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Fabrication and characterization of auxetic shape memory composite foams
Composites Part B 152 (2018) 1–7
Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Fabrication and characterization of auxetic shape memory composite foams
T
∗
Yongtao Yao, Yun Luo, Yuncheng Xu, Bing Wang, Jinyang Li, Han Deng, Haibao Lu
National Key Laboratory of Science and Technology on Advanced Composites in Special Environments, Harbin Institute of Technology, Harbin, 150080, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Auxetic foam Shape memory effect Negative Poisson's ratio
Shape memory polymers, as a kind of smart materials, play an important role in more and more fields, such as aerospace, biomedicine and intelligent clothing field and so on. Comparing with traditional materials, negative Poisson's ratio foam has excellent mechanical properties, such as double curvature, light weight, high shear resistance, auxetic etc., and it has a great potential application in the field of aerospace. Therefore, in this project, shape memory composite foam was fabricated based on the commercial soft polyurethane foam material as matrix and shape memory epoxy resin as functional phase. Negative Poisson's ratio of foam was fabricated based on its shape memory feature through a process of triaxial compression with heat treatment. Microstructure deformation was characterized after the transformation of auxetic foam. By adjusting the processing parameters, the auxetic shape memory foam with different “re-entrant” structure was obtained. Such fabricated shape memory composite foams display variable stiffness with auxetic behavior. Effective compressive and tensile modulus was obtained by compression and tension tests. The effect of processing parameters on foam Poisson's ratio was analyzed, which provided certain guiding significance for the further shape memory foam preparation.
1. Introduction Shape memory polymers (SMPs) are known as a group of smart materials, which have the ability to return to their permanent shapes upon simulation such as the change of temperature [1,2], electrical current [3–5], alternating magnetic field [6,7], light exposure [8–10], microwave [11,12], and water immersion [13,14]. In addition to the abovementioned special property of SMPs, they also have manifold stimulation sources, easy manufacture and programming and cheap, which make them have tremendous applications in multifarious fields such as aerospace engineering [15,16], textiles [17,18], biomedical engineering [19,20]. These broad applications of SMPs pushed the development of SMPs design. Recently, more and more cellular solids with shape memory behavior have been manufactured in foam [21,22] and honeycomb [23] structures. They have attracted many researchers' attention due to high deformation ability and porous feature. For example, SMP foams can serve as self-deployable structures in aerospace engineering such as the wheel for exploration rovers [24], solar sails [25] and self-healing structures [26] or absorbing impact energy [27]. Auxetic materials are known as one of the classical deformed structures, which have negative Poisson's ratio opposite to the ones belonging to conventional materials, with an unusually expansion or contraction under a tensile or compressive stress. Comparing with conventional foams, mechanical properties of ∗
Corresponding author. E-mail address: [email protected] (H. Lu).
https://doi.org/10.1016/j.compositesb.2018.06.027 Received 30 May 2018; Accepted 26 June 2018 Available online 28 June 2018 1359-8368/ © 2018 Elsevier Ltd. All rights reserved.
auxetic foams can be enhanced as a consequence of its negative Poisson's ratio property, for example, acoustic absorption [28], shear resistance [29], fracture toughness and synclastic curvature [30]. The first auxetic foam was fabricated by Lakes from commercially available open-cell polyurethane foam [31]. Scarpa et al. developed a new fabrication method to achieve auxetic foam with improved stiffness and high resilience [32,33] based on open cell thermoplastic foam. Generally, the foam was converted with auxetic behavior based on rebuild microstructure according to its thermoplastic property and rebuild reentrant cells were achieved from a quenching process with triaxial compression in a mould. The re-entrant cells unfold under tension giving rise to the negative Poisson's ratio. Thus, the rebuild structure with “re-entrant” configuration was considered to be the main reason for the negative Poisson's ratio phenomenon. In order to explain the experimentally measured values of the Poisson's ratios in terms of the microstructure of the foams, various numerical models that represent the auxetic foams have been proposed. For example, Grima et al. proposed two-dimensional model to simulate auxetic foam based on “rotation of rigid units”. McDonald et al. [34] constructed the microstructurally faithful finite element models based on 3-dimensional microtomography of auxetic foam to investigate its deformation mechanism. As we all know, the mechanical properties of materials are mainly related to their microstructure, thus, they could be controlled by controlling the microstructure of the material. Combining SMP and auxetic
Composites Part B 152 (2018) 1–7
Y. Yao et al.
characterized by the digital image correlation (XTDIC, Xi'an XinTuo 3D Optical Measurement Technology Co., Ltd) for compressive and tensile loading.
structure is a feasible way to build a smart structure with controllable and extensive morphing ability. In this case, auxetic structure could fully exerted excellent mechanical performance and deformability, while SMP could remember its deformed structure and help to extend the variability of the structure. In this study, shape memory composite foam will be fabricated with auxetic behavior. Shape memory epoxy and commercial polyurethane were employed to manufacture SMP foam with positive Poisson ratio as permanent shape. The auxetic conversion is mainly based on a process of triaxial compression with heat treatment. Relative mechanical property will be investigated in term of the foam microstructure. The relationship between processing parameter and auxetic behavior will be studied. This work hopes to provide a new idea for the preparation of smart and controllable large morphing structures.
