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Abstract

Glass reinforced epoxy composite tubes filled with 1, 2, 3, and 4 wt. % of HNC, MC, Al₂O₃, SiO₂, and SiC nanofillers were fabricated using wet-wrapping process by hand lay-up and tested under quasi-static axial loadings. Crashworthiness parameters and failure modes were recorded. Results indicated that EAC and the specimens’ failure modes are dominated by the type and wt. % of the embedded nanofillers. As compared to pristine glass/epoxy tubes, an enhancement of 230.42, 243.30, 286.43, and 336.12% in the absorbed energy (U) was attained by the addition of 1, 2, 3, and 4 wt. % of HNC, respectively. An improvement of 21.93, 87.35, 225.99, and 318.07% in U was achieved by the inclusion of 1, 2, 3, and 4 wt. % of MC, respectively. An enhancement of 17.66, 51.63, and 71.94% in U was reported by the integration of 1, 2, and 3% of nano-Al₂O₃. Whilst a reduction of 31.16% was noticed for 4 wt. % of nano-Al₂O₃. The incorporation of nano-SiO₂ and nano-SiC exhibits a reduction in U of the fabricated tubes. Composites filled with 4 wt. % of HNC has the highest load carrying capacity and EAC of 32.75 kN and 1110.84 J, respectively. So, they seem to be the best appropriate choice for energy absorbing elements. Glass/epoxy composite tubes filled with HNC, MC, and Al₂O₃ show outstanding energy absorption characteristics. However, specimens filled with SiO₂ and SiC nanofillers are ineffective in the crashworthiness applications.

Keywords

Glass/epoxy composite tubes nanofillers energy absorption quasi-static test failure modes scanning electron microscopy

Introduction

Crashworthy structure is designed such that in the event of a crash, it absorbs the impact energy in a controlled manner before the energy gets transmitted to the passenger.¹ Thin-walled structures are commonly used in automotive industries as energy absorbing components because of their lightweight and high energy absorption capacity. Many research efforts have been carried out to examine the crashworthiness performance of thin-walled structures in the literature.^2–10

Traditionally metals such as aluminum and steel have been applied for crashworthy structures. Contrary to metals, especially in compression, most composites are generally characterized by a brittle rather than ductile response to load. While metal structures collapse under crush or impact by buckling and/or folding in accordion type fashion involving extensive plastic deformation, composites fail through a sequence of fracture mechanisms involving fiber fracture, matrix cracking, fiber-matrix debonding, delamination, and interply separation. The actual mechanisms and sequence of damage of composites are highly dependent on the geometry of the structure, lamina orientation, type of trigger, and crush speed, all of which can be suitably designed to develop high energy absorbing mechanisms.^11,12 According to Carruthers,¹³ composite materials do not exhibit the ductile failure processes associated with metals and most thermoplastics (although there are exceptions). Instead, the brittle nature of most fibers and thermosets tends to generate a brittle mode of failure. This is the fundamental difference which gives rise to the very distinct energy absorption characteristics of composite materials. If polymer composites are properly designed, they can absorb high amounts of impact energy by progressive crashing and delamination.^11,14 The performance of composite (the degree of ductility or brittleness) is primarily driven by the behavior of its constituent materials. Several factors influencing the mechanical properties of polymer composites, such as fiber loadings, fiber length, fiber architecture, fiber and matrix type, chemical treatments, and hybridization by incorporating different natural/synthetic fiber/fillers.¹⁵ The combination of ductile fiber with ductile thermoplastic matrices, leads to ductile composites.¹⁶ Since the composites’ response depends on its microstructure design, comprehensive efforts are continuously being done in the direction of new composites.

Recently, researchers have focused on nanoscale reinforcements such as carbon nanotubes (CNTs),^17,18 SiO₂,^19,20 MC,^21–23 graphene,²⁴ Al₂O₃,^25,26 SiC,²⁷ and HNC^28,29 to fabricate multiscale composites. Nanoscale reinforcements have positively improved the mechanical, physical, chemical, and thermal properties of composites. It is evident in the literature that composites with nanoscale reinforcements can attain substantial enhancements in fracture toughness and energy absorption which could be significant in designing vehicle structures.^17,30

Numerous studies have reported on the energy absorption capacity (EAC) of polymer composite crashworthy structures.^31–34 On the contrary, the works related to EAC of nanofilled polymer composite crashworthy structures is inexhaustive. The contributions of prominent researchers who adopted the inclusion of nanofillers in energy absorbing composite structures are summarized in Table 1. Finally, nanocomposites are promising materials that can be utilized to produce very efficient energy absorbing structures especially in the automotive and aerospace industries.

Table 1.

Summary of some research works on nanocomposite structures for crashworthiness application in the last 10 years.

