Proceedings of the SEM Annual Conference
June 1-4, 2009 Albuquerque New Mexico USA
©2009 Society for Experimental Mechanics Inc.
Mechanics of Hypervelocity Impact Induced Damage
L. Lamberson1a, Dr. V. Eliasson1, Professor A. J. Rosakis1, Dr. M. Adams2
1
Graduate Aerospace Laboratories, California Institute of Technology,
Pasadena, CA, 91125, USA
2
Jet Propulsion Laboratory, NASA/California Institute of Technology
Pasadena, CA, 91109, USA
a
Presenting Author, email: [email protected]
ABSTRACT
Hypervelocity impact is a high-energy-density phenomenon that has the potential to alter a
spacecraft or orbiting asset trajectory, as well as structural, thermal, optical, and electrical
properties. The most challenging aspect of hypervelocity impact analysis is the fact that a
universal theory does not exist to fully describe this complex phenomenon. Hypervelocity impacts
can involve multifaceted material behavior including melting and vaporization, dissociation,
ionization and plasma formation, spallation and ejecta, mixed-phase flow, and fracture and
fragmentation, to name a few. Crater morphology, including hydrodynamic flow processes and
shock wave interactions have been proposed in literature as a means to further understand
mechanisms responsible for damage evolution. However, the majority of the current research on
hypervelocity impact focuses on post mortem analysis of the materials involved, and emphasizes
a more qualitative means of capturing the hypervelocity impact event. What is original to this work
is the fact that hypervelocity impact is experimentally investigated in situ. By combining highspeed photography and dynamic photoelasticity, target principal stress fringes, and material
interactions are illuminated in real time – leading to better, more accurate theory on hypervelocity
impacts.
Introduction
Historically, hypervelocity impact hole prediction models have been derived post mortem from
launch package and target material and geometric properties. Traditionally Whipple shields are
used to mitigate hypervelocity threats, where a thin outer wall is placed (often 5-10 cm) in front of
the inner structural wall of the spacecraft or orbiting asset [3].
440C Steel Spheres Impacting 304 Stainless Steel Plates
6
5
4
Emprical Data (2008)
Rolsten Model (1964)
Maiden Model (1965)
Ames Equation (1969)
Sorensen Model (1964)
MULTIVAR (2004)
3
2
0.9
8
Target Hole Diameter [mm]
Target Hole Diameter [mm]
Tantalum Spheres Impacting 304 Stainless Steel Plates
7
1
1.1
1.2 1.3 1.4
Velocity [km/s]
1.5
1.6
1.7
7
6
5
4
3
2
2.1
2.2
2.3
2.4
2.5
2.6
2.7
Velocity [km/s]
Figure 1: Averaged experimental results from SPHIR versus models for thin plate penetration [1].
2.8
While no model provides an absolute fit, each model contains significant limitations. For example,
the Rolsten model uses a mechanics approach and suggests the target material surrounding the
impacting projectile is displaced by radial flow. However, this particular model has no
dependence on the projectile velocity or target thickness. Thus for an equivalent density and
projectile diameter, the crater will be the same size regardless of impact speed [1].
As shown in Figure 1, all the models illustrated are developed strictly for spherical projectiles at a
normal angle of impact, yet real impacts are often from debris of irregular shape and strike at
diverse angles of obliquity. Additionally, most models are derived utilizing some amount of
empirical results often on a specific target and launch package material and size. Consequently,
these models tend to be material dependent and are not fully universal in nature.
Experimental Configuration
Experiments have been performed at Caltech/JPL's Small Particle Hypervelocity Impact Range
(SPHIR). The SPHIR facility houses a two-stage light gas gun with over a dozen observation
ports for extensive diagnostics implementation. The gas gun is capable of reaching velocities
between 0.5 to 10 km/s, and can test any potential target material and geometry at any angle of
impact. Launch packages can range from approximately 0.25 to 2.5 mm in diameter and 5 to 500
mg in weight, depending on density and shape. This particular gas gun utilizes an accelerated
reservoir firing cycle, which uses a deformable piston to compress a large volume of light gas in
an essentially isentropic compression cycle [3]. The piston is then extruded through a taper in the
high-pressure section and ruptures a Mylar disc separating the second-stage launch tube that
houses the launch package.
Dynamic photoelasticity experiments have been conducted launching nylon cylindrical slugs at
Mylar and Homalite targets between 4 to 7 km/s. A coherent, monochromatic, plane polarized
argon-ion laser beam is expanded to 100 mm and transmitted through the transparent target
specimen. Two sheets of circular polarizers (with a ¼ wave plate) are placed on both sides of the
target specimen to form a circular polariscope, which allows the stress-state in a birefringent
material to be observed. The experimental configuration is shown in Figure 2 below.
