Mapping viscoelastic and plastic properties of polymers and polymer-nanotube composites using instrumented indentation

Abstract

An instrumented indentation method is developed for generating maps of time-dependent viscoelastic and time-independent plastic properties of polymeric materials. The method is based on a pyramidal indentation model consisting of two quadratic viscoelastic Kelvin-like elements and a quadratic plastic element in series. Closed-form solutions for indentation displacement under constant load and constant loading-rate are developed and used to determine and validate material properties. Model parameters are determined by point measurements on common monolithic polymers. Mapping is demonstrated on an epoxy-ceramic interface and on two composite materials consisting of epoxy matrices containing multi-wall carbon nanotubes. A fast viscoelastic deformation process in the epoxy was unaffected by the inclusion of the nanotubes, whereas a slow viscoelastic process was significantly impeded, as was the plastic deformation. Mapping revealed considerable spatial heterogeneity in the slow viscoelastic and plastic responses in the composites, particularly in the material with a greater fraction of nanotubes.

Effect of interface surface design on the fracture behavior of bilayered composites

“…The surface microhardness measurements (five points for each specimen) were carried out using a universal Vickers device (Struers Duramin, Struers, Ballerup, Denmark), with a load of 245.3 mN being applied for 15 s. The time elapse between the point of indentation and the measurement of the deformation produced was standardized at 30 s to equate for possible elastic recovery of deformation after the point of indentation . Test indentations should be performed on a flat surface and therefore the flat interface design was chosen for surface microhardness testing.…”

Systematic research of physicochemical properties of functional photopolymers for additive manufacturing

“…The advantage of maps such as Figure 24 to Figure 27, allows assessed directly and visually that there are no local variations in the mechanical properties. Moreover, the average values extracted from these maps can be representative of the entire composites if the studied areas are large enough ( Figure 28) [70,88,89]. Figure 32 highlight the significant decrease of ' modulus and when loading the polymeric matrix with Cu NWs (Figure 30 and Figure 32(a)), and CuNi NWs (Figure 31 and Figure 32(b)), to different post-cured time.…”

“…Consequently, the advantage of maps such as Figure 29 to Figure 31 allows assessed directly and visually that there are no local variations in the mechanical properties. Moreover, the average values extracted from these maps ( Figure 32) can be representative of the entire composites if the studied areas are large enough [19,[85][86][87][88][89]. In Figure 29 to Figure 31, the darkening corresponds to higher concentrations of the nanowires embedded in the matrix of the photopolymer resin.…”

“…In fact, as the aggregates are only weakly infiltrated by the polymer, they degrade the relative properties of the polymer. On the contrary, a suitable dispersion of the nanofillers would result in optimized matrix/filler bonding and enhanced properties [70,[85][86][87][88][89]. In Figure 24 to Figure 27, the local "soft or clear" spots, which are due to the small differences in the dispersion, correspond to regions with degraded mechanical properties, probably due to Cu or CuNi aggregates poorly infiltrated by the polymeric matrix.…”

“…On the contrary, a suitable dispersion of the nanofillers would result in optimized matrix/filler bonding and enhanced properties. Furthermore, removal of entangled agglomerates is the major focus of the many methods used to disperse any filler in polymer matrices [70,[85][86][87][88][89]. Figure 55 highlight the significant variations of ' modulus and , when GO is aggregated to the polymeric matrix with GO, to different post-cured time.…”