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Fused filament fabrication (FFF) is a widely used extrusion-based additive manufacturing process due to its low operating cost and available material options. Polylactic acid (PLA) is a bio-based compostable polymer extensively used as a feedstock material in FFF. The application of PLA in engineering and medical applications is limited due to the low thermal stability associated with the polymer. The present work aims to study the effect of thermal annealing on FFF-printed PLA in static and dynamic flexural loading. Response surface methodology (RSM) is chosen as a design of experiment method to study the effect of annealing time and temperature on flexural properties of printed PLA. Annealing is found to be advantageous in increasing the dynamic flexural properties. Improved glass transition temperature, stiffness, thermal stability, and crystallinity are also evident with annealing. However, annealing is found to be detrimental in improving static flexural properties. Considering the desirability function (DF) approach, annealing time and temperature are optimised for measured static and dynamic flexural properties. Samples are further investigated in scanning electron microscope for failure mode and differential scanning calorimetry for crystalline behaviour of annealed PLA. Empirical models are proposed for predicting the flexural behaviour of printed PLA. The current study has shown that the FFF-printed PLA can be tailored to specific application requirements due to its stable characteristics for thermal loading with thermal annealing.

The low-velocity impact perforation behavior of fiber-metal laminates (FMLs) is evaluated by analytical modeling. Four different FML layups consisting of glass fiber/epoxy layers and titanium alloy...

Silicone rubber (SR) and ethylene-propylene-diene monomer (EPDM) are widely-used polymers as housing for high voltage insulators. In this work, SR/EPDM/clay nanocomposites were obtained by two-roll mill mixing for outdoor polymeric insulators. Morphology, dielectric properties, dielectric breakdown strength (DBS), and surface and volume resistivity of different weight contents of nanoclay (Cloisite 15A) incorporated in SR, EPDM, and SR/EPDM hybrid nanocomposites were characterized. In addition, the distribution of breakdown voltages was fit to the distribution of Weibull and estimated the scale and shape parameters. The polar groups of the clay particles enhanced the polarization capability of the nanocomposites. Moreover, DBS results showed an enhancement of the dielectric strength proportional to clay content. Finally, the surface and volume resistance of all nanocomposites decreases but maintains very high electrical resistance. The experimental data presented in this study will be useful for designing and manufacturing the outdoor insulators.

An epoxy-based shape memory polymer (SMP) is synthesized and examined for its deterioration in shape fixity due to springback and isothermal viscoelastic recovery at different ambient temperatures. Shape fixity depends not only on material properties but also on programming conditions. A constitutive finite deformation model is incorporated to predict the behavior of the proposed SMP and find maximum shape fixity. A programming approach is followed in which, in contrast to hot programming, the SMPs are neither heated before deformation nor cooled afterward but are deformed at ambient temperature and then stress-relaxed. The proximity of the programming temperature to the glass transition temperature plays a crucial role in determining the shape fixity of SMP. It has been found that the SMP with a glass transition temperature of 42.9 °C can achieve maximum shape fixity of 92.25% when programmed at 23 °C with 100 min stress relaxation time. Thermal contraction and dynamic tests are performed in the Dynamic Mechanical Analyzer (DMA) to determine structural relaxation properties and distinguish the programming temperature in the cold, warm or hot temperature zone. The shape memory tests are carried out using temperature-controlled UTM to determine the shape fixity and shape recovery of SMP. The SMPs are subjected to a full thermomechanical cycle with different stress relaxation times and programming temperatures.