2.2.3. Shape memory behavior tests For shape recovery tests, to avoid any possible damage in testing at a low temperature, all specimens (Er:PU = 0.5:1; 0.8:1; 1:1; respectively) compressed 50% and 85%, respectively, at Tg+20 °C. The strain of compressed specimens was kept until temperature dropped to room temperature. Finally, the foams expand at Tg+20 °C for 30 min and the height after shape recovery was tested. For Shape fixing tests, specimens are to keep the temporal shape for a long time. The samples were compressed 50% and 85% at Tg+20 °C, and that height was kept until the temperature cooled down. The height of each specimens was measured every 5 min.
2. Experiment 3. Results and discussion 2.1. Fabrication of SMP composite foam with IPN network 3.1. The morphology and structure of SMER composite foams 2.1.1. Materials In these experiments, the shape memory epoxy resin was developed in our lab according to previous report [35]. Commercially available polyurethane foam raw materials with two ingredients, A (polyester polyol, surfactant, etc.) and B (isocyanate etc.), were supplied by Shanghaishengju construction materials LTD. All chemicals involved in preparing shape memory composite foams were used as received.
In this study, scanning electron microscopy was conducted to investigate the effect of proportioning of Er on the morphology of PU/Er foam. Fig. 1 shows the results of SEM on PU/Er foam with different proportioning of Er (Er:PU = 0.5:1; 0.8:1; 1:1; respectively). It can be seen that spherical open cells structure with uniform distribution in PU/ Er foam have been obtained. The cell wall surfaces are smooth and there is no separated phase structure for those PU/Er foams with different contents. The single phase is observed for mixing PU and Er components to form interpenetrating network foams. Fig. 1(d) shows the relationship between the average diameter of open cells and Er contents. The range of diameter distribution is slightly increasing with increase of contents of the Er. As result of the foam network is mainly created based on the PU component, and the increase of the addition amount of Er is leading to the larger numbers of polyol hydroxyl groups which will speed up PU foaming rate to release more air attributing to an increase in the size of foam pore.
2.1.2. Preparation of auxetic foams Firstly, SMP PU/Er foams were fabricated based on the one-shot method. PU component provided foam structure and shape memory polymer served as shape fixing and recovering functionality. The shape memory epoxy resin (Er) solution with different weight and PU (Er:PU (A + B) = 0.5:1; 0.8:1; 1:1, respectively) were mixed for 10 S at 1000 rpm. The mixtures were subsequently poured into an open cylindrical mould to produce free-rise foams for 24 h. Then, such foams were postcured at 150 °C for 5 h in an oven in order to make shape memory Er ingredients completely cured. Secondly, the structure of SMP PU/Er foam was converted into auxetic form and its conversion process applied to the foam was similar to that of Ref. [36]. Initial foam with a cylinder structure was inserted into a smaller steel tube mould at room temperature under a biaxial compression. End tabs were employed to ensure compressive strain was applied along the mould axis. Then, the mould was placed in an oven at a temperature of 150 °C for 30 min. The auxetic foam was removed when the mould was cooled to room temperature. In this project, a series of foams with different volumetric compression ratios (0.68, 0.71, 0.78, 0.79 and 0.89, respectively) have been produced in order to achieve different auxetic properties and optimize the processing parameters.
3.2. The morphology and structure of auxetic foams Fig. 2 indicates the structure of transformed foam observed by SEM. There are three regions of foam have been chosen to evaluate their microstructure transformation: in radial direction centre region (Fig. 2(c)), near surface region (Fig. 2(b)) and longitudinal direction sheath region (Fig. 2(d)). It can be seen from Fig. 2 that cell structure transformed from open cell structure to analogical “re-entrant” structure under a biaxial compression. There is no apparent difference for microstructure in three regions of foam. That means this microstructure uniform transformed induced by a biaxial compression. Comparing with initial foam structure, auxetic foam present disordered and convoluted unit cells, with complex rib geometry (Fig. 2(e and f)). Some unit cell wall has broken ribs by a reduction of foam cell size. In contrast with former researches [34], the similar conversion results has been found that the auxetic deformation occurs primarily by the introduction of ‘kinks’ at the centres of the ribs as a result of extensive buckling (Fig. 2(e and f)). Auxetic phenomenon has been achieved mainly related to the deploying of these ‘kinks’ or bent ribs in response to uniaxial tensile loading.
2.2. Methods of characterization 2.2.1. The morphology and structure of SMER composite foams Scanning electron microscopy (SEM, VEGA3 TESCAN) was employed to characterize the microstructure deformation before and after the foams auxetic conversion. As a result of poor conductivity of sample, it was coated with Au prepared by sputter coating (KYKY2800 B, KYKY Technology Development Ltd.) for SEM analysis.