SPHIR Vacuum Chamber
mirror
SPHIR Flight Tube
Launch Package Path
Polarizer
Target
Coherent Innova-308 Laser
l=514nm
Polarizer
Beam Expander
Cordin
214-8
Camera
mirror
Pentax 67 Lens
Plano-convex Lens
Figure 2: Dynamic photoelasticity experimental configuration at SPHIR.
As governed by the stress optic law, the relationship between fringe order at a point and the local
in-plane principal stress component is:
F
(1)
σ 1 − σ 2 = 2τ max = σ N
h
where F is the material fringe constant associated with the incident light wavelength, λ, of 514
nm, h is the specimen thickness of 6.4 mm, and N is the isochromatic fringe order [2].
Discussion
The resulting isochromatic fringe pattern upon impact was captured in real time on a high-speed
camera (Cordin 214-8) with a Pentax 67 lens (Fig. 3). The P-wave speed of Mylar is shown at 2.6
km/s. A longitudinal wave train with some rotational symmetry propagates radially outwards from
the point of impact. The presence of potentially strong shear waves, as shown in (C), suggest an
axisymmetric impact event. This is consistent with the cylindrical nylon projectile used in the
experiment, as it is known to tumble down the flight tube and exhibit some degree of yaw and
pitch on impact.
A
B
C
Impact
site
1 cm
P-wave
1 μs
1 cm
5 μs
1 cm
10 μs
Figure 3: Fringe pattern sequence, nylon on Mylar, impact velocity 5 km/s, exposure time 70 ns.
Results
Dynamic photoelasticity and high-speed photography have provided the foundation to in-situ
hypervelocity impact investigation. Some of the current discoveries and next steps in research
include:
•
Examining caustics from dynamically propagating cracks initiated at the impact site,
where assuming Mode 1 dominance the stress intensity factor can be determined.
Previous
Impact
site
P-wave
1 cm
Crack
Growth
Caustics
New
Impact
site
5 μs
1 cm
20 μs
Figure 4: Fringe pattern sequence of nylon striking a Homalite disc that had previously been hit
with nylon at 5 km/s, new impact at 4.5 km/s. (Left) Illustrates P-wave advancing at roughly 2
km/s. (Right) Illustrates the caustics which appear when cracks are dynamically growing in
predominantly Mode I. The later image also shows complex fringe patterns from boundary Pwave reflections and strong shear interaction.
•
Impacting heavily loaded target materials with intentionally induced flaws or cracks to
understand the stress wave interaction and coupled damage mechanisms.
•
In the near term, exploring a more quantitative in-situ optical diagnostic technique of
Coherent Gradient Sensing, a shearing interferometry method to examine full-field
surface topology and target slope maps.
thicker_back_xfringe_0
thicker_back_yfringe_0
1000
1000
50
900
100
800
y
900
100
800
700
150
600
200
y
150
50
700
600
200
500
500
250
400
thicker_front_jacobi_flat
300
400
250
300
300
300
350
350
100
150
200
x
deformation [microns]
50
200
6
200
10
250
50
300
350
100
150
200
x
250
300
350
4
5
2
0
-5
0
-10
-2
50
50
0
0
-50
y [mm]
-4
-50
x [mm]
Figure 5: Extension of CGS capabilities, post mortem result, 3-D topological deformation mapping
of a 304 stainless steel target, 10 mm thick and 150 mm in diameter, impacted at 5.5km/s with
nylon cylindrical slug, length and diameter of 1.8mm, viewing the stern side.
Conclusions
It is apparent from the graphs presented in the introduction that more accurate predictors of
hypervelocity impact damage are needed [1]. Results in this paper illustrate the potential to
discover mechanisms responsible for overall target damage utilizing high-speed photography and
optical diagnostics in-situ.
Acknowledgements
Support from the National Science Foundation Graduate Research Fellowship, NASA
Aeronautics Scholarship, and the Department of Energy are gratefully acknowledged.
References
[1] Hill, S.A., Determination of an empirical model for the prediction of penetration hole diameter
in thin plates from hypervelocity impact, Journal of Impact Engineering, 30 (2004) 303–321.
[2] Coker, D., Rosakis, A.J., Needleman, A., Dynamic crack growth along a polymer compositeHomalite interface, Journal of the Mechanics and Physics of Solids, 51 (2003) 425–460.
[3] Piekutowski, A.J., Poormon, K.L., Impact of thin aluminum sheets with aluminum spheres up
to 9 km/s, 35 (2008) 1718–1722.