stress conditions. Several fatigue damage mechanisms were identified, including matrix cracking and fiber–matrix debonding.

In addition to the study of the influence of thermo-oxidative service environment on service performance, the clarification of the influence of different service environments on the failure mechanism of the connection structure is the foundation for further joining process optimization. Hu et al. [11] investigated the bearing performance and failure mechanisms of bolted composite interference-fit joints under thermal effects, elucidating the influence of ply stacking sequence, interference fit size, and bolt torque on joint behavior. Liu et al. [12] studied the tensile behavior of composite-aluminum four-bolt joints at different temperatures, monitoring the surface strain field and damage evolution at various loading stages. They found that higher temperatures altered the axial load distribution among the bolts, causing the loads carried by each bolt to become more uniform. Li et al. [13] tested the mechanical behavior and damage mechanisms of composite interference-fit joints under long-term low-temperature exposure. They discovered that an appropriate interference size and short-term cold exposure can significantly improve joint performance, whereas prolonged low-temperature exposure leads to performance degradation.

In summary, existing studies on the mechanical performance of composite joints under complex thermal conditions typically consider only thermal effects and neglect the coupling of thermal and time effects on structural performance. At the same time, none of the existing studies have considered the effect of the interference action between the joining holes and fasteners on the corresponding performance and failure process of CFRP joint. To address this limitation, the thermo-oxidative aging and tensile tests for composite interference-fit joints was designed that measured mechanical performance after aging, and examined degradation patterns. By combining damage observations with failure modes, the influence mechanisms of the thermo-oxidative environment on the aging and failure of composite interference-fit joints were presented for the first time.

2.1 Thermo-Oxidative Aging Test of Interference-Fit Joint Specimens

In the thermo-oxidative aging and tensile tests, specimen dimensions were chosen per ASTM D5961. Specimens were prepared with length l = 135.0 mm, width w = 36.0 mm, and thickness t = 4.0 mm. The center-to-edge distance of the bolt hole was 18.0 mm; the width-to-hole-diameter ratio (w/d) = 6 and the edge-to-diameter ratio (e/d) = 3. The joints used a single-bolt, single-lap configuration. To ensure uniform load transfer during testing and to minimize off-axis forces and additional bending moments from clamping, aluminum alloy shims of the same thickness as the specimen were bonded to both ends of the joint so that the tensile load would act directly on the joint area. The dimensions of the composite joint specimens and fasteners are shown in Fig. 1.

The interference fit was set to 0.6%, corresponding to a hole diameter of 5.964 mm, to match a standard bolt. Bolt installation was performed on a computer-controlled universal testing machine. To ensure stability, accuracy, and good coaxially during installation, a special fixture was used for precise alignment, as shown in Fig. 2. The specimen was clamped and positioned using plates, and the bolt was slowly inserted into the hole at a rate of 1.0mm/min. After assembly, a torque wrench was used to apply installation torque in 1.0N/m increments uniformly up to 10.0N/m. To investigate the effect of the thermo-oxidative environment on joint mechanical performance, the assembled joints were divided into two groups: one group of intact joints (for mechanical testing) and another with only the joint region retained (for damage evolution study under extreme thermal conditions). The intact specimens were aged in a temperature-controlled convection oven at 50.0°C, 100.0°C, and 150.0°C for 40 days. The specimens with only the joint region were aged at the same temperatures for 30 days. After aging, all specimens were removed for microstructural observation. An equivalent set of unaged specimens was prepared as a control group.

2.2 Tensile Test of Interference-Fit Joints under Thermo-Oxidative Environment

To study the evolution of mechanical performance under thermo-oxidative conditions, tensile tests were conducted on the aged composite joints according to ASTM D5961. For each aging condition, four specimens were tested. After discarding obvious outliers, the remaining data were averaged to obtain the load–displacement curves. Tensile tests were performed at room temperature (25.0°C) on an electronic universal testing machine, with a loading rate of 2.0mm/min. During the tests, the load and displacement were recorded to analyze the influence of thermo-oxidative aging on joint performance. In addition, considering that fiber buckling and breakage may occur on the surface under loading, four strain gauges were attached around the hole to monitor deformation at different circumferential positions. After testing, the failed joint specimens were collected and preserved for subsequent analysis. The tensile test setup is shown in Fig. 2.

