Mixing narrow coarse and fine coal fractions – The maximum volume fraction of suspensions

A main objective of coal-water slurry fuel (CWSF) preparation is to achieve maximum loading of coal. In the absence of a strong colloidal attractive force, the maximum loading is determined by the packing density of the particles which in turn is a function of the particle size distribution. In this study, coal fractions of different mean sizes with a narrow distribution were separated by sieving. Mixtures of different mean coarse to fine size ratios were then prepared. For each size ratio, different amounts of coarse particle contents were prepared. With these different mixtures, water was added to produce the CWSF. The maximum volume packing density, φ_max, for each mixture was determined using a rheological vane yield stress technique. The determination of φ_max involving the direct yield stress measurements of extremely high concentration suspensions is an entirely new and accurate approach. It was found that the highest φ_max was obtained when the coarse to fine ratio was ～10 located at a coarse coal content of 70 wt%. This result is consistent with that obtained by theoretical modelling of bimodal mixing of monodisperse size spheres with a size ratio of 10. At lower size ratio, φ_max obtained at the optimum coarse coal content was lower.     

Evaluation of fracture parameters of concrete from bending test using inverse analysis approach

In this study, an inverse analysis approach is developed to obtain the fracture parameters of concrete, including stress–crack opening relationship, cracking and tensile strength as well as fracture energy, from the results of a three-point bending test. Using this approach, the effects of coarse aggregate size (5–10, 10–16, 16–20 and 20–25 mm) and matrix strength (compressive strength of 40 and 80 MPa, respectively) on the fracture parameters are evaluated. For normal strength concrete, coarse aggregate size and cement matrix strength significantly influence the shape of σ–w curve. For a given total aggregate content, small aggregate size leads to a high tensile strength and a sharp post-peak stress drop. The smaller the coarse aggregate, the steeper is the post-peak σ–w curve. By contrast, in high strength concrete, a similar σ–w relationship is obtained for various aggregate sizes. The post-peak stress drop for high strength concrete is more abrupt than that for normal strength concrete. Also, the smaller the coarse aggregate size, the higher is the flexural strength. For both normal and high strength concrete, fracture energy and characteristic length are found to increase with increase of coarse aggregate size.     

The mechanism of elevated temperature intergranular cracking in heat-resistant alloys

Reheat or stress relief cracking phenomena have been reassessed in 2.25Cr1.5W heat-resistant alloys. During rupture test, time to intergranular failure increases with decreasing temperature and tensile stress and is shorter in the alloy containing a higher bulk content of phosphorus. Also the time to intergranular failure can be expressed by t = t₀·σ⁻ⁿ·exp(Q/RT) where t₀ is the proportional constant, n the stress exponent and Q the activation enthalpy. Matrix softening is accelerated under tensile stress and an active carbide growth occurs at grain boundaries oriented normal to the tensile stress direction. Because impurities segregate actively to dimples frequently observed at reheat intergranular fracture surfaces, the dimples are not micro-ductile fracture areas but the grain boundary carbide interfaces. The segregation concentration of the impurities is much higher at the grain boundary carbide interfaces than the carbide-free grain boundaries. The phosphorus segregation at the carbide interfaces of the alloy containing the higher bulk content of phosphorus is mainly replaced by the segregation of nitrogen, tin and tellurium in the alloy containing a lower bulk content of phosphorus. The elevated temperature intergranular cracking under tensile stress occurs finally due to the carbide-free grain boundary cracking following the decohesion of the grain boundary carbide interfaces.     

Theoretical analysis on pullout of anchor from anchor-mortar-concrete anchorage system

