UFC 3-260-02
30 June 2001
position. At this point on the bar, there is a hole through which the LVDT core pin falls to rest on
the B pin. This pin must be fabricated with flat sides on the shaft to provide a horizontal surface
on which the LVDT core pin rests. At the C position, the end of the bar simply rests on the
unthreaded portion of the C pin. A nut is placed on the end of the C pin to prevent excessive side
movement of the bar end. This type of bar, pin, and LVDT arrangement is provided on both sides
of the beam. Although no dimensions are provided in Figures K-1 to K-3, this type of equipment
can easily be dimensioned and fabricated to fit any size beam. Either steel or aluminum may be
used. The beam should be positioned and arranged to accommodate third point loading as
indicated in Figure K-2. As the beam bends under loading, defection at the center is measured by
determining the movement of the LVDT stems from their original positions. The LVDTs are
connected to the monitoring system to give an average deflection reading. Since it is also desired
to determine the maximum tensile strain of the beam under loading, an SR-4 strain gauge should be
attached to the lower beam surface with epoxy or some other suitable cement and should also be
connected to the monitoring system. If it is not possible to determine strain directly, a strain value
may be found using Equation K-2.
e. Test Procedure. The flexural beam test is a stress-controlled test. Therefore, an initial
specimen should be statically loaded to failure, and the stress level for the initial repetitive load
tests should be set at 50 percent of the maximum rupture load. The repetitive load test should be
conducted using a haversine wave form, a loading duration of 0.5 second, and a frequency of
about 1 hertz. To develop a strain repetition pattern, it is recommended that tests be conducted at
40, 50, 60, and 70 percent of the maximum rupture value; however, stress levels can be varied to
higher or lower levels. Data to be monitored include load, deflection along the neutral axis, strain
at the lower surface of the specimen, and number of repetitions.
f. Reporting of Test Results.
(1) Flexural Modulus. The flexural modulus should be determined at 100, 1,000, and
10,000 load repetitions or at failure. This value may be determined from load deflection data
monitored at these repetition levels using the expression
(K-1)
%
where
Ef = flexural modulus, MPa (psi)
P = maximum load amplitude, kilograms (pounds)
L = specimen length, millimeters (inches)
d = deflection at the neutral axis, millimeters (inches)
I = moment of inertia, millimeters4 (inch4)
h = specimen height, millimeters (inch)
K-2
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