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An experimental study is performed to investigate the quasi-static fracture toughness and damage monitoring capabilities of liquid metal (75.5% Gallium/24.5% Indium) reinforced intraply glass/carbon hybrid composites. Two different layups (G-0, where glass fibers are along the crack propagation direction; C-0, where carbon fibers are along the crack propagation direction) and two different weight percentages of liquid metal (1% and 2%) are considered in the fabrication of the composites. A novel four-probe technique is employed to determine the piezo-resistive damage response under mode-I fracture loading conditions. The effect of layups and liquid metal concentrations on fracture toughness and changes in piezo-resistance response is discussed. The C-composite without liquid metal demonstrated higher fracture toughness compared to that of the G-composite due to carbon fiber breakage. The addition of liquid metal decreases the fracture initiation toughness of both G- and C-composites. Scanning electron microscopy images show that liquid metal takes the form of large liquid metal pockets and small spherical droplets on the fracture surfaces. In both C- and G-composites, the peak resistance change of composites with 2% liquid metal is substantially lower than that of both no-liquid metal and 1% liquid metal composites.

An experimental study is performed to determine the head mechanics of American football helmets equipped with novel fiber energy absorbing material (FEAM). FEAM-based padding materials have substrates of textile fabrics and foam made with nylon fibers using electro-static flocking process. Both linear and angular accelerations of the sport helmets are determined under impact loads using a custom-built linear impactor and instrumented head. The effectiveness of padding materials and vinyl nitrile (VN) foam for impact loads on six different head positions that simulate two helmeted sport athletes in real-time helmet-to-helmet strike/impact is investigated. A high-speed camera is used to record and track neck flexion angles and compare them with pad effectiveness to better understand the head kinematics of struck players at three different impact speeds (6 m/s, 8 m/s, and 10 m/s). At impact speed of 6 m/s and 8 m/s, the FEAM-based padding material of 60 denier fibers showed superior resistance for angular acceleration. Although novel pads of VN foam flocked with 60 denier fibers outperformed with lowest linear acceleration for most of the head positions at low impact speed of 6 m/s, VN foam with no fibers demonstrated excellent performance for linear acceleration at other two speeds.

Chemical crosslinks known as advanced glycation end-products (AGEs) are associated with increased bone fracture risk and deteriorated bone mechanical properties. However, measurement of bone AGEs via ex vivo and in vitro methods has been limited to quantification of bulk fluorescent AGEs (fAGEs) and pentosidine only, which is a crosslinking fluorescent AGE. However, a non-crosslinking and non-fluorescent AGE such as carboxymethyl-lysine (CML) is found to be 40–100 times higher in quantity than pentosidine, but only one previous study has reported it in cortical bone, and one study reported it in trabecular bone. In our study, we wanted to investigate if accumulation of CML differs in cortical and trabecular compartments and if they are more strongly associated with bone mechanical properties than with fAGEs. We hypothesized that CML and fAGEs level would be higher in the trabecular compartment and show negative correlations to mechanical properties in cortical and trabecular bone. We obtained human cadaveric cortical and trabecular bone specimens, induced the formation of AGEs via the established in vitro ribosylation method, imaged specimens by microcomputed tomography to assess specimen geometry and microarchitecture, and mechanically tested cortical specimens by cyclic reference point indentation and fracture toughness tests and trabecular specimens by compression tests, followed by measurement of fAGEs and CML. fAGEs were 22 % higher in cortical bone (687 ± 44.8 ng Q/mg collagen) compared to trabecular bone (859 ± 317.1 ng Q/mg collagen), whereas CML levels were found to be 148 % higher in trabecular bone (6189.9 ± 866 ng/mg of protein) compared to cortical bone (924.6 ± 576.3 ng/mg of protein). Pooling the specimens from both the control and ribose groups, Spearman correlation analysis indicated that CML levels, but not fAGEs, are moderately associated with cortical porosity (r = +0.505, p ≤ 0.05) and mechanical properties such indentation depth (r = +0.460, p ≤ 0.05), total indentation depth (r = +0.440, p ≤ 0.05), and average energy dissipated (r = +0.465, p ≤ 0.05) in cortical bone. fAGEs showed a trend towards negative association with crack propagation toughness in cortical bone (r = −0.365, p = 0.055). No significant correlations were observed between CML and microarchitecture or mechanical properties in trabecular bone. CML levels were also associated with fAGEs in cortical bone (r = +0.596, p ≤ 0.05) but not in trabecular bone. Our preliminary findings indicate that CML, a non-crosslinking AGE, may affect bone material and mechanical properties differently than bulk fluorescent AGEs, given the higher accumulation of CML in each bone compartment. This study provides direction to future studies to quantify crosslinking and non-crosslinking AGEs separately as their effect on material and mechanical properties may be different and it would help identify better biomarkers for bone strength prediction.

