Introduction

Fused Deposition Modeling (FDM) is a well-established and widely adopted 3D printing technology1 that plays a key role in additive manufacturing. The basic principle involves creating a three-dimensional object by heating and extruding molten thermoplastic material in layers. The material, in filament form, is heated to a molten state and extruded through a nozzle along a predetermined path, layer by layer, until the model is complete2,3. This process is computer-controlled, with the movement of the nozzles and material deposition guided by pre-designed 3D model data, allowing the creation of high-precision, complex geometries.

FDM can process a range of materials, from common Acrylonitrile Butadiene Styrene (ABS) and Poly lactic acid (PLA) to high-performance engineering plastics like nylon, polycarbonate (PC), and polyether ether ketone (PEEK)4. Each material offers distinct advantages and limitations based on its physical and chemical properties. For instance, PLA is known for its biodegradability and biocompatibility, making it ideal for medical and educational applications. In contrast, ABS is preferred in industrial manufacturing due to its excellent mechanical properties and heat resistance5. For demanding applications, such as aerospace and automotive industries, high-performance materials like PEEK are preferred for their outstanding mechanical strength, high-temperature resistance6,7, and chemical durability. To enhance the performance of FDM-printed parts, researchers are also focusing on the development and application of reinforced composite materials, such as carbon fiber-reinforced polymers (CFRP). These materials significantly improve the strength and stiffness of composites by incorporating high-strength fibers. The advantages of FDM technology include its simplicity of operation8, low material costs, fast production speeds, and suitability for manufacturing a wide range of complex structures. Additionally, this technology demonstrates excellent comprehensive properties in terms of mechanical performance, thermal performance, and chemical performance9, making it widely applicable in fields such as prototype manufacturing, industrial design, medical devices, and educational research10.

Polyether ether ketone (PEEK) is a high-performance engineering plastic recognized for its excellent mechanical strength, high-temperature resistance, and chemical durability, widely used in aerospace, automotive, medical devices, and other demanding fields11,12,13. However, PEEK materials exhibit high melt temperatures14 and viscosities during FDM processing, placing significant demands on printing equipment and process parameters. To address these challenges, carbon fiber (CF) is introduced as a reinforcing agent to improve the molding and mechanical properties15. The addition of carbon fibers significantly increases the strength and rigidity of the composites, while also enhancing their thermal and electrical properties, resulting in superior performance across a wider range of applications. Injection molding is the most common method for preparing CF/PEEK composites16,17. However, this process requires custom molds, which are expensive, time-consuming, and complex. In contrast, FDM has emerged as the primary molding method for thermoplastic polymer composites due to its simple equipment, cost-effectiveness, and ability to produce complex parts18,19. Therefore, FDM-based preparation of CF/PEEK composites holds greater promise for various applications. CF/PEEK composites are renowned for their exceptional mechanical properties, heat resistance, chemical stability, and low density20. This composite material maintains stable performance in harsh environments and offers excellent impact and wear resistance, making it ideal for manufacturing high-strength, lightweight parts and structural components, including aerospace parts, automotive engine components, and medical devices21,22.

Extremely low-temperature environments, such as those in polar research and space exploration, impose high demands on material performance23,24. Under these conditions, materials frequently experience increased brittleness25, reduced strength, and altered fracture behavior. Therefore, an in-depth investigation of the mechanical properties and fracture behavior of CF/PEEK composites at extremely low temperatures is critical to advance their applications in these fields.

Previous studies have examined the influence of low temperatures on a wide range of materials. For instance, Jingjing Liu et al.26 investigated the notch strength and damage behavior of carbon fiber-reinforced composite laminates composed of SYT-49 S carbon fiber and SYE 20,005 epoxy resin at 20 °C and − 60 °C, reporting notable effects of low temperature on these properties. Hunt et al.27 examined the tensile strength of 3D-printed carbon fiber-reinforced polyethylene terephthalate glycol (PETG) and carbon fiber-reinforced Amphora AM1800 in liquid nitrogen, noting marked changes in tensile strength and elastic modulus as temperature decreased. These findings underscore the substantial influence of low temperatures on the mechanical properties of materials, particularly polymers and composites28,29, as low temperatures can increase brittleness and decrease toughness. Therefore, examining how low temperatures affect the mechanical properties of FDM-printed CF/PEEK composites is essential for evaluating their potential in low-temperature applications and for optimizing process parameters. Although some research has addressed FDM process parameters for CF/PEEK composites, systematic investigations into how low temperatures affect their mechanical properties and fracture behavior remain scarce30.

