Effect of Temperature on the Dynamic Mechanical Behaviors of Zr‐Based Metallic Glass Reinforced Porous Tungsten Matrix Composite

The dynamic mechanical behaviors of Zr‐based metallic glass reinforced porous tungsten matrix composite were investigated by a split Hopkinson pressure bar at different testing temperatures. The flow stress of the composite decreased, while the ductility increased with increasing testing temperature, which were attributed to that more microcracks were initiated in the tungsten phase as well as more microshear bands were induced in the metallic glass phase with increasing temperature. The failure mode of the composite is a mixture of shearing and axial splitting. The flow layer of the metallic glass phase in the shearing fracture surface at 473 K was longer and thinner than that at 223 K.     

Fracture investigation of U‐notch made of tungsten–copper functionally graded materials by means of strain energy density

The averaged strain energy density over a well‐defined control volume was employed to assess the fracture of U‐notched specimens made of tungsten–copper functionally graded materials under prevalent mode II loading. The boundary of control volume was evaluated by using a numerical method. Power law function was employed to describe the mechanical properties (elasticity modulus, Poisson's ratio, fracture toughness and ultimate tensile stress) through the specimen width. The effect of notch tip radius and notch depth on notch stress intensity factors and mode mixity parameter χ were assessed. In addition, a comparison based on fracture load between functionally graded and homogeneous W–Cu was made. Furthermore, in this research, it was shown that the mean value of the strain energy density over the control volume can be accurately determined using coarse meshes for functionally graded materials.     

Ab initio study on fracture toughness of Ti<Subscript>0.75</Subscript>X<Subscript>0.25</Subscript>C ceramics

Ab initio density functional theory calculations have been performed to evaluate the fracture toughness for selected Ti_0.75X_0.25C ceramics (X = Ta, W, Mo, Nb and V). The calculated Young’s modulus E, surface energy γ and fracture toughness K _IC of pure TiC are in a good agreement with experimental data and other theoretical calculations. The results for Ti_0.75X_0.25C system show that alloying additions increase Young’s modulus, and all but vanadium increase surface energy. It was observed that tungsten has the most significant effect on increasing Young’s modulus, while tantalum on increasing surface energy of the Ti_0.75X_0.25C system. Surface energy plays a dominated role in determining the trend of fracture toughness. Overall, tantalum and tungsten are the most effective alloying elements in increasing the fracture toughness of Ti_0.75X_0.25C ceramics.     

The contributions of microstructural characteristics and residual stress distribution to mechanical properties of AlN/W composite system

AlN–W composites were fabricated by pressureless sintering. The residual stress and mechanical properties were investigated for AlN–W composites with different tungsten contents. The fracture strength decreased and the fracture toughness increased with increase of the tungsten phase from 5 vol% to 20 vol%. It was thought that the decrease of strength would be due to the effects of relative density and the grain boundary phases. The crack tortuosity of indented specimens was gradually marked as a function of secondary phase content. In the residual stress measurement, the matrix phase of composites exhibited compressive stress distribution, which was increased with the addition of tungsten. This tendency was consistent with the results predicted by the thermoelastic stress model. From the results of the crack propagation path and the development of fracture toughness, it was thought that the residual compressive stress of matrix phase must have contributed to the development of crack propagation resistance.     

Mechanical and thermal behaviour of polyamide 6/silicon carbide nanocomposites toughened with maleated styrene–ethylene–butylene–styrene elastomer

Polyamide 6 (PA6) based nanocomposites reinforced with 1–7 wt% silicon carbide nanoparticles (SiC_p) and toughened with 20 wt% maleated styrene–ethylene–butylene–
stryrene (SEBS‐g‐MA) were fabricated by melt blending followed by injection moulding. The effects of SiC_p addition on thermal and mechanical properties of such nanocomposites were investigated. Differential scanning calorimetry tests showed that SiC_p act as effective nucleators for the crystallization of PA6 and enhance the degree of crystallinity. Mechanical measurements revealed that SiC_p addition improves the Young's modulus and yield strength of PA6/SEBS‐g‐MA 80/20 blend at the expenses of tensile ductility and impact strength. The essential work of fracture (EWF) approach under tensile mode was employed to characterize the fracture toughness of PA6/SEBS‐g‐MA/SiC_p nanocomposites. EWF results indicated that SiC_p addition reduces both the specific EWF and specific non‐essential plastic work of fracture. Thus SiC_p additions were detrimental to the fracture toughness of PA6/SEBS‐g‐MA 80/20 blends.     