Reference	Phases of nanocomposite	Type of loading	Output
Aymerich et al.³⁵	• Cloisite30B	Impact loading	An increase of 30% in EAC and 10%–15% decrease in initial crushing failure load $(P_{i p})$ were obtained over pristine laminate
Aymerich et al.³⁵	• Glass/epoxy laminate	Impact loading
Pradeep³⁶	• Nano-graphene	Quasi-static lateral loading	An increase of 10–20% in SEA was noticed for nanofilled hybrid tubes over the standard tubes tested in quasi‐static lateral crushing loadings
Pradeep³⁶	• Aluminum (Al) cylindrical tube wrapped with glass/epoxy	Quasi-static lateral loading
Silva et al.^37,38	• SiO₂ or glass-spheres or MC	Quasi-static axial loading and dynamic loading	A significant increase in specific energy absorption (SEA) was observed in PA6 reinforced with nano-SiO₂ particles and glass-spheres (GS), whereas addition of MC caused a decrease in that property. On the other hand, very little influence of the secondary reinforcement on EAC of glass/PP composites was found
Silva et al.^37,38	• Glass/polypropylene (PP) or polyamide 6 (PA6) composites	Quasi-static axial loading and dynamic loading
Velmurugan and Balaganesan³⁹	• Clay nano platelets	Impact loading and quasi-static loading	The inclusion of clay nano platelets not only protects glass/epoxy composite laminates from failure, but also enhances the stiffness as well as the energy absorption in both quasi‐static and dynamic impact conditions
Velmurugan and Balaganesan³⁹	• Glass/epoxy laminate	Impact loading and quasi-static loading
Sachse et al.⁴⁰	• MC-filled polyurethane foams	Quasi-static compression loading and low energy impact loading	The addition of MC in polyurethane foam core enhances EAC and the maximal deflection during impact. The enhancement in EAC was between 66% and 92% for PP face sheet sandwiches and 23% and 34% for PA face sheet sandwiches during point load
Sachse et al.⁴⁰	• Glass/PA and PP face sheets
Pramanik et al.⁴¹	• xGnP (exfoliated graphite nanoplatelets)/CTBN (Carboxyl Terminated Butadiene Nitrile)	Quasi-static indirect tensile loading	Under quasi-static loading, EAC of pure vinyl ester is reduced by the addition of xGnP reinforcement and even with CTBN toughening. The addition of CTBN improved EAC of xGnP (without CTBN) nanocomposites
Pramanik et al.⁴¹	• Toughened vinylester-based nanocomposites	Quasi-static indirect tensile loading
Haro et al.⁴²	• Micro and nano-fillers (Al, gamma Al₂O₃, SiC, colloidal SiO₂ and potato flour)	Ballistic impact loading	EAC of hybrid composite laminates were enhanced by deposition of micro and nano-fillers into surface of the Kevlar fabric. The EAC was achieved by deposition of Al powder followed be colloidal SiO₂ and SiC powder in that order. Addition of gamma Al₂O₃ and potato flour has produced the least effect of enhancing the EAC of the laminates
Haro et al.⁴²	• Woven Kevlar/epoxy/aluminum sheets hybrid laminate	Ballistic impact loading
Al-Lafi et al.⁴³	• Multi-walled carbon nanotubes (MWCNTs)	Low and high impact loading	PC and its MWCNTs nanocomposites with 0.1, 0.5, and 1 wt. % contents were assessed for impact resistance behavior under dynamic loadings. MWCNTs are effective in improving the impact resistance of the PC at very low filler loadings. The maximum load and the impact failure energy were increased by 320% and 350%, respectively, when only 1 wt. % MWCNTs was incorporated. For 1 wt. % MWCNTs nanocomposite, the yield stress and impact energy increased greatly by 25% and 15%, respectively, at the high strain rate of 3100 s⁻¹ as compared to the PC matrix
Al-Lafi et al.⁴³	• Polycarbonate (PC)	Low and high impact loading
Pushparaja et al.⁴⁴	• Nanoclay	Impact loading	The dispersion of nanocaly in the matrix enhances EAC of carbon/epoxy laminate (CFRP) during impact loading. Less delamination area was observed with CFRP/nanoclay than pure CFRP.
Pushparaja et al.⁴⁴	• Carbon/epoxy laminates	Impact loading
Balaganeshan et al.⁴⁵	• Nanoclay and carbon nanotubes (CNTs)	Impact loading	Nanocomposite specimens are subjected to impact loading at 0°C, 30°C, and 60°C temperatures. The presence of nanoclay and CNTs in the epoxy increases EAC of the laminate. The laminates with 3% clay and with 2% CNTs show optimum improvement in the absorbed energy (U)
Balaganeshan et al.⁴⁵	• Glass/epoxy laminates	Impact loading
Rudresh et al.⁴⁶	• Micro-fillers (SiC, Al₂O₃, molybdenum disulfide (MoS₂)	Impact loading	Synergistic effect of micro and nano-fillers has significantly improved the impact strength
	• Nano-filler (Al₂O₃)
	• Glass-carbon fibers
	• The blend polyamide 66 (PA66)/polytetrafluoroethylene (PTFE) served as a matrix material
Kumar⁴⁷	• Different nanofillers with different matrices	Impact loading	The influence of some key control parameters such as shape and size of nano-fillers, mechanical properties of nano-fillers and matrix materials, interfacial adhesion, interphase characteristics, as well as the volume fraction and dispersion of fillers in the matrix were discussed. EAC and the related properties of composites can be engineered by adding nano-scale fillers
Kumar et al.^48,49	• MWCNTs	Quasi-static lateral loading	Inclusion of MWCNTs reduced the fiber fracturing and ultimately enhanced EAC of the tubes tested under lateral quasi-static loading. Nanofilled tubes with more fabric plies revealed better crashworthiness performance compared to the traditional Al-glass/basalt composite tubes
Kumar et al.^48,49	• Al-cylindrical tube wrapped with glass and/or basalt reinforced epoxy	Quasi-static lateral loading
Kumar et al.⁵⁰	• MWCNTs	Quasi-static lateral loading	The best crashworthiness characteristics were recorded for the largest tube diameter. The lateral crashworthiness response of MWCNT filled glass/epoxy composite tubes was marginally superior to that of MWCNT filled basalt/epoxy composite tubes
Kumar et al.⁵⁰	• Al-cylindrical tube wrapped with glass and/or basalt reinforced epoxy	Quasi-static lateral loading

It is clear from the above literature that nanofillers have the prospective to be employed as a secondary strengthening agent in composite structures used as energy dissipating elements. Nanofillers play an important role in improving EAC as well as the mechanical characteristics of traditional composites. It is worth noting that inexhaustive research works have studied the quasi-static axial crushing characteristics of polymer composite tubes filled with nanofillers. To the best knowledge of the authors, no attention has been devoted to polymer composites containing HNC, MC, Al₂O₃, SiO₂, and SiC nanofillers to be used as energy dissipating elements. So, the main objective of the present study is to experimentally investigate the potentiality of different nanofillers (HNC, MC, Al₂O₃, SiO₂, and SiC) at five different loadings (0, 1, 2, 3, and 4 wt. %) on the crashworthiness performance of glass/epoxy composite tubes. Nanofilled composite tubes were fabricated using wet-wrapping process by hand lay-up and tested under quasi-static axial loading. In this study, the authors try to attain newly designed nanocomposite materials to be applied as energy dissipating elements in modern vehicles to deliver occupant safety during accidents. The proposed nanofilled composite tubes are suggested to be used as energy absorbers in the front of train structure or automobile structure, that is, anti-impact rods. These composites can also be applied as crash box in the front of the automobile and energy absorption tubes in aircraft fuselage. The proposed composites can be designed and utilized for a particular type of load into the various major structures for high-performance applications and safety equipment in major transportation industries such as aerospace, automotive, and marine industry.

Methodology

Uniaxial quasi-static compression tests with crosshead speed of 10 mm/min were performed to examine the crushing behavior of glass fiber/epoxy circular composite tubes filled with different nanofillers. The effect of wt. % of the used nanofillers on the crushing characteristics and the specimens’ failure mechanisms has been studied.

Materials

Plain weave E-glass fabric with GSM of 200 g/m² and yarn diameter of fiber is about 16 μm was selected as a reinforcement. E-glass fabric was supplied by Hebei Yuniu Fiber Glass Manufacturing Co Ltd China, Mainland. Kemapoxy 150RGL provided by Chemicals for Modern Buildings Co Ltd Egypt was chosen as a matrix material due to its good adhesive properties, superior mechanical properties, reduced degradation from water ingress, improved resistance to fatigue and micro cracking, and increased resistance to osmosis.⁵¹ It is characterized by the high resistance against mechanical stresses and chemical effects.⁶ Kemapoxy 150 RGL has been effectively used by many authors in their work.^52–61 HNC, MC, Al₂O₃, SiO₂, and SiC nanofillers were selected for this study due to the availability, good mechanical properties, and being successfully used for reinforcing polymers. In the literature, there is a little work on using the selected nanofillers in crashworthiness applications. HNC, MC, Al₂O₃, SiO₂, and SiC nanofillers were provided by Sigma-Aldrich Co Ltd United Kingdom. The specifications of the used nanofillers are as follows: HNC with diameter × length = 30–70 nm × 1–3 μm and surface area of 22.1–81.6 m²/g, MC with particle size ≤20 μm and surface area of 220–270 m²/g, Al₂O₃ with particle size, SiO₂ with particle size <50 nm and surface area of 175–225 m2/g, and SiC with particle size <100 nm and surface area of 175–225 m2/g.

Dispersion of nanofillers into epoxy resin

Nanofillers were separately added to the epoxy. The mixture was manually stirred up for about 5 min, Figure 1(a). Then the mixture was sonicated, for 3 h, using high intensity ultrasonic processor (type: Hielscher UP200S, 200 watts, 24 kHz, and amplitude of 70%), Figure 1(b). Referring to Megahed et al.,^20,26,62,63 the chosen sonication parameters ensure the good uniform dispersion of nanofillers in epoxy and break up any aggregation thus the compatibility of nanofillers with epoxy resin could be enhanced. So, improved mechanical properties could be obtained. As sonication occurs, the mixture was placed in an ice-water bath to decrease the released temperature.⁶¹ Later, the hardener was mixed with epoxy and nanofillers with a wt. ratio of 1:2 as supplier recommendation. Then, the mixture was manually stirred up for about 5 min to be ready for usage, Figure 1(c).

Figure 1.