The main aim of this study is to examine how low temperatures affect the tensile strength of carbon fiber-reinforced polyether ether ketone (CF/PEEK) composites and to explore their fracture mechanisms. A custom FDM-based 3D printer was employed to fabricate CF/PEEK composite parts with a defined fiber content. The mechanical properties were assessed through tensile strength tests conducted at different low temperatures. Finite element analysis in Workbench was conducted to compare simulation findings with experimental data, thereby confirming the experiment’s reliability. Regression analysis was used to predict the composites’ tensile strength at −200 °C. Finally, scanning electron microscopy (SEM) was performed at different temperatures to examine the fracture behavior, elucidating how low temperatures influence the material’s fracture mechanisms.

Experimental work

Preparation of composite materials

The printing feedstock used in this study is CF/PEEK filament, produced through high-temperature composite processing of staple carbon fibers and PEEK matrix. The average diameter of carbon fiber is 7 μm, supplied by Wuxi Zhishang New Materials Technology Co., Ltd. The PEEK model ESD101 is sourced from Wiggers Ltd. in the United Kingdom. The staple carbon fibers and PEEK particles were pre-dried at 150 °C for six hours to eliminate moisture. They were subsequently blended to achieve a fiber content of up to 10 wt%, as higher fiber content can result in excessive melt viscosity within the 360–400 °C range31,32.

The composites produced in this experiment contained 10 wt% carbon fibers. The material was compounded in a high-temperature twin-screw extruder at 350 °C to ensure complete melting of PEEK and uniform fiber dispersion. The composite was extruded through a die at 340–360 °C, then rapidly cooled in a water bath to form CF/PEEK filament, which was subsequently wound by a winding machine. The processed filament was dried in an oven at 260 °C for three hours to remove residual moisture and minimize the risk of bubbles or porosity during printing. The diameter of the filament is 1.75 ± 0.05 mm, the surface of the filament is smooth, the color is uniform (black), there are no obvious burrs or bubbles, and the thermal conductivity is 0.66 W/(K*m). The detailed process parameters for the printing procedure are presented in Table 1.

3D printers and processing conditions

The 3D printing system (Fig. 1) employed a high-precision linear guide in the X-axis and high-precision ball screws for Y and Z movement control. This printer employed water-cooling and a small runner fan system, while the printhead wiring was shielded from dust and debris. Samples were printed in five sets of three each for tensile strength testing at various low temperatures. The appearance and dimensions of the printed samples are presented in Table 2.

Fig. 1
figure 1

Self-developed FDM-based staple fiber 3D printer and printing principle.

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Mechanical property testing and microstructure observation

The tensile test was performed in accordance with ISO 52733, using a 100 kN electronic material testing machine. The low-temperature environment was established using a liquid nitrogen cooling system. The samples were gradually cooled to the target temperature and held there for 30 min to ensure uniform temperature distribution.

Table 1 Printing parameters.
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During the test, the samples were secured in low-temperature fixtures, and the tensile tester applied a load at a rate of 0.2 mm/min while recording real-time stress-strain data (Fig. 2)34,35. A temperature sensor continuously monitored the cooling chamber to maintain temperature stability. At the conclusion of the test, stress curves were recorded, and mechanical properties, including tensile strength and elongation, were calculated.

Fig. 2
figure 2

Stress-strain diagram.

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For fracture analysis, the fractured samples were photographed to capture their shape. The samples were sputter-coated with gold for 45 s at 10 mA using a Quorum SC7620, and the micro-morphology of the fracture surfaces was examined with a Hitachi Regulus 8100 scanning electron microscope. Images of the tensile test and damaged specimens are shown in Fig. 3.

Fig. 3
figure 3

Tensile experimental diagram of composite specimen.

