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Abstract

Line-focused infrared (LFIR) heating technology is an emerging heating method with high heat flux density/fast response rate, which can meet the demand of automatic placement of thermoplastic composites. Interlaminar performance is an important indicator of the overall quality of composites, and compaction force is a key factor affecting the quality of in-situ consolidated interlaminates. In this study, line-focused infrared heating-assisted in-situ consolidation experiments under different compaction forces were designed to investigate the law and mechanism of compaction force effects on the interlayer bonding properties. The results show that the interlayer shear strength (ILSS) increases nonlinearly with compaction force in the compaction force range of 100-400N, and the average ILSS strength of the 400N specimen increases by 17.27 MPa compared with that of 100N, which is an increase of 85.9%. The micro-morphological analysis showed that the effect of porosity on the interlayer bond strength was particularly prominent. At high compaction forces, the porosity decreased, the pure interlayer separation section of the specimen was flatter, and the interlayer contact was tighter. In addition, pressure analysis of the pressure-sensitive paper showed that the effect of the surface morphology of the raw Prepreg tows on the initial close contact between the layers was weakened as the compaction force increased, and the effective contact area increased. Finally, thermal history analysis shows that higher compaction force can reduce the interlayer contact thermal resistance and increase the temperature of 2-3 layers by 15 °C–30 °C. This paper reveals the mechanism of compaction force on key factors such as porosity and surface morphology, as well as the effect of compaction force on the temperature history, which provides a theoretical basis for optimizing the manufacturing process of composites.

Keywords

Automated fiber placement line-focused infrared heating compaction force interlaminar performance CF/PEEK

Introduction

Thermoplastic composites (TPC) have excellent mechanical properties, lightweight characteristics, better corrosion resistance, and high processing flexibility, which can effectively reduce the weight while ensuring the structural strength, making them widely used in many fields, such as aerospace, transportation, and environmentally friendly building materials.^1–3 With the continuous improvement of production demand, the molding process of thermoplastic composites is gradually developing in the direction of more efficient and precise. Automated Fiber Placement (AFP) technology has gradually become an important means in thermoplastic composites processing by virtue of its advantages of high precision, high efficiency, high flexibility and in-situ consolidation. This technology can significantly improve the lay-up precision, shorten the production cycle and reduce the material waste by precisely laying the reinforcing fiber prepreg onto the mold surface through automated equipment to form the laminated structure of composites.^2–7 However, its industrialization still faces a central challenge: the lack of stability of interlayer shear strength (ILSS), which is prone to delamination failure. In particular, for multilayer structural composites such as CF/PEEK components, the interlayer bonding strength directly determines their application range.

In AFP technology, two heating methods, hot gas torches^8–10 and laser^11–14, are very mature and widely use. Under hot gas torch heating, the maximum ILSS values found by Qureshi Z⁷ and Cai X⁸ were 49.23 MPa and 47.5 MPa, respectively. For laser heating, the maximum ILSS values reported by Eimanlou¹⁵ and Zhang M¹⁶ were 43.2 MPa and 62.8 MPa, respectively. (In contrast, laminated boards manufactured by hot pressing by material suppliers have an ILSS of 70-80 MPa.) However, they have some inherent defects: hot gas torches heating rate is slow, low energy utilization; laser price is expensive, the operation has a certain degree of danger and so on. In the past, infrared heating technology is usually only used for post-processing annealing and stress relief,¹⁷ and after the introduction of line focusing technology, the energy is effectively concentrated, and the heat flow density is greatly improved. Line-focused infrared heating technology makes its application to AFP technology possible by virtue of its fast, uniform, and controllable heating characteristics, as well as the price advantage of 4–6 times lower than that of laser. Compared with traditional heating methods, this method can heat the material surface more uniformly, slow down the temperature gradient and reduce the stress concentration.

During the AFP molding process, compaction force is a key factor affecting the interlaminar bonding properties.^13,18–20 The pressure distribution of the pressure roller determines the effective contact area and material flow characteristics, which in turn further affects the formation of pores. Porosity is a key indicator that cannot be ignored, and studies have shown that the effect of porosity on the interlayer bond strength is the most significant among the defects.²¹ Therefore, the core of the line-focused infrared heating-assisted in-situ consolidation process lies in the use of a high-energy-density infrared heat source to achieve instantaneous melting of the substrate while accurately regulating the pressure parameters of the compaction system, in order to optimize the degree of intimate contact, promote fiber infiltration, and strengthen the quality of interfacial bonding, thereby inhibiting delamination failure.

