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Carbon Nano Fibers Reinforced Composites Origami Inspired Mechanical Metamaterials

with Passive and Active Properties

Mohamed Ali E. Kshad, Clement D’Hondt, and Hani E. Naguib

Version Post-print/accepted manuscript

Citation

(published version)

Kshad, M. A. E., D’Hondt, C., & Naguib, H. E. (2017). Carbon nano

fibers reinforced composites origami inspired mechanical metamaterials with passive and active properties. Smart Materials and Structures,

26(10), 105039.

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accepted for publication/published in Smart Materials and Structures. IOP Publishing Ltd is not responsible for any errors or

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10.1088/1361-665X/aa8832.

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Carbon Nano Fibers Reinforced Composites Origami Inspired Mechanical

Metamaterials with Passive and Active Properties

Mohamed Ali E. Kshad1, Clement D’Hondt1, and Hani E. Naguib* 1,2,3

1 Department of Mechanical and Industrial Engineering, 5 King’s College Road, Toronto, ON,

M5S 3G8, Canada, University of Toronto, Canada 2 Department of Materials Science and Engineering, University of Toronto, Canada

3 Institute of Biomaterials and Biomedical Engineering, University of Toronto, Canada

* Corresponding author: naguib@mie.utoronto.ca

Abstract

Core panels used for compression or impact damping are designed to dissipate energy and to reduce the

transferred force and energy. They are designed to have high strain and deformation with low density.

The geometrical configuration of such cores plays a significant role in redistributing the applied forces to

dampen the compression and impact energy. Origami structures are renowned for affording large

macroscopic deformation which can be employed for force redistribution and energy damping. The

material selection for the fabrication of origami structures affects the core capacity to withstand

compression and impact loads.

Polymers are characterized by their high compression and impact resistance; the drawback of polymers is

the low stiffness and elastic moduli compared with metallic materials. This work is focused on the study

of the effect of Carbon Nano Fibers (CNF) on the global mechanical properties of the origami panel cores

made of polymeric blends. The base matrix materials used were Polylactic Acid (PLA) and Thermoplastic

Polyurethane (TPU) blends, and the percentages of the PLA/TPU were 100/0, 20/80, 65/35, 50/50, 20/80,

and 0/100 as a percentage of weight. The weight percentages of CNF added to the polymeric blends were

1%, 3%, and 5%. This paper deals with the fabrication process of the polymeric reinforced blends and the

origami cores, in order to predict the best fabrication conditions. The dynamic scanning calorimetry and

the dynamic mechanical analyzer were used to test the reinforced blended base material for

thermomechanical and viscoelastic properties.

The origami core samples were fabricated using per-molded geometrical features and then tested for

compression and impact properties. The results of the study were compared with previous published

results which showed that there is considerable enhancement in the mechanical properties of the origami

cores compared with the pure blended polymeric origami cores. The active properties of the origami unit

cell made of composite polymers containing a low percentage of CNF were also investigated in this

study, in which the shape memory effect test conducted on the origami unit cell.

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1. INTRODUCTION

Presently, mechanical metamaterials; materials that gain their properties from their structure rather than

from the base material composition, have received increasing interest in research due to their superior

properties that can be exploited for designing novel materials with high-end functionalities composition

[1-3]. These materials have enormous potential use for sandwich and lightweight structures in aerospace

industry and other applications, because of their attractive properties such as negative Poisson’s ratio (i.e.

auxetic materials), negative compressibility, vanishing shear modulus, etc. [1–3]. Origami-inspired

metamaterials are auxetic meta-materials, in which the material is arranged in certain patterns creating

3D-tessellation. These auxetic properties have enabled the design of new structural materials with

superior properties based on the arrangement of the structural elements [4–6]. Origami based

metamaterials, inspired by the Art of folding papers, have received particular interest for engineering

applications, such as foldable solar panels, medical stents, and sandwich core applications [7,8]. The

design of the pattern of the origami tessellations affects the auxetic properties of the resultant 3D cores

[2]. Indeed, the creation of rigidly folded 3D geometric tessellations from 2D sheets, in which, rigid faces

joined by hinges, shows a great promise for those applications by offering good mechanical properties for

low-weight and low amount of used material [9,10].

