Axial Crushing and Energy Absorption of Empty and Foam Filled Jute-glass / Epoxy Bi-tubes

Experimental work on the axial crushing of empty and polyurethane foam filled bitubular composite cone-tube has been carried out. Hand lay-up method was used to fabricate the bi-tubes using woven roving glass, jute and hybrid jute-glass/epoxy materials. The tubes were of 56 mm diameter, and the cones top diameters were 65 mm. Cone semi-apical angles of 5°, 10°, 15, 20° and 25 were examined. Height of 120 mm was maintained for all the fabricated specimens. Effects of material used, cone semi apical angle and foam filler on the load-displacement relation, maximum load, crush force efficiency, and the specific energy absorption and failure mode were investigated. Results show that the foam filler improved the progressive crushing process, increased the maximum load and the absorbed energy of the bitubes. The maximum crushing load and the specific energy absorption increased with increasing the cone semi apical angle up to 20 for the empty bi-tubes and up to 25 for the foam filled bi-tubes. Progressive failure mode with fiber and matrix cracking was observed at the top narrow side of the fractured bi-tubes as well as at the bottom surface of 20 and 25 cone semi-apical angle bi-tubes.


Introduction
Crashworthiness analysis is an important factor to design structures and to determine the capability of the structure to absorb energy and to protect vehicle occupants during collision event [1].Fiberreinforced composite materials are used now in automotive industry to provide better crashworthiness with using lightweight materials [2].The crush behaviour of composite structures offers distinct advantages for automotive applications and has been the subject of numerous investigations [3].Several studies performed on composite structures like tubes [3][4][5][6][7][8][9][10] and cones [11][12][13][14] have been carried out to investigate the energy absorption capability and failure mechanism of these structures at different testing conditions.Researchers found that, high energy absorption can be obtained during the progressive crushing of composite tubes and cones under axial loading.In addition, researchers found that the stability, mode of failure and energy absorption of thin-walled tubular components can be improved by using foam fillers [10].
The mechanism by which the structure collapses has a direct influence on the energy absorption capability [11].In this paper, a new foam filled bi-tubular arrangement is used for the aim of improving the crashworthiness under axial crushing.The main objectives of the present research are to investigate the crushing characteristics, specific energy absorption and failure mode of empty and foam filled bi-tubular cone-tube under axial compression loading.Also, the research aims to investigate the effect of cone-semi apical angle and material used on the performance of the bi-tubular arrangement.

Materials and Fabrication
Composite cone and tube specimens were fabricated by hand lay-up method using woven roving jute and glass fibers with epoxy resin matrix.Cone and tube moulds were fabricated by using zinc coated mild steel sheets.Then the sheets were bent and welded to form the tube cone shape specimens.Plywood was inserted inside these hollow moulds for support purpose.A PVC circular cross-section sheets were cut with diameters equal to the top and bottom diameters of the cone and the tube and then fixed at the mould sides.The plywood was made in a way that can be easily extracted from these moulds after the completion of the fabrication process.Figure 1 shows the geometrical configuration of the fabricated bi-tubular cone-tube.

Figure 1. Geometrical configuration of the bi-tubular cone-tube
Where; t is the wall thickness, D is the tube diameter, D t and D b are respectively the top and bottom diameters of the cone.Table 1 shows the dimensions of the fabricated bi-tubular specimens.(ߙ) is the cone semi-apical angle and H is the height of the specimen.Constant slow speed compression testing was performed using a computer-controlled servo-hydraulic Instron machine type 5584 (Figure 2).The cross-head speed was adjusted at 4 mm/min.Composite bitubular cone tube specimens were axially crushed between two parallel steel flat platens, one is static and one is moving.The fixed platen was fitted with a load cell from which the load signal was taken directly to the computer.For each test, the crush load was plotted on the Y-axis and the crosshead displacement on the X-axis.supported load higher respectively 37.97%, 38.98% and 46.40% than the empty glass, jute-glass and jute/ epoxy bi-tubes.The increase of cone semi apical angle from 5 o to 25 o increased the maximum load 25.25%, 32.20% and 33.40% respectively for the glass, jute-glass and jute/ epoxy foam filled bitubes.The increase in the load is attributed to the increase in the cross section area of the bi-tubes with the increase in the cone semi-apical angle and crush distance.With the increase in the cone semi apical angle from 5 o to 20 o , the maximum load increased 11.72%, 19.52% and 18.70% respectively for the glass, jute-glass and jute/ epoxy empty bi-tubes.Further increase in the cone semi-apical angle of the empty bi-tubes, decreased the maximum.
(a) (b) Figure 5 shows effect of cone semi-apical angle on the maximum axial compression load of the empty and foam filled bi-tubes.Cone semi-apical angles of 5 o , 10 o , 15 o , 20 o and 25 o were examined.It can be seen that the maximum load increased with the increase in the cone semi-apical angle from 5 o to 20 o and from 5 o to 25 o for the empty and foam filled bi-tubes respectively.The foam filler provide stability to the bi-tubes to withstand the applied load.Maximum load values of 141.16 kN, 112.84 kN and 85.73 kN respectively were obtained from the glass, jute-glass and jute/ epoxy foam filled bitubes of 25 o degree cone semi apical angle.The empty glass, jute-glass and jute/ epoxy bi-tubes supported maximum load values of 99.56 kN, 75.05 kN and 55.37 kN respectively for the 20 o cone semi apical angle.

