Mahesh Hosur

High strain rate compression response of carbon/epoxy laminate composites

Abstract Composite materials exhibit excellent mechanical properties over metallic materials and hence are increasingly considered for high technology applications. In many practical situations, the structures are subjected to loading at very high strain rates. Material and structural response vary significantly under such loading as compared to static loading. A structure that is expected to perform under dynamic loading conditions, if designed with the static properties, might be too conservative. Hence, it is necessary to characterize the advanced composites under high strain rate loading. In the current investigations, the response of carbon/epoxy laminated composites under high strain rate compression loading is considered using a modified split Hopkinson Pressure Bar (SHPB) setup at three different strain rates of 82, 164 and 817 s −1 . The laminates were fabricated using 32 plies of a DA 4518 unidirectional carbon/epoxy prepreg system. Both unidirectional and cross-ply laminates were considered for the study. In the case of cross-ply laminates, the samples were tested in the thickness as well as in the in-plane direction. The unidirectional laminate samples were subjected to loading along 0° and 90° directions. Dynamic stress–strain plot was obtained for each sample and compared with the static compression test result. The results of the study indicate that the dynamic strength (with the exception of through the thickness loading of cross-ply laminates) and stiffness exhibit considerable increase as compared to the static values within the tested range of strain rates.

Effect of projectile shape during ballistic perforation of VARTM carbon/epoxy composite panels

The use of carbon/epoxy composites in aircraft, marine, and automotive structural applications is steadily increasing. Robust composite structures processed using low-cost techniques with the purpose of sustaining high velocity impact loads from various threats are of great interest. An example of a low-cost process is the out-of-autoclave, vacuum assisted resin transfer molding (VARTM) technique. The present study evaluates the perforation and damage evolution created by various projectile geometries in VARTM processed carbon/epoxy laminates. A series of ballistic impact tests have been performed on satin weave carbon/epoxy laminates of 3.2 and 6.5 mm thickness, with projectile geometries representing hemispherical, conical, fragment simulating and flat tip. A gas-gun with a sabot stripper mechanism was employed to impact the samples with 50-caliber projectiles of the different shapes. The perforation mechanism, ballistic limit, and damage evolution of each laminate has been studied. The influence of projectile shape in the VARTM carbon/epoxy laminates under high velocity impact followed the analytical predictions by Wen [Compos. Struct. 49 (2000) 321; Compos. Sci. Technol. 61 (2001) 1163]. The conical shaped projectile resulted in highest ballistic limit, followed by the flat, hemispherical and the fragment simulating.

Effect of stitching and weave architecture on the high strain rate compression response of affordable woven carbon/epoxy composites

Abstract In this study, experimental investigations on stitched and unstitched woven carbon/epoxy laminates under high strain rate compression loading are discussed. Stitched/unstitched laminates are fabricated with aerospace grade plain and satin weave fabrics with room temperature curing SC-15 epoxy resin using affordable vacuum assisted resin infusion molding process. The samples are subjected to high strain rate loading using modified compression split Hopkinson’s pressure bar at three different strain rates ranging from 320 to 1149 s−1. Results are discussed in terms of unstitched/stitched configuration, fabric type and loading directions. Dynamic compression properties are compared with those of static loading. Failure mechanisms are characterized through optical and scanning microscopy.

Low-velocity impact response and ultrasonic NDE of woven carbon/ epoxy-nanoclay nanocomposites

In the present study, Nanomer® I-28E, organically modified montmorillonite nanoclay supplied by Nanocor Inc., was used to modify SC-15, a toughened epoxy system using sonication route. Different weight percentage ranging from 1—3% of nanoclay was used. The modified epoxy was then used to fabricate 15-layer plain weave carbon/epoxy composite laminates using a vacuum assisted resin transfer molding (VARTM) method. Samples of size 100 × 100 mm were cut from the laminates and were subjected to low-velocity impact loading using an instrumented drop-weight system (Dynatup Model 8210) at three different energy levels of 10, 20 and 30 J. Transient response of the samples was recorded and analyzed in terms to load-energy vs time relations. Impact damage was characterized by utilizing an ultrasonic nondestructive evaluation system (c-scan). Results of the study indicate that the infusion of nanoclay in the system reduced the impact damage though the impact response in terms of the peak load remained mostly unaltered. Reduced damage size was attributed to increased stiffness and resistance to damage progression of the nanophased laminates.

