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E composite laminates on grinding temperature
Composites Part B 157 (2019) 100–108
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Experimental study on effects of structural characteristics of C/E composite laminates on grinding temperature
T
Tao Chena,b, Hongbo Lia,b, Mengli Yea,b,∗, Chao Xianga,b, Shenlin Tiana,b a b
School of Mechanical and Electronic Engineering, Wuhan University of Technology, Wuhan, 430070, PR China Hubei Digital Manufacturing Key Laboratory, Wuhan University of Technology, Wuhan, 430070, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: C/E composites UVAG Grinding temperature Structural characteristics Fiber Bragg grating (FBG) sensors
As for ultrasonic vibration assisted grinding (UVAG) of carbon/epoxy (C/E) composites, the grinding temperature is greatly influenced by their structural characteristics (such as epoxy resin content and fiber directional angle (θ )). For revealing their relationship, C/E composite workpieces with various structural characteristics were designed and prepared; moreover, Fiber Bragg grating (FBG) sensors were embedded in each workpiece for measurement of the grinding temperature; in addition, various grinding parameters were set in UVAG experiments. Experimental results show that: 1). the grinding temperature is influenced very remarkably by the fiber directional angle of C/E composites and the grinding temperature along the direction of fibers is much higher than that reverse fiber direction; 2). the grinding temperature peaks under fiber directional angle (θ = 90°); 3). on the premise of meeting the performance of workpieces, the grinding temperature raises with the increase of epoxy resin content; 4). the change of grinding parameters has nothing to do with the relationship (the grinding temperature vs. the structural characteristics of C/E composites).
1. Introduction C/E composites have many excellent properties (such as significant anisotropy, high specific strength and rigidity, corrosion and high temperature resistance and light weight) so that they are popular in various applications. However, C/E composites are typically difficult to process due to these factors: 1) aeolotropy and non-uniformity; and 2) machinability of components. Their physical and mechanical properties include significant aeolotropy; for example, the interlayer bonding strength of laminates is only 5–20% of the tensile strength along the direction of fibers and lamination may easily occur. The C/E composite consists of carbon fibers and epoxy resin; the former is very hard and highly strong reinforced phase, which is difficult to process; on the other hand, the latter is extremely thermo-sensitive base material easily breakdown under effects of the grinding temperature, resulting in the loss of its support and protection for carbon fibers [1–3]. In recent years, numerous researches about the machining process, strength and ground surface quality of hard brittle materials and several composites were reported. Liu et al. found the four stages of material removal and the characteristics of grinding force during processing; moreover, the undeformed chip thickness has important impacts on the forming of surface defects. This study reveals the materials removal mechanism in high-speed grinding of specific composites [4]. Yang ∗
et al. investigated the critical maximum undeformed equivalent chip thickness for ductile-brittle transition (DBhmax − e ) of zirconia ceramics under different lubrication conditions theoretically and experimentally. They proposed a DBhmax − e model and carried out verifying experiments. Findings show that the model can predict relationships between DBhmax − e and lubricating conditions [5]. Zhao et al. found that a larger pore size and a higher pore concentration will reduce the strength of porous composites, while the uniformity of the pore distribution will benefit the composites strength. Their study provides a guidance for the application of porous Cu-Sn-Ti alumina composites as a bonding material for grinding wheels [6]. Dai et al. carried out grinding experiments of Inconel 718 and concluded that the minimum grinding force and specific grinding energy can be obtained at wheel speed of 120 m/s and the lost grinding power reaches the maximum value at 140 m/s under the given experimental conditions. Their conclusions can provide the basis for process optimization of hard brittle material [7]. Zhang et al. developed an improved grinding force model which considers material-removal, plastic-stacking mechanisms and the influence of lubricating conditions. The experimental results indicate that the model has a better accuracy than previous ones. In addition, the model is suitable for different lubricating conditions [8]. Ding et al. performed grinding experiments and established several models to investigate the relationships among wheel topology, undeformed chip thickness
Corresponding author. School of Mechanical and Electronic Engineering, Wuhan University of Technology, Wuhan, 430070, PR China. E-mail address: [email protected] (M. Ye).
https://doi.org/10.1016/j.compositesb.2018.08.120 Received 25 May 2018; Received in revised form 23 August 2018; Accepted 24 August 2018 Available online 27 August 2018 1359-8368/ © 2018 Elsevier Ltd. All rights reserved.
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laminates and investigated effects of the angle for cutting fibers and the cutting-edge radius on burr formation [22]. Their results indicate that burrs easily occur in the range (the angle for cutting fiber: 0∘ ≤ θ ≤ 90∘ ) when the tooling edge radius is relatively large for both milling and drilling of CFRP laminates. Their conclusions can be employed to improve the surface quality of edge trimming for CFRP laminates and the understanding of the mechanism of burr formation. Wei et al. investigated effects of carbon fibers on the corresponding fracture mechanisms in view of microcosmic and macroscopic perspectives by mean of the single-point flying cutting method [23]. Their results show that the cutting force fell in the range (fiber orientation: 0/180–15/ 165); on the other hand, it peaked at the fiber orientation (30/150). The fracture mechanism of carbon fibers was studied under various cutting conditions (such as the orientation angles (0°, 90°), and along and inverse to the fiber direction) and these findings can be utilized to reveal the mechanisms of material removal and surface formation. As for UVAG of C/E composites, measurement of the grinding temperature is relatively complicated and there is only a little study related to such theme; moreover, the study of the grinding temperature of UVAG under various fiber directional angles was seldom reported at home and abroad; thus, researches revealing effects of structural characteristics (content of epoxy resin and the directional angle of carbon fibers) of C/E composites on the grinding temperature as for UVAG shall be of great significance to the production practice.
nonuniformity and ground surface roughness. Their work developed an accurate model to predict the ground surface quality in textured CBN grinding [9]. As a new machining method, complicated spiral cutting occurs in the grinding region for ultrasonic vibration assisted grinding (UVAG) due to the ultrasonic vibration applied to single abrasive particles making the grinding performed more deeply in longer paths and the corresponding processing efficiency higher; moreover, their paths are interrupted or intertwined, the processing plane of workpiece is intermittently and repeatedly ground to gain good surface quality; thus, C/E composite materials are very appropriate to processed by UVAG. However, on one hand, the primary components (carbon fiber and resins) of C/E composite material have low thermal conductivities, resulting that heat is easily accumulated nearby the processing region and dynamic performances of the workpiece and the tool are severely influenced; in addition, absorption moisture of resin may lower its glass transition temperature so that its compressive strength is remarkably influenced in the hot and humid environment; on the other hand, the dry processing method generally taken for C/E composite material intensifies further the heat accumulation phenomena of the grinding region. Thus, the grinding heat is an important physical factor in UVAG of C/E composites [10–12]. Chen et al. designed a temperature measuring method (multi-point and multi-level layout of FBGs) to study temperature field distribution of CFRP workpieces and corresponding influencing factors and provide a reference basis for improvement of CFRP nondestructive monitoring technology and optimization of UVAG process parameters. What's more, the related temperature acquisition tests were set in accordance with requirements of the reference grating method and comparative analyses of the simulation and experimental results were performed based on the reference grating method to effectively solve the cross-sensitivity of embedded FBGs [13,14]. As for a typical anisotropic C/E composite, the change of fiber directional angle (the angle between the feed direction of a tool and the axial direction of carbon fiber: θ (Fig. 1)) is an important factor for grinding process, leading to the variation of cutting force, tool wear degree or grinding temperature [15]. From the macro view, the variation of fiber directional angle may especially significantly affect the grinding heat; moreover, the transmission rate of the grinding heat is also related to the directional angle of fibers [16]. The carbon fibers and epoxy resin of C/E composites are sensitive to the grinding temperature to varying degrees; thus, the corresponding ratio of two primary components may greatly affect transmission and distribution of the grinding temperature. Therefore, study of effects of the structural characteristics (epoxy resin content and directional angle of carbon fibers) on the grinding temperature shall be necessarily carried out [17–19]. V. Madhavan et al. thought a significant tool flank wear might lead to significant growth of the thrust and the cutting force [fiber directional angle: 0°–60°; and the periodic change force signals: 65°–80°] [20]. M. Henerichs et al. introduced the fundamental tool-geometry analysis based on the orthogonal cutting of one unidirectional CFRPmaterial [21]. Within an extensive experiment series processing forces, evaluation of abrasive tool-wear, workpiece damage or delamination depended on various tool geometries and fiber orientations. This study can supply basis for the optimization of tool-geometry. Wang et al. studied a mechanism for burr formation for edge trimming of CFRP
2. FBG temperature measurement principle The FBG sensor is a novel type of optical device with high measurement accuracy and tiny diameter (0.125 mm). Thus, FBGs are appropriate for embedded temperature measurement. When a broadband light source is transferred along optical fiber, an FBG is equivalent to a narrow bandwidth optical mirror in optical fiber; the light conforming to Bragg phase matching conditions is reflected, while light in other wavelengths is transmitted. The above description is the fundamental principle of FBG [24–27], whose relationship is as follows:
λB = 2·neff ·Λ
(1)
Where λB represents the central wavelength of the refractive light (FBG central wavelength); neff represents the effective refractive index of the FBG; Λ represents the period of the FBG. While an FBG is only under temperature, its period and refractive index will change resulting in the variation of λB . The change of λB can be expressed as follow based on FBG law:
ΔλB = KT ·ΔT
(2)
KT = (αS + ςS ) λB
(3)
Where ΔT represents the temperature difference; KT represents the FBG's temperature sensitivity coefficient, which is a constant and related to material properties of optical fiber; αS and ςS represent the thermal expansion and thermo-optical coefficients of optical fiber, respectively. However, the strength of bare FBG sensor is low, which makes itself easy to break and cannot be applied directly; moreover, the sensibility of bare FBG is not high enough. Therefore, bare FBGs need to be encapsulated for improvement of strength and sensibility. The variation of central wavelength of encapsulated FBG is described as follow:
ΔλB = [α (1 − Pe )·α (x ) + ςS ] λB ΔT
(4)
Where α (α > > αS ) represents the thermal expansion coefficient of the base material; Pe represents the elastic-optical coefficient of optical fiber; α (x ) represents the strain transmissibility of FBG due to the thermal expansion effect of the base material. While an FBG is only under stress, the change of λB can be expressed as follow based on FBG law: Fig. 1. Schematic diagram of the directional angle of fibers.
