Effect of fibre concentration,temperature and mould thickness on weldline integrity of short glass-fibre-reinforced polypropylene copolymer composites

The effect of fibre concentration, temperature and mould thickness on tensile strength of single- and double-gated injection-moulded polypropylene copolymer reinforced with 0, 10, 20, 30 and 40 wt% short glass fibre was studied at a fixed strain-rate of 7.58 × 10⁻³ s⁻¹ between 23 and 100 °C. It was found that tensile strength of single-gated mouldings, σ_c, increased with increasing volume fraction of fibres, ϕ_f in a nonlinear manner and decreased with increasing temperature in a linear manner. However, for ϕ_f values in the range 0–10% a simple additive rule-of-mixtures adequately described the variation of σ_c with ϕ_f over the entire temperature range 23–100 °C studied here. Tensile strength of double-gated mouldings like their single-gated counterparts decreased linearly with increasing temperature. The presence of weldlines significantly reduced tensile strength of double-gated composite mouldings but had little effect on tensile strength of the matrix. Weldline integrity factor, F _σ, defined as weldline strength divided by unweld strength, decreased with increasing ϕ_f but increased with increasing temperature. A linear dependence was found between F _σ and temperature. Mould thickness had no significant effect upon weld and unweld tensile strengths and consequently had no significant effect upon weldline integrity factor.     

An equation of state for porous materials under shock loading

Based on the physical interpretation of the linear equation of state (EOS) of dense solids under shock loading, which relates particle and shock speeds asU _s=C _b+gU _p, the EOS for porous solids has been developed and is expressed asU _s*=ΨC _b*+g*U _p whereC _b* andg* are effective bulk sound speed and effective inverse ultimate volume strain respectively. Ψ is a pore collapse function introduced specially to differentiate loading and unloading behaviour.C _b* andg* are derived theoretically whereas Ψ is established empirically as Ψ=f(U _p,C _b). This EOS does not call for any experimentally established material constant to describe the effect of porosity. Also its ability to describe the unloading behaviour distinguishes it from the presently available equations of state.     

Tensile and flexural properties of injection-moulded short glass fibre and glass bead ABS composites in the presence of weldlines

The effect of weldline on tensile and flexural properties of ABS reinforced with short glass fibres (ABS/GF) and spherical glass beads (ABS/GB) was investigated as a function of glass fibre and glass bead concentrations. The weldline was formed in the moulded specimens by direct impingement of two opposing melt fronts (i.e. cold weld). It was found that elastic modulus of ABS/GF composites, with or without weldlines increased linearly with increasing volume fraction of fibres (ϕ_f), according to the rule-of-mixtures for moduli. The presence of weldline reduced tensile and flexural modulus of the ABS/GF composites. Weldline integrity factor for elastic modulus of ABS/GF composites decreased linearly with increasing ϕ_f. Results showed that tensile and flexural strength of ABS/GF increased with increasing ϕ_f in a nonlinear fashion. Flexural strength was consistently greater than tensile strength for the same ϕ_f. Weldline affected both strengths in a significant way; weldline integrity factor decreased with increasing ϕ_f and was independent of loading mode. Tensile and flexural modulus of ABS/GB composites increased linearly with increasing volume fraction of glass beads (ϕ_b), showing no loading mode dependency. Although modulus of the ABS/GB system was not affected significantly by the weldline, its strength was affected, and more so in flexure than in tension. Weld and unweld strengths decreased with increasing ϕ_b in both tension and flexure according to Piggott and Leidner relationship; for the same ϕ_b, flexural strength was always greater than tensile strength. Weldline integrity factor for tensile strength of ABS/GF system was considerably lower than that for ABS/GB system but weldline integrity factor for flexural strength was almost the same for the two composite systems.     

