International Journal of Students’ Research in Technology & Management Vol 2 (05), August-September 2014, ISSN 2321-2543, pg 157-162 http://www.giapjournals.org/ijsrtm.html 157 Effect of Fiber Length on the Short-Term Flexural Creep Behavior of Polypropylene C.Subramanian *#1 , Abdulrahman Khalfan Hassan Al Mamari #2 , S.Senthilvelan #3 #1 Shinas College of Technology, Oman #2 Petroleum Development Oman, Oman #3 Indian Institute of Technology Guwahati, India 1 subra@shct.edu.om 2 a.k.ha@hotmail.com 3 ssvelan@iitg.ernet.in Abstract — Injection molded long fiber thermoplastic components are being used in recent days as a viable replacement for metals in many applications .Present work focus on the effect of fiber length on the short-term flexural creep performance of fiber reinforced thermoplastic polypropylene. Unreinforced polypropylene, 20 wt % short and 20 wt % long glass fiber reinforced polypropylene materials was injection-molded into flexural test specimens. Short-term flexural creep tests were performed for 2 h duration on molded specimen at various stress levels with the aid of in-house developed flexural creep fixture. Experimental creep performance of polypropylene composites for 2 h is utilized to predict the creep performance with the aid of four parameter HRZ model and compared with 24 h experimental creep data. Creep strain was found to be increased with respect to time for all the test materials and found to be sensitive with respect to the stress level. Test results also revealed that long fiber reinforced thermoplastic material possessed enhanced creep resistance over their counter parts and HRZ model is sufficient enough to predict creep performance of polypropylene composites over wide range of stress. Keywords- Injection molding, flexural creep, thermoplastic, creep, strain I. INTRODUCTION Due to the mass production requirement in the automotive industries, discontinuous long fiber reinforced thermoplastics (LFRT) have shown significant role in replacing metals, short fiber reinforced thermoplastics, thermoset sheet molding and bulk molding composites [1]. The common problem associated with unreinforced thermoplastics is creep under moderate to severe stress at elevated temperature. Creep resistance of thermoplastic composites is significantly improved by increase in fiber loadings [2]. Dynamic mechanical analysis (DMA) was utilized to investigate the viscoelasticity of injection-molded nylon 6/6 material reinforced with short and long glass fibers by Sepe[3] and reported an increase in creep resistance for long glass fiber reinforced nylon composites. Challa and Progelhof [4] investigated the effect of temperature on the creep characteristics of polycarbonate and developed a relationship based on Arrhenius theory to develop creep master curves. Pegoretti and Ricco [5] studied the propagation of crack under creep for varying temperature conditions for polypropylene composites and observed that speed with which the crack progresses was dependent on the test temperature. Krishnaswamy [6] performed extensive creep rupture testing on high density polyethylene pipes at various hoop stress levels and temperatures and observed the dependency of density and crystallinity towards failure. Houshyar [7] reported the improvement in creep properties with the addition of long polypropylene fibers in propylene-co-ethylene (PPE) matrix and visualized the improvement in interfacial properties. Trans-crystallization of the polypropylene matrix was observed in the PPE samples due to the thin layer of matrix on the reinforcement, which was attributed to good impregnation and wetting of the fibers. Greco et al. [8] investigated the flexural creep behavior for compression molded glass fiber reinforced polypropylene at various applied stress level. The effect of matrix crystallinity was highlighted for the improvement in creep properties for glass fiber reinforced polypropylene in their work. Acha et al. [9] studied the influence of interfacial adhesion in discontinuous jute fiber reinforced polypropylene. Relation between interfacial properties and creep deformation were investigated. Higher creep resistance was observed for polypropylene composites with good interfacial bonding which was confirmed by the observation of the composite fractured surfaces. Findley and Khosla [10] conducted creep tests for unreinforced thermoplastics; polyethylene, polyvinyl chloride and polystyrene. Approximation was carried out for the linear viscoelastic region by power law and compared the creep performance by estimating the power law coefficient and power law exponent. Liou and Tseng [11] used Findley power law to estimate the creep compliance of carbon fiber nylon composites in hygrothermal condition. Power law model was modified by Hadid et al. [12] by incorporating the time and stress dependence during creep loading of polyamide specimens and estimated four parameters for describing the deformation occurring in the material and used stress–time superposition principle to predict long-term material creep behavior of injection molded fiber glass reinforced polyamide.