Laminated polymer matrix composites (PMCs) reinforced with carbon, glass, aramids etc fibers have been extensively employed in variety of applications due to their superior properties as well as facile and economical methods of fabrication [1-3].The in-plane mechanical properties of these types of materials were dictated by the reinforcement phase (fibers) mechanical properties subsequently, the in-plane properties of PCMs are powerful enough for many applications including aerospace,ground vehicles and wind turbines. In contrast, the out-of-plane mechanical properties of the laminated composite were dominated by mechanical properties of the matrix [4, 5]. It is worth mentioning that the matrix mechanical properties are significantly less than of the mechanical properties of reinforcement phase resulting inadequate out-of-plane properties. Moreover,ply-by-ply temperament of the laminated PMCs makes them susceptible to delamination owning to creation of microcracks and subsequently, microcracks propagation through the weak resin-rich layer. Due to, this significant disadvantage laminated PMCs have not been used in critical applications [4]. Therefore,a series of researches have been carried out for enhancing the matrix mechanical properties by incorporation of a second phase such as carbon nanotubes [6, 7], micro/nanofillers [8-12]in the matrix or even improving the curing system [13, 14].However, the above mentioned methods have not been employed in fabrication of composite materials due to some limitations such as troubles due to significant increase in the viscosity of resins which adversely affect the manufacturing process and subsequently, decreases the mechanical properties of final fabricated composites [15].

目前，中药材加工过程缺乏相关的规范化管理，无论是药材的干燥方法、炮制方法还是药材的粉碎粒度，都没有相应的标准体系，致使市售的中药挥发油质量不均一。中药材加工过程规范化研究应从洗净规范化、切制规范化、干燥过程规范化、粉碎粒度规范化、炮制方法规范化入手。制定统一的中药材前处理标准，建立适合中药挥发油的质量标准管理体系，实现中药材前处理工序全过程的规范化，进而保证中药挥发油的质量稳定均一。

A method of composite strengthening was proposed by Dzenis and Reneker with the use of nanothechnology and has gathered much research interest. In this method un-oriented electrospun nanofiber mats were incorporated between layers of the reinforcement phase leading to significant improvement in the mechanical properties of the weak resin-rich area [16, 17].The method provides an inexpensive and facile technique for enhancing mechanical properties of resin-rich area with minimum influences on the fabrication processes [4]. The improvement in mechanical properties of resin-rich area, via incorporation polymeric nanofibers, has been extensively studied during recent years [18-21] as well as improvement in fracture toughness of epoxy adhesives [22-28] and neat resins [29, 30]. The researchers reported that the nanofiber breakage, nanofiber pull out, stress distribution by the nanofibers, and stress transform from weak phase to reinforcing phase are the mechanisms of mechanical properties improvement [20, 21]. Therefore, the nanofibers diameter, the type of nanofiber (mechanical properties of the nanofibers), thickness of the nanofiber mat, and the nanofibers and matrix interaction have a significant impact on the amount of mechanical properties improvement [5].

In the present research, the effect of reinforcing nanofiber,via using of the nanoparticles, on the improvement of fracture toughness of a carbon/epoxy composite was investigated. Improving mechanical properties of nanofibers by incorporating nanofillers was reported in previous works [31, 32]. Consequently, the approach is reinforcing the polyacrylonitrile(PAN) nanofibers by adding Al2O3 nanoparticles [33]. Therefore,the neat PAN and PAN-Al2O3 nanofibers were deposited on carbon fabrics. Three types of the carbon/epoxy composites namely control samples, PAN reinforced the carbon fiber reinforced polymer (CFRP) and PAN-Al2O3 reinforced CFRP panels were fabricates. The composite panels were then evaluated via mode I fracture energy assessment tests to investigate the influence of reinforcing nanofibers on the fracture toughness of a conventional carbon/epoxy composite.