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

Vacuum-assisted resin transfer molding (VARTM) is an emerging manufacturing technique that holds promise as an affordable alternative to traditional autoclave molding and automated fiber placement for producing large-scale structural parts. In VARTM, the fibrous preform is laid on a single-sided tool, which is then bagged along with the infusion and vacuum lines. The resin is then infused through the preform, which causes simultaneous wetting in its in-plane and transverse directions. An effective sensing technique is essential so that comprehensive information pertaining to the wetting of the preform, arrival of resin at various locations, cure gradients associated with thickness and presence of dry spots may be monitored. In the current work, direct current (dc) and alternating current sensing/monitoring techniques were adopted for developing a systematic understanding of the resin position and cure on plain weave S2-glass preforms with Dow Derakane vinyl ester VE 411-350, Shell EPON RSL 2704/2705 and Si-AN epoxy as the matrix systems. A SMARTweave dc sensing system was utilized to conduct parametric studies: (a) to compare the flow and cure of resin through the stitched and non-stitched preforms; (b) to investigate the influence of sensor positioning, i.e. top, middle and bottom layers; and (c) to investigate the influence of positioning of the process accessories, i.e. resin infusion point and vacuum point on the composite panel. The SMARTweave system was found to be sensitive to all the parametric variations introduced in the study. Furthermore, the results obtained from the SMARTweave system were compared to the cure monitoring studies conducted by using embedded interdigitated (IDEX) dielectric sensors. The results indicate that SMARTweave sensing was a viable alternative to obtaining resin position and cure, and was more superior in terms of obtaining global information, in contrast to the localized dielectric sensing approach.

Studies on impact damage resistance of affordable stitched woven carbon/epoxy composite laminates

This paper discusses the response of seven layer plain and satin weave carbon fabric reinforced composites fabricated using low-cost Vacuum Assisted Resin Infusion Molding (VARIM) process under low-velocity impact loading. Both stitched and unstitched laminates were tested at energy levels ranging 5-50 J using an instrumented drop-weight machine. A 3-cord Kevlar thread was used to stitch the laminate in two orthogonal grid patterns each at a 6 mm pitch: one with 25.4 mm and the other with 12.7 mm grid. Damage due to impact loading was evaluated through ultrasonic nondestructive evaluation (NDE). Results of the study showed the effectiveness of stitching in containing the damage size with 12.7 mm grid stitch samples exhibiting the least damage. Further, satin weave fabric composites exhibit better impact resistance as compared to plain weave fabric composites.

Studies on the off-axis high strain rate compression loading of satin weave carbon/epoxy composites

Abstract This paper discusses the experimental study on the response of affordable satin weave carbon/epoxy composite laminates subjected to high strain rate compression loading using a modified compression split Hopkinson’s pressure bar (SHPB) under off-axis loading. Thirty seven layer laminates were manufactured using aerospace grade woven fabrics with SC-15 epoxy resin system utilizing vacuum assisted resin infusion molding approach. Samples were subjected to high strain rate compression loading at strain rates ranging from 1092/s to 2425/s using a modified SHPB that facilitates controlled single pulse loading of the sample. Samples were tested in the inplane direction along warp (0°) and weft/fill (90°) and 15°, 30°, 45°, 60° and 75° off-axes angles. Quasi-static tests were carried out to compare with the dynamic response. Failure modes were evaluated using optical micrographs. Results of the study were analyzed in terms of peak stress, strain at peak stress, failure modes and orientation.

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.

Time Effects on Morphology and Bonding Ability in Mercerized Natural Fibers for Composite Reinforcement

Properties of cellulose-derived fibers are extremely sensitive to surface treatment. Many studies have investigated the effects of varying surface treatment parameters in natural fibers to improve fiber-matrix bonding; however, work is still needed to assist with developing better quality control methods to use these fibers in more load-bearing composites. Kenaf fibers were alkali treated, and the surface and morphology were analyzed to determine how treatment time affected the bonding sites in natural fibers. The mechanical behavior was also characterized, and tensile testing reported a 61% increase in strength and a 25% increase in modulus in fibers treated for 16 hours. The increase in tensile properties was assumed to result from increased intermolecular interaction and increased crystallinity in cellulose, which was supported by XRD. On the other hand, FTIR spectroscopy and XPS showed that the amount of hydroxyl groups needed for fiber-matrix bonding decreased at longer treatment times.

Experimental studies on the high strain rate compression response of woven graphite/epoxy composites at room and elevated temperatures

In this study, experimental investigations on affordable woven graphite/epoxy laminates under high strain rate compression loading at room and elevated temperatures are discussed. 17-layered woven graphite/epoxy laminates are fabricated with plain and satin weave fabrics with room temperature curing SC-15 epoxy resin using affordable vacuum assisted resin infusion molding (VARIM) process. Samples were tested at strain rates ranging from 200 to 1100/s at four different temperatures: room, 125, 175, and 225 F. Upper limit on the temperature was selected based on the supplier’s data sheet for SC-15 epoxy resin system, which has a dry glass transition temperature of 220 F. Failure mechanisms were characterized through optical microscopy. Failure modes were influenced by the temperature and fabric architecture. Results of the study indicate the softening of fiber–matrix interface with increasing temperature, which affects the dynamic compression strength. Satin weave samples exhibit higher compressive strength as compared to plain weave samples due to straighter fabric architecture.