After thermo-oxidative aging, the macroscopic appearance of the composite joints and the locations of observed damage in the bolted joints are shown in Fig. 3. After aging at different temperatures, the composite surface shows discoloration: the unaged joint appears black, intact, and glossy, whereas aged specimens become noticeably discolored with reduced gloss. These color and gloss changes indicate that the epoxy resin in the composite has undergone a series of physical and chemical reactions (including oxidation and dehydration) [14], which alter the material’s chemical structure and affect its surface morphology. By contrast, the titanium-alloy bolts and nuts exhibit strong oxidation resistance, with no obvious surface changes, maintaining good integrity.

Figure 4 shows the results of matrix damage in the composite joints after 30 days of aging under various conditions. It can be observed that the interference-fit bolt induces some fiber and matrix damage at the entry of the upper plate and the exit of the lower plate; however, thermo-oxidative aging has a relatively limited effect on the propagation of fiber damage in the joint, so subsequent focus is on matrix damage.

Comparing specimens aged for 30 days at different temperatures, we find that as aging temperature increases, damage induced by the interference fit expands significantly under thermal stress. As shown in Fig. 4, at 50°C aging, thermal stress causes further expansion of matrix damage and delamination, triggering extensive matrix cracking and some through-thickness cracks. Under 100°C aging, both the extent and area of damage increase markedly, mainly because the higher thermal stress leads to significant degradation of material properties, especially the matrix; its elastic modulus, stiffness, and strength are reduced, promoting further damage propagation under residual stress. At 150°C aging, the severity and area of damage further intensify, exhibiting much more pronounced damage characteristics.

The results indicate that in a thermo-oxidative environment, as aging temperature rises, the extent and range of matrix damage and delamination around the hole increase significantly. Additionally, due to the mismatch in thermal expansion coefficients between fiber and matrix, internal thermal stresses form that cause microcracking, further exacerbating matrix damage and delamination. Microscopic observation reveals that in the unaged and low-temperature (50°C) conditions, the composite shows no significant deformation and only limited damage, mainly around the hole perimeter. However, at 100°C and 150°C, obvious plastic deformation appears at the exit of the lower plate along with severe delamination and matrix damage. As thermal stress intensifies, especially under high-temperature aging, the composite experiences micro-damage, including matrix cracking and chemically-induced changes due to high-temperature degradation [15].

4.1 Strain Evolution around the Joint Hole

During tensile loading, the strain curves at different positions around the hole are shown in Fig. 5, and the locations of the strain gauges are indicated in Fig. 2(a). The significant differences depending on locations can be observed. At position #1, strain initially decreases (indicating compression) as the load increases, showing that the material in this region primarily undergoes compressive loading. As the load approaches the ultimate value, the compressive strain at #1 suddenly decreases, and with further loading, it increases again. In contrast, strains at positions #2 and #4 remain relatively unchanged throughout the loading process, indicating no significant deformation at these positions (i.e., no pronounced lateral deformation around those parts of the hole boundary). At position #3, the strain curve shows compressive strain initially, but as the load increases, it gradually transitions to tensile strain. Notably, when the load reaches 6,124 N, the compressive strain at #3 reaches its limit and reduces to zero at 12,102 N, after which tensile strain rises rapidly with further loading. Overall, the composite around the hole experiences much greater compressive deformation than tensile, suggesting that the hole boundary mainly bears compressive loading during tensile testing, likely leading to a crushing failure at the bore.

Figure 6 shows the strain–load curves for the joints under different aging conditions. In Fig. 6 (a), the curve at gauge #1 displays both linear and nonlinear stages. Under low loads, strain and load are linearly related; however, with higher aging temperatures, the slope of the curve increases markedly, indicating that aging exacerbates matrix damage and causes more severe plastic deformation. As the load further increases, the curve becomes noticeably nonlinear and exhibits a sudden drop in strain. This sudden drop is caused by severe damage at that location (such as matrix crushing, fiber–matrix debonding, and other failure modes), which rapidly reduces the compressive stress and causes the strain gauge to recover, resulting in a sharp drop. With further temperature increase, the composite shows damage under lower loads, shifting this strain-drop point leftward; after that, the compressive strain grows again.