3.3. Static mechanical analysis of conventional shape memory composite foams
2.2.2. The mechanical properties characterization The mechanical performances of the conventional and auxetic foams were quantitatively examined by ZWICK-Z010 (ZWICK Roell) at variable temperature (20 °C (room temperature), 35 °C, 50 °C, 65 °C, 75 °C, 85 °C, 100 °C, respectively). The program was set to add 0.1 N preload to ensure ideal contact between the machine and the sample and ran at a speed of 3 mm/min. The Poisson's ratio of foam was
Fig. 3 shows the stress-strain curves of shape memory composite foams (initial shape, Poisson's ratio > 0) with different shape memory polymer contents (Er:PU = 0.5:1; 0.8:1; 1:1; respectively) at room temperature. Compared with non-memory foams, it presents similar three regimes of the stress-strain compression curves for the quasi-static 2
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Fig. 1. SEM micrographs of shape memory composite foam (Poisson's ratio > 0) with different Er fraction Er:PU = 0.5:1 (a), Er:PU = 0.8:1 (b) and Er:PU = 1:1 (c); foam diameter versus proportion of Er:PU. Pore diameter was the average cell diameter in mm measured by ImageJ software from the SEM micrographies of foams.
stress-strain curve shows nearly linear increase during the compression process. In the later period of compression, the stress-strain curve has a few fluctuating points, which caused by foam structural instability due to its compression failure. The highest compressive strain is much lower than others, because of its high density leading to no more space for compacting. In contrast, only specimen 5# does not exhibit the linearity response of strain-stress curve for tensile loading, shown in Fig. 4(b). Its tensile stress rapidly reaches to platform (from 0.028 to 0.032 MPa) at very low strain (< 0.02) for porous foam structure, and when the strain > 2.2, the tensile stress rapidly increases again. It can be explained that specimen 5# endures the excessive compressed volume in the transformed foam result in more cells damage or adjacent ribs mechanical entanglement, which limits foam cells expansion within low strain in tensile loading, and when entanglement ribs are unbuckled, the tensile stress regains rapidly increase. This is consistent with our observation that the excessive compressed volume in a certain range for conversion negative Poisson's ratio foam is in favor of auxetic behavior. Comparing with auxetic foam, conventional foam exhibits relative lower elastic strain as we expect. It is attributed to cell ribs without buckling to restrict foam extension.
compression test: elastic compress, cell wall bending and densification. Fig. 3 presents compressive modulus and yield stress as a functional of temperature for compressive tests. The compressive modulus and yield stress of all specimens decrease as the temperature increase for varying Er component. At room temperature, specimens with the lowest Er fraction (30 wt%) display the highest compressive modulus and Yield stress, while the lowest compressive modulus and Yield stress has been achieved by specimen with the highest Er fraction (50 wt%). Generally, elastic property of foam is mainly associated with the synergetic effect of elastic property of cell wall and foam diameter. Er fraction increases in foam, which could efficiently increase the rigidity of cell wall and foam diameter. At room temperature, it is obviously that foam diameter play a predominant role in determining its foam mechanical property. As foam temperature increase, elastic property of PU component decease dramatically (Tg PU < Tg Er). That is why foam young's modulus was rapidly decreased with increasing of temperature for the foam with the highest PU fraction (50 wt%). In this case, the rigidity of cell wall play a predominant role in foam mechanical property compared with foam diameter at relatively higher temperature. 3.4. Static mechanical analysis of shape memory composite auxetic foam
3.4.2. Poisson's ratio analysis In order to accurate evaluate Poisson's ratio of specimen, all specimens are divided into 100 equal parts and each individual part is selected as a feature point to observe its displacement variation. Among of them, 80 feature points have been taken the average as Poisson's ratio of foam. In this case, 20 feature points at two ends of the foam were not considered, because the ends of the foam were stick on the clamps, which constrain their displacement under compression or tension loading. Fig. 5(c) shows the relationship of poisson's ratio and compression volume ratio of initial conventional foam. Auxetic foam has been successfully fabricated and negative Poisson's ratio is in the range of −0.23 to −0.43 in response to compression volume ratio from 0.68 to 0.89.
3.4.1. Compressive and tensile modulus analysis Fig. 4 shows the comparison of the foam stress-strain curves with varying compression ratio during foam conversion process. The specimens carried out at 3 mm/min for compressive tests, and very different stiffness behaviors can be observed between the samples. All specimens follow the same general trend, which the slope of specimens increases with the increase of compression volume ratio from 0 to 0.89 in both tension and compression. On the whole, auxetic foams show a stiffness increase of one order of magnitude compared with the conventional foam (before auxetic conversion) during compacting process, Fig. 4(c). The reasonable results have been obtained that the larger the volume fraction is compressed, the higher the compression modulus for transformed auxetic foam. In Fig. 4 (a), take specimen 5# for example, 3
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Fig. 2. SEM images of: (a) shape memory auxetic composite foam at macroscale; (b) near surface region and (c) centre region in radial direction; (d) in longitudinal direction region of foam; (e) and (f) deform cell wall.
Poisson's ratio of all specimens exhibits nonlinear response during tensile process. For conventional foam, accompanied with increasing of tensile strain, Poisson's ratio increases initially and then decreases. In contrast, for negative Poisson's ratio foam, auxetic behavior first increases then decreases with the increase of strain. The negative Poisson's ratio of auxetic foam increases with the increase of compressive volume ratio, except for 5# (0.89). The reason of specimen 5# with relatively lower auxetic behavior has been explained in the above paragraphs. For each auxetic specimen, the maximum value of Poisson's ratio has been achieved when it is stretched to the vicinity of its initial length, shown in Fig. 5 (e). This may be explained that the change in the
The reasonable result has been obtained that the larger the compressed initial foam volume, the more clear the auxetic behavior, except for the foam with compression volume ratio of 0.89. The maximum negative Poisson's ratio (−0.43 ± 0.05) is achieved at the compression volume ratio of 0.79. It may be caused by the excessive compressed volume (compression volume ratio > 0.79) of conventional SMP foam leading to more cells damage or adjacent ribs mechanical entanglement during the foam conversion process, which restrict the lateral expansion of auxetic foam cell under an uniaxial loading. Fig. 5(d) shows the behavior of the Poisson's ratio of conventional and auxetic foams in relation to changes under uniaxial tensile loading.