Apart from these geometric tessellations is the Miura origami structure, which was first presented by

Miura in the 1970s for sandwich core applications and solar panel deployment in space [7,8]. Miura

origami is one of the origami patterns that attracted attention due to its geometric simplicity, its wide

range of possible configurations, and its intriguing mechanical properties, which lead to great elastic

energy absorption and force redistribution. [7, 8, 11, 12]. The mechanical properties of fold core

structures and origami cores have got wide attention in the literature, Kayumov et al. [13] introduced a

mathematical model to describe the behavior of chevron type sandwich panel cores. The study of the

impact of Z-crimp structure parameters on the strength under different loading conditions were

investigated in [14], Xiang Zhou et. al. studied the mechanical properties of Miura folded cores. Heimbs

et al. [15] investigated the behavior of the sandwich made of textile-reinforced composite. Based on

Miura origami pattern, You et al.[16] examined morphing sandwich mechanisms. Numerous of research

has been done in the investigating the mathematical and geometrical parameters of foldable core

structures [17–20]. Active origami structures also get interest in the resent years, in which active materials

can be employed for folding and unfolding process [21–24].

The aim of this study is to investigate the effect of CNF on the mechanical properties of origami

structures made of multiphase polymeric blends by using pre-molded process. The material blends used

were PLA and TPU, the weight percentages of the compositions used were 100/0, 20/80, 65/35, 50/50,

20/80, and 0/100, and the reinforcement weight percentages of CNF were 1%, 3%, and 5%. The

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polymeric blends were compounded with the CNF, and then the compounded compositions were used for

fabricating the origami structures, by using compression molding.

The compression test was conducted for the fabricated cores to measure the compression and the strength

moduli of the origami cores; also, a drop mass impact test was performed to predict the transferred force

and the damped energy by the origami cores. The results demonstrated that the compression moduli

increased as a result of the increase in the CNF content; these values showed that there is large

enhancement in the moduli of compression and strength of the composite origami cores, compared with

the cores made of pure blends; while the amount of damped energy during the impact event was reduced

by the increase in the CNF percentage.

The folding/unfolding operation of origami structure is important in some origami-based applications to

be externally produced [21]. At very small, large and in remote applications, self-folding origami made of

active materials that convert various forms of energy to mechanical work, have proven their usefulness

use in many of these potential applications [25–30]. Shape memory polymers (SMP) are active materials

that have high shape memory recoverability, that can be employed in many active applications

[11,21,25,26,31,32]. This work also includes an investigation of active properties (shape memory effect)

of origami unit cell made of composite polymers. The shape memory effect test showed that there is high

recovery ratio in the PLA with low percentage of CNF samples.

2. EXPEREMENTAL WORK

2.1 MATERIALS

The goal of the use of the origami cores is to transform kinetic energy into elastic strain energy through

the elastic deformation of the core structure; therefore, the materials used to fabricate the origami

structures are the key to the global mechanical properties of the origami cores, for target applications. The

materials must be compliant to allow for the free motion of the structure. Also, since the cores should

withstand the bending loading, the material must allow the faces’ connections to deform elastically, while

also providing enough stiffness in the panels to resist bending. Therefore, two polymers were selected to

produce the material blends which are used as a matrix reinforced with Carbon Nano-Fibers (CNFs) as a

filler to enhance the mechanical properties of the material blends.

Polylactic Acid (PLA) grade (3052D) was obtained from Natureworks, LLC (USA), and Thermoplastic

polyurethane (TPU) grade (55D) was obtained from Lubrizol Engineering Polymers (USA).

PLA is a bio-based polymer which has advantageous mechanical properties, is also a bio-degradable

material, which requires low energy processability [33]. PLA is blended with (TPU) which is

characterized by strength, ductility, impact resistance and toughness [34]. Previous studies showed that

both polymers are compatible, and it is easy to compound the two material phases. Table 1, lists the

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physical properties [28], the tensile test results, and the DSC results of the base materials used in the

experimental work.

The Carbon Nanofibers CNFs used were Pyrograf, PR-19-XT-PS, with an average diameter of 150 nm,

which are available in their fiber form or as a master-batch composed of 85% of PLA and 15% of

nanofibers. The use of the master-batch is preferred for safety reasons, but the CNF fiber form can be

manipulated under fume hoods in a designated laboratory with safety equipments. The PLA/TPU weight

percentages used are similar to the previous study done for the pure blends [35], which were (100/0,

80/20, 65/35, 50/50, 20/80, 0/100), and the CNF weight percentages added to the blends were (1%, 3%,

and 5%). A twin screw micro-compounder (DSM; Geleen, Netherlands) (MICRO15) was used to produce

the material blends, by melting and mixing the pellets at a temperature range of (180 °C – 195°C) with

the screw speed of 30 rpm for 10 min; then the blends were extruded and pelletized.

