Materials testing
USF and is free at the other end, see Figure
3. The specimen is designed in a way that
maximum stress occurs at the middle of
the specimen and the maximum
displacement occurs at the free end of the
specimen. To eliminate the temperature
effect due to high test frequency, the
specimens were cooled with compressed
air during tests and the tests were
performed at a pulse-pause ratio of 50:50,
i.e. the system was set to resonate for
200ms and then stopped for the next
200ms to cool down.
Experiments for the determination of
the fatigue strength at 1E9 cycles were
performed according to the stair-case
method. When a specimen at ultrasonic
frequency failed at less than 1E9 cycles,
stress amplitude was decreased by 5MPa
for the next experiment. If the specimen
did not fail at 1E9 cycles, stress amplitude
was increased by 5MPa in the subsequent
AEROSPACETESTINGINTERNATIONAL.COM // SHOWCASE 2020 113
surface roughness, fatigue performance
after post-processing is suitable for many
applications in the aviation and medical
industry. 1,2
With the availability of the novel very
high cycle fatigue (VHCF) testing
techniques, it has been found that unlike
previous assumptions that materials do not
fail under fatigue if the applied stress is
below the so-called “endurance limit”,
materials do in fact fail under fatigue
loading even when the stresses are below
the conventional “endurance limit”. This
suggests the non-existence of such a limit
3, 4. Some alloys of both lattice types (bcc
and fcc) show a change in crack initiation
site behaviour from surface to subsurface
in a region from HCF to VHCF. 5
EXPERIMENTAL METHODOLOGY
The test samples of AlSi12 alloy were
manufactured using a commercially
available SLM system in an inert
environment using argon gas. Details of
the processing setup and parameters can
be viewed in 1,2. Quasi-static tensile tests
were carried out according to ISO 6892-
1:2009. Continuous load increase tests were
executed starting at a low stress amplitude
of 30MPa. Stress amplitude was increased
slowly at a rate of 10MPa / 1E4 cycles. Load
increase tests and constant amplitude tests
were performed at a frequency of 20Hz.
Results of process optimization, quasistatic
properties, high cycle fatigue
properties and the measurement
methodology for characterization of
process-induced defects have been
published in previous studies1,6,7. Two
types of configurations were investigated
in this study. For batch I, no pre-heating of
the base plate was applied, whereas
samples of batch II were manufactured
with base plate heating (BPH) at 200°C.
Very high cycle fatigue (VHCF) tests
were carried out with an ultrasonic fatigue
testing system at a frequency of 20kHz.
Figure 1 shows an overview of Shimadzu´s
USF-2000A testing system, while Figure 2
explains in detail the working principle
parts of the test setup. Piezoelectric
crystal is used in the actuator, which
resonates at a fixed frequency of 20kHz. In
the ultrasonic fatigue testing system,
vibrations are designed so that the
longitudinal waves transmitted through
the solid body resonate.
For fully reversed VHCF loading
(without mean stress), i.e. load ratio -1, the
specimen with geometry shown in Figure
4 is clamped only at one threaded end with
“Purity, reliability and durability are key
features to make a top-class AM product”
2
3
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