Fig. 3. Stress-strain compressive behavior (a) and Yield stress (b) of shape memory composite foams versus temperature. 4
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Fig. 4. Stress-strain compressive (a) and tensile (b) behavior of auxetic and conventional foams. (c) Map of the modulus versus compression volume ratio resulted from the compressive and tensile loading related to the conventional and auxetic foam.
Fig. 5. Pictures of conventional (a) and auxetic (b) foam under tensile loading at different strain and MATLAB treatment picture for Poisson's ratio analysis. (c) Map of the Poisson's ratio versus the compression volume ratio. (d) Poisson's ratio of the conventional and auxetic foams versus tensile strain and (e) the ratio of the length of the auxetic foam with maximum negative Poisson's ratio after stretching to the length of the original foam. 5
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Fig. 6. Shape fixation ratio versus time resulted from 50% (a) and 85% (b) compression of shape memory composite foam. (c) Shape recovery ratio versus proportion Er:PU.
process of triaxial compression with heat treatment. The effect of processing parameters on foam Poisson's ratio was analyzed. NPR increased with the increase of compression volume ratio. These values ranged from −0.23 to −0.43 in response to compression volume ratio from 0.68 to 0.79. Effective compressive and tensile modulus was obtained by compression and tension tests, which increased with the increase of compression volume ratio. Auxetic foams showed a stiffness increase of one order of magnitude compared with the conventional foam. Shape fixation and recovery ratio increased as the fraction of shape memory resin increased. The excellent shape recovery ratio was more than 99.8% at 1:1 (Er:PU).
angulations of the cell ribs as the structure is stretched or compressed. 3.5. Fixation rate and thermal actuation with temperature sensing Shape fixation ratio and shape recovery ratio of SMP foams versus shape memory polymer contents (Er:PU = 0.5:1; 0.8:1; 1:1; respectively) are presented in Fig. 6(a)-(c), respectively. As we expected that shape fixation ratio and shape recovery ratio increases as the shape memory polymer content increases. The maximum shape recovery ratio is more than 99.8% at 1:1 (Er:PU). For shape fixation ratio, all specimens present similar trends that at first they decrease rapidly and then change to be steady, with increasing of time. Compared with shape recovery behavior, they exhibit relatively low shape fixation ratio observed after 1 h (0.5:1 < 0.8:1 < 1:1). Among of them, the specimen with Er:PU = 1:1 achieves the highest fixation ratio is around 80% after 1 h. In composite foam, the main function of PU content is to build the foam structure, while Er is mainly contributed to its shape memory function. Thus, the shape memory polymer content plays the dominant role in fixing the shape of foam during deformation process. The higher recovery ratio is attributed to the good elastic performance of PU content.
Acknowledgements This research is supported by “National Natural Science Foundation of China” (Nos.11772108, 11572102). Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx. doi.org/10.1016/j.compositesb.2018.06.027. References
4. Conclusion
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In summary, this paper has proposed a feasible fabrication method for making shape memory composite foam with auxetic behavior, based on the commercial soft polyurethane foam material as matrix and shape memory epoxy resin as functional phase. Auxetic configuration conversion was achieved based on its shape memory feature through a 6
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[19] Rodriguez JN, Yu YJ, Miller MW, Wilson TS, Hartman J, Clubb FJ, Gentry B, Maitland DJ. Opacification of shape memory polymer foam designed for treatment of intracranial aneurysms. Ann Biomed Eng 2012;40:883–97. [20] Ortega JM, Hartman J, Rodriguez JN, Maitland DJ. Virtual treatment of basilar aneurysms using shape memory polymer foam. Ann Biomed Eng 2013;41:725–43. [21] Yao YT, Zhou TY, Qin C, Liu YJ, Leng JS. Styrene-based shape memory foam: fabrication and mathematical modeling. Smart Mater Struct 2016;25:105031. [22] Yao YT, Zhou TY, Yang C, Liu YJ, Leng JS. Preparation and characterization of shape memory composite foams with interpenetrating polymer networks. Smart Mater Struct 2016;25:035002. [23] Hassana MR, Scarpa F, Ruzzene M, Mohammed NA. Smart shape memory alloy chiral honeycomb. Mater Sci Eng 2008;481–482:654–7. [24] Sokolowski W, Tan S, Pryor M. Light weight shape memory self-deployable structures for gossamer applications. AIAA 2004:19–22. [25] Sokolowski W, Tan S, Willis P, Pryor M. Shape memory self-deployable structures for solar sails. Proc SPIE 2008;7267:1–14. [26] Li GQ, ASCE M, Xu T. Thermomechanical characterization of shape memory polymer-based self-healing syntactic foam sealant for expansion joints. J Transp Eng-asce 2011;137:805–14. [27] Sokolowski WM, Tan SC. Advanced