Theoretical studies on pullout of an anchor from an anchor-adhesive/mortar-concrete anchorage system have been carried out with the wide application of this anchorage system in civil engineering. Most of them focused on one interfacial debonding crack propagation from the loading end similar to the theoretical method on fiber pullout from matrix, or adopting a uniform interfacial shear stress model to calculate the tensile capacity of the anchor but with a limitary embedment length. Moreover, shear stresses were generally assumed uniformly along the thickness of the adhesive/mortar layer. Actually, the distributions of the shear stresses would be varied as both the thickness of the adhesive/mortar layer and the embedment length increase. Taking into account of this variation, the present study addressed the pullout of an anchor from an anchor-mortar-concrete anchorage system with two different boundary conditions for various embedment lengths analytically combining with the compatibility conditions and a simplified shear stress-slip relationship at the anchor-mortar interface. In the proposed analytical model, the variation of the shear stresses along the thickness of the mortar was gained. Additionally, the distributions of the tensile stresses in the anchor and the interfacial shear stresses along the embedment length were obtained under different loading conditions. After interfacial debonding, the sequences and probabilities of two interfacial debonding cracks developing from both ends of the system were analyzed according to the boundary conditions and the axial rigidities of both the anchor and concrete. Besides, the pullout load was expressed as a function of one or two debonding crack lengths. Then the maximum load P_max was gained correspondingly, as well as the critical crack lengths, by using the theories of extremum. Results show that the obtained solutions consist with the previous work from other literature if the elastic modulus of concrete is assumed to be infinite and the anchor-mortar interface is thought as rigid before debonding. Subsequently, several fundamental structural and interfacial parameters were introduced to study their influences on the calculated results using the proposed model, such as the critical crack lengths, the initial cracking load P_ini and the maximum load P_max. It was found that P_max increases linearly with the embedment length L, however, P_ini is irrelevant to L if L is very long. P_max increases monotonously with the thickness t of the mortar layer if the concrete layer keeps a constant thickness b, however, there appears a peak value in the curve of P_max varying with t if the sum of b and t is invariable. Moreover, the difference between the values of P_max under the two boundary conditions is marginal.     

Intergranular and transgranular cracking of Cu–30Zn brass during fatigue loading

Abstract

Fatigue tests have been performed on annealed α–brass to examine the dependence of fracture morphology on ?K. The cracking was predominantly intergranular at low values of ?K_Q (the stress intensity factor range when the specimen does not comply with plane strain conditions) and changed progressively to transgranular cracking at high ?K_Q values. Detailed scanning electron microscopy has been performed on the fracture surfaces of the specimens, especially from matching areas on opposite faces. It has been shown that matching extrusion and intrusion pairs as well as one–to–one matching of fine slip lines occurred on the intergranular facets indicating that plastic deformation causes the intergranular cracking. Intergranular cracking persists at low ?K_Q values even though the crack growth rate is smaller than for transgranular cracking because the latter is difficult to initiate. Transgranular cracks form only at regions of localized strain, e.g. coarse slip bands, or at cold–worked surfaces but such transgranular cracking cannot be maintained at low ?KQ values.

MST/209     

Critical conditions for the occurrence of quench cracking in an Al–Zn–Mg–Cu alloy

The occurrence of quench cracking in small cuboidal samples of aluminium alloy AA7150 was determined to be related to the maximum temperature difference (?T _max) between various locations within samples during quenching. When ?T _max between different locations is between 96 and 124 °C, there is some risk of quench cracking under various quenching conditions. When the ?T _max value is higher than 124 °C, quench cracks cannot be avoided. Quench cracks preferentially occur at sample corners and edges and are preferentially propagating in the short transverse direction–long transverse direction plane. Finite element modelling results indirectly indicate that the quench cracking should occur at the very early stages of the quenching process. Microscopy reveals that the quench cracking mode is intergranular, and cracks preferentially occur at high-angle grain boundaries with an average misorientation angle of ~42°. Moreover, quench cracks can penetrate through the whole thickness of a sample quenched from 495 into 20 °C water. Fractography reveals that no constituent particles exist in the quench fracture region, indicating that, unlike impact fracture, the occurrence of quench cracks is not dependent on the presence of coarse particles.     

An analytical model to predict the effective fracture toughness of concrete for three-point bending notched beams

An analytical model to predict the effective fracture toughness of concrete was proposed based on the fictitious crack model. Firstly, the equilibrium equations of forces in the section were formed in combination with the plane section assumption. Then a Lagrange function was presented through the equilibrium equations and the relationship formula between the effective crack length and crack tip opening displacement. Taking into account Lagrange Multiplier Method, the maximum load P_max was obtained, as well as the critical effective crack length a_c. Furthermore, was gained in an analytical manner. Subsequently, some material and structural parameters from other literatures were adopted into the proposed model for the calculation. Compared with the experimental results, most of the calculated values show a good agreement for P_max and a_c. In order to study the influence of the softening curve in the fictitious crack on the calculated fracture parameters, three series of constants determining the shape of the softening curve were chosen in the calculation. The results show that the calculated fracture parameters are not sensitive to the shape of the softening curve. Therefore, only if the elastic modulus E_c and flexural tensile strength f_r were measured, P_max, a_c and can be predicted accurately using the proposed model. Finally, the variations of the calculated fracture parameters with the specimen size and a₀/h (i.e., the ratio of the initial crack length to the depth of the specimen) were studied. It was found that both and the pre-critical crack propagation length Δa_c increase with the specimen size. However, the two parameters increase to the maximums and then decrease gradually with a₀/h. Moreover, the theories of free surface effect were utilized to explain the observed size effects.     

