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A comprehensive experimental investigation is performed to understand the damage sensing capabilities of thermoplastic intra-ply carbon/glass laminated composites embedded with Carbon Nanotubes (CNTs) under quasi-static interlaminar shear and mode-I fracture loading conditions. CNTs are dispersed in thermoplastic Elium® resin using a combination of ultrasonication and shear mixing processes. This resin mix is used in a vacuum infusion process to fabricate four different intra-ply laminated composites orientations; G-0-C-90 [Carbon fibers are in the longitudinal direction], G-90-C-0 [Glass fibers are in the longitudinal direction], G-45-C-45(R) [carbon and glass fibers are at +45°, and −45° from the longitudinal direction and form a repeating layup], and G-45-C-45(A) [carbon and glass fibers are at +45°, and −45°, but with alternating directions for each lamina in the layup]. Four-circumferential probes are utilized to obtain the piezo-resistance measurements associated with damage inside the composites under shear and mode-I loadings. In the shear experimentation, the G-45-C-45(A) orientation demonstrated the largest interlaminar shear strength, at 50% greater than the G-45-C-45(R) orientation. Both G-45-C-45(A) and G-45-C-45(R) had significantly larger shear strains compared to the G-90-C-0 and G-0-C-90 orientations. However, the best resolution in shear damage sensing was seen with the G-0-C-90 composites. In the mode-I fracture experiments, the greatest fracture initiation toughness is shown with the G-0-C-90 orientation, which is 80% greater than the G-90-C-0 orientation. Compared to all orientations, the G-0-C-90 orientation also provided the most sensitive electrical response.

An experimental study is preformed to investigate the in-situ damage sensing capabilities of intra-ply hybrid carbon/glass laminate and epoxy composites under quasi-static interlaminar shear loading. A three-dimensional electrical sensory network is generated inside the composites through embedded carbon nanotubes (CNTs) in an epoxy matrix along with the carbon fibers in the intra-ply hybrid laminates. CNTs are dispersed in the epoxy matrix using a combination of ultrasonication and shear mixing techniques. Four circumferential ring probes are used to examine the electrical response under interlaminar shear load. The effect of four different intra-ply orientations (((0–90)C, where carbon fibers are oriented along the loading direction), ((0–90)G, where glass fibers are oriented along the loading direction), ((45/−45, where glass and carbon fibers are oriented at 45o/−45o and the laminates are repeated), and ((45/−45)A, where glass and carbon fibers are oriented at 45o/−45o and the laminates are alternated)) on the shear constitutive behavior and the damage detection are discussed. Intra-ply orientations of (45/−45) and (45/−45)A showed higher interlaminar shear strength and shear strain at break compared to (0/90)C and (0/90)G orientations. Out of all four orientations, (45/−45)A provided a better resolution of electrical response for damage sensing applications.

A systematic study is reported on applying flocked energy-absorbing materials (FEAM) to designing sport and military helmet pad structures. An executed parametric study shows that the impact force absorbing (IFA) properties of FEAM elements are optimized when using (a) higher denier flock fiber (60 to 100 denier) and (b) longer flock fibers (3 to 4 mm length) at higher flock densities. Continuing work focuses on the importance of IFA/areal density ratios in helmet pad functional design. It is found that foam materials like vinyl-nitrile and ethylene vinyl acetate (EVA) inherently exhibit higher IFA/areal density (IFA/AD) ratios than FEAM material structures. With this finding, a new strategy for developing sport and military helmet pads was devised involving the combination of foam and FEAM layer elements. Here, the meritorious properties of foam materials (light weight and excellent IFA properties) and the excellent IFA and breathability (wearer comfort, sweat and heat management) properties of FEAM could be favorably encompassed. A plan was conceived and implemented whereby combination foam/FEAM test pads having high amounts of the high IFA/AD ratio VN-600 or EVA foam layer component were impact tested. By gradually introducing the more comfortable, breathable, body-heat managing FEAM layers into the helmet pad structure, some “trade-off” helmet pad configurations were designed and evaluated. Experiments showed that helmet pad designs having not more than 40% to 50% FEAM content should produce adequate IFA/AD ratio “trade-off” property helmet pad configurations.

