In this work, based on the quasi-static tensile test and acoustic emission technology, the tensile properties of two types of three-dimensional flat-knitted inlay fabrics reinforced composites are investigated, and the acoustic emission characteristic parameters of various damage mechanisms are obtained. The transverse tensile process of specimens could be divided into the elastic stage, yield stage, and fracture stage. We found that, compared with the fluctuation of the stress-strain curve in the yield stage, weft insertion yarns in composite with interlock structure broke almost simultaneously, while the composite with plain stitch broke successively. The transverse and longitudinal tensile strength of the composite with interlock structure was 44.70% and 28.63% higher than the composite with plain structure, respectively. The SEM micrographs showed that the damage mechanism of the composites was matrix fracture, fiber-matrix debonding, and fiber breakage. The amplitude ranges of the three damage mechanisms were 50–65 dB, 65–80 dB, and 90–100 dB, respectively, and the frequency ranges were 35–114 kHz, 116–187 kHz, and 252–281 kHz, respectively. Fiber-matrix debonding and matrix fracture had large cumulative AE energy, numerous events, and long duration time, while fiber breakage had the characteristics of large amplitude, high frequency, low cumulative AE energy, few events, and short duration time.
In recent years, research and engineering interest have been shifting on the polymer composites with knitted fabric as reinforcement, due to its attractive properties in rapidly forming, good forming quality, and low artificial participation [1,2,3,4,5]. Attempts have been made to reduce the processing cost of composite products with complex shapes and structures using knitted fabrics as preforms. So far, automobile components [6, 7], wind turbine blades [8,9,10], and helmet shells  are now being manufactured using knitted fabric reinforced composites. The excellent bending and drapability of knitted fabrics bring the outstanding advantages of formability and bring the disadvantages of low in-plane strength and stiffness of knitted fabric reinforced composites . However, the above disadvantages can be solved by using inlay yarn [13, 14]. For these reasons, warp-knitted axial fabric-reinforced composites are widely used in wind turbine blades, and multi-layer biaxial weft-knitted composites have been successfully used in pilot helmets, aircraft wings cover, car bodies, and other products.
Three-dimensional flat-knitted inlay (3DFKI) fabric is a knitted fabric with non-buckling reinforcement yarn. Compared with biaxial weft-knitted fabric, the 3DFKI fabric is a uniaxial three-dimensional flat-knitted fabric. Biaxial weft knitted fabric is usually produced on the modified double needle bed flat knitting machine [15, 16]. Although the fabric has outstanding mechanical properties in the transverse and longitudinal directions, it is difficult to directly produce preforms with special shaped structures and complex shapes due to longitudinal reinforcing yarn. Therefore, it is often necessary to prepare composite products with complex shapes by cutting and pressing methods. The 3DFKI fabric is made by adding weft insertion yarns in the 3D flat-knitted fabric, which can be easily realized on the conventional computer flat knitting machine, and the local knitting technology can be used to realize the integral forming of the preform with complex shape and structure. Thus, 3DFKI fabric-reinforced composites not only have good mechanical properties, but also have the advantages of good formability, low processing cost, fast production speed, and a short preparation process. It has excellent development potential in transportation, personal protection, medical devices, and other industrial fields.
The tensile test is the most critical test method to describe the mechanical properties of composites and obtain their basic mechanical properties [17,18,19]. The basic mechanical properties such as the tensile load, stress and strain, tensile modulus, and Poisson's ratio can be obtained by a tensile test, which can guide the actual production and application and evaluate and predict the mechanical properties of products. However, the tensile test can measure only the change of external force during the failure process of the composite, and it provides minimal information on damage evolution and expansion [20, 21]. In addition, the structure of textile composites is complex and anisotropic, and different damage mechanisms are activated at the same time . Consequently, the damage process of textile composites is very complex. In order to reveal the damage accumulation process of composites, in recent years, the failure mechanism of composites has been analyzed based on ultrasonic inspection [23, 24], thermography [25, 26], X-ray [27, 28], and acoustic emission (AE) [29, 30]. AE is a dynamic monitoring method that can detect the damage process of composites under load in real time and has functions such as damage detection, damage location, and damage identification [31, 32]. The working principle of AE is to connect the sensor to the sample, continuously record real-time data during the dynamic damage process of the sample, use advanced data analysis for feature extraction, link the observation data with the damage state, and determine the damage form and damage location of the sample . In a review paper published recently by Milad et al., the research progress of AE in damage detection and damage identification of composites was analyzed in detail and comprehensively . It is worth noting that there was more literature on glass fiber, carbon fiber, and aramid fiber reinforced composites and less literature on ultrahigh molecular weight polyethylene (UHMWPE) composites. UHMWPE fibers have attracted the most attention, since their strength exceeds that of carbon and aramid fibers . Compared with other high-performance fibers, UHMWPE fiber not only has high strength and modulus along the fiber axis, but also has the characteristics of low density (0.97 g/cm3) and good knitting ability. To the best of our knowledge, the literature on the use of AE technology to study the tensile properties of UHMWPE knitted composites has not yet been reported.