ΔλB = K ε ·Δε 101
(5)
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K ε = λB ·(1 − Pe )
(2). Seven types workpieces for verification of effects of fiber directional angle: Single layer thickness for all workpieces: 0.2 mm. Number of layers per workpiece: 50. Pavement ways of unidirectional laminates: 0° overlapping; 30°overlapping; 60°overlapping; 90°overlapping; 120°overlapping; 150°overlapping; 180° overlapping; (3). Five types workpieces designed to verify effects of the complexity (the number of different fiber directional angles of workpieces when other conditions remain the same) of fiber directional angles: Single layer thickness for all workpieces: 0.2 mm. Number of layers per workpiece: 50. Pavement ways of multi-directional laminates: 0°/180° overlapping; 0°/90°/180° overlapping; 0°/45°/90°/135°/180° overlapping; 0°/30°/60°/90°/120°/150°/180° overlapping; 0°/15°/30°/45°/60°/75°/90°/105°/120°/135°/150°/165°/180° overlapping.
(6)
The central wavelength of an FBG may be affected simultaneously by strain and temperature in measurement; moreover, the temperature and strain can't be distinguished by single FBG. This is the phenomenon of cross-sensitivity of FBG sensor. Taylor series expansion of Eq. (1) and the ignoring of cross and higher-order terms lead to the following equation [28,29]:
ΔλB = K ε ·Δε + KT ·ΔT
(7)
Eq. (7) reveals the fundamental for occurrence of cross-sensitivity and it indicates that ΔT and Δε affect ΔλB , respectively; moreover, two effects can be linearly superimposed. In subsequent experiments, the reference grating method is employed to solve the cross-sensitivity of FBGs. 3. Design scheme and production plan of workpieces 3.1. Design scheme The structure diagram of multi-directional laminates of C/E composite is shown in Fig. 2. Carbon fiber cloth is utilized to produce multidirectional laminates by mean of overlapping combination in accordance with directions of various fibers, and the corresponding two primary structural parameters are the epoxy resin content and the fiber directional angles. The former is reflected by the thickness of single layer carbon fiber cloth and its thickness may correspond to the production of C/E composite laminates with a specific content of epoxy resin. For studying effects of structural characteristics of C/E composites on the grinding temperature for UVAG, three groups of workpieces in various types were designed in accordance with the epoxy resin content and the fiber directional angles and by means of the control variable method, which are as follows:
3.2. Production plan The testing workpieces were made from C/E composites, whose parameters are shown in Table 1. The primary fabrication. The sizing model of a workpiece is shown in Fig. 3 (surface area of a single layer: 60 mm × 15 mm; and each workpiece: 50 layers of carbon fiber cloth; and FBG1 (FBG for measurement of the grinding temperature) embedded in the design position [the layer closest to the grinding area; horizontal position: 11.25 mm (Position: 3/4 level)] and FBG2 (FBG for measurement of vibration) embedded in the design position [the 48th layer; horizontal position: close to the edge of the workpiece to the greatest extent]. The internal strains of each workpiece due to vibration during the UVAG period were primarily measured and then the data gained by FBG1 and FBG2 were processed for elimination of any interference to temperature data of FBG1 due to vibration and solving the cross-sensitivity issue of FBG1 [30,31]. The two groups of control experiments were set as follow:
(1). Four types workpieces for verification of effects of the epoxy resin content: Pavement way: 0°/90° (θ )overlapping. Number of layers per workpiece: 50. Types of carbon fiber clothes: Single layer thickness 0.075 mm (content of resin: 45%); Single layer thickness 0.1 mm (content of resin: 40%); Single layer thickness 0.15 mm (content of resin: 35%); Single layer thickness 0.2 mm (content of resin: 30%).
1) Processing parameters same as that in experiments were set to gain the maximum temperature in grinding region by simulation (Fig. 4); afterwards, the actual processing location of the workpiece was heated to slightly above the grinding temperature for observation of the corresponding temperature change at FBG2's layout position; and it was found that temperature gained by FBG2 did not change; thus, the region for propagation of the grinding heat may not cover the embedding location of FBG2 during the UVAG period; on the other hand, data acquired by FBG2 shall be only due to vibration. 2) As for vibration applied to a workpiece (Fig. 5), the corresponding responses were observed by FBG1 and FBG2. The results show that application of the same vibration causes FBG1 and FBG2 to obtain Table 1 Properties of C/E composite materials. Material
Property
Unit
Value
Carbon fiber
Density Elastic modulus Axial tensile strength Poisson's ratio Fracture toughness Yarn specifications Density Elastic modulus Axial tensile strength Poisson's ratio Fracture toughness
g/cm3 GPa GPa / J/m2 / g/cm3 GPa GPa / J/m2
1.8 230 4.9/5 0.3 2 3000 1.2 4.5 130 0.4 500
Epoxy matrix
Fig. 2. Structure diagram of multi-directional laminates of C/E composite. 102
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Fig. 6. Arrangement of method of FBGs.
The diameter of FBGs is 0.125 mm, the thickness of single layer of carbon fiber is 0.075mm–0.2 mm (as shown in Fig. 6; thickness of Layer 2 ≥ 0.125 mm: a layer of carbon fiber cloth; thickness of Layer 2 < 0.125 mm: several layers of carbon fiber clothes; total thickness: > 0.125 mm (guarantee)). The holes (Fig. 6) are formed for successful layout of FBG. FBG1 and FBG2 were arranged in accordance with the following requirements: FBGs were embedded (Fig. 6); Layer 2 included Parts 1 and 2, between which a section was reserved for placement of FBGs; the holes for positioning FBG1 and FBG2 were fully covered with heat conduction grease and glue, respectively; afterwards, the both ends of Layers 1 and 3 were covered. Fig. 7 shows the flow chart for field preparation of workpieces.
Fig. 3. Schematic diagram of modeling a workpiece.
4. Experimental design 4.1. Experimental apparatus The experimental apparatus (Fig. 8) consists of a vertical machining center, an UVAG unit and temperature measurement FBGs. The UVAG unit is made up of an ultrasonic power supply and an ultrasonic vibrator. The ultrasonic oscillator in this paper works at the resonant frequency which is 29.7 kHz and the corresponding ultrasonic amplitude is 14 μm. The diameter of diamond grinding head is 3 mm. The temperature measurement FBGs include FBG1, FBG2 and an FBG demodulation instrument. FBGs (band: 1545–1555 nm; the encapsulation of optical fibers: electroplating copper sulfate process) were utilized in experiments. The parameters of our FBG demodulation instrument are as follows: Sampling frequency: 4 kHz Resolution: 1pm Band: 1525–1565 nm. Encapsulated FBGs were calibrated with a temperature controlled
Fig. 4. Simulation of the grinding temperature.
Fig. 5. Vibration testing apparatus procedures are glue preparation, weaving, coating glue, pre-impregnation, production (including embedding optical fibers) and drying.
Fig. 7. Flow chart for field preparation of workpieces (a: prepared carbon fiber prepregs; b: clipped single layer prepregs in various types; c: C/E composite workpieces under the vacuum drying; d: prepared and embedded optical fiber workpieces; and e: welded and calibrated temperature measurement workpieces for processing).
identical data at different positions; thus, vibration at the location of FBG1 could be described with that of FBG2 and the control experimental requirements can be met. 103
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investigate effects of the complexity of fiber directional angles on grinding temperature in UVAG for multi-directional laminates. Five types workpieces (as shown in 3.1 (3)) were machined under different processing parameters (Table 2), corresponding to experiment1/2/3/4/ 5. Experiments of group c. This group of experiments was designed to gain effects of epoxy resin content on grinding temperature in UVAG for multi-directional laminates. Four types workpieces with diverse contents of resin (as shown in 3.1 (1)) were processed under different processing parameters (Table 2) [32–35]. Furthermore, in order to gain effects of fiber directional angle on the ground surface quality of workpieces, four types unidirectional laminates whose fiber directional angle is 0°/30°/90°/150° respectively were processed at spindle speed 1500 rpm, grinding depth 1.4 mm and feed rate 100 mm/min and a scanning electron microscope was employed to observe the microscopic morphology of ground surfaces. 5. Analysis of experimental data Fig. 8. Grinding experiment field (a represents ultrasonic power supply; b represents FBG demodulator; c represents ultrasonic vibrator).