An experimental study was conducted to investigate the electro-flexure response of conductive natural fiber hybrid laminate composites. The composites were composed of laminates of jute and flax fibers, and the composites were subjected to flexural loading for the electrical and bending response. Multi-walled carbon nanotubes were shear mixed and ultrasonicated into the epoxy matrix. Short carbon fibers were reinforced in-between the laminates using “wet flocking” technique. To measure the electrical response under flexural loading, a four-point circumferential probe method was used. A parametric study was conducted to investigate flexural performance and damage sensing by varying carbon fiber lengths (150 and 350 µm) and the carbon fiber densities (500, 1000, 1500, and 2000 fibers/mm2). The addition of carbon fibers decreased the flexural strength for most of the cases, however increased the flexural strain at break for all composites of carbon fiber length of 150 µm. During the nonlinear deformation, the composites of carbon fiber length of 150 µm demonstrated a linear increase in resistance; however, that of carbon fiber length of 350 µm showed increasing slope of resistance. Overall, the composites of carbon fiber length of 350 µm showed lower resistance change at break compared to that of carbon fiber length of 150 µm.

A detailed experimental study is performed for piezo resistance damage sensing on conductive glass fiber/epoxy composites under mode-I fracture conditions. The conductive composites are fabricated by homogeneously dispersing carbon nanotubes (CNTs) within the epoxy matrix and electro-flocking short carbon fibers onto the laminates along with a vacuum infusion process. A parametric study is done on the in-situ damage sensing properties by varying the carbon fiber lengths (150 µm and 350 µm) and the carbon fiber areal densities (500, 1000, 1500, and 2000 fibers/mm2). The change in resistance is captured with a four-point probe measuring methodology by measuring the resistance through the thickness of the composite. The crack initiation toughness value of the composites containing carbon fibers showed improvement over control composites. Composites containing 350 µm length carbon fibers and 2000 fiber/mm2 not only showed the best crack initiation toughness but also provided sensitive network for detecting crack growth.

Electrical and shear behaviour of electrically conductive glass fibre/epoxy composites is studied under interlaminar shear loading. A well-connected network is developed by dispersing carbo...

An experimental investigation is performed to investigate the effect of post-processing heat treatment on the interfacial fracture toughness of bi-material additively manufactured semi-crystalline polymer composite. An asymmetric double cantilever beam (ADCB) and single-leg bending (SLB) specimens made of polylactic acid (PLA) and Nylon are considered for the mode-I and mixed-mode fracture characterization, respectively. Specimens are isothermally heated in a forced convection oven for a wide range of temperatures and durations. Fracture toughness decreases significantly for both mode-I and mixed-mode conditions when specimens are annealed below the melting temperature of PLA (150 °C). An increase of the crystallinity at the high-temperature annealing prevents the polymer chain mobility, hinders the neck growth, and provides poor intermolecular diffusion resulting in decrease of fracture toughness by 88% as compared to the unannealed specimen. Annealing at 160 °C improves the bi-material interfacial fracture toughness by a maximum of 1225% via sufficient interfacial wetting, higher molecular diffusion, and a longer polymer chain entanglement process. Material transfer, void collapse, and filament impression on the fracture surface of high temperature annealed specimen indicate a better molecular diffusion and strong interlaminar bond at the interface.

The electro-fracture studies on jute/epoxy composites are conducted for monitoring damage under mode-I interlaminar fracture loading condition. A well-connected electrical network is built inside t...

An experimental study was performed to compare the electro-mechanical response of three-dimensionally conductive woven carbon fiber/epoxy laminated composites under quasi-static uniaxial tensile and in-plane shear loading conditions. Three-dimensional (3D) electrical network was generated in these composites by embedding carbon nanotubes (CNTs) of 0.025 wt% in the epoxy and reinforcing short carbon fibers (150 μm and 350 μm long) with a fiber density of 1000 fibers/mm2 between the laminates using electro-flocking process. CNTs are effectively dispersed in the epoxy matrix using a combination of ultrasonication and shear mixing techniques. A compression molding fabrication technique was employed to fabricate composite materials. For all composite types, in general, the in-situ electrical response showed a trend of initial decrease in resistance and later increase in resistance for tensile loading. However, no noticeable increase in electrical resistance was observed until failure of the composite under shear loading conditions. Both CNTs and short carbon fibers made new contacts with neighboring carbon fiber laminates for entire duration of shear loading even though the composite was undergoing progressive failure.

