2. Thesis and Dissertations
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Item Experimental Investigation of 3d Printed Syntactic Foam Composites(National Institute of Technology Karnataka, Surathkal, 2019) Patil, Balu.; Doddamani, MrityunjayPolymer matrix composites can reduce the structural weight and result in improved fuel efficiency and performance in transportation applications. Thermoplastic matrix composites have been used for semi-structural and engineering applications. In addition to the ease of fabrication using a wide range of forming processes, thermoplastic polymers are recyclable, which is the strong driving force for their current and future applications. Rapid production of high quality components is the key to cost reduction in industrial applications. The present work is the first attempt of manufacturing syntactic foams, hollow particle filled lightweight composites using thermoplastic based fused filament fabrication /fused deposition modeling (FFF/FDM) 3D printing process. High Density Polyethylene (HDPE) is used as the matrix material and fly ash cenospheres as the filler. Development of syntactic foams with cenospheres serves dual purpose of beneficial utilization of industrial waste fly ash and reduction in the component cost. Hollow fly ash cenospheres are blended with HDPE to form cenosphere/HDPE blend and is extruded to filament form and finally fed through 3D printer for printing ecofriendly lightweight syntactic foams. Prior to filament development, thermal degradation, melt flow index (MFI) and rheological properties of cenosphere/HDPE blend are studied. MFI decreased by 39.29, 60.54 and 70.51% with increasing cenospheres content of 20, 40 and 60 vol. % respectively. Rheology study of cenosphere/HDPE blend revealed complex viscosities values are maximum at a lower frequency but decreases with an increasing frequency indicating shear thinning behaviour. Both storage and loss modulus showed an increasing trend with filler content and frequency. Single screw extruder parameters are optimized to develop ecofriendly syntactic foam filament with minimum cenosphere fracture and to obtain homogeneous mixing of constituents. The optimized parameters are used for manufacturing syntactic foams filament with 20, 40 and 60 vol.% cenosphere in HDPE matrix. Further, recycling potential of foam filament is also studied. Density of H40 (HDPE with 40 vol.% ofcenospheres) foams increased in up to two extrusion passes (2X) due to cenosphere breakage and porosity consolidation. Tensile properties of developed filaments are carried out to assess its viability into 3D printer. Tensile modulus and yield strength of neat HDPE filaments increased with each extrusion pass. Specific modulus of 3D printed H40-2X and 3X are 1.6 and 2.6 times higher than the respective filaments, however, fracture strain decreases by up to 40%. For first time extruded (1X) filament with addition of cenosphere density reduces due to intact cenosphere and void formation during extrusion, making it a 3 phase foam material. The void content and weight saving potential increases with increase in filler content and their values are higher for 3D prints than respective filament. Higher filler loading increases filament modulus by 7.72-12.79% as compared to HDPE. Among the foam filaments, H20 composition registered the highest ultimate strength (10.30 MPa) and strain at break (26.20%). Differential scanning calorimeter and X-ray diffraction analysis of neat HDPE and foam filaments crystallinity is used to assess the parametric optimization of 3D printing process. It is observed that addition of cenosphere reduced crystallinity of HDPE. HDPE and foam filaments exhibit lower crystallinity as compared to respective printed material. Coefficient of thermal expansion (CTE) of 3D printed HDPE and its foam is studied to understand warping and shrinkage phenomenon occurring during printing. It is observed that filler addition in HDPE matrix reduces CTE remarkably. Warpage of the specimen is reduced with filler content and print quality is further improvised by optimizing printer speed, layer thickness, print temperature and cooling conditions. Tensile tests are carried out on filaments and printed samples. Cenospheres addition resulted in improved tensile modulus and decreased filament strength. Tensile modulus of printed foams increases with filler content. 3D printed HDPE and foams modulus is better than respective feedstock material (filament). Tensile properties of 3D printed HDPE and foams are compared with injection molded samples. 3D printed HDPE registered higher tensile modulus and fracture strength compared to injection molding. Flexural test is conducted on 3D printed sample in two configurations (topand bottom face of print subjected to the load). Results obtained from both configurations reveals that second configuration has shown better flexural modulus and strength. Neat HDPE print did not show any fracture below 10% strain. Flexural modulus increases with cenosphere content. Highest modulus is exhibited by H60 which is 1.56 times better than neat HDPE print. Raster gaps in 3D prints lowers flexural modulus and strength as compared to fully dense injection molded sample. Quasi-static and regular strain rate compressive response is investigated on prints. Compressive behaviour of 3D printed foams follow similar trend in quasi-static and regular compressive mode as reported in fully dense injection molded two-phase foams. Modulus of neat HDPE is higher for all strain rates as compared to foams. Yield strength shows an increasing trend with strain rate. Highest specific compressive modulus and yield strength is observed for H60 and H20 respectively at 0.1 s-1 among foams. Further, HDPE matrix syntactic foam prints are characterized for their viscoelastic properties by dynamic mechanical analysis. Tests are conducted over 30-125°C temperatures. Storage and loss modulus increase with increasing volume fraction of cenospheres, with a slight difference between HDPE, H20 and H40 vol.