Viscoelastically prestressed polymeric matrix composites: An overview

Elastically prestressed polymeric matrix composites exploit the principles of prestressed concrete, i.e. fibres are stretched elastically during matrix curing. On matrix solidification, compressive stresses are created within the matrix, counterbalanced by residual fibre tension. Unidirectional glass fibre elastically prestressed polymeric matrix composites have demonstrated 25–50% improvements in impact toughness, strength and stiffness compared with control (unstressed) counterparts. Although these benefits require no increase in section dimensions or weight, the need to apply fibre tension during curing can impose restrictions on processing and product geometry. Also, fibre–matrix interfacial creep may eventually cause the prestress to deteriorate. This paper gives an overview of an alternative approach: viscoelastically prestressed polymeric matrix composites. Here, polymeric fibres are subjected to tensile creep, the applied load being removed before the fibres are moulded into the matrix. Following matrix curing, viscoelastic recovery mechanisms cause the previously strained fibres to impart compressive stresses to the matrix. Since fibre stretching and moulding operations are decoupled, viscoelastically prestressed polymeric matrix composite production offers considerable flexibility. Also, the potential for deterioration through fibre–matrix creep is offset by longer term viscoelastic recovery mechanisms. To date, viscoelastically prestressed viscoelastically prestressed polymeric matrix composites have been produced from fibre reinforcements such as nylon 6,6, ultra-high molecular weight polyethylene and bamboo. Compared with control counterparts, mechanical property improvements are similar to those of elastically prestressed polymeric matrix composites. Of major importance, however, is longevity: through accelerated ageing, nylon fibre-based viscoelastically prestressed viscoelastically prestressed polymeric matrix composites show no deterioration in mechanical performance over a duration equivalent to ∼25 years at 50℃ ambient. Potential applications include crashworthy and impact-absorbing structures, dental materials, prestressed precast concrete and shape-changing (morphing) structures

Impact and Implication of Thermal Conditioning on the Mechanical behavior of FRP Composites

Fiber reinforced composites are used extensively on a very large scale. They are subject to change in temperature and loading conditions constantly. In the experimental study, we have tried to assess the impact of temperature and conditioning time on the mechanical behaviour of glass fiber reinforced composites. Interface is the most significant part of composite structure which regulates the load transfer from matrix to fiber. Its strength is measured in terms of ILSS (Inter Laminar Shear Strength). Short beam shear tests are done at ambient and 60⁰C for conditioning times 30 minutes, 1 hour and 3 hours .The results show that at high temperature, there is initial increase in the strength of interface up to 30 minutes followed by weakening as conditioning time increases to 1 hour. This again is followed by strengthening of the interface as conditioning time extends to 3 hours. Understanding the effect of conditioning time might help us in optimization of the mechanical properties. Composite material may contain randomly spaced microvoids, incipient damage sites and microcracks with statistically distributed sizes and directions. Therefore, the local strength in the material varies in a random fashion. The failure location as well as degree of damage induced in the material will also vary in an unpredictable mode. The fractured surfaces are photographed by SEM analysis and studied. As temperature increases, the mode of failure approaches matrix cracking, fiber breakage and debonding. Each test is carried out at six different crosshead speeds, 5mm /min, 10mm/min, 50mm/min, 100mm/min, 200mm/min, and 500mm/min. ILSS decreases as crosshead speed is increased. FTIR analysis of composite specimens was carried out to interpret the reaction between matrix and fiber at the interface. DSC analysis was done to understand the deflection of glass transition temperature with the change in temperature and conditioning time. There are a lot of conflicts over this subject and this study has tried to highlight the major factors which need to be focussed upon for further improvement in the field of composites

The Effect of Glass–Kevlar 49 Fibre Loading on the Mechanical, Thermal and Physical Properties of Polypropylene Hybrid Composites