The present paper prepared two types of 3DFKI fabric reinforced composites using UHMWPE yarn. Two composites obtained were subjected to a quasi-static tensile test, and the influence of the fabric structure on its tensile properties was compared and analyzed. Moreover, the tensile tests were monitored by acoustic emission. The main objective was to correlate the damage mechanism with the AE signal, and determine the characteristics of the AE signal of the damage mechanism.
2.1 Materials and 3DFKI fabric fabrication
The UHMWPE filaments studied were purchased from Hangzhou Xiangsheng high strength fiber material Co., Ltd., Hangzhou, China. The specifications of knitting yarn and weft insertion yarn are shown in Table 1. A01 epoxy resin and B02 hardener were provided by Hualike New Material Co. LTD, Changzhou, China.
|Type||Fineness/denier||Diameter/mm||Density/g·cm−3||Tensile strength/MPa||Tensile modulus/Gpa|
|Weft insertion yarns||1,000||0.38||2,707||93|
3DFKI fabrics of two structures were prepared on the KSC-132D computer flat knitting machine (Gauge E14, Longxing, China). The knitting steps and structures of two types of fabrics (named Fabric I, Fabric II) are presented in Figures 1 and 2, respectively. The corresponding specifications of the two types of 3DFKI fabrics are shown in Table 2. Fabric I was a 3DFKI fabric that combines interlock structure, tuck stitch, and weft insertion yarns. Fabric II was a 3DFKI fabric with a plain stitch, a tuck stitch, and weft insertion yarn. The interlock structure in Fabric I belonged to double-sided fabric, and the knitting yarn ran through the upper and lower surfaces of the whole fabric, formed a three-dimensional structure with good integrity. The upper and lower surface fabrics of Fabric II were independent of each other, which were connected through the internal tuck stitch, thus forming a three-dimensional spacer structure. Therefore, the thickness of Fabric II was slightly larger.
2.2 Preparation of 3DFKI fabric reinforced composites
The vacuum-assisted resin infusion (VARI) and external pressure methods were used to prepare composite plates with the same thickness. Figure 3 shows the mold structure and composite preparation process. The specific operation process can be divided into four steps: in the first step, place a layer of demolding cloth, guide net, and vacuum film on the lower plate of the mold, then cover the upper plate, zero the four dial indicators, and then remove the upper plate. In the second step, the fabric, demolding cloth, guide net, and vacuum film are stacked together in sequence after applying demolding wax on the lower plate of the mold, and the vacuum film is sealed with sealant. In the third step, the mixed resin solution is injected under the vacuum pump pressure of 0.08 Mpa, and then the next step is carried out after the sealing performance was good. In the fourth step, cover the upper plate, and adjust the four dial indicators to 1.80 mm by adjusting the four bolts of the upper plate. The indication of the dial indicator is the thickness of the composite plate. The demolding operation was carried out after curing. The mixed resin solution was composed of A01 and B02 with a mass ratio of 2:1. The curing conditions were 5 h at 50°C and 24 h at room temperature. The resin casting was prepared by rubber mold, and the curing condition was the same as that of the composite. The parameters of the 3DFKI fabric reinforced composites (named C-I, C-II) and resin casting are shown in Table 3.
|Sample||Reinforced fabric||Thickness/mm||Density/g·cm−3||Fiber volume fraction/%|
2.3 Tensile test
Tensile experiments of 3DFKI fabric reinforced composites were conducted on a UTM5105 electronic universal testing machine at room temperature with a constant loading velocity of 10 mm/min according to the standard of GB/T 1447–2005. The sample size was 180 mm × 20 mm, the gauge distance was 50 mm, and the sample shape and size are shown in Figure 4. Five samples for each specification were used for the test.