Zhang M designed experiments based on the response surface methodology (RSM) in laser-assisted automated tape lay-up experiments to obtain an optimal process parameter solution incorporating a compaction force of 239N¹⁶ and Oromiehie E analyzed and discussed the effects of process parameters on mechanical strength by fabricating samples using different parametric conditions for short-beam strength experiments in the manufacture of AFPs based on HGT.⁹ Although pressure factors have been comprehensively studied in laser- and hot gas torches- assisted AFP processes, their specific mechanisms of action have not been systematically elucidated in the emerging scenario of line-focused infrared heating. An in-depth exploration of the mechanism of pressure influence on the interlayer bonding performance is a core issue that needs to be addressed urgently to achieve the optimization of this process.

The Short-Beam shear test is commonly used to evaluate the shear resistance of composites in the interlaminar direction and is one of the important parameters to measure the interlaminar shear strength (ILSS) of composites.^6,7 The theory of intimate contact¹² provides a theoretical framework for the action of compaction force. Therefore, the aim of this study is to investigate the variation of interlaminar bonding properties of composites under different compaction force conditions with the assistance of line-focused infrared heating. Based on the ILSS experiments, we analyzed the effect of compaction force on the interlaminar bonding strength through the in-depth study of the microscopic morphology of the interlaminar surfaces, the pressure distribution of the pressure rollers and the thermal history curves, as well as based on the theory of intimate contact, combined with the assumption of the “resin island”.²² It is expected to provide theoretical basis and practical guidance for the optimization of processing of thermoplastic composites.

Experimental procedures and methods

Materials

In this thesis, unidirectional continuous carbon fiber reinforced thermoplastic polyether ether ether ketone (CF/PEEK) prepregs of type HB90-700-12.7 with a width of 12.7 mm and a thickness of 0.135 mm provided by Jiangsu Hengbo Composites Ltd. were used, and the properties of CF/PEEK prepregs are listed in Table 1.

Table 1.

Key properties of CF/PEEK prepregs.

Targets	Data
Density	1.57 $g / {cm}^{3}$
Thickness of single layer	0.135 mm
Width	12.7 mm
Resin mass content	34% ± 3%
Resin density	1.3 $g / {cm}^{3}$
Glass transition temperature( $T_{g}$ )	143°C
Melting temperature( $T_{m}$ )	343°C

AFP equipment

In this study, a self-developed gantry-type four-degree-of-freedom thermoplastic composite Automated Fiber Placement machine with a processing area of 2000 mm × 2500 mm and a motion accuracy of 0.05 mm was used. The machine is equipped with a fiber laying head specially designed for laying up and processing 12.7 mm wide prepregs, containing unwinding, re-feeding, cutting, heating and Consolidation devices. The tape laying head is shown in Figure 1.

Figure 1.

Gantry type four-degree-of-freedom thermoplastic composite material Automated Fiber Placement machine.

Among them, the heating device is a line-focused infrared heater, as shown in Figure 2(a), which focuses infrared radiation into a heating line about 14 mm wide through the gold-sprayed reflective layer, with a heating temperature range of 50 °C–500 °C; the compaction device is a steel pressure roller (Figure 1 Hot pressure roller) fixed at the end of the biaxial cylinder, and the compaction force is precisely regulated through the closed-loop control of pressure sensors and precision electrically-controlled regulator with the compaction force precision of less than 0.5 N, as shown in Figure 2(b).

Figure 2.

(a) Schematic diagram of line-focused infrared (b) compaction force regulation method.

Experimental Design

Based on the existing studies in our laboratory²³ and with reference to the results of response surface experiments, the heating power of the line-focused infrared heater was specially selected to be 70% (980 W), and the lay-up speed was 25 mm/s. The effect of compaction force on the interlayer bonding properties is more pronounced under this laydown condition. Before lay-up, the lay-up base plate and pressure roller were carefully cleaned and leveled to ensure uniform pressure distribution, and the temperature of the thermostatic base plate was set at 36°C. The first layer of prepreg was clamped on the lay-up path of the lay-up base plate using a fixture, and the subsequent prepregs were laid up in layers according to this lay-up path through the Automated Fiber Placement machine, as shown in Figure 3. In the experiments of this research laboratory, it was found that unacceptable filament deformation occurred in the workpiece under compaction force above 400N. Referring to the related papers^9,24 and experimental phenomena, the compaction force selected for this study was investigated in the range of 100-400 N with a gradient of 100 N. Five 12.7 mm × 100 mm laminates were prepared under each compaction force, which was used to study the effect of compaction force level on the strength of the interlayer bond and to analyze the plausible reasons for the results.