2.国内研究现状。翻转课堂在我国目前的研究尚处于起步阶段，我国对翻转课堂教学的研究虽然晚于国外，但同样呈逐年上升的趋势，并以迅猛的速度展开研究。但我国对于翻转课堂的研究多集中对课堂教学的理论探讨，不具备操作性,而具体的实施策略的研究成果少之甚少，实施的深度和广度都不够，缺少翻转课堂教学与具体学科的深度融合的实践研究。其次，与美国翻转课堂实践区域大部分集中在小学课堂不同的是，对于国内翻转课堂的应用型研究大部分还是以高中、高职或大学中的应用为主，初中和小学目前的研究很少。

This paper presented an experimental study on the effect of incorporating a thin film of PAN nanofiber and Al2O3-PAN nanofibers in the interface between carbon fiber plies on the delamination behavior of the composite panels. Al2O3 nanoparticles were incorporated in PAN nanofibers to improve the mechanical behavior of them and consequently improve the delamination strength of the reinforced panel. A significant improvement was observed for the interlaminar fracture energy of both types of reinforced panels, however, the samples reinforced by Al2O3-PAN nanofibers had higher delamination strength compared to neat PAN nanofiber reinforced samples.The increased interlaminar fracture energy GIc proved that the presence of nanofibers postpone the crack initiation by reinforcing the matrix. In addition, FE-SEM micrographs revealed that during the propagation stage, the crack in Al2O3-PAN reinforced samples is forced to break a larger amount of matrix compared to virgin samples, requiring higher energy to extend.

The 10 wt% of PAN and 10 wt% of PAN containing 1 wt% of Al2O3 nanoparticles were dissolved in adequate DMF by employing magnetic stirrer for 24 h at room temperature. In order to make sure excellent dispersion of Al2O3 nanoparticles, the solution (containing Al2O3 nanoparticles) was solicited for 20 min using high intensity ultrasonic liquid processor (UP 400S, 40 KHz Sonics Vibra Cell, Hielscher, Inc). The obtained solutions were filled in the syringes with needle gauge 21 (inner diameter=0.51 mm). Before the electrospinning, the conventional unidirectional carbon fiber fabrics were placed around a 15 cm diameter grounded drum. The nanofibers were directly electrospun on the carbon fabric surfaces. The electrospinning process parameters were set the same as our previous work for both neat PAN nanofibers and Al2O3-PAN nanofibers [5] leading to a stable electrospinning process for preparation of the uniform nanofibers without any beads. Electrospinning process was hold until 1 g of nanofibers was gathered on a square meter of carbon fiber fabrics. Figure 1 schematically shows the nanofiber deposition on the surfaces of carbon fiber fabrics.

• Fracture tests were conducted on DCB specimens to study the interlaminar fracture behavior of reinforced composites.

Carbon/epoxy composite panels were prepared using hand lay-up method proceed by the vacuum assist resin transfer molding (VARTM) method. The epoxy resin and its hardener were mixed in the mass ratio 100:58 in accordance with the manufacturer’s recommendation. Two composite panels were fabricated containing 12 layers of uniaxial carbon fabric (parallel aligned) and epoxy matrix. The nanofiber reinforced panels compromised 11 layers of the neat PAN and Al2O3-PAN nanofibers in the resin-rich area, whereas the control composite did not contain any nanofibers. A 30 μm thick polyethylene film was also inserted in the mid-interface of all the specimens during the lay-up to create an initial artificial crack. Vacuum pressure was hold for 18 h in order to complete cure of epoxy resin at room temperature. Subsequently, three composite panels were separated from vacuum system and post cured in an oven at 60 °C for 30 min.