self-deployable structures for space applications. J Spacecraft Rockets 2007;44:750–4. [28] Howell B, Pendergast P, Hansen L. Acoustic Behaviour of Negative Poisson’s Ratio Materials Research and Development Report DTRC-SME-91/01. Annapolis, M U S: David Taylor Research Centre; 1991. [29] Choi JB, Lakes RS. Non-linear properties of polymer cellular materials with a negative Poisson’s ratio. J Mater Sci 1992;27:4678–84. [30] Choi JB, Lakes RS. Fracture toughness of re-entrant foam materials with a negative Poisson’s ratio: experiment and analysis. Int J Fract 1996;80:73–83. [31] Lakes R. Foam structures with a negative Poisson’s ratio. Science 1987;235:1038–40. [32] Scarpa F, Yates JR, Ciffo LG. Dynamic crushing of auxetic open-cell polyurethane foam. J Mech Sci 2002;216:1153–6. [33] Pastorino P, Scarpa F, Patsias S, Yates JR, Haake SJ, Ruzzene M. Strain rate dependence of stiffness and Poisson's ratio of auxetic open cell PU foams. Phys Status Solidi B 2007;244:955–65. [34] McDonald S, Dedreuil-Monet G, Yao YT, Alderson A, Withers P. In situ 3D X-ray microtomography study comparing auxetic and non-auxetic polymeric foams under tension. Phys Status Solidi B 2011;248:45–51. [35] Leng JS, Wu XL, Liu YJ. Effect of a linear monomer on the thermomechanical properties of epoxy shape memory polymer. Smart Mater Struct 2009;18:095031. [36] Scarpa F, Pastorino P, Garelli A, Patsias S, Ruzzene M. Auxetic compliant flexible PU foams: static and dynamic properties. Phys Status Solidi B 2005;242:681–94.
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7
Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Fabrication and characterization of auxetic shape memory composite foams
T
∗
Yongtao Yao, Yun Luo, Yuncheng Xu, Bing Wang, Jinyang Li, Han Deng, Haibao Lu
National Key Laboratory of Science and Technology on Advanced Composites in Special Environments, Harbin Institute of Technology, Harbin, 150080, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Auxetic foam Shape memory effect Negative Poisson's ratio
Shape memory polymers, as a kind of smart materials, play an important role in more and more fields, such as aerospace, biomedicine and intelligent clothing field and so on. Comparing with traditional materials, negative Poisson's ratio foam has excellent mechanical properties, such as double curvature, light weight, high shear resistance, auxetic etc., and it has a great potential application in the field of aerospace. Therefore, in this project, shape memory composite foam was fabricated based on the commercial soft polyurethane foam material as matrix and shape memory epoxy resin as functional phase. Negative Poisson's ratio of foam was fabricated based on its shape memory feature through a process of triaxial compression with heat treatment. Microstructure deformation was characterized after the transformation of auxetic foam. By adjusting the processing parameters, the auxetic shape memory foam with different “re-entrant” structure was obtained. Such fabricated shape memory composite foams display variable stiffness with auxetic behavior. Effective compressive and tensile modulus was obtained by compression and tension tests. The effect of processing parameters on foam Poisson's ratio was analyzed, which provided certain guiding significance for the further shape memory foam preparation.
1. Introduction Shape memory polymers (SMPs) are known as a group of smart materials, which have the ability to return to their permanent shapes upon simulation such as the change of temperature [1,2], electrical current [3–5], alternating magnetic field [6,7], light exposure [8–10], microwave [11,12], and water immersion [13,14]. In addition to the abovementioned special property of SMPs, they also have manifold stimulation sources, easy manufacture and programming and cheap, which make them have tremendous applications in multifarious fields such as aerospace engineering [15,16], textiles [17,18], biomedical engineering [19,20]. These broad applications of SMPs pushed the development of SMPs design. Recently, more and more cellular solids with shape memory behavior have been manufactured in foam [21,22] and honeycomb [23] structures. They have attracted many researchers' attention due to high deformation ability and porous feature. For example, SMP foams can serve as self-deployable structures in aerospace engineering such as the wheel for exploration rovers [24], solar sails [25] and self-healing structures [26] or absorbing impact energy [27]. Auxetic materials are known as one of the classical deformed structures, which have negative Poisson's ratio opposite to the ones belonging to conventional materials, with an unusually expansion or contraction under a tensile or compressive stress. Comparing with conventional foams, mechanical properties of ∗
Corresponding author. E-mail address: [email protected] (H. Lu).
https://doi.org/10.1016/j.compositesb.2018.06.027 Received 30 May 2018; Accepted 26 June 2018 Available online 28 June 2018 1359-8368/ © 2018 Elsevier Ltd. All rights reserved.