Mechanical properties of single- and double-gated injection moulded short glass fibre reinforced PBT/PC composites

Tensile and flexural properties of single-gated (SG) and double-gated (DG) injection moulded blend of polybutylene terephthalate (PBT) and polycarbonate (PC) and its composites containing 15, 20 and 30 wt.% short glass fibres were investigated. In the DG mouldings, a weldline was formed by direct impingement of two opposing melt fronts (i.e. cold weld). It was found that tensile modulus was not affected by the weldline but flexural modulus decreased in the presence of weldline. For both specimen types, modulus increased linearly with volume fraction of fibres (ϕ _f), according to the rule-of-mixtures for moduli. The weldline integrity (WIF) factor for flexural modulus decreased linearly with increasing ϕ _f. Results showed that tensile and flexural strengths for SG mouldings increase with increasing ϕ _f in a linear manner according to the “rule-of-mixtures” for strengths. The presence of weldline affected both strengths in a significant way; WIF factor decreased linearly with increasing ϕ _f and was independent of loading mode. It was noted also, that the overall fibre efficiency parameter for tensile modulus was independent of specimen type but for flexural modulus it was lower in the case of DG mouldings. In all cases, efficiency parameter for strength was considerably lower than for the modulus. Impact strength and fracture toughness of SG mouldings were significantly greater than for DG mouldings. Although these properties for SG mouldings increased with increasing ϕ _f, they decreased significantly for DG mouldings. Results showed that WIF factor for impact strength and fracture toughness decreased linearly with increasing ϕ _f.     

Effects of loading rate, notch geometry and loading mode on the local cleavage fracture stress of a C–Mn steel

In this work, notched specimens with two notch geometries were tested in two loading modes (four-point bending (4PB) and three-point bending (3PB)) at various loading rates at a temperature of − 110°C for a C–Mn steel. An elastic–plastic finite-element method (FEM) is used to determine the stress distributions ahead of notches. By accurately measuring the distances of the cleavage initiation sites from the notch roots, the local cleavage fracture stress σ _f is measured. The results obtained and combining with previous studies by the authors show that the local cleavage fracture stress σ _f is closely related to the cleavage fracture mechanism (critical events) in steels. The σ _f values do not change with loading rate, notch geometry and loading mode, as long as the critical event of cleavage fracture does not change at various testing conditions. The σ _f is mainly determined by the steel microstructure, and its scatter is mainly caused by the size distribution of the weakest constituent in steels (ferrite grain or pearlite colony with large sizes and large second phase particles) and the change of the critical events in cleavage process. The σ _f can characterize the intrinsic toughness of steels and may be used in a “local approach” model for assessing integrity of flawed structures. The σ _f values could be measured by both 4PB and 3PB tests.     

Effect of fibre concentration and strain rate on mechanical properties of single-gated and double-gated injection-moulded short glass fibre-reinforced polypropylene copolymer composites

The effect of fibre concentration, strain rate and weldline on tensile strength, tensile modulus and fracture toughness of injection-moulded polypropylene copolymer (PPC) reinforced with 10, 20, 30 and 40% by weight short glass fibre was studied. It was found that tensile modulus of single- and double-gated mouldings increased with increasing volume fraction of fibres, ϕ_f, according to additive rule-of-mixtures, and increased linearly with natural logarithm of strain rate . The presence of weldlines in double-gated mouldings led to reduction in tensile modulus which for composite containing 40% by weight short fibres was as much as 30%. A linear dependence was obtained between fibre efficiency parameter for composite modulus and for both single- and double-gated moulding. Tensile strength of single-gated mouldings, σ _c, increased with increasing ϕ_f in a nonlinear manner. However, for ϕ_f in the range 0–12% a simple additive rule-of-mixtures adequately described the variation of σ _c with ϕ_f. A linear dependence was obtained between fibre efficiency parameter for tensile strength and The presence of weldlines in double-gated mouldings reduced tensile strength by as much as 70%. Tensile strength of both single- and double-gated mouldings increased linearly with Fracture toughness of single-gated mouldings increased linearly with increasing ϕ_f. The presence of weldlines in double-gated mouldings reduced fracture toughness by as much as 60% for composite containing 40% by weight short glass fibres.     

