Effect of Graphene Addition on Crack Propagation Resistance in Glass Fibre Reinforced Polymer Matrix Composite

The effect of graphene nanoplatelets (GNPs) on enhancing the interlaminar fracture toughness of glass fiber/epoxy composites was investigated. The GnPs were physically deposited on the fiber surface by the dip coating technique. The composites were fabricated by hand layup technique followed by the compression molding process. Mode-I fracture test was conducted on the composite specimens. Crack propagation was studied by the digital image correlation (DIC) technique. Mode-I fracture toughness for composites loaded with 0.5 wt.% GnPs showed improvement by an average of 60% when compared to the pristine composites. It is concluded that the addition of GnPs produces a strong fiber/matrix interface bonding which effectively limits the crack propagation.

Processing-Mechanical Property Relationship of Hollow and Porous Carbon Fibers Fabricated by Coaxial Electrospinning

High strength hollow carbon fibers with both porous and solid shell were fabricated by a combination of coaxial electrospinning and emulsion electrospinning. In the coaxial electrospinning, a Polyacrylonitrile (PAN)/ Poly(methyl methacrylate) (PMMA)/Dimethylformamide (DMF) emulsion was used to form the porous shell and a PAN/DMF solution was used to form the solid shell. Fiber surface and cross-section morphology was studied by scanning electron microscope (SEM). Mechanical property of the hollow fibers was characterized by single fiber tensile test using microelectromechanical system devices (MEMS). The effect of pores on mechanical performance of the hollow fibers was studied. Hollow carbon fibers with porous and solid shell both showed a brittle fracture behavior. The modulus and strength of the hollow carbon fibers with solid shell was ∼ 76.1 GPa and 2.04 GPa, respectively. For the hollow carbon fibers with porous shell, the porosity led to ∼ 35 % reduction in strength. The porous fibers with the mediocre strength measured here open new horizons for combining structural functionality with energy storage, in so-called structural batteries.

Shape Memory Behavior of Carbon Composites With Functional Interlayer

Shape Memory Polymer Composites (SMPCs) are smart materials showing the structural properties of long-fiber polymer-matrix together with the functional behavior of shape memory polymers. In this study, SM carbon fiber reinforced (CFR) composites have been produced by using a SM interlayer between two CFR prepregs. Their SM properties have been evaluated in comparison with traditional structural CFR composites without the SM interlayer by using an especially designed test. Active and frozen forces are measured during a thermo-mechanical cycle in the three-point bending configuration. Experimental results show that SMPCs are able to fix a temporary deformed shape by freezing high stresses.

A State-of-the-Art Review on Solid-State Metal Joining

This paper aims at providing a state-of-the-art review of an increasingly important class of joining technologies called solid-state welding. Among many other advantages such as low heat input, solid-state processes are particularly suitable for dissimilar materials joining. In this paper, major solid-state joining technologies such as the linear and rotary friction welding, friction stir welding, ultrasonic welding, impact welding, are reviewed, as well as diffusion and roll bonding. For each technology, the joining process is first depicted, followed by the process characterization, modeling and simulation, monitoring/diagnostics/NDE, and ended with concluding remarks. A discussion section is provided after reviewing all the technologies on the common critical factors that affect the solid-state processes such as the joining mechanisms, chemical and materials compatibility, surface properties, and process conditions. Finally, the future outlook is presented.

Nickel/Alumina Metal Matrix Nanocomposites Obtained by High-Energy Ball Milling and Spark Plasma Sintering

Metal matrix nanocomposites (MMNCs) are anticipated to offer significantly better performance than existing superalloys. Nickel/alumina nanocomposite samples were fabricated with a powder metallurgy method, combining high-energy ball milling (HEBM) and spark plasma sintering (SPS). The objective of this research is to determine the effect of alumina nanoparticle fraction and HEBM parameters on the powder preparation and sintering processes, and resultant microstructure and properties. Nickel-based powders containing various fractions (1, 5 and 15 vol.%) alumina nanoparticles were prepared by HEBM. The initial particle sizes were 44 μm and 50 nm for nickel and alumina, respectively. The milling process was conducted by starting with mixing at 250 rpm for 5 min, followed by cycling operation at high and low speeds (1200 rpm for 4 min and 150 rpm for 1 min). Samples at different milling times (30, 60, 90 and 120 min) of each composition were obtained. Scanning electron microscopy (SEM) was used to evaluate the dispersion of nanoparticles in the powders at different milling times. SPS technique was used for consolidation of the prepared powders. SEM images showed that alumina nanoparticles are homogeneously dispersed in the metal matrix in the sample containing 15 vol.% alumina. Hardness measurements in cross sections of SPSed samples showed higher values for Ni/Al2O3 MMNC compared to pure Ni.