A comprehensive experimental parametric study is performed to investigate the electro-mechanical response of novel three-dimensional conductive multi-functional carbon/epoxy composite materials. Three-dimensional conductive network is generated by embedding multi-wall carbon nanotubes in the epoxy matrix and reinforcing short carbon fibers between the carbon fabric laminates. An open mold compression method is utilized to fabricate the composite materials. Wet electro-up-flocking technology is employed to reinforce 150-µm and 350-µm length carbon fibers vertically at varying densities (500, 1000, 1500, 2000 fibers/mm2) between each laminate to analyze the effects these parameters have on electrical resistivity, tensile properties, and electro-mechanical response to quasi-static tensile loading. A high-resolution four-point circumferential ring probe is used to obtain electrical measurements. The resistivity of the composites having 150 µm flocked carbon fibers did not show significant change with increase in flock density; however, composites of 350-µm length carbon fibers showed a clear decrease in resistivity by a factor of 10 when the flock density increased from 500 to 2000 fibers/mm2. The electro-mechanical response of composites without short carbon fibers is inconsistent and jagged compared to that of flocked composites. The composites having 350-µm long carbon fibers showed a longer duration of initially decreasing resistance due to the applied tensile load when compared to that of 150 µm flocked carbon fibers. However, composites with no short carbon fibers registered maximum value of percentage change in resistance at the break point of tensile loading.

An experimental study is performed to investigate the dynamic fracture of additive manufactured Acrylonitrile Butadiene Styrene (ABS). A single edge notched bending (SENB) specimen with three orientations, namely horizontal builds with 45°/−45° (H45), 0°/90° (H90) raster orientations, and vertical builds with layers perpendicular to the pre-crack (V0) are considered for this study. In addition, a novel toughening mechanism is explored by changing the surface topology to deflect the crack paths. A modified split Hopkinson pressure bar with a copper pulse shaper (to increase the raising time of incident loading pulse) is used to conduct a three-point bend impact experiment to characterize the dynamic fracture initiation toughness and crack dynamics of 3D printed specimens. Real-time crack initiation and propagation is captured by using a high-speed video camera. Using the load history diagram, accurate fracture initiation load is found to determine dynamic fracture initiation toughness. Fracture initiation toughness is increased by 138% for a V0 specimen configuration compared to H90. Three different sized circular patterns (with diameters of 1, 1.75 and 2.5 mm) and a square pattern (with a length of 1.53 mm) are considered to observe the effect of surface topology on the dynamic fracture initiation toughness. Introducing surface pattern to the specimen increases the fracture toughness by 58% as compared to specimens without surface pattern. Surface pattern also exhibits two steps of crack growth and decreases the initial crack propagation velocity significantly for all three orientations. Additionally, higher fracture initiation toughness is achieved with the increase in the size of the pattern and the change of the pattern shape.

An experimental study is performed to investigate the electro-mechanical response of three-dimensionally conductive multi-functional glass fiber/epoxy laminated composites under quasi-static tensile loading. To generate a three-dimensional conductive network within the composites, multi-wall carbon nanotubes are embedded within the epoxy matrix and carbon fibers are reinforced between the glass fiber laminates using an electro-flocking technique. A combination of shear mixing and ultrasonication is employed to disperse carbon nanotubes inside the epoxy matrix. A vacuum infusion process is used to fabricate the laminated composites of two different carbon fiber lengths (150 µm and 350 µm) and four different carbon fiber densities (500, 1000, 1500, 2000 fibers/mm2). A four circumferential probe technique is employed to measure the in-situ electrical resistance of composites under tensile load. Although composites of both carbon fiber lengths showed significant decrease of sheet resistance under no mechanical load conditions, composites of 350 µm long carbon fibers showed the lowest resistivity of 10 Ω/sq. Unlike the resistance values, composites of 350 µm carbon fibers showed a significant decrease in Young’s modulus compared to 150 µm counterparts. For the electro-mechanical response, composites containing carbon fibers of 150 µm long demonstrated a maximum value of percentage change in resistance. These results were then compared to both 350 µm and no added carbon fibers under quasi-static tensile loading.