Acoustic emission test was carried out according to ASTM E2478-11. Double channel DS5 full information AE signal analyzer manufactured by Beijing Soft Island Times Technology Co., Ltd. was used for the analysis. During the tensile test, the collected AE signals were processed by the system software to obtain the data of action time, amplitude, energy, and peak frequency. Figure 5 shows the schematic diagram of the AE testing system.
3 Results and discussion
3.1 Tensile behavior of composites
The tensile stress-strain curves and parameters of the two composites are shown in Figure 6(a)–(c). It can be seen from Figure 6(a) that the transverse stress-strain curves of the two composites can be divided into three stages. The first stage was the elastic stage, where the stress and the strain were in an apparent linear relationship, and the stress increased with the increase of strain. At this stage, due to the small strain, the knitting yarn in the fabric was still in a bent state, and the resin matrix and the weft insertion yarn bore the tensile load together. With the increase of load and tensile deformation, the weft insertion yarn gradually bore the main load, giving full play to the performance of the non-buckling reinforcing yarn. The second stage was the yield stage, in which the stress began to decrease slightly after reaching the peak value, the linear relationship between stress and strain was destroyed, and the stress fluctuated with the increase of strain. These characteristics were more evident in the tensile curve of C-II-0°. In this stage, the weft insertion yarn continued to bear the external load. It could be inferred from the fluctuation of the stress that the weft insertion yarn began to break. Compared with C-II, the peak force of C-I in the yield stage was higher, and the duration was shorter. This indicates that the weft insertion yarn of C-I broke almost simultaneously under the external load, while the weft insertion yarn of C-II broke successively. The third stage was the failure stage. The stress dropped sharply and remained at a certain height, which indicates that the weft insertion yarn inside the composite had completely failed, and the knitting yarn began to bear the external force. With the increase of strain, the stress remained in a high range and fluctuated continuously, which indicates that the knitting yarn inside the composite was gradually destroyed. When the composite was stretched longitudinally, it mainly bore the external force through the loop pillar . Therefore, the longitudinal stress-strain curves of the two composites were relatively simple.
Figure 6(b)–(c) shows the tensile strength and modulus of the two composites. The tensile strength and modulus of composites depend on fiber volume content, component materials, and reinforcement structure . It could be seen that the transverse tensile strength and modulus of the two composites were much higher than their longitudinal tensile strength due to the addition of weft insertion yarns. Notably, although the fiber volume content of C-I was slightly lower than that of C-II, the transverse tensile strength of the former was 44.70% higher than that of the latter. Compared with C-II with the spacer structure, the interlock structure in C-I with better integrity had a noticeable binding effect on the weft insertion yarn, which made the force of the weft insertion yarn more balanced. In addition, it could be seen from the cross-section diagram of two fabrics in Figure 2 that there were more contact points between the basic structure of Fabric I and the weft insertion yarn. Contact points increased the friction resistance to the weft insertion yarn during the tensile process, and also helped to improve the force uniformity and consistency of the weft insertion yarn at different positions. This is also why the elongation at the break of C-I was about 51.12% higher than that of C-II. Since the wale density of the two fabrics was the same and the number of weft insertion yarns inside the fabric was the same, the transverse modulus of the two composite materials was similar. Moreover, we compared the longitudinal tensile properties of the two composites, and it was found that the longitudinal tensile strength and modulus of C-I were 28.63% and 41.85% higher than that of C-II, respectively. The tensile properties of composites mainly depended on the mechanical properties of reinforced fabrics. It could be seen from the cross-sectional structure of the fabric that the difference between the two types of fabrics was that the interlock loops inside Fabric I were overlapped with each other, while the plain stitch loops inside Fabric II were independent on both sides of the fabric. It is because of this compact structure that the tensile properties of C-I were higher.
