ABL Friction Machine Description
Test Purpose
The purpose of completing friction sensitivity tests is to measure the sensitiveness of a material to friction insults. The transition from no reaction to reaction must be determined to evaluate in-process risks. The in-process potential from friction scenarios is compared against the material response. For example below is shown a plot of an example transition from no-reaction at low frictional forces (per area) to reaction at increasing frictional forces. The frictional force must be expressed in terms of force per area at a given speed in order to compare to in-process scenarios and in-process frictional areas and speeds.
Test Variations
There are multiple pieces of friction equipment. The test equipment should be able to closely mimic in-process conditions (surface types and roughness) and relate to the in-process frictional forces by the expression of reaction probability in-terms of friction forces per area at a given speed. The ABL Friction machine has been used by Hercules, ATK, SMS, and others to closely mimic in-process conditions and obtain reaction probabilities at various frictional forces per area at various speeds 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 |
|---|---|---|---|
| Pressure applied to sample | Used to relate sensitivity data (psi) to in-process pressures (N/m2) | Pressures used to mimic process equipment conditions | Pressure range on the hydraulic ram from 25-1000 psi |
| Slider or Anvil speed | Provide a uniform and repeatable speed for each trial that can be related to in-process conditions of the friction insult | Other speeds may be used to simulate specific process equipment conditions | Typically up to 8 ft/sec (2.4 m/sec) |
| Hydraulic ram | Consistent loading of sample beneath the stationary wheel throughout the test | Ram diameter is typically 1 inch | |
| Thickness of friction wheel | Strongly influences the amount of pressure applied to the sample; must be of sufficient thickness to resist the test loads without structural deformation | Other thicknesses may be used to simulate specific process equipment components | Standardized to 0.125 ± 0.001 in (0.318 cm) |
| Surface finish of friction wheel and anvil | Provide a consistent friction insult per area | MIL-STD-1751A Method 1021 references 63 µin for a surface finish for the ABL Friction test surfaces | Standardized to 1.3 – 1.8 µm (50 – 70 µin) for at least six (friction wheel) or three (friction anvil) random locations on at least one friction wheel or two friction anvils from each grinding group. There are standardized ABL Friction Wheel and Anvil Surfacing Procedures. Other surface finishes may be used to simulate specific process equipment components. |
| Material and hardness of friction wheel and anvil | Provide a standardized material and 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” 55 – 62 |
| Parallelism of friction wheel and anvil | Provide a flat, even contact area for a uniform friction insult that can be related to in-process conditions | A contact pattern other than square could result in a localized high friction insult being applied to the sample | Friction loads less than 25 psi can result in uneven contact |
| Flatness of the friction anvil | Provide a uniform surface so additional friction forces are not introduced by surface inconsistencies | Standardized to ≤0.002 in | |
| 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 or length | Provide sufficient sample to cover the area directly under the wheel | Amount of sample on the anvil standardized to cover approximately an area 0.25-in long by at least 0.125-in wide starting under the wheel | |
| 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. | |
| Slide length | Provide a consistent and repeatable friction insult | Standardized to 1 ± 0.05 in | |
| Slider tightness | Minimal lateral movement and sliding friction (no binding) | Standardized to a tightness of fit ≤0.040 in | |
| 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 (particulate velocities > 1000 in/sec) | |
| 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 frictional force per area to the material response. |
ABL Friction Setup Example