The material removal mechanism of CFRP is mainly divided into three types: the lamination fracture mechanism when the feed direction of the cutting tool is the same as the axial direction of the carbon fiber, the extrusion shear fracture mechanism when the feed direction of the cutting tool is in an acute angle with the axial direction of the carbon fiber, and the bending shear fracture mechanism with a blunt angle between the feed direction of the cutting tool and the axial direction of the carbon fiber. Therefore, different fiber directional angles will result in different material removal mechanisms in grinding process. From a macro point of view, the change of fiber directional angle has a great impact on the grinding temperature. At the same time, the propagation rate of grinding temperature is also related to the fiber directional angle. Therefore, the distribution of grinding temperature field in the workpiece after grinding will change with the change of the fiber directional angle. The sensitivity of carbon fiber and epoxy resin to grinding temperature is different in C/E composites, so the proportion of the two components has a great influence on the propagation and distribution of grinding temperature. Therefore, it is necessary to study the influence of fiber directional angle and resin content of C/E composites on grinding temperature.
Fig. 9. Thermostatic water bath calibration experiment.
tank (Fig. 9) to gain the relationship between central wavelength drift and temperature of each FBG. Then calibrated FBGs were embedded in various types C/E composite workpieces for real-time measurement of grinding temperature.
1) Fig. 10 shows the experimental results of group a and the measured directional angles of fibers are 0°, 30°, 60°, 90°, 120°, 150° and 180°, respectively. As for UVAG of C/E composite workpiece, the grinding temperature is greatly influenced by the directional angle of fibers, which is concretely embodied as follows: while θ < 90∘, the grinding temperature rises along the growth of the directional angle (θ ) of fibers; while θ = 90∘, the grinding temperature reaches maximum; and while θ > 90∘, the grinding temperature falls along the growth of the directional angle (θ ) of fibers. Moreover, the grinding temperature corresponding to the direction of running fibers (θ < 90∘) is significantly higher than that corresponding to the direction of inverse fibers (θ > 90∘). Thus, a higher grinding temperature may easily occur in the direction of running fibers while other processing conditions remain the same.
4.2. Experimental method To reveal effects of structural characteristics (the fiber directional angle and the epoxy resin content) of C/E composites on the grinding temperature, processing experiments were primarily divided into three groups as follow (the grain size of diamond electroplating tool: 120#): Experiments of group a. This group of experiments was design to obtain effects of fiber directional angle on grinding temperature in UVAG for unidirectional laminates. Seven types workpieces with different fiber directional angles (as shown in 3.1 (2)) were machined under various processing parameters (Table 2). Experiments of group b. This group of experiments was designed to
Fig. 10(a), Fig. (b) and Fig. (c) also show the effects of the grinding depth, spindle speed and feed rate on the grinding temperature. While the directional angle (θ ) of fibers remains the same, the grinding temperature will rise along with the growth of the grinding depth, spindle speed or feed rate; at the same time, the grinding temperature is affected by the grinding depth to the greatest extent; on the other hand, it is found that the change of grinding parameters (grinding depth, spindle speed and feed rate) has almost little effect on the relationship (the grinding temperature vs. the directional angle of fibers) for unidirectional laminates. When the directional angle of carbon fibers is 90°, absolute values of
Table 2 Experimental conditions for UVAG. Experiment
Spindle speed (r/min)
Grinding depth (mm)
Feed rate (mm/ min)
1
500, 1000, 1500, 2000, 2500 1500 1500
1.4
180
1, 1.2, 1.4, 1.6, 1.8 1.4
180 100, 140, 180, 220, 260
2 3
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Fig. 11. The grinding temperature vs. the complexity of fiber directional angles for multi-directional laminates.
of the angle between the direction of grinding speed and the axial direction of carbon fibers. This will lead to lower grinding force and grinding heat. In addition, the heat spreads more easily to other locations of the workpiece than the measurement plane because of the fiber directional angle. These factors may contribute to lower grinding temperature. Furthermore, the variation of processing parameters only influences the grinding heat generated during UVAG, but not change the material removal mechanism and the thermal conductivity of workpieces. Thus, the tendency between grinding temperature and fiber directional angles did not alter under different machining parameters.
Fig. 10. The grinding temperature vs. the fiber directional angle for unidirectional laminates.
2) Fig. 11 presents the experimental results of group b. These results indicate that the grinding temperature measured during UVAG first rises and then falls consistently; moreover, the measured grinding temperature corresponding to experiment 2 peaked and that corresponding to experiment1 was minimized; but there's little variation in the overall temperatures. The above phenomena demonstrate that effects of the complexity of fiber directional angles of C/E composite workpiece on the grinding temperature depend on the ratio of carbon fiber layers with fiber directional angle nearby 90°; and the
the angle between the direction of grinding speed and the axial direction of carbon fibers are relatively small in most cutting positions. This will cause most carbon fibers to fracture due to extrusion and stretching, which in turn increases the grinding force and grinding heat. What's more, the heat will spread more easily to the measurement plane (plane perpendicular to the feed direction) because of the better conductivity of carbon fibers. These factors eventually result in higher grinding temperature. In other situations, the carbon fibers are more likely to break owning to shearing resulted from larger absolute values 105
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grinding temperature rises with the increase of this ratio (Fig. 11). Fig. 11(a), Fig. (b) and Fig. (c) also show effects of the grinding depth, spindle speed and feed rate on the grinding temperature; for each experiment, the grinding temperature rises along with the grinding depth, spindle speed or feed rate; moreover, the grinding depth has the greatest influence on grinding temperature; in addition, it is found that any change of grinding parameters (grinding depth, spindle speed or feed rate) almost does not influence the relationship (the grinding temperature vs. the directional angles of fibers) for multidirectional laminates. There are two main factors affecting grinding temperature as follow: grinding heat generated during UVAG and the thermal conductivity of the workpieces. The former factor will be related to the ratio of carbon fiber layers with fiber directional angle close to 90° in the workpiece and the higher this radio, the more grinding heat will be generated during the process. The later factor will be determined by the number of the different fiber directional angles in the workpiece and the greater this number, the better the thermal conductivity of workpieces will be. In addition, the better thermal conductivity results in lower grinding temperature when the grinding heat remains the same. In different situations, the dominated factor determining the final grinding temperature is different. In all, from experiment 1 to experiment 5, the thermal conductivity of workpieces is getting better because of the more complicated structure and the grinding heat generated in experiment 1 is higher than that in experiment 2 due to the workpieces' structure. For experiment 1 and experiment 2, the dominated factor is grinding heat, while for experiment 2/3/4/5, the leading factor is the thermal conductivity of workpieces. The dominated factor remains the same when the processing parameters change. This is the reason that the grinding temperature peaks at expreiment2 and the changing process parameters have little influence on the trend shown in Fig. 11. 3) Fig. 12 shows the experimental results of group c. As for UVAG of C/ E composite workpieces, the grinding temperature rises along with the growth of epoxy resin content of workpieces while other processing conditions are the same, resulted from that the thermal conductivity of epoxy resin is far less than that of carbon fibers [1]; moreover, the grinding temperature increases more and more rapidly. Fig. 12(a), Fig. (b) and Fig. (c) also indicate the effects of the grinding depth, spindle speed and feed rate on the grinding temperature. While the epoxy resin content remains the same, the grinding temperature increases along with the growth of the grinding depth, spindle speed or feed rate; moreover, the grinding temperature is influenced by the grinding depth to the greatest extent; in addition, it is found that any change of grinding parameters (grinding depth, spindle speed and feed rate) has almost little effect to the relationship (the grinding temperature vs. the content of resin). 4) Fig. 13 presents the macroscopic and microscopic appearance of ground surfaces. In order to observe the microstructure more clearly, the magnification of the first three pictures is 300 times, and the last three pictures is 1000 times. Fig. 13(a) and (b) show that with the increase of fiber directional angle, the macroscopic surface of workpieces is substantially degraded and the number of burrs near the edge of workpieces raises greatly, respectively. Fig. 13(c) shows that with the increase of fiber directional angle, the microcosmic surface of workpieces is significantly deteriorates. The proportion of intact fiber bundles is reduced and the number of carbon fibers that have fallen off from the matrix resin increases. What's more, the degree of destruction of matrix resin gradually deepens, and
Fig. 12. The grinding temperature vs. the content of resin under various grinding parameters.
