Abstract

I. INTRODUCTION

II. EXPERIMENTAL DETAILS

III. RESULTS

IV. DISCUSSION

V. CONCLUSION

REFERENCES

Abstract

     Nanocomposites have been developed as the third-generation insulating materials. This article introduces the crosslinked polyethylene (XLPE) nanocomposite dielectric doped with a small amount of nano-hexagonal boron nitride ( h -BN), which exhibits excellent insulation properties under high dc voltage. The test results show that XLPE/ h -BN nanocomposites possess significant improvements in dc breakdown strength and 0.1 wt% nano h -BN doping can actually increase the dc breakdown strength by 39%. The space charge properties of the XLPE/ h -BN obtained by the pulsed electro-acoustic (PEA) measurements and the direct current integrated charge [DCIC- Q(t) ] measurements show that the positive homocharge injection under dc voltage is obviously suppressed, and the electrical conductivity under relatively high electric field decreases. The thermally stimulated current results show that the nano h -BN doping introduces only deep charge traps (above 1 eV) to the XLPE matrix. The trap energy is deeper than that of the original traps, and this is the main factor leading to the improvement of the dc insulation properties. Besides, the doping of h -BN does not introduce impurities or change the crystallinity of XLPE. This article presents the novel XLPE/ h -BN nanocomposites with excellent dc insulation properties and indicates the large potential of the nano h -BN-doped insulation materials.

Introduction

     NANOCOMPOSITES have excellent dielectric properties and have become the third-generation insulating materials [1]. Nano-doping can improve corona resistance and electrical tree characteristics as well as regulate dielectric properties of the polymer matrix. More importantly, the dc insulation properties can be enhanced to various degrees by inorganic nanoparticle doping, such as the suppression of the charge injection and transportation, as well as the increase of the breakdown strength. With the construction of high-voltage direct current (HVDC) transmission line in a large scale, the power industries have proposed stricter requirements on the dc insulation performance of insulating materials. Therefore,nanocomposite dielectrics may be more suitable for dc insulation systems in the future, and nanocomposite materials with higher insulation performance need to be continuously developed. For example, crosslinked polyethylene (XLPE)- based nanocomposites are expected to be used in the new generation of HVDC cable.

Conclusion

     The XLPE/h-BN nanocomposite with high dc breakdown strength and suppressed charge injection features were introduced in this study. It is useful to reach the conclusions as follows.

     1) XLPE/h-BN nanocomposite samples with loading content ranging from 0.1 to 3 wt% were prepared successfully. The dc Eb of the nanocomposite sample shows at most a 39% increase under 0.1 wt% loading.

     2) The effect of h-BN doping on the charge trap characteristics of XLPE is a significant increase in the density of deep traps, and this is the main cause for the improvement of dc insulation properties.

Abstract

1. Introduction

2. Experimental section

3. Results and discussion

4. Conclusions

Acknowledgment

References

Abstract

     The addition of graphene as an efficient nano-filler to different polymer matrices lead to a substantial improvement in different physical properties which, as a result, pave the way towards promising applications. In this work, Polystyrene (PS) was reinforced with different low loadings of graphene oxide (GO). The latter was prepared according to the Hummers method. The incorporated weight fractions were (0.01, 0.05, 0.1, 0.25, 1.0) wt. %. Solution blending was the adopted approach to prepare the nanocomposites. The resulted nanocomposites were characterised compared to the neat polymer. Many tools were employed to investigate the structure, morphology, thermal, and thermo-mechanical behaviour of GO, neat polymer, and the nanocomposites. The structural and morphological outcome for GO and PS was confirmed compared to the literature. The outcome confirmed as well the good dispersion of GO nano-sheets in the PS. The thermal and thermo-mechanical properties were enhanced for the nanocomposites compared to the neat polymer. The results emphasised the improvement of nanocomposites’ performance as the inclusion of GO went higher.

Introduction

     Carbon based nano-fillers for a range of applications have recently attracted a considerable attention due to their exceptional mechanical and physical properties such as high strength, high stiffness and high thermal and electrical conductivities. Many of research groups have prepared and study the features of these materials according to their low cost, low weight and ease of processing.[1] Graphite, fullerene, carbon nano-tubes and the recently discovered material, graphene, are derived from this fascinating material, carbon, and they are the most widely studied allotropes of interest to technologists and researchers due to their unique applications in various fields.