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 the composites by electro-flocking microcarbon fibers between the laminates and dispersing carbon nanotubes in the matrix. The effect of carbon fiber length and density on the electro-fracture behavior is studied. The change in electrical resistance is measured using a novel four-probe method through the thickness direction of cantilever beam test specimen. Results show that the maximum increase of fracture initiation toughness is 75% by the composite with carbon fibers length of 350 μm and fiber density of 2000 fibers/mm2 compare to composites with no carbon fibers. For all composite types, the increase in resistance is not noticeable before crack initiation. The cycles of fracture energy build up and release during the crack propagation are accurately detected onsite by the composite with carbon fiber length of 350 μm and fiber density of 2000 fibers/mm2.

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.

In the original publication [...]

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.

An experimental investigation was performed to understand the quasi-static shear response of additively manufactured (AM) acrylonitrile butadiene styrene (ABS) via fusion deposition modeling (FDM). A modified flat hat-shaped (FHS) specimen configuration was used for shear testing. The main aim of this study was to investigate the effect of four different shear angles (0°, 5.44°, 13.39°, and 20.83°) and three printing orientations (vertical build, 0°/90°, and 45°/−45°) on the shear constitutive response and shear performance of FDM-printed ABS. Scanning electron microscopy images of the failure surface were used to explain the shear response of the material. The flow shear stress of the shear stress-strain response for vertically printed specimens demonstrated a monotonic increase up to a peak shear stress and then decrease at the end of the shear zone, while for 0°/90° specimens, an increasing trend until the peak value at the end of the shear zone was observed. With increasing shear angles, all specimens printed with three printing orientations exhibited increasing shear zone size and shear strength, and the 0°/90° specimens exhibited the highest shear strength for all four shear angles. However, the specimens of the 45°/−45° orientation demonstrated the highest increase in shear strength by about 60% and in the shear strain at the end of shear zone by about 175% as the shear angle was increased from 0° to 20.83°.

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.

In this study, an experimental investigation has been performed to understand the electromagnetic interference-shielding effectiveness (EMI-SE) of glass fiber/epoxy laminated composites embedded with carbon nanotubes (CNTs) and Fe3O4 nanoparticles, reinforced with micro carbon fibers along the thickness direction. Micro carbon fibers were reinforced along the thickness direction between the laminates using an electro-flocking process and a vacuum infusion process used to fabricate the composites. The EMI-SE of the composites was measured in the X-band frequency range (8–12 GHz). The effect of carbon fibers of three different lengths (80 µm, 150 µm, and 350 µm) with two different fiber densities (1000 and 2000 fibers/mm2) and two different amounts of Fe3O4 nanoparticles (0.5 and 1 wt.%) on total SE, absorption, and reflection was investigated. Due to the synergetic effect of Fe3O4 nanoparticles, CNTs, and carbon fibers, the final EMI shielding of the composites was mainly dominated by the absorption process. The absorption was more pronounced in the composites of longer carbon fibers with improved electrical conductivity. The presence of Fe3O4 nanoparticles also enhanced total SE values with improved magnetic permeability. The composite with micro carbon fibers of 350 µm length and 2000 fibers/mm2 density with 1 wt.% of Fe3O4 nanoparticles showed the maximum value of total SE.

A comprehensive experimental study is performed to investigate the electro-mechanical response of jute/epoxy composites for in situ damage detection under in-plane shear loading conditions. By embedding carbon nanotubes (CNTs) in epoxy matrix and also reinforcing carbon fibers (CFs) using electro-flocking process between the jute laminates, a three-dimensional sensory network is generated inside the composites. A parametric study is conducted to examine the effect of two different weight percentages of CNTs (0.025% and 0.1%), two different CFs lengths (150 and 350 μm), four different CF flock densities (500, 1000, 1500, and 2000 fibers mm−2), and two different laminate orientations ((0-0-0-0)T and (0-90-0-90)T) on the generation of electrical conductive network and its role on damage detection. The electrical resistance values are measured using four circumferential ring probes measurement system. Results indicated that (0-0-0-0)T orientation showed 400% improvement in interlaminar shear strength compared to (0-90-0-90)T composites. CF flocking between the laminates experienced maximum increase of 50% in shear strength compared to non-flocked composites. For all composite types, there is no change in resistance during the elastic deformation of shear loading. Beyond elastic deformation, the increase in CF length and its density showed similar electrical response. The increase in CNTs concentration and laminate orientations did not show significant change in electrical response under shear loading conditions. [ABSTRACT FROM AUTHOR]

Finite element simulations were performed to determine the contact behavior of radio frequency (RF) micro-electro-mechanical (MEM) switch contact surfaces under monotonic and cyclic loading conditions. Atomic force microscopy (AFM) was used to capture the topography of RF-MEM switch membranes and later they were analyzed for multi-scale regular as well as fractal structures. Frictionless, non-adhesive contact 3D finite element analysis was carried out at different length scales to investigate the contact behavior of the regular-fractal surface using an elasto-plastic material model. Dominant micro-scale regular patterns were found to significantly change the contact behavior. Contact areas mainly cluster around the regular pattern. The contribution from the fractal structure is not significant. Under cyclic loading conditions, plastic deformation in the 1st loading/unloading cycle smooth the surface. The subsequent repetitive loading/unloading cycles undergo elastic contact without changing the morphology of the contacting surfaces. The work is expected to shed light on the quality of the switch surface contact as well as the optimum design of RF MEM switch surfaces. [ABSTRACT FROM AUTHOR]