Effect of Specimen Thickness on High Strain Rate Properties of Kevlar/Polypropylene Composite Procedia Engineering 173 ( 2017 ) 694 – 701 1877 7058 © 2017 The Authors Published by Elsevier Ltd This is[.]
Available online at www.sciencedirect.com ScienceDirect Procedia Engineering 173 (2017) 694 – 701 11th International Symposium on Plasticity and Impact Mechanics, Implast 2016 Effect of Specimen Thickness on High Strain Rate Properties of Kevlar/Polypropylene Composite Hemant Chouhana,b,*, Neelanchali Asijaa, Shishay Amare Gebremeskela, Naresh Bhatnagara a Department of Mechanical Engineering, Indian Institute of Technology Delhi, New Delhi, 110016, India b Department of Mechanical Engineering, Amity University Uttar Pradesh, 201313, India Abstract 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 specimens of 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 © 2017 The Authors Published by Elsevier Ltd This is an open access article under the CC BY-NC-ND license © 2016 The Authors Published by Elsevier Ltd (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the organizing committee of Implast 2016 Peer-review under responsibility of the organizing committee of Implast 2016 Keywords: Kevlar; Thermoplastic matrix; High strain rate; Aspect ratio Introduction The need for light weight body armor had always been in demand This resulted in phenomenal growth of fiber reinforced plastic composites in last five decades The high tenacity fibers coupled with low specific gravity matrix * Corresponding author Tel.: +91-011-26591139; fax: +91-011-26582053 E-mail address: hemant.chouhan78@gmail.com 1877-7058 © 2017 The Authors Published by Elsevier Ltd This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the organizing committee of Implast 2016 doi:10.1016/j.proeng.2016.12.151 Hemant Chouhan et al / Procedia Engineering 173 (2017) 694 – 701 695 results into a composite system perfectly suited for the ballistic applications With ever increasing demand, the need to study the material behaviour under high strain rate loading becomes essential The quasi-static testing done on universal testing machines cannot reveal the materials response under high strain rate of loading Therefore, there was a strong need of a testing scheme to study the material behaviour under dynamic high strain rates of loading This need was fulfilled in the form of a unique test facility developed by Kolsky [1], named as split Hopkinson pressure bar test apparatus It has been reported in literature that most of the material whether metallic or non-metallic have strain rate dependent properties Allazadeh et al [2], studied steel, aluminum, wood and graphite-epoxy composite under high stain rate of loading and reported difference in rate dependent behaviour of all these materials Similarly, numerous studies revealed rate dependent behaviour of metals and composite Nolting et al [3] studied different grades of naval steel They optimized SHPB system to suit their materials and reported rate dependency of three grades of naval steel An increase in strength and peak strain was reported for different metals with increasing strain rate of loading [4][5] The rate dependent behaviour of FRC’s has also been studied Song et al [6], studied S-2 glass/SC15 composite along thickness direction and in-plane direction under high rate loading on SHPB and the peak strength obtained were 700 MPa and 500 MPa for peak strain rate with in the experimental strain rate range Xuan et al [7], studied woven carbon/epoxy under dynamic loading and reported dynamic strength limit higher than quasi-static strength limit Woo et al [8][9], studied Kevlar composite under high strain rate loading using acoustic emission along with SHPB and reported higher strength as a function of growing strain rates of loading The peak stress acquired by Kevlar composite varied in range of 160 ~ 370 MPa for an experimental strain rate range of 1182 ~ 1460 s -1 Ramadhan et al [10], studied the effect of three different specimen thicknesses on dynamic properties of Kevlar/epoxy and Aluminum laminated panels under ballistic impact and