     Graphene is a planar honeycomb lattice that is a two dimensional, one atom thick carbon sheet. Its properties include high thermal conductivity, high intrinsic electron mobility, optical transmittance of 98%, large specific surface area and high Young’s modulus.[3] These properties offer graphene the potential for numerous different applications in many disciplines such as sensors, energy conversion, storage devices, solar cells and reinforced composites.[4] To enable the exploitation of these unique properties in applications, graphene and its derivatives have been successfully prepared using different routes such as bottom-up chemical vapour deposition and top down exfoliation of graphite by means of oxidation, intercalation and/or sonication.[5] As a consequence of their high G-G interactions (kind of non-covalent interaction between electron rich G system and another molecule), graphene sheets are not directly mixed with the polymer matrices to produce polymer nanocomposites as the graphene sheets tend to stack into large aggregates. For this reason, graphene oxide is usually incorporated into polymer matrices for making high quality polymer nanocomposites, as it has better compatibility with polymers, forms a uniform dispersion and offers a possibility of mass production.

Conclusions

     In this study, different low loadings of GO were incorporated homogenously into a PS matrix and different structural, thermal properties were studied as well as the nanomechanical behaviour for the polymer, and the nanosheets imaged in the cryogenically fractured surface. The incorporation of low loadings of the GO led to improvement in thermal and thermomechanical performance as the results of TGA, DSC and DMA indicated. As the homogenous dispersion for the nanosheets in the polymer matrix is a prerequisite for a good performance, this kind of dispersion was verified via different microscopic techniques carried out in this study. These techniques included OM, SEM and TEM. According to the very low weight fractions used in this study, no sharp peaks of GO appeared in Raman spectroscopy for the nanocomposites. Weak shoulders appeared in FTIR diagram and tiny humps can be seen in the curves of XRD. These can be attributed to the low loadings of GO in the PS that led to dilution of GO in the polymer matrix. In general, the incorporation of GO in PS using low loadings led to improve thermal and thermomechanical performance for the nanocomposites compared to the neat polymer. Td, Tg, and storage modulus were increased as the weight fraction of GO went higher.

This article reports the effects of different dosages of silane coupling agent (KH-550) on the properties of geopolymer that have been cured for 360 days. The mineral phase formation and microstructure changes of geopolymer were analyzed combined with mechanical properties, XRD, FT-IR, SEM and 29Si NMR. The results show that adding an appropriate amount of silane coupling agent can improve the toughness of geopolymer, and the best performance is obtained when the silane coupling agent content is 0.1 wt%. The highest compressive strength and flexural strength of geopolymer samples containing 0.1 wt% silane coupling agent cured for 90 days reached 51.4 MPa and 12.93 MPa, respectively. The gel phase in the geopolymer is significantly enriched after adding silane coupling agent. Combined with 29Si NMR analysis, it can be seen that the doping of silane coupling agent promotes the polymerization reaction, and gradually develops towards low-aluminum bonded silica and Q4(0Al) structure. The decrease in Q4(0Al) structure of the geopolymer samples cured for 360 days indicates the weakening of the geopolymerization reaction and also explains the decrease in strength after long-term curing.

Introduction

Concrete is one of the most widely used building materials, traditionally produced by using ordinary portland cement (OPC) as major binder. However, the environmental problems caused by the emission of large amounts of carbon dioxide (CO2) during the production of cement have attracted more and more attention [[1], [2], [3], [4], [5]]. The production of 1 ton of cement is accompanied by the production of 0.6–1 ton of CO2, and it is accompanied by the production of other harmful gases [6,7]. This promotes the use of geopolymer, a new type of green cementitious material, and readily available raw materials such as metakaolin (MK), fly ash (FA) and ground blast furnace slag powder (GGBS) can be used for preparation. In addition, geopolymer can also reduce CO2 emissions by 26–45% without losing economic benefits [8,9]. The excellent properties of geopolymer gradually make it an alternative to OPC [10]. Recently, more researches have shown that the use of alkaline activators can activate cementitious materials, and can directly prepare geopolymer concrete without using OPC [[11], [12], [13]]. The zeolite-like gel (N-A-S-H) with a high degree of polymerization can be produced through the polymerization reaction of the silico-alumina material under alkaline conditions. The basic structure of geopolymer is a three-dimensional network structure with random distribution of [SiO4]4- and [AlO4]5- tetrahedron, and alkali metals distribute between network pores to balance the electric charge [14].