obvious surface defects such as pits occur. 6. Conclusions Various workpieces were first designed and prepared in accordance 106
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processing experiments under different parameters and real-time measurement of the grinding temperature of C/E composite workpieces was performed to gain the effects of their structural characters on the grinding temperature during UVAG process. (1) As for UVAG of C/E composite workpieces, the grinding temperature is greatly affected by the fiber directional angle, resulted from its influences on grinding heat and the heat distribution of workpieces. While θ = 90∘, most carbon fibers fracture due to extrusion and stretching which cause higher grinding force and grinding heat, moreover, the heat spreads more easily to the measurement plane; while θ < 90∘ or θ > 90∘, the carbon fibers are more likely to break owning to shearing which leads to lower grinding force and grinding heat, moreover, the heat spreads more easily to other locations of the workpieces than the measurement plane. Those factors result in the phenomena as follow: while θ < 90∘, the grinding temperature rises along the growth of the fiber directional angle (θ ); while θ = 90∘, the grinding temperature reaches maximum; while θ > 90∘, the grinding temperature falls along the growth of the fiber directional angle (θ ). (2) Effects of the complexity of the fiber directional angle of C/E composite workpieces on the grinding temperature, which is eventually determined by grinding heat and the thermal conductivity of workpieces, can be divided into two aspects: on the one hand, the ratio of carbon fiber layers with fiber directional angle nearby 90° affects the grinding heat and the larger this ratio, the more heat generated; on the other hand, the complexity of fiber directional angle influences the thermal conductivity of workpieces when other conditions remain the same and the larger the number of different fiber directional angles of workpiece, the better the thermal conductivity. Furthermore, in different situations, the dominant aspect will be different. (3) As for component of C/E composite workpieces, the grinding temperature is influenced by the epoxy resin content, mainly resulted from the poor thermal conductivity of resin. The measured results indicate that the grinding temperature rises with the growth of epoxy resin content while the processing conditions remain the same on the premise that performance requirements of workpieces are satisfied. (4) The change of grinding parameters (grinding depth, spindle speed and feed rate) greatly influences the grinding temperature; and the grinding depth shall be dominant; on the other hand, the variations of the grinding parameters bring about little effect on material removal mechanism and the thermal conductivity of workpieces and therefore the relationship (the grinding temperature vs. its structural characteristics (the directional angle of fibers and the content of resin))almost remains the same. The C/E composite materials are typical representative of fiber reinforced composites therefore the conclusions above can provide guidance for grinding temperature studies of other fiber reinforced composites whose structures and properties are like those workpieces employed in this paper. Acknowledgments The authors would like to thank the Science and Technology Department of Hubei Province (Grant Number: 2015BAA022) for funding this study. References
Fig. 13. The effects of the fiber directional angles on the grinding quality.
[1] Benedetti A, Fernandes P, Granja JL, Sena-Cruz J, Azenha M. Influence of temperature on the curing of an epoxy adhesive and its influence on bond behavior of NSM-CFRP systems. Compos B Eng 2016;89(3):219–29. [2] Cai JG, Lin ZY. Research on temperature measurement of high speed drilling tool
with structural characteristics (the directional angle of fibers and the epoxy resin content) of C/E composite; and then FBGs were embedded in the setting locations during the preparation of workpieces; finally, 107
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[3]
[4] [5]
[6] [7]
[8]
[9]
[10]
[11]
[12]
[13]
[14] [15] [16]
[17]
[18] Ma FJ, Jiang YC, Zhang SF, Liu Y, Sha ZH, Su C. Influence analysis of fiber angle on cutting performance for carbon fiber reinforced composite. J Dalian Jiaot Univ 2016;37(6):37–42. [19] Jiang TY, Li WB. Analysis of influence of angle of 2 Abrasive grain on grinding. Tool Technology 2017;51(3):72–5. [20] Madhavan V, Lipczynski G, Lane B, Whitenton E. Fiber orientation angle effects in machining of unidirectional CFRP laminated composites. J Manuf Process 2015;20:431–42. [21] Henerichs M, Vob R, Kuster F, Wegener K. Machining of carbon fiber reinforced plastics: influence of tool geometry and fiber orientation on the machining forces. CIRP Journal of Manufacturing Science and Technology 2015;9:136–45. [22] Wang FJ, Yin JW, Ma JW, Jia ZY, Yang F, Niu B. Effects of cutting edge radius and fiber cutting angle on the cutting-induced surface damage in machining of unidirectional CFRP composite laminates. Int J Adv Manuf Technol 2017;91:3107–20. [23] Wei YY, An QL, Cai XJ, Chen M, Ming WW. Influence of fiber orientation on singlepoint cutting fracture behavior of carbon-fiber/epoxy prepreg sheets. Materials 2015;8:6738–51. [24] Zhu DD, Li WX, Li ZQ, Wang JJ. A distributed fiber Bragg grating system for simultaneous measurement of the strain and temperature. Acta Metrol Sin 2008;29(1):29–32. [25] Lee CL, Lee RK, Kao YM. Design of multichannel DWDM fiber Bragg grating filters by Lagrange multiplier constrained optimization[J]. Optic Express 2006;14(23):11002–11. [26] Guan B, Liu Z, Kai G, Ge C, Dong X. Temperature sensor based on fiber Bragg grating[J]. Journal of Transcluction Technology 1999;15(2):90–3. [27] Zhan YG, Cai HW, Xiang SQ, Qu GH, Wang XZ. Study on high resolution fiber Bragg grating temperature sensor[J]. Chin J Laser B 2005;32(1):83–6. [28] Zhang B, Yan GS, Deng YJ. Cross-sensitivity of fiber grating sensor measurement[J]. Journal of Applied Optics 2007;28(5):614–8. [29] Li TL, Tan YG, Wei L, Zhou ZD, Zheng K, Guo YX. A non-contact fiber Bragg grating vibration sensor[J]. Rev Sci Instrum 2014;85(1):81–4. [30] Zhang XW, Nin TG. Study of cross-sensitivity in fiber grating sensors. Optical Fiber & Electric Cable 2007;2:1–4. [31] Chen LJ. Solution to cross-sensitivity of fiber grating sensors. Digital Communication 2012;39(6):15–7. [32] Lu D, Cai LG, Cheng Q, Li ZK. Finite element simulation of ultrasonic vibration assisted turning of carbon fiber reinforced polymer composite. J Vib Shock 2015;34(14):110–5. [33] Wang KJ, Liu X, Li H, Wang LY. Experimental study on ultrasonic vibration aided micro grinding temperature. Sci Technol Eng 2016;16(29):54–8. [34] Kang RK, Ma FJ, Dong ZG. Ultrasonic assisted machining of difficult-to-cut material. Aeronautical Manufacturing Technology 2012;412(16):44–9. [35] Chen T, Ye ML, Liu SL. Measurement of ultrasonic assisted grinding temperature based on fiber Bragg grating (FBG) sensor. Int J Adv Manuf Technol 2017;1:1–10.
made of C/E composite material. Machine Building & Automation 2014;44(2):201–14. Wen Q, Guo DM, Gao H, Zhao D. Comprehensive evaluation method for carbon/ epoxy composite hole-making damages. Acta Mater Compos Sin 2016;33(2):265–72. Liu CJ, Ding WF, Yu TY, Y CY. Materials removal mechanism in high-speed grinding of particulate reinforced titanium matrix composites[J]. Precis Eng 2018;51:68–77. Yang M, Li CH, Zhang YB, Jia DZ, Zhang XP, Hou YL, Li RZ, Wang J. Maximum undeformed equivalent chip thickness for ductile-brittle transition of zirconia ceramics under different lubrication conditions[J]. Int J Mach Tool Manufact 2017;122:55–65. Zhao B, Yu TY, Ding WF, Li XY. Effects of pore structure and distribution on strength of porous Cu-Sn-Ti alumina composites[J]. Chin J Aeronaut 2017;30(6):2004–15. Dai CW, Ding WF, Zhu YJ, Xu JH, Yu HW. Grinding temperature and power consumption in high speed grinding of Inconel 718 nickel-based superalloy with a vitrified CBN wheel[J]. Precis Eng 2018;52:192–200. Zhang YB, Li CH, Ji HJ, Yang XH, Yang M, Jia DZ, Zhang XP, Li RZ, Wang J. Analysis of grinding mechanics and improved predictive force model based on material-removal and plastic-stacking mechanisms[J]. Int J Mach Tool Manufact 2017;122:81–97. Ding WF, Dai CW, Yu TY, Xu JH, Fu YC. Grinding performance of textured monolayer CBN wheels: undeformed chip thickness nonuniformity modeling and ground surface topography prediction[J]. Int J Mach Tool Manufact 2017;122:66–80. Tam LH, Zhou A, Yu Z, Qiu QW, Lau D. Understanding the effect of temperature on the interfacial behavior of CFRP-wood composite via molecular dynamics simulations. Compos B Eng 2016;109:227–37. Liu SL, Chen T, Wei YX, Wu CQ. Study on surface quality of CFRP after rotary ultrasonic face grinding. Advanced Processing Technology of Composites 2016;510(15):57–61. Hu AD, Chen Y, Fu YC, Xu JH, Liu SQ. Effects of ultrasonic Vibration-assisted grinding on cutting force and surface quality of CFRP. Acta Mater Compos Sin 2016;33(4):788–96. Chen T, Ye ML, Liu SL, Deng Y. Experimental study on cross-sensitivity of temperature and vibration of embedded fiber Bragg grating (FBG) sensors[J]. Optoelectron Lett 2018;14(2):92–7. Chen T, Ye ML, Deng Y, Liu SL. Study of temperature field for UVAG of CFRP based on FBG[J]. Int J Adv Manuf Technol 2018;96:765–73. Liu HL, Zhang JB, Wang Z, Zhang JP, Li G. Cutting tool selection in CFRP and AFRP machining. Aerospace Materials and Technology 2013;43(4):95–8. Ye XZ, Wang DZ, Liu DJ, Yang XP, Yu DM, Jing JW. Experimental study on tool wear of milling carbon fiber composites. Mechanical and Electrical Technology 2014;3:75–7. Wen L, Jiang LP, Zhang HZ, Cai XJ, An QL, Chen M. Test on thermal characteristics when orthogonally free machining carbon fiber reinforced plastics unidirectional laminates. Acta Mater Compos Sin 2015;32(5):1469–79.
