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A Study on the Ability to Analytically Predict Mechanical Failure due to a Pyrotechnic Shock Environment.

Speaker: Sean P. KeonAuthors: Sean P. Keon

Meryl R. Mallery

Ensign-Bickford Aerospace & Defense Company

June 26-28, 2007Uniquely Designed Test Specimens

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Pyrotechnic Shock Testing Presentation Overview

• General Overview of Study • Pyrotechnic Shock and the Effect on Hardware• Structural Analysis Techniques• Test Specimen Description • Determination of Resonant Frequency• Pyrotechnic Shock Testing• Structural Analysis of Pyrotechnic Shock• Technical Observations• Go-Forward Plan

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General Overview of Study

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F SRS Point7:+XF SRS Point8:+YF SRS Point9:+ZF Overall level -3db specF Overall level nominal specF Overall level +6db spec

Choose a Design Concept- Keep it simple- Uni-axial failure mode- Low Resonant Freq.

Subject Test Specimens to Real Pyro-Shock- Accelerometers installed on each specimen- Which ones broke? - How do the results compare to analytical prediction?

Determine Actual Resonant Frequency- Perform 1G Sine Sweep- Compare to Analysis

Analyze Design Concept- Select appropriate material- Predict Resonant Frequency- Analyze for break strength

Re-Calibrate Analytical Model and Failure Predictions- How well does the SRS reflect the damage potential of a given shock test?

Adjust the Test Parameters based on Re-Calibration- Option #1 – adjust breaking strength of test unit.- Option #2 – adjust magnitude of pyrotechnic shock source

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General Overview of StudyObjectives

• Acquire data to validate and refine EBA&D’s structural analysis process for assessing design susceptibility to a pyrotechnic shock environment. – Currently, EBA&D’s structural analysis process lacks empirical test data.– What are the best analytical tools to use for such a structural analysis? – What pyrotechnic shock data is the most useful when assessing damage

potential? Is it the Shock Response Spectrum? Is it the Pseudo Velocity Response Spectrum?

• Creation of an innovative test tool, with tunable resonant frequency and breaking strength.– Pyrotechnic shock overtest monitor.– Validation tool for assessing accelerometer data accuracy at low

frequency.

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General Overview of StudyObjectives

• Continue to further understand the real energy present in a pyrotechnic shock event.– EBA&D will segway our understanding of pyrotechnic shock into an

ability to build components that are effectively designed to perform in the rigorous environments required by our customers.

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• Definition of Pyrotechnic Shock is: “The response of a structure to high-frequency, high-magnitude stress waves that propagate throughout the structure as a result of an explosive event….” [Davie, Bateman, Ch. 26 S&V Handbook]

– Results in a near instantaneous velocity change in the test fixture and test specimens.

• Pyrotechnic Shock can be mechanically destructive. – EBA&D has witnessed mechanical failure of many

metal (aluminum & steel) components.• Shear Pins, Fasteners, Separation Planes

Pyrotechnic ShockThe Effect on Hardware

LSC Sheath Deformation, End Seal Destruction

Mechanical Failure of Electrical Connector

Location of Failure

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• Structural Analysis of Ordnance Disconnect, during the design phase, did not predict the mechanical failure that eventually occurred.

• Structural Analysis for Pyrotechnic Shock does not appear to be straight-forward.– What magnitude of pyrotechnic shock overtest

should be anticipated? +6dB, +12dB?– Does the Shock Response Spectrum

provide us with enough information to performa thorough structural analysis?

Mechanical Failure of Fastener

Location of Failure

Ordnance Disconnect for FCDCAs: Mechanical Failure at separation plane due to pyrotechnic shock test.

Pyrotechnic ShockThe Effect on Hardware

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Pyrotechnic ShockStructural Analysis Techniques

• Current EBA&D Analytical Approach for Shock Modeling– Evaluate Design for critical regions that may be most affected

by shock loads.– Import CAD representation of part which has been defeatured,

as necessary for efficient modeling, into ANSYS Workbench 11.0.

– Obtain material properties – typically minimum strength properties.

– Determine resonant frequency of the component.– Review overall shock requirements and determine shock input

at component resonant frequency.– Review customer required safety factors.

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Pyrotechnic ShockStructural Analysis Techniques

– Run nonlinear static structural analysis w/equivalent static acceleration.

– Analysis method known to be conservative due to the short duration nature of a pyrotechnic shock event.

• Additional Analytical Options– Conduct a Fully Explicit Dynamic Analysis in LSTC LSDYNA

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Pyrotechnic Shock StudyTest Specimen Description

• Simplistic design with a uni-axial failure mode. • Target Resonant Frequency, 300 – 500 Hz.• Targeted to break when subjected to 1000 G’s at its resonant frequency. • Material: 7075-T7351

– Notch sensitive material• Machined Notch Defines the “Break” Location

– Designed to minimize tolerance stack-up on critical design features– Single notch on one side of beam

reduces tolerance variations on notch base thickness.

• Different weight steel cylinders used to adjust part resonant frequency and breaking strength.– (Small, Medium and Large)

Basic Test Specimen, No Added Weight

Design Goal: Test Specimen with adjustable breaking strength and resonant frequency.

