Lecture 11 Flashcards
Sensitivity to friction
Assessing explosive performance
- Place a small quantity of explosive onto a sliding block
- Apply a load with known weight
- Hit the sliding block using a pendulum and observe any evidence of initiation
- Repeat 100 times to ensure reproducibility
- Different frictional surfaces are often considered
- Can also explore more advanced properties
How does the sensitivty to impact approach work?
Pendulum is released from a known height and causes a frictional force with the load and we observe the initiation of the explosive
Why are repeats necessary in the sensitivity to friction approach?
The test has uncertainties associated with it
What advanced properties can be explored via the sensitivity to friction approach?
- Type of cleavage of explosive (even to a specific miller plane). The point at which is cleaving can impact the design.
- Effect of environmental conditions (e.g. moisture, temperature, humidity).
- We want our explosive to be able to operate within lots of different environmental conditions and we want to understand how it they are initiated in those conditions
What can we change in the sensitivity to friction approach?
We can change the type of coarseness on top of the load
Sensitivity to impact
Assessing Explosive Performance
- Primarily carried out using drop towers
- A small quantiy of explosive is placed underneath the impactor
- A known weight is dropped from increasing heights
- High speed camera used to monitor ignition event and how much has exploded
- Use new sample for every drop height to prevent impact-induced sensitiveness
- Plot height vs ignition event using median drop height
- Compare them to standards like TNT on the arbitary scale
What should you do if a sample hasn’t exploded during a performance analysis?
Even if it hadn’t initiated you would still have to use a new sample as they weight could have caused cracking or morphological change that could have changed it
Sensitivity to Sparks and Discharge
Assessing Explosive Performance
- A capacitor is charged using a high potential source is used to mimic a spark
- A small quantity of explosive is placed on a roller
- The sample is gradually wound upwards toward the discharge electrode
- At a critical distance energy is released via a spark and initiation is monitored
- Testing starts with a high spark energy
- Repeat measurements are made gradually reducing the spark energy until no initiation events occur
How does the sensitivity to sparks and discharge approach work?
- The discharge electrode is stored within a capacitor
- The capacitor is charged and the sample is moved towards it as the arms can move in both directions.
- When it gets to the critical point we get arcing which causes a spark to hit our material through the explosive and we see if it initiates
- Understanding the critical difference and relative potential is important
- Its difficult to predict the relative energy in terms of a spark to initiate an explosive. So you start high and then go down from there if it initiates
What levels are assessed during the sensitivity to sparks and discharge approach?
- Average static shock from a human
- The maximum energy from initiation devices
- The mass, shape and size of sample are considered
- This can significantly change the result due to arcing of the spark changing
- It can affect how the current can travel through or around the sample
- Environmental conditions can also be examined
- Multiple samples tested at each energy level to ensure reproducibility
- Understandng the voltage we need to apply through the detonators is an important paramter. If this is too high then it is difficult to achieve.
Sensitivity to heat
Assessing Explosive Performance
- Small quantities of explosive are placed into vials and then holes within a metal block (typically 6 to 12)
- The vials have a conductive base
- The block is heated slowly at a fixed temperature gradient set by regulations. This caused conduction to the explosive samples in the vials
- The temperature of ignition is monitored using a high-speed camera
- Experiment repeated 10 times to ensure reproducibility of sample
- External environmental conditions are carefully controlled (e.g. ambient heat, humidity)
Why do the external environmental conditions need to be carefully controlled in a sensitivity to heat approach?
- The rate of heating is impotyance as it can effect the way the explosive will initiate. This can be effected by the ambient temperature which is why it needs to be carefully controlled.
- Explosives have different properties and sensitivities when they are wet so we need to know the humidity when conducting these experiements
Why does the sensitivity to heat approach require less repeats?
- We can test many samples at the same time and see if they initiate at the same time
- So less repeats needed as we can test many samples at the same time
What does the Abel Test measure and what is it used for?
- The Abel Test measures the decomposition of energetic materials into NOx gases
- Used for nitrocellulose, smokeless powders, rocket fuels & nitro containing compounds
What can be used to examine the chemical stability of certain explosives
Heating