auxetic foams can be enhanced as a consequence of its negative Poisson's ratio property, for example, acoustic absorption [28], shear resistance [29], fracture toughness and synclastic curvature [30]. The first auxetic foam was fabricated by Lakes from commercially available open-cell polyurethane foam [31]. Scarpa et al. developed a new fabrication method to achieve auxetic foam with improved stiffness and high resilience [32,33] based on open cell thermoplastic foam. Generally, the foam was converted with auxetic behavior based on rebuild microstructure according to its thermoplastic property and rebuild reentrant cells were achieved from a quenching process with triaxial compression in a mould. The re-entrant cells unfold under tension giving rise to the negative Poisson's ratio. Thus, the rebuild structure with “re-entrant” configuration was considered to be the main reason for the negative Poisson's ratio phenomenon. In order to explain the experimentally measured values of the Poisson's ratios in terms of the microstructure of the foams, various numerical models that represent the auxetic foams have been proposed. For example, Grima et al. proposed two-dimensional model to simulate auxetic foam based on “rotation of rigid units”. McDonald et al. [34] constructed the microstructurally faithful finite element models based on 3-dimensional microtomography of auxetic foam to investigate its deformation mechanism. As we all know, the mechanical properties of materials are mainly related to their microstructure, thus, they could be controlled by controlling the microstructure of the material. Combining SMP and auxetic
Composites Part B 152 (2018) 1–7
Y. Yao et al.
characterized by the digital image correlation (XTDIC, Xi'an XinTuo 3D Optical Measurement Technology Co., Ltd) for compressive and tensile loading.
structure is a feasible way to build a smart structure with controllable and extensive morphing ability. In this case, auxetic structure could fully exerted excellent mechanical performance and deformability, while SMP could remember its deformed structure and help to extend the variability of the structure. In this study, shape memory composite foam will be fabricated with auxetic behavior. Shape memory epoxy and commercial polyurethane were employed to manufacture SMP foam with positive Poisson ratio as permanent shape. The auxetic conversion is mainly based on a process of triaxial compression with heat treatment. Relative mechanical property will be investigated in term of the foam microstructure. The relationship between processing parameter and auxetic behavior will be studied. This work hopes to provide a new idea for the preparation of smart and controllable large morphing structures.
2.2.3. Shape memory behavior tests For shape recovery tests, to avoid any possible damage in testing at a low temperature, all specimens (Er:PU = 0.5:1; 0.8:1; 1:1; respectively) compressed 50% and 85%, respectively, at Tg+20 °C. The strain of compressed specimens was kept until temperature dropped to room temperature. Finally, the foams expand at Tg+20 °C for 30 min and the height after shape recovery was tested. For Shape fixing tests, specimens are to keep the temporal shape for a long time. The samples were compressed 50% and 85% at Tg+20 °C, and that height was kept until the temperature cooled down. The height of each specimens was measured every 5 min.
2. Experiment 3. Results and discussion 2.1. Fabrication of SMP composite foam with IPN network 3.1. The morphology and structure of SMER composite foams 2.1.1. Materials In these experiments, the shape memory epoxy resin was developed in our lab according to previous report [35]. Commercially available polyurethane foam raw materials with two ingredients, A (polyester polyol, surfactant, etc.) and B (isocyanate etc.), were supplied by Shanghaishengju construction materials LTD. All chemicals involved in preparing shape memory composite foams were used as received.
In this study, scanning electron microscopy was conducted to investigate the effect of proportioning of Er on the morphology of PU/Er foam. Fig. 1 shows the results of SEM on PU/Er foam with different proportioning of Er (Er:PU = 0.5:1; 0.8:1; 1:1; respectively). It can be seen that spherical open cells structure with uniform distribution in PU/ Er foam have been obtained. The cell wall surfaces are smooth and there is no separated phase structure for those PU/Er foams with different contents. The single phase is observed for mixing PU and Er components to form interpenetrating network foams. Fig. 1(d) shows the relationship between the average diameter of open cells and Er contents. The range of diameter distribution is slightly increasing with increase of contents of the Er. As result of the foam network is mainly created based on the PU component, and the increase of the addition amount of Er is leading to the larger numbers of polyol hydroxyl groups which will speed up PU foaming rate to release more air attributing to an increase in the size of foam pore.
2.1.2. Preparation of auxetic foams Firstly, SMP PU/Er foams were fabricated based on the one-shot method. PU component provided foam structure and shape memory polymer served as shape fixing and recovering functionality. The shape memory epoxy resin (Er) solution with different weight and PU (Er:PU (A + B) = 0.5:1; 0.8:1; 1:1, respectively) were mixed for 10 S at 1000 rpm. The mixtures were subsequently poured into an open cylindrical mould to produce free-rise foams for 24 h. Then, such foams were postcured at 150 °C for 5 h in an oven in order to make shape memory Er ingredients completely cured. Secondly, the structure of SMP PU/Er foam was converted into auxetic form and its conversion process applied to the foam was similar to that of Ref. [36]. Initial foam with a cylinder structure was inserted into a smaller steel tube mould at room temperature under a biaxial compression. End tabs were employed to ensure compressive strain was applied along the mould axis. Then, the mould was placed in an oven at a temperature of 150 °C for 30 min. The auxetic foam was removed when the mould was cooled to room temperature. In this project, a series of foams with different volumetric compression ratios (0.68, 0.71, 0.78, 0.79 and 0.89, respectively) have been produced in order to achieve different auxetic properties and optimize the processing parameters.
