Experimental Study on Double-Walled Copper and Carbon/Epoxy Composite Tubes under the Axial Loading

All the mentioned samples have the same initial length, tube wall thickness and filling conditions; but, their diameters are different. Fig. 5 shows diagram of axial compression load versus displacement of the specimens C-01 and C.R-02 with the same external diameter, in which, the specimen C-01 is a carbon/epoxy composite tube with wall thickness of 1 mm and the specimen C.R-02 is a solid carbon/epoxy composite rod. The figure indicates that by increasing tube diameter in the silicon sealant-filled condition of composite tubes, axial compression load enhances. As shown in Fig. 5, initial peak load of the specimen C.R-02 is almost three times bigger than the initial maximum load of the specimen C-01.

Experimental observations show that fracture occurs during the axial compression test on the solid composite tube and so, a sudden decrement is considered in axial load of the specimen C.R-02 in the graph. Also, Fig. 5 shows that energy absorption by the carbon/epoxy composite rod is more than the corresponding empty carbon/epoxy composite tube.

Specific Absorbed Energy (SAE) is a very important parameter in design of energy absorbers. Fig. 6 compares SAE of carbon/epoxy composite tubes with empty and silicon sealant-filled conditions as well as, a solid carbon/epoxy composite rod. The specimens C-01, 03 and 04 are empty carbon/epoxy composite tubes with the same initial length and tube wall thickness and with different diameters. Also, the specimens C.F-05, 06 and 07 are filled carbon/epoxy composite tubes with silicon sealant having different tube diameters and the same characteristics. The figure shows that in silicon sealant-filled condition of carbon/epoxy composite tubes, when tube diameter decreases, SAE by the structure increases, but, in empty composite tubes, there is an optimum diameter for tubes and an empty tube with optimum diameter has the highest performance, in viewpoint of the ratio of absorbed energy/mass. On the other hand, in the filled composite tubes, specimens with smaller diameters are better energy absorbers; but, in the empty ones, by increasing tube diameter, SAE of some cases decreases and SAE of some other cases increases.

The specimens C-04 and C.F-07 are the same tubes with different filling conditions of empty and silicon sealant-filled, respectively. There is a similar order between the empty and filled specimens C-01 and C.F-06 and also, between the specimens C-03 and C.F-05. The figures shows that in all the cases, when carbon/epoxy composite tube is filled with silicon sealant-filler, SAE of the structure decreases in comparison with the corresponding empty one. It means that in viewpoint of crashworthiness, an empty carbon/epoxy composite tube is preferred to the corresponding silicon sealant-filled composite tube. In other words, filling carbon/epoxy composite tubes with silicone sealant is not a suitable method to increase specific absorbed energy by the structure.

Also, Fig. 6 illustrates that specific absorbed energy by the solid carbon/epoxy composite rod C.R-02 is less than the corresponding values of most of the cases. It means that carbon/epoxy composite cylinder with hollow profile is preferred to the corresponding solid structure.

3.2. Effects of Copper Tube Diameter

Fig. 7 depicts load-displacement curves of the specimens CP-08 and CP-09. Both of the samples are made of copper with the same wall thickness and length, while having different diameters. Based on the fact that the area under the load-displacement curve indicates energy absorption of the specimen, the figure shows that copper tube with larger diameter has more energy absorption compared to the other one. Fig. 8 shows SAE of the two above mentioned specimens. According to the figure, it is found that in the studied cases, SAE increases by decreasing the copper tube diameter.

3.3. Copper Tubes Reinforced with Carbon/Epoxy Composite Tubes

The effects of tube diameter and different filling conditions on energy absorption behavior of the specimens are investigated.

