Laser-Shock Spall Experiments in Aluminum II:
INSTITUTE
FOR
SHOCK
Interface Measurements
Thomas M. Bersano,* Illinois Wesleyan University
Institute for Shock Physics
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
PMMA Backing
t3
a)
Spall
plane
t2
Target and Laser
Characteristics
Real Time Measurements
• 1064nm wavelength laser beam
• 6061-T6 or 1100-H14 aluminum targets
approximately 110 m thick
• PMMA with vapor deposited Al mirror
Δt
b)
Pullback Vel.
Target
Interface Velocity
Impactor
• Velocity Profiles (Left)
• Stress-Velocity Plots (Top)
• Soundspeed and Spall Plane
Distance (Bottom)
t1
Spall Plane Distance
Time
t1
t2
t3
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
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”
Soda-lime substrate
Laser 1064nm
15 ns FWHM
Brass Spacer
PMMA Window
~ 220 m
VISAR
Fig 3 : A simplified diagram of the VISAR system.
Laser Interferometertry
Chromium
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
Deposited Al
Mirror (< 1 m)
Al Flyer (~54 m)
PHYSICS
6061 Al Target
Fig 2: A diagram of the target set-up.
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)
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
Aluminum Aluminum Aluminum
Δt
Spall Plane Indicates
Thickness
Alloy
Soundspeed Peaks Distance
Spall?
(μm)
(μm/ns)
(ns)
(μm)
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
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
* Advised by Yoshi Toyoda and Y. M. Gupta