Drop Fixture Design, Fabrication, and Testing for

Ballistic Shock Events

Jonathan F. Holmes, Wiley Holcombe, Frank Marrs, Alessio Medda, Stewart Skiles

ITEA Test Instrumentation Workshop, May 17, 2012

Agenda

• Introduction

• Background of Ballistic Shock

• Integrated Blast Effects Sensor System (I-BESS)

• Drop Test Fixture Design and Development

• Test Results

• Lessons Learned

• Closing Statements

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Background of Ballistic Shock

• Impact testing is not a new concept – provides a good intermediate test prior to expensive live fire test events

• The modal response of electronics is critical to understanding the expected acceleration level

• A device with a 100 Hz natural frequency can experience 502 g’s whereas a device with 1,000 Hz natural frequency can experience 5,020 g’s

*Excerpt from MIL-STD-810G Method 522 3

I-BESS Description

• Integrated Blast Effects Sensor System (I-BESS) – Sponsored by the Army’s Rapid Equipping Force (REF)

– Intended to serve as a field ready data collection system to assist our understanding of blast effects as they relate to traumatic brain injury

• The system contains several LRUs with three items containing accelerometers for data acquisition – Dynamic Even Recorder (DER)

– Various Interface Adapter (VIA)

– Soldier Body Unit (SBU)

4

Critical I-BESS Components

SBU

5

m1

m2

h

impact pad

Drop Test Fixture Requirements

• Internal design commissioned for vertical drop fixture

• Immediate need for DER evaluation

– Capable of ± 6,000 g pulses

– Operational over range of -40 to 71°

• Test apparatus needed to meet test needs

– Several impacts per hour

– Record up to 10 kHZ

– Operational as soon as possible

– Quickly change out test articles

C

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Drop Test Fixture Calculations

• Needed bounding values to determine sizes of components and forces involved

– COR tests with high speed video and oscilloscope

– Compared to published values for sphere impacts

• Drop height ranged from 23 to 44 ft for 6,000 g over 1 ms – requirement reduced to 0.5 ms for practicality

𝜏 = 6.46𝜌2/5 ∙𝑅

𝑈01/5

𝐸2/5 𝑎𝑝𝑒𝑎𝑘 = 𝑈0 ∙

𝜋

𝜏∙

(1 + 𝐶𝑂𝑅)

(1 + 𝑚)

𝑣 = 2 ∙ 𝑎𝑝𝑒𝑎𝑘 ∙ ℎ 𝐹(𝑥) = −2𝐸 𝑅

3(1 − 𝜎)∙ 𝑥3/2

Basic equations used for initial calculations

7

Drop Test Apparatus Design

• Required analysis to verify performance in excess of 20 ft drop heights

• Impact pad easily changed for wear and achieving different shock pulses

• Spherical face on drop hammer to provide repeatable results

8

Drop Test High Speed Video

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Test Fixture and Data Acquisition

http://www.ni.com/compactrio

• Collection system utilized:

– Endevco accelerometers

– National instruments DAQ hardware

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Test Fixture Reference Data

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Design of Experiments

• Initial testing limited by schedule as quick assessment of dynamic event recorder devices was required

• Lower acceleration levels acceptable for initial tests

– Tests not intended to test vendor devices to failure

Drop

Height

Approx. Peak

Acceleration DER Temperature Impact Material

Replications

(per vendor) Drop Axis

35” 900g, 1ms Room Temperature .75" 90A 2 Z

23” 600g, 1ms Approx. -20degC .75" 90A 4 Z

23” 600g, 1ms Approx. 65degC .75" 90A 4 Z

47" 1000g, .9ms Room Temperature .75" 90A 2 Z

59" 1200g, .85ms Room Temperature .75" 90A 2 Z

35" 3000g, .4ms Room Temperature 1" 75D 2 Z

47" 3200g, .35ms Room Temperature 1" 75D 2 Z

59" 3000g, .3ms Room Temperature 1" 75D 2 Z

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Typical Vendor Test Results

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Lessons Learned

# Phase Description

1 Design Ensure all basic bolt load calculations are performed on major test devices. In particular, shear and tensile loading on all major items needs to be analyzed for appropriate safety factors prior to operation.

2 Design Pay attention to safety of personnel when designing dangerous machines. Proper procedures must be in place to avoid compromising the safety of test staff.

3 Calibration Binding in test article support rails was resolved by reviewing high speed videos of the test article. High speed imaging, when available, offers a unique evaluation of complex kinematic systems that can only be properly appreciated when made easily available to development engineers.

4 Calibration A significant exercise took place in finding appropriate impact materials. It was determined that a Shore 90A material that was .75” thick and a Shore 75D material that was 1.0” thick provided good performance as shown in Table 1.

5 Calibration Mounting to a concrete floor was initially done with a large mat of double sided adhesive on a foam carrier. This proved inefficient and had to be modified to use four concrete anchors.

6 Testing The ease of access of the test carriage support proved to be cumbersome. Future changes will be required to ease the burden of the tester from this difficult task.

7 Testing The reference accelerometer may require additional support beyond the screws attaching the tri-axial support block. A small shoulder machined on the mounting surface limits the shear forces on these attachment screws. Similarly, proper torqueing is necessary for all major components.

8 Testing In order to achieve the desired goal of 6,000 g a higher drop level would be required but determined to be unnecessary for initial evaluation experiments. The drop height is limited to the ceiling height of the test area in practice as the structural analysis was performed to guarantee safe operation beyond a 30 foot drop.

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Closing Statements

• Machine development within schedule goals and for a cost of approximately $15,000 in materials

• Developing shock testing internally proved to be valuable to GTRI for many reasons – Results available immediately

– Better understanding of shock testing to the entire development team

• Preliminary developmental shock tests are crucial to supporting successful live fire test events

Special thanks to the Rapid Equipping Force as they continue to provide emerging technological solutions

quickly to our soldiers in the field

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