Dependence of the empirical model parameters of the surface resistance of a high-temperature superconducting film on the parameters of the technological process

The dependence of the surface resistance on the substrate heater temperature has been investigated for the purpose of optimizing the conditions for preparing YBa₂Cu₃O_7−δ films. It is shown that the resistance R _sur is highly sensitive to the accuracy of maintaining the substrate holder temperature to minimize the surface resistance of the films and maximize the parameter γ, which determines the temperature-dependent curves σ(t) and λ_L(t). Pis’ma Zh. Tekh. Fiz. 23, 79–84 (August 12, 1997)     

Length-scale distributions of the concentration pulsations of a passive impurity in a homogeneous turbulent flow

A relation for calculating the probability density function f _t ^λ (ϕ) of the length scales of a passive concentration field in homogeneous turbulence has been obtained by consideration of the joint statistics of the concentration field and its gradient. The closed equation derived for f _t ^λ (ϕ) has been solved numerically using the data of direct numerical modeling of homogeneous turbulence for the mean characteristics involved in the equation as the coefficients. The results obtained for different values of the Schmidt number have been compared. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 78, No. 6, pp. 131–142, November–December, 2005.     

Prediction of fracture toughness for heat-resistant steels considering specimen dimensions. Report 2. Ductile fracture

A model of ductile failure of a body with a crack has been developed which enables predicting fracture toughness on the upper shelf of the fracture toughness temperature dependence taking into account the influence of the stress state. The model is based on the physical-mechanical model of ductile failure which is controlled by the critical value εf reached by plastic strain at the crack tip ε _i ^ρ . In this case it is assumed that both the ε _i ^ρ value, which precedes the crack growth onset by the mechanism of pore coalescence, and the critical strain εf are functions of specific stress state parameters, namely: the critical strain is a function of the stress state triaxiality σ _m /σ _n (σ _m is the hydrostatic stress, σ _i is the stress intensity), and ε _i ^ρ is a function of the parameter χ introduced, which is an explicit function of all three principal local stresses in the process zone at the crack tip and which defines the degree to which the stress state approaches the plane strain conditions for a body of specified thickness. The model developed has two modifications one of which enables predicting fracture toughness of large-size bodies from the results of testing only small cylindrical specimens without cracks (smooth and with a circular recess) and the other from the results of testing small cylindrical specimens and small specimens with a crack. Translated from Problemy Prochnosti, No. 2, pp. 5–19, March–April, 1997.     

Two Domains of the Parameter Space of the 2D Hubbard Model for High-Tc Superconductors

High-T _c superconducting cuprates have two types of Fermi surfaces: simple-2D-tight-binding-band type (LSCO type) and the much deformed one (Bi2212 type). The difference is attributed to that of band parameter values, i.e., t′ ∼ −0.1 and t″ ∼ 0 versus t′ ∼ −0.3 and t″ ∼ 0.2 in terms of the second- and third-neighbor transfer energies t′ and t″, respectively (energy unit is the nearest-neighbor transfer energy t). Assuming a moderate value of on-site Coulomb energy U ∼ 6 and performing the variational Monte Carlo computation, we found that the two superconducting parameter domains exist in fact around these parameter sets, respectively, in which superconductivity predominates over spin density wave (SDW) due to the latter being at the brink of vanishing. Stripes were obtained in the first domain but tend to disappear in the second. In the latter domain there seems to exist parameter sets for which superconductivity appears without doping.     

