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Three-Point Bending Fatigue Test Results
The loading conditions in the three-point fatigue tests were 60
to 85 % of the static three-point flexural strength. The test
conditions are shown in Table 4, and the S-N curves are shown
in Fig. 3.
Fig. 5 Examples of Observations Using the SEM Servopulser
(1) Before the test (2) 719 cycles, magnification ×30 (3) 719 cycles, magnification ×50
(4) 719 cycles, magnification ×100 (5) 5,500 cycles, magnification ×13 (6) 5,500 cycles, magnification ×100
Table 4 Three-Point Bending Fatigue Test Conditions
Observations Using the SEM Servopulser
The SEM Servopulser combines a scanning electron microscope
(SEM) with a fatigue testing machine (Servopulser). This enables
the extension of cracks to be observed in detail during tests.
From the S-N curve in Fig. 3, observations using the SEM
Servopulser were performed up to a maximum loading stress of
535 MPa. Fig. 4 shows a photograph of the test specimen and
test jig. To make the test specimen more easily observed using
the SEM, the test specimen was coated with gold in advance.
Fig. 5 shows images observed using the SEM Servopulser. Fig. 5
(1) shows the test specimen directly below the indenter before
the test. Fig. 5 (2) to (4) show the test specimen close to the
indenter when the number of cycles was 719. The SEM
magnification ratio was 30, 50, and 100, respectively. From Fig. 5
(2) to (4), compression failure, including the rupture of fibers,
was observed on the left side of the indenter. Fig. 5 (5) and (6)
show the test specimen when the number of cycles was 5500
with a magnification ratio of 13 and 100, respectively. From Fig.
5 (5), it can be seen that cracking occurred not only on the
indenter side but also on the opposite side. In Fig. 5 (6), a new
matrix crack that was not observed in Fig. 5 (4) was observed, so
it is clear that the damage had progressed.
Fig. 4 Test Specimen and Test Jig
Indenter
Specimen
Support point
Support point Fig. 3 S-N Curve
Note: Each blue point is one fatigue test result. The point with an arrow
indicates that even at the prescribed number of cycles (10 million) the
specimen did not fail
Max. Loading Stress [MPa]
Number of Cycles to Failure [Cycles]
Maximum Loading Stress: 377, 409, 440, 472, 503, 535 [MPa]
(60 to 85 % of the static flexural strength)
Stress Ratio: 0.1
Frequency: 20 Hz
Number of Tests: n = 2
Distance between Supports: 30 mm
Indenter/Support Point: R2
Apparatus Used: EHF-L
(1) (2) (3)
(4) (5) (6)
Indenter
Specimen