Experimental Investigations of Fracture Process in Concrete by Means of X‐ray Micro‐computed Tomography

The paper describes investigation results on fracture in notched concrete beams under quasi‐static three‐point bending by the X‐ray micro‐computed tomography. The two‐dimensional (2D) and three‐dimensional image procedures were used. Attention was paid to width, length, height and shape of cracks along beam depth. In addition, the displacements on the surface of concrete beams during the deformation process were measured with the 2D digital image correlation technique in order to detect strain localisation before a discrete crack occurred. The 2D fracture patterns in beams were numerically simulated with the finite‐element method using an isotropic damage constitutive model enhanced by a characteristic length of micro‐structure. Concrete was modelled as a random heterogeneous four‐phase material composed of aggregate, cement matrix, interfacial transitional zones and air voids. The advantages of the X‐ray micro‐computed tomography were outlined.     

Aligned Single‐Crystalline β‐Si3N4 Whiskers Prepared with SHS Process

Aligned single‐crystalline β‐Si₃N₄ whiskers with high aspect ratio were first prepared via a Self‐propagating high temperature‐synthesis (SHS) process, by using tungsten powders as catalysts. The as‐synthesized Si₃N₄ whiskers typically have uniform diameters of 400 nm, length about 200 µm, and exhibit smooth and straight surfaces. Above all, the products possesses a perfect aligned structure, which is quite different from the reported β‐Si₃N₄ whiskers. Elastic bending modulus of individual whiskers was measured by in‐situ TEM process, the average value of elastic bending modulus of individual as‐synthesized whiskers was 488 GPa. Results revealed that tungsten powders plays an significant effects on the morphology of Si₃N₄ whiskers.     

Effect of TaC addition on the microstructures and mechanical properties of Ti(C, N)-based cermets

The microstructures of the prepared Ti(C, N)-based cermets with various TaC additions were studied using X-ray diffractometry (XRD) and scanning electron microscopy (SEM). Mechanical properties such as transverse rupture strength (TRS), fracture toughness (K_1C) and hardness (HRA) were also measured. The results showed that the grain size of the cermets decreased with increasing TaC addition, but too high TaC addition resulted in agglomeration of the grains. An increasing TaC addition increased the dissolution of tungsten, titanium, molybdenum and tantalum in the binder phase. The hardness of the cermets decreased slightly with increasing TaC addition. The transverse rupture strength was the highest for the cermets with 5 wt.% TaC addition, which was characterized by fine grains, homogeneous microstructure and the moderate thickness of rim phase in the binder. The fracture toughness of the cermets with TaC addition from 0 to 5 wt.% decreased obviously, which resulting from decreased grain size. The further decreasing of fracture toughness for the cermets with 7 wt.% TaC addition was due to increased porosity and interfacial tensile stress.     

Strangpressen von verschleißbeständigen Fe‐Basis MMC

Hot extrusion of wear resistant Fe‐base metal matrix composites (MMC) Increasing demands on technical surfaces, i.e. thermal load, corrosion or wear, often prompt the development of tailored materials or coatings. In highly abrasive environments the progress in powder metallurgy has lead to the production of highly wear‐resistant materials based on metal‐matrix composites (MMC). Such materials are produced from a metal matrix (MM) based on Fe, Ni or Co and additional hard phases (HP), such as carbides, nitrides, borides or oxides. Moreover, powder metallurgical techniques can be used to adapt the particle size, the distribution and the content of the hard phases to the wear system on a large scale. HIP cladding is an established method of producing such MMC, but due to its near net shape capsule technique it is quite expensive. Because of this reason hot direct extrusion of capsules filled with powder blends was researched in a DFG‐Project as a method of producing long cylindrical products. Aiming at a high abrasive wear resistance, powder blends of hardenable steels with additions of fused tungsten carbide (WSC) or titanium carbides (TiC) were used. The extruded MMC were investigated with respect to their densification and microstructure, their bending strength and their wear resistance.     