Communication in Physical Sciences 2020, 5(2): 99-105 Author: E. Danladi, P.A.P. Mama, S.A. aro, M.T. Isa, E. R. Sadiku and S.S. Ray Received 04 April 2020/Accepted 20 April 2020 In order to improve properties of polypropylene, , hybrid composites of GlassKevlar reinforced polypropylene were fabricated by using the compression molding technique. Mechanical, thermal and physical properties of the hybrid composites were investigated. The percentage fibre loadings were 06/07, 12/07 and 11/13 (i.e., GF/KF). The effect of stacking sequence on the properties was also investigated and reported. The fibre loading increase the toughness of the hybrid composites with GF/KF (11/13) having the highest. The observed high damping was attributed to alteration in molecular motion in the hybrid composites. Hybridization did not significantly affect the density of the produced composites. The increase in water absorption capacities of the hybrid composites with increasing Kevlar fibre contents was attributed to the presence of hydrogen bonds in the Kevlar fibre

The experimental and numerical analysis of the ballistic performance of elastomer matrix Kevlar composites

In this paper, the behavior of high-velocity impact of Kevlar fabric and elastomer composites was investigated both experimentally and numerically. The experimental tests were performed by a gas gun device and hemispherical projectiles at different velocities, ranging from 122 m/s to 152 m/s for 2- and 4-layer samples. The penetration resistance of these composites during impact was determined using ABAQUS/Explicit. The present study's novelty lies in choosing the finite element model for Kevlar fabric and elastomer matrix in composites with nonlinear behavior to estimate the damage mechanism in the impact zone. For this purpose, the material model of the formable was used to define the damage criteria for Kevlar, and the material model of the VUMAT was used to consider the non-linear behavior and damage evolution of elastomer matrix with one of the damage criteria. Then, the dynamic behavior of the laminate was studied by a Split Hopkinson Pressure Bar. The effect of the number of layers, the shape of the projectile, the energy absorption and failure mechanisms were studied. The verification of this numerical model with experimental observations showed good agreement. The results reveal that elastomeric composites can cause to increase energy absorption and reduce the damaged area

Polymer fibre composites : investigation into performance enhancement through viscoelastically generated pre-stress

In this research, the performance and further development of viscoelastically pre-stressed polymer matrix composites (VPPMCs) was investigated. Pre-stressed composite samples with continuous unidirectional fibres are produced by applying a tensile load to polymeric fibres to induce tensile creep. After removing the load, the fibres are moulded in a polyester resin. Following resin curing, compressive stresses are imparted by the viscoelastically strained fibres as they attempt to recover their strain against the surrounding solid matrix material. Prior to this study, VPPMCs using nylon 6,6 fibres increased impact energy absorption and flexural modulus by 30-50% relative to control (un-stressed) counterparts. The current work contributes to ongoing efforts in VPPMC research by expanding the knowledge of existing VPPMC materials and identifying the potential for an alternative, mechanically superior polymeric fibre.For nylon 6,6 fibre-based VPPMCs, the effects of Charpy impact span settings and fibre volume fraction (3-17% Vf) were investigated. The effects of commingling nylon pre-stressing fibres with Kevlar fibres to produce hybrid VPPMCs was also evaluated. Moreover, as an alternative to nylon fibre, the viscoelastic characteristics and subsequent VPPMC performance of polyethylene (UHMWPE) fibre was investigated. Charpy impact and three-point bend tests were used to evaluate VPPMC samples against control (un-stressed) counterparts. In addition, microscopy techniques were applied to impact-tested samples, to analyse fracture behaviour.For the nylon fibre-based VPPMCs, it was found that improvements in impact energy absorption from pre-stress depended principally on shear stresses activating fibre-matrix debonding during the impact process. Scanning electron microscopy of impact-tested samples revealed visual evidence of pre-stress impeding crack propagation. A short span setting (24 mm) showed greater increases in energy absorption of 25-40%, compared with samples tested at a larger span (60 mm) which gave increases of 0-13%. The results suggest that there is an increasing contribution to energy absorption from elastic deflection at larger span settings; this causes lower energy absorption as well as reducing any improvements from pre-stress effects. However, this effect was suppressed by the addition of Kevlar fibres (to produce hybrid VPPMCs), which promoted more effective energy absorption at the larger span. Moreover, bend tests on the hybrid composites demonstrated that pre-stressing further enhanced flexural modulus by ~35%.The viscoelastic characteristics of UHMWPE fibres indicated that these fibres could release stored energy for pre-stressing over a long time period. This was effectively demonstrated with UHMWPE fibre-based VPPMCs using three-point bend tests, i.e. flexural modulus increased by 25-35% from pre-stressing with no deterioration observed over the time scale investigated (~2 years). Also, these VPPMCs absorbed ~20% more impact energy than their control counterparts, with some batches reaching 30-40%. Although fibre-matrix debonding is known to be a major energy absorption mechanism, this was not evident in the UHMWPE fibre-based VPPMCs. Instead, evidence of debonding at the skin-core interface within the UHMWPE fibres was found. This is believed to be a previously unrecognised energy absorption mechanism.This work contributes to a further understanding of the viscoelastic properties of polymeric fibres and insight into the field of pre-stressed composite materials. The findings support the view that VPPMCs can provide a means to improve impact toughness and other mechanical characteristics for composite applications

