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Codes Involved in the Project:

IMPACT CODE VALIDATION & BENCHMARK PROJECT

A collective effort of the impact cratering modeling community

The objective of this project is to provide a set of information to serve as a standard for comparison and validation of impact cratering and fragmentation modeling codes. The information set consists of two parts. The first contains a description of the initial conditions and measurements from laboratory and field experiments that can be used to validate code calculations over a wide a range of event sizes and geological materials. The second part consists of several hypothetical impact events of varying complexity that serve as benchmarks for comparison of the numerics and physical models employed in various codes.

Different codes work better for different problems. For example, simulations of disruption events exercise different aspects of the material models than do simulations of cratering events. Similarly, the material properties that are important in studies of impact melting and vaporization differ from those in studies of late-time crater collapse. Thus, it is important to characterize the best applications for various codes, and provide this information to the scientific community to help prevent incorrect use of codes. Toward this end, this project includes simulations that test a gamut of physical mechanisms involved in impact events and includes comments on the applicability of various codes wherever possible.

At the same time, it is recognized that codes are in a continual state of development. Details such as the version of a code and the specific input file used in a given simulation are recorded. Additionally, the information set is designed to be extensible to accommodate future improvements and the development of new codes.

 

Comments on modeling impact events

 

In validation of the codes, the dominant uncertainties lie in the type, extent and accuracy of the material modeling. For the purposes of choosing benchmarks, it is important to know what benchmark exercises what part of the material model. For example, in the early time coupling, melt and vapor are determined by the thermodynamic models, and of course, sufficient numerical resolution. For just the initial flow field in a nonporous material, a Mie-Gruneisen or a Tillotson equation of state (EOS) suffices. Porosity adds considerable complexity, in which case an explicit crush model is necessary. (A low-density material is not the same as a porous one. The low density does not have the "P-dV" work of the crush in it.) A nonlinear elastic model such as a Murnaghan model will not have the energy dissipation necessary at the higher velocities.

For melt, one only needs to determine which material reaches a particular pressure, then the material is known to unload into melt states even if the code may not explicitly model the phase change during the isentropic expansion. For vapor, one must have explicit three-phase equations, and details such as the blow-off velocities depend on both the front side and the backside of the vapor dome.

Gravity must be included in all simulations of the later stages of crater growth for large craters, and for any size of crater in cohesionless materials such as dry sand or transient craters in water. Although gravity is not difficult to model, it is important to get an initially quiescent pressure and density state compatible with the EOS to avoid waves running around in the problem having the magnitude of the initial pressure.

The various forms of strength models must also be considered, including both yield models and fracture models. For ductile metals, a Von Mises or Tresca yield condition is sufficient. For granular geological materials with little cohesion, such as dry soils, there must be a pressure component to the yield strength, a Mohr-Coloumb or a Drucker-Prager model is generally used. There is little evidence of significant size or rate effects in the shear strength for non-cohesive materials such as dry sand.

For geological materials such as rocks, the cohesive components of the yield strength become important, so that tensions are possible. The tensile strength is often observed to be strongly rate dependent, at least in small-scale experiments. In large impacts the strain rates may be sufficiently slow rate that the rate dependence is not too important. However, large-scale rocks certainly behave differently than small lab samples usually picked for their pristine character. For impact disruption events, it is the tensile part of the failure envelope that is of primary importance, while for cratering the shear part is dominant.

Finally there is the case of highly porous materials, in which craters have been observed to form mostly by compaction (as opposed to shear). In this case, an accurate porous compaction model is needed.

 

Validation Tests

Numerous laboratory and field experiments of cratering and fragmentation have been conducted under a variety of conditions. The validation part of this project employs a set of experimental results that provides stringent tests of the physical models in used in codes.

The experimental results are drawn both from laboratory studies of impacts and from field tests of explosion cratering. Laboratory tests are useful because they provide direct information on impact events. However, they are necessarily conducted at small scales. Field explosion tests are complementary in that they provide important data at much larger size scales.