Carbon Nanofibers are fillers used in polymers when multifunctional properties are needed [36,37]. In our

case, the properties that we need to improve are mainly the strength of the origami cores and the thermal

properties (for active properties).

Al-Saleh et al. [38,39] introduced multiple reviews on CNF/polymer composites’ properties that outline

the various improved properties, taking in the consideration the optimal method to prepare those

composites. The fibers need to be well dispersed and distributed in the polymer matrix to minimize the

stress concentration, and to improve the uniformity of the stress distribution. One important factor is the

fiber diameter, which improves the tensile properties due to a diminution of the number of defects, the

contact area between fillers and polymer increases, as well as the fiber flexibility. The flexibility allows

the fibers to keep their aspect ratio increasing the stress transfer, and enhancing its mechanical properties.

It has also been proven that the CNF increases the complex viscosity, storage modulus and loss modulus

of the matrix polymer. CNF could also change the polymer crystallinity and affect the transition

temperatures. Al-Saleh et al. [38,39] also considered how to blend polymers and CNF by melt-spinning to

get a good dispersion, and to maintain the aspect ratio of the fibers.

Table 1. The physical, mechanical and the thermal properties used to fabricate origami cores

Material Density (g/cm3) E (MPa) Melting Temperature Tg (C)

PLA 1.24 1060 145 - 160

TPU 1.16 34.0 ~ 181

2.2 MATERIAL BLENDS MICROSTRUCTURE

Small samples from the composite blends were frozen by using liquid nitrogen and fractured for imaging

the cross section. The samples were prepared and placed on stubs, and then they were coated with

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platinum ions using SC7620 Sputter Coater, Polaron, for 3 minutes. The microstructure images were

obtained by using scanning electron microscope micro-scope (JSM-6060, JEOL) (SEM).

The SEM images of the blended composites are illustrated in figure 1, in which figures a, b and c, show

the SEM images of PLA with 1, 3 and 5 wt.% of CNF respectively, it is clear that, the CNF fibers were

homogeneously distributed in the polymeric matrix. Figures 1 d, e, and f, show the SEM images of the

80/20 PLA/TPU with 5 wt.% CNF, 50/50 PLA/TPU with 5 wt.% CNF, and neat TPU with 5 wt.% CNF,

the images show that the CNF were uniformly distributed in matrix in both 80/20 and 50/50 PLA/TPU

composites, while there is slight CNF agglomeration in the case of neat TPU.

Figure 1. SEM micrographs of composite blends, magnification factor 20000X, a) PLA with 1 wt% CNF, b) PLA with 3wt% CNF, c) PLA with 5 wt% CNF, d) 80/20 PLA/PTU with 5 wt% CNF, e) 50/50

PLA/PTU with 5 wt% CNF, f) TPU with 5 wt% CNF.

2.3 FABRICATION PROCESS

To fabricate the origami core structures, a special compression mold was used; the mold was designed to

transfer the geometrical features of the origami structure, to the fabricated cores (see figure 2) [35]. The

pelletized material blends were uniformly spread in the mold to ensure the homogeneous distribution of

the material, and to avoid bubbles occurring in the samples; the mold was initially coated with silicon

based mold-release spray, in order to easily remove the samples. The upper and the lower parts of the

mold were set at a temperature of 20C above the melting temperature of the material blends for 30

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minutes, allowing the blended material to melt. Then, after the upper and lower temperatures were

stabilized, the mold was sealed using fiber reinforced rubber, and was closed without applying pressure

for 15 minutes; in this stage, the melted material flowed through the geometrical features of the origami.

In the next stage, a pressure of (3 - 3.5) tons was applied to the compression mold for 15 minutes; and

then finally, the pressure was released and the mold was immersed in a cold-water bath for the

recrystallization process. The melting temperatures of the material blends were experimentally measured

using DSC. Table 2 lists the fabrication parameters used for each material blend. In this process, it was

noticed that with the higher amount of CNF the material blends get more viscous and more material

leakage was observed; therefore, thicker sealing rubber was used to reduce the material leakage. The

values of the molding temperatures were taken to be about 20 °C above the melting range of the

composites, because the large mold size cannot allow the prediction of the temperature of the mold center.

