Design of Octo-Strain 

Device for Sheet Metal 

Testing using Neutron 

Diffraction

Eric-Paul Tatman

Advisor: Dr. Justin Milner
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Motivation

• Automotive lightweighting Research

• Meet demand to develop automobiles 

with increased fuel efficiency

• Improvements in crash test ratings

• Create material models that accurately 

represent mechanical properties of 

material

• Saves cost by producing correct metal forming 

dies that require zero rework because of 

incorrect modeling

Figure 1: Chart displaying steel development [1] 
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Motivation- Continued

• Traditional uniaxial testing is not an accurate enough measurement for 

the prediction of complex multiaxial deformation

Figure 2: Various Strain Paths for Octo-Strain Device Figure 3: Strain Paths when Forming Automotive Parts 

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• A device used for multiaxial sheet metal testing

• Designed for specialized 

test specimens with 

eight arms

• Eight linear actuators 

apply tensile or 

compressive forces to 

the test specimen

Introduction- What is Octo-Strain?

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Figure 5: Strain Space for Octo-Strain 

[2]

Figure 4: Assembled Octo-Strain Ring
Introduction- Why use Neutron 

Diffraction?

• Unlike traditional tensile testing, cross-sectional area is difficult to calculate 

for complex geometries

• Neutrons are non-destructive with high penetration depth (cm)

• X-ray Diffraction penetrates surface millimeters in comparison 

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Figure 6: Comparing traditional tensile test specimen to Octo-Strain test specimen

σσ =

FF

AA σσ =

FF

AA



Crosssectional 

Area and 

forces that 

act on the 

gauge area is 

unknown

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Introduction- Stress-Strain 

Relationship

Figure 8: Finding Strain using Digital Image Correlation Figure 9: Finding Stress using Neutron Diffraction

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λ θ =2 sin d hkl

Figure 7: Speckle Pattern Used for Strain [3]

Bragg’s Law

εε =

∆LL

LL

ε =

dd − dd0

dd0

Hooke’s Law

σσxx = � EE

1 + νν)(1 − 2νν [ 1 − νν εx + νν εy + εz

σσyy = � EE

1 + νν)(1 − 2νν [ 1 − νν εyy + νν εx + εz

σσ � zz = EE

1 + νν)(1 − 2νν

Camera’s [ 1 − νν εzz + νν εx + εy

Sample Lights Lattice Strain
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Introduction- Stress-Strain 

Relationship

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Figure 11: Neutron Beam Path Location Figure 10: CAD model of Octo-Strain Test Specimen

Neutron 

Beam Path

Incident Beam

Diffracted Beam
Introduction- Benefits of test 

specimen geometry

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• Increased failure strain 

by over 2x

• Increased strain 

homogeneity within 

gauge area

• Early failure due to 

sample geometry of 

Cruciform

Figure 12: Strain Path of Octo-strain test specimen and cruciform

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• 50 kN force applied to each of the eight 

loading arms

• Allows for testing of High-Strength Metals

• Easy to assemble and make adjustments

• Keep weight of design under 500 pounds

• Max. Capacity of 3-axis measurement table

• Stepper Motors cannot protrude from 

device

• This will allow for an increased range of 

measurable scattering angles

Design Requirements

Figure 13: Previous 10 kN design

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CAD Modeling

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• Creo Elements direct drawing CAD software was used to model all components 

of Octo-Strain

• From 3D models, engineering drawings were made and sent to machine shop

Figure 14: Isometric and Front View of Octo-Strain Device

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• Determined a timing belt would 

work best for driving screw-jack

• Calculated Torque of 26.3 Nm to 

drive screw-jack

• 1:1 gear ratio, 25 tooth, 30 mm 

wide pulley

• Bearing eliminates radial load 

on motor shaft

Drive System

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50 kN Lifting Force

26.3 Nm of torque at drive shaft 

required (includes Break-away Torque)

Figure 15: Free-body diagram of forces acting on screwjack [4]

Figure 16: CAD model of completed drive assembly

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Design Requirements- Continued

• Transmission and Reflection beam paths are possible

• Provides Measurement of all three principle directions

Figure 17: Top and Side View of BT8 Beam Path

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Primary 

Beam

Neutron 

Detector

Beam Stop

Reflection Transmission
FEA Results

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• Determined max deflection of test stand when Device is assembled into base

Figure 18: FEA Result of Max. Deflection due to gravity (0.37mm) Figure 19: Assembled Octo-Ring without Test Stand

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FEA Results- Continued

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• Determined max deflection with loading condition of 50 kN (Compression)

Figure 20: FEA Result of deflection due to 50 kN load in compression on Octo-Ring 

(0.04 mm)

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Von Mises Stress 

47.76 MPa

Yield Strength of 

Aluminum 6061 = 

276 Mpa
FEA Results- Continued

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• Determined max deflection with loading condition of 50 kN (Tension)

Figure 21: FEA Result of deflection due to 50 kN load in tension on Octo-Ring (0.04 mm)

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Von Mises Stress 

47.76 MPa

Yield Strength of 

Aluminum 6061 = 

276 Mpa
FEA Results- Continued

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• Determined max deflection with worst case loading condition of 50 kN

Figure 22: FEA Result of deflection due to 50 kN load on Octo-Ring (0.40 mm)

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Von Mises Stress 55.14 

MPa

Yield Strength of 

Aluminum 6061 = 276 

Mpa
Comparison of Designs

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Figure 23: CAD model of 50 kN redesign compared to existing 10 kN design

Timing Pulley 

Drive system Increased Load Capacity 

and larger Load cells

Larger diameter 

Octo-ring with slim 

profile

Bearing to aid in 

sample alignment

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Redesigned Grips for 

faster sample changes
Conclusions

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• With this device, material property data will be collected that will lead to the 

adoption of generation three high strength steels in the automotive industry

• Increased load capacity 

• Allows for testing of high-strength metals

• By using pulley and drive system, significant space is saved

• Increased range of measurable scattering angles

• FEA was performed to verify structural integrity of device

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Future Work

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• Get all parts machined from engineering 

drawings

• Assemble Octo-Strain device

• Begin Testing of high-strength metals

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Acknowledgements

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SURF Program

Dr. Justin Milner

Dr. Thomas Gnaupel-Herold

Dr. Julie Borchers

Dr. Joseph Dura

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References

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[1] Courtesy of World Auto Steel

[2] Courtesy of Dr. Justin Milner

[3] Courtesy of Correlated Solutions

[4] "Worm Gear Screw Jacks." Thomsonlinear. Web.

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Questions?