Steps of fabricating glass/epoxy circular composite tubes filled with nanofillers. All the steps were carried out at room temperature (25°C). (a) Adding nanofillers to the epoxy. The mixture was manually stirred up for about 5 min [71]; (b) The mixture was sonicated, for 3 hours [52, 53, 62]; (c) The hardener was mixed with the epoxy and nanofillers. The mixture was manually stirred up for 5 min [6]; (d) The mixture was spread upon glass fabric [72]; (e) Impregnation of the fabric with matrix using a roller [6]; (f) Impregnation of the fabric with matrix using a brush [6]; (g) Wrapping the impregnated fabric around a circular aluminum mandrel [6]; (h) Final products were left to be cured for 7 days [71].

Fabrication of nanofilled composite specimens

In this study, wet-wrapping process by hand lay-up was used to fabricate glass/epoxy circular composite tubes filled with HNC, MC, Al₂O₃, SiO₂, or SiC nanofillers. Hand lay-up is the most basic fabrication method for thermoset composites.⁶⁴ Due to the simplicity, low-cost option, and minimal infrastructural requirements of hand lay-up fabrication process,⁶⁵ it has been used by several researchers.^6,66–69 Hand lay-up process also allows creating simple and complex shapes.⁷⁰ The steps of the fabrication process are summarized as follows:

⁃ The epoxy-hardener-nanofiller mixture was uniformly spread upon the glass fabric, Figure 1(d).

⁃ The glass fabric was impregnated using a roller and a brush, Figure 1(e) and (f).

⁃ The impregnated glass fabric was wrapped around a circular aluminum mandrel with 33 mm outer diameter. Each composite tube was made of 12 glass plies, Figure 1(g).

⁃ The primarily constructed tubes needed 7 days at room temperature for complete curing.

⁃ After complete curing of fabricated tubes, the aluminum mandrel was removed and the fabricated tubes were visually inspected for geometrical irregularities or material imperfections, Figure 1(h).

⁃ The consolidated tubes were cut into 60 mm length. Description, and the measured geometrical parameters of the fabricated composite tubes are shown in Table 2.

Table 2.

Description, and the measured geometrical parameters of nanofilled glass/epoxy composite tubes.

Type of nanofiller	wt. %	Average inner diameter (mm)	Average thickness (mm)	Average length (mm)	Average mass (g)
No filler	--------	32.64	5.84	60.89	30.03
HNC	1	32.84	5.90	59.82	31.73
	2	32.12	6.32	60.21	32.22
	3	32.19	6.30	59.7	32.37
	4	32.49	6.41	60.38	33.42
MC	1	32.86	5.25	60.32	33.05
	2	32.35	6.38	60.08	32.21
	3	32.25	6.25	60.12	34.52
	4	31.84	6.99	60.12	33.98
Al₂O₃	1	32.70	5.83	60.34	32.35
	2	33.11	6.00	60.25	32.45
	3	32.46	6.87	60.21	33.64
	4	32.30	6.74	60.37	33.77
SiO₂	1	32.66	5.62	60.16	30.18
	2	32.72	5.90	61.05	31.55
	3	32.34	6.28	60.08	31.96
	4	32.20	6.55	61.28	33.11
SiC	1	32.52	5.89	60.16	30.75
	2	32.62	6.22	59.01	31.01
	3	32.97	5.30	59.85	31.90
	4	32.51	6.25	59.02	30.58

Quasi-static compression test

Quasi-static axial compression tests were carried out to determine the crashworthiness parameters of the fabricated composite tubes using a universal testing machine with a capacity of 100 kN (Type: Jinan WDW, China (Mainland)) at a crosshead speed of 10 mm/min. The advantages of quasi-static axial compression tests over the impact tests are as follows:¹²

1. Quasi-static tests are simple, easy to control, and available in low cost.

2. Impact tests require very expensive equipment to follow the crushing process because the whole crushing takes place in a split second. Hence, quasi-static tests are used to study the failure mechanisms in composites, by selection of appropriate crush speeds.

Two flat steel plates were set parallel to each other prior to the start of the test. The performance of crashworthy composite tubes can be quantitatively evaluated via the obtained load–displacement curves by estimating some key parameters such as the initial crushing failure load $(P_{i p})$ , average crushing load $(P_{a v g})$ , crushing force efficiency (CFE), absorbed energy (U), specific absorbed energy (SEA), and spring back (SB). Figure 2 schematically illustrates a typical load–displacement curve to identify the key parameters mentioned above as follows:

• Initial crushing failure load $(P_{i p})$ : is the first peak load at the end of the pre-crushing stage and it is directly linked to the degree of possible harm to passengers.

• Average crushing load ( $P_{a v g}$ ) is considered as the average force over the effective crushing distance ( $l_{m a x}$ ), which should maintain high level to acquire superior energy absorption capacity⁷³

P_{a v g} = \frac{\int_{0}^{l_{m a x}} P (l) d l}{l_{m a x}}

(1)

where

P (l)

is the instantaneous crushing load.

• Crushing force efficiency (CFE) is defined as the ratio of $(P_{a v g})$ to ( $P_{i p}$ ). High CFE value means that the energy absorber could dissipate large amounts of energy while minimizing the injury to occupants during crashing events⁷⁴

C F E % = \frac{P_{a v g}}{P_{i p}} \times 100

(2)

• Absorbed energy (U) represents the energy dissipated during the crushing process, which can be calculated by the work done by the crushing force

U = \int_{0}^{l_{\max}} P (l) d l

(3)

• Specific energy absorption (SEA) is the energy absorbed per unit mass of the energy absorber⁷⁴

S E A = \frac{U}{m_{c}}

(4)

where

m_{c}

is the crushed mass of the energy absorber. Large SEA value is one of the most essential objectives in lightweight design.

• Spring back (SB), which reflects the capability of the structure to retain its original shape after being crushed. Zero spring back of the configuration indicates that it was destroyed during crushing with no possibility of springing back.⁷⁵

Figure 2.

Schematic view for identifying the crashworthiness parameters.⁶⁴

Results and discussion

The effect of the addition of different nanofillers (HNC, MC, Al₂O₃, SiO₂, and SiC) at five different loadings (0,1,2,3, and 4 wt. %) on the crashworthiness performance of glass/epoxy composite tubes was investigated. Three samples were tested for each case and the average was selected. Referring to,^76–82 The most beneficial effect is in general attained with a small wt. % of nanofillers, that is, less than 5 wt. % in which mechanical properties are generally improved. So, the authors decided to study the effect of adding 0–4 wt. % of nanofillers.

Crushing response and deformation histories

Load–displacement curves, energy absorption, and the deformation histories obtained from quasi-static axial compression tests for unfilled and nanofilled glass/epoxy composite tubes are shown in Figures 3–12. The results, presented in Figures 3–12, are for the most representative sample for each composite type. Generally, it was noted from Figures 3(a), 5(a), 7(a), 9(a), and 11(a) that the behaviors of all composite tubes are nearly linear till they come up to the first peak load then a load drop can be recorded. The magnitudes of the first peak load and load drop are governed by the type of the incorporated nanofiller and its wt. %. A load fluctuation around the average crushing load in the post-crushing stage was noticed for all tested composites. Then an increase in the load was noticed as the compaction zone was reached.

Figure 3.

(a) Load–displacement and (b) energy absorption-displacement curves of glass/epoxy composite tubes filled with HNC.

Figure 4.

Deformation histories of glass/epoxy composite tubes filled with HNC under quasi-static axial loadings.

Figure 5.

(a) Load–displacement and (b) energy absorption-displacement curves of glass/epoxy composite tubes filled with MC.

Figure 6.

Deformation histories of glass/epoxy composite tubes filled with MC under quasi-static axial loadings.

Figure 7.

(a) Load–displacement and (b) energy absorption-displacement curves of glass/epoxy composite tubes filled with Al₂O₃.

Figure 8.

Deformation histories of glass/epoxy composite tubes filled with Al₂O₃ under quasi-static axial loadings.

Figure 9.