1- Introduction

2- Calculation details

3- Results and discussion

4- Conclusion

References

Abstract

High spin-polarized materials are expected for the development of spintronic devices. In this work, we are aiming to find new spintronic materials in Mn-based binary, ternary and quaternary Heusler alloys by using the first-principles calculations. We investigated structures, electronic properties, magnetic, tetragonal distortion and mechanic properties of Mn3Ga, Mn2YGa (Y = V, Nb, Ta) and ScMnVGa. The results show that Mn3Ga, ScMnVGa type-I, and type-II have lower energy in antiferromagnetic (AFM) states than ferromagnetic (FM) and paramagnetic (NM). Mn2YGa (Y = V, Nb, Ta) compounds are more stable in FM states. The results of electronic and magnetic properties indicate that Mn3Ga is a gapless half-metallic antiferromagnet (Gapless HM-AFM). Mn2VGa and Mn2NbGa belong to gapless half-metallic ferrimagnets (Gapless HM-FE). Mn2TaGa is ferrimagnets. ScMnVGa type-I is half-metallic antiferromagnets (HM-AFM) and type-II belongs to spin gapless semiconductors with AFM (SGSs-AFM). Inspired by Ferromagnetic shape memory alloys (FAMAs), we performed tetragonal deformation of those compounds, and we predicted that Mn3Ga, Mn2YGa (Y = V, Nb, Ta) have possible martensitic transformations. Finally, the mechanical stability and elastic properties of above-mentioned in both cubic and tetragonal structures were discussed in detail.

Introduction

Spintronics is considered to be one of the most promising subjects in the information age because spintronic devices use not only the state of charge but electronic spin to transfer and store information, which speeds up information transmission and reduces energy losses. The findings of Giant Magnetoresistance (GMR), Colossal Magnetoresistance (CMR) and Tunnel Magnetoresistance (MGR) accelerate the development of spintronics. Despite great potential advantages, spintronics still faces some challenges, such as, generate high spin injectors. However, the discovery of half-metallic ferrimagnet has improved this problem, which is an ideal semiconductor spin-injection source due to it has 100% spin polarizability. Among different halfmetallic materials, Heusler alloys are prominent especially owing to high Curie temperature and flexible electronic structures [1,2]. Since De Groot et al. [3] discovered the half-metallic ferromagnets of NiMnSb, while they were calculating the energy band using the planewave method, this intermetallic materials with unusual electronic structures arouse people’s attention. In the big family of Heusler alloys, Co-based Heusler alloys [4–6] usually show magnetism, and the source of this property is direct interaction between Co-Co atoms. Cu-based Heusler alloys [7–9] are typical representative of non-magnetic elements show ferromagnetic after high-ordered, Pd-based Heusler alloys [10,11] which contain rareearth elements is superconducting, Ni2Mn-based Heusler alloys [12–17] have both ferromagnetic properties and thermoelastic martensitic transformation, which realizing magnetic-field-induced strain, shape memory effect, magnetoresistance effect and other application functions. At present, most research focuses on ternary Mn-based Heusler alloys, and they usually behave ferromagnetic or ferromagnetic [18–21]. Actually, besides half-metallic ferromagnets and ferrimagnets, half-metallic antiferromagnets also worthy of study. Not only it has 100% spin polarizability, but its total magnetic moment is zero. Thus, it means that when HM-AFMs are applied to spintronic devices, and those alloys will get the feature of the low-stray field and low energy loss. HM-AFMs were discovered in Heusler alloys [22] and perovskite compounds [23]. Besides, the martensitic transformation was found in some Mn-based Heusler alloys [24–26].