Design of Nano-Filled PET Sheets With Enhanced Barrier Properties

Barrier properties are achieved in PET food packaging by using additives, coatings or multi-layers. An analytical model to predict the contamination during multiple recycling steps of this packaging is proposed. This model shows that lower contents of non-PET materials should be achieved to promote a valid recycling chain. A possible solution is using nano-technologies because of the very small amount of added material. Results are shown in the case of PVD coatings with aluminum target, and PET nano-composites. In both cases, less than 0.1 wt% of aluminum is able to reduce the oxygen transmission ratio of the PET packaging down to 50% of the virgin PET sheet.

Friction Stir Lap Welding of Thin AA6061-T6 and AISI304 Sheets at Different Values of Pin Penetrations

The present paper focuses on the influence of pin penetration on joint strength achieved in friction stir lap welding between AA6061-T6 and AISI304. Penetration of pin into the steel substrate, which is placed below the aluminium, has an influencing role in achieving a good weld. Beyond a certain depth of penetration of pin into the steel, non-uniform thicker intermetallic compound and the weld defects have been found. Defects were found to be located in the stir zone. Thus, an excessive penetrations of the pin into the steel matrix has been produced a detrimental effect to the weld strength. An optimum penetration of pin has been experimentally found out.

A Study on Laser Welding of Titanium and Stainless Steel

Joining of dissimilar metals and alloys has been envisioned since a long time with specific high end applications in various fields. One such combination is austenitic stainless steel grade SS304 and commercial grade titanium, which is very difficult to join under conventional fusion process due to extensive cracking and failure caused by mismatch in structural and thermal properties as well as formation of the extremely brittle and hard intermetallic compounds. One of the methods proposed in literature to control the formation of intermetallics is by fast cooling fusion process like laser beam welding. The present study has been done on laser welding of titanium and stainless steel AISI 304 to understand the interaction of these materials during laser welding at different laser power and welding speed which could yield different cooling rates. Two types of cracks were observed in the weld joint, namely longitudinal cracks and transverse cracks with respect to the weld direction. Longitudinal cracks could be completely eliminated at faster welding speeds, but transverse cracks were found little influenced by the welding speed. The thermal history, i.e. melt pool lifetime and cooling rate of the molten pool during laser welding was monitored and a relation between thermo-cycle with occurrence of cracks was established. It is inferred that the longitudinal cracks are mainly due to the formation of various brittle intermetallic phases of Fe and Ti, which could be minimized by providing relatively less melt pool lifetime at high welding speeds. The reason of the transverse cracks could be the generation of longitudinal stress in weld joint due to the large difference in the thermal expansion coefficient of steel and titanium. In order to mitigate the longitudinal stress laser welding was carried out with a novel experimental arrangement which ensured different cooling rates of these two metals during laser welding. With this the tendency of transverse cracks also could be minimized significantly.

Interaction of a Sliding Wedge and a Metallic Specimen With a Near-Surface Inhomogeneity

The problem of a hard wedge sliding against a metal substrate has been studied extensively for its importance in tribo-plasticity and deformation processing. Here we explore the effect of introducing a single, near-surface plastic inhomogeneity (termed as a pseudograin) in a metal substrate using Lagrangian finite element (FE) analysis. The pseudograin is allowed to be softer or harder than the surrounding material. The effects of sliding parameters like the size and location of the pseudograin, friction and indenter geometry are also studied. Interestingly, the introduction of the pseudograin can lead to production of surface folds / self-contacts, and acutely-inclined, near-surface, crack-like features, which cannot be reproduced by homogeneous specimens. In fact, this tribosystem is phenomenologically very rich, despite differing from classical triboplastic systems of Challen, Oxley and Torrance only by way of the inhomogeneity. Despite its simplicity, the model replicates several experimentally observed features of surface folding, and is a minimal model to obtain folding in sliding. The occurrence of surface folds and concomitant residual surface damage points to the important role played by microstructure-related inhomogeneities in determining surface quality in deformation processing operations (e.g. repeated sliding to generate UFG surfaces) and is also a potentially new mode of sliding wear.

Gaussian Process Model for Touch Probing

In this paper, we present an approach to determine probing points for the CMM (coordinate measurement machine) measurement. A surface uncertainty based approach is developed to maximize the amount of information acquired by the touch probing of the surface deviations caused by machining errors. The surface uncertainty is modeled by the Gaussian process model and the probing points are selected to minimize the maximum surface uncertainty (surface variance) conditioned on the touch probings at the selected points. The algorithm has been tested with various numerical examples and has been applied in real machining scenarios. The surface reconstruction error based on the developed algorithm is 50% smaller than uniform sampling. The experiments of on machine probing has validated that the selected points can adequately capture the machining errors.