3.1 Free Bulge Tube Hydroforming Without Axial Feeding
For investigating the maximum amount of bulge in both kinds of bi-layer tubes, samples of tubes with an initial length of 250 mm are used. In the first method (bulge without axial feeding), the maximum applied internal pressure for bilayer AL-6063 and FMTLs is 38 and 950 MPa, respectively. Increasing the internal pressure of the tubes more than this amount caused occurs excessive thinning and tearing in tubes, and applying an internal pressure of less than 38 and 950 MPa causes a decrease in the height of the bulge. Figure 8 shows the parts obtained from simulation tests in the first type at the pressure mentioned above. It is obvious that by increasing the pressure, the diameter of the bulge region of the deformed tubes increased, and since there is an insufficient flow of material in the peripheral route of the expansion region, severe thinning occurs in the bulge region. Figure 9 displays the deformed tubes at the primary pressure of 39 and 1000 MPa, where the maximum thickness reduction and, as a result the tear defect occurred.
The bursting phenomena were predicted based on the thinning ratio in this simulation. According to the thinning ratio in bi-layer AL-6063, it can be bulged by 70%. The following figures demonstrate the bulge height profiles in the different pressures in bi-layer AL and CFRP-AL tubes. As shown, bi-layer AL tubes can be bulged until 38 MPa and burst at 39 MPa, while the FMTLs can bear pressure until 950 MPa and burst at 1000 MPa.
Based on Fig. 10, the bulge height of bi-layer AL-6063 and FMTLs is slightly different, so the maximum bulge height of bi-layer AL is obtained at 38 MPa with 28.96 mm while it is 30.03 mm for bi-layer CFRP-AL at 950 MPa.
3.2 Free Bulge Tube Hydroforming with Axial Feeding
On the Other hand, to increase the bulge height and improve the thickness distribution of tubes, axial feeding was employed. In the second method, the tubes’ internal pressure has been considered 19 and 475 MPa for bilayer AL-6063 and FMTLs before moving the punches. Initially, the amount of axial feeding was defined as 10 mm. In this method, at the beginning of the process the punches don’t have any movement. The punches move after reaching the internal pressure to the pressure mentioned above, and after that, the amount of the internal pressure and axial feeding increase simultaneously. Figure 11 shows the loading path in both types of tubes.
3.3 Investigating Hydroforming of Tubes with an Initial Length of 250 mm with Different Pressure Paths
To improve the height of the bulge and achieve the maximum amount of bulge and improve the thickness distribution in the tubes with other pressure paths, three pressure paths have been utilized by FEM. The examined paths with the axial feeding rate of 7, 10, and 13 mm with a final pressure of 38 MPa for bi-layer AL-6063 and an ending pressure of 950 MPa for FMTLs are displayed in Fig. 12.
Also, the geometric shape obtained from the simulation for the pressure paths of Fig. 12 is shown in Fig. 13.
The following figures illustrate the bulge height profiles at different pressures with axial feeding in bi-layer AL and CFRP-AL tubes.
According to the above figure, in the bi-layer AL-6063 tube, due to the lack of coordination between the internal pressure and the axial feeding, wrinkling phenomena occur. For this reason, the height of the bulge has been decreased than the bulge height with a pressure of 38 MPa without axial feeding. Also, it is obtained that the bulge height increases by increasing the amount of axial feeding. Unlike bi-layer AL-6063, in the FMTLs, the bulge height has been decreased in the path (3). To the excessive internal pressure and lack of coordination between the internal pressure and the amount of axial feeding, wrinkling occurs at the edge of the tubes. For this reason, the tubes cannot move and the material cannot flow toward the bulge area as it is shown in the above figure, like bi-layer AL-6063, the bulge height increases by increasing the amount of axial feeding. As can be seen, the height of the bulge in both types of tubes is the same. Although, the bulge height is calculated based on the path (3) and path (2) in the bi-layer AL and FMTLs, respectively. Furthermore, the thickness distribution is separately investigated to examine the influence of the carbon fiber on the AL tube. Figure 15 displays the thickness distribution at each layer of tubes.
According to Fig. 15, the thickness in the bulge area of the inner and the outer tubes is investigated to examine the thickness distribution of bi-layer AL-6063 and FMTLs. As can be seen in Fig. (15 a, b), the thickness variation obtained by FE simulation shows a uniform distribution in the longitudinal direction in both layers of bi-layer AL-6063 while in the FMTLs, due to the carbon fiber has fabric structure, and it is applied high internal pressure, the thickness distribution shows a fluctuating path at the bulge area. In both types of tubes, the highest thinning ratio is in the bulge zone's central section, and it gradually reduces when it goes from the center toward the ends of the tube. Based on the obtained data of thickness distribution, in both tubes of bi-layer AL, the maximum thinning of the AL-6063 is 1.05 mm while, in the FMTLs, it is 1.15 and 1.365 mm for AL-6063 and carbon fiber, respectively. Due to the presence of wrinkling in the external tube of FMTLs in paths 2 and 3, there is a significant thickening in the edge of the tube.
3.4 Deformation Behavior Under Axial Compression
Whenever a tube is compressed, it will collapse either asymmetrically or non-symmetrically, based on tube property. For structural application, progressive axial folding has been considered an efficient energy-absorbing [27].
Figure 14 and Fig. 15 show the axial compression responses of the bi-layer AL-6063 and bi-layer Al and carbon fiber tube after compression tests. As shown in Fig. 16, the instability occurring during compression of the bi-layer Al tube is a uniform axisymmetric folding. When the folding is initiated, the axial stiffness of the tube is significantly reduced, and the folding gradually accumulates with a reduced load. In the case of the bi-layer Al-carbon fiber tubes shown in Fig. 17, instability occurs owing to non-axisymmetric local deformations.
The load–displacement curves are shown in Fig. 18. The peak loads at instability for bi-layer AL-carbon fiber is more than for bi-layer AL tube, but the instability initiation of the carbon fiber-AL tube occurs slightly earlier than that of the bi-layer AL tube owing to the non-axisymmetric buckling with regard to finite element results. According to the load–displacement curves after instability, the stiffness of the AL-carbon fiber tube under compression is weaker than that of the bi-layer AL tube.
3.5 Load-Bearing Capacity Under Lateral 3-point Bending
To evaluate the structural performance of the tubes, 3-point bending experimental tests were performed. Figure 19 and Fig. 20 illustrate the deformation under 3-point bending experimental tests for two different kinds of bi-layer tubes.
Figure 21 shows the obtained load–displacement curves. The load increases in the primary deformation-resistant zone, decreasing gradually in the bending collapse zone. The load-carrying capacity of double-layer AL tubes is higher than that of double-layer carbon fiber tubes.