reported change in material behaviour as a function of specimen thickness Nomenclature Hr Ht C0 E AB AS Ls t X FRC SHPB K-PP reflected strain transmitted strain elastic wave velocity in the bars Young’s modulus of elasticity of bar material cross-section area of the bar cross-section area of the specimen specimen length time duration poisson’s ratio fiber reinforced composite split Hopkinson pressure bar Kevlar-Polypropylene Kevlar composites have been used with thermosetting polymers for ballistic applications However, the need arises for using thermoplastic polymers as matrix to further reduce the weight of FRC armor A number of experimental works on varying specimen aspect ratios are available in literature Knowing that ideal aspect ratio resulting into negligible inertial effects is given as ξͲǤͷX [11] The objective of present study is to characterize three different thicknesses of Kevlar-thermoplastic composite system under high strain rate of loading The 16, 24 and 30 layered Kevlar-Polypropylene composite with constant diameter of cylindrical specimen results into three different aspect ratios (0.3, 0.42 and 0.53) were tested under high strain rate loading on SHPB Kevlar composite having 24 layers is close to theoretical requirement and other two will serve as lower and higher value of aspect ratio The test results are of importance due to the fact that whereas, SHPB test reveals the properties on the basis of one-dimensional loading, the physical loading while in service is three-dimensional in nature Hence, the test results 696 Hemant Chouhan et al / Procedia Engineering 173 (2017) 694 – 701 will help understand the effect of varying number of Kevlar mat layers on high strain rate loading of identical diameter composite specimen Experimental Details An experimental SHPB set-up was developed in-house for testing of various materials under high strain loading (Fig 1) The major sub-components of SHPB apparatus are gas gun, striker bar, incident bar, transmission bar, momentum trap, speed sensor, signal conditioner & amplifier and data acquisition system [12] Majority of works reported in literature used steel grades for testing of composites To keep the impedance mismatch within close range softer materials like Aluminum should be preferred However, lack of hardness results in deformation of Aluminum bars Therefore, Titanium bars having diameter of 16 mm were used Titanium is having lower density than steel and much harder than Aluminum The specifications of Titanium bars and strain gauges used for compressive SHPB development are presented in Table Fig Compressive Split Hopkinson Pressure Bar schematic arrangement Table Specifications of SHPB Set-up Bar properties Material Density Modulus of Elasticity Elastic wave speed Bar Diameter Incident bar length Transmission bar length Striker lengths Strain gauge properties Gauge factor Resistance Initial excitation voltage Titanium (Ti6Al4V) 4430 kg/m3 113.8 GPa 5068 m/s 16 mm 1200 mm 1200 mm 240 mm 2.12 350 Ohms 2.5 Volts 2.1 Operation of SHPB The striker bar acquires velocity when the pressurised Nitrogen gas is released by operating solenoid value controlled by a PLC suitably programmed for the purpose The impact of striker bar onto the incident bar generates elastic compressive stress wave in the incident bar When the elastic stress wave crosses the strain gauge mounted at mid of incident bar the same is recorded in the form of voltage change generated due to change in resistance of strain gauge The stress wave then enters the specimen placed at the end of incident bar When the stress wave passes through the specimen, a part of stress wave is reflected back into the incident bar and rest of the wave passes through the specimen into the transmission bar The strain/deformation history of specimen is recorded in the form of voltage signal generated due to change in resistance of strain gauge on incident and transmission bar The voltage 697 Hemant Chouhan et al / Procedia Engineering 173 (2017) 694 – 701 signal received from incident and transmission bar gives reflected and transmitted pulse voltage