A custom designed dynamic mechanical apparatus was constructed to measure the tan(δ) (loss tangent) properties of Flocked (layer) Energy Absorbing Materials (FEAM). Flocked Impact Energy Absorbing (IEA) materials derive their dynamic mechanical impact absorbing properties from the compressional deformation (bending , buckling) and inter-fiber friction of nominally upright flocked fibers in the flock fiber composite assembly. The reported work summarizes the dynamic mechanical, low strain, linear viscoelastic tan(δ) properties of FEAM composite layer structures at frequencies from 25 to 100 Hz and a temperature range of 0 °C–50 °C. The effect of tan(δ) at various flock densities (fibers/mm 2) was also measured. These studies were carried out to gain information and insight into the role that a material’s dynamic mechanical properties have on the overall IEA performance of sport (and military) apparel pad structures. Results show that the tan(δ) of FEAM materials increases as the measuring frequency and flocked layer’s flock density increase. Tan(δ)’s behavior at various temperatures (0 °C–50 °C) did not follow an expected trend; the highest tan(δ) value was observed to occur at room temperature. Nevertheless, the obtained tan(δ) information on these various FEAM configurations should provide some guidance in designing optimized FEAM sport (and military) pad IEA composites.

A combination of computational and experimental investigation is performed to study additively manufactured (AM) polymers for enhanced fracture properties. Single edge notch tension specimens made of acrylonitrile-butadienestyrene (ABS) materials through fused deposition modeling with various build/raster orientations are studied, namely, horizontal builds with 45°/−45° (45–45) or 0°/90° (0–90) raster orientations, and vertical builds with layers perpendicular to the notch (V0). The measured fracture properties were found to highly depend on the build/raster orientations and crack kinking was observed in 45–45 samples to follow the weak inter-filament weld-lines. The extended finite element method (XFEM) using cohesive segment approach with anisotropic damage initiation and evolution criteria was developed to capture the dependency of fracture behaviors on build/raster orientations. Numerical parametric studies further show that the inter-filament bonding strength could be tuned to create alternate crack paths for maximum energy dissipated in AM polymer fracture. Finally, toughening mechanisms using topological patterns on the sample surface to deflect crack paths are demonstrated in experiments. This study sheds light on optimization of AM polymers for enhanced fracture properties.

A comprehensive experimental investigation was performed to study the dynamic compressive constitutive response of novel auxetic Kevlar®/epoxy laminated composites. Strain rate response was investigated using the split Hopkinson pressure bar (SHPB) test setup. Laminated composites were fabricated using the vacuum infusion process. Short Nylon fibers were flocked between the laminates with different flock densities and flock length. For obtaining dynamic force equilibrium in SHPB experiments, a copper pulse shaper was used to increase the rising time of incident pulse. To have a comparison, woven Kevlar®/epoxy composites were also characterized at similar strain rates. In addition, quasi-static tests were also performed on both woven and auxetic laminated composites for completeness of the study. For quasi-static loading conditions, auxetic composites showed higher peak strain and lower peak stress compared to woven composites. For non-flocked composites, both auxetic and woven composites showed rate dependency. Woven composites provided 353% increase in peak stress when the strain rate increased from 1200 s−1 (low) to 3300 s−1 (high). However, in the same conditions, auxetic composites showed only 155% increase in peak stress. For different flocking conditions, woven composites showed rate dependency for all strain rates, but auxetic composites demonstrated rate dependency only from low to medium strain rates. Both auxetic and woven composites experienced shear failure under quasi-static compression, where auxetic composites failed at higher shear angle of 37°, but woven composites had a failure angle of 30°. For impact loads, under no flocking condition, woven composites did undergo severe edge failure at all strain rates, but auxetic composites showed a sign of edge failure only at high strain rates. With the flocking condition, auxetic composites had through thickness shear failure and woven composites experienced splitting and fibrillation of Kevlar® fibers.

