INSTITUTE FOR SHOCK PHYSICSLaser-Shock Spall Experiments in Aluminum II:

Interface Measurements

Institute for Shock PhysicsThomas M. Bersano,* Illinois Wesleyan University

Objectives

• Examine tensile fracture in aluminum

alloys at short time scales (10-8 s)

• Role of PMMA backing material window

-Changing the tensile pulse characteristics

• Determine agreement of spall plane

prediction with microstructural evidence

Laser-Shock Spall Experiments

• Laser vaporizes the chromium layer,

launching the flyer

• Flyer impact creates stress pulse in the

material

• Tension Occurs!

• Spall occurs as material is “torn apart”

Fig 1: a) Plane waves traveling through both the flyer and the target collide

causing the material to experience tension. b) Pullback velocity indicates tensile

loading has occurred. This velocity difference is used to calculate stress data.1

Target and Laser

Characteristics

• 1064nm wavelength laser beam

• 6061-T6 or 1100-H14 aluminum targets

approximately 110 m thick

• PMMA with vapor deposited Al mirror

Soda-lime substrate PMMA WindowBrass Spacer

Chromium

Al Flyer (~54 m)

Deposited Al

Mirror (< 1 m)

6061 Al Target

Laser 1064nm

15 ns FWHM

VISAR

~ 220 m

Fig 2: A diagram of the target set-up.

Laser Interferometertry 2

• Laser light reflects off a moving

surface

• Reflected light splits and travels

through two legs of the same length

• Etalons slow one leg of light creating

a time difference τ

• Analyzing light intensity at the

detectors gives Δu, a change in

velocity

Fig 3 : A simplified diagram of the VISAR system.

Real Time Measurements

Post Shock Data

• Targets polished to

clean non-spall markings

• Viewed by optical

microscopy

• Only two samples

showed any sign of spall

• 1100-H14 did not show

tensile fracture Fig 8: Two 6061-T6 targets at 20x magnification. (Top) After polishing

this target doesn’t show any sign of spall. (Bottom) There is an evident

spall plane across the middle of the target.

Conclusions

• Laser-shock induced tensile damage possible with a PMMA backing

• Post shock data for aluminum, backed with PMMA, are inconsistent

• Better polishing and greater magnification are needed to inspect for tensile fracture

References

[1] X. Chen, J.R. Asay, S.K. Dwivedi, D.P. Field, “Spall behavior of aluminum with varying

microstructures.” J. Appl. Phys. 99 (2006).

[2] D.H. Dolan, “Foundations of VISAR analysis.” Sandia National Laboratories Report. (2006)* Advised by Yoshi Toyoda and Y. M. Gupta

Aluminum Thickness

(μm)

Aluminum Alloy

AluminumSoundspeed

(μm/ns)

Δt Peaks (ns)

Spall Plane Distance

(μm)

Indicates Spall?

103 6061-T6 6.372 18.5 59 YES

114 6061-T6 6.372 25.4 81 YES

116 6061-T6 6.372 21.4 68 YES

102 6061-T6 6.372 18.3 58 YES

117 1100-H14 6.494 26.0 84 YES

145 1100-H14 6.494 27.7 90 YES

Impactor Target PMMA Backing

Spall

plane

t1

t2

t3

Inte

rfac

e V

eloci

ty

Timet1 t2 t3

Δt

Pull

bac

k V

el.

a) b)

Spall Plane Distance

• Velocity Profiles (Left)

• Stress-Velocity Plots (Top)

• Soundspeed and Spall Plane

Distance (Bottom)