responsible for specimen strain, strain rate and stress induced in the specimen Quarter bridge arrangement based on Wheatstone bridge was used for recording the strain developed on the bars The strain gauges were powered by signal conditioner and amplifier and the data received in the form of voltage was stored by NI-6115 onto a dedicated PC From these measured voltage signals using one-dimensional wave propagation theory, the strain, strain rate and stress induced in the specimen can be assessed 2.2 Specimen preparation and design Flat composite laminates of Kevlar-129 (Grade: 802F, Make: DuPont) with high impact Polypropylene (Grade: Repol CO15EG, Make: Reliance Polymers) were fabricated by compression molding The Kevlar mat was used as provided by the manufacturer The PP polymer was available in the form of granules By the process of film extrusion, the polymer was converted into film of 0.02mm thickness The bonding of PP with Kevlar is not good, therefore, 10% Malic Anhydride grafted Polypropylene (MAgPP) was blended to enhance the bonding between fiber and PP matrix Compression molding was done at 2000 C, 10 bar pressure under 500 mm of Hg vacuum To achieve three different aspect ratios of specimen the number of Kevlar layers were kept as 16, 24 and 30, respectively Laser machining was done on 400W continuous wave fiber laser (Model: RS400, SPI lasers, UK) to get cylindrical specimen of K-PP composite from flat composite laminates The laser machining was done perpendicular to flat surface of laminate The diameter of specimen were kept constant at 11.5 mm, resulting into theoretical aspect ratio of 0.3, 0.42 and 0.53 respectively, with in an experiment tolerances of ±5% error Fiber volume fraction and density plays a significant role in deciding the mechanical properties of composites Desolution by solvent was performed to determine fiber volume fraction of composite laminates Xylene at 80 ˚C (atmospheric pressure) for hours is used to dissolve polymeric matrix content of the K-PP composite specimen The fiber volume fraction and density determined are presented in Table Table Fiber volume fraction of Kevlar-Polypropylene laminates No of Kevlar Layers 16 24 Density (kg/m3) 1170 1140 Fiber volume fraction (%) 61.48 60.70 30 1010 58.63 Pulse shapers are recommended to modify the shape of incident pulse It has been reported to use a pulse shaper giving rise time identical to material response reported by transmission wave when experimented without a pulse shaper [13] Thus, Linatex a natural rubber was optimized to diameter of 3.1 mm and thickness of 1.4 mm to be used as pulse shaper High Strain Rate Testing One-dimensional wave propagation in elastic bars with particle motion in longitudinal direction is the basis for development of SHPB Complete theory, instrumentation and methodology of SHPB is explained in literature [14][15] The derivations of SHPB are based on assumptions, which are prerequisite to achieve one-dimensional loading of the specimen That includes, homogenous state of stress in the specimen, negligible friction effects, negligible inertial effects and negligible wave dispersion along with perfect flat ends for contact with the bars However, due to continuous stress wave attenuation as a function of fiber-matrix deboning the condition of homogenous state of stress in the specimen is not successfully fulfilled in SHPB of composites [16][17] The analytical relations for determination of specimen strain, strain rates and stress as function of time are as follows: The strain rate, ɂሶ s(t) = ሺ ଶబ ௦ ሻɂ ሺݐሻ The average strain, Hs(t)= േሺ ଶେబ ௦ (1) ௧ ሻ ɂ ሺݐሻǤ ݀ݐ (2) 698 Hemant Chouhan et al / Procedia Engineering 173 (2017) 694 – 701 The average stress, V(t) = േܧ ా Ht(t) (3) During the SHPB testing, two methods of determining strain rate are prevailing In general, the strain rate of homogenous materials is determined by initial slope of strain rate-time curve However, due to stress wave attenuation there is substantial variation in reflected strain wave In this case, Nakai et al [18] suggested that strain rate can be calculated by dividing