A comprehensive experimental investigation was performed to understand damage detection capabilities of intra-ply glass/carbon composites embedded with carbon nanotubes under mode-I and mode-II fracture. Novel modified four circumferential probes were employed on fracture test specimens to capture piezo-electrical response. The effect of three different laminate orientations (G-0-C-90, C-0-G-90, and G-45-C-45) on both mode-I and mode-II fracture initiation toughness and the electrical-fracture response were discussed. C-0-G-90 demonstrated higher mode-I fracture toughness, while G-0-C-90 composites exhibited higher mode-II fracture toughness. The fracture toughness values were related with scanning electron microscopy images and electrical response was correlated well with the crack dynamics.

In this study, a comprehensive experimental investigation is performed to observe the influence of printing process parameters on the interfacial fracture toughness of bi-material additively manufactured composites. A single leg bending (SLB) specimen made of Polylactic acid (PLA) and Nylon is loaded under the quasi-static three-point bending condition to initiate the fracture between the PLA/Nylon interface. Six different process parameters (bed temperature, printing temperature, printing speed, printing orientation, layer height, and thickness ratio) are considered to investigate the fracture toughness. It is observed that process parameters have a significant impact on the bi-material interfacial fracture toughness. Due to the increase of crystallinity of the polymer, the fracture toughness decreases by about 40% when the bed temperature increases from 60 °C to 100 °C. However, with improved molecular diffusion, the fracture toughness is enhanced by 95% with the increase of the printing temperature. Although printing speed has not any significant impact on fracture toughness, thinner layers provide a better bond strength and polymer wetting, resulting in a higher fracture initiation toughness compared to thicker layers. Due to the presence of extra voids and anisotropic behavior, the specimens of 0o/90o raster-orientation demonstrate about 75% lower fracture toughness compared to those of +45o/-45o orientated specimens.

Numerical studies were performed to determine electrical contacts in carbon nanotubes (CNTs)-embedded epoxy under quasi-static tensile loading conditions. A realistic CNT network in epoxy was generated by random orientation and waviness by employing the Monte Carlo method. Two kinds of contacts between CNTs, namely Type-I (overlap) and Type-II (in-plane), were considered in generating a conductive network. A commercial finite element package, COMSOL, was used to determine the distance between these contacts and variation of these contacts during elastic deformation under tensile loading. Numerical predictions were later compared with the analytical results derived from experimental findings. The effect of initial f-ratio and the amount of waviness of CNTs on the variation of f-ratio (which is determined as a ratio of Type-II contacts to total contacts) was investigated as a function of axial strain. The initial f-ratio as well as the extent of waviness showed a significant effect on the change in the type of contacts with increasing axial strain values.

Polyhydroxyalkanoates (PHAs) are a family of biodegradable and biocompatible polymers produced by several species microorganisms that possess favorable mechanical properties (e.g. strength and elongation properties). Different types of PHA polymers have been used in medical applications. However, in order to better understand the use of this polymer in the different applications, a thorough understanding of the kinetics of in vivo degradation is one of the major requirements. In this study, poly(3-hydroxybutyrate) (PHB) was subcutaneously implanted in mice and incubated for 2, 4, 8, or 16 weeks. After removal from the animal, the strength, elongation, mass loss, and enthalpy of the PHB were tested for each time point. From these data, a mathematical model was generated by Rayleigh's method of dimensional analysis, where polymer strength over tissue contact time could be predicted. To prove the model, previous data obtained by our group were used: poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) [P(HB-co-HHx)] incubation in the presence of human embryonic kidney cells (HEK). It was found that the developed model was aligned with experimental results, could predict the strength of the polymer when in contact with cells, and the predicted strength follows the trend of the experimental data. Also, the dimensionless constant (K) value associated with the model is different for both experiments, where this constant, produced via experimental data, is used for construction of a homogeneous equation. Copyright © 2017 John Wiley & Sons, Ltd.