Background: Interleukin-21 (IL-21) is a relatively new immune-enhancing, multifunctional and pleiotropic cytokine that plays an essential role in controlling chronic viral infections. It is a protein that in humans is encoded by IL-21 gene, mapped on chromosome 4 and 180 kb from IL-2 gene and the mRNA encoding the product is 616 nucleotides long. IL-21 is expressed in activated human CD4+ T cells but not in most other tissues. In addition, IL-21 expression is up-regulated by Th2 and Th17 subsets of T helper cells, as well as T follicular cells. Further IL-21 is expressed in NKT cells regulating the functions of these cells. IL-21 may be a critical factor in the control of persistent viral infections such as HIV. The intrinsic ability of IL-21 to promote the longevity of B cells should benchmark its role in sustained production of neutralizing antibodies by proliferating and differentiating B cells committed to cognate stimulating antigenic determinants. Nonetheless, immune response to identical or related antigenic insults are quite heterogeneous enlisting complex factors including ethnic and HLA elements in determining the final outcome. Present study aims to explore the levels of IL-21 in a select segment of ART recipients of East Delhi covered by NACP, Govt. of India. Methods: Estimation of levels of IL-21 was done using ELISA test employing commercially available kit (QAYEE-BIO Kit for Interleukin-21). The method of ELISA used for the present study was sandwich ELISA. Results: The mean levels of IL-21 in the study of 40 ART recipients were 1178.21±927.063. There were no controls included in the study. The range of IL-21 were 150-6100. In one patient IL-21 levels was not detected. Conclusion: This study attempts a clinical correlation with the levels of a cytokine which could help in a better understanding of the immunobiology of the disease. In the current study, the levels of IL-21 in samples of ART recipients were detected. In all ART patients the levels of Interleukin-21 were detected except one. Majority of patients showed moderate levels of IL-21. But 3 patients showed high levels of IL-21 >2000 pg/ml who were not associated with opportunistic infections after 10 to 12 months of ART treatment. Therefore, further studies with a higher sample size may be required to arrive at a statistically significant correlation.

For crashworthiness applications, hollow composite tubes have been proposed and studied as impact-bearing members in earlier studies to replace metal tubes and beams which resisted impact by bending. This study focusses on the effect of the addition of hollow glass microballoons (GMB) in composite tubes for compressive loading. Epoxy containing 0 to 0.3 volume fraction of GMB is used with woven glass fabrics to fabricate the specimens. The mean load and energy absorbed while progressively crushing the tubes are experimentally obtained. The variation of these properties with the addition of GMB is analyzed. It is found that the compression properties improve with the addition of hollow glass particles. To explain the different modes of energy dissipation, the basic mechanical properties of the GMB-filled composites are obtained by various mechanical tests. These results are also used in an energy-based model to predict the properties of the tubes under compression.

A reusable passive radio frequency identification (RFID) sensor is reported for wireless strain measurement on metals. The characteristics of the proposed sensor are assessed on aluminum tensile specimens. The received signal strength intensity (RSSI) from the RFID sensor and the frequency corresponding to the maximum RSSI are used to measure the specimen strain. Besides strain, the yield, ultimate tensile strength and failure points of the specimen could also be identified from the RSSI measurements and correlated with strain gauge load cell data. Digital image correlation measurements gathered during tensile tests confirm that the sensor does not deform during tensile loading and is reusable.

In this study, the deformation and energy absorption characteristics of a novel hybrid tube is investigated both experimentally and numerically. The architecture of the proposed hybrid tube composed of an inner conventional tube and a co-axially arranged outer auxetic tube. Quasi-static compressive tests were conducted on the auxetic tubes, the conventional tubes and the hybrid tubes. A three-dimensional digital image correlation technique was used to monitor the evolution of radial contraction of the tubes. Measured performance of the tubes were compared in terms of force, energy absorption, and specific energy absorption. Finite element (FE) models were also developed for the three types of tubes, analyzed using ANSYS/LS-DYNA, and validated by experimental measurements. Both experimental and numerical results showed that the presence of the auxetic tube in the hybrid tube alters the deformation mode of the conventional tube by exerting a radial force. Parametric studies were performed to investigate the effects of the outer auxetic tube, wall thickness of the inner conventional tube, the failure strain, and yield strength of the outer tube’s material on mechanical performance of the hybrid tube. Increasing the yield strength of the outer auxetic tube improved both the energy absorption and specific energy absorption characteristics of the hybrid tube significantly.

A new auxetic structure was designed with rigid diamonds incorporated to enhance the strength of a star-shaped auxetic structure along with the symmetry in the present work. The new structure was fabricated through Multi-Jet Fusion process and subjected to quasi-static compression tests for static analysis. Finite element model was developed using ABAQUS and validated by experimental results in terms of deformation mode , nominal stress-strain curve, and Poisson’s ratio of the auxetic structure. Theoretical model was also developed to predict the Poisson’s ratio of a unit cell of the designed auxetic structure. The theoretically predicted Poisson’s ratio is in tandem with that of obtained from a finite element model, which utilized periodic boundary conditions. Parametric study using the theoretical model revealed that the angle of the inclined members has shown a significant effect on the Poisson’s ratio. The newly proposed structure exhibited 36% better specific plateau stress than a conventional 3D star-shaped auxetic structure of similar geometrical parameters.

