MadSci Network: Physics

Re: bullet impact with steel plate

Date: Mon Sep 3 17:14:09 2001
Posted By: Jennifer Anderson, Grad student, Geological Sciences, Brown University
Area of science: Physics
ID: 998492340.Ph

It is amazing how much damage a high-speed impact of one material into 
another can do.  In your example of a lead projectile impacting into a steel 
plate at approximately 4000 feet per second (~1.2 km/s) a hemispherical 
cavity or crater is formed.  The ejecta from this impact have traveled up to 
100 meters in distance, and, even though the ejected particles are fairly 
small in size, they were moving fast enough to etch glass and punch through 

A number of experiments involving impact crater formation have observed 
similarly fast-moving ejecta.  In fact, the earliest ejected particles from 
impact craters can be moving as fast as the initial projectile that impacted 
the surface.  In your example, that would mean that the fastest moving 
ejected particles, at the very earliest times after impact, could be moving 
at 4000 feet per second (~1.2 km/s).  Even particles ejected halfway through 
the crater formation could be moving fast enough to etch glass 100 meters 
away.  These ejected particles are moving fast enough that they create small 
craters when they impact a surface – these craters are called "secondary 
craters" because they were formed by ejecta from the first, or primary, 
crater.  Secondary craters are observed in laboratory experiments as well as 
on planetary bodies, such as the Moon.

For more information on impact craters, check out the Canadian National 
Geophysics Terrestrial Impact Crater Database:

A general text that includes a good section on Impact Cratering is "The New 
Solar System" by J K Beatty, CC Petersen and A Chaikin.  (1999, 4th Edition, 
but any edition will do).  Cambridge Univ. Press.

If you’d like to read some scientific papers that deal with ejection 
velocities, let me recommend:

M J Cintala, L Berthoud & F. Horz (1999) Ejection-velocity distributions 
from impacts into course-grained sand.  Meteoritics and Planetary Science, 
Vol 34, p.605-623.
This paper discusses experiments where the ejecta particle velocities were 
measured.  Graphs within this paper show how the ejecta velocity starts off 
very high and decreases with distance from the crater center.

K R Housen, R M Schmidt & K A Holsapple (1983) Crater ejecta scaling laws: 
Fundamental forms based on dimensional analysis.  Journal of Geophysical 
Research, Vol 88, p. 2485-2499.
This paper is more of a theoretical treatment of ejecta velocities, but 
compares a number of experimental results with theory.

J L B Anderson, P H Schultz & J T Heineck (2001) Oblique impact ejecta flow 
fields: An application of Maxwell’s Z Model.  Lunar and Planetary Science 
Conference 32, #1352.  (Available online at:
meetings/lpsc2001/pdf/1352.pdf )
The experiments that I’ve been doing directly measure impact crater ejecta 
velocities.  Check out my most recent abstract that includes a graph showing 
the velocities we’ve measured in the laboratory – very fast indeed!

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