Six samples from each composition were fabricated to be used for compression and impact tests. Figure 2

shows the fabricated origami core sample. For blended material compositions, a dog-bone shaped samples

and rectangular samples were produced according to ASTM 638 and ASTM D638 standards using

injection molding process. Those samples were used for the tensile testing and the DMA to investigate the

mechanical and the dynamic mechanical properties of the blended compositions.

Figure 2. a) Multi-stage compression mold used to fabricate origami cores, b) Composite origami

structures made by molding process

2.4 DIFFERENTIAL SCANNING CALORIMETRY (DSC) TEST

Differential scanning calorimetry provided by TA Instruments (Q50 TGA) was used to characterize the

thermal properties of the blended materials’ composition. Small material masses, ranging between 10 -

15g, were cut from each material blend composite and panned in aluminum pans; the temperature cycle

covered the range of -50 C to 180 C, the cooling/heating rate was 10C/min, and heat/cool/heat cycles

were conducted to predict the thermal properties of the tested samples.

b) a)

)

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2.5 DYNAMIC MECHANICAL ANALYSIS (DMA) TEST

Dynamic mechanical analyzer provided by TA Instruments (DMA Q800) was used to test the viscoelastic

properties of the blended composite materials. The test was carried out on the rectangular samples

prepared by injection molding according to the ASTM D638 standard. A sinusoidal load with a frequency

of 5 Hz is applied under a temperature ramp from 30°C to 85°C. The storage and the loss moduli

were measured for each sample.

2.6 PASSIVE PROPERTIES OF COMPOSITE ORIGAMI CORES

2.6.1 COMPRESSION TEST OF COMPOSITE ORIGAMI CORES

To measure the capacity of withstanding the compression loads, the fabricated composite origami cores

were tested in compression, using a compression machine. Three samples from each composition were

prepared and tested. The samples were placed between two thick-rigid plates connected to the testing

machine to ensure the uniform distribution of the compressive load (figure 3). The load was applied

gradually at the rate of 5mm/min (ASTM D1621/94 standard), and the load deformation values were

recorded; then, the elasticity moduli and the compressive strength moduli were obtained.

Figure 3. Composite origami cores placed between thick-rigid plates in the compression test

2.6.2 IMPACT TEST OF COMPOSITE ORIGAMI CORES

For the impact event, a custom drop-weight impact setup was used to measure the impact force received

in the other side of the origami sandwiched cores, during the impact event (figure 4).

The test setup has a dynamic load sensor (Dytran, 1060V) placed in the lower side of the testing plate.

This load sensor is connected to the data acquisition system, allowing the recording of the force-time

values of the impact event, and then the force and energy transferred were obtained. In the test, the impact

weight used was 1.104 Kg, and the height was 66.5cm.

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Figure 4. a) Composite origami core sandwiched between two plates, b) Impact test setup.

2.7 ACTIVE PROPERTIES

In order to investigate the active properties of the origami unit cell, the origami core made of PLA with

0.1% CNF was fabricated, and the unit cell was split for shape test. The shape memory test started with

compressing the origami unit cell using Instron (5848) testing machine, with a strain rate of 5mm/min as

per ASTM D695. The test was done at a temperature of 80C, which represents the glass transition

temperature of the composite material. Then the relaxation test was run under the same thermal conditions

to remove the residual stresses from the samples, from which the stress-time relation was recorded. After

relaxation test, the deformed origami unit cell sample was fixed. Finally, the deformed sample was kept

under a uniform temperature of 80 C, allowing the origami unit cell geometry to recover; the changing

height versus time recorded and the recovery factor was calculated.

3. RESULTS AND DISCUSSION

3.1 THERMAL PROPERTIES OF COMPOSITE MATERIAL BLENDS

3.1.1 DIFFERENTIAL SCANNING CALORIMETRY (DSC)

Figure 5 shows the heat flow curves of the composite blends, which were obtained using DSC. The

curves show the thermal behavior of the material tested. The glass transition temperature range (Tg), and

the melting temperatures of the composites were taken from the test, to decide the fabrication conditions

of the composite blends. Table 2 lists the values of the Tg, the melting temperature ranges of blended

composites Tm, and the molding temperature ranges used in the fabrication process.

