In this review, recent progress in the mechanical, thermal and interfacial properties of graphene/CNT multiphase polymer composites is examined. Progress in the mechanical and thermal properties of CNT (1D) and graphene (2D) nanostructure materials is also reviewed and compared. Furthermore, this review highlights the improvements in the mechanical, thermal and interfacial properties of two- and three-phase composites, owing to the addition of graphene/CNT. In particular, analysis of several notable papers on hybrid composites (graphene/CNT) provides an intensive review of synergetic effects on the overall properties of the corresponding composites. This holistic review describes an improved interface between fibers and a nanofiller-reinforced matrix. Although the presence of nanofillers even at low loadings confers an overall improvement in composite properties, the exact ratios of individual fillers and combined forms remain to be discussed in depth. Finally, potential applications, current challenges and future perspectives for use of these multiphase composites are discussed with regard to their extraordinary capabilities and promising developments in graphene/CNT family-based composite materials.

Carbon nanotube- and graphene-reinforced multiphase polymeric composites: review on their properties and applications

Tensile, fracture, and fatigue crack growth properties of a 3D printed maraging steel through selective laser melting

A strain rate sensitive constitutive model for quenched boron steel with tailored properties

This experimental study investigates the combined effect of the three primary Additive Manufacturing (AM) build orientations (0°, 45°, and 90°) and an extensive array of heat treatment plans on the plastic anisotropy of maraging steel 300 (MS1) fabricated on the EOSINT M280 Direct Metal Laser Sintering (DMLS) system. The alloy's microstructure, hardness, tensile properties and plastic strain behaviour have been examined for various strengthening heat-treatment plans to assess the influence of the time and temperature combinations on plastic anisotropy and mechanical properties (e.g. strength, ductility). A comprehensive visual representation of the material's overall mechanical properties, for all three AM build orientations, against the various heat treatment plans is offered through time – temperature contour maps. Considerable plastic anisotropy has been confirmed in the as-built condition, which can be reduced by aging heat-treatment, as verified in this study. However, it has identified that a degree of transverse strain anisotropy is likely to remain due to the AM alloy's fabrication history, a finding that has not been previously reported in the literature. Moreover, the heat treatment plan (6h at 490 °C) recommended by the DMLS system manufacturer has been found not to be the optimal in terms of achieving high strength, hardness, ductility and low anisotropy for the MS1 material. With the use of the comprehensive experimental data collected and analysed in this study, and presented in the constructed contour maps, the alloy's heat treatment parameters (time, temperature) can be tailored to meet the desired strength/ductility/anisotropy design requirements, either for research or part production purposes.

Plastic anisotropy of additively manufactured maraging steel: Influence of the build orientation and heat treatments

Pulsed eddy current testing (PECT) method for electrical conductivity measurement of ferromagnetic metallic materials is proposed. Based on time-domain analytical solutions to the PECT model of ferromagnetic plates, the conductivity and permeability are determined via an inverse problem established with the calculated and measured values of induced voltage. PECT method for conductivity measurement is verified by the four-point probe method on three carbon steel plates. In addition, the effects of the amplitude of pulsed excitation current and the lift-off of probe coils on measurement results are studied. PECT is an innovative, non-contacting method with good repeatability for electrical conductivity measurement.

Electrical conductivity measurement of ferromagnetic metallic materials using pulsed eddy current method

2xxx series Al-alloys are widely employed in structural applications due to their good mechanical properties. During heat treatment of these alloys, solution treated parts sometimes mixed with age hardened parts during handling. This result in difficulty in distinguishing between solution treated and aged parts of various grades. Moreover, it is also necessary to separate improper aged parts from properly treated parts. The traditional methods of characterization of different heat treated parts are hardness, tension testing and microscopy, however these are destructive in nature and sometimes not desired particularly for finished products. The main purpose of this paper is characterization of material properties of 2xxx series Al-alloys by eddy current and ultrasonic NDE techniques so that the inspection can be carried out effectively in the shortest possible time. Three wrought Al-alloys of 2xxx series (AA 2014, AA 2024 and AA 2219) were homogenized followed by solution heat treatment and age hardening treatments at specific temperatures for 1–16 h. The changes in hardness and microstructure during heat treatments were determined by traditional material characterization methods and then correlated with electrical conductivity, sound velocity and attenuation coefficient obtained through Nondestructive Evaluation (NDE) techniques. Results demonstrated an excellent correlation between hardness and sound velocity, whereas extend of aging can be easily predicted by electrical conductivity, and attenuation coefficient measurement. Investigation suggested a way towards the non-destructive detection and characterization of material properties when conventional testing methods are not applicable.