When validating a code it is important to consider as many aspects of the impact process as possible. For example, one must assure that a simulation not only predicts the correct final size of a crater, but also the kinematics of the crater growth, material flow, ejection, etc. Therefore, the test results included here were selected to encompass as many observables as possible and to sample a wide a range of experimental conditions. Results are included for simple materials, such as water and metal, and materials such as soil and rock, whose constitutive behavior is quite complex.

The validation test cases are summarized in Table 1 below. Each case is given a unique designator of the form VXYn, where XY is one of IC (impact crater), EC (explosion crater), IF (impact fragmentation) or EF (explosive fragmentation), and n is an integer.

The number of tests in the validation suite is fairly large, and will undoubtedly grow with time. It is not expected that every code will be applied to the full suite of tests. For example, many impact codes do not currently implement a pressure-dependent strength model. However, because the suite includes materials ranging from very simple to very complex, virtually all impact codes should be applicable to at least some of the test conditions.

The results for impacts in water eliminate the need for a strength model. Furthermore, gravity only needs to be included to model the later stages of crater growth, as it approaches its maximum volume before collapsing. Hence the water tests are relatively uncomplicated and should be the starting point for validation testing.

The impact event in an aluminum target is the next step in complexity. In this case, gravity is not needed, and a simple Von Mises criterion is sufficient to model the strength as noted in the previous section.

Results are included for impact cratering in dry sand. In this case, the strength model needs no cohesion, but must include the pressure dependence. In order to exercise the pressure dependence, results are included for dry sand, which has a friction angle of 35 deg, and glass beads, which have a friction angle of about 20 deg.

Several experiments are included for alluvium targets. This material model requires both a cohesion and pressure dependence. Results are provided for both small-scale impact and for large-scale explosion craters. The field tests were selected to cover a wide range of sizes and as many diagnostics as possible. Models of the explosion tests may either employ a model of the explosive detonation or simply an initial region of high pressure, as outlined in Appendix A.

Simulation of the experiments in porous targets requires a model of porous crush-up, which must be coupled appropriately with the strength model, which is not included in most codes at the time of this writing.

Experiments are included for impact cratering and disruption of rock. As noted in the previous section, simulations of these events must consider the rate dependent strength of rock.

Finally, the test suite contains the results of explosive fragmentation tests using targets made from weakly cemented basalt. The experiments were conducted in a pressurized atmosphere as an analog of the gravitational overpressure in large asteroids. These are presently the only experimental results that address the shattering of large, gravity-dominated asteroids.

Details of the initial conditions, material properties and test results are provided in Appendix A. This information, along with summaries of the corresponding code calculations will be made available to the public on the web at http://keith.aa.washington.edu/craterdata/.

Table 1. Summary of Validation Tests

 

ID

Source

Target

Remarks

VIC1

1.9 km/s pyrex sphere

Water

Transient crater growth in simple material

VIC2

4.6 km/s glass sphere

Water

Transient crater growth in simple material

VIC3

1 km/s steel sphere

Water

Pressure histories in simple material

VIC4

8 km/s aluminum sphere

Aluminum

Final crater size in simple material w/strength

VIC5

2 km/s polyethylene cylinder

Dry Sand

Crater growth, tracer particle motions

VIC6

2 km/s polyethylene cylinder

Dry Sand

Final crater profile at 500G

VIC7

6 km/s aluminum sphere

Glass beads

Final crater size in low-friction-angle material

VIC8

2 km/s polyethylene cylinder

Alluvium

Final crater profile in cohesive soil

VEC1

20 Tons TNT, 10m deep

Alluvium

Final profile, mound growth, tracers (Stagecoach)

VEC2

500 Tons TNT, 38m deep

Alluvium

Final profile, tracers, ground motion (Scooter)

VEC3

4.4 KTons ANFO, surface

Alluvium

Final crater size (Minor Scale)

VEC4

100 KTons Nuc., 190m deep

Alluvium

Final crater profile, mound growth, tracers (Sedan)