Fiber Reinforced Polymer (FRP) anchors are an effective method to increase the bond strength and/or ensure load path continuity between FRP materials and the concrete substrate when FRP materials are used as Externally Bonded Reinforcement (EBR) to strengthen and/or repair existing structures. While advances in developing a design methodology have been made on the fiber rupture and concrete cone failure modes for FRP anchors, the FRP-to-FRP bond behavior has received limited research attention. In an effort to develop design equations to calculate FRP-to-FRP bond capacity to be used by engineers, an extensive experimental program was undertaken to characterize the behavior of adhesively bonded FRP-to-FRP lap joints. Two force-based models to calculate the FRP-to-FRP bond capacity were proposed considering the influence of the critical bond length on lap joint behavior. A study to characterize the statistical properties of the experimental data was undertaken, and 95 and 99.87 percentile models were developed based on the statistical distribution of the experimental data set. Main conclusions inferred from the study and ideas for future work are also presented.

Introduction

Externally Bonded Fiber Reinforced Polymer (EBR-FRP) systems are widely used as a method for strengthening of existing structures. FRP materials feature a high strength to weight ratio, which is one of the main advantages of using FRP materials to improve the capacity of existing structures. FRP sheets, consisting of a varying number of fiber tows interweaved together, are one of the most commonly used products in EBR-FRP systems. The sheets are saturated with epoxy resins to form a composite matrix and then adhered to the external surface of the structure, but premature debonding of the FRP sheets from the concrete substrate is one of the main drawbacks of EBR-FRP systems [1]. Another common problem is the obstructions caused by structural or nonstructural elements that prevent the complete wrapping of the structure. Both of these problems limit the FRP design strain that the engineers can use when implementing EBR-FRP systems. The use of FRP anchors has been identified as one possible method to minimize premature FRP-to-concrete debonding [2,3], by transferring load from the FRP sheet directly into the RC structure. FRP anchors (as shown in Fig. 1 [4]) consist of a bundle of fibers splayed in a fan shape and bonded onto the FRP sheet at one end, with the bundle of fibers being embedded into the structure at the other end. FRP anchors typically feature a high concentration of stresses at the section where the fan transitions into the dowel, commonly known as the key portion of the anchor.

Deterioration of cement/casing adhesion in wellbore scenarios can result in unwanted and potentially harmful leakage with the potential of serious repair costs. In this work, the authors explore the use of self-healing polymers added to conventional wellbore cements as a way to bring about self-healing and readhering (to casing) properties to the composite. Self-healing capability was demonstrated by permeability analysis showing that polymer-cement composites reduce flow by 50–70% at cement bulk and at the cement/steel interface. Use of atomistic simulations imply that these polymers have good wetting properties on the steel surfaces. Interactions between steel/polymer and cement/polymer are complementary, resulting in a wider range of bonding patterns. Cracks seem to expose under-coordinated sites that result in more bonding interactions, which agrees well with the permeability measurements showing high degree of healed cracks and cement-steel interfacial gaps together with an overall increased in structural integrity of these advanced polymer-cement composite materials.

Introduction

Wellbore integrity is a significant environmental consideration in industries which use deep production wells such as in geothermal energy production. During wellbore construction, cement and cement composites are injected into the annulus between the geologic formation and the wellbore casing to hydraulically isolate production zones from overlying aquifers[1]. When applied using strict industry standards, cement and cement composites can extend the life of a producing well as well as protect the near surface environment. The potential for short life of wells and expensive remediation costs can hinder the development of geothermal energy despite of the fact that a large number of reserves of this clean energy alternative exist in the United States and around the globe[2]. A study of over 380,000 wells worldwide found that nearly 7% of wells experience wellbore failure[3] with one of the main reasons being the high temperature  (up to 400 °C), thermal cycles, and chemically corrosive (typically hypersaline, CO2 and H2S rich) environments[4] typical of low and high temperature geothermal systems. Failure of the wellbore cement can be due to a combination of chemical degradation, fracturing, and deboning from the host rock or well casing. Wellbore integrity issues are most common in the form of leakage pathways allowing for unwanted fluid migration. Cement bonding to the interface of both the casing and host rock has been identified as one of the most significant wellbore integrity issues[5].