the area under the strain rate-strain curve up to maximum strain under loading by the maximum strain 3.1 Calibration of SHPB apparatus Calibration of SHPB apparatus is essential to validate the conditions of one-dimensional wave theory and assess the accuracy of apparatus During calibration, both the incident and transmission bars are pressed together without a specimen Lubricant layer is applied in between the bars to minimize the friction In this condition both the bars can be treated as single bar Experimentation was done in this state and resulting voltage signals on incident and transmission bar were recorded and analyzed Fig (a) depicts voltage signals received on incident and transmission bar This voltage pulse is converted to equivalent force versus time curve for both incident and transmitted pulse (Fig 2(b)) Force history obtained for incident and transmitted pulse is F1 and F2 respectively Both the forces F1 and F2 matches well, indicating identical stress state in both the bars This ensures SHPB is perfectly aligned, friction free and ready for experimentation 0.1 50 Incident pulse (v) Transmitted pulse (v) 0.0002 -0.1 0.0004 0.0006 Force (kN) Voltage 0 F1 F2 40 30 20 10 -0.2 -0.3 0 0.00005 0.0001 0.00015 0.0002 Time (s) Time (s) (b) (a) Fig Calibration results of compressive SHPB setup (a) strain gauge signals and (b) comparison of force vs time behaviour derived from strain gauge signals Results and Discussion High strain rate compressive behaviour of 16, 24 and 30 layered Kevlar-PP composite is studied To calculate dynamic stress-strain properties, the data recorded in the form of incident, reflected and transmitted wave voltage is suitably fed to equations (1) to (3) Strain rates attained and resulting compressive properties for all the experiments is presented in Table The strain rate is defined at its peak and average for all the experiments in Table For 16 layered K-PP composite, a uniformly rising stress with increasing strain rate is noted (Fig (a)) A rising slope of stress-strain curve as function of strain rate is observed till a strain rate of 5442 s-1 Further higher striker bar velocity only resulted in damaged specimen Associated with damaged specimen was higher strain and strain rate due to increased area under stress-stain curve For strain rates below limiting value of 5442 s-1, a stress unloading curve was observed and the resulting permanent strain was noted below 0.2 After limiting strain rate, peak strain increased as a function of increasing strain rate, without much increase in peak stress The limiting strain rate, therefore refers to the strain rate at which the specimen just fails and there is neither a stress unloading curve nor a gradual stress fall A secondary loading curve was observed for further higher strain rate of loading at 9965 s-1 For 24 layered K-PP composite, a phenomenal change was observed in stress-strain behaviour for strain rates below limiting strain rate of 4048 s-1 A loop was formed at the peak of stress when strain starts dropping but stress continuous to grow (Fig (b)) This phenomenon indicates growing strain signal on transmission bar as a function 699 Hemant Chouhan et al / Procedia Engineering 173 (2017) 694 – 701 of weakening strain signal on incident bar After limiting strain rate of loading, an insignificant growth in modulus is observed with marginally higher peak stress However, a significant growth in peak strain continues after limiting strain rate A secondary loading curve for 24 layered K-PP composite was noted at strain rate of 4379 s-1 For 30 layered K-PP composite, the growth pattern of stress was found different than the previous two cases (Fig (c)) Whereas, a linear growth in plastic region was observed after insignificant stress growth in linear elastic region for 16 and 24 layered K-PP composite, a non-linear growth pattern in plastic region was observed in case of 30 layered K-PP composite The permanent strain for strain rates below limiting strain rate was in a narrow band After acquiring limiting strain rate, 30 layered K-PP composite also resulted in higher strain