3.2 Tensile damage and fracture mechanism
The tensile damage morphology of composites is shown in Figure 7. SEM micrographs of the specimen after tensile failure are presented in Figure 8. It can be seen in Figure 8 that there were three damage mechanisms of matrix fractures, fiber-matrix debonding, and fiber breakage. Among them, matrix fractures and fiber-matrix debonding were the primary failure mechanism. The fracture morphology of the two composites in the same loading direction was similar, as shown in Figure 7. The damage morphology after transverse tensile failure had the characteristics of a long fracture area and no clear fracture, while the longitudinal direction had a clear and concentrated fracture. Before the tensile fracture of fabric-reinforced composites, the yarns supported the main load, so the damage morphology of the sample was related to the state of the yarn in the fabric. In the longitudinal direction of the reinforced fabric, there was the only yarn with coil structure, and the structure was simple. In the tensile process, when the bending coil and the matrix broke, the sample failed, and the fracture was clear. At the same time, during the longitudinal stretching process, the loop pillar of the coil bore the main load and the deformation was small, so the fracture area was more concentrated. In the transverse direction of the reinforced fabric, there were not only the yarn with the coil structure but also the weft insertion yarn in the straight state. In the transverse stretch, the weft insertion yarn broke first, and then with the increase in strain, the knitting yarn broke, and the fabric structure was destroyed. When the knitting yarn broke, the needle loop arc and sinker loop arc were deformed larger, and the transverse tensile fracture area of the composite was longer. Additionally, due to the large transverse elongation of the basic structure, there were still some yarns that are not broken from tensile failure.
3.3 Acoustic emission analysis
Figure 9 exhibits the load and amplitude versus time for composites and resin casting. The parameter characteristics of two types of composites in transverse tension are shown in Figures 9(a) and (c). It could be seen that the AE signal was mainly concentrated between 50–80 dB in the elastic stage of tension, and the primary damage mechanism in this stage was matrix fractures and fiber-matrix debonding. With the increase in load and time, the AE signal increased gradually, indicating that the damage was aggravated. In the yield stage, the amplitude of the AE signal was 50–100 dB, which contained a small amount of high amplitude AE signal (90–100 dB). This indicates that in addition to matrix fractures and fiber-matrix debonding, fiber breakage also occurred in the composites. The amplitude range of UHMWPE fiber breakage was consistent with the reference . There was no damage on the surface of the two composites in the actual tensile process, because the fiber breakage at this stage was the breakage of the weft insertion yarn in the sample. The time and load of the first high amplitude AE signal of C-I were 53 s and 4,243 N, respectively, and the corresponding time and load of C-II were 34 s and 3,110 N, respectively. It could be seen that the starting time of weft insertion yarns of C-I was later than that of C-II, and had higher strength. This result shows that the tensile properties of C-I were better. In addition, the duration of the yield stage of C-I was 8 s, while that of C-II was 36 s, which again proved the fiber breakage in C-II did not occur simultaneously. In the failure stage, the range of the AE signal was also between 50–100 dB, indicating that damage mechanisms such as matrix fractures, fiber-matrix debonding, and fiber breakage coexist. However, the load at this stage was lower than that at the yield stage because the knot strength of the yarn was much lower than the tensile strength of the non-crimp yarn . Therefore, the fiber breakage at this stage was caused by the fracture of the knitting yarn at the bending state. Figure 9(e) shows the tensile load and amplitude versus time for resin casting. It could be seen that the amplitude range of matrix fracture was 50–65 dB, so it could also be inferred that the fiber-matrix debonding was 65–80 dB. The number of AE signals in longitudinal tension was much lower than in transverse tension, as illustrated in Figures 9(b) and (d). This phenomenon was due to the simple structure of the fabric in the longitudinal direction, which was stressed by the loop pillar, so the deformation was small and the stretching process lasted for a short time. It could be seen that during the stretching process, the AE signal amplitude was mainly maintained at about 50 dB, and the number was small. Before the composite failed, the AE signal increased rapidly, and the high amplitude signals of 70–80 dB and 90–100 dB appeared.
In order to further explore the relationship between the damage mechanism and the dynamic response characteristics of AE signal, on the one hand, and distinguished and quantified various damage mechanisms, on the other, we took the transverse tension of C-I as an example. The feature extraction of AE signal data was carried out based on an advanced data analysis method. Based on the three main damage mechanisms observed in SEM micrographs, namely matrix fractures, fiber-matrix debonding, and fiber breakage, the AE signals were divided into three classes by K-Means clustering analysis. Figure 10 shows the clustering results of AE signals in the transverse tensile test of C-I. The peak frequency range of Class-1 was 35–114 kHz, showed low frequency; the peak frequency range of Class-2 was 116–187 kHz, presented intermediate frequency; the peak frequency range of Class-3 was 252–281 kHz, indicated as high frequency. From earlier studies, in the fabric reinforced composites, the frequency of matrix fractures was lower, the frequency of fiber breakage was the highest, and the frequency of fiber-matrix debonding was between the two frequencies [39,40,41]. For 3DFKI fabric reinforced composites, Class-1 represented matrix fractures, Class-2 represented fiber-matrix debonding, and Class-3 represented fiber breakage. Table 4 shows the classification ratio of AE signals in the transverse tensile test of C-I. The data showed that the fiber-matrix debonding was the primary failure mode, accounting for 69.33%. This is mainly due to the poor interface bonding between the epoxy resin and UHMWPE . The fiber breakage accounted for a minor proportion, and the matrix fractures were in the middle.