The DSC results show that the addition of CNF in the base polymer matrix does not significantly change

the glass transition temperature of the TPU, which ranges from -36°C to -31°C or of PLA, which ranges

from 56°C to 60°C. The general behavior is that these temperatures increase with a higher PLA

composition in the PLA/TPU polymer blend, and a higher CNF composition in the composite.

a) b)

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Table 2. The DSC Results of the Material blends

CNF

(w%)

Material Blends Composition

(PLA/TPU) w%

Glass Transition

Temperature Range Tg (C)

Molding Temperature

range (C)

1% 100/0, 80/20,65/35, 50/50, 20/80, 0/100 (-34.55 – 58.65) 210 - 230

3% 100/0, 80/20,65/35, 50/50, 20/80, 0/100 (-32.35 – 59.30) 210 - 220

5% 100/0, 80/20,65/35, 50/50, 20/80, 0/100 (-31.85 – 58.9) 200 - 210

a) Composite blends with 1 w% CNF b) Composite blends with 3 w% CNF

c) Composite blends with 5 w% CNF

Figure 5. Thermal behavior of composite blends with CNF

3.1.2 DYNAMIC MECHANICAL ANALYSIS (DMA)

DMA results show the mechanical viscoelastic properties of the composite polymeric blends; those

results are illustrated in figure 6, which shows the storage and the loss moduli of the composite blends. At

low temperature, the macromolecules remain stiff, and do not resonate with the sinusoidal load, while at

high temperature; the molecular segments become mobile and then resonate with the load. By analyzing

the curve, we can clearly see the glass transition of PLA between the energy elastic state and the energy

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entropy state shown by the sudden drop in storage modulus and the pronounced peak for loss modulus.

The behavior of PLA/TPU blends shows that, PLA dominates the viscoelastic response up to 50/50

PLA/TPU composition, and then deviates to TPU behavior. The glass transition of PLA has major

consequences on the viscoelastic properties for high PLA composition up to 50/50 PLA/TPU. Moreover,

CNF tends to increase the storage moduli in a significant way, from 1% to 3% CNF, while the loss

moduli are only slightly improved. From the experiments on 5% CNF, 80/20 PLA/TPU shows

significantly improved moduli, while there seems to be a stagnation starting at 65/35 PLA/TPU

composition and below. For 50/50 PLA/TPU, the DMA results do not show a significant variation in

dynamic moduli, but the 5% CNF loss modulus seems inferior to the 3% CNF. With the actual results for

20/80 PLA/TPU and Neat TPU, there was no significant improvement of the DMA moduli with the

addition of CNF.

a) PLA/TPU Blends with 1% CNF b) PLA/TPU Blends with 1% CNF

c) PLA/TPU Blends with 3% CNF d) PLA/TPU Blends with 3% CNF

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e) PLA/TPU Blends with 5% CNF f) PLA/TPU Blends with 5% CNF

Figure 6. Storage and loss moduli of PLA/TPU blends with CNF

3.2 PASSIVE PROPERTIES

3.2.1 COMPRESSION TEST RESULTS

The results of the modulus of elasticity of the CNF reinforced blended composite parent materials are

shown in figure 7. The compression tests results of the origami composite cores are shown in figures 8 –

10, in which figure 8 shows a comparison of the modulus of elasticity values of the composite origami

cores. It is clear that, the 5% CNF samples have the highest values of the elastic modulus in the 20/80,

65/35, and 50/50 PLA/TPU samples, while it overlaps with the values of 3% CNF in the case of pure

PLA, pure TPU and the 20/80 PLA/TPU samples. The 1% CNF samples always have the lowest modulus.

The strength of composite origami cores is illustrated in figure 9, which clearly shows that the 5%

samples have the highest strength values compared with the other compositions, similarly to the behavior

of the elastic modulus.

In figure 10, the toughness of the origami cores is illustrated, and it is obvious that, samples with high

percentages of CNF absorbed higher energy than the samples with low CNF content, but with plastic

deformation and fracture through the crease lines, as observed in the samples with high PLA (100% and

80%), while samples with lower PLA percentage showed elastic behavior during the compression test,

and were able to recover after releasing the load.

In general, the elastic modulus and the strength of the composite origami cores decrease in response to the

increase of the TPU composition, which is compatible with the behavior of pure blended origami results

[35]. Samples with high PLA content tend to fracture at the crease lines, while the samples with high TPU

content have the lower elasticity modulus and able to withstand high strains without fracturing.

