What are the two main ways explosives can be detonated?
Direct and Indirect
What are direct ways of detonating explosives?
- Electrical
- Mechanical (percussion)
- Deflagration/detonation
What are indirect ways to detonate explosives?
- Magnetism
- Acoustic
- Radar
Where is an example of a percussion detonation and how does it work?
In blasting caps
* Pulling the trigger moves a hammer which hits a cartridge case
* The percussion sets off a low explosive
* This compresses the anvil that sets off the powder explosive in the powder chamber
* The flash hole allows gas to escape
What causes percussion and why can the detonation be problematic?
Caused by localised heating upon decreased volume
* Hammer compresses the voids of air - gas gets hot and initiates explosion
* Can compress air so much to generate sufficient heat to cause cracks in he explosive material
* Detonation can be non-uniform (wont be controlled)
* Can be easily detonated accidentally from dropping it
What are stabbing based detonation systems?
- Needle is stabbed into explosive material
- Causes hotspots through high localised friction
- Needle can get bent causing a misfire (have a blunt end to decrease chances of bending)
- Friction and percussion can combine in a stabbing detonator
- Some materials are more sensitive to stab and others to percussion
Electrical detonators
- Uses an electrical current to heat up the explosive causing it to detonate
- Low power (not a lot of current passing through) devices often use lead styphnate to start detonation sequence as it is highly sensitive
- High power detonators use higher currents
- More controlled
What is an explosive train?
Where one explosive sets off another, which then sets off another
* Deceases in sensitivity
What is a slapper detonator?
- Indirect electrical detonator
- Uses high electrical current to indirectly detonate an explosive
- Electrical current vapourises a thin piece of metal (slapper plate) that is then propelled into the explosive material
- Initiates an explosive shock with a percussive force
- More controlled - easy to set up but hard to accidentally go off
- Can have multiple slappers from different directions to compress
What are blasting caps and what are different types?
A small explosive device used to detonate a more powerful explosive
* Pyrotechnic Fuse Type
* Solid pack electric type
* Match or fusehead electric type
* Exploding bridgewire type
How does a pyrotechnic fuse type blasting cap work?
Light a fuse that burns away and causes the next explosive to start burning (explosive train)
* Crimping point to stop fuse falling out
How does a solid pack electric type blasting cap work?
Wire heats up and detonates primary explosive
* electrical type
How does a match or fusehead electric type blasting cap work?
Wire heats up and pyrotechnic burns which allows for a time delay
* Can have a controlled time delay to have explosives go off at different times
How does a exploading bridgewire type blasting cap work?
Wire heats up rapidly and vapourises the material to cause initiation
What are the three chemical compositions in blasting caps?
- Pyrotechnic ignition mix - metal with a salt (metal makes it very tracible so need to use metals that arent commonly found in the background environment) - Ti/Zr/B + KClO4/KNO3
- Primary explosive - lead styphnate + lead azide + Al (ASA)
- Output explosive - small amount of high explosive - TNT, PETN
What are the stages of an explosive train in blasting caps?
- Primary explosive
- Pyrotechnic
- Propellant
- Primary explosive
- Booster
- Main charge (secondary)
What are the three stimuli that can set off the fuse types in blasting caps?
- Acoustic - sound
- Proximity (radar) - when things come into a certain distance it will become initiated
- Magnetism - as shsips travel through the water, they build up electrical charge which induces a change in the magnetic field, this change causes detonation
A lot of things can cause acoustic waves but not many will cause a change in magnetic field
What is one way that ships reduce its magnetic field when travelling as to not initiate detonation?
Degaussing the vessle (putting wires over the ships)
Controlling primary explosives and detonators
- Detonators usually require primary explosives - tertiary explosives (ANFO) needs a secondary detonator
- Control explosive material to control the detonators
- Detonator contains the most metal in the explosive
- Hard to synthesise many primary explosives (wont be able to detonate it without this)
