4.6
The effect of shell material
The material of the detonator shell is an important aspect of detonator design. The most commonly used metals are aluminium and copper, but manufacturers have been introducing new metal alloys in an effort to balance the requirements for low cost, efficiency and reliability. Experimental studies have shown that the initiation capability of the detonator is influenced by the type of the shell [42, 44], but the physical mechanism behind this effect is not well understood. The significance of the material of the shell is also evident when considering the physical processes involved in the generation of the blast wave. The detonation wave initiated in the base charge, induces a shock wave in the metal shell and subsequently in water through a series of transmissions and reflections at the material interfaces which depend on material properties. This section compares the blast waves generated from detonators of different metal shell in order to identify the nature and extent of the influence of the detonator shell.
The materials considered are aluminium, copper and steel. They differ in density with aluminium being the lighter (2710 kg m−3) whereas copper (8930 kg m−3) and steel (8030 kg m−3) have approximately three times larger density. The materials also have different sound speed with copper having the lowest value (4600 m s−1), aluminium the highest (6220 m s−1) and steel approximately in the middle (5680 m s−1). These translate to different specific acoustic impedance values which is a quantity that determines the ratio of transmitted and reflected wave at an interface. Lastly, these materials also differ in terms of yield strength. Copper has the lowest yield point at 140 MPa and will exhibit more plastic deformation, followed by aluminium (298 MPa) and steel (1370 MPa) which has the highest value.
The flow field after the initiation of the copper detonator is shown in figure 4.10. The generated blast wave is initially ellipsoidal and progressively transforms into a spherical wave as it expands, as in the case of the aluminium shell. However, the initial wave transmitted in water (figure 4.10a) is seen to have multiple secondary peaks of equivalent strength. This is caused by strong reflections at the interfaces of the shell and leads to typical reverberations or ’ringing’ in the metal [64]. Once the reactions in the explosive have completed, the resulting blast wave has a complex structure and variable strength along its front which was observed in all materials. The deformation of the cup and shell is similar to the aluminium case for times up to 10 µs. In later times, the copper shell continues to feature the inverse corner and has more anisotropic expansion compared to aluminium due to lower yield strength leading to more extensive plastic
4.6. The effect of shell material 0 1 2 3 [GPa] −0.01 0.15 0.3 0.45 [GPa] −20 10 40 70 [MPa] (a) 2 µs (b) 10 µs (c) 43 µs
Figure 4.10: Plots of pressure from the numerical solution of the blast wave generated by the detonation of the explosive charge in a detonator with 0.5 mm copper shell. The blast wave is asymmetric in the near field similar to the aluminium case, but the internal reflections lead to multiple peaks of approximately half the strength of the blast wave generated in the case of aluminium.
4.6. The effect of shell material 0 1 2 3 4 5 6 7 8 9 10 250 255 260 265 270 275 280 Pressure [MP a ] Time [µs] Aluminium Copper Steel
(a) 400 mm in the normal direction
0 1000 2000 3000 4000 5000 6000 0 1 2 3 4 5 6 Pressure [MP a ] Time [µs] Aluminium Copper Steel
(b) 1 mm in the normal direction
0 2000 4000 6000 8000 10000 12000 14000 0 1 2 3 4 5 6 Pressure [MP a ] Time [µs] Aluminium Copper Steel
(c) 1 mm in the axial direction
Figure 4.11: Comparison of the pressure pulse generated underwater for 0.5 mm thick detonators for different shell materials.
4.6. The effect of shell material
flow.
The comparison of the pressure pulses obtained at selected points in the near and far field of the detonators is shown in figure 4.11 for the three shell materials examined. The three materials show little variation in the generated blast wave when observed in the far field but have stark differences in the near field. The peak pressure for the copper and steel detonators at the distance of 1 mm is a little over 2.5 GPa which is half the value of the aluminium detonator. Similar behaviour is observed in the axial direction. The pulse of the aluminium detonator has almost double the pressure peak of the copper and steel detonators.
Furthermore, the copper and steel detonators also feature multiple peaks which are a result of stronger reflections of the shock wave within the copper/steel shell. This is caused by the higher impedance difference of copper/steel and water which means that the shock wave is mostly reflected at the interface of metal and water and only a small portion is transmitted outside. The shock wave is also reflected at the inner interface between metal and explosive products where the impedance mismatch is also large. This results in more pressure peaks of lower amplitude compared to the case of aluminium case.
As the blast wave expands to larger distances, the stronger pressure peaks merge with the slower ones ahead while the weaker ones behind decrease further and are incorporated into the tail of the blast wave. After this equilibration process, only a single peak remains. At the distance of 400 mm the blast wave has peak pressure of around 9 MPa for both aluminium and copper and slightly lower for steel.
Figure 4.12 shows the decay of shock pressure with distance in the normal direction for all materials considered. The higher peak pressures observed in aluminium extend to a distance of 10 mm while copper and steel exhibit similar peak pressures and decay. At larger distances the blast wave from all configurations shows nearly identical values of pressure and the decay rate specified by the power law 𝑟−1.321(7) describes all materials within error. Similar results are observed for the blast wave in the axial direction.
Overall the blast waves from these configurations are seen to differ only in the near field. As the wave expands, the blast waves converge and they are practically the same at the distance of 400 mm. As a result, the shock energies measured at this point would be similar for all detonators which would mean that they have equivalent initiation capability, according to the European standard test. However, the strength of the shock is the leading factor in the initiation of heterogeneous solid explosives [65, 66]. The initial pressure pulse must be above a threshold to ensure the reliable initiation of
4.7. The effect of the shell thickness