Weave S2-glass Research Articles

Experimental and numerical studies on the repeated low-velocity impact response and damage accumulation in woven S2-glass fibre/epoxy composites

In this paper, the repeated low-velocity impact response of woven S2-glass/epoxy composites is studied. The impacts were performed with energies from 18.4 to 59.2 J using a drop-tower apparatus, and a post-mortem analysis after each impact was employed to assess the impact response. A damage index was used to describe the changes in impact response due to repeated impacts. Finite element simulations considering both interlaminar and intralaminar failure modes were performed. The results showed that the impact force and bending stiffness decreased with the number of impacts, while impact duration and maximum central displacement increased. The shape of the damaged area was also affected. The numerical results showed that interlaminar damage initiated at most interfaces during the first impact, followed by in-plane propagation in the next impacts. Also, intralaminar damage initiated at the backside of the laminate, and then in-plane and through-thickness propagations followed until penetration occurred.

Micromechanical Strength Modeling and Investigation of Stitch Density Effects on 3D Orthogonal Composites

A 3D orthogonal woven S2-glass composite is investigated using finite element micromechanics to characterize the stiffness and the strength. The methods are applied to a targeted parametric investigation of the effects of stitch density on strength properties and potential benefits of through-thickness reinforcement, such as resistance to transverse shear and delamination, with some consequent loss of in-plane properties. Direct modeling of the exact microstructure from scanning electron microscope visualization provides a precise knowledge of the mechanics and the failure modes of the microstructure under various loading conditions. Modeling results are verified by comparison to experimental data. In-plane stiffness and strength are predicted with 90% or better accuracy. Transverse shear stiffness was less well predicted, but strength was still predicted within 86% accuracy.

Dynamic compression behavior of integrated core sandwich composites

Sandwich composites are widely used in many structural applications due to their superior bending stiffness, low weight and excellent thermal insulation, etc. Many innovative improvements have been made in the construction of sandwich composites in recent years to improve their impact resistance. Evaluation of dynamic properties of such laminates is of vital importance to the designer. In the current study, foam filled 3D integrated core sandwich composite laminates with and without additional face sheets were fabricated using vacuum assisted resin infusion molding process in multiple steps. For additional facing, plain weave S2-glass and twill weave carbon fabrics were used on top and bottom sides of the panels in four different monolithic (S2-glass only, carbon only) and hybrid (glass + carbon, one with S2-glass layer outside and other with carbon layer outside) combinations. High strain rate (HSR) response of integrated core sandwich composites was determined using a split Hopkinson pressure bar setup (SHPB) under compression loading at strain rates ranging from 742 to 1546 s −1. Dynamic response of different types of samples was compared in terms of the peak stress, modulus, strain at peak stress and failure modes.

Studies on Stitched Woven S2 Glass/Epoxy Laminates under Low Velocity and Ballistic Impact Loading

Experimental investigations were carried out to study the response of stitched S2-Glass/SC-15 Epoxy composites under low and ballistic impact loading. Laminates were fabricated using plain weave S2-Glass fabric with SC-15 resin system employing low-cost Vacuum Assisted Resin Infusion Molding (VARIM) process. Both unstitched and stitched laminates were considered. For stitching a 3-cord Kevlar thread was used. Dry fabric preforms were stitched through the thickness to form square grids of size 25.4 and 12.7 mm. Low velocity impact tests were carried out on 5 layer stitched and unstitched laminates of size 90× 90 mm at energy ranging 10-80 J using an instrumented impact testing machine. Transient responses of the laminates were measured and analyzed. For ballistic impact testing, laminates of size 300×300 mm were tested using a gas-gun facility. The panels were impacted using sabot-assisted fragment simulating projectiles. Energy absorbed by the laminates was calculating by measuring inlet and exit velocities. All the samples considered in the study were nondestructively evaluated using ultrasonics for their damage size. Results of the study indicate that stitching considerably enhanced the damage resistance of the laminates by restricting the size of damage as also improving ballistic limits.

Manufacturing and low-velocity impact characterization of foam filled 3-D integrated core sandwich composites with hybrid face sheets

In the current study, foam filled 3-D integrated core sandwich composite laminates with and without additional face sheets were fabricated using vacuum assisted resin infusion molding process in multiple steps. For additional facing, plain weave S2-glass and twill weave carbon fabrics were used on top and bottom sides of the panels in four different monolithic (S2-glass only, carbon only) and hybrid (glass + carbon, one with S2-glass layer outside and other with carbon layer outside) combinations. Square samples of 100 mm sides were subjected to low-velocity impact loading using an instrumented impact testing machine at three energy levels of 15, 30 and 45 J. Three samples were tested at each energy level. Impact parameters like peak load, time to peak load, deflection at peak load, and absorbed energy were evaluated and compared for different types of laminates. Failure modes were studied by sectioning the samples and observing under optical microscope. Results of the study indicate that the provision of additional face sheets considerably enhances the damage resistance. Of different facings, glass facing sustained maximum load followed by glass–carbon hybrid, carbon–glass hybrid, and carbon facings.