Fracture mechanism in coarse grained HAZ of HSLA steel welds

• 1.1. The effects of M-A constituent on the micromechanism of fracture processes in the coarse grained HAZ of HSLA steel welds were investigated by examining the initiation of voids and microcracks in sectioned tensile specimens.
• 2.2. The coarse grained HAZ includes two locally brittle microstructures, whose toughness values are strongly affected by the amount of M-A constituents.
• 3.3. Voids and microcracks are observed to initiate at the M-A constituents by the shear cracking process, i.e., cracking or decohesion of the M-A constituents at the orientation of 40– 50 degree to the tensile axis when the M-A constituents are approximately parallel to the tensile axis.
• 4.4. The void initiation strain in the coarse grained HAZ are very low because of premature void nucleation or brittle fracture at M-A constituents, confirming that the M-A constituents is the main metallurgical factor which governs the toughness of the coarse grained HAZ.

     

Low-cycle fatigue-crack initiation study in René 95

A microstructural study of fatigue deformation and cracking was conducted on René95, which is a thermomechanically processed superalloy developed for use as discs in advanced gas turbine engines. Optical, replica, scanning and transmission electron microscopy were used in order to study the deformation structures and mode of cracking during crack initiation under low-cycle fatigue. As in a previous tensile study, it was found that the deformation occurred very homogeneously throughout the material. This is believed to be due to the slip dispersive effect of the substructure in the warm worked grains and the very small size of the necklace grains. The study also showed that the number of load cycles to produce crack initiation can be strongly affected by brittle constituents of the microstructure, such as MC carbides. It was found that the specimens that had shorter lives were characterized by MC carbide cracking at the site of the crack initiation, whereas those which had longer lives under the same conditions of loading and temperature were characterized by only slip band cracking with no evidence of MC carbide cracking or decohesion in influencing the initiation.     

Impact of particle packing mix design method on fracture properties of natural and recycled aggregate concrete

The fracture properties of four types of concrete prepared using natural coarse aggregate and recycled coarse aggregate and conventional and particle packing method (PPM) of mix design approaches are studied. The three‐point bending (TPB) test is performed using three different sizes of single edge notched beam. The fracture energy is calculated from the load‐CMOD curve obtained in the TPB test, and in this process the load‐CMOD curve is curtailed at 2% of the depth of the beam. Based on CTOD_c and w₁ relationship, appropriate softening function is used to estimate the double‐K fracture parameters. The fracture energy and fracture toughness parameters of recycled aggregate concrete (RAC) is inferior to the natural aggregate concrete (NAC). The PPM mix design improves the fracture properties of concrete in comparison to the conventional mix design approach. The fracture properties of PPM mix designed RAC are comparable to that of NAC prepared using conventional method.     

Search for Enhancement of the Isobaric Thermal Expansion Coefficient of Superfluid 4He near T �� by?a?Heat Current

We report an experimental search for the enhancement of the isobaric thermal expansion coefficient (?? _P ) of superfluid ⁴He near the superfluid transition by a heat current (Q). The experiment was carried out using the hot volume technique at constant sample pressure of 1 bar. Liquid helium was contained in a thermal conductivity cell, and a constant heat current, Q=10 or 100 ??W/cm², was supplied from below through the sample column. We performed a sample density calculation based on existing helium properties known in the literature and a proposed enhancement ???? _P (Q). Both calculations, with or without the ?? _P enhancement, agree qualitatively with the measurement. The lack of definitive differentiation indicates that the ?? _P enhancement cannot be definitively resolved by our measurement in spite of applications of high-resolution thermometry and pressure regulation.     

Measurement of interface strength in Al/SiC particulate composites

Fracture in particulate-reinforced metal-matrix composites is initiated by particulate cracking and interface decohesion, and crack propagation occurs through the matrix, particulate and interface. A ‘critical stress partition’ model is described which considers the proportions of matrix, particulate and interface for which the fracture stress is exceeded. Tensile tests and microhardness measurements are reported for SiC/Al metal-matrix composites having particulate volume fractions of 0–20%. Measurements of the fractions of cracked and interface-debonded particulate before and after final fracture are combined with the fracture model to calculate the interface strength, σ_int′. The values of σ_int′ obtained are 469 MPa for uncoated SiC particulate and 438 MPa for particulate coated with a thin layer of Al₂O₃ to prevent interface reaction. The tensile results indicate that the weaker interfaces promote interface debonding and increase percent elongation.     