In the present work, the neat epoxy and different orientations [0°, 45°, 90°, (45°/-45°/45°) s, (±45°/0°/90°) s] of unidirectional carbon/epoxy composites are experimentally studied under tensile and impact loading. The notched impact tests are performed using the Izod impact machine to obtain the energy absorption of neat epoxy and different CFRP composites which is required for effective design of bullet proof jackets and military vehicles. The micro mechanical analysis is employed to determine the shear properties of a matrix using the tensile properties. Using classical laminate theory [CLT], the theoretical tensile properties are determined. The SEM fractography analysis is used to observe the damage mechanisms of neat epoxy and different orientations of CFRP composites subjected to tension and impact loading.

Hepatitis B virus (HBV) and Hepatitis C virus (HCV) infections are major global health problems. In 2009, the national Integrated Disease Surveillance Programme (IDSP) began conducting surveillance across all Indian states for epidemic-prone diseases including viral hepatitis. Hence this retrospective study was conducted to identify trend changes in HBV and HCV epidemiology over a period of 7 years at a tertiary care hospital in Delhi. Enzyme-linked immunoassay result entries for Hepatitis B surface antigen (HBsAg) and Hepatitis C antigen and/or anti-HCV antibody were analyzed. The 7-year seroprevalence of HBV and HCV was found to be 3.084% and 5.381% respectively. The seroprevalence of HBsAg was maximum in 2010. Since then the seroprevalence has shown a small decrease. Government of India has included HBV vaccine in the National Universal Immunization Program in the entire country in 2011-12. We consider the reduction in HBsAg seroprevalence to represent a true fall in HBV transmission due to vaccination and/or public consciousness. In the present study, HCV infection appeared to be more common than HBV infection. During last few years, seroprevalence of HCV also increased, which is possibly attributable to an increase in the population of Delhi and a heavy load of the migrant population. These figures may be important for estimation of the disease burden in this region. The study provides a fine reference for future studies, because of the large number of cases investigated over a long period of 7 years and draws attention to the lacunae in ongoing immunization programs.

Influence of interphase material and clay particle shape on the effective elastic properties of epoxy/clay nanocomposites (ECNs) containing randomly oriented exfoliated nanoclay particles is investigated by using the Mori–Tanaka method and a hybrid method. The effective particle concept is used to model the clay particle surrounded by the interphase material. Non-uniform distribution of the aspect ratio (AR) of clay particles is modelled by using the Gauss and rectangular distributions. The results indicate that the interphase thickness, interphase stiffness, clay AR distribution and clay particle shape have a significant influence on the Young's modulus of ECNs, but not on the Poisson's ratio. However, Poisson's ratio of the interphase material has a considerable influence on the Poisson's ratio of ECNs. Finally, a comparison of analytical and experimental results is presented.

Auxetic materials, due to its negative Poisson’s ratio, shrink laterally when compressed axially and expand laterally when pulled axially. A re-entrant diamond structure was developed by replacing the vertical walls in a conventional re-entrant structure with diamond cells, which featured cross-linking members to make them rigid. The incorporation of the rigid diamond unit cells increased the stiffness, strength, and energy absorption of the structure. A validated finite element (FE) model was adopted from previous work, and the structures were compressed at a speed of 5 m/s in the FE model. The independent geometrical parameters of the re-entrant diamond unit cell were re-entrant wall length (L1 ), diamond angle (θ2 ), and diamond wall length (L2 ). The FE model based on these values was thus used to investigate the influence of geometrical parameters (θ2 & L2/L1 ) on the deformation mode, stiffness, strength, and specific energy absorption (SEA) of the structure. The value of diamond angle (θ2 ) varied from 40° to 90° at intervals of 10°, while the length ratio (L2/L1 ) varied from 0.7 to 1.2 in increments of 0.1. The specific strength was used in the discussion to account for differences in the relative density of re-entrant diamond structures.