By means of an electron microscope, images of fracture surface have been captured to investigate the failure mechanisms in different reinforcing configurations. Figure 5 shows the FE-SEM images of the fracture surfaces of control composite (Fig. 5(a)),composites reinforced with PAN nanofibers (Fig. 5(b)) and Al2O3-PAN nanofibers (Fig. 5(c)). Comparing fracture surface of control composite with composites reinforced with thin layer of the nanofibers reveals that fracture surface of the control composites is comparatively smooth with oriented fracture features.This type of fracture was resulted from the propagation of microcracks that initiated at the place of stress concentration. In contrast, for reinforced composites with PAN and Al2O3-PAN nanofiber (Fig. 5(b) and (c)), it can be observed that the roughness of the fracture surface significantly increased. Higher roughness of fracture surfaces confirms that the attendance of nanofibers in the resin-rich area could avert the microcracks, conducting the cracks along more zigzag paths, and subsequently the resistance to crack propagation would be increased. Voids and holes were also observed on the fracture surfaces of the composite reinforced with the neat PAN and Al2O3 nanofibers owning to nanofibers pullout and deboning from the matrix. It was reported that, the nanofiber breakage and pullout are the mechanisms attributed for improvement in fracture toughness of the composite [20, 35]. Furthermore, comparing Fig. 5(b) with 5(c) reveals that the fracture surfaces of the composite reinforced with Al2O3 nanofibers is rougher than the fracture surfaces of composite containing neat PAN nanofibers. Therefore, it can be concluded that incorporating Al2O3 nanoparticles increased the mechanical properties of the nanofibers, subsequently more energy needed for nanofiber breakage and better stress distribution(through the resin-rich area) and stress transform from resinrich area to reinforcing phase (carbon fiber) were occurred. Considering the samples reinforced by PAN nanofibers the local failure mode around the carbon fibers is a combination of adhesive and cohesive failures while a complete cohesive failure was observed for the samples reinforced by Al2O3-PAN nanofibers. This can be due to probable impact of Al2O3 particles on improvement of the bonding energy between the enriched resin and the carbon fibers.

The double cantilever beam (DCB) samples have been prepared with 130 mm length, 25 mm width, initial crack of 45 mm.DCB samples were cut from composite panels using water jet.Steel hinges were glued on both tips of the samples to apply the load. DCB fracture tests have been performed to calculate the mode I energy release rate GIc, using Eq. (1), from the modified beam theory (MBT) according to ASTM D 5528 [34]:where P is the load, δ is the load point displacement, b is the specimens width, and a is the delamination length.

Fig. 1. Schematic illustration of the nanofiber deposition on the surfaces of carbon fiber fabric.

Fig. 2. a FE-SEM image of Al2O3-PAN nanofiber. b distribution of the nanofiber diameter.

HIGHLIGHTS

主要观察缼齿蓑藓的枝叶形态和细胞性状，并参照Yu等[7]的方法进行测量。缼齿蓑藓配子体枝叶13个形态性状具体见表2。

能否在不同地形条件下实现平稳运动，是一个机器人是否具有实际使用价值的评判标准。斜面是自然界中常见的地形，一些崎岖的地形也可以等效为斜面。因此，研究四足机器人在斜面上的运动控制具有重要意义。经典控制和现代控制在设计过程中一般是基于被控对象的精确数学模型。对于四足机器人来说，因其具有非线性、高耦合、模型复杂等特点，采用传统的控制理论和方法往往存在局限性，难以取得满意的控制效果[1]。模糊控制是建立在模糊集合论基础上的一种基于语言规则与模糊推理的控制理论，在控制非线性、高耦合及参数和结构不确定的复杂过程中具有较好的控制效果。

• Al2O3 -PAN nanofiber reinforced composites had higher fracture energy improvements compared to the PAN nanofiber reinforced composites.

According to the experimental data, the fracture energies of control samples, PAN reinforced samples and Al2O3-PAN samples were 0.832 ± 0.07 N/mm, 0.981 ± 0.08 N/mm, and 1.219± 0.13 N/mm, respectively. Experimental results show a general improvement of fracture energy on both PAN and Al2O3-PAN reinforced samples, which are about 18% and 47% higher than the control samples. Nevertheless, the improvement values for Al2O3-PAN reinforced CFRP samples were higher than the samples reinforced with PAN nanofibers, which can be a result of different failure mechanisms in these samples.