indicating consistency of performance of K-PP composite With increasing strain rate of loading, the shape of stress-strain curve at peak changes from a continuous curve to sharp peak followed by gradual fall The strain recovery is recorded maximum for 24 layered K-PP Indicating importance of ideal specimen aspect ratio After limiting strain rates of loading insignificant change is noted in stress as a function of rising strain rates It may be noted that for strain rates below limiting stress a significant change in both the slope of stress-strain curve and peak of stress is observed With increasing number of fiber layers, the position of initiation of secondary loading curve shifts on strain scale from 0.7 for 16 layered K-PP to 0.35 for 24 layered K-PP and 0.26 for 30 layered K-PP composite, respectively The peak stress attained by 16 layered K-PP composite was maximum at 753 MPa when loaded at limiting strain rate of 5442 s-1, whereas, higher number of layers resulted into lower peak stress and stain rate 738 MPa and 661 MPa at 4048 s-1 and 2837 s-1 for 24 and 30 layered K-PP composite, respectively 5442 s·¹ 600 6708 s·¹ 9965 s·¹ 400 2548 s·¹ 4048 s·¹ 4105 s·¹ 4379 s·¹ 800 3307 s·¹ Stress (MPa) Stress (MPa) 800 200 600 400 200 0 0.2 0.4 Strain 0.6 0.8 0.2 (a) Stress (MPa) 800 Strain (b) 0.4 0.6 2101 s·¹ 2837 s·¹ 600 3481 s·¹ 3515 s·¹ 400 200 0 0.2 Strain (c) 0.4 0.6 Fig Effect of aspect ratio on dynamic compressive behaviour of K-PP composite compression molded at 10 bar (a) 16 layered K-PP composite, (b) 24 layered K-PP composite and (c) 30 layered K-PP composite Fig depicts the aspect ratio effect on high strain rate behaviour of K-PP composite (average aspect ratio mentioned in bracket for respective number of Kevlar layers) The stress growth is presented till limiting strain rate is achieved Since, after limiting strain rate specimen is failed and resulting strain growth increases strain rates For a given peak stress, thin specimen acquired highest strain rate of loading The K-PP specimen with minimum thickness resulted in highest strain rates of loading 30 layered K-PP composite resulted in lowest strain rates for identical peak stress value The 24 layered composite results lies in between the two extremes A linear curve fitting 700 Hemant Chouhan et al / Procedia Engineering 173 (2017) 694 – 701 is adopted to reveal the nature of stress growth as a function of strain rate for different aspect ratios of specimen The linear curve fits well for K-PP composites upto limiting strain rates having different aspect ratios The coefficient of determination indicating closeness with the fitted regression line is above 0.99 for all the aspect ratios 800 Stress 700 y = 0.3517x - 342.18 y = 0.1741x - 26.733 R² = 0.996 R² = 0.9996 y = 0.1679x - 157.13 R² = 0.9979 600 500 K-PP-16 (0.3) K-PP-24 (0.42) K-PP-30 (0.53) 400 300 2000 3000 4000 Strain rate (s-1) 5000 6000 Fig Effect of aspect ratio on K-PP composite loaded up to limiting strain rates Table 3: Strain rate effect on compressive properties of Kevlar-Polypropylene composites No of Kevlar layers K-PP-16 K-PP-24 K-PP-30 Strain rate (s-1) Strain at peak Fracture Modulus ‘F’ Peak Average stress Peak stress (MPa) (MPa) Toughness (MPa) 4522 3307 0.192 392 2041 52.81 7340 5442 0.284 753 2653 152.55 10413 6708 0.2393 725 3001 190.63 14730 9965 0.24 662 2908 223.17 3364 2548 0.160 418 2607 32.47 5456 4048 0.219 738 3370 83.48 5637 4105 0.213 764 3587 110.69 5800 4379 0.193 672 3472 127.68 2897 2101 0.151 400 2651 36.39 3950 2837 0.157 661 4215 76.69 5099 3481 0.180 572 3171 99.12 5196 3515 0.172 582 3377 100.52 Conclusion To reveal the effect of aspect ratio on high strain rate behaviour of K-PP composite SHPB testing was done Following conclusions may be drawn on basis of high strain rate testing of 16, 24 and 30 layered K-PP composite Fracture modulus grows as a function of strain rate of loading, till limiting strain rates of loading Growth pattern of stress and strain were found linear in nature till limiting strain rate, but the toughness continues to follow growth even further at higher strain rates of