posted on 2022-01-21, 14:11authored byJared
W. Strutton, Newell H. Moser, Edward J. Garboczi, Abby R. Jennings, Brandon Runnels, Jena M. McCollum
To determine if full-strain measurements
are sufficient in measuring
interfacial phenomena in additively manufactured thermosets, we devised
a testing method to examine the effect of interface history (i.e.,
time between interface formation and original material deposition)
on localized mechanical properties. Interfaces were formed in diglycidyl
ether of bisphenol A (DGEBA)/diethylenetriamine (DETA) epoxy resins
with various DETA concentrations. Times to the gel point and full
cure were assessed by rheology and Fourier transform infrared spectroscopy
(FTIR) for each composition. From here, dogbone samples were fabricated
with an interface formed at either the gel point or at full cure for
each composition. Key findings show that tensile strength and Young’s
modulus deteriorate globally with the presence of an interface regardless
of the history or initiator (DETA) concentration. Samples with an
interface demonstrate high strain regions near the interface prior
to fracture. Micro X-ray computed tomography revealed high density
regions at the interface that increased in number with both cure times
and initiator concentrations. FTIR revealed that the interface demonstrated
a higher cure completion than the sample interior, resulting in a
stiffer epoxy localized at the interface versus the bulk. These findings
were confirmed by atomic force microscopy modulus mapping at the interface.
Finally, computational modeling of epoxy in uniaxial tension with
an increasing number of stiff inclusions demonstrated that inclusion
contents correlated with increased, localized stress concentrations.
These findings will aid in the understanding of fracture phenomenon
in additively manufactured thermosets and point to digital image correlation
as a useful tool in epoxy interface detection.