Progressive failure of brittle rocks with non‐isometric flaws: Insights from acousto‐optic‐mechanical (AOM) data

Uniaxial compression tests combined with nondestructive testing techniques are performed to explore the roles of non‐isometric flaws in crack developments in brittle rocks. The acoustic emission (AE) rate‐process theory is adopted to analyze fracture‐related AE event rate characteristics. The full‐field optical method is applied to detect cracking modes. Experimental results show that AE activity is quite active when the matrix microcracking is dominant, while after each macrocracking event, AE activity becomes inactive because of the stress release. Multiphysical data for each tested flaw configuration faithfully confirm the rupture progressivity. The larger the flaw length ratio, the lower the peak stress (also peak axial strain and elastic modulus), as well as the more progressive the cracking process. Moreover, ultimate failure is triggered by the shear fracturing from the relatively long flaw. The short flaw is conditionally involved in ultimate failure when the stress buildup effect dominates. Finally, the fracture mechanism of brittle rocks with non‐isometric flaws is revealed.     

Packaging‐related properties of protein‐ and chitosan‐coated paper

The mechanical and gas‐barrier properties of paper and paperboard coated with chitosan–acetic acid salt (chitosan), whey protein isolate, whey protein concentrate and wheat gluten protein were studied. Paper sheets were solution‐coated using a hand applicator. In addition, bi‐layer composites of wheat gluten and paper or paperboard were produced by compression moulding, and the chitosan solution was also applied on paperboard using curtain coating. Young's modulus, fracture stress, fracture strain, tearing strength, air permeance and oxygen permeability were assessed. The mechanical and air permeance measurements of solution‐coated paper showed that chitosan was the most effective coating on a coat weight basis. This was due to its high viscosity, which limited the degree of penetration into the paper. The proteins, however, also enhanced the strength and toughness of the paper. Compression‐moulded wheat gluten/paper or paperboard, as well as curtain‐coated chitosan paperboard laminates, showed oxygen barrier properties comparable to those of paper and paperboard coated with commercial barrier materials. None of the composites could be delaminated without fibre rupture, indicating good adhesion between the coatings and the substrates. Copyright © 2005 John Wiley & Sons, Ltd.     

Heterogeneity effects on mixed‐mode I/II stress intensity factors and fracture path of laboratory asphalt mixtures in the shape of SCB specimen

Edge cracked semi‐circular shape specimen subjected to three point bend loading is a favourite test specimen for determining fracture toughness of asphalt mixtures. However, in the vast majority of previous experimental works, the homogeneous medium assumption has been considered for determining the stress intensity factor and geometry factors of asphalt mixtures tested with this test configuration. As a more realistic model and in order to consider the effects of heterogeneity on corresponding values of stress intensity factors, the asphalt mixture was modelled as a two‐phase aggregate/mastic heterogeneous mixture and its fracture behaviour was investigated using numerical models of asymmetric semi‐circular bend (ASCB) specimens. The generation and packing algorithm was employed to randomly distribute the aggregates with different shapes and sizes inside the mastic part. The effect of the mechanical properties of asphalt mixture (elastic modulus and the Poisson's ratios of aggregates and mastic), coarse aggregates distribution and crack length were studied on modes I and II geometry factors by means of extensive two‐dimensional finite element analyses. Moreover, the effect of the elastic modulus of asphalt mixture components was evaluated on the fracture path using the maximum tangential stress criterion. It was shown that crack tip location, elastic modulus of aggregates and mastic are the most important affecting parameters on the magnitude of modes I and II geometry factors. It was also shown that the geometry factors are not sensitive to the Poisson's ratios of aggregates and mastic. In addition, fracture cracking path is affected by the elastic modulus of the asphalt mixture components such that, depending on the difference between the stiffness of stiffer coarse aggregates and softer mastic part, the crack may propagate either through the aggregates, mastic or interface of aggregate/mastic.     

The feasibility of synthesis tungsten nitride in magnesium‐tungsten oxide‐melamine ternary system via mechanochemical method

Different routes have been investigated to synthesize tungsten (W) – tungsten nitride (W₂N) nanocomposite powder by mechanochemical method in magnesium–tungsten oxide–melamine ternary system. In stoichiometric mixture, reduction of tungsten oxide by magnesium took place after 30 minutes of milling in mechanically induced self‐sustaining reaction (MSR). A large amount of heat generated from magnesiothermic reaction, makes the used melamine unstable which causes the formation of undesirable tungsten carbide phase. By separating magnesiothermic reduction from nitride formation reaction, no thermal degradation of melamine occurred; however, milling alone has not brought much change in the used melamine to form tungsten nitride. Dispersion of reaction heat by adding magnesium in three‐stages (during 3 hours of milling) has been capable of preparing maximum value of tungsten nitride phase. According to scanning and transmission electron microscope images, the range of particle size was within 100 nanometer.     