Figure 3.

Schematic diagram of line focused infrared heating assisted molding.

Interlayer shear strength test

The ILSS test was carried out according to ASTM D2344 to characterize the interlaminar bond strength of the composites, and the standard dimensions l×b×h=(18 ± 1)mm × (6 ± 0.2)mm × (3 ± 0.1)mm were selected. Firstly, select the zones on the workpiece where the warping degree meets the standard (≤0.015 mm/20 mm) through the flat plate transmittance method, then cut them roughly with a 0.33 mm diamond wire saw, and finally grind the specimens with sandpaper according to the standard dimensions to the qualified range. The test equipment was a 5969 universal testing machine (load capacity of 50 kN) manufactured by Instron, with a loading upper indenter diameter of 6 mm, a specimen support seat diameter of 2.8 mm, and a span L of (12 ± 0.3)mm, and loaded at a rate of 1 mm/min for 3 mm (thickness of the specimen) or until the experimental piece breaks in the interlaminar layer (as shown in Figure 4).The ILSS was calculated according to the standard experimental formula (1)²⁵:

τ_{m} = \frac{3}{4} \times \frac{F}{b h}

(1)

where F is the maximum force value before fracture measured during the test, N; b and h are the width and thickness of the specimen, mm.

Figure 4.

ILSS test method.

DIC (Digital Image Correlation Technology) acquisition and analysis is accomplished through the MatchID system. High-contrast scattering spots are sprayed on the surface of the experimental parts; image sequences before and after ILSS experiments are captured using a high-resolution camera; the displacement of the scattering spots in the sub-region is tracked by a 2D MatchID software algorithm, and the displacement field jumps are analyzed frame by frame to reconstruct the crack extension trajectory. The DIC identifies micrometer-sized strain-concentration zones, which are visible to the naked eye long before the emergence of the cracks. (Figure 5)

Figure 5.

DIC experimental setup.

Macro-expression

Fuji pressure-sensitive paper was used to test the pressure level and distribution during the molding process. Pressure-sensitive paper is divided into two layers, A-film and C-film, cut into long strips of 100 mm in length and slightly wider than the tape, placed face to face in the middle of two prepregs, and laid according to the conditions of laying the tape. Under pressure, the micro color capsule on the A-film leaks out colorant and the colorant on the C-film reacts to develop color. The C layer after color development is scanned with a printer and compared with a standard colorimetric card. Testing revealed that when the film with a rated strength of 2.5 MPa began to change color, discrete adhesion points appeared at the interlayer interface. Therefore, 2.5 MPa is considered to be the effective threshold for intimate contact, and all experiments in this study used 2.5 MPa color-changing film.

High temperature resistant thermocouples of type K were used for thermal history monitoring. The thickness of the KTC used was approximately 0.1 mm and the filament width was 1 mm, which was chosen to ensure that the thermocouples would not break during compaction. The KTCs were collected at 50 Hz. In the experiment, the “0” layer of prepreg is fixed on the laying track as the bottom layer, and five independent channels of thermocouples are evenly arranged in suitable positions every 60 mm (avoiding the initial power instability and resin-enriched areas at the end), so that the temperature sensing point is close to the upper surface of layer 0. Subsequent placement of the prepreg is the first layer, the temperature measured by the thermocouple is absorbed through the layer of prepreg heat transfer to the lower surface temperature. And so on layer 2, layer 3, ....... . The temperature measurement method is shown in Figure 6.

Figure 6.

Temperature measurement methods and equipment.

Microscopic Characterization

The porosity of the experimental pieces was measured using a light microscope. The cross section of the laminate was obtained by cutting with a diamond wire saw and inlaid into a metallographic inlay solution for grinding and polishing, and five points were randomly taken for imaging under an optical microscope. The obtained images were analyzed in gray scale by ImageJ-win64 software to obtain the average value of porosity.

The plane of the non-extruded part of the SBS test natural fracture at each compaction force (hereafter referred to as the pure interlayer separation fracture surface) was intercepted. The fracture surfaces were not touched and were cleaned only with dry, clean, low-pressure air. Ultra-depth-of-field images were obtained by scanning with a Keyence VK-X1100 3D Contour Morphology Scanning System from Keyence Japan Ltd. and analyzed by post-processing software.