|Classification||Frequency range / kHz||Number of AE signals||Ratio / %|
The increasing trend of AE signal could well reflect the dynamic damage evolution process of composites during loading . AE energy is very sensitive to the initiation of damage, evolution, and ultimate failure of composites in tensile tests . Figure 11 illustrates the various damage mechanism and the total cumulative AE energy versus time for C-I. There were three intervals (0–60 s, 60–156 s, 157–240 s) corresponding to the sharp increase of cumulative energy curve value. That the number of AE signals increased considerably indicates the occurrence of significant damage in the material. The reinforced fabric of C-I consisted of three structures, i.e., the weft insertion, tuck, and interlock stitch. From the yarn length of various structures, it could be judged that the weft insertion yarn broke first, followed by the tuck yarn, and finally the interlock yarn when the fabric stretched in the transverse direction. Therefore, we inferred that the 0–60 s corresponded to the weft insertion yarn fracture, the 60–156 s corresponded to tuck stitch fracture, and the 157–240 s corresponded to the interlock structure failure. It can also be seen in Figure 11 that the cumulative AE energy curves of fiber-matrix debonding and matrix fracture are consistent with the total cumulative AE energy curves, and the cumulative AE energy of the two damage mechanisms was much higher than that of fiber breakage. Table 5 shows the ratio of AE energy, AE events, and duration for the various damage mechanisms. It can be seen that matrix fracture and fiber-matrix debonding had the characteristics of large cumulative AE energy, numerous events, and long duration time, while fiber breakage was just the opposite.
|Damage mechanisms||AE energy / mV·ms||Ratio / %||AE events||Ratio / %||AE signal duration / μs||Ratio / %|
For this paper, two types of 3DFKI fabric-reinforced composites were prepared using UHMWPE yarn, and the tensile properties of two composites were studied based on acoustic emission technology. The following conclusions can be summarized:
The transverse tensile process of two composites can be divided into the elastic, yield, and fracture stages. Compared with C-II, the stress-strain curve of C-I in the yield stage had the characteristics of small fluctuation, long duration, and large peak force, indicating that the weft insertion yarns of C-I broke almost simultaneously, while the weft insertion yarn of C-II broke successively.
The transverse tensile strength and modulus of the composites were greatly improved due to the addition of weft insertion yarns. The structure of the reinforced fabric had a significant effect on the tensile properties of the composites. The longitudinal tensile strength and modulus of C-I were 28.63% and 41.85% higher than that of C-II, respectively. Due to the fact that the interlock structure had a more prominent binding effect on the weft insertion yarn, the transverse tensile strength of C-I was 44.70% higher than that of C-II.
The amplitude ranges of matrix fracture, fiber-matrix debonding, and fiber breakage were 50–65 dB, 65–80 dB, 90–100 dB, respectively, and the frequency ranges were 35–114 kHz, 116–187 kHz, and 252–281 kHz, respectively. Fiber-matrix debonding and matrix fracture had large cumulative AE energy, numerous events, and long duration time, while fiber breakage had the characteristics of large amplitude, high frequency, low cumulative AE energy, few events, and short duration time.
The 3DFKI fabric-reinforced composites with interlock structure have better tensile properties. When we combine it with the forming technology of flat knitting, it is possible to develop knitted composites with excellent mechanical properties and complex shapes. In addition, acoustic emission characteristic parameters of different damage mechanisms of UHMWPE knitted composites can provide a reference for damage detection and damage identification of wind turbine blades, storage tanks and other products.
The authors acknowledge the financial support from the National Science Foundation of China (61772238, 11972172), the Fundamental Research Funds for the Central Universities (JUSRP22026), the Taishan Industry Leading Talents (tscy20180224).
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© 2021 Jiangtao Tan et al., published by De Gruyter
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