Tribological behavior of Zr-based bulk metallic glass sliding against polymer,ceramic, and metal materials

The tribological properties of zirconium-based bulk metallic glass (BMG) sliding against polymers, steels, and ceramics at different loads and speeds were investigated. Acoustic emission (AE) technology was used to analyze wear states. The frictional coefficients of the BMG slid against the steel and ceramic balls were high but decreased with increased applied normal load and sliding speed. As the steel balls were more ductile than the ceramic ones, the steel–BMG sliding pairs generated weaker AE signals and exhibited larger wear rates. The BMG tested against the polymer balls had much lower and more stable frictional coefficients than the ones against the steel or ceramic balls because of transferred polymer layers on the BMG surfaces. The BMG against the polymer balls also exhibited the highest AE signals among the three types of counter materials used, indicating that abrasive wear dominated in the polymer-BMG sliding pairs. These results demonstrated the potential application of the BMG as a new tribomaterial that could be an alternative to the traditional crystalline metals for various counter materials.     

Allotropic transformation induced stacking faults and discontinuous coarsening in a γ-γ′ Co-base alloy

The stability of a Co-based alloy designed to possess a microstructure comprising of L1₂, γ′ Co₃Ti-type precipitates embedded in an A1, γ Co solid solution matrix has been investigated. The alloy showed acute microstructural instabilities upon ageing at 700 °C, resulting in the degeneration of the γ-γ′ aggregate into i) a faulted Co-based martensite and Co₃Ti and ii) a lamellar aggregate of A3-Co and Co₃Ti. The faulted Co-based phase was formed by isothermal diffusionless transformation of the metastable A1-phase, whilst the lamellar aggregate was a discontinuous reaction product.     

Numerical investigation of onset and evolution of LVI damages in Carbon–Epoxy plates

In order to study the onset and the evolution of low velocity impact damages in Carbon–Epoxy plates, a numerical investigation has been led. A detailed finite element model has been created by using the finite element code Abaqus® which, thanks to the different implemented algorithms, allowed considering both intra-laminar and inter-laminar failure criteria.In particular, the numerical modelling technique of such failure criteria allowed predicting delamination growth, by using special purpose-elements (cohesive elements) and fiber and matrix failure, by using Hashin criteria.Moreover, with the aim to reduce the required CPU time, a global/local finite element modelling approach has been proposed.For validation purpose, numerical results have been compared with data from two sessions of experimental impact tests. The considered impact energy values are 6 J, 10 J and 13 J respectively.     

A review on basalt fibre and its composites

In recent years, both industrial and academic world are focussing their attention toward the development of sustainable composites, reinforced with natural fibres. In particular, among the natural fibres (i.e. animal, vegetable or mineral) that can be used as reinforcement, the basalt ones represent the most interesting for their properties. The aim of this review is to illustrate the results of research on this topical subject. In the introduction, mechanical, thermal and chemical properties of basalt fibre have been reviewed. Moreover, its main manufacturing technologies have been described. Then, the effect of using this mineral fibre as reinforcement of different matrices as polymer (both thermoplastic and thermoset), metal and concrete has been presented. Furthermore, an overview on the application of this fibre in biodegradable matrix composites and in hybrid composites has been provided. Finally, the studies on the industrial applications of basalt fibre reinforced composites have been reviewed.     

Preparation and characterization of water-borne epoxy shape memory composites containing silica

Shape memory silica/epoxy composites were successfully prepared by hydrolysis of tetraethoxysilane (TEOS) within the epoxy matrix via latex, freeze-drying, and hot-press molding method. The silane coupling agent 3-triethoxysilylpropylamine (KH550) was introduced to improve the interfacial properties between the in-situ generated silica particle and epoxy matrix. The morphology structure and the effect of the content of the in-situ formed silica on the mechanical and shape memory properties of the silica/epoxy composites were studied. The experimental results indicated that the silica particles were homogenously dispersed and well incorporated into the epoxy matrix. Significant improvements were achieved in the mechanical property of the organic–inorganic hybrid materials. The silica/epoxy composites exhibited high shape recovery and fixity ratio approximately 100% even after 10 thermo-mechanical cycles.     