ENHANCING BALLISTIC IMPACT RESISTANCE OF POLYMER MATRIX COMPOSITE ARMORS BY ADDITION OF MICRO AND NANO-FILLERS

Improving the ballistic impact resistance of hybrid polymer matrix composites through addition of micro- and nano-particles as fillers is the principal goal of this research. Development of light-weight ballistic plates, made of polymer matrix composites with improved ballistic resistance, can offer a solution of shielding with lighter, thinner, stronger and less expensive materials than the conventional ballistic plates. The use of micro- and nano-particles in low concentrations can achieve this goal without compromising the density or strength of the new armor plates. Firstly, laminated hybrid composites consisting of aluminum alloy plates, epoxy resin and Kevlar® fabrics were developed. Shear thickening fluid (STF) made of nano-particles of colloidal silica (SiO2) was impregnated into Kevlar® fabrics to determine its effect on the energy absorption behavior of the composites. STF decreased the tendency of Kevlar® fibers to rupture during projectile penetration, and thus, increased its impact energy absorption performance when compared to the samples made of Kevlar® neat fabrics (containing no STF). Similar laminated hybrid composites were subsequently built through impregnation of micro- and nano-particles of aluminum, gamma alumina, silicon carbide, colloidal silica and potato flour into Kevlar fabrics by mixing these particles with polyethylene glycol. The obtained laminates were evaluated to determine their impact resistance and energy absorption capabilities under ballistic impact. The plates containing aluminum and colloidal silica nano-powders have the highest energy absorption capability of between 679 up to 693 J for plate thickness and areal density of about 10.8 mm and 1.9 g/cm2, respectively. These laminates can meet the protective requirements for levels IIA, II, and IIIA to resist ballistic impact from pistols caliber 9 mm. In another approach, hybrid composite armor plates based on high density polyethylene (HDPE) were prepared by using 10 wt.% of Kevlar® short fibers, and 20 wt.% chonta palm wood, potato flour, colloidal silica or gamma alumina particles. Addition of colloidal silica and gamma alumina nano-particles improve stiffness by 43.5% and increase impact energy absorption capability by 20%, compared to control sample, which is HDPE containing 10 wt.% Kevlar® short fibers. Hybrid bio-composites made of 10 wt.% Kevlar® short fibers and varying amount of chonta wood particles (10, 20, 30 wt. %), as additional reinforcement, were also developed and investigated. The hybrid composite plates containing 10 wt.% chonta palm wood micro-particles exhibited the highest energy absorption capability of 62.4 J, which is equivalent to 19.5 % improvement over control specimens: HDPE reinforced with 10 wt.% Kevlar® short fibers. Finally, bio-composites made of HDPE reinforced with varying fractions of micro-particles of chonta palm wood (10, 20, 25, and 30 wt. %) were developed and characterized. The ballistic impact performance of the biocomposites containing 25 wt.% chonta palm wood particles exhibited the highest energy absorption of 53.4 J, which represents a 41.3% improvement over the unreinforced HDPE specimens with similar thickness and density