Influence of atomized powder size on microstructures and mechanical properties of rapidly solidified Al–Cr–Y–Zr aluminum alloys

Rapidly solidified powders of Al–5.0Cr–4.0Y–1.5Zr (wt%) were prepared by using a multi-stage atomization-rapid solidification powder-making device. The atomized powders were sieved into four shares with various nominal diameter level and were fabricated into hot-extruded bars after cold-isostatically pressing and vaccum degassing process. Influence of atomized powder size on microstructures and mechanical properties of the hot-extruded bars was investigated by optical microscopy, X-ray diffraction, transmission electronic microscopy with EPSX and scanning electron microscopy. The results show that the fine atomized powders of rapidly solidified Al–5.0Cr–4.0Y–1.5Zr aluminum alloy attains supersaturated solid solution state under the exist condition of multi-stage rapid solidification. With the powder size increasing, there are Al₂₀Cr₂Y (cubic, a = 1.437 nm) and Ll₂ Al₃Zr (FCC, a = 0.407 nm) phase forming in the powders, and even lumpish particles of Al₂₀Cr₂Y appearing in the coarse atomized powders, as can be found in the as-cast master alloy. Typical microstructures of the extruded bars of rapidly solidified Al–5.0Cr–4.0Y–1.5Zr aluminum alloy can be characterized by fine grain FCC α-Al matrix with ultra-fine spherical particles of Al₂₀Cr₂Y and Al₃Zr. But a small quantity of Al₂₀Cr₂Y coarse lumpish particles with micro-twin structures can be found, originating from lumpish particles of the coarse powders. The extruded bars of rapidly solidified Al–5.0Cr–4.0Y–1.5Zr aluminum alloy by using the fine powders eliminated out too coarse powders have good tensile properties of σ_0.2 = 403 MPa, σ_b = 442 MPa and δ = 9.4% at room temperature, and σ_0.2 = 153 MPa, σ_b = 164 MPa and δ = 8.1% at high temperature of 350 °C.     

Modelling the effect of creep–fatigue interaction on crack growth

Time-dependent creep–fatigue crack growth (CFCG) is a major consideration in estimating the remaining life of elevated temperature components. Fracture mechanics approaches have proven useful in providing a framework for characterizing crack growth under service conditions, and in defining safe operating conditions and selecting inspection criteria and intervals. Experimental and analytical approaches have been developed to characterize crack growth under combined creep and fatigue loading conditions using (C_t )_avg as the crack tip parameter. The analytical approaches that have been proposed to characterize CFCG are limited in their application because they do not completely account for the effect of creep–fatigue interactions in modelling crack tip deformation, and thus, accurately estimating the (C_t )_avg value. A new creep-reversal parameter, C_R , is defined in this study to quantify the extent of creep–fatigue interaction at the crack tip, and is used in an analytical scheme, suitable for components, for calculating (C_t )_avg . This approach does not rely on any simplifying assumptions regarding the extent of reinstatement of C_t , which is dependent on the amount of creep reversal due to cyclic plasticity, during the unloading part of a trapezoidal loading waveform cycle. The (C_t )_avg values calculated by this approach compare well with the experimentally obtained values for compact type (CT) specimens, thus providing an experimental verification of the approach.     

Frequency effect on the electrical and dielectric properties of (TlGaS2) 1 ? x(TlInSe2)x (x = 0.005, 0.02) single crystals

The electrical properties (loss tangent (tanδ), real (ɛ) and imaginary (ɛ″) parts of complex dielectric permittivity, and ac conductivity across the layers (σ_ac)) of (TlGaS₂)_{1 − x} (TlInSe₂) _x (x = 0.005, 0.02) layered single crystals have been studied in the frequency range f = 5 × 10⁴ to 3.5 × 10⁷ Hz. The results demonstrate that the dielectric dispersion in the crystals has a relaxation nature. Almost throughout the frequency range studied, their ac conductivity follows the relation σ_ac ∼ f ^0.8, characteristic of hopping conduction through localized states near the Fermi level. The Fermi-level density of states (N _F ), the spread of their energies, the mean hop time τ and distance R, and the concentration of deep traps determining the ac conductivity of the crystals (N _t ) have been estimated. With increasing x in (TlGaS₂)_{1 − x} (TlInSe₂) _x , N _F and N _t increase, while τ and R decrease.     