Experimental study was performed to investigate fracture and impact properties of novel Auxetic Kevlar® laminated composites. For comparison, standard Kevlar® woven composites with and without polyurethane treatment were also considered in this study. For these three composites, short nylon fibers of two different fiber lengths and three different fiber densities were flocked between laminates. Vacuum infusion process along with optimized compaction was employed to fabricate composites. The double cantilever beam configuration was used to investigate the fracture properties. The Auxetic Kevlar® composites showed a significant improvement of 225% in fracture toughness compared to regular woven Kevlar® composites. Furthermore, the initiation toughness was increased by 577% with the application of flocking in Auxetic Kevlar®. During impact testing, the Auxetic Kevlar® reinforced composites showed a significant reduction in damaged area compared to woven counterpart. On the other hand, the reduction in damaged area influenced the reduction in impact energy absorption.

Natural fibers are lighter, less expensive, and less abrasive than synthetic fibers such as glass or carbon, and provide a resource that is both renewable and biodegradable. This study uses surface treatments and flock fiber based Z-axis reinforcement technology to improve the strength and fracture toughness properties of natural fiber composites (NFCs), thus increasing their ability to bear loads. The enhancement of performance properties will expand applications of NFCs in industries such as automotive and housing/construction industries. NFCs will also provide a more sustainable, environmentally friendly, and inexpensive structural material alternative over traditional resin reinforced glass or carbon fiber composite materials.

A detailed experimental study was performed on the piezo-resistive damage sensing of novel electrically conductive inter/intra glass/carbon fiber hybrid epoxy laminated composites reinforced with micro carbon fibers along the thickness direction between the laminates using electro-flocking technique. The electrical resistance is determined by four-circumferential probe measurements under tensile and flexure loads. Intralaminar hybrid composites’ mechanical and electrical properties showed a dependency on layup angle, with glass fibers along the loading direction showed the highest initial resistance value (1180 Ω) and ultimate tensile strength (400 MPa). Intra-ply composites with [±45/∓45] layup orientation demonstrated the lowest ultimate tensile strength (57 MPa) and the highest elongation at break (9.7%). Mechanical and electrical properties of interlaminar hybrid composites showed a dependency on the number of carbon fiber laminates. More carbon fiber laminates within the composite layup produces a lower initial resistance value (0.37 Ω) and higher ultimate tensile strength (247 MPa). In both intra and inter composites cases, with the addition of short carbon fibers, the initial resistance decreased. In some cases, with the addition of short carbon fibers the failure mechanisms changed. Composites of all types experienced much higher percentage change in resistance under flexural loading due to severe interlaminar failure compared to corresponding cases under tensile loading conditions. These results are useful for designing composite structures with emphasis on structural health monitoring.

Damage monitoring of conductive glass fiber/epoxy laminated composites is performed by observing the electro-fracture response under dynamic mixed-mode fracture loading. A three-dimensional conductive network is generated in these composites by embedding carbon nanotubes (CNTs) in the matrix and reinforcing short carbon fibers (80 µm, 150 µm, and 350 µm) between the laminates. A modified split Hopkinson pressure bar setup is used to load the mixed-mode open notch flexure (MONF) specimens. A modified four-point probe technique coupled with a high-frequency data acquisition system is employed to capture the electrical response within process zone of the fracture specimens. A high speed video camera is used to capture the dynamic crack propagation. Due to the bending effect, a decreasing trend in resistance change is observed before the damage initiation in composites with no carbon fibers. However, short carbon fiber reinforced composites show a minimal change in resistance before the crack initiation due to less amount of CNTs in them and moreover short carbon fibers did not change their position during elastic deformation. After crack initiation, the rate of increase of resistance is almost five times higher in composites containing short carbon fibers of 80 µm than to those of 150 µm.