Tearing the interlayer bond that was not disrupted by the SBS experiment, Scanning electron microscope (SEM) was used to observe the microstructure of the ILSS tear surface of the experimental piece. A portion of the tear surface was intercepted to cut it into 5 mm × 5 mm specimens, sprayed with gold and observed under high voltage at 10 kV.

Results and discussion

Results of ILSS experiments

The Interlaminar shear strength test is one of the important means of characterizing the interlaminar mechanical properties of plywood. This paper focuses on the effect of compaction force on ILSS during layup. The measured ILSS properties at 100N, 200N, 300N and 400N were 20.1123 ± 1.4104 MPa, 24.8952 ± 0.9458 MPa, 27.5333 ± 0.7988 MPa and 37.3820 ± 2.2051 MPa, respectively, and the detailed results are shown in Figure 7.

Figure 7.

Graph of ILSS statistical results.

Experiments show that the compaction force has a significant modulation effect on the CF/PEEK interlaminar shear strength (ILSS) with a line-focused infrared heater as the heat source: when the compaction force is increased from 100N to 400N, the ILSS strength exhibits a nonlinear growth characteristic, in which the strength jump phenomenon occurs in the interval from 300N to 400N (an increase of up to 35.8%), and the average ILSS intensity of the 400N specimen increased by 85.9% compared with that of 100 N. The DIC image processing results also proved this conclusion. Figure 8 shows the images of the experimental failure moments obtained by the high-speed camera and the results of crack visualization obtained by post-processing with the crack module of DIC software: the 100-200N test specimens experienced multilayer failure, and the weak interfaces triggered multiple concentrations of stress, leading to the damage of the interlayer slippage and delamination coupling, which indicated that the interlayer bonding strength was generally low; the 300N test specimens experienced only single-layer failure, and the cracks showed a steady expansion; In contrast, the 400N test piece showed observable localized failure only when the upper indenter was displaced by 1.33 mm, and the damage occurred only in a small number of the weakest regions, with the best bonding performance.

Figure 8.

Post-processing result diagram of DIC and crack module.

Porosity analysis

Porosity, as one of the key defective factors affecting the interlayer bond strength,²¹ was systematically characterized and analyzed in this study. In order to quantitatively characterize the porosity features, optical microanalysis was used to observe the cross-section of the specimen perpendicular to the fiber direction: the white regular circular area corresponds to the carbon fiber cross-section, the gray area is the resin-enriched phase, and the black irregular area characterizes the pore defects. Gray scale analysis based on ImageJ-win64 software showed that the porosity was 5.33%, 3.97%, 2.88%, and 1.68% under 100-400N compaction force conditions in order (Figure 9). Calculations yielded decline rates of 25.5%, 27.5%, and 41.7%, respectively, which coincided with the ILSS enhancement rate.

Figure 9.

Porosity photomicrograph (50x, local magnification is 200x) (a)100N (b) 200N (c) 300N (d) 400N.

Figure 9(a)–(d) show the typical morphological features of the pore distribution under different compaction forces, respectively: under the condition of low consolidation force of 100 N (Figure 9(a)), significant macroscopic cracks are visible in the specimens with jagged features at the edges, which is the result of the fibers not being adhered by the resin infiltration. The simultaneous presence of large-sized bubble structures in the interlayer bonding region indicates that interlayer bonding occurs only in localized discrete regions. When the compaction force was increased to 200 N (Figure 9(b)), macroscopic cracks evolve into a high density of microcracks clustered in some of the interlayers (the area marked by the red circle in Figure 9(b)), which had a tendency to rapidly connect to form macro cracks under the effect of the interlaminar shear stresses, and then triggered the interlaminar fracture failure. 300 N compaction force specimens (Figure 9(c)) showed a unique structural feature: the boundary of the interlaminar layers tended to be blurred but the uneven distribution of fibers was significant (area indicated by arrows in Figure 9(c)). Comparison with the original prepreg tow morphology (Figure 10) shows that this phenomenon stems from the intrinsic inhomogeneity of the fiber distribution in the raw material. At this time, the interlaminar region is mainly characterized by the presence of isolated distribution of micron-sized pores (identified by the red circle in Figure 9(c)), which, according to the literature,²⁶ are prone to be the beginning for crack initiation in the interlaminar shear (ILSS) test. When the compaction force reaches 400 N (Figure 9(d)), the specimen exhibits optimal interlaminar bonding quality: the interlaminar interface is completely blurred, the resin-enriched areas are homogenized by the compaction force effect, and the uniformity of fiber distribution is significantly improved. Only a trace amount of submicron pores existed in the cross section, and the distribution of pores was characterized by a shift from interlaminar to intralaminar. This phenomenon has similarity with the pore evolution pattern observed by Amanat et al.²⁷ in laser transmission welding of PEEK films, which may be related to the evaporation process of water absorbed within the matrix.