Fig. 3. Typical load-displacement curves of the tested DCB specimens.

Fig. 4. Typical R-curves of the tested DCB specimens.

The morphology of Al2O3 nanofiber and the fractured surfaces of the composites were analyzed by employing a field emission scanning electron microscope (FE-SEM), Hitachi S-4300, Japan. Before morphological investigations, the surfaces of the nanofibers and fractured composites were coated with a fine layer of gold. As mentioned in our previous work the average diameter of the neat PAN nanofiber and its standard deviation were obtained 380 and 70 nm, respectively [5]. Figure 2 shows the FE-SEM micrograph of Al2O3-PAN nanofibers and diameter distribution of the nanofibers for 50 diameter measurement using Image J software. From Fig. 2, it can be observed that the nanofibers containing Al2O3 nanoparticles were fabricated uniformly without any signs of beads. The average nanofiber diameter and its standard deviation were measured 417 and 112 nm, respectively. Therefore, the average of Al2O3-PAN nanofibers is 10% more than average of neat PAN nanofibers diameter. This increasing in nanofiber diameter can be attributed to growing of solution viscosity due to addition of the Al2O3 nanoparticles.

Fig. 5. FE-SEM images of fractured surfaces of a control composite, b composite reinforced with neat PAN nanofibers, and c composite reinforced with neat Al2O3-PAN nanofibers.

As reported by Brugo and Palazzetti [36], UD samples are less affected by Nylon 6.6 nanofibers due to the fact that the crack tends to cross plies and not to propagate in the same interface the nanolayer is laid. According to their results, higher fracture energy enhancement values were obtained for Nylon 6.6 nanofiber reinforced woven carbon plies. The same conclusion can be made in the case of PAN and Al2O3-PAN reinforced samples.Although the improvement of fracture behavior was considerable, however, it is expected to have higher improvements for woven plies, which can be examined by further studies.

PAN (Mw=150000 g/gmol) and N, N-dimethylformamide(DMF, 99.8%) were provided from Sigma-Aldrich. The epoxy resin (EPONTM Resin 828), and its curing agent (EPIKURETM Curing Agent F205) were supplied by Hexion Inc. The conventional carbon fiber fabric (300 g·m-2) was purchased from Jinsor-Tech Industrial Co. Al2O3 nanoparticles (99%, diameter average=20 nm)were purchased from nanosany Co.

References

[1]X. Li, Y. Yan, L. Guo, et al., Effect of strain rate on the mechanical properties of carbon/epoxy composites under quasi-static and dynamic loadings, Polymer Testing 52 (2016) 254–264.

[2]X. Li, L. Wu, L. Ma, et al., Effect of temperature on the compressive behavior of carbon fiber composite pyramidal truss cores sandwich panels with reinforced frames, Theoretical and Applied Mechanics Letters 6 (2016) 76–80.

[3]R.E. Neisiany, S.N. Khorasani, J. Kong Yoong Lee, et al., Encapsulation of epoxy and amine curing agent in PAN nanofibers by coaxial electrospinning for self-healing purposes, RSC Advances 6 (2016) 70056–70063.

[4]X.F. Wu, A.L. Yarin, Recent progress in interfacial toughening and damage self-healing of polymer composites based on electrospun and solution-blown nanofibers: An overview, Journal of applied polymer science 130 (2013) 2225–2237.

[5]R.E. Neisiany, S.N. Khorasani, M. Naeimirad, et al., Improving Mechanical Properties of Carbon/Epoxy Composite by Incorporating Functionalized Electrospun Polyacrylonitrile Nanofibers, Macromolecular Materials and Engineering 302 (2017)1600551.

