2.1. Experimental Design
Unidirectional carbon fiber reinforced epoxy resin-based composite laminate was used. It was provided by Zhejiang University. This material uses carbon fiber as the reinforcing phase and epoxy resin as the matrix phase. The carbon fiber bundle is embedded in the resin in the same direction through autoclave or vacuum diversion. The unidirectional composite material forms after curing. The bundles are stacked at a staggered angle, and then cemented and solidified by resin to form a carbon fiber composite laminate.
To explode the unidirectional carbon fiber reinforced epoxy resin-based composite laminate and to meet the requirements of the containment experiment, three blasting and cutting pretests were carried out on the laminate. The tests included normal and lateral opening charge explosive tests on the composite plate and linear charge-shaped jet cutting, the blasting separation method that was most suitable was determined by comparing the separation effects of the three blasting cuts in the pretests. The experimental objects were 120 mm × 120 mm × 15 mm and 120 mm × 120 mm × 25 mm laminate.
The normal opening charge is to open two through 10 mm diameter holes in the middle of the laminate for charging and to make guide grooves along the 90° direction on both sides of the charging hole, as shown in
Figure 1a. During the test, the end of the charging hole was closed with thin copper and desensitized RDX detonating cord was inserted into the charging hole. After charging was completed, the charging hole was sealed with paper, and the flanks of two nonel detonators of the same model were fixed with tape to the center of the two holes, as shown in
Figure 1b.
The lateral opening charge was drilled in the thickness direction of the laminate. A 6 mm through-hole was drilled in the 90° direction of the flat plate for charging, as shown in
Figure 1c. For charging and detonation convenience, a detonating cord that was 8 cm longer than the composite board was inserted into the lateral through-hole. The flanks of the detonator were fixed, and the detonating cord was extended with tape, as shown in
Figure 1d.
The principle of linear shaped jet cutting is that after the charge desensitized RDX in the cutter was detonated, the product of explosion and shock wave squeeze the V-shaped explosive cover and push it to form a jet that rushes to the laminate and cuts it off. The schematic is shown in
Figure 2. The shell of the cutter and the V-shaped explosive cover were made from 1 mm thick copper. The shell and explosive cover were attached with glue. The internal charge was desensitized RDX, and the port was closed with plasticine. The two cutters were fixed symmetrically on the front and back of the composite board and exploded by detonating cord, as shown in
Figure 3.
2.2. Test Results and Discussion
The explosion cut result of the charge in the normal opening is shown in
Figure 4. One of the charge holes on the 15 mm thick laminate was not detonated because it was not attached tightly to the main detonation zone of the detonator, and therefore the composite board did not break. Two charge holes on the 25 mm thick laminate were detonated. Fibers around the charge holes were damaged, which damaged the fibers and matrix, and expanded the through-hole to the groove. The laminate broke, and the two broken parts were ~1.3 m apart. The laminate broke because, under this charging structure, the explosion generated a cylindrical shock wave, which propagated from the charging hole to the surroundings, and combined with explosion gas to expand the opening. The prefabricated guide groove expanded and the laminate broke. In [
10], the thickness of the composite board was 8.5 mm, and the three normal opening methods did not successfully separate the composite board, because the thickness of the composite plate was too small, and a large amount of the gas generated by explosions escaped in the thickness direction and could not effectively act on the laminated plate. As the shock wave opening effect became smaller, it was difficult to expand the opening to the prefabricated guide groove, and the laminate would not broke. If the laminate is to be separated successfully, the prefabricated guide groove must be expanded to the edge of the charge hole, and at the same time, the plate cannot be broken in advance, which is too difficult to operate, and therefore, for small thickness laminates, this method is difficult to implement. The two fractured plates were 1.3 m apart in this pretest because the explosion shock wave provided a large additional lateral force to the fractured two plates in the fracture direction during the expansion and propagation, which resulted in a long distance after the composite plate was separated. It is difficult to meet the requirement that the laminate breaks only in the containment test without excessive residual speed and fragment splash.
The results of the explosive cutting of the lateral opening charge are shown in
Figure 5. Two laminates of different thicknesses were cut, but along the thickness direction, the laminate disintegrated because the huge explosive energy blew it into many pieces. The shock wave pressure that was generated by the explosion was greater than the Y-direction tensile strength of the laminate, which made the laminate fracture. However, this laminate was composed of many single-layer carbon fiber boards that had been superimposed by a specific method, and the tensile strength in the thickness direction was low. When the shock wave was transmitted from the inner hole of the plate to the interface between the plate and the air in the thickness direction, since the acoustic impedance of air was much smaller than that of laminates, most of the stress wave was reflected back to form tensile stress wave that was opposite to the propagation direction. The peak pressure of the tensile stress wave was greater than the tensile strength of the laminate in the thickness direction, therefore, the laminate disintegrated in the thickness direction. The experimental results did not meet the requirements of the containment test.
The results of the linear charge-shaped jet cutting are shown in
Figure 6. Two laminates of different thicknesses were broken. The two parts of the 15 mm thick laminate are ~0.7 m apart, and the two parts of the 25 mm thick laminate are ~0.4 m apart. After the explosion, a certain degree of damage resulted on the surface of the incisions of the two laminates with different thicknesses. In
Figure 7, the area enclosed by the red frame is the damage area of the plate after the laminate was cut. The damage area of the 15 mm thick plate was about 27.9% of the entire sample, and the damage area of the 25 mm thick plate was about 28.36% of the entire sample. This damage meant that a considerable number of additional fragments were produced. Fragments result because the explosion height is not set between the cutters and the laminate, and the jet reaches the laminate before it is formed sufficiently. Large lateral forces result during cutting, which result in a long distance between the two parts after the laminate is separated, causing a large degree of damage to the laminate surface.
The pretest results show that normal opening charge blasting cutting is not conducive to cutting thin laminate, whereas the broken board retains a higher speed, explosive cutting of the lateral opening charge, and the laminate is easy to delaminate. Therefore, the above two experimental methods do not meet the requirements of the containment experiment. After linear charge-shaped jet cutting, although damage occurs near the cut surface of the composite plate, and the fractured composite plate has a higher lateral velocity, the damage can be reduced by adjusting the explosion height and amount of explosive. Therefore, among the three cutting methods, linear charge-shaped jet cutting is the best way to separate the composite boards.
After the composite plate has been separated, additional fragments splash the least when the surface damage is a minimum, and the minimum lateral velocity of the broken plate occurs when the plate breaks under the minimum explosive amount. In
Section 3, a numerical simulation method is described that is used to obtain the explosion height that can minimize the degree of surface damage to the composite plate and maximize the penetration depth of the jet (the minimum amount of explosive when cutting the composite plate of the same thickness) to optimize the linear charge-shaped jet cutting and to meet the requirement of the containment experiment that composite board is separated under the premise of an uneven blade thickness.