Figure 10.

Photomicrograph of unprocessed raw material.

Based on the theory of intimate contact, the increase of compaction force can effectively increase the effective contact area between material interfaces. In this study, the pure interlayer separation sections of SBS experimental natural fracture under 100N and 400 compaction forces were characterized by three-dimensional morphology (Figure 11), and the surface profile characteristics were obtained by using 100xmagnification and 650% height difference enhancement mode. The analysis of the results showed that: significant morphological defects existed in the cross section of the 100N specimen, in which the typical bubble structure (Figure 10(a)) presented an extreme height difference of 154.5 μm, and the ten-point average roughness $R_{z x}$ = 105.342 μm, measured along the fiber direction through the lowest point of the bubble, confirms that the minimum diameter of the bubble exceeds this critical value; the bulge structure formed by the accumulation of localized fibers was observed on the side wall in the consolidation direction, this is assumed that the fiber rearrangement phenomenon is generated by the flow of the molten matrix when the pressure roller exerts a pushing action on the incompletely discharged air bubbles. This phenomenon reveals that the gas inside the bubbles cannot be discharged in time under low-pressure conditions, resulting in the formation of permanent hole defects in the cured matrix. Figure 11(b) demonstrates a unique depression-type defect in the interlayer region of the 100N specimen, with ten-point average roughness $R_{z y}$ in the vertical fiber direction of 113.216 μm. Conformal characterization suggests that the defect may have originated from in-face fiber wrinkles or prepreg placement deviations during the manufacturing process, and that such structural distortions could not be repaired by making the inside of the depression in intimate contact with each other in the low-consolidation-force condition. In comparison, the cross-section morphology of the 400N specimens improved significantly (Figure 11(c) and (d)), with the ten-point average roughness $R_{z}$ values along the fiber and its perpendicular profile were reduced to 26.694 μm and 33.305 μm respectively. By statistically analyzing five groups of sections, the arithmetic mean surface roughness $R_{a}$ of 100N and 400N specimens were 17.585 μm and 6.702 μm, respectively, with a decrease of 59.6%. The lower surface roughness indicates that the material interface achieves fuller physical contact, which effectively promotes the mechanical interlocking effect of molecular chain diffusion and thus strengthens the interlayer bonding properties.

Figure 11.

Three-dimensional profile scan and roughness measurement results (100×, 650% elevation) (a) Typical bubble at 100N (b) Typical depression at 100N (c) Typical undulation at 400N (d) Typical depression at 400N.

The degree of intimate contact measured by the bearing area curve method (BAC)²⁸ strongly supports this conclusion. The degree of intimate contact was quantitatively characterized by plotting the bearing ratio curves (Figure 12) of the original and single-layer prepreg tows processed with 100-400N compaction force using a 3D contour scanning system under the same process parameters (heating power of 980W and lay-up rate of 25 mm/s). The measured degrees of intimate contact between layers at 100-400N compaction force were 44.14%, 50.33%, 54.35%, and 62.19%, respectively. The experimental data verified the mechanism of high compaction force molding on the enhancement of interfacial contact quality: greater compaction force results in more intimate contact between layers. This finding is in high agreement with the positive compaction force-contact area correlation predicted by the intimate contact theory, which is compatible with the intimate contact theory.

Figure 12.

Surface bearing ratio curves of the original and each compaction force processed monolayer Prepreg tows (BAC method) (a) Original, 100N, 200N (b) Original, 300N, 400N.