[6]M.R. Ayatollahi, S. Doagou-Rad, S. Shadlou, Nano-/Microscale Investigation of Tribological and Mechanical Properties of Epoxy/MWNT Nanocomposites, Macromolecular Materials and Engineering 297 (2012) 689–701.

[7]R. Baptista, A. Mendão, F. Rodrigues, et al., Effect of high graphite filler contents on the mechanical and tribological failure behavior of epoxy matrix composites, Theoretical and Applied Fracture Mechanics 85 (2016) 113–124.

[8]J. Ma, L.T.B. La, I. Zaman, et al., Fabrication, Structure and Properties of Epoxy/Metal Nanocomposites, Macromolecular Materials and Engineering 296 (2011) 465–474.

[9]M. Naeimirad, A. Zadhoush, R.E. Neisiany, Fabrication and characterization of silicon carbide/epoxy nanocomposite using silicon carbide nanowhisker and nanoparticle reinforcements,Journal of Composite Materials 50 (2016) 435–446.

[10]S. Salimian, A. Zadhoush, M. Naeimirad, et al., A review on aerogel: 3D nanoporous structured fillers in polymer-based nanocomposites, Polymer Composites (2017) .

[11]S.N. Khorasani, S. Ataei, R.E. Neisiany, Microencapsulation of a coconut oil-based alkyd resin into poly(melamine–urea–formaldehyde) as shell for self-healing purposes, Progress in Organic Coatings 111 (2017) 99–106.

[12]F. Safaei, S.N. Khorasani, H. Rahnama, et al., Single microcapsules containing epoxy healing agent used for development in the fabrication of cost efficient self-healing epoxy coating, Progress in Organic Coatings 114 (2018) 40–46.

[13]R. Akbari, M.H. Beheshty, M. Shervin, Toughening of dicyandiamide-cured DGEBA-based epoxy resins by CTBN liquid rubber, Iranian Polymer Journal 22 (2013) 313–324.

[14]H. Jamshidi, R. Akbari, M.H. Beheshty, Toughening of dicyandiamide-cured DGEBA-based epoxy resins using flexible diamine, Iranian Polymer Journal 24 (2015) 399–410.

[15]B. De Schoenmaker, S. Van der Heijden, I. De Baere, et al., Effect of electrospun polyamide 6 nanofibres on the mechanical properties of a glass fibre/epoxy composite, Polymer Testing 32(2013) 1495–1501.

[16]A. Zucchelli, M.L. Focarete, C. Gualandi, et al., Electrospun nanofibers for enhancing structural performance of composite materials, Polymers for Advanced Technologies 22 (2011)339–349.

[17]R. E. Neisiany, J.K.Y. Lee, S. N. Khorasani, et al., Facile strategy toward fabrication of highly responsive self-healing carbon/epoxy composites via incorporation of healing agents encapsulated in poly(methylmethacrylate) nanofiber shell,Journal of Industrial and Engineering Chemistry 59 (2018)456–466.

[18]Q. Chen, Y. Zhao, Z. Zhou, et al., Fabrication and mechanical properties of hybrid multi-scale epoxy composites reinforced with conventional carbon fiber fabrics surface-attached with electrospun carbon nanofiber mats, Composites Part B: Engineering 44 (2013) 1–7.

[19]S.R. Dhakate, A. Chaudhary, A. Gupta, et al., Excellent mechan-ical properties of carbon fiber semi-aligned electrospun carbon nanofiber hybrid polymer composites, RSC Advances 6 (2016)36715–36722.

[20]R.E. Neisiany, J.K.Y. Lee, S.N. Khorasani, et al., Self-healing and interfacially toughened carbon fibre-epoxy composites based on electrospun core–shell nanofibres, Journal of Applied Polymer Science 134 (2017) 44956.

[21]R.E. Neisiany, J.K.Y. Lee, S.N. Khorasani, et al., Towards the development of self-healing carbon/epoxy composites with improved potential provided by efficient encapsulation of healing agents in core-shell nanofibers, Polymer Testing 62 (2017)79–87.