Design criterion for fatigue strengthening of riveted beams in a 120-year-old railway metallic bridge using pre-stressed CFRP plates

This study presents a design criterion developed for fatigue strengthening of a 120-year-old metallic railway bridge in Switzerland and presents a pre-stressed un-bonded reinforcement (PUR) system developed to apply the strengthening. The PUR system uses carbon fiber reinforced polymer (CFRP) plates; however, unlike conventional pre-stressed CFRP reinforcement methods, preparation of the existing metallic bridge surface is not required. This decreases the time required for on-site strengthening procedures. The principle of the constant life diagram (CLD) and two fatigue failure criteria (Johnson and Goodman) are described. Analytical formulations are developed based on the CLD method to determine the minimum CFRP pre-stress level required to prevent fatigue crack initiation. The PUR system uses an applied pre-stress force to reduce the mean stress level (and stress ratio) to shift an existing fatigue-susceptible metallic detail from the ‘at risk’ finite life regime to the ‘safe’ infinite life regime. The applied CLD method is particularly valuable when the stress history of the detail is not known and it is difficult to assess the remaining fatigue life. Moreover, it is shown that the currently adopted approach in many structural codes which emphasizes stress range as the dominant parameter influencing fatigue life are non-conservative for tension–tension stress patterns (i.e., stress ratios of 0 < R < 1). Analyses show that the modified Johnson formula accurately reflects the combined effect of stress range, mean stress level, and material properties, and offers a relatively easy design procedure. Details of a retrofit field application on members of a riveted wrought iron railway bridge are given. A wireless sensor network (WSN) system is used for long-term monitoring of the on-site CFRP stress levels and temperature of the retrofitted details. WSN measurements indicate that increases in ambient temperature result in increased CFRP pre-stress levels.     

Multiobjective design of viscoelastic laminated composite sandwich panels

The optimal design of laminated sandwich panels with viscoelastic core is addressed in this paper, with the objective of simultaneously minimizing weight and material cost and maximizing modal damping. The design variables are the number of layers in the laminated sandwich panel, the layer constituent materials and orientation angles and the viscoelastic layer thickness. The problem is solved using the Direct MultiSearch (DMS) solver for multiobjective optimization problems which does not use any derivatives of the objective functions. A finite element model for sandwich plates with transversely compressible viscoelastic core and anisotropic laminated face layers is used. Trade-off Pareto optimal fronts are obtained and the results are analyzed and discussed.     

Evaluation of thermal,mechanical, and electrical properties of PVDF/GNP binary and PVDF/PMMA/GNP ternary nanocomposites

Graphene nanoplatelet (GNP) was incorporated into poly(vinylidene fluoride) (PVDF) and PVDF/poly(methyl methacrylate) (PMMA) blend to achieve binary and ternary nanocomposites. GNP was more randomly dispersed in binary composites compared with ternary composites. GNP exhibited higher nucleation efficiency for PVDF crystallization in ternary composites than in binary composites. GNP addition induced PVDF crystals with higher stability; however, PMMA imparted opposite effect. The binary composite exhibited lower thermal expansion value than PVDF; the value further declined (up to 28.5% drop) in the ternary composites. The storage modulus of binary and ternary composites increased to 23.1% and 53.9% (at 25 °C), respectively, compared with PVDF. Electrical percolation threshold between 1 phr and 2 phr GNP loading was identified for the two composite systems; the ternary composites exhibited lower electrical resistivity at identical GNP loadings. Rheological data confirmed that the formation of GNP (pseudo)network structure was assisted in the ternary system.     

Nesting effect on the mode II fracture toughness of woven laminates

The mode II fracture toughness is evaluated for carbon fibre T700-epoxy reinforced woven laminates using the end notch flexure set-up. The analysed woven composites have a different tow size (3K/12K). Three different nesting/shifting configurations are applied to the plies at the fracture surface. Corrected Beam Theory with effective crack length method (CBTE) and Beam Theory including Bending rotations effects method (BTBE) are evaluated for obtaining mode II fracture toughness. During data post-processing, the importance of the bending angle of rotation and the test configuration is observed to be important. The results show that crack propagation under mode II is more stable if the matrix is evenly distributed on the surface. The nesting does not significantly affect mode II fracture toughness values, although a greater presence of matrix on the delaminated area increases its value.     

Uncertainty quantification for multiscale modeling of polymer nanocomposites with correlated parameters

We propose a stochastic multiscale method to quantify the correlated key-input parameters influencing the mechanical properties of polymer nanocomposites (PNCs). The variations of parameters at nano-, micro-, meso- and macro-scales are connected by a hierarchical multiscale approach. The first-order and total-effect sensitivity indices are determined first. The input parameters include the single-walled carbon nanotube (SWNT) length, the SWNT waviness, the agglomeration and volume fraction of SWNTs. Stochastic methods consistently predict that the key parameters for the Young’s modulus of the composite are the volume fraction followed by the averaged longitudinal modulus of equivalent fiber (EF), the SWNT length, and the averaged transverse modulus of the EF, respectively. The averaged longitudinal modulus of the EF is estimated to be the most important parameter with respect to the Poisson’s ratio followed by the volume fraction, the SWNT length, and the averaged transverse modulus of the EF, respectively. On the other hand, the agglomeration parameters have insignificant effect on both Young’s modulus and Poisson’s ratio compared to other parameters. The sensitivity analysis (SA) also reveals the correlation between the input parameters and its effect on the mechanical properties.