Studies have shown that the copolymer poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) [P(HB-co-HHx)] possesses favorable mechanical properties for use in medical supplies and products (e.g., sutures, scaffolds, bone plates). One of the major under-addressed issues associated with the use of biodegradable, bio-based PHA polymers in resorbable medical products is the correlation between the mechanical properties and the in vivo material degradation over time. In this study, P(HB-co-17 mol% HHx) matrices were mechanically tested after either incubation in cultures of human embryonic kidney cells (HEK) for in vitro degradation studies for up to 4 weeks, or inserted into Danio rerio (zebrafish) tissues for in vivo degradation studies for up to 7 weeks. The mechanical properties and scanning electron microscopy (SEM) images of the degraded materials were examined and later correlated to understand the degradation phenomenon. Our results show that Young’s modulus of P(HB-co-17 mol%HHx) during in vitro studies decreased from 3.26 to 2.42 GPa within 4 weeks, and in vivo breakdown resulted in a significant decrease in Young’s modulus with a decrease from 3.26 to 0.51 GPa and a mass loss of 59 % within 7 weeks. SEM images showed the development of pores and cracks on the surface of the material over time. Plasticization and recrystallization were observed and likely play a role in the alteration of mechanical properties.

Microstructural, tensile, and fracture characterizations of cylindrically forged forms of aluminum alloys AA7085-O and AA7175-T7452 were performed. Mechanical and fracture properties were investigated along radial, circumferential, and longitudinal directions to determine directional dependency. American Society for Testing and Materials (ASTM) test methods (ASTM E8-04 and ASTM E1820) were employed for both the tensile and fracture characterizations, respectively. The tensile and fracture properties were related to microstructure in each direction. The strength, elongation at break, and ultimate tensile strength of AA7085-O were higher than those of AA7175-T7452. AA7175-T7452 alloy failed in a brittle manner during fracture studies. AA7085-O outperformed AA7175-T7452 on fracture energy in all of the orientations studied. Smaller grain sizes on the planes normal to circumferential and longitudinal directions showed improvement in both elongation at break and fracture energy values compared to those of radial direction. Scanning electron microscopy images demonstrated cleavage fracture in AA7175-T7452 and transgranular fracture in AA7085-O.

Natural biodegradable sandwich materials were fabricated and mechanical characterization was performed under flexural and impact loading conditions. The sandwich structure consists of hardwood as face plates, mushroom foam as a core, and natural glue as an adhesive. Two different hardwood layers were used to investigate the effect of thickness and the stiffness on the flexural and impact properties. Under impact loads, two different projectile velocities were employed to understand their effect on energy absorption of the sandwich materials. It was found that the sandwich made using high stiffness wood panels showed improved flexural properties as well as maximum energy absorption at higher impact velocities. The damage of front and rear sides of impact specimens was analyzed to understand the effect of impact velocity on damage zone. The damage area on the rear side of the specimen decreased significantly with high stiffness wood face plates at lower projectile velocity. The increase is energy absorption is better supported by the increase in damage area on both front and rear sides of sandwich materials at higher projectile velocity. Both flexural properties and energy absorption demonstrated great promise to use these sandwich structures for structural load bearing applications.

In this study, an experimental investigation is performed to observe the electromechanical response of CB (carbon black)/Acrylonitrile butadiene styrene (ABS) additive manufactured composite under quasi-static (tensile, shear, and mode-I fracture) and dynamic (mode-I fracture) loading conditions for the potential damage sensing applications. Dog bone tensile, double v-notch shear, and single edge notch bending (SENB) specimen printed with three different configurations (0°/90°, +45°/-45°, and 0°) are considered for the quasi-static condition. A modified split Hopkinson pressure bar along with high-speed video camera is used for dynamic fracture experiments. Four-point probe technique coupled with a high-resolution data acquisition system is employed to obtain the real-time electrical response. In the case of tensile loading, +45°/-45° printed specimens show a nonlinear change of electrical resistance due to unique failure mode. Under the shear loading, electrical resistance remains unchanged during the elastic deformation. After the damage evolution, +45°/-45° printed specimens exhibit a higher rate of change in electrical resistance due to alignment of the filaments along the maximum principle shear stress direction. For both static and dynamic fracture loading, a minimal change of electrical resistance is observed before crack initiation. However, after the crack initiation, a sharp change of electrical resistance for 0°/90° printed specimens indicates a faster crack propagation as compared to the +45°/-45° printed specimens.

