TM 5-822-13/AFJMAN 32-1018
Figure C-l. Repeated Flexure Apparatus.
beam center and to position the centers of the two reactions clamps 6 inches from the beam center. Double
layers of Teflon sheets are placed between the specimen and the loading clamps to reduce friction and longitudi-
nal restraint caused by the clamps.
(2) After the beam has reached the desired test temperature, repeated loads are applied. Duration of a
load repetition is 0.1 second with 0.4-second rest periods between loads. The applied load should be that which
produces an extreme fiber stress level suitable for flexural fatigue tests. For fatigue tests on typical bituminous
concrete paving mixtures, the following ranges of extreme fiber stress levels are suggested:
Temperature, degrees F.
Stress Level Range, psi
55
150 to 450
70
75 to 300
85
35 to 200
The beam center point deflection and applied dynamic load are measured immediately after 200 load repetitions
for calculation of extreme fiber strain
The test is continued at the constant stress level until the specimen
fractures. The apparatus and procedures described have been found suitable for flexural fatigue tests at tem-
peratures ranging from 40 to 100 degrees F. and for extreme fiber stress levels up to 450 psi. Extreme fiber
stress levels for flexural fatigue tests at any temperature should not exceed that which causes specimen fracture
before at least 1,000 load repetitions are applied.
(3) A set of 8 to 12 fatigue tests should be run for each temperature to adequately describe the relation-
ship between extreme fiber strain and the number of load repetitions to fracture. The extreme fiber stress
should be varied such that the resulting number of load repetitions to fracture ranges from 1,000 to 1,000,000.
f. Report and presentation of results. The report of flexural fatigue test results should include the following:
(1) Density of test specimens.
(2) Number of load repetitions to fracture, N,.
(3) Specimen temperature.
(4) Extreme fiber stress,
The flexural fatigue relationship is plotted in figure C-2.
C-2
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