In order to further analyze the causes of pore formation, quantitative cross-sectional morphology analysis was carried out on the original and monolayer prepreg tows treated with different compaction forces (100-400 N). Figure 13 shows the optical microscopic comparison images of the raw material and the molded specimens with different compaction forces, in which the yellowish curves characterize the boundary contours after the heat-force coupling. The morphology analysis shows that with the increase of compaction force from 100N to 400N, the curvature radius of the surface profile curve of the Prepreg tow gradually increases, and the smoothness of the interface is significantly improved. Quantitative statistics show that the peak-to-valley difference between the surface undulations of the 100N and 200N specimens is more than 20 μm, with an average amplitude of about 15 μm, which is in good agreement with the arithmetic mean surface roughness $R_{a}$ value (17.585 μm) in the three-dimensional profile test. This phenomenon can be attributed to the significant difference in thermal expansion coefficients between carbon fibers and resin matrix.²⁹ When the processing temperature is higher than the melting point $T_{m}$ of the resin, the viscosity of the matrix decreases, resulting in the weakening of the fiber binding force, and the release of the residual stress triggers the fiber to “elastically rebound” (bulking behavior), resulting in the formation of the typical surface bumpy structure (red marking area in Figure 13(a) and (b)). This periodic undulation structure will form a resin flow obstacle during the lamination process, and permanent interlayer porosity will be generated in the resin-poor region when the peaks and valleys of neighboring laminates are misaligned.³⁰ However, it is noteworthy that the surface flatness of the 300N and 400N specimens is better than that of the original Prepreg tows (Figure 13(c) and (d)), confirming that the high consolidation force condition can achieve dynamic repair of interfacial defects by promoting lateral resin percolation.²² This repair mechanism involves two synergistic processes: (1) it enhances the shear coupling at the fiber-resin interface and suppresses the fiber rebound due to stress relaxation²⁹; and (2) the high consolidation force drives the molten resin to fill the microscale gaps and increase the effective bonding area.³¹

Figure 13.

Out-of-face undulation of single-layer prepreg at various compaction forces.

The experimental results reveal the mechanism of compaction force regulation of pores from the perspective of interfacial morphology evolution: the compaction force can enhance the interfacial constraining force and resin percolation capacity, and the synergistic effect of the two can reverse the rebound of residual stress caused by the difference in the coefficients of thermal expansion of the materials, and realize the suppression and healing of the interlayer voids.

Pressure distribution analysis

For processes with large time-applied pressure durations, the initial degree of intimate contact is often neglected. However, this is not the case for in situ consolidation processes.¹⁸ In this work, the modulation law of compaction force on the initial contact state of CF/PEEK was tested by high-temperature pressure-sensitive paper. As shown in Figure 14(a), the pressure field distribution of in-situ consolidation recorded by using high-temperature pressure-sensitive paper (working temperature ≥300°C) indicates that the pressure distribution under the low-pressure condition of 100N exhibits a significant non-uniformity (coefficient of variation CV = 0.203), and only 37.3% of th area has reached the effective contact threshold (≥2.5 MPa). As the compaction force increases to 400N, the percentage of area under effective pressure reaches 95.8%, and the coefficient of variation CV of the pressure field decreases to 0.067, which confirms that the high-pressure condition is able to break through the limitation of the intrinsic surface heterogeneity of the material.

Figure 14.

Scan of pressure distribution of pressure sensitive paper testing (a) Pressure distribution under different compaction force at high temperatures (b), (c), (d) A rubber wheel was used to simulate the elastic state of high-temperature melting at room temperature, and the pressure distribution of three different Prepreg tows was measured repeatedly at different compaction forces (e) Effective action width of the pressure wheel under different compaction force at high temperatures.

It is worth noting that batch differences in the surface morphology of the original Prepreg tows³² can significantly affect the initial contact state. For this reason, this study innovatively uses room-temperature rubber experiments to simulate the high-temperature molten state (Figure 14(b)–(d)), and excludes the interference of material variations by reproducibly loading the same section of the strip. The experimental data show that as the compaction force increases, the image always continues to expand the area and deepen the color on the basis of the previous compaction force (the purple circle area in Figure 14(b)–(d)), and there is an obvious progressive relationship from 100N to 400N, which indicates that the plastic deformation of the surface micro-convexity dominates the contact mechanism under high pressure conditions; When the compaction force exceeds 300N, more than 80% of the height difference of the surface micro-convexity is effectively collapsed, prompting the contact interface to change from discrete contact mode to continuous contact mode; the images of each group tend to be complete and uniform rectangle under the compaction force of 400N, which reveals that the higher pressure can gradually eliminate the unfavorable effect of surface morphology through the multi-scale deformation mechanism (elasticity → elasto-plasticity → complete plasticity).

