A Comparison of Corrosion-Resistant Steel (18 Percent Chromium - 8 Percent Nickel) and Aluminum Alloy (24st) Page: 4 of 13
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N.A.C.A. Technical Note No. 560 3
indicate what can be expected from a notched bar subject-
ed .to stresses to produce fracture, are not true indexes
of the actual energy absorption power of materials. It is
considered that the designer is more interested in the ca-
pacity of a member to resist injury when subjected to a
suddenly applied load within or close to the elastic range
than he is in the work done in fracturing the member. It
is believed that resiliency, or elastic recovery from en-
ergy loads, would be more pronounced on materials of the
same cross-sectional area with low moduli of elasticity
than on those with high moduli when subjected to identical
conditions of impact. The assumption is made that the work
of deformation is equal to the energy of the blow. Consid-
ering the two formulas on page 42 of Johnson's "Materials
of Construction" (reference 1) for stress under axial im-
pact and for a beam under impact of a center load, it is
apparent that the elastic modulus is a major consideration.
These formulas are as follows:
W (h + e) = 2 AL (axial impact) (1)
(h+ f) = R S AL (center load) (2)
6 C E
W, weight falling
h, distance dropped
e, maximum deformation
S, maximum intensity of stress corresponding
E, modulus of elasticity
A, area of cross section
f, maximum deflection
It is therefore submitted that the tests on actual
built-up structures, measuring deformation, and deflec-
tion under a suddenly apPlied load or by-a drop tesT of
the structure itself, are conducive of more accurate de-
terminations of impact resistance within the range in
which the designer is primarily interested than are the
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Sullivan, J. E. A Comparison of Corrosion-Resistant Steel (18 Percent Chromium - 8 Percent Nickel) and Aluminum Alloy (24st), report, March 1936; (digital.library.unt.edu/ark:/67531/metadc54153/m1/4/: accessed December 18, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.