PSI - Issue 14

Vivek Khare et al. / Procedia Structural Integrity 14 (2019) 215–225 Vivek khare, Shubham srivastava, Sudhir kamle, G.M. Kamath / Structural Integrity Procedia 00 (2018) 000 – 000

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1. Introduction

Polymer nanocomposites are now being excessively employed in most of the aerospace and automotive industry due to their light weight, high strength to weight ratio and fatigue resistant properties in addition to low cost and ease of manufacturing than aluminum and steel as studied by Tuttle et al. (1986) and Pegoretti et al. (2004). These materials when fabricated with thermoplastic matrix such as polypropylene (PP), offers excellent resistance to deformation. Thermoplastics exhibits viscoelastic behavior due to strong dependencies over time, stress levels, frequency of loading and temperature, therefore creep characteristics of these materials has resulted in better understanding of their structural behavior suggested by Dutta et al. (2000). Jia et al. (2011) concluded that uncovered creep strain reduces with the decrease in temperature compared to polymer matrix. They studied long term creep behavior at low and high stress levels and found that low stress loading corresponds to recovery of uncovered strain better than high stress levels. Pegoretti et al. (2004), Ganß et al. (2007) and Khare et al. (2017) has obtained that the creep compliance can be decreased in thermoplastics filled with nanotubes and layered silicates. Khare et al. (2018) studied creep and recovery behavior of PP nanocomposites and found that creep strain and compliance were reduced by reinforcing MWCNTs up to 1% weight fraction. Papanicolaou et al. (2011) found creep compliance as time and stress levels dependent and increases with stress. Linear viscoelastic region was determined by creep compliance curves. Present study was initiated by the fabrication of polypropylene nanocomposites with two different grades of MWCNT (pristine and – COOH functionalized) using solution casting method. Mechanical characterization was performed with quasi static uniaxial tensile tests to obtain stress levels of characterization, monitor strength and failure strain. Based on the tensile test results, a series from low to high stress levels is selected for cyclic tensile tests and creep tests at room temperatures to showcase the transition and change in properties from linear to nonlinear viscoelastic region. Since polymers exhibits time and temperature dependent behavior, dynamic tests were performed to monitor thermal effects and stiffness under dynamic loading conditions. A threshold of Linearity limit for stress and time was proposed by obtaining successive superimposed creep compliance curve with respect to creep loading time and further validated by running repeated loading and unloading cycles for 1 MPa. It was noted that beyond 1 MPa and 5 secs of loading, there is a transition from linear to nonlinear behavior. Moreover, it was also found that – COOH functionalized 1% MWCNT increases the strength of nanocomposites up to 54.08% and 2% pristine MWCNT decreases strength up to 31.44%. Creep tests results showed that creep strain has a strong influence on magnitude of loading, time of loading and filler functionalization. Creep strain was minimum for 0.5% pristine sample and maximum for 1% functionalized MWCNT sample.

Nomenclature PNC

polymer nanocomposites multi walled carbon nanotubes

MWCNT

multi walled carbon nanotubes, surface functionalized with – COOH

MWCNT-COOH

PP iPP

polypropylene

isotactic polypropylene

P F

pristine multi walled carbon nanotubes

-COOH functionalised multi walled carbon nanotubes

TGA SDT

thermo gravimetric analysis

simultaneous DSC (differential scanning calorimetry) and TGA analysis

FESEM

field emission scanning electron microscopy

DMA

dynamic mechanical analysis

MWCNT-PP

multi walled carbon nanotubes polypropylene nanocomposites

MWCNT-COOH-PP

-COOH functionalised multi walled carbon nanotube polypropylene nanocomposites