In Fig. 6 (b) and 6 (d), positions #2 and #4 show little effect from aging temperature, indicating that under all aging conditions, these regions undergo no significant lateral deformation. However, their strain curves fluctuate more as load increases, with the amplitude of fluctuation also increasing with aging temperature. This suggests that although these positions do not deform laterally appreciably, the applied longitudinal load still induces some lateral strain fluctuations, which become more pronounced as internal damage intensifies with aging. In Fig. 6 (c), position #3 is significantly affected by aging. The strain–load curves at #3 initially decrease and then increase. For the unaged and 50°C aged specimens, the tensile strength (the load at which tensile strain appears) is relatively high (approximately 14,152 N and 12,547 N, respectively). With aging temperatures of 100°C and 150°C, the tensile strength decreases, so tensile strain appears at lower loads (around 10,578 N and 9,152 N, respectively). This is because higher aging temperatures cause more severe internal damage; as the aging-induced damage accumulates, obvious matrix crushing and interface debonding occur even at the initial loading stage.

4.2 Load-Carrying Performance of Bolted Joints

The load–displacement curves of the composite interference-fit joints under various aging conditions are shown in Fig. 7. The results show a clear linear region in the initial loading stage, indicating that early in loading the joint’s load capacity is primarily governed by friction between the mating surfaces. However, as aging temperature increases, the slope of the load–displacement curve decreases and the bearing stiffness drops significantly. In this study, the bearing stiffness decreased from 11.2 kN/mm at room temperature to 7.3 kN/mm at 150°C, a reduction of 34.8%. As the external load increases further, the load–displacement curve becomes nonlinear. When displacement reaches a certain critical value (due to a large tilt angle of the bolt), the bolt bends and fractures, causing a sudden drop in the load.

Overall, the bearing capacity of the composite joint changes significantly in a thermo-oxidative environment. After 40 days of aging at 50°C, 100°C, and 150°C, the maximum bearing capacities of the joints decrease to 17,124 N, 14,416 N, and 13,317 N, which correspond to 98.8%, 83.2%, and 76.9% of the unaged specimen’s capacity, respectively. At 50°C, thermo-oxidative aging slightly degrades the matrix mechanical properties, causing only a minor reduction in load capacity. When the aging temperature reaches 100°C and 150°C (near the glass transition temperature of the composite), the resin matrix undergoes significant chemical changes and the material’s plasticity increases markedly. In particular, under 150°C aging the load–displacement curve exhibits much more pronounced nonlinearity, indicating significant plastic deformation at the hole periphery and shear failure at the fiber–matrix interface; consequently, the curve is smoother under this condition.

4.3 Failure Modes of Bolted Joints

Figure 8 compares cross-sections of the composite interference-fit joints after tensile failure under different aging conditions: (a) unaged, (b) 50°C, (c) 100°C, and (d) 150°C aging for 40 days. In all cases, there is severe damage at the bolt/composite interface, and the bolt tilt angle differs among the conditions. For the unaged specimen, its mechanical properties remain intact. In the quasi-static tensile test, the composite undergoes a series of damage modes including matrix crushing, fiber breakage, and delamination, which propagate horizontally along the hole. For the specimens aged 40 days at 50°C, 100°C, and 150°C, the bolt axis tilt angles after failure increase to 6.4°, 11.6°, and 11.0°, respectively. Compared to the unaged sample, these final tilt angles are significantly larger, indicating that the composite’s mechanical properties have degraded substantially under the thermo-oxidative environment. Moreover, as the aging temperature rises, the composite’s internal chemical structure softens, further increasing the tilt and damage.

Figure 9 shows the microscopic morphology of the compression (bearing) failure under different aging conditions: (a) unaged, (b) 50°C, (c) 100°C, and (d) 150°C after 40 days of aging. In all cases, both the upper and lower composite plates in the bearing region exhibit clear damage, characterized by shear and compression failure patterns. Under tensile load, the upper plate hole wall is continuously compressed by the bolt shank, causing fiber buckling. At the edge of the washer, as the clamp-up restraint in the thickness direction weakens under compression, the composite experiences shear and delamination damage at the fiber–matrix interface. For the unaged specimen, since its structure and properties remain as originally, quasi-static loading induces matrix crushing, fiber buckling, and delamination that propagate horizontally around the hole, resulting in a compression failure angle of about 31.7°.