loading, due to increasing specimen strain as a function of strain rate Thin specimen proved to be more advantageous over thick specimen, as far as peak stress is the criteria A peculiar behaviour of loop formation at peak stress was observed in case of 24 layered K-PP composite For a given striker bar velocity, varying strain rates were attained by the specimen of different thicknesses Secondary loading of specimen was depicted in all cases, expect 30 layered K-PP composite Hemant Chouhan et al / Procedia Engineering 173 (2017) 694 – 701 Acknowledgments The authors are thankful to IRD-IITD for grand challenge (MI00810) for granting this research project References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] H Kolsky, An Investigation of the Mechanical Properties of Materials at very High Rates of Loading, Proc Phys Soc London B 62 (1949) 676–700 M.R Allazadeh, M.K Itani, S.N Wosu, Compression of the Material Characteristics of Steel, Aluminum, Wood and Woven Graphite Epoxy Composites in Response to High Strain Rate Load, Adv Mater Sci Appl (2012) 13–30 A.E Nolting, R Arsenault, M Bolduc, Increased accuracy of SHPB test apparatus to better evaluate naval steels, Procedia Eng 10 (2011) 2268–2273 M Peroni, G Solomos, V Pizzinato, Impact behaviour testing of aluminium foam, Int J Impact Eng 53 (2012) 74–83 W Zhang, P Hao, Y Liu, X Shu, Procedia Engineering Determination of The Dynamic Response of Q345 Steel Materials by Using SHPB, Procedia Eng 24 (2011) 773–777 A.P Ground, Quasi-static and Dynamic Compressive Behaviors of a S-2 Glass/SC15 Composite, 37 (2003) 1723–1744 Z Song, Z Wang, H Ma, H Xuan, Mechanical behavior and failure mode of woven carbon/epoxy laminate composites under dynamic compressive loading, Compos Part B Eng 60 (2014) 531–536 S Woo, T Kim, High strain-rate failure in carbon/Kevlar hybrid woven composites via a novel SHPB-AE coupled test, Compos Part B 97 (2016) 317–328 S.C Woo, T.W Kim, High-strain-rate impact in Kevlar-woven composites and fracture analysis using acoustic emission, Compos Part B Eng 60 (2014) 125–136 A.A Ramadhan, A.R Abu Talib, A.S Mohd Rafie, R Zahari, High velocity impact response of Kevlar-29/epoxy and 6061-T6 aluminum laminated panels, Mater Des 43 (2013) 307–321 E D H Davis and S.C Hunter, The dynamic compression test of solids by the method of the split Hopkinson pressure bar, J Mech Phys Solids 11 (1963) 155–179 S.A Gebremeskel, N Asija, A Priyanshu, H Chouhan, N Bhatnagar, Design Customization and Development of Split Hopkinson Pressure Bar for Light and Soft Armour Materials, Glob Jr Res Eng XIV (2014) 62–75 K.S Vecchio, F Jiang, Improved Pulse Shaping to Achieve Constant Strain Rate and Stress Equilibrium in Split-Hopkinson Pressure Bar Testing, Metall Mater Trans A 38 A (2007) 2655–2665 M Firdaus, H Akil, Z Arifin, Static and dynamic compressive properties of mica / polypropylene composites, Mater Sci Eng A 528 (2011) 1567–1576 M V Hosur, J Alexander, U.K Vaidya, S Jeelani, High strain rate compression response of carbon / epoxy laminate composites, Compos Struct 52 (2001) 405–417 N.K Naik, V Ch, V.R Kavala, Hybrid composites under high strain rate compressive loading, Mater Sci Eng A 498 (2008) 87–99 N.K Naik, K.S Pandya, J.R Pothnis, T.A Gelu, Revisiting Kolsky bar data evaluation method, Compos Struct 111 (2014) 446–452 T Yokoyama, K Nakai, Determination of the impact tensile strength of structural adhesive butt joints with a modified split Hopkinson pressure bar, Int J Adhes Adhes 56 (2015) 13–23 701 ... Conclusion To reveal the effect of aspect ratio on high strain rate behaviour of K-PP composite SHPB testing was done Following conclusions may be drawn on basis of high strain rate testing of. .. 4000 Strain rate (s-1) 5000 6000 Fig Effect of aspect ratio on K-PP composite loaded up to limiting strain rates Table 3: Strain rate effect on compressive properties of Kevlar-Polypropylene composites... deboning the condition of homogenous state of stress in the specimen is not successfully fulfilled in SHPB of composites [16][17] The analytical relations for determination of specimen strain, strain