%, at all temperatures. Storage modulus decreased with increasing temperature for neat HDPE and foam prints. Storage and loss modulus decrease with increasing temperature in the range of 30-125°C, while Tan δ increases. Structureproperty correlations of all the investigated properties are presented with the help of exhaustive SEM images to understand underlying mechanisms. Property maps for selected test conditions are presented for comparative analysis between FFF/FDM based 3D printing of eco-friendly lightweight syntactic foam prints and other processing routes used for thermoplastics. This work is an effort towards making wide material choices availability for FFF based 3D printing industries. Finally, the potential for using the optimized parameters of 3D printing is demonstrated by printing several industrial components as a deliverable of of this work.Item Experimental Investigation of Glass Microballoon/Hdpe Syntactic Foam Composite(National Institute of Technology Karnataka, Surathkal, 2018) M. L, Jayavardhana; Doddamani, MrityunjayPolymer matrix composites inherit good specific values and reduced structural weight making them more promising in automobiles and aerospace applications. Thermoplastic polymers are moldable into different shapes, recyclable and reusable leading to wide usage in semi-structural and engineering applications. These materials are widely used in consumer products and industrial components. Reducing weight of thermoplastic components has been always a high priority in transportation, aerospace, consumer products and underwater vehicle structures. Their current and future potential are driven by processing flexibilities using variety of industrial scale manufacturing techniques and material innovations therein like in foams. Foams are lightweight cellular materials that are widely used in applications such as packaging, thermal insulation, sound absorption, underwater vehicle structures and as the core in sandwich structures used in aircraft. Rapid production of such high quality foam components for industrial applications reduces matrix material requirement and the associated cost. The present study is focused on developing an industrial scale compression molding based processing method for glass microballoon/high density polyethylene (GMB/HDPE) syntactic foams and studying their mechanical properties to develop structure-property correlations. Although glass hollow particle filled lightweight syntactic foams with thermoset matrices have been studied in detail, studies on thermoplastic syntactic foams are scarce. Despite continued interest in developing lightweight thermoplastic syntactic foams, they have not been studied extensively with focus on volume fraction and wall thickness variations. Matrix material used in the present investigation is high density polyethylene (HDPE) and the filler is glass microballoon (GMB), both in as received conditions. Syntactic foam (SF) developed by glass microballoons have benefits like low density, good dimensional stability, high stiffness, material saving and reduced component cost without compromising the specific properties. Blending of GMB in HDPE is carried out using a Brabender mixer with processing parameters optimized for minimal filler breakage. The optimized parameters are used for manufacturing HDPE syntactic foam lumps (brabender output) with 20, 40 and 60 volume % glass microballoon. SF lumpsare processed through compression molding route to form SF sheets that are used for mechanical characterization. In total NINE types of syntactic foams are prepared with three different GMB true particle densities (200, 270 and 350 kg/m3) varying by 20, 40 and 60 volume % in HDPE resin. Different density particle resemble varying wall thickness of GMBs. Lower and higher density values represent thin and thick walled GMBs. Neat HDPE samples are also prepared with similar processing condition as that of foams for comparative analysis. Minimum of five replicates are tested and average values are used for analysis. Uniform distribution of GMBs is observed through micrography affirming the good quality of GMB/HDPE syntactic foam sample processed through the adopted compression molding route. Experimental and theoretical densities of developed syntactic foams are computed. Measured density of all the syntactic foams is lower than neat HDPE resin. Weight saving potential of 10-36% is observed by using GMBs in HDPE matrix. For all particles types, GMB failure is observed to be the highest for syntactic foams containing 60 vol. % GMBs. Increasing glass microballoon content increases particle to particle interaction during processing resulting in particle breakage. Additionally, increasing wall thickness makes GMBs stronger resulting in reduced particle fracture. Particle failure forms glass debris that gets embedded in HDPE matrix. Although fractured particles do not provide reduction in density as planned, they still help in replacing more expensive HDPE resin. Tensile