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Adhesives are viscoelastic polymeric materials with rate-dependent behaviour. A personal body armour for higher threat perception consists of hard ceramic facing adhesively joined to a comp...

The presented study investigates the effect of moisture absorption on the compressive high strain rate performance of Ultra High Molecular Weight Polyethylene-Synthetic Rubber composite along the thickness direction. Split Hopkinson Pressure Bar is employed to investigate the dynamic compressive properties. The cylindrical test specimens were machined from flat composite laminate fabricated by compression molding. Two types of specimens were used for high strain rate testing. First set of specimen was oven dried at 65°C and second set was dipped in potable water as per NIJ standards. For identical incident energy, different strain rates and stresses were attained by the dry and wet composites, respectively. Impact loading of wet composite resulted in physically intact specimen at the lowest loading rate only; higher loading rates resulted in the macroscopic damage. Experimental results are presented as stress-strain–strain rate plots and a surface fitting approach is adopted to create a phenomenological model capable of describing the overall material response.

The presented study compares the compressive high strain rate behavior of Aluminum (Al) specimen fabricated from the powder as received, with the effect of ball milling and incorporation of high rock salt percent. Four different types of sample specimens were fabricated by compacting Aluminum powder followed by sintering in Argon atmosphere at 600 °C for 3 hours. Sample types comprised of (i) Aluminum powder as received, (ii) ball-milled Aluminum powder for 24 hours, (iii) Aluminum powder with 33% rock salt and (iv) specimen of type three boiled in a water bath, respectively. The compressive high strain rate testing was done on a 16 mm Titanium bars based Split Hopkinson Pressure Bar (SHPB). For identical incident energy, different strain rates and stresses were attained by these four different types of Al samples. Ball milling of Al powder for 24 hours resulted in significant property variation, which enhanced the peak stress under compressive high strain rate loading. However, the damage pattern shifted from ductile in case of non-milled powder to partly brittle damage for ball milled Al specimens. Incorporation of higher rock salt percent was meant to produce relatively low-density Al. The resulting compressive stresses recorded were much lower and the corresponding damage was brittle in nature. Boiling in water bath dissolved salt but could not enhance the properties of Al specimen. The study reveals the importance of Al powder processing on the rate dependent stress-strain behavior of Aluminum specimen fabricated by powder metallurgy route.

Shear thickening fluids (STFs) are a special class of field responsive non-Newtonian fluids which exhibit transition from low viscosity to high viscosity state when these are subjected to shear deformation, particularly when the shear rate exceeds a critical value termed as the critical shear rate (CSR). Due to this unique characteristic of STFs, these are generally used for vibration mitigation or shock absorbance such as in vibration dampeners, hip protection pads, in protective gear for athletes etc. From the last two decades, STFs have found application in the field of ballistics, particularly in the development of special class of STF-intercalated armours called Liquid Body Armours (LBAs). These new age armours are lighter in weight and more flexible as compared to conventional heavy armours, which, infact seriously affect the mobility and agility of the soldier, especially in combat situations. Although, exhaustive studies are available which show the improvement in impact resistance of STF-treated high performance fabrics, but there are limited studies which explore the efficacy of STF treatment method. In this study, an attempt is made to understand this aspect. The low velocity impact studies were conducted on drop tower machine, while high velocity impact studies were accomplished on in-house designed and fabricated Split Hopkinson Pressure Bar (SHPB) experimental set-up. It was observed that when STF was kept in liquid form between layers of ballistic fabrics, the composite exhibited reduced performance, whereas, STF-treated ballisic composites exhibited enhanced impact toughness at high strain rates in SHPB testing.

Shear thickening fluids (STFs) are a special class of field responsive fluids which exhibit change of phase from liquid to solid when subjected to imposed shear. This remarkable characteristic of STFs has led to their application in soft body armor technologies, leading the development of concept of Liquid Body Armors (LBAs). LBAs extensively rely upon the symbiotic relationship between STF and high strength FRP (Fiber Reinforced Polymer) composites comprising of high tenacity fibers such as Kevlar, UHMWPE (Ultra High Molecular Weight Poly Ethylene) etc. Although in the past, STFs have been widely characterized by rheometers at low strain rates (≤10 3 s −1 ), but in actual practical scenarios, body armours encounter much higher strain rates (of the order of 10 5 –10 7 s −1 ) under a ballistic impact. The main objective of this study is to capture the dynamics of STFs at such high strain rates by employing Split Hopkinson Pressure Bar (SHPB) technique. The STF sample was synthesized by dispersing 67.5 wt.% of 100 nm silica powder in Poly Propylene Glycol (PPG) and ethanol, using ultrasonic homogenization method. The mechanical response of STF was studied at high strain rates in terms of the stress-strain behavior and variation of the impact toughness with the loading rate of the fluid specimen. It was observed that the impact toughness of STF increased progressively with the specimen loading rate. The peak stress and peak strain rate attained in the SHPB tests were 147 MPa and 22,100 s −1 , respectively. The characteristic transition time of STF was found to be in the range of 13–25 µs.