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Table 2 Conforming to a type I geometry of ISO527-2.
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Finite element analysis of tensile strength

To validate the tensile test results, a three-dimensional tensile simulation model of the CF/PEEK composite was developed using the finite element software Abaqus. Considering specimen symmetry and computational efficiency, all degrees of freedom at the fixed end were constrained, while only axial displacement was allowed at the loading end to simulate actual fixture constraints. The simulation results are presented in Fig. 4. The model uses C3D8R solid elements (eight-node linear reduced integration), which provide good numerical stability and are well-suited for simulating the mechanical behavior of composite materials. A global mesh size of 1 mm was applied. The matrix was modeled as an ideal elastic–plastic material, using the von Mises equivalent stress criterion36 to define failure. Temperature-dependent elastic modulus and tensile strength were derived from experimental data fitting. No explicit interface debonding elements were included in the model; however, simulation results indicate that significant stress concentration occurs in the central region at low temperatures, potentially leading to fiber–matrix interfacial failure. Stress distribution contours at various temperatures are shown in Fig. 5a–e. The simulated ultimate tensile stresses were 81.83 MPa (25 °C), 76.54 MPa (− 25 °C), 62.05 MPa (− 75 °C), 31.62 MPa (− 125 °C), and 11.28 MPa (− 175 °C). This trend indicates that the tensile strength of the CF/PEEK composite gradually decreases as the temperature decreases. To further validate the simulation, the predicted results are compared with stress–strain data from experimental measurements.

Fig. 4
figure 4

Homogenization equivalent treatment.

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Fig. 5
figure 5

Cloud view of finite element analysis of composites under various temperature conditions (a) 25 °C (b) −25 °C (c) −75 °C (d) −125 °C (e) −175 °C.

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Results and discussion

Tensile properties of CF/PEEK composites

This study measured the tensile strength of CF/PEEK composites at five temperatures: 25 °C, −25 °C, −75 °C, −125 °C, and − 175 °C. The tensile strengths are shown in Fig. 6a. The results show a significant decrease in tensile strength with decreasing temperature.

Fig. 6
figure 6

Tensile strength versus modulus of elasticity as a function of temperature (a) Tensile strength (b) modulus of elasticity.

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At 25 °C, the tensile strength was 81.93 MPa, which decreased slightly to 76.63 MPa at −25 °C, a reduction of 6.5%. At −75 °C, the tensile strength dropped significantly to 63.23 MPa, an 18% decrease. The tensile strength continued to decrease at lower temperatures: 32.13 MPa at −125 °C (a 60.8% decrease) and 12.83 MPa at −175 °C, an 84.3% reduction from the original strength at 25 °C.

In contrast, the elastic modulus exhibited a different trend. Initially, it decreased slightly but then increased as the temperature decreased (Fig. 6b). At 25 °C, the modulus of elasticity was 7.1 GPa, decreasing slightly to 6.9 GPa at −25 °C and 6.8 GPa at −75 °C. However, it increased to 7.3 GPa at −125 °C and 7.8 GPa at −175 °C. We believe that it may be related to the glass transition temperature (Tg) of PEEK. The Tg of PEEK is approximately 143 °C, and the testing temperatures in this study were all lower than its Tg, indicating that the material is in a glassy state. As the temperature decreases, the movement of polymer segments further weakens, and the stiffness of the material increases, resulting in an increase in elastic modulus37. However, further research is needed in future work to investigate the reduction of elastic modulus.

The trends in elongation at break and sectional shrinkage further confirm the impact of low temperatures on material toughness (Fig. 7). Elongation at break decreased sharply from 3.53% at 25 °C to 3.39% at −25 °C, 2.82% at −75 °C, 1.02% at −125 °C, and 0.1% at −175 °C. This decline indicates a significant increase in brittleness and a drastic reduction in ductility at lower temperatures. Similarly, sectional shrinkage decreased from 0.9% at 25 °C to 0.8% at −25 °C, 0.5% at −75 °C, and to 0.3% and 0.08% at −125 °C and − 175 °C, respectively, indicating a significant weakening in material deformation at lower temperatures.

Fig. 7
figure 7

Plot of elongation at break and section shrinkage versus temperature change.

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The sharp decrease in tensile strength at low temperatures is likely due to changes in the material’s internal microstructure. The PEEK matrix weakens, and the interfacial bonding between the carbon fibers and matrix weakens, leading to a decline in overall mechanical properties. Low temperatures may also induce the expansion of microcracks in the matrix, exacerbating the material’s fracture behavior38,39,40. At extremely low temperatures, such as −125 °C and − 175 °C, the composites may undergo brittle transitions, further reducing tensile strength and ductility. Morphological analysis of the fracture surfaces at different temperatures revealed significant changes in crack morphology and propagation paths due to low temperatures. These observations provide important insights into the fracture mechanism of CF/PEEK composites in low-temperature environments.