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Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Experimental study on effects of structural characteristics of C/E composite laminates on grinding temperature
T
Tao Chena,b, Hongbo Lia,b, Mengli Yea,b,∗, Chao Xianga,b, Shenlin Tiana,b a b
School of Mechanical and Electronic Engineering, Wuhan University of Technology, Wuhan, 430070, PR China Hubei Digital Manufacturing Key Laboratory, Wuhan University of Technology, Wuhan, 430070, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: C/E composites UVAG Grinding temperature Structural characteristics Fiber Bragg grating (FBG) sensors
As for ultrasonic vibration assisted grinding (UVAG) of carbon/epoxy (C/E) composites, the grinding temperature is greatly influenced by their structural characteristics (such as epoxy resin content and fiber directional angle (θ )). For revealing their relationship, C/E composite workpieces with various structural characteristics were designed and prepared; moreover, Fiber Bragg grating (FBG) sensors were embedded in each workpiece for measurement of the grinding temperature; in addition, various grinding parameters were set in UVAG experiments. Experimental results show that: 1). the grinding temperature is influenced very remarkably by the fiber directional angle of C/E composites and the grinding temperature along the direction of fibers is much higher than that reverse fiber direction; 2). the grinding temperature peaks under fiber directional angle (θ = 90°); 3). on the premise of meeting the performance of workpieces, the grinding temperature raises with the increase of epoxy resin content; 4). the change of grinding parameters has nothing to do with the relationship (the grinding temperature vs. the structural characteristics of C/E composites).
1. Introduction C/E composites have many excellent properties (such as significant anisotropy, high specific strength and rigidity, corrosion and high temperature resistance and light weight) so that they are popular in various applications. However, C/E composites are typically difficult to process due to these factors: 1) aeolotropy and non-uniformity; and 2) machinability of components. Their physical and mechanical properties include significant aeolotropy; for example, the interlayer bonding strength of laminates is only 5–20% of the tensile strength along the direction of fibers and lamination may easily occur. The C/E composite consists of carbon fibers and epoxy resin; the former is very hard and highly strong reinforced phase, which is difficult to process; on the other hand, the latter is extremely thermo-sensitive base material easily breakdown under effects of the grinding temperature, resulting in the loss of its support and protection for carbon fibers [1–3]. In recent years, numerous researches about the machining process, strength and ground surface quality of hard brittle materials and several composites were reported. Liu et al. found the four stages of material removal and the characteristics of grinding force during processing; moreover, the undeformed chip thickness has important impacts on the forming of surface defects. This study reveals the materials removal mechanism in high-speed grinding of specific composites [4]. Yang ∗
et al. investigated the critical maximum undeformed equivalent chip thickness for ductile-brittle transition (DBhmax − e ) of zirconia ceramics under different lubrication conditions theoretically and experimentally. They proposed a DBhmax − e model and carried out verifying experiments. Findings show that the model can predict relationships between DBhmax − e and lubricating conditions [5]. Zhao et al. found that a larger pore size and a higher pore concentration will reduce the strength of porous composites, while the uniformity of the pore distribution will benefit the composites strength. Their study provides a guidance for the application of porous Cu-Sn-Ti alumina composites as a bonding material for grinding wheels [6]. Dai et al. carried out grinding experiments of Inconel 718 and concluded that the minimum grinding force and specific grinding energy can be obtained at wheel speed of 120 m/s and the lost grinding power reaches the maximum value at 140 m/s under the given experimental conditions. Their conclusions can provide the basis for process optimization of hard brittle material [7]. Zhang et al. developed an improved grinding force model which considers material-removal, plastic-stacking mechanisms and the influence of lubricating conditions. The experimental results indicate that the model has a better accuracy than previous ones. In addition, the model is suitable for different lubricating conditions [8]. Ding et al. performed grinding experiments and established several models to investigate the relationships among wheel topology, undeformed chip thickness
Corresponding author. School of Mechanical and Electronic Engineering, Wuhan University of Technology, Wuhan, 430070, PR China. E-mail address: [email protected] (M. Ye).
https://doi.org/10.1016/j.compositesb.2018.08.120 Received 25 May 2018; Received in revised form 23 August 2018; Accepted 24 August 2018 Available online 27 August 2018 1359-8368/ © 2018 Elsevier Ltd. All rights reserved.
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laminates and investigated effects of the angle for cutting fibers and the cutting-edge radius on burr formation [22]. Their results indicate that burrs easily occur in the range (the angle for cutting fiber: 0∘ ≤ θ ≤ 90∘ ) when the tooling edge radius is relatively large for both milling and drilling of CFRP laminates. Their conclusions can be employed to improve the surface quality of edge trimming for CFRP laminates and the understanding of the mechanism of burr formation. Wei et al. investigated effects of carbon fibers on the corresponding fracture mechanisms in view of microcosmic and macroscopic perspectives by mean of the single-point flying cutting method [23]. Their results show that the cutting force fell in the range (fiber orientation: 0/180–15/ 165); on the other hand, it peaked at the fiber orientation (30/150). The fracture mechanism of carbon fibers was studied under various cutting conditions (such as the orientation angles (0°, 90°), and along and inverse to the fiber direction) and these findings can be utilized to reveal the mechanisms of material removal and surface formation. As for UVAG of C/E composites, measurement of the grinding temperature is relatively complicated and there is only a little study related to such theme; moreover, the study of the grinding temperature of UVAG under various fiber directional angles was seldom reported at home and abroad; thus, researches revealing effects of structural characteristics (content of epoxy resin and the directional angle of carbon fibers) of C/E composites on the grinding temperature as for UVAG shall be of great significance to the production practice.
nonuniformity and ground surface roughness. Their work developed an accurate model to predict the ground surface quality in textured CBN grinding [9]. As a new machining method, complicated spiral cutting occurs in the grinding region for ultrasonic vibration assisted grinding (UVAG) due to the ultrasonic vibration applied to single abrasive particles making the grinding performed more deeply in longer paths and the corresponding processing efficiency higher; moreover, their paths are interrupted or intertwined, the processing plane of workpiece is intermittently and repeatedly ground to gain good surface quality; thus, C/E composite materials are very appropriate to processed by UVAG. However, on one hand, the primary components (carbon fiber and resins) of C/E composite material have low thermal conductivities, resulting that heat is easily accumulated nearby the processing region and dynamic performances of the workpiece and the tool are severely influenced; in addition, absorption moisture of resin may lower its glass transition temperature so that its compressive strength is remarkably influenced in the hot and humid environment; on the other hand, the dry processing method generally taken for C/E composite material intensifies further the heat accumulation phenomena of the grinding region. Thus, the grinding heat is an important physical factor in UVAG of C/E composites [10–12]. Chen et al. designed a temperature measuring method (multi-point and multi-level layout of FBGs) to study temperature field distribution of CFRP workpieces and corresponding influencing factors and provide a reference basis for improvement of CFRP nondestructive monitoring technology and optimization of UVAG process parameters. What's more, the related temperature acquisition tests were set in accordance with requirements of the reference grating method and comparative analyses of the simulation and experimental results were performed based on the reference grating method to effectively solve the cross-sensitivity of embedded FBGs [13,14]. As for a typical anisotropic C/E composite, the change of fiber directional angle (the angle between the feed direction of a tool and the axial direction of carbon fiber: θ (Fig. 1)) is an important factor for grinding process, leading to the variation of cutting force, tool wear degree or grinding temperature [15]. From the macro view, the variation of fiber directional angle may especially significantly affect the grinding heat; moreover, the transmission rate of the grinding heat is also related to the directional angle of fibers [16]. The carbon fibers and epoxy resin of C/E composites are sensitive to the grinding temperature to varying degrees; thus, the corresponding ratio of two primary components may greatly affect transmission and distribution of the grinding temperature. Therefore, study of effects of the structural characteristics (epoxy resin content and directional angle of carbon fibers) on the grinding temperature shall be necessarily carried out [17–19]. V. Madhavan et al. thought a significant tool flank wear might lead to significant growth of the thrust and the cutting force [fiber directional angle: 0°–60°; and the periodic change force signals: 65°–80°] [20]. M. Henerichs et al. introduced the fundamental tool-geometry analysis based on the orthogonal cutting of one unidirectional CFRPmaterial [21]. Within an extensive experiment series processing forces, evaluation of abrasive tool-wear, workpiece damage or delamination depended on various tool geometries and fiber orientations. This study can supply basis for the optimization of tool-geometry. Wang et al. studied a mechanism for burr formation for edge trimming of CFRP
2. FBG temperature measurement principle The FBG sensor is a novel type of optical device with high measurement accuracy and tiny diameter (0.125 mm). Thus, FBGs are appropriate for embedded temperature measurement. When a broadband light source is transferred along optical fiber, an FBG is equivalent to a narrow bandwidth optical mirror in optical fiber; the light conforming to Bragg phase matching conditions is reflected, while light in other wavelengths is transmitted. The above description is the fundamental principle of FBG [24–27], whose relationship is as follows:
λB = 2·neff ·Λ
(1)
Where λB represents the central wavelength of the refractive light (FBG central wavelength); neff represents the effective refractive index of the FBG; Λ represents the period of the FBG. While an FBG is only under temperature, its period and refractive index will change resulting in the variation of λB . The change of λB can be expressed as follow based on FBG law:
ΔλB = KT ·ΔT
(2)
KT = (αS + ςS ) λB
(3)
Where ΔT represents the temperature difference; KT represents the FBG's temperature sensitivity coefficient, which is a constant and related to material properties of optical fiber; αS and ςS represent the thermal expansion and thermo-optical coefficients of optical fiber, respectively. However, the strength of bare FBG sensor is low, which makes itself easy to break and cannot be applied directly; moreover, the sensibility of bare FBG is not high enough. Therefore, bare FBGs need to be encapsulated for improvement of strength and sensibility. The variation of central wavelength of encapsulated FBG is described as follow:
ΔλB = [α (1 − Pe )·α (x ) + ςS ] λB ΔT
(4)
Where α (α > > αS ) represents the thermal expansion coefficient of the base material; Pe represents the elastic-optical coefficient of optical fiber; α (x ) represents the strain transmissibility of FBG due to the thermal expansion effect of the base material. While an FBG is only under stress, the change of λB can be expressed as follow based on FBG law: Fig. 1. Schematic diagram of the directional angle of fibers.
