Hassan Mahfuz

Fatigue crack growth and life prediction of foam core sandwich composites under flexural loading

Abstract Fatigue crack growth of foam core sandwich beams loaded in flexure has been investigated. Sandwich panels were manufactured using an innovative co-injection resin transfer molding process. S2-glass fiber with epoxy resins was used as face sheets over a PVC foam core. Testing was performed in a three-point flexure mode utilizing a newly designed fixture such that the localized indentation damage was minimal. Extensive fatigue data were generated for the S – N diagram and crack growth was monitored to develop a model for life prediction. The first visible sign of damage initiation was a core–skin debond parallel to the beam axis. This debond propagated slowly along the top interface and eventually kinked into the core as shear crack and then grew in an unstable manner resulting in total specimen collapse. A fatigue model based on this crack growth has been developed and validated with experiments.

Enhancement of strength and stiffness of Nylon 6 filaments through carbon nanotubes reinforcement

We report a method to fabricate carbon nanotube reinforced Nylon filaments through an extrusion process. In this process, Nylon 6 and multiwalled carbon nanotubes (MWCNT) are first dry mixed and then extruded in the form of continuous filaments by a single screw extrusion method. Thermo gravimetric analysis (TGA) and differential scanning calorimetry (DSC) studies have indicated that there is a moderate increase in Tg without a discernible shift in the melting endotherm. Tensile tests on single filaments have demonstrated that Young’s modulus and strength of the nanophased filaments have increased by 220% and 164%, respectively with the addition of only 1wt.% MWCNTs. SEM studies and micromechanics based calculations have shown that the alignment of MWCNTs in the filaments, and high interfacial shear strength between the matrix and the nanotube reinforcement was responsible for such a dramatic improvement in properties.

High Strain-Rate Behavior of Plain-Weave S-2 Glass/Vinyl Ester Composites

Thick-section composites made from plain-weave S-2 glass fabric (24 oz./sq. yard) and vinyl ester (411-C50) resin have been tested over a wide range of strain-rates (200-1600 s -1) using a compression split Hopkinson pressure bar (SHPB) with a momentum trapping device. Experiments were performed in two material directions: thickness and fill. Three different types of specimens having rectangular cross sections were tested with thickness ranging from 3.8 mm to 12.7 mm. The strain-rate effects on maximum stresses and maximum non-linear strains have been characterized. The dominant failure modes of the material have been determined through optical and scanning electron microscopy (OM and SEM). It has been identified that the dynamic ultimate stress and failure strain is higher than the corresponding quasi-static values. The ultimate stress is found rate insensitive for both thickness and fill direction loading. The non-linear failure strain is also found to be rate insensitive in the case of thickness direction loading; however, the failure strain increases with strain-rate in the case of fill direction loading. The dominant dynamic failure modes in thickness direction loading are compressive matrix cracking, fiber breakage, and lateral flow of fiber bundles. In the fill direction loading, the dynamic failure modes are kink band formation, delamination, transverse matrix cracking, and longitudinal splitting.

MANUFACTURING AND CHARACTERIZATION OF CARBON NANOTUBE/POLYETHYLENE COMPOSITES

The present study describes a method to fabricate polymer matrix nanocomposites by reinforcing multi-walled carbon nanotubes through an extrusion process. Linear low density polyethylene (LLDPE) powder and multi-walled carbon nanotubes (CNTs) are first dry mixed and extruded in the form of filaments by a single screw extrusion process. After extrusion, the filament is partially cooled by chilled air, dried, and continuously wound in a spool. The filaments are then laid in roving, stacked in a unidirectional fashion, and consolidated in a compression molding machine to come up with laminated composites. Thermo gravimetric analysis (TGA) has been performed to compare the thermal stability of as-fabricated composites with the neat polymer. The TGA result shows that the extruded composites are thermally more stable than their neat counterparts. The crystalline nature of CNTs and of as-fabricated composites were identified by X-ray diffraction (XRD) studies. The XRD results indicate that the nanocomposite materials are more crystalline than the neat systems, and the differential scanning calorimetry studies also confirmed the same trend. The scanning electron microscopy result showed that the sizes of extruded neat and nanophased filaments were about 117 and 73 µm, respectively. Tensile coupons from the consolidated panels were then extracted both in longitudinal (0◦) and in transverse (90◦) directions and tested in a Minimat Tester. It was found that with the addition of 2% by weight of CNTs in LLDPE, the tensile strength and modulus of the composite has increased by about 34 and 38%, respectively. The (0◦ )a nd (90 ◦) coupons have also demonstrated that there are directional effects in the tensile response, which is believed to have been caused by the alignment of CNTs during the extrusion process. It is our understanding that such improvement in properties is because of the increase in crystallinity of the polymer due to CNT infusion, and also due to the alignment of CNTs in the extrusion direction in the nanocomposites.