In contrast, for specimens aged 40 days at 50°C, 100°C, and 150°C, the compression failure direction angles increase to approximately 39.3°, 45.2°, and 52.3°, respectively. In addition, the gap between the hole wall and bolt is larger in the aged specimens than in the unaged specimen, indicating significant bending deformation of the surface material under compression, with some material displaced into the gap. This is due to pronounced mechanical degradation of the composite under thermo-oxidative aging. At high temperature, in particular, the matrix and fiber–matrix interface strength drop substantially, leading to more severe fiber damage, matrix damage, delamination, and shear failure at the load-bearing surface. Furthermore, high temperatures induce chemical changes in the composite that soften the material, thereby increasing the compression failure angle and exacerbating damage.

Under all aging conditions, the lower plate hole wall is dominated by compression damage, with some material migrating along the thickness direction under compressive stress, causing material buildup. Gaps also appear between the hole wall and bolt, indicating the hole diameter has enlarged under the tensile load. For the unaged specimen, the bolt preload strongly constrains the lower plate in the thickness direction, effectively suppressing shear and delamination damage. Nevertheless, even in this case the joint still exhibits matrix crushing, fiber buckling, and delamination that propagate horizontally from the hole. After 40 days of aging at 50°C, 100°C, and 150°C, the composite’s matrix and interface strengths are significantly weakened by thermo-oxidation, accelerating the aging process. This causes severe degradation of interface bonding strength and matrix properties, resulting in more material buildup on the upper side of the lower plate. In addition, preload relaxation reduces the constraint on the lower plate, leading to a wider area and more severe shear damage and widespread delamination.

CFRP interference-fit joints are extensively used in aerospace structures due to their high strength-to-weight ratio. Long-term exposure to high-temperature oxidative environments, however, can significantly degrade composite materials and compromise joint reliability. This study quantifies the influence of thermo-oxidative aging on damage evolution, strain distribution, and load-bearing performance of CFRP bolted joints. The key findings are summarized below:

Thermo-oxidative aging alone (without external load) induced internal damage in the composite joint. Fiber–matrix thermal expansion mismatch produced compressive stress around the fastener, causing extensive matrix cracking and delamination at both the hole entry and exit.
Local strain around the bolt hole increased with aging temperature: both compressive and tensile strains grew, with compressive strain more sensitive to temperature. The joint exhibited no significant lateral deformation.
Tensile tests of aged composite joints demonstrate that thermo-oxidative aging markedly degrades tensile bearing performance. With higher aging temperatures, the joint stiffness and strength decrease by up to 34.8% and 23.1%, respectively, and the area and extent of matrix damage and delamination increase substantially. The upper plate’s compression failure angle increases from 31.9° to 52.3°, and the lower plate exhibits material migration along the thickness, leading to material buildup.
Under elevated aging temperatures, the joint’s tensile stiffness and strength fell by up to 34.8% and 23.1%, respectively, reflecting a marked reduction in load-bearing capacity. The extent of matrix cracking and delamination increased significantly. Notably, the upper laminate’s bearing failure angle increased from 31.9° to 52.3°, and the lower laminate exhibited localized material migration (pile-up).

Funding Declaration

This research was supported by the National Natural Science Foundation of China (Grant Nos. 52035005, 52375234, 52275459, 52475485, 52375461, 52405509). The funders had no role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Author Contribution

Xuda Qin – conceived the study and wrote the first draft.Yanwei Xu – co‑designed experiments, supervised tests, and edited the manuscript.Gongbo Feng – ran thermo‑oxidative ageing tests and collected micrographs.Linjing Gui – performed tensile tests, processed data.Shipeng Li – conducted microscopy and prepared Figures 8–9.Hao Li – carried out statistical analysis and plotted results.Guoyu Fu – provided funding, resources, and final approval.