ΔλB = K ε ·Δε 101
(5)
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K ε = λB ·(1 − Pe )
(2). Seven types workpieces for verification of effects of fiber directional angle: Single layer thickness for all workpieces: 0.2 mm. Number of layers per workpiece: 50. Pavement ways of unidirectional laminates: 0° overlapping; 30°overlapping; 60°overlapping; 90°overlapping; 120°overlapping; 150°overlapping; 180° overlapping; (3). Five types workpieces designed to verify effects of the complexity (the number of different fiber directional angles of workpieces when other conditions remain the same) of fiber directional angles: Single layer thickness for all workpieces: 0.2 mm. Number of layers per workpiece: 50. Pavement ways of multi-directional laminates: 0°/180° overlapping; 0°/90°/180° overlapping; 0°/45°/90°/135°/180° overlapping; 0°/30°/60°/90°/120°/150°/180° overlapping; 0°/15°/30°/45°/60°/75°/90°/105°/120°/135°/150°/165°/180° overlapping.
(6)
The central wavelength of an FBG may be affected simultaneously by strain and temperature in measurement; moreover, the temperature and strain can't be distinguished by single FBG. This is the phenomenon of cross-sensitivity of FBG sensor. Taylor series expansion of Eq. (1) and the ignoring of cross and higher-order terms lead to the following equation [28,29]:
ΔλB = K ε ·Δε + KT ·ΔT
(7)
Eq. (7) reveals the fundamental for occurrence of cross-sensitivity and it indicates that ΔT and Δε affect ΔλB , respectively; moreover, two effects can be linearly superimposed. In subsequent experiments, the reference grating method is employed to solve the cross-sensitivity of FBGs. 3. Design scheme and production plan of workpieces 3.1. Design scheme The structure diagram of multi-directional laminates of C/E composite is shown in Fig. 2. Carbon fiber cloth is utilized to produce multidirectional laminates by mean of overlapping combination in accordance with directions of various fibers, and the corresponding two primary structural parameters are the epoxy resin content and the fiber directional angles. The former is reflected by the thickness of single layer carbon fiber cloth and its thickness may correspond to the production of C/E composite laminates with a specific content of epoxy resin. For studying effects of structural characteristics of C/E composites on the grinding temperature for UVAG, three groups of workpieces in various types were designed in accordance with the epoxy resin content and the fiber directional angles and by means of the control variable method, which are as follows:
3.2. Production plan The testing workpieces were made from C/E composites, whose parameters are shown in Table 1. The primary fabrication. The sizing model of a workpiece is shown in Fig. 3 (surface area of a single layer: 60 mm × 15 mm; and each workpiece: 50 layers of carbon fiber cloth; and FBG1 (FBG for measurement of the grinding temperature) embedded in the design position [the layer closest to the grinding area; horizontal position: 11.25 mm (Position: 3/4 level)] and FBG2 (FBG for measurement of vibration) embedded in the design position [the 48th layer; horizontal position: close to the edge of the workpiece to the greatest extent]. The internal strains of each workpiece due to vibration during the UVAG period were primarily measured and then the data gained by FBG1 and FBG2 were processed for elimination of any interference to temperature data of FBG1 due to vibration and solving the cross-sensitivity issue of FBG1 [30,31]. The two groups of control experiments were set as follow:
(1). Four types workpieces for verification of effects of the epoxy resin content: Pavement way: 0°/90° (θ )overlapping. Number of layers per workpiece: 50. Types of carbon fiber clothes: Single layer thickness 0.075 mm (content of resin: 45%); Single layer thickness 0.1 mm (content of resin: 40%); Single layer thickness 0.15 mm (content of resin: 35%); Single layer thickness 0.2 mm (content of resin: 30%).
1) Processing parameters same as that in experiments were set to gain the maximum temperature in grinding region by simulation (Fig. 4); afterwards, the actual processing location of the workpiece was heated to slightly above the grinding temperature for observation of the corresponding temperature change at FBG2's layout position; and it was found that temperature gained by FBG2 did not change; thus, the region for propagation of the grinding heat may not cover the embedding location of FBG2 during the UVAG period; on the other hand, data acquired by FBG2 shall be only due to vibration. 2) As for vibration applied to a workpiece (Fig. 5), the corresponding responses were observed by FBG1 and FBG2. The results show that application of the same vibration causes FBG1 and FBG2 to obtain Table 1 Properties of C/E composite materials. Material
Property
Unit
Value
Carbon fiber
Density Elastic modulus Axial tensile strength Poisson's ratio Fracture toughness Yarn specifications Density Elastic modulus Axial tensile strength Poisson's ratio Fracture toughness
g/cm3 GPa GPa / J/m2 / g/cm3 GPa GPa / J/m2
1.8 230 4.9/5 0.3 2 3000 1.2 4.5 130 0.4 500
Epoxy matrix
Fig. 2. Structure diagram of multi-directional laminates of C/E composite. 102
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Fig. 6. Arrangement of method of FBGs.
The diameter of FBGs is 0.125 mm, the thickness of single layer of carbon fiber is 0.075mm–0.2 mm (as shown in Fig. 6; thickness of Layer 2 ≥ 0.125 mm: a layer of carbon fiber cloth; thickness of Layer 2 < 0.125 mm: several layers of carbon fiber clothes; total thickness: > 0.125 mm (guarantee)). The holes (Fig. 6) are formed for successful layout of FBG. FBG1 and FBG2 were arranged in accordance with the following requirements: FBGs were embedded (Fig. 6); Layer 2 included Parts 1 and 2, between which a section was reserved for placement of FBGs; the holes for positioning FBG1 and FBG2 were fully covered with heat conduction grease and glue, respectively; afterwards, the both ends of Layers 1 and 3 were covered. Fig. 7 shows the flow chart for field preparation of workpieces.
Fig. 3. Schematic diagram of modeling a workpiece.
4. Experimental design 4.1. Experimental apparatus The experimental apparatus (Fig. 8) consists of a vertical machining center, an UVAG unit and temperature measurement FBGs. The UVAG unit is made up of an ultrasonic power supply and an ultrasonic vibrator. The ultrasonic oscillator in this paper works at the resonant frequency which is 29.7 kHz and the corresponding ultrasonic amplitude is 14 μm. The diameter of diamond grinding head is 3 mm. The temperature measurement FBGs include FBG1, FBG2 and an FBG demodulation instrument. FBGs (band: 1545–1555 nm; the encapsulation of optical fibers: electroplating copper sulfate process) were utilized in experiments. The parameters of our FBG demodulation instrument are as follows: Sampling frequency: 4 kHz Resolution: 1pm Band: 1525–1565 nm. Encapsulated FBGs were calibrated with a temperature controlled
Fig. 4. Simulation of the grinding temperature.
Fig. 5. Vibration testing apparatus procedures are glue preparation, weaving, coating glue, pre-impregnation, production (including embedding optical fibers) and drying.
Fig. 7. Flow chart for field preparation of workpieces (a: prepared carbon fiber prepregs; b: clipped single layer prepregs in various types; c: C/E composite workpieces under the vacuum drying; d: prepared and embedded optical fiber workpieces; and e: welded and calibrated temperature measurement workpieces for processing).
identical data at different positions; thus, vibration at the location of FBG1 could be described with that of FBG2 and the control experimental requirements can be met. 103
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investigate effects of the complexity of fiber directional angles on grinding temperature in UVAG for multi-directional laminates. Five types workpieces (as shown in 3.1 (3)) were machined under different processing parameters (Table 2), corresponding to experiment1/2/3/4/ 5. Experiments of group c. This group of experiments was designed to gain effects of epoxy resin content on grinding temperature in UVAG for multi-directional laminates. Four types workpieces with diverse contents of resin (as shown in 3.1 (1)) were processed under different processing parameters (Table 2) [32–35]. Furthermore, in order to gain effects of fiber directional angle on the ground surface quality of workpieces, four types unidirectional laminates whose fiber directional angle is 0°/30°/90°/150° respectively were processed at spindle speed 1500 rpm, grinding depth 1.4 mm and feed rate 100 mm/min and a scanning electron microscope was employed to observe the microscopic morphology of ground surfaces. 5. Analysis of experimental data Fig. 8. Grinding experiment field (a represents ultrasonic power supply; b represents FBG demodulator; c represents ultrasonic vibrator).