(a) Load–displacement and (b) energy absorption-displacement curves of glass/epoxy composite tubes filled with SiO₂.

Figure 10.

Deformation histories of glass/epoxy composite tubes filled with SiO₂ under quasi-static axial loadings.

Figure 11.

(a) Load–displacement and (b) energy absorption-displacement curves of glass/epoxy composite tubes filled with SiC.

Figure 12.

Deformation histories of glass/epoxy composite tubes filled with SiC under quasi-static axial loadings.

For more illustration, the variations of the energy absorption with the displacement of the tested composite specimens are showed in Figures 3(b), 5(b), 7(b), 9(b), and 11(b). Generally, the energy absorption increases with the displacement. The type of the nanofiller and its wt. % have significant effects on the variants of the energy absorption against displacement, mainly after the initial peak load. Each load–displacement curve is correlated with the representative specimen deformation histories for convenience in viewing the failure progress as shown in Figures 4, 6, 8, 10, and 12.

It is clear from Figure 3(a) that the specimens with no nanofillers behave linear till approach the first peak value of 21.46 kN at about 3.16 mm. As shown in Figure 4(a), matrix fretting and interlaminar cracks start to occur at the top and bottom of the tube due to the local stress concentration and a sudden sharp load drop to 13.21 kN at 5.19 mm can be noticed. With increasing the crushing displacement, specimens with no nanofillers gradually deform plastically and yield load–displacement profiles with low oscillations’ amplitudes, that is, smooth curve in post crashing zone. At (6.21 kN, 40 mm), specimens go into compaction zone and the axial load increases rapidly. Bending, wrinkling, and global buckling of the tube walls can be noted due to the buckling of the fiber. Figure 3(b) shows that the energy absorption increases nonlinearly with the displacement.

Tubes filled with HNC

As shown in Figure 3(a), specimens filled with 1,2,3, and 4 wt. % of HNC behave nearly linear till they approach the first peak value of (22.37 kN at 3.02 mm), (26.67 kN at 3.34 mm), (29.58 kN at 2.98 mm), and (32.75 kN at 2.5 mm), respectively. As shown in Figure 4, micro-fragmentation damage appears at the upper and/or lower ends of the tubes and then the load drops to (17.92 kN at 5.07 mm), (21.72 kN at 5.78 mm), (25.50 kN at 5.43 mm), and (25.68 kN at 4.61 mm), respectively. In the post crashing zone, the fluctuations of load-displacement curve are quite significant. Matrix fretting and interlaminar cracks occurred at the top and/or bottom of the tube accompanied with matrix cracking. The cracks propagate along the longitudinal axis of the tubes. Further propagation of these cracks leads to lamina bending and splaying accompanied with the formation of internal and external folds. The compaction stage for specimens filled with 1,2,3, and 4wt. % of HNC starts at (17.77 kN and 50.10 mm), (20.69 kN and 46.69 mm), (23.67 kN and 47.86 mm), and (23.00 kN and 49.92 mm), respectively, and the axial load rapidly increases.

For specimens filled with different wt. % of HNC, the energy absorption increases linearly with the displacement as shown in Figure 3(b). As wt. % of HNC increases, the energy absorption capacity increases. This is because the low concentration of HNC up to 4 wt. % leads to a better dispersion of HNC in the epoxy and a reasonable wettability of glass fiber with the nanomodified epoxy. This result agrees with that reported by Zhang et al.⁸³ HNC is considered as an ideal modifier for polymer modification due to its excellent mechanical properties and large aspect ratio.⁶¹

Tubes filled with MC

Figure 5(a) shows that specimens filled with 1,2,3, and 4 wt. % of MC behave linear till they reach the first peak of (21.67 kN at 2.01 mm), (24.14 kN at 3.43 mm), (28.41 kN at 3.44 mm), and (31.74 kN at 2.31 mm), respectively. After the peak, a sharp load decrease to (8.82 kN at 4.38 mm), (17.52 kN at 6.17 mm), (14.78 kN at 6.88 mm), and (24.83 kN at 4.91 mm) was noticed. Low oscillations in the post-crushing zone of load–displacement curves were noticed for 1 and 2 wt. % of MC. Whilst obvious oscillations were recorded for 3 and 4 wt. % of MC. For 1 wt. % of MC, transverse shear crack growth in the circumferential direction of the tube was recorded. Also, matrix fretting and interlaminar cracks started to occur at the top and bottom of the tube due to the local stress concentration. Progressive buckling with repeated folding of the tube wall is the dominant failure mode, Figure 6(a). Matrix fretting and delamination at the top and bottom of the specimen followed by splaying and fragmentation of the tube skin and bending of the fronds were noticed for 2,3, and 4 wt. % of MC, Figure 6(b)–(d). The compaction stage starts at (14.81 kN and 40.00 mm), (14.51 kN and 40.61 mm), (16.15 kN, 47.29 mm), and (24.84 kN, 47.68 mm) for specimens filled with 1,2,3, and 4 wt. % of MC, respectively.

The energy absorption increases nonlinearly with the displacement for specimens filled with 1 and 2 wt. % of MC. On contrary, it increases linearly for specimens filled with 3 and 4 wt. % of MC, Figure 5(b). Glass/epoxy composite tubes filled with 4 wt. % MC showed the highest absorbed energy as compared to 0, 1,2, and 3 wt. %. This was attributed to exfoliated dispersion of 4 wt. % MC in the epoxy matrix.⁸⁴ The addition of C30B nanoclay improved the impact strength as they formed a tortuous path, but this was found to be good only for clay loadings lower than 5 wt. %. Higher clay loadings deteriorated the properties which was again attributed to nanoparticle agglomeration.⁷⁹

Tubes filled with nano-Al₂O₃

Figure 7(a) shows that specimens filled with 1, 2, 3, and 4 wt. % of nano-Al₂O₃ have similar trend. They behave linear till the first peak value of 22.53 kN at 3.98 mm, 26.275 kN at 3.15 mm, 27.11 kN at 2.73 mm, and 14.70 kN at 3.72 mm for specimens filled with 1, 2, 3, and 4 wt. % of nano-Al₂O₃, respectively. This trend is followed by load drops to 8.03 kN at 8.18 mm, 15.66 kN at 6.66 mm, 14.38 kN at 6.28 mm, and 6.41 kN at 7.79 mm for specimens filled with 1, 2, 3, and 4 wt. % of nano-Al₂O₃, respectively. In the post-crushing zone, low oscillations around the average loads can be noticed for specimens filled with 1, 2, and 3 wt. % of nano-Al₂O₃. Whilst no oscillations were recorded for specimens filled with 4 wt. % of nano-Al₂O₃. The curves start to enter the compaction zone at (12.63 kN, 40.64 mm), (11.78 kN, 41.00 mm), (17.00 kN, 42.00 mm), and (5.08 kN, 37.00 mm) for specimens filled with 1, 2, 3, and 4 wt. % of nano-Al₂O₃, respectively. During the compression of the tubes, interlaminar shear cracks and delamination at the top and bottom of the tubes accompanied with fiber fracture can be observed, Figure 8. As shown in Figure 7(b), the energy absorption increases nonlinearly with the displacement for all tubes filled with nano-Al₂O₃. Nano-Al₂O₃ agglomeration in samples with 4 wt. % loading contributed toward limiting the improvement in energy resistance. Similar result was recorded by Rafiq et al.⁷⁹ for glass/epoxy filled with 3 wt. % of I.30E nanoclay. Nayak et al.⁸⁵ reported that alumina increases the impact energy of epoxy-based composite compared to other SiO₂ and TiO₂ modifiers.

