Faculty Publications
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Item Dynamic impact resistance of composite sandwich panels with 3-D printed polymer syntactic foam cores(DEStech Publications, 2021) Tewani, H.R.; Bonthu, D.; Bharath, H.S.; Doddamani, M.; Prabhakar, P.Polymer-based syntactic foams find use in the marine industry as primary structural materials due to their inherent lightweight nature and enhanced mechanical properties relative to pure HDPE. 3-D printing these materials circumvents the use of joining assemblies, enabling the production of complex shapes as standalone structures. Although the quasi-static response of these 3D printed foams has been well studied independently in recent years, their dynamic impact resistance and tolerance as potential core material for sandwich panels have not been the focus. Moreover, 3D printing is known to impart directionality in the printed syntactic foams, which may introduce failure mechanisms typically not observed in molded foams. It is therefore important to investigate the mechanics of 3-D printed syntactic foam core composite sandwich structures under impact loading and characterize their failure mechanisms for establishing dynamic impact resistance. To this end, 3-D printed syntactic foams have been developed using rasters of High-Density Polyethylene (HDPE) and Glass MicroBalloon (GMB) fillers by adopting the Fused Raster Fabrication (FFF) technique. The current study is performed to assess the impact performance of these composite foam cores based on the volume fraction of fillers and print orientation. The weight percentage of GMB fillers in printed specimens ranges from 0% to 60% in increments of 20%. This study presents the impact response of these composite sandwich panels at different energy levels, in compliance with ASTM D7136/D7136M - 20. Observations suggest that an increase in GMB % in HDPE matrix improves the impact performance in terms of the peak load of the material, but the failure behavior becomes brittle to an extent. Observing the failed specimens under a Micro-CT scanner captures the failure morphologies and helps characterize failure processes during impact. It is noticed that core materials with higher GMB content are prone to individual raster breakage and delamination at the back face, in addition to debonding between individual rasters. Specimens printed along the longer dimension (y-direction) impart more warping in the final sandwich structures than that of specimens printed along the shorter dimension (x-direction). Therefore, they are more susceptible to delamination at the back face. Addition of GMB fillers mitigate the tendency of the sandwich panels to warp. © 2021 36th Technical Conference of the American Society for Composites 2021: Composites Ingenuity Taking on Challenges in Environment-Energy-Economy, ASC 2021. All Rights Reserved.Item Mechanical behavior of 3D printed syntactic foam composites(Elsevier Ltd, 2020) Bharath, H.S.; Sawardekar, A.; Waddar, S.; Jeyaraj, J.; Doddamani, M.A three-dimensional printed (3DP), polymer based syntactic foams are developed using hollow glass micro balloons (GMB) dispersed in high density polyethylene (HDPE). This work presents the buckling and vibration response of 3D printed foams subjected to axial compression. The buckling load is estimated using Modified Budiansky Criteria (MBC) and Double Tangent Method (DTM) through the load–deflection plots. The first three natural frequencies and their mode shapes are computed as a function of axial compressive load. It is noted that the natural frequency reduces with an increase in axial compressive load. It is also observed that with an increase in GMB %, the natural frequencies and critical buckling load increases. In mode-1, the natural frequency decreases in pre-buckling regimes and increases exponentially in post-critical loading conditions. Analytical solutions obtained from the Euler-Bernoulli-beam theory are compared with experimental results. It is noted that the fundamental frequency approaches zero when the axial load is equal to the critical load. The critical buckling load is estimated through the vibration correlation technique and compared with the results obtained using DTM and MBC methods. The property map is plotted for buckling load against the density of various composites. © 2020 Elsevier LtdItem Three-dimensional printed lightweight composite foams(American Chemical Society service@acs.org, 2020) Bharath, H.S.; Bonthu, D.; Prabhakar, P.; Doddamani, M.The goal of this paper is to enable three-dimensional (3D) printed lightweight composite foams by blending hollow glass