The material removal mechanism of CFRP is mainly divided into three types: the lamination fracture mechanism when the feed direction of the cutting tool is the same as the axial direction of the carbon fiber, the extrusion shear fracture mechanism when the feed direction of the cutting tool is in an acute angle with the axial direction of the carbon fiber, and the bending shear fracture mechanism with a blunt angle between the feed direction of the cutting tool and the axial direction of the carbon fiber. Therefore, different fiber directional angles will result in different material removal mechanisms in grinding process. From a macro point of view, the change of fiber directional angle has a great impact on the grinding temperature. At the same time, the propagation rate of grinding temperature is also related to the fiber directional angle. Therefore, the distribution of grinding temperature field in the workpiece after grinding will change with the change of the fiber directional angle. The sensitivity of carbon fiber and epoxy resin to grinding temperature is different in C/E composites, so the proportion of the two components has a great influence on the propagation and distribution of grinding temperature. Therefore, it is necessary to study the influence of fiber directional angle and resin content of C/E composites on grinding temperature.
Fig. 9. Thermostatic water bath calibration experiment.
tank (Fig. 9) to gain the relationship between central wavelength drift and temperature of each FBG. Then calibrated FBGs were embedded in various types C/E composite workpieces for real-time measurement of grinding temperature.
1) Fig. 10 shows the experimental results of group a and the measured directional angles of fibers are 0°, 30°, 60°, 90°, 120°, 150° and 180°, respectively. As for UVAG of C/E composite workpiece, the grinding temperature is greatly influenced by the directional angle of fibers, which is concretely embodied as follows: while θ < 90∘, the grinding temperature rises along the growth of the directional angle (θ ) of fibers; while θ = 90∘, the grinding temperature reaches maximum; and while θ > 90∘, the grinding temperature falls along the growth of the directional angle (θ ) of fibers. Moreover, the grinding temperature corresponding to the direction of running fibers (θ < 90∘) is significantly higher than that corresponding to the direction of inverse fibers (θ > 90∘). Thus, a higher grinding temperature may easily occur in the direction of running fibers while other processing conditions remain the same.
4.2. Experimental method To reveal effects of structural characteristics (the fiber directional angle and the epoxy resin content) of C/E composites on the grinding temperature, processing experiments were primarily divided into three groups as follow (the grain size of diamond electroplating tool: 120#): Experiments of group a. This group of experiments was design to obtain effects of fiber directional angle on grinding temperature in UVAG for unidirectional laminates. Seven types workpieces with different fiber directional angles (as shown in 3.1 (2)) were machined under various processing parameters (Table 2). Experiments of group b. This group of experiments was designed to
Fig. 10(a), Fig. (b) and Fig. (c) also show the effects of the grinding depth, spindle speed and feed rate on the grinding temperature. While the directional angle (θ ) of fibers remains the same, the grinding temperature will rise along with the growth of the grinding depth, spindle speed or feed rate; at the same time, the grinding temperature is affected by the grinding depth to the greatest extent; on the other hand, it is found that the change of grinding parameters (grinding depth, spindle speed and feed rate) has almost little effect on the relationship (the grinding temperature vs. the directional angle of fibers) for unidirectional laminates. When the directional angle of carbon fibers is 90°, absolute values of
Table 2 Experimental conditions for UVAG. Experiment
Spindle speed (r/min)
Grinding depth (mm)
Feed rate (mm/ min)
1
500, 1000, 1500, 2000, 2500 1500 1500
1.4
180
1, 1.2, 1.4, 1.6, 1.8 1.4
180 100, 140, 180, 220, 260
2 3
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Fig. 11. The grinding temperature vs. the complexity of fiber directional angles for multi-directional laminates.
of the angle between the direction of grinding speed and the axial direction of carbon fibers. This will lead to lower grinding force and grinding heat. In addition, the heat spreads more easily to other locations of the workpiece than the measurement plane because of the fiber directional angle. These factors may contribute to lower grinding temperature. Furthermore, the variation of processing parameters only influences the grinding heat generated during UVAG, but not change the material removal mechanism and the thermal conductivity of workpieces. Thus, the tendency between grinding temperature and fiber directional angles did not alter under different machining parameters.
Fig. 10. The grinding temperature vs. the fiber directional angle for unidirectional laminates.
2) Fig. 11 presents the experimental results of group b. These results indicate that the grinding temperature measured during UVAG first rises and then falls consistently; moreover, the measured grinding temperature corresponding to experiment 2 peaked and that corresponding to experiment1 was minimized; but there's little variation in the overall temperatures. The above phenomena demonstrate that effects of the complexity of fiber directional angles of C/E composite workpiece on the grinding temperature depend on the ratio of carbon fiber layers with fiber directional angle nearby 90°; and the
the angle between the direction of grinding speed and the axial direction of carbon fibers are relatively small in most cutting positions. This will cause most carbon fibers to fracture due to extrusion and stretching, which in turn increases the grinding force and grinding heat. What's more, the heat will spread more easily to the measurement plane (plane perpendicular to the feed direction) because of the better conductivity of carbon fibers. These factors eventually result in higher grinding temperature. In other situations, the carbon fibers are more likely to break owning to shearing resulted from larger absolute values 105
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grinding temperature rises with the increase of this ratio (Fig. 11). Fig. 11(a), Fig. (b) and Fig. (c) also show effects of the grinding depth, spindle speed and feed rate on the grinding temperature; for each experiment, the grinding temperature rises along with the grinding depth, spindle speed or feed rate; moreover, the grinding depth has the greatest influence on grinding temperature; in addition, it is found that any change of grinding parameters (grinding depth, spindle speed or feed rate) almost does not influence the relationship (the grinding temperature vs. the directional angles of fibers) for multidirectional laminates. There are two main factors affecting grinding temperature as follow: grinding heat generated during UVAG and the thermal conductivity of the workpieces. The former factor will be related to the ratio of carbon fiber layers with fiber directional angle close to 90° in the workpiece and the higher this radio, the more grinding heat will be generated during the process. The later factor will be determined by the number of the different fiber directional angles in the workpiece and the greater this number, the better the thermal conductivity of workpieces will be. In addition, the better thermal conductivity results in lower grinding temperature when the grinding heat remains the same. In different situations, the dominated factor determining the final grinding temperature is different. In all, from experiment 1 to experiment 5, the thermal conductivity of workpieces is getting better because of the more complicated structure and the grinding heat generated in experiment 1 is higher than that in experiment 2 due to the workpieces' structure. For experiment 1 and experiment 2, the dominated factor is grinding heat, while for experiment 2/3/4/5, the leading factor is the thermal conductivity of workpieces. The dominated factor remains the same when the processing parameters change. This is the reason that the grinding temperature peaks at expreiment2 and the changing process parameters have little influence on the trend shown in Fig. 11. 3) Fig. 12 shows the experimental results of group c. As for UVAG of C/ E composite workpieces, the grinding temperature rises along with the growth of epoxy resin content of workpieces while other processing conditions are the same, resulted from that the thermal conductivity of epoxy resin is far less than that of carbon fibers [1]; moreover, the grinding temperature increases more and more rapidly. Fig. 12(a), Fig. (b) and Fig. (c) also indicate the effects of the grinding depth, spindle speed and feed rate on the grinding temperature. While the epoxy resin content remains the same, the grinding temperature increases along with the growth of the grinding depth, spindle speed or feed rate; moreover, the grinding temperature is influenced by the grinding depth to the greatest extent; in addition, it is found that any change of grinding parameters (grinding depth, spindle speed and feed rate) has almost little effect to the relationship (the grinding temperature vs. the content of resin). 4) Fig. 13 presents the macroscopic and microscopic appearance of ground surfaces. In order to observe the microstructure more clearly, the magnification of the first three pictures is 300 times, and the last three pictures is 1000 times. Fig. 13(a) and (b) show that with the increase of fiber directional angle, the macroscopic surface of workpieces is substantially degraded and the number of burrs near the edge of workpieces raises greatly, respectively. Fig. 13(c) shows that with the increase of fiber directional angle, the microcosmic surface of workpieces is significantly deteriorates. The proportion of intact fiber bundles is reduced and the number of carbon fibers that have fallen off from the matrix resin increases. What's more, the degree of destruction of matrix resin gradually deepens, and
Fig. 12. The grinding temperature vs. the content of resin under various grinding parameters.
