Influence of Silicon Carbide on Direct Powder Bed Selective Laser Process (Sintering/Melting) of Alumina

The powder bed selective laser process (sintering/melting) has revolutionised many industries, including aerospace and biomedicine. However, PBSLP of ceramic remains a formidable challenge. Here, we present a unique slurry-based approach for fabricating high-strength ceramic components instead of traditional PBSLP. A special PBSLP platform capable of 1000 °C pre-heating was designed for this purpose. In this paper, PBSLP of Al2O3 was accomplished at different SiC loads up to 20 wt%. Several specimens on different laser powers (120 W to 225 W) were printed. When the SiC content was 10 wt% or more, the chemical interaction made it difficult to process. Severe melt pool disturbances led to poor sintering and melting. The structural analysis revealed that the micro-structure was significantly affected by the weight fraction of SiC. Interestingly, when the content was less than 2 wt%, it showed significant improvement in the microstructure during PBSLP and no effects of LPS or chemical interaction. Particularly, a crack pinning effect could be clearly seen at 0.5 wt%.

Experimental studies on mechanical properties and toughening mechanisms of PA6/zeolite nanocomposites

In recent years, polymer/inorganic nanocomposite have attracted the attention of researchers because of the many superior properties. Incorporation of nano-zeolite as a spherical nano-particle, into the PA6 matrix, can improve the stiffness-toughness properties simultaneously. PA6/zeolite nanocomposites standard mechanical testing specimens, containing different fractions (2.5, 5 and 7.5 wt.%) of nanosized zeolite particles were produced by utilizing a twin screw extruder and injection molding. The mechanical properties were characterized and the morphology was studied using scanning electron microscopy (SEM). The SEM micrograph showed that nano-zeolite were uniformly distributed in the PA6 matrix. The incorporation of nano-zeolite into PA6, increased the tensile strength, tensile modulus, flexural strength, flexural modulus and impact strength. Moreover, nano-zeolite stiffen and toughen PA6 simultaneously, and optimal properties were achieved at 5 wt.% of PA6/zeolite in flexural strength and impact strength. A number of strengthening mechanisms including crack pinning and crack deflection were detected for nanocomposite gear samples.

The Role of Synergies of MWCNTs and Carbon Black in the Enhancement of the Electrical and Mechanical Response of Modified Epoxy Resins

Nano-reinforced composites are widely studied by the scientific community. The main factors affecting the final nanocomposite performance are the filler type and content, as well as the duration of the dispersion. In this work, we report the effects of Multi-Walled Carbon Nano Tubes (MWCNTs) and milled Carbon Black (CB) dispersion in epoxy resin on the electrical and mechanical properties of the resulting composites. Impedance Spectroscopy (IS) was utilized to assess the dielectric properties of the specimens. The mechanical properties were evaluated by fracture toughness tests, while Scanning Electron Microscopy (SEM) was performed to study the influence of the reinforcement on the failure mechanisms acting on the fracture surfaces of the specimens. IS results for epoxy/CNT systems revealed the creation of a 3D conductive network for concentrations above 0.3 wt. %, while CB did not result in the formation of such a network for filler contents up to 2 wt. %. However, the synergistic effect of CNTs/CB was successfully manifested by both the optimal electrical properties and the 81% enhanced fracture toughness in comparison to the neat resin. Fractography confirmed the aforementioned results and revealed the fracture mechanisms of all systems, such as crack pinning and deflection, and particle pull-out phenomena.

Effects of HfC Particles on Crack Initiation and Propagation Behavior of WC/Co Composites

Tungsten carbide composites were prepared by cold-pressing and hot-pressing sintering; fracture toughness and bending strength of the specimens were tested. The microstructures of HfC/WC/Co composites were observed with the SEM. The mathematical models were established to investigate the relationship between stress intensity factors of crack straight-through, crack deflection, and crack bifurcation with crack length, based on the crack propagation energy release rate. The simulation software ABAQUS was used to verify the four crack propagation methods of crack straight-through, crack deflection, crack bifurcation and crack pinning. The simulation results show that adding appropriate amount of HfC can effectively improve the fracture toughness and bending strength of the composites. The homogeneous distribution of HfC and Co in the matrix has a significant effect on the improvement of the strength and toughness of the composites, and the improvement mechanism is to disperse or transfer the stress at the crack tip to HfC by crack deflection, crack bifurcation, crack pinning, transcrystalline fracture, etc. As a result, the stress concentration at the crack tip in the matrix is reduced, and the toughness of the composites is improved.