3.2. The morphology and structure of auxetic foams Fig. 2 indicates the structure of transformed foam observed by SEM. There are three regions of foam have been chosen to evaluate their microstructure transformation: in radial direction centre region (Fig. 2(c)), near surface region (Fig. 2(b)) and longitudinal direction sheath region (Fig. 2(d)). It can be seen from Fig. 2 that cell structure transformed from open cell structure to analogical “re-entrant” structure under a biaxial compression. There is no apparent difference for microstructure in three regions of foam. That means this microstructure uniform transformed induced by a biaxial compression. Comparing with initial foam structure, auxetic foam present disordered and convoluted unit cells, with complex rib geometry (Fig. 2(e and f)). Some unit cell wall has broken ribs by a reduction of foam cell size. In contrast with former researches [34], the similar conversion results has been found that the auxetic deformation occurs primarily by the introduction of ‘kinks’ at the centres of the ribs as a result of extensive buckling (Fig. 2(e and f)). Auxetic phenomenon has been achieved mainly related to the deploying of these ‘kinks’ or bent ribs in response to uniaxial tensile loading.
2.2. Methods of characterization 2.2.1. The morphology and structure of SMER composite foams Scanning electron microscopy (SEM, VEGA3 TESCAN) was employed to characterize the microstructure deformation before and after the foams auxetic conversion. As a result of poor conductivity of sample, it was coated with Au prepared by sputter coating (KYKY2800 B, KYKY Technology Development Ltd.) for SEM analysis.
3.3. Static mechanical analysis of conventional shape memory composite foams
2.2.2. The mechanical properties characterization The mechanical performances of the conventional and auxetic foams were quantitatively examined by ZWICK-Z010 (ZWICK Roell) at variable temperature (20 °C (room temperature), 35 °C, 50 °C, 65 °C, 75 °C, 85 °C, 100 °C, respectively). The program was set to add 0.1 N preload to ensure ideal contact between the machine and the sample and ran at a speed of 3 mm/min. The Poisson's ratio of foam was
Fig. 3 shows the stress-strain curves of shape memory composite foams (initial shape, Poisson's ratio > 0) with different shape memory polymer contents (Er:PU = 0.5:1; 0.8:1; 1:1; respectively) at room temperature. Compared with non-memory foams, it presents similar three regimes of the stress-strain compression curves for the quasi-static 2
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Fig. 1. SEM micrographs of shape memory composite foam (Poisson's ratio > 0) with different Er fraction Er:PU = 0.5:1 (a), Er:PU = 0.8:1 (b) and Er:PU = 1:1 (c); foam diameter versus proportion of Er:PU. Pore diameter was the average cell diameter in mm measured by ImageJ software from the SEM micrographies of foams.
stress-strain curve shows nearly linear increase during the compression process. In the later period of compression, the stress-strain curve has a few fluctuating points, which caused by foam structural instability due to its compression failure. The highest compressive strain is much lower than others, because of its high density leading to no more space for compacting. In contrast, only specimen 5# does not exhibit the linearity response of strain-stress curve for tensile loading, shown in Fig. 4(b). Its tensile stress rapidly reaches to platform (from 0.028 to 0.032 MPa) at very low strain (< 0.02) for porous foam structure, and when the strain > 2.2, the tensile stress rapidly increases again. It can be explained that specimen 5# endures the excessive compressed volume in the transformed foam result in more cells damage or adjacent ribs mechanical entanglement, which limits foam cells expansion within low strain in tensile loading, and when entanglement ribs are unbuckled, the tensile stress regains rapidly increase. This is consistent with our observation that the excessive compressed volume in a certain range for conversion negative Poisson's ratio foam is in favor of auxetic behavior. Comparing with auxetic foam, conventional foam exhibits relative lower elastic strain as we expect. It is attributed to cell ribs without buckling to restrict foam extension.
compression test: elastic compress, cell wall bending and densification. Fig. 3 presents compressive modulus and yield stress as a functional of temperature for compressive tests. The compressive modulus and yield stress of all specimens decrease as the temperature increase for varying Er component. At room temperature, specimens with the lowest Er fraction (30 wt%) display the highest compressive modulus and Yield stress, while the lowest compressive modulus and Yield stress has been achieved by specimen with the highest Er fraction (50 wt%). Generally, elastic property of foam is mainly associated with the synergetic effect of elastic property of cell wall and foam diameter. Er fraction increases in foam, which could efficiently increase the rigidity of cell wall and foam diameter. At room temperature, it is obviously that foam diameter play a predominant role in determining its foam mechanical property. As foam temperature increase, elastic property of PU component decease dramatically (Tg PU < Tg Er). That is why foam young's modulus was rapidly decreased with increasing of temperature for the foam with the highest PU fraction (50 wt%). In this case, the rigidity of cell wall play a predominant role in foam mechanical property compared with foam diameter at relatively higher temperature. 3.4. Static mechanical analysis of shape memory composite auxetic foam
3.4.2. Poisson's ratio analysis In order to accurate evaluate Poisson's ratio of specimen, all specimens are divided into 100 equal parts and each individual part is selected as a feature point to observe its displacement variation. Among of them, 80 feature points have been taken the average as Poisson's ratio of foam. In this case, 20 feature points at two ends of the foam were not considered, because the ends of the foam were stick on the clamps, which constrain their displacement under compression or tension loading. Fig. 5(c) shows the relationship of poisson's ratio and compression volume ratio of initial conventional foam. Auxetic foam has been successfully fabricated and negative Poisson's ratio is in the range of −0.23 to −0.43 in response to compression volume ratio from 0.68 to 0.89.