Tubes filled with nano-SiO₂

It was noted from Figure 9(a) that all specimens behave linear till the first peak values of 6.83 kN at 2.50 mm, 10.15 kN at 2.52 mm, 11.89 kN at 4.70 mm, and 11.27 kN at 4.62 mm for tubes filled with 1, 2, 3 and 4% of nano-SiO₂, respectively. After the peak loads, load drops for all tested tubes were recorded. The levels of load drops were 3.67 kN at 5.97 mm, 5.26 kN at 5.44 mm, 5.86 kN at 6.52 mm, and 5.98 kN at 6.90 mm for tubes filled with 1, 2, 3, and 4% of nano-SiO₂, respectively. This may be attributed to nano-SiO₂ agglomeration which acted as stress concentrators. Also, this may be due to the bigger particle size than the abovementioned three nanofillers, that is, HNC, MC, and Al₂O₃. This result is consistent with that obtained by Nayak et al.⁸⁵

Slight fluctuations around the average load values in the post-crushing zone was recorded for all cases. For 1 and 2% of nano-SiO₂, transverse shear crack growth in the circumferential direction of the tube center was reported accompanied with progressive compression of the composite tubes, Figures 10(a) and (b). For 3 and 4% of nano-SiO₂, the dominant failure mechanism is the tube global buckling, Figure 10(c) and (d). The starting points of the compaction stages for tubes filled with 1, 2, 3, and 4% of nano-SiO₂ are (4.68 kN, 39.00 mm), (4.29 kN, 35.81 mm), (3.81 kN, 34.14 mm), and (3.11 kN, 38.18 mm), respectively. Nonlinear trend was noticed for the energy absorption-displacement curves of all specimens filled with SiO₂.

Tubes filled with nano-SiC

Figure 11(a) shows that all specimens filled with 1,2,3, and 4 wt. % of nano-SiC behave linear till approach the first peak of 8.91 kN at 3.97 mm, 11.98 kN at 4.34 mm, 11.98 kN at 4.06 mm, and 11.72 kN at 4.60 mm, respectively. Sharp load drops after the peaks were noticed for all specimens. Stable and smooth behavior in the post-crush zones were recorded. The dominant failure mechanism is global buckling, Figure 12.

Nonlinear trend was recorded for the energy-displacement curves of all specimens filled with nano-SiC. The agglomerates and associated cracking may be responsible for decreasing the energy absorption. This result is consistent with that recorded by Landowski et al.⁸⁶

It is clear from Figures 3, 5, 7, 9, and 11 that the load increases rapidly at higher displacement. The region after the post-crushing zone is known as the compaction region. At this stage, the load value increased nonlinearly and rapidly due to debris accumulation and the mass of the tube has been fully compressed. The major energy absorption was contributed from matrix fracture and fragmentation. As a result, the energy absorbed by the structure was minimal. At the compaction zone, the load increased drastically indicating material densification.⁸⁷ In axial crushing, load increased at higher displacement, that is, at the beginning of compaction zone. The specimen enters the compaction zone when it is fully damaged and cannot resist more load. At this moment, the machine table supports the specimen, carries, and resists the load, so the load increases. This is consistent with that obtained by Abdewi et al.⁸⁸

Crashworthiness parameters

Peak and average crushing loads ( $P_{ip}, P_{avg}$ )

$P_{i p}$ is one of the most important parameters for assessing the crushing response because at low-speed or low-energy impacts, it is necessary for the structure to have no permanent deformation as this deformation would be considered as undesirable damage.^89,90 It is obvious from Figures 13 and 14 and Table 3 that the type of the used nanofiller and its wt. % significantly affect the values of $P_{i p}$ and $P_{a v g}$ . For the unfilled glass/epoxy composite tubes, $P_{i p}$ and $P_{a v g}$ are 21.46 and 6.38 kN, respectively.

Figure 13.

Peak load ( $P_{i p}$ ) of axially compressed glass/epoxy composite tubes filled with different nanofillers.

Figure 14.

Average load ( $P_{a v g})$ of axially compressed glass/epoxy composite tubes filled with different nanofillers.

Table 3.

Test results of glass/epoxy composite specimens filled with different types of nanofillers.

Type of nanofiller	wt. %	Sample no	$P_{ip}$ , kN	$P_{avg}$ , kN	CFE	U, J	SEA, J/g	Spring back
No filler	--------	Value	21.46	6.38	0.3	254.71	11.62	52.36
		SD.	1.27	0.92	0.03	35.45	1.67	2.07
		CV	6.21	15.02	10.56	12.12	13.34	7.42
HNC	1%	Value	22.37	16.79	0.75	842	32.78	51.2
		SD.	1.67	1.08	0.03	48.8	2.07	1.56
		CV	12.02	7.18	6.08	7.16	10.26	9.5
	2%	Value	26.67	19.10	0.72	874	33.48	25.2
		SD.	1.97	0.94	0.02	60.18	1.65	2.05
		CV	10.69	10.65	2.64	14.26	13.98	11.30
	3%	Value	29.58	20.54	0.69	984	37.24	23.54
		SD.	1.6	1.05	0.02	46.18	1.47	1.85
		CV	5.66	14.28	9.44	12.99	13.35	10.52
	4%	Value	32.75	22.21	0.68	1111	39.16	20.13
		SD.	1.22	1.06	0.03	40.97	0.89	1.03
		CV	10.27	6.29	3.99	6.30	2.32	8.56
MC	1%	Value	21.67	6.51	0.30	311	11.66	52.79
		SD.	1.26	0.39	0.02	9.53	0.46	1.5
		CV	5.86	5.72	7.79	3.04	3.54	5.01
	2%	Value	24.14	11.75	0.49	477	20.88	48.74
		SD.	2.04	0.7	0.05	35.68	1.12	2.44
		CV	14.75	5.98	9.35	7.56	5.39	6.24
	3%	Value	28.41	17.54	0.62	830	29	30.25
		SD.	1.03	1.06	0.05	34.86	1.46	2.98
		CV	3.59	11.80	8.96	7.83	4.93	9.86
	4%	Value	31.74	22.27	0.70	1065	38.46	17.42
		SD.	1.12	0.61	0.05	47.03	1.21	1.52
		CV	3.62	2.81	7.01	4.55	3.24	5.35
Al ₂ O ₃	1%	Value	22.53	7.38	0.33	300	12.26	54.83
		SD.	0.47	0.15	0.01	5.01	0.41	1.95
		CV	2.06	2.03	1.74	1.68	3.35	2.95
	2%	Value	26.28	9.40	0.36	386	16.10	51.37
		SD.	1.20	0.72	0.03	17.64	1.35	1.82
		CV	4.73	7.92	9.41	4.64	9.55	3.45
	3%	Value	27.11	10.42	0.38	438	18	46.57
		SD.	1.44	1.05	0.03	42.58	1.74	1.66
		CV	5.36	12.53	4.90	9.85	10.53	3.65
	4%	Value	14.70	4.75	0.32	175	7.92	22.28
		SD.	1.86	0.08	0.04	4.12	0.16	1.99
		CV	13.16	1.26	12.56	2.39	2.03	4.12
SiO ₂	1%	Value	6.83	3.22	0.47	125	6.25	57
		SD.	0.63	0.10	0.04	8.4	0.41	0.85
		CV	9.40	3.08	8.22	6.80	6.60	4.21
	2%	Value	10.15	3.65	0.36	131	6.52	55.19
		SD.	1.35	0.23	0.03	4.8	0.39	1.21
		CV	13.40	6.66	17.57	3.72	5.99	6.35
	3%	Value	11.89	3.84	0.32	132	6	53.35
		SD.	0.98	0.18	0.02	15.72	0.94	1.98
		C.V.	8.56	4.74	5.25	11.91	13.07	7.05
	4%	Value	11.27	3.66	0.32	140	5.64	58.35
		SD.	0.84	0.36	0.01	8.51	0.56	1.04
		CV	7.44	9.84	2.62	6.23	9.85	2.39
SiC	1%	Value	8.91	2.55	0.29	93	4.35	57.82
		SD.	0.88	0.11	0.02	8.84	0.59	1.01
		CV	10.24	4.15	8.41	8.81	13.38	8.40
	2%	Value	11.98	3.38	0.28	132	5.37	56.15
		SD.	0.87	0.26	0.04	10.31	0.33	0.78
		CV	7.43	8.22	12.72	8.18	6.18	2.01
	3%	Value	11.98	3.86	0.32	148	6	57.72
		SD.	1.07	0.35	0.02	5.79	0.31	1.59
		CV	9.74	9.80	2.41	3.95	5.01	4.89
	4%	Value	11.72	3.60	0.31	150	6.18	58.50
		SD.	0.72	0.27	0.01	7.15	0.12	0.76
		CV	6.21	7.53	3.00	4.79	2.02	3.04