Regression analysis predictive model

Establishment of regression equations

In investigate the relationship between tensile strength \(y\) and temperature \(x\), the data were plotted in a scatter diagram (Fig. 8), revealing a roughly linear correlation between \(y\) and \(x\). The regression equation for the univariate linear regression is then obtained

$${\hat{y}} = b_{0} + bx .$$
(1)
Fig. 8
figure 8

Plot of regression equation fit.

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Where \({b}_{0}\) and \(b\) are the regression coefficients of the regression equation.

Ultimately, solving yields the regression equation as

$$\begin{array}{c}{\hat{y}}=80.74+\left(0.37^{\circ}\right){x}_{\:} \end{array}$$
(2)
$$\bar{x} = \frac{1}{5}\sum\nolimits_{{t = 1}}^{5} {x_{t} } = - 75;\;\bar{y} = \frac{1}{5}\sum\nolimits_{{t = 1}}^{5} {y_{t} } = 53.726.$$
(3)

The regression line must pass through the point (−75, 53.726). To determine the line, choose a value for \({x}_{0}\), substitute it into the regression equation \({{\hat{y}}}=80.74+\left(0.37^{\circ}\right){x}_{0}\), and calculate the corresponding \({{\hat{y}}}_{0}\). Then, connect the points (−75, 53.726) and (\({x}_{0}\), \({{\hat{y}}}_{0}\)) to form the regression line, which is plotted on the scatter diagram.

Analysis of variance (ANOVA) and test of significance for regression equations

The differences between the tensile strengths \({y}_{1},\:{y}_{2},\:...,\:{y}_{5}\) are caused by two aspects: (1) the different values of the temperature \({x}_{\:}\); (2) the influence of other factors (including test error). In order to test the regression equation, it is first necessary to decompose the variance caused by them from the total variance of \(y\). The variance between the five test values \(y\) can be expressed as the sum of the squares of the deviations between the test values y and their arithmetic mean \(\bar{y}\) called the total sum of squares of the deviations to look at and is written as

$$S=\sum\:_{t=1}^{N}\:({y}_{t}-\bar{y}{)}^{2}=0.37$$
(4)

The total sum of squared deviations can be decomposed into two parts, namely

$$\begin{array}{c}S=U+Q \end{array}.$$
(5)

U is the regression sum of squares, reflecting the portion of the total variation in y that is due to the linear relationship between x and y. Q is the residual sum of squares, which represents the effect on y of all factors other than the linear effect of x on y (including the nonlinear effect of the experimental error x on y, and other uncontrolled factors), and which is the portion of the variation that could not be reduced by consideration of the linear relationship between x and y alone.

U and Q are usually calculated according to the following formula:

$$\begin{array}{c}\begin{array}{c}\:\\\:\text{U}={b}^{2}\sum\:_{t=1}^{N}\:({x}_{t}-\bar{x} {)}^{2}=3340.1218 \\\end{array} \end{array}$$
(6)
$$\begin{array}{c}Q=S-U=210.1418 \end{array}$$
(7)

The test of significance of regression equations is usually done using the F-test, so the calculation of the statistic F

$$\begin{array}{c}F=\frac{U/1}{Q/(N-2)}=47.6838 \end{array}.$$
(8)

Check the F-distribution table and compare it with the calculated F-value.

$$\begin{array}{c}F\ge\:{F}_{0.01}\left(1,\:\:N-2\right)=34.12 \end{array}$$
(9)

The regression is thus considered significant at the 0.01 level.

The tensile strength of the composite at −200 °C is expected to be 7.64 MPa, as predicted by regression Eq. (2).

Analysis of tensile damage patterns and fracture mechanisms

Figure 9 presents SEM micrographs of tensile fracture surfaces and interlaminar arrangements for samples at five different temperatures.

Fig. 9
figure 9

(a) Shape of carbon fiber after fracture (b) interface gap.