This study systematically investigates the effect of Shear Thickening Fluid (STF) treatment on the high strain rate properties of UHMWPE (Ultra High Molecular Weight Poly Ethylene) composites. Spherical nanosilica particles of size 100 nm were used for the synthesis of STF. The high strain rate impact studies were accomplished on in-house designed and fabricated Split Hopkinson Pressure Bar (SHPB) experimental set-up. Compression moulded UHMWPE variant Gold Shield® was used as the ballistic composite. Both STF-treated as well as neat Gold Shield® specimens were subjected to high strain rate impact testing. From the experimental results and Fractography studies it was revealed that STF treatment enhanced the ballistic resistance of Gold Shield® composite material. In the SHPB experiments, the improved ballistic performance of STF-treated Gold Shield® specimens was manifested in terms of higher peak stress, specimen strain rates and impact toughness.

High temperature blast and ballistic resistant light weight fibre reinforced plastic composites are very much in demand. Accordingly, the present investigation is focused on the quasi-static and high strain rate behaviour of Basalt/Polyetherimide (PEI) composite. Nine different types of composite samples were fabricated by compression moulding. The moulding pressures and temperatures were varied in the range of 15–25 bars and 325 °C−375 °C, respectively. Quasi-static testing was done at the rate of 3 mm/min to establish the optimum moulding parameters. High strain rate compression tests in the strain rate range of 900/s−3200/s were performed on the optimum moulded composite specimens. High strain rate testing revealed that 40 layered Basalt/PEI composite moulded at 25 bar pressure and 350 °C attained highest stress of 932 MPa whereas, the same was found to attain 446 MPa under quasi-static loading conditons. Thus, establishing the rate depedency of Basalt/PEI composite. Damage studies revealed that Basalt/PEI undergoes brittle damage, typically resulting in shear plane angle of 42°−45°. SEM micrographs revealed brittle fibre failure and matrix free fibre bundles, indicating the importance of process parameter optimiation.

In the present work, dynamic compression response of polypropylene (PP) based composites reinforced with Kevlar/Basalt fabrics was investigated. Two homogeneous fabrics with Kevlar (K3D) and Basalt (B3D) yarns and one hybrid (H3D) fabric with a combination of Kevlar/Basalt yarns were produced. The architecture of the fabrics was three-dimensional angle-interlock (3D-A). Three different composite laminates were manufactured using vacuum-assisted compression molding technique. The high strain rate compression loading was applied using a Split-Hopkinson Pressure Bar (SHPB) set-up at a strain rate regime of 3633–5235/s. The results indicated that the dynamic compression properties of thermoplastic 3D-A composites are strain rate sensitive. In all the composites, the peak stress, toughness and modulus were increased with strain rate. However, the strain at peak stress of Basalt reinforced composites (B3D, H3D) decreased approximately by 25%, while for K3D specimens it increased approximately by 15%. The K3D composites had a higher strain rate as compared to the B3D and H3D composites. In the case of K3D composite, except strain at peak stress, remaining dynamic properties were lower than the B3D composite, however, hybridization increased these properties. The failure mechanisms of 3D-A composites were characterized through macroscopic and scanning electron microscopy (SEM).

Neat polypropylene (PP) and PP reinforced with nano clay platelets were tested at a fixed quasi-static strain rate on UTM and at corresponding limiting strain rates on SHPB (Split Hopkinson Pressure Bar). Enactment with minimum possible thickness of specimens cum maximum impact pressure on SHPB experiments is found important to set limiting strain rate. Combination of stress-strain responses of the material systems from both high strain rate impact and quasi-static loading is used to estimate the representative strain rate sensitivity index ‘m’ values: A new approach more justified than the trend of using strain rates limited the quasi-static range. Particular constant strain value of 0.0125 m/m and room temperature of 21±3 o C at which ‘m’ values are calculated were considered. The increase in weight fraction of nano-reinforcement was found to increase strain rate sensitivity index of PP to the level comparable with the lower bound of the range for superplastic alloys.

Characterization of Kevlar-Polypropylene based composite material system under high strain rate loading has been investigated using Split Hopkinson Pressure Bar (SHPB) test for varying specimen aspect ratios. Flat laminates of 16, 24 and 30 layered Kevlar composite were compression molded and laser machining to get cylindrical specimensof desired aspect ratios. Based on SHPB experiments, stress-strain plots were obtained and analysed to reveal compressive material behaviour as function of growing strain rate. The peak stress, strain and toughness exhibited considerable increase with growing strain rate of loading. With increasing strain rates peak specimen stress increased by 90%, for lowest thickness composite. The aspect ratio studies suggests application of thin laminates for better performance of composite laminates.