microballoons (GMBs) with high density polyethylene (HDPE). To that end, lightweight feedstock for printing syntactic foam composites is developed. The blend for this is prepared by varying the GMB content (20, 40, and 60 volume %) in HDPE for filament extrusion, which is subsequently used for 3D printing. The rheological properties and the melt flow index (MFI) of blends are investigated for identifying suitable printing parameters. It is observed that the storage and loss modulus, as well as complex viscosity, increase with increasing GMB content, whereas MFI decreases. Further, the coefficient of thermal expansion of HDPE and foam filaments decreases with increasing GMB content, thereby lowering the thermal stresses in prints, which promotes the reduction in warpage. The mechanical properties of filaments are determined by subjecting them to tensile tests, whereas 3D printed samples are tested under tensile and flexure tests. The tensile modulus of the filament increases with increasing GMB content (8-47%) as compared to HDPE and exhibit comparable filament strength. 3D printed foams show a higher specific tensile and flexural modulus as compared to neat HDPE, making them suitable candidate materials for weight-sensitive applications. HDPE having 60% by volume GMB exhibited the highest modulus and is 48.02% higher than the printed HDPE. Finally, the property map reveals a higher modulus and comparable strength against injection-and compression-molded foams. Printed foam registered 1.8 times higher modulus than the molded samples. Hence, 3D printed foams have the potential for replacing components processed through conventional manufacturing processes that have limitations on geometrically complex designs, lead time, and associated costs. © © 2020 American Chemical Society.Item 3D printing of syntactic foam cored sandwich composite(Elsevier B.V., 2020) Bonthu, D.; Bharath, H.S.; Gururaja, S.; Prabhakar, P.; Doddamani, M.In additive manufacturing, fused filament fabrication (FFF) based three-dimensional printing (3DP) is one of the most popular rapid processing technologies. The key benefit of 3DP is the ability to build integrated, complex, and tailored components. Optimization of feedstock material and associated 3DP process to achieve the required properties for various applications has been an important research field in the recent past. The main objective of this paper is 3DP of syntactic foam cored sandwich composite all at once (skin-core-skin printing in sequence at once). High density polyethylene (HDPE) and glass microballoon (GMB) reinforced (20, 40, and 60 by volume%) HDPE blends are fed into the extruder for manufacturing respective filaments. These HDPE and HDPE/GMB filaments are further sent to the FFF based printer to realize skin and syntactic foam core respectively of the sandwich. The selection of substrate for printing and the crucial issues associated with the printing of core and sandwich composites are analyzed. The present work successfully demonstrates that by optimizing the printing parameters, good quality core and sandwich structures can be printed all at once without any defects. Finally, the suitable printing parameters are used as boundary conditions in finite element (FE) simulation of sandwich for carrying out thermomechanical analysis to analyze the thermal stress distributions in the prints. © 2020Item Effect of axial compression on dynamic response of concurrently printed sandwich(Elsevier Ltd, 2021) Bharath, H.S.; Waddar, S.; Bekinal, S.I.; Jeyaraj, J.; Doddamani, M.In this work, the sandwich is concurrently realized using high density polyethylene (HDPE) skins and syntactic foam core through three-dimensional printing (3DP). Syntactic foam core is printed using lightweight feedstock filaments having glass microballoons (GMBs) by 20–60 vol% embedded in HDPE. These lightweight filaments are used as feed material in FFF (fused filament fabrication) based three-dimensional printer. The concurrently printed sandwich is loaded axially in a compressive mode for investigating the influence of GMB loading on buckling and natural frequency. The experimental load–deflection data and modal analysis are utilized for estimating critical buckling load and natural frequencies, respectively, under axial compression. Increasing GMB content enhances load to buckle and frequency of the printed sandwiches. The natural frequency decrease with higher compressive loads. Furthermore, the fundamental natural frequency increases exponentially when these printed sandwiches are subjected to axial compression loads that are higher than the load required for critical