2. Thesis and Dissertations
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Item Machinability Characteristics in Drilling of Glass Microballoon /Epoxy Syntactic Foam(National Institute of Technology Karnataka, Surathkal, 2019) Ashrith, H. S.; Doddamani, MrityunjayPolymer composites are steadily substituting the conventional materials in aerospace, marine, automobile and many other engineering applications owing to their unique properties such as lightweight feature combined with high specific strength and superior corrosion resistance. Weight reduction of composite materials is of great interest in aerospace, marine and automobile applications to meet the stringent guidelines of fuel consumption and emissions in the coming years. Structural weight reduction without compromising the desired properties can be achieved by using a unique class of composite called syntactic foams, wherein the matrix is filled with hollow particles called microballoons. Even though the composites are produced to near-net shape, drilling is unavoidable during final stage of production process for the assembly of various structural components using fasteners. Many problems arise during drilling of composites due to non-homogeneous and anisotropic nature of the material. Nearly 60% of the composite parts are rejected during aircraft assembly due to drilling induced damages. The focus of the present study is to achive good quality holes in drilling of glass microballoon/epoxy syntactic foams by selecting appropriate process parameters. In the present investigation, epoxy resin (LAPOX L-12) is used as the matrix resin and borosilicate glass microballoon (GMB) is used as hollow filler without any surface treatment. Syntactic foams are fabricated by dispersing 20, 40 and 60 vol.% GMBs in epoxy matrix using manual stirring method. Nine different types of syntactic foams specimens with 20, 40 and 60 vol.% of GMBs are fabricated using three different densities (varying wall thickness) of GMBs (SID-200Z: 200 kg/m3, SID-270Z: 270 kg/m3 and SID-350Z: 350 kg/m3). All the prepared samples are coded as per EYYY-R convention. Epoxy resin is denoted by ‘E’ and ‘YYY’ represents density of GMBs. Neat epoxy specimens are also fabricated under similar processing conditions for comparison. Extensive micrography of fabricated foams confirms the uniform distribution of GMBs in the epoxy matrix without forming the clusters. Experimental density of all the fabricated syntactic foams is lower than neat epoxy resin. Density of foams decreases with decreasing GMB wall thickness and increasing volume fraction of GMBs. Density reduction in the range of 18-53% is noted as compared to neat epoxy indicating significant weight saving potential of the proposed syntactic foams.Experiments are conducted using vertical computer numerical control machine and TiAlN coated tungsten carbide twist drills of varying diameter based on full factorial design (FFD). Cutting speed (v), feed (f), GMB content (R), GMB wall thickness (w) and drill diameter (D) are taken as input parameters, while thrust force, surface roughness, specific cutting coefficient, cylindricity, exit side circularity error and exit side damage factor are considered as responses for evaluating the quality of drilled hole. Three levels for each input process parameters (v: 25, 75 and 125 m/min; f: 0.04, 0.08 and 0.12 mm/rev; R: 20, 40 and 60 vol.%; w: 0.716, 0.925 and 1.080 µm; D: 8, 12 and 16 mm) are selected to consider the nonlinear effects among the parameters. Experiments are repeated for three times and the average values are used for analysis. Mathematical models based on response surface methodology (RSM) are developed using Minitab 14 software for analyzing the influence of the input parameters on the measured responses. Adequacy of the developed mathematical models is confirmed using analysis of variance. Higher R-squared values indicate that the developed mathematical models can be effectively used as a tool in industrial practices to predict the machinability characteristics of GMB reinforced epoxy foams during drilling. Individual and interaction effect of process parameters on the responses are analyzed using RSM based mathematical models. Individual effects are studied by varying one parameter at a time in the mathematical models while keeping all the remaining process parameters at the intermediate levels. Two parameters are varied at the same time while keeping the other parameters at the intermediate level in the mathematical models to study the interaction effect of process parameters on the chosen responses. Thrust force is found to be increasing with increasing feed and drill diameter, while it decreases with increasing GMB content. Thrust force of all the foams is found to be lower as compared to neat epoxy resin. Thrust force is observed to be decreased by 40-55% as compared to neat epoxy due to the incorporation of GMBs. Drill diameter, feed and GMB content have a significant effect on the thrust