obvious surface defects such as pits occur. 6. Conclusions Various workpieces were first designed and prepared in accordance 106
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processing experiments under different parameters and real-time measurement of the grinding temperature of C/E composite workpieces was performed to gain the effects of their structural characters on the grinding temperature during UVAG process. (1) As for UVAG of C/E composite workpieces, the grinding temperature is greatly affected by the fiber directional angle, resulted from its influences on grinding heat and the heat distribution of workpieces. While θ = 90∘, most carbon fibers fracture due to extrusion and stretching which cause higher grinding force and grinding heat, moreover, the heat spreads more easily to the measurement plane; while θ < 90∘ or θ > 90∘, the carbon fibers are more likely to break owning to shearing which leads to lower grinding force and grinding heat, moreover, the heat spreads more easily to other locations of the workpieces than the measurement plane. Those factors result in the phenomena as follow: while θ < 90∘, the grinding temperature rises along the growth of the fiber directional angle (θ ); while θ = 90∘, the grinding temperature reaches maximum; while θ > 90∘, the grinding temperature falls along the growth of the fiber directional angle (θ ). (2) Effects of the complexity of the fiber directional angle of C/E composite workpieces on the grinding temperature, which is eventually determined by grinding heat and the thermal conductivity of workpieces, can be divided into two aspects: on the one hand, the ratio of carbon fiber layers with fiber directional angle nearby 90° affects the grinding heat and the larger this ratio, the more heat generated; on the other hand, the complexity of fiber directional angle influences the thermal conductivity of workpieces when other conditions remain the same and the larger the number of different fiber directional angles of workpiece, the better the thermal conductivity. Furthermore, in different situations, the dominant aspect will be different. (3) As for component of C/E composite workpieces, the grinding temperature is influenced by the epoxy resin content, mainly resulted from the poor thermal conductivity of resin. The measured results indicate that the grinding temperature rises with the growth of epoxy resin content while the processing conditions remain the same on the premise that performance requirements of workpieces are satisfied. (4) The change of grinding parameters (grinding depth, spindle speed and feed rate) greatly influences the grinding temperature; and the grinding depth shall be dominant; on the other hand, the variations of the grinding parameters bring about little effect on material removal mechanism and the thermal conductivity of workpieces and therefore the relationship (the grinding temperature vs. its structural characteristics (the directional angle of fibers and the content of resin))almost remains the same. The C/E composite materials are typical representative of fiber reinforced composites therefore the conclusions above can provide guidance for grinding temperature studies of other fiber reinforced composites whose structures and properties are like those workpieces employed in this paper. Acknowledgments The authors would like to thank the Science and Technology Department of Hubei Province (Grant Number: 2015BAA022) for funding this study. References
Fig. 13. The effects of the fiber directional angles on the grinding quality.
[1] Benedetti A, Fernandes P, Granja JL, Sena-Cruz J, Azenha M. Influence of temperature on the curing of an epoxy adhesive and its influence on bond behavior of NSM-CFRP systems. Compos B Eng 2016;89(3):219–29. [2] Cai JG, Lin ZY. Research on temperature measurement of high speed drilling tool
with structural characteristics (the directional angle of fibers and the epoxy resin content) of C/E composite; and then FBGs were embedded in the setting locations during the preparation of workpieces; finally, 107
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[3]
[4] [5]
[6] [7]
[8]
[9]
[10]
[11]
[12]
[13]
[14] [15] [16]
[17]
[18] Ma FJ, Jiang YC, Zhang SF, Liu Y, Sha ZH, Su C. Influence analysis of fiber angle on cutting performance for carbon fiber reinforced composite. J Dalian Jiaot Univ 2016;37(6):37–42. [19] Jiang TY, Li WB. Analysis of influence of angle of 2 Abrasive grain on grinding. Tool Technology 2017;51(3):72–5. [20] Madhavan V, Lipczynski G, Lane B, Whitenton E. Fiber orientation angle effects in machining of unidirectional CFRP laminated composites. J Manuf Process 2015;20:431–42. [21] Henerichs M, Vob R, Kuster F, Wegener K. Machining of carbon fiber reinforced plastics: influence of tool geometry and fiber orientation on the machining forces. CIRP Journal of Manufacturing Science and Technology 2015;9:136–45. [22] Wang FJ, Yin JW, Ma JW, Jia ZY, Yang F, Niu B. Effects of cutting edge radius and fiber cutting angle on the cutting-induced surface damage in machining of unidirectional CFRP composite laminates. Int J Adv Manuf Technol 2017;91:3107–20. [23] Wei YY, An QL, Cai XJ, Chen M, Ming WW. Influence of fiber orientation on singlepoint cutting fracture behavior of carbon-fiber/epoxy prepreg sheets. Materials 2015;8:6738–51. [24] Zhu DD, Li WX, Li ZQ, Wang JJ. A distributed fiber Bragg grating system for simultaneous measurement of the strain and temperature. Acta Metrol Sin 2008;29(1):29–32. [25] Lee CL, Lee RK, Kao YM. Design of multichannel DWDM fiber Bragg grating filters by Lagrange multiplier constrained optimization[J]. Optic Express 2006;14(23):11002–11. [26] Guan B, Liu Z, Kai G, Ge C, Dong X. Temperature sensor based on fiber Bragg grating[J]. Journal of Transcluction Technology 1999;15(2):90–3. [27] Zhan YG, Cai HW, Xiang SQ, Qu GH, Wang XZ. Study on high resolution fiber Bragg grating temperature sensor[J]. Chin J Laser B 2005;32(1):83–6. [28] Zhang B, Yan GS, Deng YJ. Cross-sensitivity of fiber grating sensor measurement[J]. Journal of Applied Optics 2007;28(5):614–8. [29] Li TL, Tan YG, Wei L, Zhou ZD, Zheng K, Guo YX. A non-contact fiber Bragg grating vibration sensor[J]. Rev Sci Instrum 2014;85(1):81–4. [30] Zhang XW, Nin TG. Study of cross-sensitivity in fiber grating sensors. Optical Fiber & Electric Cable 2007;2:1–4. [31] Chen LJ. Solution to cross-sensitivity of fiber grating sensors. Digital Communication 2012;39(6):15–7. [32] Lu D, Cai LG, Cheng Q, Li ZK. Finite element simulation of ultrasonic vibration assisted turning of carbon fiber reinforced polymer composite. J Vib Shock 2015;34(14):110–5. [33] Wang KJ, Liu X, Li H, Wang LY. Experimental study on ultrasonic vibration aided micro grinding temperature. Sci Technol Eng 2016;16(29):54–8. [34] Kang RK, Ma FJ, Dong ZG. Ultrasonic assisted machining of difficult-to-cut material. Aeronautical Manufacturing Technology 2012;412(16):44–9. [35] Chen T, Ye ML, Liu SL. Measurement of ultrasonic assisted grinding temperature based on fiber Bragg grating (FBG) sensor. Int J Adv Manuf Technol 2017;1:1–10.
made of C/E composite material. Machine Building & Automation 2014;44(2):201–14. Wen Q, Guo DM, Gao H, Zhao D. Comprehensive evaluation method for carbon/ epoxy composite hole-making damages. Acta Mater Compos Sin 2016;33(2):265–72. Liu CJ, Ding WF, Yu TY, Y CY. Materials removal mechanism in high-speed grinding of particulate reinforced titanium matrix composites[J]. Precis Eng 2018;51:68–77. Yang M, Li CH, Zhang YB, Jia DZ, Zhang XP, Hou YL, Li RZ, Wang J. Maximum undeformed equivalent chip thickness for ductile-brittle transition of zirconia ceramics under different lubrication conditions[J]. Int J Mach Tool Manufact 2017;122:55–65. Zhao B, Yu TY, Ding WF, Li XY. Effects of pore structure and distribution on strength of porous Cu-Sn-Ti alumina composites[J]. Chin J Aeronaut 2017;30(6):2004–15. Dai CW, Ding WF, Zhu YJ, Xu JH, Yu HW. Grinding temperature and power consumption in high speed grinding of Inconel 718 nickel-based superalloy with a vitrified CBN wheel[J]. Precis Eng 2018;52:192–200. Zhang YB, Li CH, Ji HJ, Yang XH, Yang M, Jia DZ, Zhang XP, Li RZ, Wang J. Analysis of grinding mechanics and improved predictive force model based on material-removal and plastic-stacking mechanisms[J]. Int J Mach Tool Manufact 2017;122:81–97. Ding WF, Dai CW, Yu TY, Xu JH, Fu YC. Grinding performance of textured monolayer CBN wheels: undeformed chip thickness nonuniformity modeling and ground surface topography prediction[J]. Int J Mach Tool Manufact 2017;122:66–80. Tam LH, Zhou A, Yu Z, Qiu QW, Lau D. Understanding the effect of temperature on the interfacial behavior of CFRP-wood composite via molecular dynamics simulations. Compos B Eng 2016;109:227–37. Liu SL, Chen T, Wei YX, Wu CQ. Study on surface quality of CFRP after rotary ultrasonic face grinding. Advanced Processing Technology of Composites 2016;510(15):57–61. Hu AD, Chen Y, Fu YC, Xu JH, Liu SQ. Effects of ultrasonic Vibration-assisted grinding on cutting force and surface quality of CFRP. Acta Mater Compos Sin 2016;33(4):788–96. Chen T, Ye ML, Liu SL, Deng Y. Experimental study on cross-sensitivity of temperature and vibration of embedded fiber Bragg grating (FBG) sensors[J]. Optoelectron Lett 2018;14(2):92–7. Chen T, Ye ML, Deng Y, Liu SL. Study of temperature field for UVAG of CFRP based on FBG[J]. Int J Adv Manuf Technol 2018;96:765–73. Liu HL, Zhang JB, Wang Z, Zhang JP, Li G. Cutting tool selection in CFRP and AFRP machining. Aerospace Materials and Technology 2013;43(4):95–8. Ye XZ, Wang DZ, Liu DJ, Yang XP, Yu DM, Jing JW. Experimental study on tool wear of milling carbon fiber composites. Mechanical and Electrical Technology 2014;3:75–7. Wen L, Jiang LP, Zhang HZ, Cai XJ, An QL, Chen M. Test on thermal characteristics when orthogonally free machining carbon fiber reinforced plastics unidirectional laminates. Acta Mater Compos Sin 2015;32(5):1469–79.
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