Theoretical modeling and experimental validation for electro-shear behavior of carbon nanotubes (CNTs) embedded epoxy under quasi-static shear loading are performed. The theoretical modeling incorporates electron tunneling between CNTs and the electrical resistance change due to shear deformation of the epoxy matrix between CNTs. Two different contact types (head to head and overlap) and three different deformation connections (Type-I, Type-II, and Type-III) are considered to model the electrical network. Using the constitutive relations, the model relates the matrix deformation between the CNTs to the macroscale deformation of the composite. To validate the modeling results, experiments are performed using a four circumferential ring probe method. A parametric study is performed to investigate the effect of the total number of initial contact types, ratio of contact types, and number of potential Type-II deformation connections on electrical response. The theoretical predictions are validated with experiments for the first two stages of deformation and an optimal combination of deformation connections are obtained to have a close match with experimental findings.

Experiments are performed to understand the electro-bending behavior of conductive jute/epoxy curved composites for monitoring of damage under four-point bending. A well-connected electrical network in the laminated curved composite is obtained by wet flocking micro carbon fibers between laminates and dispersing carbon nanotubes (CNTs) within the matrix. By embedding only 0.1 wt% of CNTs into the composites, the effects of carbon fiber length and density on electro-bending behavior is studied. The normalized curved beam strength (curved beam strength/thickness) of composites marginally increased for a case of carbon fiber length of 350 µm and fiber density of 500 fibers/mm2 compared to composites with no carbon fibers, however for all other cases, it showed slight decrease. The electro-bending behavior of composites of carbon fiber length of 150 µm showed higher percentage resistance change both at crack initiation point as well as at failure of specimen compared to those of carbon fiber length of 350 µm. The longer fiber (350 µm) composites provided rich conductive network and formed new connections to compensate the increase in resistance due to damage growth.

A comprehensive experimental impact characterization study of novel impact energy absorbing (IEA) materials under impact shear loading is conducted. Electro-flocking process is employed to fabricate the novel fiber based padding materials, also known as flocked energy absorbing materials (FEAM). FEAM IEA panels are prepared by flocking 1–3 mm long, 6–60 denier nylon fibers onto a planar polyester fabric sheet. The impact energy absorption under combined pre-compression loading and shear impact loading is investigated using a custom built guided weight drop tower along with a specially designed pre-compression and shear loading fixture for this study. A parametric study is performed where the effect of fiber material properties such as flock fiber length , diameter and flock density (number of flock fibers per area) on IEA is investigated and they are later compared with that of Vinyl Nitrile (VN) foam materials . Padding material based on FEAM configurations showed remarkable improvement when compared directly to VN foam with a 135% increase in shear strain energy density for the high impact velocity loading condition. Additionally, for low velocity impact conditions, the FEAM based padding materials out performed with a 49% increase in shear strain energy density as compared to VN foam.

Early-stage damage detection could provide better reliability and performance and a longer lifetime of materials while reducing maintenance time of a variety of structures and systems. We investigate the early-stage damage formation and damage evolution in advanced multi-functional laminated aerospace composites embedded with a very small amount of carbon nanotubes (CNTs) in the matrix material and short carbon fibers along the Z-direction to reinforce the interlaminar interfaces. The three-dimensional (3-D) conductive network formed by the CNTs and the flocked carbon fibers allows for sensitive in-situ damage detection in materials in addition to providing improved mechanical properties such as superior fracture toughness for damage tolerance. We optimize several parameters such as fiber length, diameter, and density to generate an effective 3-D electrical conductive network, and characterize the responses of these composites under mechanical loading to investigate damage formation and evolution, advancing science and technology towards superior damage-tolerant and zero-maintenance structural materials.

The objective of this study was to develop a predictive model for cell seeding depth in electrospun scaffold as a function of fiber stiffness. Electrospun scaffolds (micron and submicron) and 3T3 fibroblasts are used as scaffold–cell systems under vacuum seeding conditions. Atomic force microscopy is used to determine the Young's modulus (E) as a function of fiber diameter. A higher E value led to a lower depth of cell seeding (closer to the surface) indicating that nanofibrous scaffolds offer higher resistance to cell movement compared to microfibrous scaffold. An energy balance model was developed to predict cell seeding depth as a function of E for various vacuum pressures. Experimental data was used in the model to extract unknown parameters to predict cell seeding depth as a function of vacuum pressure for different stiffness scaffolds. © 2014 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 103: 2289–2299, 2015.