In previous research, the fractal dimensions of fractured surfaces of vinyl ester based nanocomposites were estimated applying classical method on 3D digital microscopic images. The fracture energy and fracture toughness were obtained from fractal dimensions. A noteworthy observation, the strain rate dependent ductile-to-brittle transition of vinyl ester based nanocomposites, is reinvestigated in the current study. The candidate materials of xGnP (exfoliated graphite nanoplatelets) reinforced and with additional CTBN (Carboxyl Terminated Butadiene Nitrile) toughened vinyl ester based nanocomposites that are subjected to both quasi-static and high strain rate indirect tensile load using the traditional Brazilian test method. High-strain rate indirect tensile testing is performed with a modified Split-Hopkinson Pressure Bar (SHPB). Pristine vinyl ester shows ductile deformation under quasi-static loading and brittle failure when subjected to high-strain rate loading. This observation reconfirms the previous research findings on strain rate dependent ductile-to-brittle transition of this material system. Investigation of both quasi-static and dynamic indirect tensile test responses show the strain rate effect on the tensile strength and energy absorbing capacity of the candidate materials. Contribution of nanoreinforcement to the tensile properties is reported in this paper.
Vinyl ester based composites are mostly considered in applications such as pipelines and chemical storage tanks. The ester groups in the molecular structure are susceptible to water degradation by hydrolysis. The vinyl ester molecule features fewer ester groups, hence, exhibits better resistance to water and to some chemicals [
Characterizing material strength and energy absorption response of materials at higher strain rates has gained increasing attention from multiple researchers. Several attempts [
In this paper, previous research on investigation of fractured surface of the candidate materials [
Five different Derakane 510A-40 vinyl ester thermoset nanocomposite panels [
The nanoparticles are exfoliated and homogeneously dispersed in polymer matrix by applying sonication technique. The homogeneous exfoliation and dispersion is performed in 1 gal container for 4 hours, followed by 4 passes through a flow cell connected to a 100 W sonicator. The resin solution was mixed for 2 min with FlackTek speed mixer at 3000 rpm. The solution of vinyl ester resin with nanoreinforcement and toughening agent is poured into a mold, kept at room temperature for 30 minutes, and then postcured at 80°C for 3 hours [
Typical SEM and TEM morphology of nanoparticle dispersion. (a) Edge view of xGnP flake [
The failed specimens subjected to low velocity impact [
(a) 3D image of typical fractured surface at 1000 times magnification and (b) closed-contour at 10
Determination of surface fractal dimension from the slope of the regression line on scatter-plot of logarithmic area versus perimeter [
The molded nanocomposite panels are machined into disk specimens with a diameter of 12.7 mm using carbide tipped tool in CNC machine. Traditional split-Hopkinson Pressure Bar (SHPB) apparatus is modified [
SHPB setup for indirect tensile tests including Laser Occluding Expansion Gage (LOEG) system.
Induced tensile stress in circular disk specimen along transverse direction due to applied compressive loading.
Three samples in each candidate material group were selected for evaluation. The average response is reported along with the maximum data-scatter.
Quasi-static indirect tensile experiments [
Quasi-static indirect tensile test setup with LOEG.
The quasi-static indirect tensile response of pure vinyl ester specimens (Figure
Posttest photographs of (a) pure vinyl ester sample elliptically deformed without diametrical splitting and (b) nanoreinforced composite sample with diametrical splitting.
The quasi-static indirect tensile stress-strain history for pure vinyl ester, xGnP reinforced, and CTBN toughened samples are shown in Figures
Quasi-static indirect tensile response. (i) Typical stress versus strain behavior, (ii) strength, and (iii) energy absorbing capacity of (a) graphite platelet reinforced and (b) with additional CTBN toughened vinyl ester nanocomposites (note: pure vinyl ester specimens did not fail within the 10 kN load cell limit of test equipment used for quasi-static testing).
Figure
High-strain rate indirect tensile response from SHPB tests. (i) Typical stress versus strain behavior, (ii) strength, and (iii) energy absorbing capacity of (a) graphite platelet reinforced and (b) additionally CTBN toughened vinyl ester nanocomposites.
It can be observed that the tensile strength of pure vinyl ester remains unchanged with
Previous research on fractal analysis of fractured surface revealed the strain rate dependent ductile-to-brittle transition [
Strain rate dependent ductile-to-brittle transition of fracture propagation mechanism investigated by surface fractal analysis [
In present research, comparative observation of Figures
About 25% increment in tensile strength is observed at high-strain rate loading (Figures
Surface fractal analysis, in previous research, depicted the ductile or brittle fracture propagation mechanism, depending upon the rate of loading. In the current research, quasi-static and high-strain rate experimental investigations characterize the effect of strain rate and the contribution of xGnP reinforcement along with CTBN toughening on the indirect tensile properties of vinyl ester based nanocomposites. Tensile strength and energy absorbing capacity of pure vinyl ester are reduced by the addition of xGnP reinforcement and even with CTBN toughening under quasi-static loading. Addition of CTBN marginally improved the energy absorbing capacity of the only xGnP reinforced (without CTBN) nanocomposites under quasi-static loading. Tensile strength of pure vinyl ester remains almost the same with the addition of xGnP reinforcement and even with CTBN toughening under high-strain rate loading. Energy absorbing capacity of pure vinyl ester is improved with addition of xGnP reinforcement under high-strain rate loading. Pure vinyl ester shows ductile-to-brittle transition from quasi-static to high-strain rate loading. Tensile strength observed in quasi-static test is increased at high-strain rate loading for these candidate nanocomposites. The energy absorption capacity of pure vinyl ester is adversely affected under high-strain rate loading.
The authors declare that there is no conflict of interests regarding the publication of this paper.
Partial support for this research was provided by Office of Naval Research, Solid Mechanics Program (Dr. Yapa D. S. Rajapakse, Program Manager) Grant no. N00014-7-1-1010 and US Army Research Office under the DOD-PIRT sub-contract through North Carolina A & T University Grant no. 300223243A. The vinyl ester nanocomposite panels were manufactured by Dr. Larry Drzal’s group at Michigan State University.