Modern and sophisticated ammunition systems have necessitated the development of advanced ballistic protection personal armour systems that are damage resistant, flexible, light-weight and possess high energy absorbing/dissipating capacity. Although, exhaustive studies are available which show the improvement in impact resistance of STF-treated high performance fabrics, but there are limited studies which explore the efficacy of STF impregnation method. In this study, an attempt is made to understand this aspect. The low velocity impact studies were conducted on drop tower machine, while high velocity impact studies were accomplished on Split Hopkinson Pressure Bar (SHPB). High impact Poly Propylene Co-polymer (CO15EG) and UHMWPE variants Gold Shield® and Spectra Shield® were chosen for this study. It was observed that when STF was kept in liquid form between layers of ballistic fabrics, the composite exhibited reduced performance, whereas, when STF was impregnated in a ballistic fabric it had a synergistic effect to enhance the impact toughness of ballistic composite.

For the first time, the influence of moisture ingress and egress on the dynamic performance of commercial grades of Ultra-High-Molecular-Weight-Polyethylene (UHMWPE) composites is investigated. Two different grades of UHMWPE prepregs comprising Thermoplastic Elastomer (TPE) and Polyurethane (TPU) matrix were compression moulded and cylindrical specimens were cut-out for the high strain rate testing on a Split Hopkinson Pressure Bar (SHPB) apparatus. For identical incident energies, different strain rates were attained by the dry, wet and forced dried specimens of both the matrices. The experimental results indicate that the stress, strain and toughness significantly increase the pseudo properties with increasing rates of loading, irrespective of the type of conditioning process adopted. Moisture ingress reduced the strength of TPE and TPU matrix based composite by half and quarter of the strength of its dry composite. Moisture egress resulted in higher property enhancement of TPU based composite as compared to TPE based composite. The study thus substantiates the importance of moisture-proofing the composites meant for ballistic applications even from minor damage.

Kevlar ® fabric reinforced plastics (KFRPs) are specialized composites with multiple layers of fabrics (∼ranging from 20 to 50 layers) designed for high impact applications. In this work, Kevlar-129 fiber was reinforced with polyetherimide (PEI) and polypropylene (PP) to obtain two groups of laminates in three configurations i.e. 16, 24 and 30 layers. Holes of diameter 11.6 mm were profile cut using fiber laser machining system operating at 1070 nm wavelength. Effects of polymeric matrix on the failure of the Kevlar – polyetherimide (K-PEI) and Kevlar – polypropylene (K-PP) laminates were characterized by studying the following: (i) threshold laser power required to make the holes (ii) surface morphology using scanning electron microscopy (iii) damage zone along the laser cut path using scanning acoustic microscopy and optical microscopy. K-PEI laminates underwent material separation at much lesser line energy (ratio of laser power to velocity) than K-PP laminates during laser machining. Scanning electron microscopy (SEM) was used to further analyze the laser cut surfaces. A prominent observation on the laser-irradiated surface was: less recast/resolidified polymer covered the Kevlar fabric in K-PEI as compared to a thicker polymeric layer in K-PP. Heat affected zone and damage factors were evaluated using scanning acoustic microscopy (SAM). Surface roughness and kerf width were also analyzed to understand the effect of laser machining of Kevlar laminates.

Quasi-static tensile test of UHMWPE fiber-reinforced composite laminate is challenging to perform due to low interlaminar shear strength and low coefficient of friction. Tensile tests proposed in the literature were conducted and limitations associated with each method led to the evolution of a new method. Tensile test of single-ply was realized as the best representative of tensile strength of a composite than tensile test of UHMWPE laminate. A fixture was developed for single-ply tests which increased friction and provided the mechanical constraint to slipping. The fixture is easy to fabricate and has provided repeatable results for eight grades of UHMWPE fiber-based (0/90) fabrics. Reported tensile strengths are in quite high range of 900–1500 MPa.

As a major challenge, development of light-weight fibre reinforced polymer (FRP) composite body armour, characterization of candidate matrix polymers at high strain rate impact is the focus in this research. Polypropylene (PP) and the nano-composites with 1-5% by weight of NC (nanoclay) platelets are the candidates considered. In the characterization phase, high strain rate impact and quasi-static loading tests were performed to figure out the limiting (failure) responses. Comparison between the material systems is, subsequently, made to nominate one matrix configuration. Enhancements of mechanical properties with increase in weight percentage of the nanoparticles are observed at both quasi-static and dynamic loadings. Observations of dispersed imposed failure modes, development of novel model for failure modulus and evaluation of peak strength values are also attempted. Keywords : Nano-composite, Impact behaviour, Analytical modelling, Polypropylene, Failure mode.