[22]H. Khoramishad, S.M.J. Razavi, Metallic fiber-reinforced adhesively bonded joints, International Journal of Adhesion and Adhesives 55 (2014) 114–122.

[23]M.R. Ayatollahi, A.N. Giv, S.M.J. Razavi, et al., Mechanical properties of adhesively single lap-bonded joints reinforced with multi-walled carbon nanotubes and silica nanoparticles, The Journal of Adhesion 93 (2017) 896–913.

[24]S.M.J. Razavi, R.E. Neisiany, M.R. Ayatollahi, et al.,, Fracture assessment of polyacrylonitrile nanofiber-reinforced epoxy adhesive, Theoretical and Applied Fracture Mechanics(${ref.ref_year}) (In Press).

[25]E. Esmaeili, S.M.J. Razavi, M. Bayat, et al., Flexural behavior of metallic fiber-reinforced adhesively bonded single lap joints,The Journal of Adhesion 94 (2018) 453–472.

[26]S.M.J. Razavi, M.R. Ayatollahi, E. Esmaeili, et al., Mixed-mode fracture response of metallic fiber-reinforced epoxy adhesive,European Journal of Mechanics - A/Solids 65 (2017) 349–359.

[27]A. Nemati Giv, M.R. Ayatollahi, S.M.J. Razavi, et al., The effect of orientations of metal macrofiber reinforcements on mechanical properties of adhesively bonded single lap joints, The Journal of Adhesion (2018) .

[28]S.M.J. Razavi, M.R. Ayatollahi, A. Nemati Giv, et al., Single lap joints bonded with structural adhesives reinforced with a mixture of silica nanoparticles and multi walled carbon nanotubes,International Journal of Adhesion and Adhesives 80 (2018)76–86.

[29]Q. Chen, L. Zhang, M.-K. Yoon, et al., Preparation and evaluation of nano-epoxy composite resins containing electrospun glass nanofibers, Journal of Applied Polymer Science 124 (2012)444–451.

[30]Y. Zhao, T. Xu, X. Ma, et al., Hybrid multi-scale epoxy composites containing conventional glass microfibers and electrospun glass nanofibers with improved mechanical properties, Journal of Applied Polymer Science 132 (2015) 42731.

[31]M.S. Enayati, T. Behzad, P. Sajkiewicz, et al., Fabrication and characterization of electrospun bionanocomposites of poly(vinyl alcohol)/nanohydroxyapatite/cellulose nanofibers, International Journal of Polymeric Materials and Polymeric Biomaterials 65 (2016) 660–674.

[32]M.S. Enayati, T. Behzad, P. Sajkiewicz, et al., Crystallinity study of electrospun poly (vinyl alcohol) nanofibers: effect of electrospinning. filler incorporation, and heat treatment, Iranian Polymer Journal. 25 (2016) 647–659.

[33]Q. Wang, W.-L. Song, L.-Z. Fan, et al., Facile fabrication of polyacrylonitrile/alumina composite membranes based on triethylene glycol diacetate-2-propenoic acid butyl ester gel polymer electrolytes for high-voltage lithium-ion batteries, Journal of Membrane Science 486 (2015) 21–28.

[34]ASTM, 5528-94a, Standard test method for Mode I interlaminar fracture toughness of unidirectional fiber-reinforced polymer matrix composites, Annual book of ASTM standards, 15 (1994).

[35]Q. Chen, L. Zhang, Y. Zhao, et al., Hybrid multi-scale composites developed from glass microfiber fabrics and nano-epoxy resins containing electrospun glass nanofibers, Composites Part B: Engineering 43 (2012) 309–316.

[36]T. Brugo, R. Palazzetti, The effect of thickness of Nylon 6.6 nanofibrous mat on Modes I–II fracture mechanics of UD and woven composite laminates, Composite Structures. 154 (2016)172–178.