The inclusion of 1, 2, 3, and 4 wt. % of HNC to glass/epoxy composite tubes exhibits an enhancement of 4.24, 24.31, 37.85, and 52.65% in $P_{i p}$ and an enhancement of 163.17, 199.37, 221.94, and 248.12% in the $P_{a v g}$ , respectively. As wt. % of HNC increases up to 4 wt. %, $P_{i p}$ and $P_{a v g}$ increase. This is because the low concentration of HNC causes a good dispersion of HNC in the epoxy and consequently leads to a satisfactory wetting of glass fiber with the nano-modified epoxy. Thus, adding nanoparticles into epoxy matrix improves the adhesion between glass fibers and matrix. Good fiber/matrix adhesion allows good load transmission between glass fiber and epoxy matrix. This result agrees with that reported by Zhang et al.;⁹¹ Abd El‐Baky and Attia.^71,92 As stated by Young et al.;³⁰ Kumar et al.,⁴⁸ composites with nanofillers attain visible enhancements in fracture toughness and energy absorption which could be important in designing vehicle structures. As reported by Fathy et al.,⁸⁰ the enhancement in the properties can be explained by the large specific surface area of the nanoparticles, which leads to an increase in the mechanical properties of the epoxy matrix.

The addition of 1, 2, 3, and 4 wt. % of MC exhibits an enhancement of 1.00, 12.51, 32.39, and 47.91%, respectively, in $P_{i p}$ and an enhancement of 2.04, 84.17, 174.92, and 249.06% in $P_{a v g}$ , respectively. The remarkable enhancement in $P_{i p}$ and $P_{a v g}$ of nanofilled glass/epoxy composite tubes may be attributed to the large surface area/volume ratio of nanofillers which generates an extreme interfacial area surrounding the nanoparticle. The adjacent polymer chains are disordered forming interphase zones of different properties.^52,53,55 According to Kwon et al.,⁹³ the existence of nanofillers could fill in the surface cracks of fiber reinforced composites and increase the radius of the crack tip to avoid the stress concentration. Consequently, the mechanical properties could be enhanced.

An improvement of 5.01, 22.47, and 26.33% in $P_{i p}$ and an enhancement of 15.67, 47.34, and 63.32% in $P_{a v g}$ were achieved by the integration of 1, 2, and 3 wt. % of nano-Al₂O₃, respectively, as compared to neat glass/epoxy tubes. Whilst a reduction of 31.51 and 25.55% in $P_{i p}$ and $P_{a v g}$ were noted by the inclusion of 4 wt. % of nano-Al₂O₃. It is obvious from Figures 13 and 14 that the incorporation of Al₂O₃ in epoxy resin shows a positive effect on $P_{i p}$ and $P_{a v g}$ up to 3 wt. %. The deterioration effect at 4 wt. % of Al₂O₃ may be resulted from the aggregation of nano-Al₂O₃ and poor wetting behavior due to the high viscosity of epoxy.⁹⁴

The incorporation of SiO₂ and SiC nanoparticles adversely affect the crashworthiness characteristics of glass/epoxy composite tubes. The incorporation of 1, 2, 3, and 4 wt. % of nano-SiO₂ causes a reduction in $P_{i p}$ with 68.17, 52.69, 44.58, and 47.49% and a reduction of 49.53, 42.79, 39.81, and 42.63% in $P_{a v g}$ , respectively. The addition of 1, 2, 3, and 4 wt. % of nano-SiC exhibits a reduction of 58.49, 44.19, 44.19, and 45.40% in $P_{i p}$ and a reduction of 60.03, 47.02, 39.50, and 43.57% in $P_{a v g}$ , respectively, Figures 13 and 14.

Crush force efficiency

Crush force efficiency (CFE) is another significant measure of crushing performance.⁹⁵ As the value of CFE moves toward the unity, the improved is the performance of the energy absorbing structure as the absorber is crushed at a value close to $P_{i p}$ , hence minimizing the changes in the deceleration, as desired from any energy absorber design. On the other hand, the deviation of CFE away from the unity refers to a rapid change in the deceleration and this should be avoided in vehicle design.⁹⁶ As noticed from equation (2), CFE relates $P_{a v g}$ to $P_{i p}$ and hence it indicates the shape of load–displacement curve. The peak and post-crush load-carrying capacities of specimens undergoing progressive folding are mostly attributed to the flexural stiffness of the composite lamina.⁶ As shown in Figure 15 and Table 3, CFE of unfilled tube is 0.30 which is obviously affected by the addition of different wt. % of nanofillers. An enhancement of 152.46, 140.83, 133.55, and 128.06% was attained by the addition of 1, 2, 3, and 4 wt. % of HNC to glass/epoxy composite tubes, respectively. Also, the inclusion of 1, 2, 3, and 4 wt. % of MC displays an improvement of 1.03, 63.68, 107.66, and 135.99% in CFE of composite tubes, respectively. The incorporation of 1, 2, 3, and 4 wt. % of nano-Al₂O₃ exhibits an enhancement of 10.15, 20.31, 29.28, and 8.70% in CFE for the fabricated tubes, respectively. As compared to neat glass tube, an enhancement of 58.54, 20.93, 8.61, and 9.26% in CFE was achieved by the incorporation of 1, 2, 3 and 4% of nano-SiO₂, respectively. The addition of 1 and 2 wt. % of nano-SiC has a reduction of 3.70 and 5.08% in CFE of glass/epoxy composite tubes, respectively. On contrary, an improvement of 8.40 and 3.34% in CFE was attained by the integration of 3 and 4% of SiC, respectively.

Figure 15.

Crush force efficiency (CFE) of axially compressed glass/epoxy composite tubes filled with different nanofillers.

Absorbed energy (U)

It is clear from Figure 16 and Table 3 that the type of nanofiller and its wt. % significantly affect the values of U. For unfilled composite tubes, U is 254.71 J. An enhancement of 230.42, 243.30, 286.43, and 336.12% in U was attained by the addition of 1, 2, 3, and 4 wt. % of HNC to glass/epoxy composite tubes, respectively. This improved performance is attributed to the toughening mechanism of HNC such as crack deflection, crack front pinning, and crack bridging. Because of HNC bridging, the micro-cracks were stabilized and stopped developing into large and harmful cracks. This observation also suggests that the adhesion between the HNC and the matrix was quite strong. Some bridging HNC broke when the crack opening force exceeded their fracture strength. The behavior of HNC is similar to that of multi-walled carbon nanotubes (MWCNTs).^97,98 The proposed toughening mechanism of epoxy/HNC nanocomposites was related to several phenomena, including the formation of micro-cracks, nanotubes bridging, pull-out and breaking, crack deflection, and clusters resisting the advance of cracks.^98,99 Similar mechanisms were recorded by Choe et al.¹⁰⁰ and Mirjalili and Hubert¹⁰¹ for different nano reinforcements. Figure 17 Schematic description of HNC toughening mechanisms.