Dynamic mechanical analyses and flexural fatigue of PVC foams

Abstract Flexural fatigue tests were performed on cross-linked PVC foams of densities in the range from 75 to 300 kg/m3 at a frequency of 3 Hz and at a stress ratio, R=0.1. S–N diagrams were generated, and the failure mechanisms were examined. The fatigue behavior was found to be similar to structural materials with a fatigue strength that increased with increased foam density. The final failure event was catastrophic due to crack propagation initiating at the tension side of the specimen. SEM analyses performed on the R300 foam specimen revealed cell wall cracking and densification of the foam during the earlier part of the fatigue life. This densification contributed to stiffening of foam specimens. Viscoelastic parameters of the foams were determined using a dynamic mechanical analyzer over a frequency range of 1–10 Hz. For the virgin specimens it was found that the viscoelastic moduli and damping ratio were quite independent of frequency over this range of frequencies. Viscoelastic parameters were also extracted at intermediate stages of fatigue life and the residual properties were studied with respect to the number of fatigue cycles. It was observed that the variation of dynamic mechanical analysis (DMA) parameters with the number of fatigue cycles was consistent with the variation of residual global stiffness of the material. Experimental details and the analyses of fatigue behavior are presented in this paper.

Enhanced stab resistance of armor composites with functionalized silica nanoparticles

Traditionally shear thickening fluid (STF) reinforced with Kevlar has been used to develop flexible armor. At the core of the STF-Kevlar composites is a mixture of polyethylene glycol (PEG) and silica particles. This mixture is often known as STF and is consisted of approximately 45 wt % PEG and 55 wt % silica. During rheological tests, STF shows instantaneous spike in viscosity above a critical shear rate. Fabrication of STF-Kevlar composites requires preparation of STF, dilution with ethanol, and then impregnation with Kevlar. In the current approach, nanoscale silica particles were dispersed directly into a mixture of PEG and ethanol through a sonic cavitation process. Two types of silica nanoparticles were used in the investigation: 30 nm crystalline silica and 7 nm amorphous silica. The admixture was then reinforced with Kevlar fabric to produce flexible armor composites. In the next step, silica particles are functionalized with a silane coupling agent to enhance bonding between silica and PEG. The ...

Dynamic compression of cellular cores: temperature and strain rate effects

Abstract Cross-linked polyvinyl chloride closed-cell foams were examined under quasi-static and high strain rate compression loading using a servo-hydraulic testing machine and a modified split Hopkinson pressure bar apparatus consisting of polycarbonate bars for strain rates up to 1900 s −1 . Three foam densities were examined viz. 75, 130, and 300 kg/m 3 . Each core density has been subjected to compressive loading at room and elevated temperatures. A reverse trend in failure modes was observed when moving from room to elevated temperatures at high strain loading, which was not found in quasi-static testing at elevated temperatures. Accordingly, post-impact tests were conducted to evaluate the residual strength of the foam cores subject to elevated temperatures and HSR. Results of the post-impact test revealed that the foam cores are still capable of taking some loading. The residual strength of cores was fairly constant regardless of temperature therefore recovery of volume does not signify an increase in residual strength of cores.

Low Velocity Impact Response of Resin Infusion Molded Foam Filled Honeycomb Sandwich Composites

In this study the low-velocity impact and post-impact response of low-cost resin infusion molded sandwich composites utilizing a foam filled honeycomb core with graphite and S2-glass fabric facesheets (skins) has been investigated. The foam filled honeycomb core provides combined advantages of the traditional foam core and honeycomb sandwich composites in that it possesses high shear and bending stiffness, and cell wall stability. The low velocity impact response of 101.6 mm x 101.6 mm sandwich plates is studied at five energy levels representative of damage initiation and propagation. The low velocity damage is correlated to ultrasonic C-scan images, vibration resonance frequency and optical microscopy observations. The results indicate that the damage tolerance is enhanced by the foam filled honeycomb core and that load required to initiate damage is independent of the facesheet type for any specific core/facesheet thickness. The sandwich composites with S2-glass facesheets are found to possess more damage tolerance as compared to the graphite facesheets.

Effect of processing conditions and material properties on the debond fracture toughness of foam-core sandwich composites: experimental optimization

Abstract The structural performance and reliability of the foam-core sandwich composites are known to be dependent on the strength of the core–skin bonding. Mechanical tests have repeatedly demonstrated that the failure modes for the sandwich during flexural, compression, and tension loading are first triggered by the failure of the interface or the sub-interface zones between the core and the skin. Once this failure mode sets in, core shear and delamination progress rapidly, leading to the final failure of the sandwich construction. The strength of the core–skin bonding depends on the chemical reactions taking place during the cure process. The effect of processing parameters and material properties on the core–skin bonding strength were investigated experimentally. The skin–core debond fracture toughness was measured using Tilted Sandwich Debond specimens. Verifying the heuristics developed in the previous part of this paper [Srinivasagupta et al., Compos. Part A, in press], we achieved a 78% increase in debond fracture toughness with elevated temperature processing, and observed reduced variability with higher suction pressures. We also saw increase in debond fracture toughness with foam density, validating the assumption that interfacial bonding controls the debond fracture toughness. An increase in resin uptake with foam density was an interesting observation from these experiments.

Strain rate effects on sandwich core materials: An experimental and analytical investigation

Abstract Poly-vinyl chloride (PVC) based closed-cell foams were tested at different strain rate under compression loading ranging from 130s–1750 s −1 using a modified Split Hopkinson Pressure Bar (SHPB) apparatus, consisting of polycarbonate bars. Foams with different density and microstructure were examined. The attainment of stress equilibrium within the specimen at various strain rates was examined. It was found that the stress equilibrium was reached early at lower strain rate as compared to higher strain rate. Both the peak stress and absorbed energy were found to be dependent on foam density and strain rate, although foam density was found to be a more dominating factor. A model based on unit cell geometry of the closed-cell foam was also developed to predict the absorbed energy at high strain rate. The proposed model is found to be promising in predicting the energy absorption during high strain rate loading.