VIC9

2 km/s polyethylene cylinder

50% Porous

Final profile (500G) in highly porous soil

VIC10

2 km/s polyethylene cylinder

70% Porous

Final profile (500G) in highly porous soil

VIC11

X km/s XXX

Gabbro

Crater profile and subsurface fracturing

VEC4

0.42 KTon Nuc, 34 m deep

Basalt

Crater/ejecta profile, est of rupture zone (Danny Boy)

VEC5

0.5 KTon TNT, surface

Basalt

Crater/ejecta profile (Sailor Hat)

VIF1

0.6 km/s Al cylinder

Granite

Collisional fragmentation of 1.9 cm target

VIF2

0.6 km/s Al cylinder

Granite

Collisional fragmentation of 3.2 cm target

VIF3

0.6 km/s Al cylinder

Granite

Collisional fragmentation of 17 cm target

VIF4

0.6 km/s Al cylinder

Granite

Collisional fragmentation of 34 cm target

VEF1

X gm PETN

Grout

Explosive frag of weakly cemented basalt

VEF2

X gm PETN

Grout

Exp frag of weak cem basalt, 75 psi overpressure

VEF3

X gm PETN

Grout

Exp frag of weak cem basalt, 1000 psi overpressure

Benchmark Tests

A fundamental part of this project includes a set of simple benchmark simulations that should be carried out to test a code against other codes routinely used for impact modeling calculations. This approach provides a standard method for comparing codes against each other, tests a codes capabilities and accuracy for different types impacts, and provides a public record of performance for the same set of test cases.

The set of benchmark simulations includes relatively simple cases, such as impact into water, which should be addressable by virtually all impact codes, as well as more complicated cases that are meant to test the behavior of material models. The main purpose here is not to validate the accuracy of a code, but rather to compare its results against those of other codes. Hence, the benchmark includes some events that cannot be performed in the lab or in the field, and involves detailed comparisons of quantities that generally cannot be measured in experiments.

The simulations are divided into two classes. The early-time modeling focuses on the early stages of the impact, dealing with the propagation of the shock wave through the target and projectile. Therefore, the modeling should focus on maximum shock pressure, shock pressure decay, internal energy, temperature, melting/vaporization, and tracer particle histories during crater growth.

For shock levels and melt/vaporization estimates, gravity can be ignored in the calculations. Additionally, it is not necessary to have a sophisticated EOS, as long as the available EOS does a good job at modeling the Hugoniot. This means that EOS can be as simple as Mie-Gruneisen. All that needs to be determined is if the material reaches the threshold shock pressure associated with melting/vaporization upon unloading from the shock state. A strength model is also not needed for the early stage calculations.

The second class contains the late-time modeling, which focuses on the slowing and possible collapse of the transient crater. In this case, both gravity and a good strength model are important. As noted in a previous section, the necessary complexity of the strength models varies with the material being modeled. For ductile metals a von Mises or Tresca yield condition is sufficient, whereas granular materials with little strength, like dru soils, must include a pressure component to the yield strength, such as in the Mohr-Coulomb or Drucker-Prager models. For rocky materials the cohesive component of the yield strength, which allows tension, becomes important. For cratering it is the shear part of the failure envelope that is crucial, and for disruption it is the tensile part of the failure envelope.

The late-time model results should focus on crater depth, radius, and volume as a function of time, transient cavity characteristics, final crater shape, tracer histories, and stress/strain fields.

Table 2 summarizes the benchmark simulations. Each simulation is identified by a designator of the form BXn, where X is either E (early time) or L (late time), and n is an integer. The details of the initial conditions for the simulations are provided in Appendix B.