On the Response of Stitched Woven S2-Glass/Epoxy Composites to High Strain Rate Compression Loading

Experimental investigations were carried out to understand the response of stitched/unstitched S2-Glass/SC-15 epoxy composites to high strain rate compression loading using a Split Hopkinson Pressure Bar set-up. Fifteen layers of plain weave S2-glass fabric were used to fabricate stitched/unstitched laminates with a low viscosity room temperature curing SC-15 epoxy resin system. For stitching, a 3-cord Kevlar thread was used. The fabric preforms were stitched through the thickness in a lock-stitch pattern, maintaining a pitch of 6 mm. The fabric was stitched along the length and width, forming a 25.4 mm stitch grid. Four different types of samples were considered. Sample configuration was designated according to the location of the stitch line with reference to the loading direction. Dynamic and static responses of the laminate under compression were compared. Failure modes were studied using optical microscopy. For high strain rate loading, samples of dimension 22.5 × 22.5 × 9.5 mm were tested at five different pressure settings of the Hopkinson Bar Set-up. Results were analysed in terms of peak stress, strain at peak stress, modulus and failure modes. Results of the study showed that, under higher strain rates loading, the stitched laminates exhibited a confining effect. The primary mode of failure for unstitched samples was fibre microbuckling with multiple delaminations. For stitched samples, fibre microbuckling and micro-delaminations were the major modes of failure and were contained within the bounds of the stitch. Overall, stitched laminates will provide increased damage resistance, which will prove very crucial under crash situations.

Manufacturing and low-velocity impact characterization of hollow integrated core sandwich composites with hybrid face sheets

In the current study, 3-D hollow integrated core sandwich composite laminates with and without additional face sheets were fabricated using vacuum assisted resin infusion molding process in multiple steps. For additional facing, plain weave S2-glass and twill weave carbon fabrics were used on top and bottom sides of the panels in four different monolithic (S2-glass only, carbon only) and hybrid (glass + carbon, one with S2-glass layer outside and other with carbon layer outside) combinations. Square samples of 100 mm sides were subjected to low-velocity impact loading using an instrumented impact testing machine at 3 energy levels of 15, 30 and 45 J. Three samples were tested at each energy level. Impact parameters like peak load, time to peak load, deflection at peak load, and absorbed energy were evaluated and compared for different types of laminates. Failure modes were studied by sectioning the samples and observing under optical microscope. Results of the study indicate that the provision of additional face sheets considerably enhances the damage resistance. Of different facings, glass facing sustained maximum load followed by glass–carbon hybrid, carbon–glass hybrid, and carbon facings.

Experimental investigations on the response of stitched/unstitched woven S2-glass/SC15 epoxy composites under single and repeated low velocity impact loading

In the current investigation, damage resistance of stitched/unstitched S2-glass/epoxy composites is studied. Five layer stitched/unstitched plain weave S2-glass woven fabric composite laminates were manufactured using toughened SC15 epoxy resin system. For stitching, two configurations: one with 25.4 mm grid and other with 12.7 mm grid, were used with 6 mm pitch. Damage resistance was evaluated by subjecting 100 × 100 mm samples to low velocity impact loading at energy levels ranging 10–80 J using DYNATUP Model 8210, an instrumented impact test system. Three samples were tested at each energy level. The extent of damage was evaluated using ultrasonic C-scan. Results of the study indicate that stitching confined the damage size. As the damage was low over most of the energy range, the study was further extended to determine the effect of repeated impact loading. Under this study, laminates were subjected to repeated impact loading up to maximum of 40 impacts at energy levels ranging from 10 to 50 J. Results of repeated impact study are reported in terms of peak load, absorbed energy and projected damage area. All the laminates, sustained repeated impact loading up to 30 J. Beyond 30 J, laminates failed to carry repeated loading beyond certain numbers, which depended on the laminate configuration.