3.3.1. Effects of Carbon/Epoxy Composite Tube Diameter

In the specimens C.C-10, C.C-11 and C.C-12, carbon/epoxy composite tubes with different diameters Are positioned concentrically into copper tube of 19 mm diameter and then both of them are filled with silicone sealant (type G in Table 2). Fig. 9 and Fig. 10 show load-displacement and energy-displacement diagrams of three above mentioned specimens, respectively. Comparison of the curves in Fig. 9 indicates that when diameter of composite tube reduces, axial load of double-walled copper and carbon/epoxy composite tubes with silicon sealant-filler between them and into the inner tube increases. Also, Fig. 10 shows that absorbed energy by the specimens C.C-10, C.C-11 and C.C-12 are equal to 338.2, 268.5 and 222.6 J, respectively. It means that by increasing diameter of carbon/epoxy composite tube, energy absorption of the structure increases. On the other hand, when total absorbed energy is the main parameter in design of an energy absorber, smaller diameter of composite tubes into the cupper ones is suggested.

3.3.2. Effects of Copper Tube Diameter

Load-displacement and energy-displacement diagrams of the specimens C.C-12 and C.C-15 are shown in Fig. 11 and Fig. 12, respectively. The figures indicate that in double-walled copper and carbon/epoxy composite tubes with silicon sealant-filler between them and into the inner tube (type G in Table 2), by enhancing copper tube diameter, axial load and energy absorption of specimens increase due to the higher volumes of silicon sealant-filler and copper tube material in the specimen C.C-12. Furthermore, Fig. 13 depicts an experimental comparison with SAE of the above mentioned specimens. As shown in the figure, it is found that when diameter of carbon/epoxy composite diameter reduces in the filled double-walled tubes with the same copper tube diameter, SAE decreases. Also, by decreasing copper tube diameter in the specimens with the same diameter of carbon/epoxy composite tube, SAE increases.

Figure 9. Comparison of load–displacement diagrams of copper tubes reinforced by carbon/epoxy composite tubes with different diameters

Figure 10. Comparison of energy–displacement diagrams of copper tubes reinforced by carbon/epoxy composite tubes with different diameters

It means that according to the ratio of absorbed energy/mass, filled double-walled structures with smaller diameter of copper tube and larger diameter of composite tube are suggested.

3.3.3. Effects of Filling Conditions

Load-displacement curves of the specimens C.C-13 and C.C.E-16 with the same geometrical characteristics are depicted in Fig. 14. The only difference between these specimens lies in filling condition, so that the specimen C.C-13 is filled by silicone sealant completely (type G in Table 2), but in the specimen C.C.E-16, the gap between the tubes is only filled by silicone sealant (type F in Table 2). By comparing the results in Fig. 14, it can be concluded that energy absorption of the specimen which is completely filled is more than the other one due to higher volume of silicon sealant-filler. SAE of double-walled copper and carbon/epoxy composite tubes with different filling conditions is shown in Fig. 15. The figure shows that in the specimens with the same copper and carbon/epoxy tube diameters, filling condition G (table 2) has more energy absorption than the filling condition F. On the other hand, in double-walled copper and carbon/epoxy composite tubes, when silicon sealant is used as the filler between the tubes and also, into the inner tube, a better energy absorber structure is obtained, compared to using silicon sealant-filler just between the tubes. Also, in Fig.15, two specimens C.C.R-18 and C.C.R-19 are compared. In both of the specimens, a solid carbon/epoxy composite rod of 10 mm diameter is used instead of carbon/epoxy composite tube. Diameter of copper tube in the specimen C.C.R-19 is larger than the copper tube in the specimen C.C.R-18. Comparison shows that, decreasing the copper tube diameter in concentrically solid carbon/epoxy composite rod and copper tube with silicon sealant-filler between them, leads to an increase in SAE of the structure.