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At room temperature (Fig. 9a1), the fracture surfaces of short carbon fiber composites exhibit numerous fiber pull-outs and slightly dispersed interlaminar gaps (Fig. 9b1). The surfaces are concave and uneven, with bumps and voids formed by carbon fibers pulled out with the PEEK matrix, showing no significant tensile deformation. At −25 °C and − 75 °C (Fig. 9a2,a3), clusters of short carbon fibers oriented in various directions, along with numerous cross-sectional pores, are visible. This indicates reduced tensile energy consumption and decreased tensile strength, with no significant interlayer separation observed (Fig. 9b2,b3). For samples at −125 °C and − 175 °C (Fig. 9a4,a5), the fracture surfaces become relatively flat, and the pull-out lengths of short carbon fibers are noticeably shorter than those at higher temperatures. The interlayer gaps are larger, particularly at −175 °C (Fig. 9a5,b5), where the shortest fiber pull-out lengths and the largest gaps suggest a decline in the interface properties between the short carbon fibers and the PEEK matrix.

This observation is consistent with the lowest measured tensile strengths. SEM analysis reveals significant changes in fracture morphology and interlaminar gaps as the temperature decreases. At lower temperatures, the fracture surfaces become flatter, fiber pull-outs shorter, and interlayer gaps larger, reflecting weakened interface properties between carbon fibers and the PEEK matrix. These changes account for the substantial decline in tensile strength at extremely low temperatures. Based on Fig. 2 and the interlayer bonding condition of the − 175 °C sample in Fig. 9, it is speculated that the decrease in tensile strength may be related to changes in the microstructure of the material at low temperatures. Under low-temperature conditions, the PEEK matrix in the CF/PEEK composite material becomes more fragile, and the interfacial bonding strength between the carbon fibers and the polyether ether ketone matrix weakens, leading to a decrease in the overall mechanical properties of the composite material. This study offers critical insights into the fracture mechanisms of CF/PEEK composites under low-temperature conditions, which is essential for their application in extreme environments.

Conclusions

This thesis examines the mechanical properties and fracture behavior of carbon fiber reinforced polyether ether ketone (CF/PEEK) composites under low-temperature conditions. Through experimental testing and data analysis, the following key conclusions were drawn: (1) The mechanical properties of CF/PEEK composites show significant temperature dependence. As the temperature decreases, tensile strength, sectional shrinkage, and elongation at break all decrease. Notably, at −125 °C and − 175 °C, the tensile strength decreases to 32.13 MPa and 12.83 MPa, respectively, indicating a marked increase in material brittleness at extremely low temperatures. (2) The elastic modulus of CF/PEEK composites initially decreases, then increases as the temperature drops, suggesting that the material’s rigidity is enhanced at extremely low temperatures, likely due to the glass transition of the matrix polymer. (3) At low temperatures, CF/PEEK composites primarily show fiber pull-out and matrix cracking. As the temperature decreases, the material exhibits more brittle fracture behavior, particularly at −125 °C and − 175 °C. The fracture mode shifts from plastic to brittle, with flatter fracture surfaces and more pronounced fiber tensile fractures. This is attributed to low-temperature-induced internal crack propagation and stress concentration, resulting from decreased matrix toughness and weakened fiber-matrix interfacial bonding.

This study clarifies how low temperatures affect the mechanical properties and fracture behavior of CF/PEEK composites, providing valuable insights for their engineering applications in cold environments. Future research should explore different fiber contents and types, and integrate numerical simulations with theoretical analysis to enhance the material’s performance in extreme conditions. This approach will enhance the material’s applicability in ultra-high altitudes, polar research, and deep-sea exploration, providing both theoretical foundations and practical guidance for these challenging fields.

Since low-temperature mechanical testing relies on a liquid nitrogen cooling system, it is crucial to ensure that the test specimens reach thermal equilibrium at extreme temperatures during the experiment. The use of liquid nitrogen significantly increases economic costs. This study balances experimental feasibility and cost-effectiveness, focusing solely on the tensile properties and failure mechanisms of CF/PEEK composites under low-temperature conditions, without exploring other mechanical property metrics. However, in practical applications, materials may encounter more complex load forms (such as impact, bending, and fatigue loads), so it is necessary to expand multi-dimensional mechanical property testing to comprehensively assess their low-temperature adaptability.