A Study of X-Ray analysis of fatigue fracture surface

An X-ray analysis was made of the fatigue fracture surface of SM50A steel. The residual stress and the half-value breadth of the diffraction intensity curve were examined on and under the fracture surface. An analytical study was also made of the residual stress and strain near the fatigue crack surface by using the finite element method. Emphasis was on finding the correlation between the residual stress or the half-value breadth and the applied stress intensity factor of K_max or 2K. It was found that both the residual stress and the halfvalue breadth distributions under the fracture surface were useful for the estimation of the monotonie plastic zone size or K_max.The correlation of the absolute value of the residual stress with K_max or 2K, however, was not clear in the present study. From the analytical study, it was found that the effect of the surface contact due to fatigue crack closure on the residual stress was less important and limited to the thin layer near the crack surface.     

Synthesis, characterization, and dielectric properties of Ba(Ti1−xSnx)O3 nanopowders and ceramics

We prepared Ba(Ti_1−xSn_x)O₃ powders and ceramics by means of the sol-gel process, with dibutyltin dilaurate as the Sn precursor. The samples were characterized by means of Fourier-transform infrared spectroscopy, X-ray diffraction, transmission electron microscopy, and scanning electron microscopy, and also determined the dielectric properties of the ceramics. The powders synthesized by means of the sol-gel process had a grain size on the nanometer scale, with the grains mainly composed of a cubic BaTiO₃ phase. Sn can disperse into BaTiO₃ more uniformly in the sol-gel technique using dibutyltin dilaurate as the Sn precursor. With increasing Sn concentration, the grain size of the Ba(Ti_1-xSn_x)O₃ ceramics increased and the maximum dielectric constant (?_max) first increased and then decreased. At a Sn concentration of 5 mol%, ?_max reached its maximum value (19,235).     

Cohesive zone simulation of grain size and misorientation effects on hydrogen embrittlement in nickel

The size and misorientation effects on hydrogen embrittlement of a four grain nickel aggregate are studied with the help of hydrogen informed cohesive zone model. The grain misorientation angle is parameterized by fixing the lower grains while rotating the upper grains about the out-of-plane axis. Brittle failure of the grain aggregate is observed and nominal strength obtained. In the crack-free situation, the grain misorientation exerts an obvious weakening effect on the nominal strength, which is most pronounced at misorientation angles around 20°. Such trend applies to the pre-cracked situation but is much less pronounced. Both misorientation and pre-crack lead to size effect. The nominal strength shows a decreasing trend with the grain size, indicating that grain refinement tends to improve the load bearing capacity, which coincides with the observation in practice. Further, it is shown that the size effect diagram without hydrogen can be divided into three regimes. The conclusions apply to the case with hydrogen, except that the trend of the size effect curve can be affected by large grain sizes due to the longer absolute distance of hydrogen diffusion. These results provide guidelines for grain boundary engineering and for nanomechanical tests aiming at calibrating the intergranular decohesion parameters.     

Hydrogen assisted intergranular cracking in steels

McMahon suggested that interface decohesion at grain-boundary carbides and precipitates is the mechanism of hydrogen assisted intergranular cracking, HAIC, in high strength steels. In general, cleavage of grain-boundary carbides, adhesion failure or interface decohesion at grain-boundary carbides and precipitates, and crack-tip shear slip along the grain boundary could be the mechanisms of HAIC. Hydrogen reduces cleavage strength, adhesion strength and the resistance to shear slip; therefore, hydrogen assists intergranular cracking. A method of identifying such mechanisms is suggested. A generalized theory of hydrogen assisted cracking is deduced. Brittle crystals cleave on their cleavage planes. Cleavage cracking of such crystals is anisotropic. When the crack-tip stress intensity factor, K, is low, the tortuous cracking process from the anisotropy results in rapidly increasing Stage-I crack growth rate with respect to K. The mechanism of the crack growth threshold, K_TH, is also discussed.     