Palanivelu et al. [19] perform some valuable experiments on nine different geometrical shapes of composite tubes in viewpoint of their quasi-static crushing performance to understand effects of geometry, dimension and triggering mechanism on progressive deformations of small-scale composite tubes. They manufacture different geometrical shapes of composite tubes by hand lay-up technique using unidirectional E-glass fabric (with single and double plies) and polyester resin. 144 quasi-static tests are conducted for all nine geometrical shapes with different thickness/diameter ratios (t/D) and two triggering profiles: 45^o chamfering and tulip pattern with an included angle of 90^o. Their experimental results [19] show that in composite samples with wall thickness of 1 mm, specific absorbed energies by all different geometrical shapes of composite tubes with two different triggering profiles except for a type of conical circular tube are in the range of 3630-17300 J/kg. Comparing the recently reported results by Palanivelu et al. [19] and numerical results of the present article in Fig. 15, it is found that specific absorbed energy by the specimen C.C.R-18 that is a concentrically solid carbon/epoxy composite rod and copper tube with silicon sealant-filler between them is almost 21000 J/kg and it is higher than specific absorbed energies by all eight types of different geometrical shapes of composite tubes with two different triggering profiles. It novelty of the present article in introducing solid carbon/epoxy composite rods as a central core into copper tubes, comparing with the previous published work by Palanivelu et al. [19].

Abedi et al. [15] investigate folding process of square and rectangular metal columns under the quasi-static axial loading and study effects of polyurethane foam-filler on energy absorption behavior of the columns.  For this purpose, they introduce a new theoretical deformation model for column and the foam-filler and derive some equations for predicting instantaneous axial force versus displacement through the process, based on the energy method. Furthermore, they prepare some metal specimens with rectangular and square cross-sections and use them in the folding test between two rigid platens. Their experimental results show that during the axial compression of aluminium and brazen columns, energy absorption capability by metal columns increases through the folding progress due to the polyurethane foam-filler; but, at the commencement of the process, compression axial load increases from zero and when the load reaches a peak value, decremental trend starts. On the other hand, during the axial compression of metal columns, axial load has intensive variations through the process, and in viewpoint of energy absorption, it is an undesirable trend. But, the sketched curves in Fig. 4 show that silicone sealant-filler into the carbon/epoxy composite tubes causes an approximate constant load through the process. Constant load during plastic deformations is a valuable advantage of energy absorbers and the present work introduces the new filler of silicone sealant into the carbon/epoxy composite tubes with this valuable advantage during the axial compression between two rigid platens in the quasi-static condition.

1. 4.       Conclusion

Circular copper tubes reinforced with carbon/epoxy composite tubes were tested in the axial compression condition and their energy absorption capabilities were measured. Load–displacement and energy–displacement diagrams of the specimens were sketched and energy absorption was calculated per unit of mass.. The effects of different geometrical characteristics such as carbon/epoxy composite tube diameter, copper tube diameter and the effects of filling conditions were studied. The experiments showed that absorbed energy of circular copper tubes that were filled with carbon/epoxy composite tubes was higher than a sole copper tube.

Also, it is found that copper tubes have less absorbed energy per unit of mass, compared to corresponding empty carbon/epoxy composite tube and also, a filled composite tube by the silicone sealant. The present research suggests filled double-walled structures with smaller diameter of copper tube and larger diameter of composite tube by considering the ratio of absorbed energy/mass. Also, the experiments show that when silicon sealant is used as the filler between the tubes and also, into the inner tube in double-walled copper and carbon/epoxy composite tubes, a better energy absorber structure is obtained, compared to using silicon sealant-filler just between the tubes.

Acknowledgements

The authors gratefully acknowledge Eng. Majid Moeinifard (M.Sc. student in Tarbiat Modares University, TMU) for his contribution in this research. Also, the authors gratefully acknowledge Strength of Materials Laboratory in Yasouj University for its supports in preparing the specimens and also, machine test.

Figure 13. Comparison of SAE of reinforced copper tubes with carbon/epoxy composite tubes (type G in Table 2)

Figure 14. Load-displacement curves of specimens with the same geometrical characteristics and different filling conditions

Figure 15. SAE of copper tubes reinforced with carbon/epoxy composite tubes by different filling conditions

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