Comparing these results with previous results of pure polymeric blends origami [35], it can clearly

observe a significant improvement of the strength and the modulus of the origami cores by the increase of

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the CNF, Figures 8 and 9 show comparison of the compressive modulus of elasticity and the strength of

origami cores made of composite blends and pure blends [35].

Figure 7. modulus of elasticity of the CNF reinforced

blended composite parent materials

Figure 8. Comparison the modulus of elasticity of the

pure blended origami [35] with composite origami cores

Figure 9. Comparison the strength of pure blended

origami [35] with composite origami cores.

Figure 10. Toughness of composite origami cores

3.2.2 IMPACT TEST RESULTS

The study in the previous section showed that the addition of the CNFs to the PLA/TPU blends improves

the elastic modulus, and the strength of the composite origami cores. In the case of impact event, the

higher strength altered the ability of the base material to work as a compliant mechanism, and thus

decreases the absorbed force and diminishes the energy absorption by the cores, compared to pure

PLA/TPU cores. Origami cores with high percentage of CNF tend to fracture more than those of pure

polymer blends. Most of the samples made of high PLA/CNF composition break under the impact point

(figure 11), and it is observed that the fracture propagated through the crease lines, which is the weakest

region in the origami structures. Some PLA with high percentage of CNF samples, which are more rigid

and brittle were fractured even through the rigid faces of the origami structures (figure 11C).

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Figure 11. (a) Fractured PLA+1%CNF sample under the point of impact, (b) Propagation of the fracture

through crease line on a PLA+1%CNF sample, (c) fractured PLA+3%CNF sample

Composite origami core samples with 80/20 PLA/TPU also experienced fracture through the crease lines,

when they were tested multiple times. This can be an indicator for the existence of internal micro-

fractures during the tests. Figure 12 shows a sample of force-time response curve, which was used to

calculate the transferred energy. The presented results in figure 13 show the higher maximum force

transferred proportional with the higher amount of the CNF. These values of the maximum force

transferred are still lower than the values of force transferred by steel, or polycarbonate plates tested in the

same conditions, which proves the efficiency of origami core structures in distributing the impact loads

through its unique geometrical features.

The behavior of the composite origami cores seems to present two different zones, with the 65/35

PLA/TPU being in the middle. The samples above this composition showed brittle behavior. Samples

with neat PLA showed fractures and 80/20 PLA/TPU might have internal fracture as explained above.

This explains that these samples transferred forces to the bottom side until they got facture, which

decreased the transferred force they might transmit during the impact event, and the slight difference in

the force transmitted between neat PLA and 80/20 PLA/TPU, because of the small amount of TPU

allowed the distribution of the impact force before fracture.

The samples with PLA/TPU below 65/35 showed a flexible behavior allowed them to distribute the

incoming impact force through the origami structure, and as a result transferred less force to the other

sandwich side. The neat TPU samples showed a slightly higher transferred force than the 20/80

PLA/TPU, because of the rubbery behavior of the TPU that allows the cores to densify and transfer force.

The 65/35 PLA/TPU composition benefits from the rigid behavior without showing fracture and then

transferred more force to the bottom side of the core. In Figure 14 the maximum impact energy

transferred by the composite origami cores is illustrated. It shows a higher value with the increased

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amount of CNF, which might act against the brittle fracture of the samples. Moreover, the samples that

seem to transfer the least energy are 20PLA/80TPU, due to their high flexible behavior, allowing

compliant mechanisms on the creases, but preventing a high deformation of the faces. Neat TPU with

CNF seems to be the composition that transfers the most energy, as explained before, due to the rubbery

behavior, causing it to endure important elastic deformation without any fracture and transfer force on a

large scale of time compared to the other compositions. The decreasing transferred energy for TPU with

increasing amount of CNF might be due to the improvement of the samples’ rigidity. The results of the

impact testing showed that the addition of CNF increases the force transferred by the origami cores and

decreases the damping efficiency of the cores. Figures 13 and 14 illustrate the comparison of impact force

and impact energy transferred by origami cores made of composite origami and pure blended origami

[35]. Table 3 lists a comparison values of the maximum force transferred by composite origami cores and

the maximum force transferred by plane polycarbonate and steel plats tested in the same condition [35].