Shear thickening is a non-Newtonian flow behavior characterized by the increase in apparent viscosity with the increase in applied shear rate, particularly when the shear rate exceeds a critical value termed as the critical shear rate (CSR). Due to this remarkable property of shear-thickening fluids (STFs), they are extensively used in hip protection pads, protective gear for athletes, and more recently in body armor. The use of STFs in body armor has led to the development of the concept of liquid body armor. In this study, the effect of particle size is explored on the low and high strain rate behavior of nanosilica dispersions, so as to predict the efficacy of STF-aided personal protection systems (PPS), specifically for ballistic applications. The low strain rate study was conducted on cone and plate rheometer, whereas the high strain rate characterization of STF was conducted on in-house fabricated split Hopkinson pressure bar (SHPB) system. Spherical nanosilica particles of three different sizes (100, 300, and 500 nm) as well as fumed silica particles of four different specific surface areas (Aerosil A-90, A-130, A-150, and A-200), respectively, were used in this study. The test samples were prepared by dispersing nanosilica particles in polypropylene glycol (PPG) using ultrasonic homogenization method. The low strain rate studies aided in determining the CSR of the synthesized STF dispersions, whereas the high strain rate studies explored the impact-resisting ability of STFs in terms of the impact toughness and the peak stress attained during the impact loading of STF in SHPB testing.

Aluminum-Silicon carbide (Al-SiC) metal matrix composite (MMC) have gained importance as a new class of material capable of serving the future generation brake material requirements. The presented study deals with the compressive high strain rate behavior of Al-SiC composite comprising 15% SiC by weight. The MMC specimens were fabricated by powder metallurgy route. Three different ball milling times were selected for the study purpose after optimizing all other processing parameters. Incorporation of SiC invariably enhanced the hardness, quasi-static and dynamic compressive strength of Al-SiC composite. Hardness was noted to enhance as a function of increasing ball milling time. However, the quasi-static and dynamic compression properties were recorded highest for the ball milling time of 240 minutes. The study confirms that minor addition of a suitable reinforcing material can significantly improve the properties of an MMC. Also, increasing the ball mill time may not necessarily enhance all the material properties.

peer-reviewed The present study deals with the quasi-static and high strain rate characterization of Kevlar-129 based thermoplastic composites. Two different thermoplastic matrices, namely, Polypropylene (PP) and Polyetherimide (PEI) were used to manufacture composite laminates. Quasi-static compression tests were performed at strain rates of 0.041 s−1 and 0.045 s−1. High strain rate tests were performed using a split Hopkinson pressure bar apparatus within the strain rates ranging from 2548 s−1 to 4379 s−1. Stress-strain relations reveals the rate-sensitive behaviour of composites. Kevlar/PP (K-PP) showed higher peak stress under quasi-static loading as compared to the high strain rate test. Comparable peak stresses were revealed under quasi-static and high strain rate loading for Kevlar/PEI (K-PEI) composite. Also, high strain rate compression properties such as peak stress, peak strain and toughness of K-PP were 25%, 27% and 6% higher than that of the K-PEI composite. The failure mechanisms of both the composites were characterized through macroscopic and scanning electron microscopy. K-PP failed majorly due to matrix crush and fibre failure while K-PEI failed due to shear cracking. Damage study reveals that a single fibre based composite system can be tailored to act as an energy-absorbing or dissipating material system by varying the thermoplastic matrix materials

peer-reviewed In this study, the influence of hybridization on the compression response of thermoplastic matrix-based composites under high strain rate loading was investigated. The intra-ply and inter-ply hybrid composites were manufactured with Kevlar/Basalt yarns as the reinforcements with Polypropylene as a matrix. Cylindrical composite specimens were laser cut from the flat compression moulded laminates. The composite specimens were loaded under high strain rate using split-Hopkinson pressure bar setup at strain rates ranging from 2815/s to 5481/s. The study revealed differences in the rate-dependent growth of peak stress, peak strain and toughness with the strain rate. Intra-ply hybrid composites with alternate weaving of Kevlar and basalt yarns exhibited highest peak stress as compared to the Inter-ply hybrid composites (alternate layers of Kevlar and basalt fabrics) and another intra-ply composite containing Kevlar in the warp and basalt in the weft direction. Whereas in inter-ply hybrid composite, with Kevlar as the loading face attained higher stress, while composite with Basalt as the loading face attained higher strain. SEM micrographs revealed that Kevlar on the loading face can bear the impact with lesser delamination as compared to the Basalt on the loading face. Damage studies revealed that Kevlar fiber surface loading results in higher stress as compared to basalt (brittle) surface loading with lower overall damage. PUBLISHED peer-reviewed