Table 2. Summary of Benchmark Tests

 

ID

Source

Target

Remarks

BE1

1 km Al sphere, 5 km/s

Water

Early-time low speed impact in water

BE2

1 km Al sphere, 20 km/s

Water

Early-time high speed impact in water

BE3

1 km Al sphere, 5 km/s

Aluminum

Early-time low speed impact in metal

BE4

1 km Al sphere, 20 km/s

Aluminum

Early-time high speed impact in metal

BE5

1 km Al sphere, 5 km/s

Dry Sand

Early-time low speed impact in dry soil

BE6

1 km Al sphere, 20 km/s

Dry Sand

Early-time high speed impact in dry soil

BL1

1 km Al sphere, 5 km/s

Water

Late-time low speed impact in water

BL2

1 km Al sphere, 20 km/s

Water

Late -time high speed impact in water

BL3

1 km Al sphere, 5 km/s

Aluminum

Late -time low speed impact in metal

BL4

1 km Al sphere, 20 km/s

Aluminum

Late -time high speed impact in metal

BL5

1 km Al sphere, 5 km/s

Dry Sand

Late -time low speed impact in dry soil

BL6

1 km Al sphere, 20 km/s

Dry Sand

Late -time high speed impact in dry soil

AUTODYNE, CTH, SALEB, SAGE, SOVA, SPH, ZEUSS, ALE3D, GEODYN

 

Natalia Artemieva nata_art [at] mtu-net.ru

Erik Asphaug asphaug [at] emerald.ucsc.edu

Willy Benz Willy.Benz [at] phim.unibe.ch

James Cazamias cazamias1 [at] llnl.gov

Rob Cocker robc [at] lanl.gov

Gareth Collins gareth [at] lpl.arizona.edu

David Crawford dacrawf [at] sandia.gov

Galen Gisler grg [at] lanl.gov

Keith Holsapple holsapple [at] aa.washington.edu

Kevin Housen kevin.r.housen [at] boeing.com

Boris Ivanov ivanov [at] lpl.arizona.edu

Don Korycansky kory [at] es.ucsc.edu

Jay Melosh jmelosh [at] lpl.arizona.edu

Patrick Michel michel [at] obs.nice.fr

Dugan O'Keefe dinosr [at] aol.com

Elisabetta Pierazzo

Sarah Stewart sstewart [at] eps.harvard.edu

Zibi Turtle turtle [at] lpl.arizona.edu

 

Appendix A

 

 

Description of validation experiments

VIC1 - Initial Conditions

Description: 1.9 km/s impact of a pyrex sphere at normal incidence into a water target. This was a quarter-space experiment in which the target fixture had a thick Plexiglas window as its front face. The projectile struck the target just inside the interface between the water and the window, allowing the crater growth to be viewed against the window.

Diagnostics: Various snapshots of the profile of the transient crater.

Projectile:

 Radius:

 Velocity:

 Angle: Normal to target surface

 Material:

 Density:

 Mass:

 Shape:

Environment:

 Pressure:

 Gravity:

Target:

 Material: water

 Density: 1.0 gm/cm3

 Properties: include any relevant material property information

Comments: Simulations of quarter-space experiments should use.

VIC1 - Results

Include plots of the results. Web site will also have data files (tab-separated text).

Comments:

 

 

Appendix B

 

 

Description of benchmark simulations

BE1 - Initial Conditions

Description: Impact of a 1-km aluminum sphere into a water target.

 

Diagnostics: Provide

Projectile:

 Radius: 0.5 km

 Velocity: 5 km/s

 Angle: Normal to target surface

 Material: Aluminum

 Density: 2.7

 Shape: Sphere

Environment:

Gravity: not required (or 1G if required by code)

Strength model: not required

Target:

 Material: water

 Density: 1.0 gm/cm3

 EOS model:

Simulation duration:

Tracer locations:

Mesh:

 X extent 10 km in from impact point to edge of mesh.

 Z extent: -6 km to +5 km (water for z<=0, vacuum for z>0)

 Spatial resolution:

 

Comments:

Code Diagnostics

  • Peak shock pressure, temperature, energy, density
  • Shock decay: pressure, particle velocity (temperature?)
  • Crater morphology: diameter, depth, ??
  • Melting/vaporization (shock pressure contours)
  • Ejecta distribution and speed
  • Post-impact temperature distribution
  • Stress/strain distribution

 

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