MIL-DTL-8971F
4.6.2.4 Flexural strength. Determine the average diameter (D) in inches, effective length
(L) in inches, and breaking load (P) in pounds of the test samples using the procedures in
4.6.2.4.1. Calculate and record the flexural strength of each test sample in psi using the formula
below. As an alternative to the methodology given in this specification, a methodology may be
approved in advance by the Defense Logistics Agency-Aviation, Standardization Office (DLA-
GS). For example, an approved, optional method is ASTM C651, "Standard Test Method for
Flexural Strength of Manufactured Carbon and Graphite Articles Using Four-Point Loading at
Room Temperature." To be approved by DLA-GS, the methodology must be documented and
submitted for approval.
1.697 × P × L
Flexural strength (psi) =
D3
NOTE:
1. If shapes other than round are used, the formula must be revised as appropriate.
4.6.2.4.1 Test procedures.
a. Set up a Universal Test Machine with a test fixture equivalent to figure 4. Make sure
that the test fixture is centered on the universal test machine. Set up and operate the universal test
machine in accordance with standard laboratory procedures. Use a test range on the universal test
machine such that the fracture of the test sample occurs at or above 10 percent of the load test
range and at or below 95 percent of the load test range.
b. Accurately measure the diameter of each test sample. Make sufficient measurements
along the length and around the circumference of the test sample to determine the average
diameter of test sample to 0.5 percent. Calculate and record the average diameter (D) of the test
sample in inches. Accurately measure the distance between the highest points of the bearing
rods on the lower span of the test fixture to an accuracy of 0.5 percent. Record this distance as
the effective length of the test samples (L) in inches.
c. Center a test sample on the bearing rods on the bottom span of the test fixture with at
least 1/2 inch of test sample extending past the bearing rods on each end of the fixture. Align the
upper portion of the test fixture so that the upper bearing rods make contact with the test sample
at points that divide the effective length of the test sample into three equal lengths (see figure 4).
Adjust the Universal Test Machine to apply a slight downward force on the test sample sufficient
to hold the test sample in place.
d. Reset the maximum force pointer on the universal test machine. Apply a continuously
increasing unidirectional vertical load to the test sample such that the sample fractures between
45 and 60 seconds after the start of the application of the load. The rate of increase in the load
should be constant. Record the load (P), in pounds, that was being applied at the time the test
sample fractured. NOTE: Disregard the test if the fracture of the test sample did not occur
within the middle 43 percent of the effective test sample length.
13
For Parts Inquires submit RFQ to Parts Hangar, Inc.
© Copyright 2015 Integrated Publishing, Inc.
A Service Disabled Veteran Owned Small Business