buckling. The load–deflection data and frequency obtained experimentally are compared with numerical predictions deduced using finite element analysis (FEA), which are noted to match well. © 2020 Elsevier LtdItem Flexural response of 3D printed sandwich composite(Elsevier Ltd, 2021) Bharath, H.S.; Bonthu, D.; Gururaja, S.; Prabhakar, P.; Doddamani, M.Among many lightweight materials used in marine applications, sandwich structures with syntactic foam core are promising because of lower water uptake in foam core amid face-sheets damage. HDPE (high-density polyethylene) filament is used to 3D print sandwich skin, and glass microballoon (GMB) reinforced HDPE syntactic foam filaments are used for the core. The optimized parameters are used to prepare blends of 20, 40, and 60 vol% of GMB in HDPE. These foamed blends are extruded in filament form to be subsequently used in commercially available fused filament fabrication (FFF) based 3D printers. The defect-free syntactic foam core sandwich composites are 3D printed all at once (skin-core-skin printing in sequence at once) using optimized printing parameters and characterized for the flexural behavior. The result reveals that the addition of GMB increases both specific modulus and strength in sandwich composites and is highest for the sandwich having a core with 60 vol% of GMB. The measured properties of sandwiches are compared with a respective core to study the effect of the sandwich. It is observed that flexural strength, fracture strength, and strain of foam core sandwiches registered superior response than their respective cores. The experimental results are found in good agreement with theoretical predictions. Finally, the failure mode of the printed sandwich is also discussed, and it is observed that none of the 3D printed syntactic foam core sandwiches fractured due to shear failure. © 2021 Elsevier LtdItem Dynamic behavior of concurrently printed functionally graded closed cell foams(Elsevier Ltd, 2021) Dileep, B.; Prakash, R.; Bharath, H.S.; Jeyaraj, J.; Doddamani, M.In this work, functionally graded foams (FGFs) of closed cell types are three-dimensionally printed (3DP) concurrently. These closed cell syntactic foams are manufactured by reinforcing 20, 40, and 60 vol% hollow glass microballoons (GMBs) in the high density polyethylene (HDPE) matrix and are investigated for their mechanical buckling and free vibration response. The critical buckling load (Pcr) of the FGFs are evaluated using the Double Tangent Method (DTM), Modified Budiansky Criteria (MBC), and Vibration Correlation Technique (VCT). It is observed that Pcr evaluated by all three methods are in good agreement. Among all FGFs, FGF-2 exhibited higher buckling strength with 22–26% higher than FGF-1 and FGF-3. Under no-load and uniaxial compressive loads, the first three natural frequency of FGFs and their corresponding damping factors are evaluated. At first mode, the natural frequency of FGFs decreases in the pre-buckling zone and started increasing in the post-buckling zone. Damping factor exhibited reverse trend compared to the trend shown by the natural frequencies. Among all FGFs, FGF-2 (20-40-60 GMB gradation) exhibited better natural frequency. Experimental results are compared with a finite element based simulation results. © 2021 Elsevier LtdItem Mechanical response of additively manufactured foam: A machine learning approach(Elsevier B.V., 2022) Neelam, R.; Kulkarni, S.A.; Bharath, H.S.; Powar, S.; Doddamani, M.This paper uses ensemble and automated machine learning algorithms to predict the mechanical properties (tensile and flexural strength) of a three-dimensionally printed (3DP) foamed structure. The closed cell foams were made from the most commonly used thermoplastic, High-Density Polyethylene (HDPE). The hollow glass microspheres are infused in HDPE at varying volume %. The available data on these foams' mechanical properties are used by the chosen machine learning (ML) algorithms to propose the best suited algorithm for such a three-phased microstructure as these closed cell foams exhibit. Finally, the strength predictions from the models were validated using experimental data. The models were trained with nozzle temperature, bed temperature, and force values as input parameters. The output parameters predicted were the tensile and flexural strength. LightGBM outperforms all other models in terms of performance among ensemble-based models, while H2OAutoML outperforms all other models. All the ML algorithms produced models with greater than 95% accuracy. Finally, memory and time consumption for each model are presented. © 2022 The Authors