Studies on the Influence of Through-the-Thickness Reinforcement on Low-Velocity and High Strain Rate Response of Woven S2-Glass/Vinyl Ester Composite

Due to their inherent weakness in the thickness direction, laminated fiber reinforced composites are susceptible to undergo large delamination damage as well as splitting when subjected to transverse loading and microbuckling under in-plane compressive loading. In the current work, an effort is made to improve the transverse strength by providing discrete 3D reinforcement in the form of pins and stitching the laminate in the thickness direction. The effectiveness of the 3D reinforcement is compared with 2D-Plain laminates. The laminates were made of 15 layers of 2 × 2 twill weave S2-glass fabric and vinyl ester C-50 resin system. Specimens of size 75 × 100mmwere subjected to low-velocity impact loading at energy levels of 20, 30 and 40 joules. The effectiveness of 3D-reinforcement to in-plane dynamic loading was studied using a Compression Split Hopkinson’s Pressure Bar at three different strain rates of 327/s, 436/s and 544/s using cubic samples of size 6.0mm. The stitched samples were subjected to high strain rate loading in two configurations. In the first, the sample (referred to as 3D-Stitch1) had stitching along the loading direction at the center while in the second; the sample (referred to as 3D-Stitch2) had an additional stitch line along the width at the center. The results of the study indicate that, reinforcement using stitching confines the delamination growth under low-velocity impact loading. Reinforcement in the form of pins did not show considerable improvement in the damage containment. However, under high strain rate compressive loading, 3D-Stitch1 and 3D-Pin reinforced laminates demonstrated higher compressive strengths at the strain rates of 327/s and 436/s. Stitch2 and 2D-Plain samples, in comparison, exhibited lower strength. Both 2D-Plain and 3D-Pin reinforced laminates exhibited increasing strength with the increase in the strain rate. Whereas, stitched laminates exhibited increase in strength from the strain rate of 327/s to 436/s before dropping in magnitude at 544/s. Thiswas attributed to the localized damage in the fabric due to piercing of the needle during stitching resulting in resin pooling.

Assessment of flow and cure monitoring using direct current andalternating current sensing in vacuum-assisted resin transfer molding

Vacuum-assisted resin transfer molding (VARTM) is an emergingmanufacturing technique that holds promise as an affordable alternative totraditional autoclave molding and automated fiber placement for producinglarge-scale structural parts. In VARTM, the fibrous preform is laid on asingle-sided tool, which is then bagged along with the infusion and vacuumlines. The resin is then infused through the preform, which causessimultaneous wetting in its in-plane and transverse directions. An effectivesensing technique is essential so that comprehensive information pertaining tothe wetting of the preform, arrival of resin at various locations, curegradients associated with thickness and presence of dry spots may bemonitored. In the current work, direct current (dc) and alternating currentsensing/monitoring techniques were adopted for developing a systematicunderstanding of the resin position and cure on plain weave S2-glass preformswith Dow Derakane vinyl ester VE 411-350, Shell EPON RSL 2704/2705 and Si-ANepoxy as the matrix systems. A SMARTweave dc sensing system was utilized toconduct parametric studies: (a) to compare the flow and cure of resin throughthe stitched and non-stitched preforms; (b) to investigate the influence ofsensor positioning, i.e. top, middle and bottom layers; and (c) toinvestigate the influence of positioning of the process accessories, i.e. resin infusion point and vacuum point on the composite panel. The SMARTweavesystem was found to be sensitive to all the parametric variations introducedin the study. Furthermore, the results obtained from the SMARTweave systemwere compared to the cure monitoring studies conducted by using embeddedinterdigitated (IDEX) dielectric sensors. The results indicate that SMARTweavesensing was a viable alternative to obtaining resin position and cure, and wasmore superior in terms of obtaining global information, in contrast to thelocalized dielectric sensing approach.

Static and High Strain Rate Compression Response of Thick Section Twill Weave S-2 Glass/Vinyl Ester Composites Manufactured by Affordable Liquid Molding Processes

Fiber reinforced composites, due to their higher specific strength and specific stiffness, are replacing many metallic structures. Of these, thick composite laminates are of high interest in various, millitary, transportation and marine applications for their use in ballistic and shock protection. One such application is in Composite Armored Vehicle (CAV) integral armor comprising of thick section composite that serves as the primary load-bearing component. The current solution of the structural backing laminate utilizes an S2-glass/epoxy system processed using automated fiber placement method. While proven structurally suitable, this method is time consuming as well as expensive. This paper presents several alternative cost-effective manufacturing solutions for fabricating composite laminates of 20 mm (0.8 in.) nominal thickness (made of 45 layer, 2 × 2 twill weave S2-glass with 933 sizing/vinyl ester C-50 resin), consisted with them CAV application in focus. They include Vacuum Assisted Resin Transfer Molding (VARTM) and Vacuum Assisted Resin Infusion Modeling (VARIM) and their variations. The effectiveness of different affordable processing approaches adopted in fabricating the structural laminate is compared in terms of static and dynamic compression response of the laminations. Static studies have been conducted on thick composites using specimen based on Army Material Technology Laboratory’s (AMTL) recommendation for thick section composites, while dynamic response is studied on cubic specimen samples using a Split Hopkinson Pressure Bar (SHPB).