Some problems in experimental fracture mechanics

A standard procedure for the determination of fracture toughness K_IC is discussed. The insufficiency of the existing K_ic determination confidence criteria is stressed and the following criteria are proposed instead: φ_max ? 1.5%; $\text{σ}{}_{\mathrm{fr}}{}^{\mathrm{net}}\text{σ}{}_{\mathrm{0.2\; ?\; 0.8}}$, in conjunction with the old criterion $\text{P}{}_{\max }\text{P}{}_{\mathrm{Q}}\text{? 1.1}$. Determination of K_IC from P_max should be used instead of from P_Q.A method for the determination of a point on the “force-displacement” diagram corresponding to crack growth initiation is set forth. The method is based on specimen compliance tests under repeated load-relief cycles. The crack growth initiation point is used to determine both the critical crack opening and plane strain fracture toughness. The indefinite effect of the growing crack (in the ease of crack opening or Cherepanov-Rice integral calculations) is thereby eliminated. Necessity is emphasized to determine the share of the J-integral which contributes to fracture process. A method for plotting the elastic displacement diagram is proposed which allows on the basis of preliminary estimates to determine fracture toughness of small-sized specimens without using special setups. The area ratio between the plastic and elastic strain diagrams is proposed to be adopted as fracture type criterion. Certain experiments to determine crack resistance of material specimens are described and discussed.     

Influence of loading levels on RCF life and failure mode of Ni-based alloy and WC–Ni ceramic composite coatings

This paper studied influence of loading levels on rolling contact fatigue (RCF) performance of NiCrBSi/WC–Ni composite coating. The results showed that abrasion, spalling, delamination, and rolling cracking were four kinds of main failure modes related with contact stress. Under relatively high contact stress, intense deformation of NiCrBSi particles resulted in rolling cracking failure. Furthermore, RCF life can be predicted by established lognormal distribution and σ_max − N model. The study also found that RCF life was closer to expectation E(N), higher failure probability of the coating. And RCF life became more discrete and more difficult to predict as contact stress increased.     

Relationship of particle stimulated nucleation,recrystallization and mechanical properties responding to Fe and Si contents in hot-extruded 7055 aluminum alloys

The variations of coarse intermetallic particles in hot-extruded 7055 aluminum alloys with 0.041 wt%Fe and 0.024 wt% Si increasing to 0.272 wt% Fe and 0.134 wt% Si were investigated.The particle stimulated nucleation(PSN) behaviors for different kind of coarse particles were detailly analyzed by EBSD.Moreover,the effect of PSN responding to Fe and Si contents on recrystallization and tensile properties of 7055 alloys was evaluated.With increasing Fe and Si contents,the size and number density of coarseη/S particles are reduced,while the number densities of coarse Al_7Cu_2 Fe and Mg_2Si particles are both increased and the coarse Al_7Cu_2 Fe particles transform from rod-like to irregular.More PSN recrystallized grains with predominant orientations deviated from the extruded fiber textures are stimulated by the irregular Al_7Cu_2 Fe and Mg_2Si particles,because a higher degree of local non-uniform deformation is produced.The rod-like Al_7Cu_2 Fe particles cause the greatest degree of local non-uniform deformation owing to the largest aspect ratio,but the shape also restricts the area of particle deformation zone(PDZ) resulting in fewer PSN recrystallized grains.The irregular η/S particles give rise to the lowest degree of local non-uniform deformation and fewest PSN recrystallized grains with the major orientations close to the extruded fiber textures.Consequently,despite the number and size of coarse η/S particles are reduced,the proportion of high angle grain boundaries(HAGBs) is increased and the extruded fiber textures are weakened with Fe and Si contents increasing,because of the increased Al_7Cu_2 Fe and Mg_2Si particles.The strength is slightly declined by the weakened 111//ED(extrusion direction) fiber texture,while the elongation is reduced for a larger number of coarse particles and more HAGBs with higher Fe and Si contents.     

Long term high temperature oxidation resistance of a nanoceria coated 316 SS under dry air conditions

In this work, nanoceria dip coated and uncoated 316 stainless steels were exposed to dry air at 1073–1273 K for times of up to 250 h. From this work, measured activation energies, Q = 256 kJ mol⁻¹ and Q = 240 kJ mol⁻¹ were found for coated and uncoated 316 SS, respectively. In the coated steel, the activation energy for oxidation cannot be attributed to a single mass transport mechanism. In addition, the scale morphologies, as well as the dominant oxide phases were determined by X-ray diffraction means. It was found that in the nanoceria dip coated steels, the scale was fine grained and highly adherent. Oxidation at increasing temperatures in the coated steels favored the development of the spinel ((Mn, Cr)₃O₄ structure and at 1273 K with the presence of Fe₂O₃ was severely hindered. In contrast, in the uncoated steel, a relatively thick scale, predominantly Fe₂O₃ developed and it exhibited severe damage through spallation and detachment from the steel substrate. Also, the resultant grain structure was rather coarse and it consisted of faceted grains with continuous nucleation/growth at grain ledges.