test is conducted at a constant strain rate of 5 mm/min strain rate on trimmed GMB/HDPE foam samples as per ASTM D638-14. Tensile modulus is observed to be highest for the thick walled microballoon having highest filler content as compared to neat resin. Increasing filler content and the wall thickness increases modulus, effect of volume fraction being more prominent. Ultimate tensile strength is seen to be decreased by 32-66% with increasing filler content as compared to neat HDPE. The fracture strength of all the GMB/HDPE foams is 1.3-2.9 times lower than that of the neat HDPE. Neverthless, specific modulus is highest for syntactic foam with thick walled microballoon at 60 vol. % filler loading as compared to neat resin and other foams. Specific strength of GMB/HDPE foams is less compared to neat resin. Highervalues of specific tensile modulus affirm the use of these syntactic foams for weight sensitive applications demanding higher modulus in molded components. Further, tensile test is carried out for lower strain rates (1.6×10-5, 1.6×10-4 and1.6×10-3 s-1). Highest tensile modulus is observed in foams with thin walled microballoons at highest filler loading as compared to neat HDPE at 1.6×10-3 s-1 strain rate. The effect of wall thickness on the modulus of syntactic foams with the same GMB volume fraction is greater at lower strain rates compared to higher ones. Tensile modulus is found to be relatively insensitive to GMB wall thickness variations. Ultimate tensile strength decreases with increasing filler content. Compared to neat HDPE, syntactic foams fracture at lower strain. The fracture strength of all the developed syntactic foams is 1.5-3 times lower than that of the neat HDPE. No clear trend is observed for specific tensile strength. GMB/HDPE foams samples are subjected next to flexural test as per ASTM D790-10. Foams exhibited higher flexural modulus as compared to neat HDPE. Flexural modulus increases while strength decreases with increasing filler content. Additionally, increase in wall thickness increases the flexural modulus. Specific flexural modulus and strength of SF with 350 kg/m3 particle density having 60 vol. % GMB and 200 kg/m3 GMB having 60 vol. % are observed to be 147 and 8% higher compared to neat HDPE samples. Flexural properties are sensitive to volume fraction variations as compared to wall thickness variation. Two theoretical approaches, Porfiri-Gupta and Bardella- Genna model are used to estimate tensile and flexural modulus of syntactic foams. Bardella-Genna model predicts values closer with experimental results for all GMB/HDPE foams tested under tensile (except lower strain tests) and flexural conditions. Outcome of existing literature on tensile and flexural studies is compared with the experimental results of the present work is presented in the form of property maps which helps in material selection for the material scientist/design engineer based on the suitable application. Quasi-static compressive behavior of GMB reinforced HDPE syntactic foams are investigated next. Compression molded GMB/HDPE sheets are subjected to 0.001,0.01 and 0.1 s-1 strain rates. Compressive modulus of foams is higher compared to neat HDPE. Increasing strain rates and decreasing filler content increases yield strength for all the foams investigated compared to neat HDPE. Yield strain and energy absorption of GMB/HDPE foams increases with an increasing strain rate and wall thickness. Specific compressive modulus and strength of GMB/HDPE foams are superior and are comparable to neat HDPE. GMB/HDPE foam achieved high stiffness to weight ratio making them suitable for wide variety of applications. Porfiri-Gupta model based on differential scheme predicts a good estimate of compressive modulus for all the type of GMB/HDPE foams. Property maps are exhibited to present comparative studies of quasi-static compression with existing literature. Further, GMB/HDPE foams are characterized for viscoelastic properties by dynamic mechanical analysis. Test is ramped from 35-150°C at a rate of 5°C/min with the deformation occurring at a constant frequency of 1 Hz. With increase in temperature, storage and loss modulus decreases while tanδ increases with increase in filler loading and wall thickness. Storage modulus and loss modulus increases with increasing wall thickness and volume fraction of GMBs. Damping factor (tanδ) shows an increasing trend with increase in GMB content and wall thickness. Damping factor is less sensitive to glass microballoon content as compared to storage and loss modulus. Structure-property correlations of all the investigated properties are presented with the help of exhaustive SEM images to understand underlying mechanisms. Finally the behavior of material is analyzed using the crystallinity measurement. Crystallinity is observed to be highest for the HDPE as compared to GMB/HDPE foams. Inclusion of GMB decreases the crystallinity signifying stiffness rise of the polymer backbone resulting in ductile to brittle behavioral change. Developed GMB/HDPE syntactic foams achieved better physical and mechanical properties as compared to other thermoplastic foams studied in recent past as exhibited by property maps. Consumption of expensive matrix is reduced by dispersing GMBs leading to lower cost of these syntactic foams. GMB/HDPE foams developed in the present work have a weight saving potential of 36% with betterspecific mechanical properties making them candidate material in weight sensitive and buoyant applications.