Characterization of Material Properties of 2xxx Series Al-Alloys by Non Destructive Testing Techniques

Multi-scale composite material was developed by combining carbon fabric and multi-wall carbon nanotubes (MWCNTs) in epoxy matrix. Varying concentrations of MWCNTs were ultrasonically dispersed in epoxy resin and applied on carbon fabric followed by curing. Tensile and flexural tests were conducted to observe the influence of MWCNTs on the mechanical properties of carbon fiber-reinforced plastic (CFRP). Coefficient of thermal expansion (CTE), thermal stability and glass transition temperature were determined through dilatometry, thermo-gravimetric analysis and differential scanning calorimetry, respectively. The MWCNTs dispersion in the epoxy was characterized through optical microscopy and scanning electron microscopy. Fractured surfaces of tensile samples were examined through high-resolution scanning electron microscope to investigate the failure mechanism. Tensile and flexural strengths were improved by 60 and 54%, respectively, on addition of 0.25 wt% MWCNTs in CFRP composite. This enhancement was attributed to the micro-crack bridging effect of the MWCNTs. Nanotubes acted as nano-stitches by holding the matrix together, thereby increasing the load bearing capability. Glass transition temperature and CTE of the CFRP was reduced by addition of MWCNTs whereas MWCNT–CFRP was found thermally stable up to 350 
 $${^{\circ }}$$
 C. Microscopy results showed that the nanotubes were well dispersed in the epoxy matrix. However, higher weight fraction of MWCNTs in the epoxy had promoted agglomeration.

Mechanical and Thermal Properties of Multi-scale Carbon Nanotubes–Carbon Fiber–Epoxy Composite

This study was undertaken to gain a better understanding of microstructures obtained by multipass gas tungsten arc welding in maraging steel grade 250. Metallography and microhardness measurements were carried out on sheet and welded joints in as-welded and post-weld aged conditions. It was found that there was a significant amount of reverted austenite formed on cell boundaries of weld metal after aging at 758-823 K for 3-5 h, and was stable at room temperature. Aging at higher temperatures led to an increase in the continuous network of patchy austenite along the cell boundaries. The reason for the above, in our opinion, is the concentrational heterogeneity which characterizes the microstructure of maraging steel welds. No reverted austenite was observed in as-welded specimens. Solution annealing at 1093 K for 1 h did not completely eliminate the chemical heterogeneity associated with weld structures. However, homogenizing at 1373 K produced homogenous structure that on subsequent aging produces austenite-free lath martensitic structure.

Investigation into Microstructures of Maraging Steel 250 Weldments and Effect of Post-Weld Heat Treatments

This article discusses a detailed failure analysis conducted on high density polyethylene which acts as the liner of composite compresses natural gas cylinder. Leakage from the cylinder was observed after about 2000 cycles of hydrostatic pressure testing at 250 bars. Visual inspection revealed that the leakage occurred from the circumferential fusion joint between the cylinder and dome section. The cylinder and dome sections were produced from different techniques and joined together by using a heated tool butt welding process conducted by a local manufacturer. The joined components work as an integral part. The investigation was carried out using various techniques including mechanical, thermal and metallurgical examination. Fractography of the failed joint surface showed stepwise marks typical of a fatigue failure. Mechanical testing results showed that the strength of dome section was significantly lower than that of the cylinder section. Moreover, both the tensile and fatigue strength of joint was also almost half that of the cylinder. The fracture surface of the broken welded joint showed brittle failure. The melting points of both the cylinder and the dome sections were found almost same by differential scanning calorimeter but large difference was found between the melt point indexes of both sections. The results suggested that the failure was actually a stress cracking via a process of slow crack growth, which occurred due to use of a dome material having inferior properties and very high melt flow index. These properties inhibited proper fusion and resulted in a poor weld joint. Consequently, the weld joint of lower strength eventually failed in macroscopically brittle manner upon cyclic loading.

Failure Analysis of High Density Polyethylene Butt Weld Joint

This paper presents the results of novel one-step quenching and partitioning (Q&P) heat treatment conducted on medium carbon low alloy steel sheet. Samples were austenitised at 1193 K followed by interrupted quenching at 473 K for different partitioning times and finally they were quenched in water. Dilatometry was employed for selection of treatment temperatures. Optical and scanning electron microscopy was carried out to examine the microstructural changes. Volume fraction of retained austenite was measured by x-ray diffraction technique. Resulting microstructures were correlated with the mechanical properties such hardness, tensile strength, elongation, impact absorbed energy, etc. The notch tensile and fracture toughness properties of Q&P steels are still lacking therefore notch tensile strength and plain strain fracture toughness tests were conducted and results are reported here. Results of Q&P treatments were also compared with the properties obtained by conventional Quenching and Tempering (Q&T) and normalizing treatments. Optimum strength-ductility balance of about 2000 MPa tensile strength with 11% elongation was achieved in samples quenched at 473 K and isothermally partitioned for 100 s. Higher ductility of Q&P steel was attributed to the presence of 6.8% film-type interlath retained austenite. Fine-grained martensitic structure with high density of interphase boundaries imparted ultrahigh strength. It was further noted that the impact toughness, notch tensile strength and fracture toughness of 1000 s partitioned samples was higher than 100 s partitioned samples. Possible reasons for high toughness are synergetic effect of recovery of dislocations, partial loss of martensite tetragonality and precipitation of fine transition carbides.

Fawad Tariq

Papers

Characterization of Material Properties of 2xxx Series Al-Alloys by Non Destructive Testing Techniques

Mechanical and Thermal Properties of Multi-scale Carbon Nanotubes–Carbon Fiber–Epoxy Composite

Investigation into Microstructures of Maraging Steel 250 Weldments and Effect of Post-Weld Heat Treatments

Failure Analysis of High Density Polyethylene Butt Weld Joint

One-Step Quenching and Partitioning Heat Treatment of Medium Carbon Low Alloy Steel