1- Introduction

2- Experimental methods

3- Results and discussion

4- Conclusions

References

Abstract

This study deals with the development of drum brake liner for a multi-utility vehicle possessing a hydraulic brake system by varying 7 weight % of steel fiber and stainless steel fiber each, in friction composite formulations. The developed friction composites were tested for physical, chemical, corrosion, mechanical, thermal properties, and tribological characteristics, under near-actual conditions using an inertia dynamometer as per industrial standards. Finite element analysis software (ANSYS) analysis was performed to show the thermal stress distribution of the developed friction composites at the maximum temperature rise due to heat generated during brake stops, and an extensive evaluation method was used to rank the composites. The study concludes that the brake factor of the stainless steel fiber-based friction composite produces stable performance in all conditions with a lower liner temperature rise of 340 ° C and lower thermal stress at 4.255294 MPa. However, the steel fiber-based composites produced high performance at the beginning but deteriorated after a certain period due to higher levels of corrosion and a high temperature rise of 361 ° C resulting in a negative fade (−0.84%) and more thermal stress (5.619102 MPa). The primary plateau, secondary plateau, back transfer of drum wear debris, and the distribution of constituents on the worn surface of the developed composites in a resin matrix were identified and studied using a scanning electron microscope (SEM) equipped with energy-dispersive spectroscopy.

Introduction

The brake is a mechanical component for converting kinetic energy into thermal energy using friction [1]. A brake system is classified as a drum brake or disc brake based on its mating surface (construction). Drum brakes contain a set of brake liners made up of friction material bonded or riveted to a curvilinear backing plate and pressed against a rotating part called the rotating drum. In the case of a disc brake system, a disc rotates with the wheel and when the brakes are applied, a set of pads made up of friction material bonded with a backing plate present in the caliper assembly actuate towards the disc, causing the vehicle to stop. Due to energy transformation in this zone, heat dissipation takes place in the form of conduction and convection. This heat also causes the formation of friction film at the mating interface by degrading low, thermally stable, and polymeric ingredients [2]. Thus, the brake friction materials are a cocktail of 15 to 20 ingredients required to achieve desired characteristics like stable friction, low wear rate, low noise, and improved performance in all environmental conditions.

Degradation due to exposure to ultraviolet (UV) radiation is an important durability challenge with glass fiber reinforced polymer (GFRP) composite. Design and construction guidelines of GFRP suggest using UV protection paint to prevent GFRP degradation. In this study we examine the possible use of multi-walled carbon nanotubes (MWCNTs) dispersed in epoxy matrix to produce UV-resistant GFRP composite. We suggest that MWCNTs can result in a significant improvement to UV degradation resistance in the GFRP. Direct tension tests of GFRP coupons incorporating 0.25 wt%, 0.50 wt%, and 1.0 wt% of MWCNTs show inherent stability and good resistance to UV degradation. Microstructural analysis shows the ability of MWCNTs to resist polymer backbone disassociation caused by UV radiation thus preventing UV degradation in GFRP. Scanning electron microscopy (SEM) images show MWCNTs can resist microcracking caused by UV radiation and thus improve UV degradation resistance of GFRP.

Introduction

Glass fiber reinforced polymers (GFRP) composite represents an excellent material alternative for reinforcing and strengthening reinforced concrete structures because of its high strength-toweight ratio, long-term corrosion resistance, relatively low cost and ease of installation [1]. New manufacturing methods, like pultrusion and filament winding, have made it possible to use GFRP sections as frame and truss elements in FRP structures. GFRP is also used in windmill blades, airplanes and other industrial applications. GFRP is made by reinforcing polymer matrix (typically epoxy, polyester or vinyl ester) with continuous glass fibers. In this composite, the polymer matrix is the weak link and is susceptible to changes in physical properties [2]. When used in construction applications, GFRP bars and sections are prone to different types of environmental exposures including temperature, humidity, and ultraviolet (UV) radiation. It is well documented that GFRP has low resistance to UV radiation [3–6]. UV radiation experienced by the Earth’s surface has a wavelength range of 290–400 nm, which lies within the intensity known to cause bond disassociation in polymers [6,7]. The surface of the polymer degrades because of the photo-initiated oxidation that takes place due to UV exposure. This oxidation breaks the skeletal bonds in the polymers making an adverse impact on its stiffness and strength [6,8].