Figure 16.

Absorbed energy (U) of axially compressed glass/epoxy composite tubes filled with different nanofillers.

Figure 17.

Schematic description of HNC toughening mechanisms.

Moreover, it has been demonstrated that HNC improves the adhesion between the fiber/matrix interface.¹⁰² Also, when the micro-cracks due to the quasi-static axial load were met with HNC, they might be stabilized by crack bridging of HNC. This result is consistent with that obtained by Ye et al.¹⁰³ for HNC nanofilled carbon/epoxy. Also, there is an improvement of 21.93, 87.35, 225.99, and 318.07% in U was attained by the addition of 1, 2, 3, and 4 wt. % of MC, respectively. Two different toughening mechanisms were observed. First, due to the increase in elongation to brake. Second, due to the increase in the material impact strength and stiffness.³⁷ The previous studies on various composite materials, tested in the similar experiments,^104–106 show that the materials which fail with extensive delamination and fragmentation can absorb much higher energies than those which tend to fail in a brittle manner. That is why it can be concluded that the ability of a composite structure to dissipate the impact energy is highly dependent on the mode of fracture.

The improvement in U could be directly related to the matrix properties which were influenced by the dispersion of MC. It is obvious from Figure 16 that the addition of nano-Al₂O₃ up to 3 wt. % shows a positive effect on U of glass/epoxy composite tubes. An improvement of 17.66, 51.63, and 71.94% in U was attained by the integration of 1, 2, and 3% of nano-Al₂O₃, respectively, as compared to neat glass tube. Whilst a reduction of 31.16% was noticed for glass/epoxy composite tubes filled with 4 wt. % of nano-Al₂O₃. The increasing trend of the absorbed energy could be explained by the strengthening effect of the matrix, and fiber/matrix interface, but the deterioration effect resulting from the agglomeration of nano-Al₂O₃ and poor wetting behavior due to the high viscosity of epoxy. This result agrees with that obtained by Zhai et al.⁹⁴

The incorporation of 1, 2, 3, and 4 wt. % of nano-SiO₂ exhibits a reduction of, 50.79, 48.71, 48.03, and 45.22% in U of the fabricated tubes, respectively. Also, the incorporation of 1, 2, 3, and 4 wt. % of nano-SiC attains a reduction in U by 63.41, 48.18, 41.97, and 41.13%, respectively. Silva et al.³⁷ recorded that adding SiO₂, glass-spheres (GS), and MMT into glass reinforced polyamide 6 (PA6) conical structures reduces the EAC of the material. Results showed that by changing the secondary filler material, it is possible to change the micro-mechanism of the crash and therefore control the energy absorption characteristics of the composite. The transition from ductile to brittle fracture mode due to adding SiO₂ and SiC nanoparticles was clearly demonstrated as a main reason for the reduced EAC.

Specific absorbed energy

The use of SEA concept is essential when comparing the energy absorption capabilities of energy absorbers fabricated from different materials.⁷⁵ Commonly, the larger the value of SEA, the more efficient is the energy absorber.⁷⁴ Figure 18 displays SEA of the tested composite tubes. It was noticed that unfilled tube has SEA of 11.62 J/g. The addition of 1, 2, 3, and 4 wt. % of HNC exhibits an enhancement of 182.15, 188.19, 220.53, and 237.05% in SEA of glass/epoxy composite tubes, respectively.

Figure 18.

Specific absorbed energy (SAE) of axially compressed glass/epoxy composite tubes filled with different nanofillers.

The addition of 1, 2, 3, and 4 wt. % of MC displays an enhancement of 0.36, 79.74, 149.67, and 231.01% in SEA of neat glass, respectively. The incorporation of 1, 2, and 3 wt. % of nano-Al₂O₃ attains an improvement of 5.57, 38.56, and 51.40% in SEA of glass/epoxy composite tubes, respectively. Whilst the incorporation of 4 wt. % of nano-Al₂O₃ gives a reduction of 31.80% in SEA value. The incorporation of 1, 2, 3, and 4 wt. % of nano-SiO₂ attains a reduction of 46.23, 43.84, 44.70, and 51.43%, in SEA value, respectively. Also, a reduction of 62.54, 53.75, 45.67, and 46.84% in SEA was obtained by the addition of 1, 2, 3, and 4 wt. % of nano-SiC, respectively.

Spring-back

It is obvious from Figure 19 that SB value decreases with increasing wt. % of HNC. That is because SB is associated with the mode of failure. SB of specimens which progressively fail is lower than that of specimens which fail in buckling mode. The same trend for SB values were noticed for glass/epoxy composite tubes filled with MC and Al₂O₃. For glass/epoxy tubes filled with SiO₂ or SiC nanoparticles, there is no obvious trend. SB values of glass/epoxy composite tubes filled with SiO₂ or SiC nanoparticles are higher than those of glass/epoxy composite tubes filled with HNC, MC, and Al₂O₃.

Figure 19.

Spring back (SB) of axially compressed glass/epoxy composite tubes filled with different nanofillers.

Silva et al.³⁷ reported that a significant improvement can be obtained at nanofiller’s content of 0.5–5 %. The unique properties of nanocomposites come from the large number of interfacial effects, existing due to the high surface-area-to-volume ratio of nanofillers. In this perspective¹⁰⁷ reported that the crashworthiness and energy absorption properties of composites could be enhanced by reinforcing the polymer matrix system with active nanoparticulate fillers. Incorporation of nanofillers with proper filler dispersion in the matrix can enhance the interlaminar strength and the energy absorption capacity of the composite.¹⁰⁸

Scatter in test results

Many factors can affect test results, such as specimen manufacturing, preparation, handling, storage, test rig design, and experimental technique. The statistical scatter, that is, coefficient of variation (CV), in the experimental test results, has been obtained as presented in Table 3. The maximum CV for $P_{i p}$ was observed to be 14.75 for 2 wt. % of MC. The maximum CV for $P_{a v g}$ was observed to be 15.02 for specimens with no nanofillers. The maximum CV values for U, SEA, and SB were observed to be 14.26, 13.98, and 11.30 for 2 wt. % of HNC, respectively. The maximum CV for CFE was observed to be 17.57 for 2 wt. % of SiO₂. The obtained results reflect a visible accuracy for all the tested composite tubes.

Visual inspection of damage

The ability of a composite structure to absorb energy was found to be highly dependent on the mode of fracture. Materials which fail in a progressive manner, with extensive delamination and fragmentation, can absorb much higher energies than those which tend to fail in a brittle manner.³⁷ Five failure modes were noticed during the quasi-static compression test of nanofilled glass/epoxy composite specimens. Figure 20 represents the photos of the top views of the crushed specimens. These failure modes can be categorized and identified as follows:

Figure 20.

Top views of the final crushed specimens.

Mode I: The axial crushing mechanism of the tubes is dominated by pure normal macro fragmentation progressive mode, followed by extensive matrix deformation, starting from the lower end of the tube. The crushed materials act like a debris wedge forcing the axial material to both of inside and outside of the tube. The crushing process consists of sliding of the inward and outward fronds, splaying and fragmentation of the tube skin, longitudinal cracks, bending of the fronds and the crushed matrix with destroyed fibers.¹⁰⁹ This failure mode was observed for composite tubes filled with 3 wt. % of HNC.

Mode II: This mode is very similar to Mode I. The only difference between Modes I and II is where the initial fracturing occurs; in Mode I, it happens at the bottom of the tube, while in Mode II, it happens at the top. Because of the contributions of friction between fiber layers and bending, Modes I and II had higher energy absorption capacity than Mode V.¹¹⁰ Since the apparatus is symmetric, it is important to note that Modes I and II are mechanically identical. However, it was discovered that the failure of some specimens began at the top end, while the failure of other specimens began at the bottom end. So, they were categorized as Mode I and Mode II⁷⁴ registered a similar classification. This failure mode was observed for composite tubes filled with 4 wt. % of HNC and composite tubes filled with 3 and 4 wt. % of MC.