Figure 12. Sample of impact force-time response

Figure 13. Comparison of the max. force

transferred by pure blended origami cores [35]

with composite origami cores

Figure 14. Comparison of maximum impact

energy transferred by pure blended origami

cores[35] with composite origami cores

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Table 3. Comparison between the maximum force transferred by composite origami cores and other

plane materials

CNF (w%) Material Blends Composition

(PLA/TPU) w% Maximum force transferred (N)

1% 100/0, 80/20,65/35,

50/50, 20/80, 0/100 (2574.26 – 3559.76)

3% 100/0, 80/20,65/35,

50/50, 20/80, 0/100 (3019.65 - 3829.513)

5% 100/0, 80/20,65/35,

50/50, 20/80, 0/100 (3184.39 - 4094.45)

Polycarbonate (11432.9) [35]

Steel (20852.0) [35]

3.3 ACTIVE PROPERTIES

3.3.1 SHAPE MEMORY EFFECT

The four steps of the shape memory test were performed on the composite origami unit cell, the samples

with high percentages of CNF showed no recovery during the shape memory test, the 1%, 3%, and the

5% CNF composite samples have showed no response during the recovery test. This can be explained

because of the existence of the carbon fibers which does not allow the material to flow during the heating

process, even when the material reaches the glass transition temperature. The amount of fibers affects the

flow of the polymer particles, and increasing the stiffness of the composites which resist the activation

force required to move the part.

In addition, the CNF affected the compression resistance when the samples heated during the

compression stage of the shape memory test, in which crack has been observed along the crease lines on

the origami unit cell (figure 15). The samples with small amount of CNF (0.1 wt %) showed high

response and high recovery ratio, due to the thermal response of the composite and the less amount of the

CNFs, the unite cell faces were able to move during the recovery test. Figure 16 shows a compressed

composite origami unit cell made of PLA + 0.1 wt % CNF after shape fixing.

The compression-relaxation test results are shown in figure 17; the figure shows the maximum stress

reached by the origami unit cell was 0.348 MPa, and the tested sample relaxed in about 300 sec. The

recovery test showed that the sample was able to recover its original height in about 63 sec. Figure 18

shows the height recovery versus time, and figure 19 shows the unit cell height recovery during time. This

results show that there is an enhancement in the recovery time compared with the recovery time of the

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pure PLA samples tested in our previous work[40], in which the pure PLA origami sample toke more

than 450 sec to recover 85% from its original height.

Figure 15. Cracked composite Origami unit cell made

of (PLA + 3wt % CNF) Figure 16. Deformed composite origami unit cell made

of low percentage of CNF (PLA + 0.1wt % CNF)

Figure 17. Stress relaxation test of composite

origami sample made of PLA + 0.1wt% CNF

Figure 18. Recovery shape of composite origami

sample made of PLA + 0.1wt % CNF

Figure 19. The origami unit cell recovery (height)

From the same material composition (PLA + 0.1 wt % CNF), origami samples were fabricated and tested

for compression and impact resistance. The results demonstrated that the elastic modulus was 8.55 MPa,

and the maximum force transferred by impact was 419 .5 95 N. These results are comparable with the

values obtained from the PLA+CNF samples.

4. CONCLUSION

The way the origami metamaterial can redistribute compression and impact forces make it qualified for

potential use in future applications, as a light-weight sandwich core. The use of CNF with the material

blends to fabricate composite origami structures significantly enhances the overall mechanical properties

17

of the origami cores in compression, in which the compressive modulus and the strength of the origami

cores were increased by the increase of the CNF. By noting that samples with high PLA content face

fracture through the crease lines, while low PLA samples showed more elastic behavior. The impact

testing showed a higher transferred force and energy for higher carbon nanofiber compositions, because

of the increasing in the rigidity of the samples. 65PLA/35TPU showed the highest transferred force

because of the coupling effect of rigidity, flexibility and fiber reinforcement, while the higher PLA

composition samples showed major brittle behavior and the lower PLA composition samples showed

flexible behavior. The DSC test results showed an improvement in glass transition for both TPU and PLA

with the increasing amount of CNF, and the DMA results showed a significant increasing of the storage

modulus and a slight improvement of the loss modulus. Active composite origami structures showed

fractures along crease lines and low recovery ratio, composite origami with low CNF percentages (0.1%)

showed good stress relaxation behavior and high recovery ratio.

ACKNOWLEDGMENTS

The authors would like to acknowledge the following agencies for financial support: The Natural Science

and Engineering Research Council (NSERC) of Canada, the Canada Research Chair Program, the Libyan

Ministry of Higher Education and scientific research, Tripoli.

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