Thermal history analysis

Two stages are required to achieve bonding at the interface between layers, including self-bonding in addition to intimate contact,^20,29,33 and the onset and process of bonding is closely related to temperature. In this study, the indirect modulation of interfacial properties by compaction force through temperature is revealed by monitoring the paving exothermic history by thermocouples. Figure 15(a) shows the maximum temperature of each layer in five independent channels at 70% heating power (980 W) and 25 mm/s lay-up speed, the red and orange dashed lines in the figure are the melting temperature $T_{m}$ and the glass transition temperature $T_{g}$ . Peak temperature measurements revealed average peak temperatures of 372.84°C, 309.28°C, 272.02°C, 230.54°C, 205.56°C, and 177.92°C for layers 1-6, respectively, which all exceeded $T_{g}$ of CF/PEEK with a gradient decay. It has been shown that self-bonding of amorphous chains in semi-crystalline polymers proceeds when the temperature is above $T_{g}$ ^18,34. This suggests that the interfacial bonding of the layers is actually the result of undergoing multiple thermal-pressure coupling actions. Other analyses have pointed out that the presence of microcrystals hinders the diffusion process below the melting temperature,^30,31,35 which, together with the very high viscosity of the matrix at lower temperatures,³⁶ leads to a rather small interfacial strength formed by welding in the vicinity of $T_{g}$ ³³, and therefore the contribution of layers 4-6 to the interface bonding is negligible.

Figure 15.

(a) Maximum temperature of each layer at 70% heating power (980W) and 25 mm/s lay-up speed (b) Temperature history curves of layers 1, 2 and 3 at 100N and 400N measured by 3-channel measuring meter.

Figure 15(b) further compares the temperature history curves of layers 1-3 at 100N and 400N compaction force measured by the same thermocouple 3-channel. It was found that as the line-focused infrared heating device approaches, the 1, 2, and 3 layers warm up rapidly in sequence; when the pressure roller is close, the infrared radiation will be gradually partially blocked by the pressure roller wheel body, and the temperature growth rate slows down gradually; until it is almost completely masked, the temperature reaches the maximum value and starts to cool down rapidly, and the pressure roller arrives to realize the cured adhesive bonding after about 0.6 s. The temperature history is shown in Figure 15(b) for the same thermocouple with 3 channels. The width of the compacted area of the stationary pressure roller was measured to be 5 mm by Fuji high-temperature pressure-sensitive paper (Figure 14(e)), which means that the pressure retention time was only 0.2s (Figure 15(b) red bar area shows the actual pressure history). The difference in temperature between the two at the moment of consolidation of the first layer is not significant, being more than 380°C; from the second layer onwards the difference becomes obvious: the consolidation temperature of the 400N is close to $T_{m}$ , whereas the consolidation temperature of the 100N is about 15°C lower; and the difference is even more pronounced in the third layer, where the difference in temperature reaches 30°C. This shows that the heat under small compaction force is neither retained in the 1st layer nor transferred quickly to the 2nd and 3rd layers, but is rapidly dissipated in the air and the pressure rolls, and is poorly utilized. The reason for this phenomenon is attributed to the fact that the high compaction force increases the degree of intimate contact between the layers and thus reduces the thermal resistance, allowing the heat received by layer 1 to be transferred to the subsequent layers as quickly and as non-destructively as possible. It has been shown that the melt bonding of pure PEEK near its $T_{m}$ is most drastically affected by temperature.^29,31 It is proved that the temperature enhancement by large compaction force creates better conditions for molecular chain diffusion, which can effectively promote interlayer bonding. It was also found that the thermal history of line-focused infrared heating had a relatively large difference compared with laser-assisted heating, and the warming effect on the subsequent layup was more obvious,³⁷ which was a result of different heating systems. It indicates that the warming mechanism of large pressure on subsequent layups under line-focused infrared heating has a more obvious effect on interlayer bonding.

Notably, the thermal history analysis also revealed that the instantaneous consolidation characteristic of the rigid rollers (contact time 0.2 s) leads to the presence of residual temperature at the interface after compaction: the interior of the specimen maintains a temperature above $T_{g}$ after the rollers have passed. This results in regions where residual gases are present being prone to rebound expansion after the compaction force disappears, forming large intra- or interlayer voids,^32,38 which could be the reason for the large air bubbles found between the layers of the 100N test piece in the 3D contouring experiments. This phenomenon highlights the central role of compaction force on gas evacuation, especially in the rigid pressure roller system, where the compaction force amplitude directly determines the suppression effect of interfacial defects.

