Fig.2 Spectro microscopy images for strut thickness measurement (a)
Cubic Irregular (b) Star Regular with thickness measurement in μm.
Quasi-static and fatigue
testing
The mechanical tests were carried out under two loading conditions,
monotonic quasi-static and cyclic. They were carried out according to
the standard ISO 13314:2011 for metallic foams49. Both
tests were carried out at room temperature using an INSTRON universal
testing machine with a load capacity of 100kN. The displacement was
measured using an Instron LVDT. A constant crosshead speed of 1mm/min
and a data sampling frequency of 1kHz was employed. For each
configuration, one specimen was tested under monotonic compression
condition and four specimens under cyclic condition.
The stress-strain curve obtained from the monotonic testing condition is
then used to acquire the monotonic Young’s modulus (Em),
0.2% offset yield strength (σy) and the maximum
compressive strength (σmc). Cyclic tests were performed
to obtain the cyclic Young’s modulus (Ec) after
stabilization of the stress-strain response. The specimens were loaded
between 20-70% of the yield load obtained from the monotonic testing
condition using a triangular shape wave for five cycles.
High cycle compression-compression fatigue testing was carried out for a
minimum of 12 specimens for each topology. RUMUL resonating fatigue test
machine was used with an R-ratio of 0.1 in compression. The applied
maximum load was chosen in the range 0.1-0.8 yield load to obtain the
S-N curves36,50. It was verified that the contact
surfaces of the specimens were flat with no irregularities. A frequency
drop of 1 Hz during the test was considered as an indicator of fatigue
failure in the specimens, and the number of cycles at that specific
moment was recorded as fatigue life. The S-N curve was subsequently
obtained by fitting the data points using a curve fitting expressed by
equation 2, where σmax is the maximum stress,
Nf is the number of cycles to failure, and
C1, C2, m are the curve fitting
parameters. The run-out condition for the specimens was set as
107 cycles and the fatigue strength of the specimens
was calculated at 106 cycles using the S-N curves. The
scatter of the fatigue data (S2) is calculated by
equation 3, where σmax-i is the ithmaximum stress, σ’max-i is the ithestimated maximum stress, n is the number of data points and p is the
number of parameters in equation 2 and the standard deviation of the
fatigue strength was calculated as indicated in the
reference18. Investigation of the fractured surface
was carried out by using a JOEL JSM-IT300LV scanning electron
microscope.
\(\sigma\max{=C1+}\frac{C2}{(Nf)m}\ \)………… (2)
\(S2=\ \frac{\sum_{i=1}^{n}{(\sigma max-i-\sigma^{{}^{\prime}}max-i)2}}{n-p}\)……………(3)
To obtain the deformation mechanism and the crack propagation planes
characterizing any cellular structure considered herein, some dedicated
fatigue tests were carried out at a frequency of 5Hz using an MTS 809
Axial/Torsional test system with a load capacity of 100 kN. To evaluate
the failure sequence during the fatigue loading, a high speed camera
with a capturing frequency of 30 Hz was used to record the whole
duration of the fatigue tests. The frames from the captured video have
been indicated in the results and discussion section.