D2ME 2016
The crush force efficiency (CFE) is important factors to measure the crush performance and to evaluate the crashworthiness of the energy absorber component.The CFE for the composite tubes under lateral loading is determined using the following equation [2,5].
Where F ୫ୣୟ୬ and F ୫ୟ୶ are the mean and maximum crush failure loads respectively.Maximum absorption capacity is obtained when the crush force efficiency approach to unity cartographic damage [15].On the other hand as the crush force efficiency decreases, the absorption capacity decreased resulted in a catastrophic failure [16].
Figure 6 shows the crush force efficiency of the axially crushed bi-tubes.As shown, crush force efficiency of the foam filled bi-tubes is significantly higher than the empty bi-tubes.Maximum crush force efficiency of 0.91, 0.86 and 0.84 were obtained by the foam filled glass, jute-glass and jute/ epoxy bi-tubes respectively.Maximum crush force efficiency of 0.82, 0.79 and 0.74 were obtained from the empty glass, jute-glass and jute/ epoxy bi-tubes respectively.Figure 6.Crush force efficiency versus cone semi-apical angle for the empty and foam filled bi-tubes

Specific Energy Absorption
The specific energy absorption (SEA) is defined as the total absorbed energy (E t ) per unit mass (m) of the absorbed structure as in the following equation [4,8,13] : Where the total absorbed energy is the work done by the crushing force represented by the area under the axial force versus axial displacement curve and can be determined as follows [4,8,13] : Where F is the crush force in axial direction, and δ is the displacement in axial direction.The specific energy absorption of the axially crushed tubes is shown in Figure 7.It can be seen that, the specific energy absorption increased with the increase in the cone semi-apical angle.The energy absorbed by the foam filled bi-tubular cone-tubes is found higher than that obtained by the empty bi-tubes for all the cone semi-apical angles tested.Maximum specific energy absorption values of 1056 J/kg, 9115 J/kg and 6885 J/kg were obtained by the glass, jute-glass and jute/epoxy foam filled bi-tubes respectively.Maximum specific energy absorption values of 9562J/kg, 8553 J/kg, and 5902 J/kg were obtained by the empty bi-tubes of glass, jute-glass and jute/epoxy respectively

Failure Mode
Figure 8 shows the fractured specimens of empty and foam filled bi-tubular cone-tubes tested under axial compression load.The fracture initiated at the narrow upper end of the specimens at the region of the highest stress.It has been found that more fiber splitting obtained from the foam filled bi-tubes.
The more fiber splitting provides stable crushing process with higher load to cause failure.

Conclusions
The main conclusions that could be drawn from this investigation are: 1.The maximum load, crush force efficiency and the energy absorption increased with increasing cone semi-apical angle up to 20 o for the empty bi-tubes and up to 25 o for the foam filled bi-tubes.
2. Maximum axial load of 141.16 kN, 112.84 kN and 85.73 kN were obtained from the foam filled glass, Jute-glass and jute/ epoxy bi-tubes respectively with the cone semi-apical angle of 25 o .The maximum loads obtained by the empty bi-tubes were 99.56 kN, 75.05 kN and 55.37 kN respectively with the cone semi-apical angle of 20 o .3. The crush force efficiency of the foam filled bi-tubes is higher than the empty bi-tubes.Maximum crush force efficiency of 0.91, 0.86 and 0.84 were obtained from the foam filled glass, jute-glass and jute/ epoxy bi-tubes.The values were 0.82, 0.79 and 0.74 for the empty bi-tubes.4. Maximum energy absorption of 10561 J/kg, 9115 J/kg, and 6885 J/kg were obtained from the foam filled glass, jute-glass and jute/ epoxy bi-tubes respectively with the cone semi-apical angle of 25 o .The maximum energy absorbed by the empty bi-tubes were; 9562 J/kg, 8553 J/kg, and 5902J/kg respectively with the cone semi-apical angle of 20 o . 5. The bi-tubular specimens were fractured by progressive failure splaying mode with fiber and matrix cracking at the top narrow side of the specimens.Further cracks were noticed at the bottom surface of 20 o and 25 o cone semi-apical angle bi-tubes.

Figure 5 .
Figure 5. Maximum load versus cone semi-apical angle for the empty and foam filled bi-tubes

Figure 7 .
Figure 7. Specific Energy absorption of the axially crushed empty and foam filled bi-tubes

Figure 8 .Figure 9 .
Figure8shows the fractured specimens of empty and foam filled bi-tubular cone-tubes tested under axial compression load.The fracture initiated at the narrow upper end of the specimens at the region of the highest stress.It has been found that more fiber splitting obtained from the foam filled bi-tubes.The more fiber splitting provides stable crushing process with higher load to cause failure.Fiber break and matrix cracking were observed at the bottom end of the empty bi-tubular specimens of 20 o and 25 o cone semi apical angles.Samples of the fractured specimens are shown in Figure9.The specimens were of 5 o cone semi-apical angle.It has been found that the foam filled bi-tubular specimens fractured by progressive crushing splaying mode.Failure mode Specimen typeEmpty bi-tube Foam filled bi-tube ߙ, (degree) 5 25 5 25

Table 1 . Dimensions of the cone and tube arrangements Specimen type
ߥ 12 =0.29 and ߩ = 1600 kg/m 3 .The properties of the glass/epoxy specimens were E 11 =52.25 GN/m 2 , G 12 =3.08GN/m 2 , G 23 =4.20 GN/m 2 , ߥ 12 = 0.35 and ߩ= 2100 kg/m3.Where E 11 is the modulus of elasticity in the longitudinal direction, G 12 and G 23 are the In-plane and out of plane modulus of rigidity, ߥ12 is the minor Poisson's ratio and ߩ is the density.For tubes with foam filler, polyurethane foam of 20 kg/m 3 density was used to fill the gap between the inner tube and the outer cone.