MBOM Impact Machine Description
Test Purpose
The purpose of completing impact sensitivity tests is to measure the sensitiveness of a material to drop-weight impact. The transition from no reaction to reaction must be determined to evaluate in-process risks. The in-process potential energy from impact scenarios is compared against the material response. For example below is shown a plot of an example transition from no-reaction at low impact energies to reaction at increasing energies. The height must be expressed in terms of energy per area in order to compare to in-process energies and in-process contact areas.
Test Variations
There are multiple pieces of drop-weight impact equipment. The test equipment should be able to closely mimic in-process conditions and relate to the in-process energy by the expression of reaction probability in-terms of drop weight energy. The modified bureau-of-mines (MBOM) impact machine has been used by Hercules, ATK, SMS, and others to obtain reaction probabilities at various drop-weight impact energies for more than 25 years.
Key Parameters
Sample conditionRepeatably obtain results for a given sample Substances are tested in the worst-case or smallest-particle-size in-process condition.
| Key Parameter | Objectives | Origin | Specs |
|---|---|---|---|
| Applied impact energy (calibrated) | Used to relate sensitivity data to in-process energies | Historical drop height to imparted energy correlation based on a 6.76 N frictional force | |
| Drop Mass | Source of energy | Other drop masses may be used to simulate process equipment conditions | Mass standardized to 2.00 ± 0.05 kg |
| Drop Height | Source of energy | Typically up to 116 cm; calibration and positioning system should provide minimal drop height variability | |
| Drop Mass Guides | Minimal sliding friction (no binding) which can be qualified by drop time from a specified height | ||
| Intermediate Mass Assembly | Transfer energy from the drop mass into the sample through the impact insert, providing a uniform, repeatable strike | Other intermediate mass assemblies may be used that are equally effective | Standardized to 1.25 kg |
| Impact Insert Diameter | Provide a consistent and repeatable impact insult per area | Other diameters may be used to simulate specific process equipment conditions | Standardized to 0.500 ± .002 in |
| Anvil and Insert Surface Finish | Provide a consistent impact insult per area | Standardized to 50 – 70 µin for at least two (impact insert) or three (impact anvil) random locations on at least two impact inserts or anvils from each grinding group. There are standardized MBOM Impact Insert and Anvil Surfacing Procedures. Other surface finishes may be used to simulate specific process equipment components. | |
| Material and hardness of impact insert and anvil | Provide a standardized hardness | Material and hardness of common process equipment tooling; other materials or hardnesses may be used to simulate specific process equipment components | Standardized as MGR tool steel (A8) with a hardness of Rockwell “C” 50 – 55 |
| Surface contact between anvil and insert | Provide a uniform impact surface so that sensitivity data can be related to in-process energies | A contact pattern other than a whole, full circle will result in localized high and low energy points applied to the sample | Contact pattern visually verified to be a whole, full circle using NCR paper, carbon paper, or equivalent; otherwise the impact insert and/or anvil are replaced |
| Sample thickness | Replicate an in-process condition or otherwise repeatably test a standardize thickness | Other sample thicknesses may be used to simulate specific process equipment conditions | Standardized to a monolayer of material; thickness of slices are typically 0.033 ± 0.004 in |
| Sample quantity | Provide a uniform distribution of material so that energy is not imparted to a localized area | Insufficient quantity to cover the entire impact insert will result in uncharacteristically high energies applied to the sample | Place enough material on the anvil to cover an area in excess of the 0.2 square inch area of the impact insert; slices to have a minimum edge length of 5/8 in |
| Test Temperature and Humidity | Determine the temperature/ humidity dependence of the sample’s sensitivity as applicable. Energetics typically become more sensitive with an increase in temperature. | Standardized as ambient temperature and humidity. In-process conditions should be used as possible. | |
| Reaction Detection | Observation, detection, and documentation of a material’s response to the test stimuli | High-speed video, gas analyzer | |
| Characterization of reaction types | Provide consistent standards for judgment and evaluation of a material | Flash/ flame, audible report, smoke, flame trace, significant sample consumption, decomposition (change in color or odor), jetting | |
| Number of trials | Adequately determine the sensitivity result (characterization curve) with the fewest number of trials | At least 30 trials |
Indicators
| Indicators | Detection Method | Assessment |
|---|---|---|
| Reaction upon insult | Detection of explosion (high-speed video, gas analysis, etc.) | The trial is considered “+” at the ONSET of material reaction. |
| Transition probability | Transition probability most efficiently defined by use of an algorithm such as SEQ, Neyer, or other step sized optimized method | Risk is assessed based on the comparison of the in-process energy to the material response energy. |
MBOM Impact Setup Example
Example Video of Flash and Audible Report (+)
Example Video of Jetting (+)