Mode III: Matrix fretting and delamination at the top and bottom of the specimen followed by splaying and fragmentation of the tube skin and bending of the fronds were noticed. It can be said that mode III is a combination of mode I and mode II. This combined failure mode was observed for composite tubes filled with 1 wt. % of HNC and composite tubes filled with 2 wt. % of MC.

Mode IV: Transverse shear cracks develop in the circumferential direction of the tube, followed by axial shear cracks, fiber deboning, splaying at the top and/or bottom of the tube, laminar bundle fracture, and compression of the composite tube in this mode. This failure mode can be noticed for composite tubes filled with 2 wt. % of HNC and composite tubes filled with 1 wt. % of MC.

Mode V: It has a plastic hinge-like formation and progressive buckling with repeated folding. This is like thin-walled metals and plastic tubes being crushed. This mode was noticed for all other composite tubes.

Scanning electron microscopy

Figure 21 shows SEM of axially crushed nanofilled glass/epoxy composite tubes. The common failure signs are fiber breakage, matrix cracking, fiber/matrix debonding, and delamination. Glass/epoxy composite tubes filled with HNC shows a perfect bond between fiber/matrix with low matrix cracking and low fiber pullout as shown in Figure 21. This ensures that the incorporation of HNC to epoxy leads to a better resistance of glass/epoxy composite to crushing loads. Glass fibers are closely bonded with each other and effectively covered by nanofilled matrix.

Figure 21.

SEM of axially crushed nanofilled glass/epoxy composite specimens.

For composite tubes filled with SiO₂ and SiC, extensive cavities occurred between the adjacent layers. This cavity reflects the weak fiber/matrix bonding. The low crushing resistance of glass/epoxy composites filled with nano-SiO₂ and nano-SiC may be attributed to the agglomeration of nano-SiO₂ and nano-SiC. Since agglomeration in the interfacial region weakens the fiber/matrix interface and permits an easy path for a crack to propagate.⁵²

Applications

The proposed nanofilled composite tubes are suggested to be used as energy absorbers in the front of train structure or automobile structure, that is, anti-impact rods. These composites can also be applied as crash box in the front of the automobile and energy absorption tubes in aircraft fuselage. Nanofilled composites into energy absorption tube or crash box can be utilized and designed for a particular type of load into the various major structure for high-performance application and safety equipment in major transportation industries such as aerospace, automotive, and marine industry, Figures 22 and 23.

Figure 22.

Safety component device of cylindrical tube as energy absorbers in automobile/train application.^111,112

Figure 23.

Application of energy absorption tubes in aircraft fuselage.^64,113

State-of-the-art

State-of-the-art studies of the crushing of composite tubes reveal different factors affecting their crashworthiness. Of those factors, the constituent materials of the composite are of great interest. In the current study, glass/epoxy composite tubes reinforced with five types of nanofillers, that is, halloysite nanoclay (HNC), montmorillonite clay (MC), alumina (Al₂O₃), silica (SiO₂), and silicon carbide (SiC) were tested under a quasi-static crushing scenario to investigate the best composite in terms of the crushing load and the energy-absorption capacities (EAC). This article provides a state-of-the-art of industrial applications. A state-of-the-art review on EAC of nanofilled polymer composites is introduced.

Future directions

Polymer composites are sensitive to the environment such as temperature and moisture, which are known to influence stiffness, failure modes, and strength properties. For the design of nanofilled crashworthy composite structures, which will work in a wide range of environmental conditions, it is important to understand what influence environmental effects might have on crush failure modes and SEA. Different nanofillers may be used. Different loading conditions may be applied, that is, dynamic (drop weight impact) and quasi-static (lateral or oblique) loadings. This should be supported by numerical simulation to reduce cost and to expedite the understanding of the structure’s response.

Conclusions

Light weight comes to be a significant issue for energy efficiency in automotive. It arises needing for developing innovative materials that combine both weight reduction and safety issues. Throughout this work, the applicability of different nanofillers to polymer composite materials is discussed regards to the fulfillment of these requirements. An experimental study was performed to investigate the crashworthiness and failure mechanisms of glass fiber reinforced polymer composite tubes filled with different nanofillers and different wt. % of these nanofillers. Nanofilled tubes were fabricated using wet wrapping by hand lay-up process and tested under quasi-static axial loading conditions. From the experimental tests performed in this study, the main conclusions are as follows:

• The addition of nanofillers have a significant effect on EAC of glass fiber reinforced epoxy composite tubes. Changing the type and wt. % of the nanofiller can change the failure micro-mechanism, and therefore, control the energy absorption characteristics of the composite.

• The incorporation of 1, 2, 3, and 4 wt. % of HNC leads to an improvement of 230.42, 243.30, 286.43, and 336.12% in U of glass/epoxy composite, respectively. The inclusion of 1, 2, 3, and 4 wt. % of MC leads to an improvement of 21.93, 87.35, 225.99, and 318.07% in U of pristine glass/epoxy tubes, respectively. The effect of HNC is obviously greater than that of MC.

• Compared to pure glass/epoxy composite tubes, an enhancement of 17.66, 51.63, and 71.94% in U was recorded by the inclusion of 1, 2, and 3 wt. % of nano-Al₂O₃, respectively. On contrary, the incorporation of 4 wt. % of nano-Al₂O₃ reduces U by 31.16%.

• The addition of SiO₂ and SiC nanofillers has a negative effect on U of glass/epoxy composite tubes. A reduction of 50.79, 48.71, 48.03, and 45.22% was recorded by the addition of 1, 2, 3, and 4 wt. % of nano-SiO₂ and a decrease of 63.41, 48.18, 41.97, and 41.13% was found by the inclusion of 1, 2, 3, and 4 wt. % of nano-SiC, respectively.

• It is essential to replace energy absorbers of vehicles by novel nanocomposite materials to deliver occupant safety during accidents. The proposed nanofilled composite tubes are suggested to be used as energy absorbers in the front of train structure or automobile structure, that is, anti-impact rods. These composites can also be applied as crash box in the front of the automobile and energy absorption tubes in aircraft fuselage.

• Overall results indicate that glass/epoxy composite tubes filled with 4 wt. % of HNC or filled with 4 wt. % of MC are appropriate to be employed as energy dissipating elements in vehicles due to their remarkable energy absorbing abilities. On contrary, glass/epoxy composite tubes filled with nano-SiO₂ or nano-SiC are not recommended to be used as energy dissipating elements.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Marwa A Abd El-baky

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Novel energy absorbent composites for crashworthiness applications

Abstract

Keywords

Introduction

Methodology

Materials

Dispersion of nanofillers into epoxy resin

Fabrication of nanofilled composite specimens

Quasi-static compression test

Results and discussion

Crushing response and deformation histories

Tubes filled with HNC

Tubes filled with MC

Tubes filled with nano-Al2O3

Tubes filled with nano-SiO2

Tubes filled with nano-SiC

Crashworthiness parameters

Peak and average crushing loads ( P ip , P avg )

Crush force efficiency

Absorbed energy (U)

Specific absorbed energy

Spring-back

Scatter in test results

Visual inspection of damage

Scanning electron microscopy

Applications

State-of-the-art

Future directions

Conclusions

Footnotes

Declaration of conflicting interests

Funding

ORCID iD

References

Tubes filled with nano-Al₂O₃

Tubes filled with nano-SiO₂

Peak and average crushing loads ( $P_{ip}, P_{avg}$ )