An Assessment of Mechanical Behavior on High Temperature and Different Volume Fraction of Glass Fiber Reinforced Polymer Composites

Fiber reinforced polymer (FRP) composite materials are the primary choice in various structural and high performance application facilitating the need from the last four decades. High specific strength, high specific modulus, high stiffness to weight ratio, and design flexibility enables FRP composite materials to be used in a large number of critical structural components in aircrafts, satellite structures, various automobile components, wind turbine blades, sport goods etc. The mechanical properties of glass fiber/epoxy composite is significantly altered by high temperature and volume fraction which exhibits the various types of the failure modes (e.g. delamination sites, debonding, fiber pullout regions, crack propagation front, striations and bubble bursting in the matrix). The glass/epoxy composites were prepared for two different volume fraction of 50/50 and 60/40 and SBS samples were thermally conditioned at 500c at ambient and for different time duration period of 1hr, 5hr and 7hr. Interlaminar shear behaviour may be used to characterize FRP composite material.DSC analysis shows Tg value increases with increase in thermal conditioning time w.r.t ambient Tg value for glass/epoxy composites. From the FTIR analysis we observe the band at 550-650 cm-1 is the spectra range of 50/50 volume fraction of the glass/epoxy system with the shifting of bandwidth with decrease in thermal conditioning time

Compressive Failure Behaviour of Kevlar Epoxy and Glass Epoxy Composite Laminates Due to the Effect of Cutout Shape and Size with Variation in Fiber Orientations

The increasing trend of constructing components made of composite laminates is due to the flexibility in tailoring their properties and high strength-to-weight ratio. Nevertheless, most practical components involve cutout features for fastening and these cutouts could reduce significantly the strength of the laminate.  Due to its importance, many studies were conducted to study the effect of circular cutouts however, there is lack of information regarding the effect of various cutout shapes. Therefore, this study aims to investigate the compressive failure behavior of Kevlar Epoxy and Glass Epoxy composite laminates due to the effect of cutout shape and size with variation in fiber orientations. Finite element software, ANSYS were used to simulate the deformation and failure behavior of the laminates under compressive load. Prior to that, mesh convergence analysis and numerical validation were performed. Failure analysis was conducted for various cutout shapes (square cutout, diamond cutout, and circular) and size, based on Maximum Stress Theory. The results show that the existence of the cutouts on the composite laminates have reduced up to ten times the strength of the laminated composite plates. This information in regards to the failure behavior is important when designing components made of composite laminates under compression

Compressive Failure Behaviour of Kevlar Epoxy and Glass Epoxy Composite Laminates Due to the Effect of Cutout Shape and Size with Variation in Fiber Orientations

The increasing trend of constructing components made of composite laminates is due to the flexibility in tailoring their properties and high strength-to-weight ratio. Nevertheless, most practical components involve cutout features for fastening and these cutouts could reduce significantly the strength of the laminate.  Due to its importance, many studies were conducted to study the effect of circular cutouts however, there is lack of information regarding the effect of various cutout shapes. Therefore, this study aims to investigate the compressive failure behavior of Kevlar Epoxy and Glass Epoxy composite laminates due to the effect of cutout shape and size with variation in fiber orientations. Finite element software, ANSYS were used to simulate the deformation and failure behavior of the laminates under compressive load. Prior to that, mesh convergence analysis and numerical validation were performed. Failure analysis was conducted for various cutout shapes (square cutout, diamond cutout, and circular) and size, based on Maximum Stress Theory. The results show that the existence of the cutouts on the composite laminates have reduced up to ten times the strength of the laminated composite plates. This information in regards to the failure behavior is important when designing components made of composite laminates under compression