3.4.1. Compressive and tensile modulus analysis Fig. 4 shows the comparison of the foam stress-strain curves with varying compression ratio during foam conversion process. The specimens carried out at 3 mm/min for compressive tests, and very different stiffness behaviors can be observed between the samples. All specimens follow the same general trend, which the slope of specimens increases with the increase of compression volume ratio from 0 to 0.89 in both tension and compression. On the whole, auxetic foams show a stiffness increase of one order of magnitude compared with the conventional foam (before auxetic conversion) during compacting process, Fig. 4(c). The reasonable results have been obtained that the larger the volume fraction is compressed, the higher the compression modulus for transformed auxetic foam. In Fig. 4 (a), take specimen 5# for example, 3
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Fig. 2. SEM images of: (a) shape memory auxetic composite foam at macroscale; (b) near surface region and (c) centre region in radial direction; (d) in longitudinal direction region of foam; (e) and (f) deform cell wall.
Poisson's ratio of all specimens exhibits nonlinear response during tensile process. For conventional foam, accompanied with increasing of tensile strain, Poisson's ratio increases initially and then decreases. In contrast, for negative Poisson's ratio foam, auxetic behavior first increases then decreases with the increase of strain. The negative Poisson's ratio of auxetic foam increases with the increase of compressive volume ratio, except for 5# (0.89). The reason of specimen 5# with relatively lower auxetic behavior has been explained in the above paragraphs. For each auxetic specimen, the maximum value of Poisson's ratio has been achieved when it is stretched to the vicinity of its initial length, shown in Fig. 5 (e). This may be explained that the change in the
The reasonable result has been obtained that the larger the compressed initial foam volume, the more clear the auxetic behavior, except for the foam with compression volume ratio of 0.89. The maximum negative Poisson's ratio (−0.43 ± 0.05) is achieved at the compression volume ratio of 0.79. It may be caused by the excessive compressed volume (compression volume ratio > 0.79) of conventional SMP foam leading to more cells damage or adjacent ribs mechanical entanglement during the foam conversion process, which restrict the lateral expansion of auxetic foam cell under an uniaxial loading. Fig. 5(d) shows the behavior of the Poisson's ratio of conventional and auxetic foams in relation to changes under uniaxial tensile loading.
Fig. 3. Stress-strain compressive behavior (a) and Yield stress (b) of shape memory composite foams versus temperature. 4
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Fig. 4. Stress-strain compressive (a) and tensile (b) behavior of auxetic and conventional foams. (c) Map of the modulus versus compression volume ratio resulted from the compressive and tensile loading related to the conventional and auxetic foam.
Fig. 5. Pictures of conventional (a) and auxetic (b) foam under tensile loading at different strain and MATLAB treatment picture for Poisson's ratio analysis. (c) Map of the Poisson's ratio versus the compression volume ratio. (d) Poisson's ratio of the conventional and auxetic foams versus tensile strain and (e) the ratio of the length of the auxetic foam with maximum negative Poisson's ratio after stretching to the length of the original foam. 5
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Fig. 6. Shape fixation ratio versus time resulted from 50% (a) and 85% (b) compression of shape memory composite foam. (c) Shape recovery ratio versus proportion Er:PU.
process of triaxial compression with heat treatment. The effect of processing parameters on foam Poisson's ratio was analyzed. NPR increased with the increase of compression volume ratio. These values ranged from −0.23 to −0.43 in response to compression volume ratio from 0.68 to 0.79. Effective compressive and tensile modulus was obtained by compression and tension tests, which increased with the increase of compression volume ratio. Auxetic foams showed a stiffness increase of one order of magnitude compared with the conventional foam. Shape fixation and recovery ratio increased as the fraction of shape memory resin increased. The excellent shape recovery ratio was more than 99.8% at 1:1 (Er:PU).
angulations of the cell ribs as the structure is stretched or compressed. 3.5. Fixation rate and thermal actuation with temperature sensing Shape fixation ratio and shape recovery ratio of SMP foams versus shape memory polymer contents (Er:PU = 0.5:1; 0.8:1; 1:1; respectively) are presented in Fig. 6(a)-(c), respectively. As we expected that shape fixation ratio and shape recovery ratio increases as the shape memory polymer content increases. The maximum shape recovery ratio is more than 99.8% at 1:1 (Er:PU). For shape fixation ratio, all specimens present similar trends that at first they decrease rapidly and then change to be steady, with increasing of time. Compared with shape recovery behavior, they exhibit relatively low shape fixation ratio observed after 1 h (0.5:1 < 0.8:1 < 1:1). Among of them, the specimen with Er:PU = 1:1 achieves the highest fixation ratio is around 80% after 1 h. In composite foam, the main function of PU content is to build the foam structure, while Er is mainly contributed to its shape memory function. Thus, the shape memory polymer content plays the dominant role in fixing the shape of foam during deformation process. The higher recovery ratio is attributed to the good elastic performance of PU content.
Acknowledgements This research is supported by “National Natural Science Foundation of China” (Nos.11772108, 11572102). Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx. doi.org/10.1016/j.compositesb.2018.06.027. References
4. Conclusion
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In summary, this paper has proposed a feasible fabrication method for making shape memory composite foam with auxetic behavior, based on the commercial soft polyurethane foam material as matrix and shape memory epoxy resin as functional phase. Auxetic configuration conversion was achieved based on its shape memory feature through a 6
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