Muscle-like fatigue-resistant hydrogels by mechanical training

Skeletal muscles possess the combinational properties of high fatigue resistance (1,000 J/m2), high strength (1 MPa), low Young’s modulus (100 kPa), and high water content (70 to 80 wt %), which have not been achieved in synthetic hydrogels. The muscle-like properties are highly desirable for hydrogels’ nascent applications in load-bearing artificial tissues and soft devices. Here, we propose a strategy of mechanical training to achieve the aligned nanofibrillar architectures of skeletal muscles in synthetic hydrogels, resulting in the combinational muscle-like properties. These properties are obtained through the training-induced alignment of nanofibrils, without additional chemical modifications or additives. In situ confocal microscopy of the hydrogels’ fracturing processes reveals that the fatigue resistance results from the crack pinning by the aligned nanofibrils, which require much higher energy to fracture than the corresponding amorphous polymer chains. This strategy is particularly applicable for 3D-printed microstructures of hydrogels, in which we can achieve isotropically fatigue-resistant, strong yet compliant properties.

Study of the Surface Roughness Evolution of Pinned Fatigue Cracks, and its Relation to Crack Pinning Duration and Crack Propagation Rate Between Pinning Points

Changes in surface morphology have long been thought to be associated with crack propagation in materials. In this paper, we study the changes in the surface profile of the crack-tip plastic zone with an attempt to understand the relationship between the plasticity-induced surface profile changes and the crack growth behavior. Center crack specimens were electropolished and etched to reveal the grain structure for white light interferometer (WLI) imaging prior to and during fatigue testing. After growing the crack to a predetermined pre-crack length, a viewing zone was selected outside of the plastic zone of the pre-crack. The surface profile of the viewing zone was imaged using a WLI microscope at selected fatigue cycle intervals. An image processing algorithm was developed to evaluate the changes of the surface profile. We observed that the crack growth rate is not uniform at the microscopic scale; the crack growth was retarded at crack pinning points and the crack grows at a faster rate while propagating between the pinning points. Relatively large surface topology changes were observed to be constrained to the area surrounding the tip of pinned cracks. However, there was an avalanche of surface changes covering the entire monotonic zone upon the crack being released from a pinned location. Interestingly enough, minor or no measurable surface changes could be seen for propagating cracks. These results indicate a surface roughness change threshold may exist for predicting the duration during which a crack is pinned. Results suggest the threshold and crack propagation rate between pinning locations may be functions of the amplitude of the stress intensity factor.

Mechanical and Thermal Properties of Al2O3-Filled Epoxy Composites

The paper presents studies of influences of platelet-shape alumina content on the morphology, mechanical and thermal properties of Al2O3/epoxy composites. The alumina particles were pre-treated with surface modifier, and its addition contents were ranged from 17 to 40vol.%. It was found that tensile strength and thermal conductivity increased with the increase of the alumina contents. The fracture surface investigation showed that several strengthening mechanisms, including particles pull-out, crack pinning, plastic void growth and deformation were the main factors for the increments of the tensile strength of the Al2O3/epoxy composites. Comparing with pure epoxy polymer, the creep strain values at 70 °C for 1800 s and recovery strain after 3600 s of 40vol.%Al2O3/epoxy composites was lower as 0.23% and 70%, respectively. It was due to effective prohibition of the slippage and disentanglement of epoxy polymer molecules by the rigid Al2O3.

Modified fracture properties of cement composites with nano/micro carbonized bagasse fibers

A novel cost-effective alternative in the form of nano/micro carbonized particles produced from waste bagasse fibers has been explored to modify the mechanical properties and fracture pattern of the resulting cementitious composites. Carbonized bagasse particles were produced at Politecnico di Torino and characterized by Raman spectroscopy and scanning electron microscopy. When added with cement paste up to 1 wt% in six different proportions, the carbonized bagasse particles were found effective in significant enhancement of mechanical strength as well as fracture toughness. From micro-graphical observations it is evident that these heterogenic inclusions either block the propagation of micro cracks which has to deviate from its straight trajectory and has to follow the carbon nano/micro particles contour or distribute it into multiple finer cracks. Crack contouring along the carbonized particle, crack pinning, crack diversions and crack branching are the mechanisms which can explain the increase of toughness in the composite samples.