Auxetic materials exhibit interesting mechanical properties. In this work, a novel auxetic unit cell was proposed with an aim to improve the strength and energy absorption of the auxetic structure constituting these unit cells. In the developed unit cell, the vertical member of a conventional re-entrant unit cell was replaced by a diamond. The corresponding structure was named re-entrant diamond structure. Specimens were fabricated using Multi-Jet Fusion (MJF) technique and experimentally tested to investigate their mechanical performance under uniaxial quasi-static and dynamic compression. Finite element model was also developed in ABAQUS/Explicit and validated using experimental results. Moreover, in order to improve stiffness of the diamonds, a cross-link member was introduced in each diamond. Subsequently, a parametric study was conducted to study the influence of diamond angle (θ2) and length ratio (L2/L1) on the deformation mode, stress–strain curve, Poisson’s ratio, and energy absorption of the re-entrant diamond structures with and without cross-links. This study revealed that larger diamond angle (θ2) and smaller length ratio (L2/L1) are desirable for high strength. Furthermore, the re-entrant diamond structures with and without cross-links were compared with the conventional re-entrant and hexagonal honeycombs in terms of strength and energy absorption.

In this study, the mechanical performance of a three-dimensional (3D) re-entrant structure subjected to dynamic compression was experimentally investigated using a high-speed Instron machine. Quasi-static tests were conducted for comparison. All samples were fabricated by multi-jet fusion technology. A finite element (FE) model was developed using ABAQUS/Explicit and validated against the dynamic test results in terms of deformation mode, plateau stress, and Poisson's ratio. The validated FE model was subsequently employed to study the effects of the number of unit cells, geometrical parameters (aspect ratio, H/L, and re-entrant angle, θ) of the re-entrant unit cell, and compressive velocity. The results showed that the structure with H/L = 1 and θ = 45° had the highest specific energy absorption (SEA) when compressed in the Z direction and the structure with H/L = 1.1 and θ = 75° had the highest SEA when compressed in the X direction. Moreover, the performance of the 3D re-entrant structure was compared with that of a 2D re-entrant structure with the same geometrical parameters. The energy absorption of the 3D re-entrant structure was also compared with other popular cellular structures using an Ashby chart.

The dynamic characteristics of the Polypropylene honeycomb (PPHC) sandwich composites have been investigated under various temperatures (30°,35°,40°,45°,50°,55°,60°, 65°,70°,75° and 80°C) and different orientations (0° and 90°) of the glass fibers in the composites. Since the thermal properties of the constituent materials (glass fiber, epoxy resin and PPHC core) of the PPHC sandwich composites are different and the in-plane effect of the composites varies with the two different orientations (0° and 90°) of the fibers, the variation of the loss factor under the various temperatures are also different for these orientations. A two stage layup technique has been used to fabricate the sandwich composite specimens. Impulse technique associated with the half power bandwidth method, has been used to evaluate the natural frequency and damping values of the sandwich composite under different temperatures.

Conventionally, shape memory programming involves deforming the material at a temperature higher than the glass transition (\(T_\mathrm {g}\)) and subsequently cooling the material below \(T_\mathrm {g}\) while holding the deformation to fix the temporary shape. Alternatively materials with reversible plasticity shape memory (RPSM) property can be programmed at temperatures well below the glass transition temperature which offers several advantages over conventional programing. Here, the RPSM property of multi-walled-carbon-nanotube (MWCNT) reinforced epoxy nanocomposites is investigated. A commercially available epoxy resin is tailored to realize RPSM effect and the properties are enhanced with the addition of nano-fillers in the epoxy matrix. This report systematically investigates the effect of MWCNT addition on the mechanical, thermal and RPSM properties of the epoxy matrix. The RPSM performance under different programming conditions like strain rate, strain level and stress relaxation time is studied. Results reveal that all samples show excellent shape memory properties under various programming conditions. The addition of MWCNT resulted in a significant improvement in mechanical and shape memory properties. Further the RPSM mechanism is explained using a thermo-viscoelastic-viscoplastic model and the model is used to predict the RPSM behavior of the nanocomposite under different programming conditions. As a result, this study shows that by varying the parameters like glass transition temperature, filler content and programming conditions the material can be designed for applications in self-healing systems and smart structures.

Thin walled structures are used as energy absorbing elements in various applications. These elements, when axially loaded, fold progressively which make them as good energy absorbing elements. The circular tube proves to be a popular energy absorber because it provides a reasonably constant operating force which is the primary characteristics of the energy absorber. Square and rectangular tubes are widely used in automobile structures as these cross sections are suitable for welding with other components in the structure and hence highly preferred. In the present study annealed steel tubes of the above three shapes are made for the same cross - sectional area and are subjected to axial compression to find out the efficiency of these tubular sections. The energy absorbing characteristics of these cross sections are studied under static and dynamic loading conditions. The effect of impact velocity is studied by keeping the striking mass constant and varying the drop height. The experimental results are compared with the available analytical models and good agreement is found.