force while the effect of cutting speed is found to be insignificant. v125f0.04R60D8 is the optimum condition for minimizing thrust force of E200 and E270 foams while performing machining at v25f0.04R60D8 minimizes the thrust force of E350 syntactic foam. Extensive microscopy is conducted on the drilledspecimens to understand crack initiation and propagation mechanisms. Surface roughness of the drilled hole is measured using Mitutoyo surftest with a cut-off length of 0.8 mm. As compared to neat epoxy, the surface roughness of syntactic foams increases by 14-20 times. However, surface roughness in foams decreases with increasing GMB volume fraction. Surface roughness is strongly governed by drill diameter and cutting speed. Minimum surface roughness for E200 and E270 foams is obtained at v25f0.12R60D16, while v25f0.12R60D12 is found to be optimum for E350 foam. Specific cutting coefficient increases with increasing drill diameter and decreasing feed. Increasing GMB content significantly decreases specific cutting coefficient by 40-55% as compared to neat epoxy specimens. v25f0.12R60D8 is the optimum condition for E350 foam, while machining at v125f0.12R60D8 is found to be beneficial for E200 and E270 foams for minimizing specific cutting coefficient. Coordinate measuring machine is used to measure the cylindricity, exit side circularity and maximum diameter of drilled hole for damage estimations. Cylindricity of the foams increases with increasing the cutting speed, feed and drill diameter. Increasing GMB content decreases the cylindricity by 46-69% as compared to neat epoxy. Drill diameter, feed and GMB content have a significant effect on cylindricity of drilled holes. v25f0.04R60D8 is noted to be the optimum conditions for E200 and E270 foams while v75f0.04R60D8 parametric setting is most suitable for thick-walled (E350) foams to minimize cylindricity. Circularity error increases with increasing cutting speed and drill diameter, while it decreases with increasing feed and GMB content. Increasing the microballoon volume fraction decreases the circularity error of foams by 18-67% as compared to neat epoxy. Circularity error of the holes is highly influenced by drill diameter followed by GMB volume fraction and wall thickness. v25f0.12R60D8 is the optimum condition for minimizing the circularity error of all the type of foams. The damage factor is dependent on the thrust force developed during drilling process. Drill diameter, feed and GMB content have a significant effect on damage factor of the drilled holes. Optimum conditions for minimizing damage factor is observed to same as that of thrust force. A reduction in the damage factor by 26-42% is noted in foams with increasing GMB content as compared to neat epoxy. Optimum conditions based on response surfacemethodology for minimizing all the responses are not same and the trade-off among various process parameters necessitates multi-response optimization. In the present work, grey relation analysis (GRA) is used for finding a specific combination of process parameters for minimizing all the response at the same time to obtain a good quality hole in drilling GMB/Epoxy syntactic foams. According to GRA, v125f0.08R60D8 is the optimal condition for producing a quality hole in E200 foams, whereas v25f0.12R60D8 is found to be optimal for E270 and E350 syntactic foams. Higher GMB content is preferred in the foams from drilling operations perspective, which is also beneficial for weight sensitive applications. Influence of GMB wall thickness on the responses is studied by keeping the GMB content at 60 vol.%, as higher filler content significantly improves the hole quality. Response surface plots for varying wall thickness of GMBs are plotted using the developed mathematical models to study the interaction effects among input process parameters. Increasing microballoon wall thickness from w0.716 to w1.080 increases thrust force, specific cutting coefficient and damage factor by 40%. Surface roughness, cylindricity and circularity error of drilled holes are significantly affected by GMB wall thickness and is found to be decreased by 30, 41 and 56% respectively. Combination of higher particle wall thickness and feed with lower cutting speed and drill diameter (v25f0.12w1.080D8) is the optimum condition for producing a sound hole quality as observed from GRA. Hole quality is highly influenced by drill diameter followed by the interaction between cutting speed and GMB wall thickness. Finally, microscopy is conducted to analyze the shape and size of chips produced during drilling. Cutting tools are inspected using a confocal microscope post drilling operation and micrographs show negligible tool wear due to the superior wear resistance of TiAlN coating. Observations and parameters settings explored in this work offers guidelines for the industrial practitioners to produce quality holes in drilling of GMB reinforced epoxy composites.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.