1- Introduction

2- Experimental investigation

3- Numerical modelling

4- Discussion and assessment of current design methods

5- New design proposal

6- Conclusions

References

Abstract

The stability and design of laser-welded stainless steel I-section beam-columns are explored in this study. Owing to the high precision and low heat input of laser-welding, structural cross-sections produced using this fabrication method have smaller heat affected zones, lower thermal distortions and lower residual stresses than would typically arise from traditional welding processes. Eighteen laser-welded stainless steel beam-columns were tested to investigate the member buckling behaviour under combined compression and bending. Two I-section sizes were considered in the tests: I-50 × 50 × 4 × 4 in grade EN 1.4301 and I-102 × 68 × 5 × 5 in grade EN 1.4571 austenitic stainless steel. The two cases of minor axis bending plus compression and major axis bending plus compression with lateral restraints were investigated. The initial loading eccentricities in the beam-column tests were varied to provide a wide range of bending moment-to-axial load ratios. The test results obtained herein and from a previous experimental study were used to validate finite element (FE) models, which were subsequently employed for parametric investigations to generate further structural performance data over a wider range of cross-section sizes, member lengths and loading combinations. The obtained test and FE results were utilized to evaluate the accuracy of the beam-column capacity predictions according to the current European and North American design provisions and a recent proposal by Greiner and Kettler. Finally, an improved approach for the design of stainless steel I-section beam-columns is proposed.

Introduction

Stainless steel is becoming increasingly used in the construction industry owing to its corrosion resistance, aesthetic appeal, favourable structural properties and a range of other beneficial characteristics. A recent addition to the stainless steel product range is that of laserwelded sections. Laser-welding is a fabrication method which uses lasers to locally melt and fuse together individual metallic elements into a range of complete structural sections without the use of filler material. Compared to conventional arc welding, laser-welding enables the heat input to be kept to a minimum, and thus leads to lower thermal distortions and residual stresses. Since their recent introduction to the construction industry, there has only been some initial research [1–4] into the structural behaviour of laser-welded stainless steel sections, and their design is not explicitly covered in current structural design provisions. The design of beam-column members generally features interaction formulae, with the bending moment resistance and compressive member resistance as end points. Previous research has been carried out into the behaviour and cross-section resistance of stainless steel Isections in bending [5–11] and the member buckling behaviour of stainless steel I-section members in compression [7,12–15]. Research has been performed on stainless steel beam-columns, but focussing on hollow sections [16–19], while studies and data on I-section beamcolumn are scarce. In 2000, Burgan et al. [7] conducted eight stainless steel welded I-section beam-column tests under compression and major axis bending. More recently Zheng et al. [16] carried out five stainless steel welded I-section beam-column tests with different buckling lengths to investigate their global stability.

1- Introduction

2- Calculation methods

3- Comparison of the two approaches

4- Conclusion

References

Abstract

Fatigue design optimization of composite material structures is limited by the classical approaches used, derived from knowledge based on metal fatigue. Other approaches exist to describe damage mechanisms of composites but they cannot always be applied at the structure scale because of their complexity. However, assumptions can be made in the case of beam structures to reduce the structural investigation at the section scale. With these assumptions this paper proposes to compare a progressive fatigue damage model written at the ply scale to the normative approach for the assessment of wind turbine blade section design. It is shown that the normative approach is very conservative and the progressive fatigue damage model provides very useful information to understand damage propagation at the structure scale.

Introduction

At the present time, the wind power industry is facing the double challenge of further increasing the size of turbines for off-shore production and making them lighter for land-based production, so that they can be adapted for use in less windy areas. The main stumbling block has been identified in the hope that the design of the blades can be optimised to meet these challenges, and it concerns the precision of their design under fatigue. A wind turbine blade is generally designed to be in rotational operation for 20 years in wind fields that by their very nature are variable. This creates structures that are highly subject to fatigue. We are talking about 108 cycles approximately, which makes fatigue over a long lifetime the main design criterion [1]. In addition to high resistance to fatigue, the material used for the blades should have a low mass, which has a direct impact on stress, sufficient stiffness so that it does not impair the aerodynamic properties of the turbine, and a cost that makes wind energy competitive. With all these elements combined, it soon became clear that glass fibre non-crimp fabrics (NCF), with possibly carbon fibres locally with polyester or epoxy matrices, were required for medium and large sized blades. Although long fibre composites have high resistance to fatigue, there is as yet no commonly accepted design method devoted specifically to these materials and the normative approach currently used in the context of blade certification is based on a transposition of existing knowledge of the fatigue behaviour of metals [2]. In this study, the normative approach is compared with a progressive fatigue damage model written at the ply scale.