N. Forintos and T. Czigany, “Multifunctional application of carbon fiber reinforced polymer composites: electrical properties of the reinforcing carbon fibers—a short review,” Compos. Part B: Eng., vol. 162, pp. 331–343, 2019.
Y. Zheng, C. Zhang, Y. Tie, et al., “Tensile properties analysis of CFRP-titanium plate multi-bolt hybrid joints,” Chin. J. Aeronaut., vol. 35, no. 3, pp. 464–474, 2022.
M. Genedy, R. Chennareddy, E. M. Soliman, et al., “Improving shear strength of bolted joints in pultruded glass fiber reinforced polymer composites using carbon nanotubes,” J. Reinf. Plast. Compos., vol. 36, no. 13, pp. 958–971, 2017.
J. Yu, J. Liu, and B. Y. Fan, “Research progress on composite structural connections,” Fiber Composites, vol. 41, no. 2, pp. 96–103, 2024.
B. Cysne, A. P. Fulco, E. Guerra, et al., “Accelerated aging effects on carbon fiber/epoxy composites,” Compos. Part B: Eng., vol. 110, pp. 298–306, 2017.
M. H. Haque, P. Upadhyaya, S. Roy, et al., “Changes in flexural properties and microstructures of carbon fiber bismaleimide composite after exposure to a high temperature,” Compos. Struct., vol. 108, pp. 57–64, 2014.
S. A. Khan, H. A. Khan, A. Khan, et al., “Investigation of the mechanical behavior of FDM processed CFRP/Al hybrid joint at elevated temperatures,” Thin-Walled Struct., vol. 192, art. no. 111135, 2023.
U. A. Zain, S. A. Khan, H. A. Khan, et al., “Classification of progressive failure and mechanical behavior of dissimilar material hybrid joints at varying temperatures,” Thin-Walled Struct., vol. 182, art. no. 110212, 2023.
Z. Kapidzic, H. Ansell, J. Schon, et al., “Fatigue bearing failure of CFRP composite in biaxially loaded bolted joints at elevated temperature,” Compos. Struct., vol. 127, pp. 298–307, 2015.
Z. Kapidzic, H. Ansell, J. Schon, et al., “Fatigue bearing failure of CFRP composite in bolted joints exposed to biaxial variable amplitude loading at elevated temperature,” Compos. Struct., vol. 142, pp. 71–77, 2016.
J. Hu, S. Mi, Z. Yang, et al., “Experimental investigation on bearing behavior and failure mechanism of bolted composite interference-fit joints under thermal effects,” Eng. Fail. Anal., vol. 131, art. no. 105830, 2022.
X. Liu, L. Zhang, K. Huang, et al., “Experimental investigation of temperature effects on the tensile behavior of composite-aluminum four-nail bolted joints,” Polym. Compos., vol. 45, no. 5, pp. 4281–4294, 2024.
J. Li, W. Guo, P. Zou, et al., “Multistage mechanical behavior and damage mechanism of composite interference-fit joints subject to long-term low-temperature aging,” J. Braz. Soc. Mech. Sci. Eng., vol. 45, no. 11, art. no. 602, 2023.
Y. Ren, L. Xu, Y. Sun, et al., “Study on oxidation behavior during the process of recycling carbon fibers from CFRP by pyrolysis,” J. Environ. Manage., vol. 347, art. no. 119103, 2023.
Y. Kobayashi and S. Kobayashi, “Effect of long-term high-temperature atmospheric exposure on damage progress and mechanical properties for carbon fiber/polycyanate 90° unidirectional composites,” Adv. Compos. Mater., vol. 24, no. 1, pp. 105–123, 2015.

No competing interests reported.

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You are reading this latest preprint version

Damage Formation and Performance Degradation of CFRP Bolted Joints Under Thermo- Oxidative Aging Process

Status:

Journal Publication

Version 1

Abstract

Figures

1 Introduction

2 Experimental Set-up

2.1 Thermo-Oxidative Aging Test of Interference-Fit Joint Specimens

2.2 Tensile Test of Interference-Fit Joints under Thermo-Oxidative Environment

3 Damage Analysis of Composite Joint Structures

4 Effects of Thermo-Oxidative Aging on Mechanical Performance of CFRP Bolted Joints

4.1 Strain Evolution around the Joint Hole

4.2 Load-Carrying Performance of Bolted Joints

4.3 Failure Modes of Bolted Joints

5 Conclusions

Declarations

Author Contribution

References

Additional Declarations

Status:

Journal Publication

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