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Pyrotechnic Shock StudyTest Specimen Description

Steel Cylinder

Accelerometer Mounting Port

Test Specimen Mounting Holes

(2 places)

Main Body measures 1 inch x 1 inch

Material: 7075 Aluminum

Notch Thickness of

0.070 inch Uni-Axial Failure Mode

“Z-Axis”

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Pyrotechnic Shock StudyDetermination of Resonant Frequency

• The resonant frequency was determined for test samples #7, #8, #9 using ANSYS 11.0 and compared to measured values from a 1G sinusoidal sweep on a vibration table.

NOTE: resonant Frequency predictions are within 2.7% of measured values.

Test Specimen Undergoing Sine

Sweep Test

Test Specimen

No.

Notch Wall Thickness(inches)

Small, Medium, or

Large Cylindrical

Mass

Added Weight Due to

Cylinder and Bolt

(grams)

Predicted Resonant Frequency

Determined Resonant Frequency

(Sine Sweep)

7 0.0657 Large 70.14 342 344

8 0.0669 Medium 58.00 395 398

9 0.0715 Small 46.40 482 469

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• Designing for specific resonant frequencies has been tricky.– The addition of mass to promote mechanical failure causes a

significant decrease inresonant frequency.

• Acceleration values decrease at a rate of ~ 10 dB/Oct with decreasing resonant frequency.

– Despite having the highest breaking strength, the Test Specimen with the Small Weight exhibited failure more often than the Medium and Large Weight.

Pyrotechnic Shock StudyDetermination of Resonant Frequency

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Pyrotechnic Shock Testing

• Three (3) Test Specimens mounted per shock test– 1 w/ Small Weight ≈ 480 Hz (resonant frequency)– 1 w/ Medium Weight ≈ 400 Hz– 1 w/ Large Weight ≈ 350 Hz

• Each Test Specimen had an Accelerometer mounted to it.– PCB 350B02 Accelerometer– Measuring Z-Axis Only

• Aluminum Pyro-Shock Fixturewith welded shelf for productmounting.

• Typical Shock Source: 10 feet of 25 gr/ft Detonating Cord.– PETN Explosive Core

Typical Shock Test Set-Up

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NOTE: Acceleration at PNF for Specimens 1, 2, 3, are from the 3rd Pyrotechnic Shock

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• Exact Value from Shock Response Spectrum is used for comparison to analytical model.

• The 3 SRS Curves (to the right) represent test specimens 4, 5, 6.– Data is fairly consistent across

the three.• Test Specimen #4.

– Permanently Deformed from pyrotechnic shock.

– At ƒres = 485 Hz, Accel. = 2,515 G’s

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Structural Analysis for Pyrotechnic Shock

• ANSYS 11.0 Test Specimen Model

Fixed Support

1000 G Body Load

7075 T7351 Material Properties

MIL-HDBK-5HUltimate Strength = 67,000 psiYield Strength = 57,000 psiElongation = 6%

Certified Inspection ReportUltimate Strength = 74,500 psiYield Strength = 64,600 psiElongation = 15%

Analysis Results are presented with the material properties specified on the Certified Material Inspection Report

Notch Thickness

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Plastic Strain Contours Spec. #8.0669 Notch Wall Thickness

Medium Cylinder1000 G Load All Contours in Red

exceed Plastic Strain of the Material, which is15%

Fracture Predicted


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Plastic Strain Contours Spec. #9.0715 Notch Wall Thickness

Small Cylinder1000 G Load

Fracture is NOT Predicted

Max Strain Shown is 1.9%


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• Why does the analysis predictions and test not correlate well?– Does SRS provide an accurate representation of load the part sees? – Is specimen damping different from SRS assumption (Q=10)?– Is the short duration shock load not accurately modeled with static

analysis?


Test Specimen

No.

Small, Medium, or

Large Cylindrical

Mass

Added Weight Due to

Cylinder and Bolt

(grams)

Predicted Fracture

w/1000 G's

Test ResultsFracture?

4 Small 46.4 No Yes2500 G's

7 Large 70.14 Yes Now/1100 G's

8 Medium 58.00 Yes Now/1300 G's

9 Small 46.40 No Now/2250 G's

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Pyrotechnic Shock StudyTechnical Observations

• Test Specimens are withstanding significantly higher acceleration values (as extracted from the SRS) than was analytically predicted by the process described on Slide 10.

• Experiencing difficulty in causing mechanical failure in test specimens with resonant frequency less than 450 Hz.

• Uni-Axial Failure Mode has been effective. Permanent deformation has been easy to identify.

• Noteworthy results for Test Specimen with Small Weight:– No evidence of deformation with Accel. = 2,250 G’s (#9)– Permanent deformation with Accel. = 2,500 G’s (#4)

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Pyrotechnic Shock StudyGo-Forward Activities

• Further Review of Pyrotechnic Shock Data– Review Power Spectral Density of Accel. Time History.– Create Pseudo Velocity Response Spectrum plots.

• Test Specimen Modification– Focus on Test Specimen with Small Weight– Lower Breaking Strength by Reducing Notch Thickness

• 0.055” thickness– ƒres = 369 Hz with Small Weight– Predicted Breaking Load: < 1000 G’s

• 0.060” thickness– ƒres = *TBD*– Predicted Breaking Load *TBD*

• Future Plan to Design X and Y Axis Test Specimens

• Future Plan to Study Shear Pin Failure

Test Specimen #2“Desired” Mechanical Failure

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