SEM analysis

SEM observation (Figure 16) shows that: with the increase of compaction force from 100N to 400N, the plastic deformation characteristics of the fracture surface is gradually significant, specifically as follows: 100N specimens (Figure 16(a) and (b)): the fracture surface shows a typical brittle fracture morphology, and a large number of unadhered areas and interlayer pores can be seen; 200-300N transition specimens (Figure 16(c) and (d)) show brittle-tough transition characteristics, and the local area of plastic deformation; 400N specimens (Figure 16(e) and (f)), the plastic deformation area extends to the entire interface, and a large number of fiber pull-out occurs. It was again demonstrated that the high compaction force condition significantly optimized the interfacial bonding quality by promoting resin flow and fiber infiltration in the infrared focused heating system.

Figure 16.

SEM images of fracture surfaces of ILSS test samples (a) 100N, 200× (b) 100N, 400× (c) 200N, 200× (d) 300N, 200× (e) 400N, 500× (f) 400N, 200×.

Conclusions

The mechanism of compaction force on the interlaminar shear strength (ILSS) of line-focused infrared heating-assisted molded CF/PEEK laminates was revealed by interfacial morphology, pressure field distribution and thermal history analysis, and the following conclusions were drawn:

(1) Under line-focused infrared heating system, ILSS showed a nonlinear positive correlation with compaction force. When the compaction force increased from 100 to 400 N, the average strength of ILSS increased by about 17.27 MPa, which was 85.9%, and the performance was significantly improved. High compaction force can reduce porosity, promote interface plasticization, change the failure mode from multi-layer brittle damage to single-layer ductile damage, and also achieve surface morphology homogenization through plastic deformation of microbumps, weakening the negative impact of the original Prepreg tow surface undulation on the interfacial contact.

(2) Porosity is significantly negatively correlated with the interlayer bond strength, which has a prominent effect on the ILSS results. Specimens fabricated under low compaction force causes inadequate interlaminar intimate contact and insufficient resin squeeze flow, manifesting as elevated porosity and microscopically rough interfacial morphology, while the high compaction force promotes intimate contact between the layers, which results in flat interlayers and low porosity. Comparison of the surface morphology of single-layer Prepreg tows reveals that larger compaction force can inhibit the expansion behavior of the material and realize the close fit between layers.

(3) Thermal history analysis shows that 400N compaction force raises the interface temperature of 2-3 layers by 20 °C–30 °C compared to 100N, effectively promoting molecular chain diffusion and enhancing the interlayer intimate contact. This heat transfer enhancement effect stems from the fact that the interlayer thermal resistance decreases as the intimate contact improves with increasing compaction force. In addition, the interfacial rebound effect induced by residual heat and the instantaneous consolidation characteristics of the rigid rolls further emphasize the importance of high compaction force to allow the escape of gases.

(4) The mechanism of compaction force on the interlayer shear strength of line-focused infrared heating-assisted molded CF/PEEK laminates revealed in this study is the underlying physical principle of the consolidation process of all thermoplastic composites. Regardless of the thermoplastic resin matrix, adequate infiltration, low porosity and intimate contact are always the key to obtaining high ILSS. The core physical principles and key conclusions are generalizable to a considerable extent, but the idiosyncrasies brought by different material systems and heating methods need to be taken into account: the thermal history curves can vary considerably under different heat sources and need to be analyzed specifically.

Footnotes

Author contribution

Jianglin Liu: conceptualization, resources, guidance, and funding acquisition; Fengwei Liu: writing-original draft, data organization, and visualization; Hui Li: writing-review and editing; Ting Wu: writing-review and editing; Xiaoxiang Zhang: writing-review and editing; Yinhui Li: writing-review and editing; and Jianguo Liang: project management, supervision, validation, and funding acquisition.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

This work was supported by the National Science and Technology Major Program (NS&T) [grant number 2024ZD0701601]; National Natural Science Foundation of China [grant number 52075361]; Shanxi Province Science and Technology Major Project [grant number 20201102003]; Shanxi-Zheda Institute of Advanced Materials and Chemical Engineering [grant number 2022SX-TD021]; Lvliang Science and Technology Guidance Special Key R\&D Project [grant number 2022XDHZ08].

ORCID iD

Fengwei Liu

Data Availability Statement

Date will be made available on request.*

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Effect of line-focused infrared heating-assisted CF/PEEK in-situ consolidation compaction force on interlayer shear strength

Abstract

Keywords

Introduction

Experimental procedures and methods

Materials

AFP equipment

Experimental Design

Interlayer shear strength test

Macro-expression

Microscopic Characterization

Results and discussion

Results of ILSS experiments

Porosity analysis

Pressure distribution analysis

Thermal history analysis

SEM analysis

Conclusions

Footnotes

Author contribution

Declaration of conflicting interests

Funding

ORCID iD

Data Availability Statement

References