A Study of Microstructures and Properties of High‐Carbon Si‐Cr Cast Steel Containing Rare Earth and Titanium

The microstructure, tensile and impact behaviour of high‐carbon Si‐Cr cast steel containing rare earth (RE) and titanium have been determined after austempering. The additions of RE and titanium refined the primary austenite grain size resulting in improving toughness. The addition of silicon handicapped the formation of carbide and carbide‐free bainitic ferrite and carbon enriched retained austenite could be obtained in the austempering structures of high‐carbon Si‐Cr cast steel, which had excellent mechanical properties and abrasion resistance. Moreover, the basic tendency of the mechanical properties of high‐carbon Si‐Cr cast steel influenced by the austempering temperature was that the hardness and tensile strength reduced and the impact toughness and fracture toughness increased with increasing temperature. The comprehensive properties were the best while austempering at 330^oC.     

Mixed finite element method for coupled thermo‐hydro‐mechanical process in poro‐elasto‐plastic media at large strains

A mixed finite element for coupled thermo‐hydro‐mechanical (THM) analysis in unsaturated porous media is proposed. Displacements, strains, the net stresses for the solid phase; pressures, pressure gradients, Darcy velocities for pore water and pore air phases; temperature, temperature gradients, the total heat flux are interpolated as independent variables. The weak form of the governing equations of coupled THM problems in porous media within the element is given on the basis of the Hu–Washizu three‐filed variational principle. The proposed mixed finite element formulation is derived. The non‐linear version of the element formulation is further derived with particular consideration of the THM constitutive model for unsaturated porous media based on the CAP model. The return mapping algorithm for the integration of the rate constitutive equation, the consistent elasto‐plastic tangent modulus matrix and the element tangent stiffness matrix are developed. For geometrical non‐linearity, the co‐rotational formulation approach is utilized. Numerical results demonstrate the capability and the performance of the proposed element in modelling progressive failure characterized by strain localization and the softening behaviours caused by thermal and chemical effects. Copyright © 2005 John Wiley & Sons, Ltd.     

Bio‐Inspired,Self‐Toughening Polymers Enabled by Plasticizer‐Releasing Microcapsules

A new concept for the design of self‐toughening thermoplastic polymers is presented. The approach involves the incorporation of plasticizer‐filled microcapsules (MCs) in an intrinsically rigid and brittle matrix polymer. The intriguing adaptability that this simple tactic enables is demonstrated with composites composed of a poly(lactic acid) (PLA) matrix and 5–20% w/w poly(urea‐formaldehyde) (PUF) MCs that contained hexyl acetate as plasticizer. At low strain (<1.5%), the glassy PLA/MC composites remain rigid, although the intact MCs reduce the Young's modulus and tensile strength by up to 50%. While the neat PLA shows brittle failure at a strain of around 2.5%, the composites yield in this regime, because the MCs rupture and release their plasticizing cargo. This effect leads up to 25‐fold increase of the elongation at break and 20‐fold increase of the toughness vis‐à‐vis the neat PLA, while the impact on modulus and ultimate stress is much smaller. Ballistic impact tests show that the self‐toughening mechanism also works at much higher strain rates than applied in tensile tests and the operating mechanism is corroborated through systematic thermomechanical studies that involved dynamic mechanical testing and thermal analysis.     

