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National Advisory Committee for Aeronautics, Technical Notes - The Elastic Constants for Wrought Aluminum Alloys

Since the evaluation of an elastic constant of a metal
involves the precise determination of the strains resulting
from stresses set up in the material by applied leads, it is
essential that“the loads be known within suitable limits of
accuracy. The loads are usually applied by means of one of
the many types and sizéshof commercially available testing
machines. Even though well—recOgnized standard methods (refl-
erence l) of verification of testing machines are followed in
calibrating the machines to be used in determining the elastic
constants, Yet it should be noted that these standard methods
of verifications are primarily intended to cheek the.machines
within limits suitable.for acceptance testing of materials
against product specifications. Present verification stand—
ards require that a testing machine shall have errors in its
lead readings not greater'than'il percent, within the loading
range to.bc used.

For the precise determinatien'ef the.elastie constants
of metals; smaller limits for the load errors are necessary.
whis may’not require a testing machine with-smaller errors
but (a) the magnitude of the errors must be known within
closer’limits; (b) the sensitivity'of the machine must be
commensurate with the smaller”errqf limits; (c) the repro-
ducibilityIOf the indicated loads must be within about the
same smaller limits; and (d) the loads must be indicated
within suitable limitsxundor the conditions used during test-
ing.

In attempting to meet these'requiremcnts the Aluminum
Research Laboratories have made three separate calibrations
of each load rangp.p£-the=testing machine to be used; the
average of these three sets of data compared with the data
from the National Bureau of Standards? report on the enli+
bratihg device used: and the loads indicated by the testing
machine corrected.in accordance with the observed errore.
Comparisons'ef the individual readings for each of the dif—
ferent-leads used in the'three calibration runs'show the
sensitivity and reproducibility'limits obtaining3- Very slew
rates of loading are used-to minimize if not eliminate any
dynamic effects.