Method 522.2 of MIL-STD-810G CN1 defines ballistic shock as “a high-level shock that generally results from the impact of projectiles or ordnance on armored combat vehicles”. Typical engagements of interest also include Kinetic Energy projectiles, land mines, and improvised explosive devices. For the purposes of this TOP, ballistic shock is generally referred to as the sudden high-rate loading resulting from under body blast (UBB) testing designed to assess the crew-survivability of military vehicles. Historical testing conducted in both areas have proven the relative similarities between the two environments.

Typical load bench used to quantify an accelerometer's base strain sensitivity prior to testing.

Live-fire and other ground tests conducted with modern instrumentation have proven that the damage potential of ballistic shock is not only to the test item, but also to the instrumentation used to quantify the severity of the event. One of the primary interests of ballistic shock studies is to ensure that armored vehicles and their occupants survive the encounter while retaining their mission capabilities. To help assess this, the instrumentation used to quantify the survivability of the event must itself survive the test and provide accurate data.

One of the primary means of collecting ballistic shock data is through the use of accelerometers. Capturing valid acceleration data in this environment can be particularly challenging. Many preparations must be taken to ensure the highest likelihood of obtaining a valid record. For example, data acquisition systems must be placed several hundred feet from the test pad in a hardened bomb-proof enclosure, and instrumentation cabling must be protected from fragmentation with underground troughs, steel beams, and flexible ballistic wrapping. Care must be taken to ensure that instrumentation cabling isn't susceptible to triboelectric effects resulting from blast overpressure exposure. Proper strain relief techniques must be used on accelerometer cabling to ensure cable “whip” is reduced as much as possible. Mounting surfaces must be prepped according to manufacturer's recommendations and the proper mounting torque must be used to mount the accelerometer. Data acquisition systems must also be configured to eliminate data aliasing and out-of-band energy contamination.

Even if all of the necessary preparatory steps are followed, it can still prove difficult to capture accurate acceleration data in the ballistic shock environment. This is because most, if not all, high-g accelerometers used in this environment are vulnerable to errors and damage from the broad frequency and high amplitude of the mechanical input. Most high-g silicon MEMS accelerometers commercially available have two main vulnerabilities: base-strain sensitivity and resonance susceptibility. High-g accelerometers have been produced that can measure upwards of 200,000g but are oftentimes undamped and have high resonant frequencies (hundreds of Kilohertz (kHz)). Their high-resonant frequency, low-damping design also means there will be a large amplification at the resonance of the seismic mass. Q-Factors of up to 1,000X have been identified.

The broad spectrum of ballistic shock almost guarantees some magnitude of frequency content at or near the resonant frequency of the accelerometer. This leaves the accelerometer extremely susceptible to resonance during a ballistic shock event as little power is required to excite the resonant frequency. Large deflections of vehicle subfloors subjected to underbody blast load can create a strain at the base of the sensor that can cause a DC acceleration output from the accelerometer. Though brief, these DC offsets can accumulate to significant error when integrating acceleration data to obtain velocity information.

For these reasons, it is often necessary to perform an “initial validation” of accelerometers to qualify their performance in the ballistic shock test environment before being utilized in a test for record.

This work was done by the Ballistics Instrumentation Division (TETD-AT-SLB) for the Army Test and Evaluation Command. ARL-0205

This Brief includes a Technical Support Package (TSP).
Test Operations Procedure (TOP) 01-1-070 Initial Validation of Ballistic Shock Transducers

(reference ARL-205) is currently available for download from the TSP library.

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This article first appeared in the October, 2017 issue of Aerospace & Defense Technology Magazine.

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