Out‐of‐Plane 3D‐Printed Microfibers Improve the Shear Properties of Hydrogel Composites

One challenge in biofabrication is to fabricate a matrix that is soft enough to elicit optimal cell behavior while possessing the strength required to withstand the mechanical load that the matrix is subjected to once implanted in the body. Here, melt electrowriting (MEW) is used to direct‐write poly(ε‐caprolactone) fibers “out‐of‐plane” by design. These out‐of‐plane fibers are specifically intended to stabilize an existing structure and subsequently improve the shear modulus of hydrogel–fiber composites. The stabilizing fibers (diameter = 13.3 ± 0.3 µm) are sinusoidally direct‐written over an existing MEW wall‐like structure (330 µm height). The printed constructs are embedded in different hydrogels (5, 10, and 15 wt% polyacrylamide; 65% poly(2‐hydroxyethyl methacrylate) (pHEMA)) and a frequency sweep test (0.05–500 rad s^?1, 0.01% strain, n = 5) is performed to measure the complex shear modulus. For the rheological measurements, stabilizing fibers are deposited with a radial‐architecture prior to embedding to correspond to the direction of the stabilizing fibers with the loading of the rheometer. Stabilizing fibers increase the complex shear modulus irrespective of the percentage of gel or crosslinking density. The capacity of MEW to produce well‐defined out‐of‐plane fibers and the ability to increase the shear properties of fiber‐reinforced hydrogel composites are highlighted.     

The effect of iron additions on the microstructure and properties of the “Tribaloy” Co-Mo-Cr-Si wear resistant alloys

The effect of iron additions in the range 5 to 15 wt % iron inclusive on the microstructure and properties of two Co-Mn-Cr-Si wear resistant alloys (Tribaloys T400 and T800) has been investigated. Iron additions were found to stabilize the f c c form of the cobalt solid solution, to give a fully eutectic matrix and to decrease the volume fraction of the primary Laves phase. These microstructural modifications have little effect on the plane strain fracture toughness but result in a significant increase in the modulus of rupture. The addition of iron induces only minor changes in the corrosion and oxidation resistance of T800, whereas the performance of T400 deteriorates.     

Processing and Characterization of In Situ (TiC–TiB2)p/AZ91D Magnesium Matrix Composites

In this paper, a practical and cost‐effective processing route, in situ reactive infiltration technique, was utilized to fabricate magnesium matrix composites reinforced with a network of TiC–TiB₂ particulates. These ceramic reinforcement phases were synthesized in situ from Ti and B₄C powders without any addition of a third metal powder such as Al. The molten Mg alloy infiltrates the preform of (Ti_p + B₄C_p) by capillary forces. The microstructure of the composites was investigated using scanning electron microscope (SEM)/energy dispersive X‐ray spectroscopy (EDS). The compression behavior of the composites processed at different conditions was investigated. Also, the flexural strength behavior was assessed through the four‐point‐bending test at room temperature. Microstructural characterization of the (TiB₂–TiC)/AZ91D composite processed at 900 °C for 1.5 h shows a relatively uniform distribution of TiB₂ and TiC particulates in the matrix material resulting in the highest compressive strength and Young's modulus. Compared with those of the unreinforced AZ91D Mg alloy, the elastic modulus, flexural and compressive strengths of the composite are greatly improved. In contrast, the ductility is lower than that of the unreinforced AZ91D Mg alloy. However, this lower ductility was improved by the addition of MgH₂ powder in the preform. Secondary scanning electron microscopy was used to investigate the fracture surfaces after the flexural strength test. The composites show signs of mixed fracture; cleavage regions and some dimpling. In addition, microcracks observed in the matrix show that the failure might have initiated in the matrix rather than from the reinforcing particulates.     

Production and Anisotropic Tensile Behavior of Resin‐Metal Interpenetrating Phase Composites

Metal‐polymer composites can be used to synthesize material properties. A variety of interpenetrating phase composites have been produced by spontaneously infiltrating porous short‐fiber preforms with unsaturated polyester resin under vacuum conditions. Porous preforms are fabricated by compacting and sintering short 304 stainless steel fibers from cutting stainless steel fiber ropes. Tensile experiments are conducted, and fractographs are examined via scanning electron microscopy. The results reveal that the tensile strength, elongation at maximum stress, and elasticity modulus of the IPCs increase with the increasing fiber fractions and exhibit anisotropy in different directions. The tensile strength and elongation at maximum stress are significantly improved compared with the consistent preforms. A nonlinear elastic behavior and sawtooth‐like fluctuation during yield deformation are noted. Compared with the through‐thickness direction, a higher tensile strength and larger elongation at maximum stress are observed in the in‐plane direction. Finer‐diameter fibers can improve the strength and increase the elongation at maximum stress. The tensile fracture surfaces show a mixture of brittle and plastic fracture characteristics.