Chemical potential and the self-energy correction of spinors: Further results on the large-U hubbard model

The spinor-holon effective Hamiltonian proposed by Zou and Anderson is used to calculate the chemical potential and the self-energy correction of spinors, taking into account the effect of spin exchange interaction. In the superfluid phase, the transition temperature is sensitive to various choices of the on-site Coulomb repulsionU and the band parametert. The correction due to spin fluctuations is extremely important for small doping fractions δ. In the region of δ ≤ 8t/π² U, the effect of paramagnon fluctuations renders the mean field theory invalid. The paramagnetic susceptibility χ in the long wavelength limit is almost independent of temperatureT at high temperatures. For fixedT, χ increases with doping δ while for fixed δ, χ decreases withT, in qualitative agreement with experiments.     

Correlation dependences between the mechanical characteristics of corrosion-resistant steels

A stability factor of austenite A _γ is proposed for corrosion-resistant steels. By using this factor, the equilibrium phase-structural state of steels can be quantitatively evaluated according to their chemical composition. The factor A _γ is determined by the values of the chromium and nickel equivalents. This factor, together with the generalized Scheffler diagram, enables one to deduce the correlation equations for the mechanical characteristics of steels σ_u, σ_0.2, δ, and ψ. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 1, pp. 54–60, January–February, 2007.     

Dynamics of Drop Coalescence on a Surface: The Role of Initial Conditions and Surface Properties

An investigation of the coalescence of two water drops on a surface is presented and compared with drop spreading. The associated capillary numbers are very low (< 10⁻⁵). The drops relax exponentially towards equilibrium. The typical relaxation time t_c decreases with contact angle. t_c is proportional to the drop size R, thus defining a characteristic velocity U^* = R/t_c. The corresponding U* values are smaller by many orders of magnitude than the bulk hydrodynamic velocity (U = σ /η, with σ the gas–liquid surface tension and η the viscosity). The dynamics of receding (coalescence) and spreading motion is found to be of the same order when coalescence or spreading is induced by a syringe. The dynamics of coalescence induced with the syringe deposition is systematically faster by an order of magnitude than condensation-induced coalescence. This disparity is explained by the coupling of the contact line motion with the oscillation of the drop observed for syringe deposition but absent for condensation-induced coalescence. Paper presented at the Fifteenth Symposium on Thermophysical Properties, June 22–27, 2003, Boulder, Colorado, U.S.A.     

The temperature dependence of the strength of adhesion between epoxy-rubber glues and steels

We have studied the temperature dependence of the strength of adhesion between epoxy-rubber glues and steel 45 measured at a fixed loading rate. It is established that the adhesion strength σ exhibits a linear decrease with increasing temperature, predicted by the kinetic theory of fracture, only in the temperature interval between the characteristic values T _t and T _b. Below T _t, the adhesion strength is independent of the temperature, while at T>T _b, the slope of the σ(T) curve decreases with increasing temperature. Deviations from the kinetic theory predictions are explained by a change in the fracture mechanism (with tunneling transitions being involved in the fracture at low temperatures) and by variation of the parameter γ (at elevated temperatures). The values of the activation energy and activation volume of the fracture are determined.     

Effect of rubber particle size on the impact tensile fracture behavior of MBS resin with a bimodal particle size distribution

We performed impact tensile fracture experiments on methylmethacrylate–butadiene–styrene (MBS) resin with small and large particles in a bimodal size distribution, and examined the effects of particle size on fracture behavior by fixing the total rubber content (28 wt%) and the small particle size (about 140 nm), and varying the size of large particles (about 490 nm or 670 nm). Dynamic load P′ and displacement δ′ of single-edge-cracked specimens were measured using a Piezo sensor and a high-speed extensometer, respectively. A P′−δ′ diagram was used to determine external work U _ex applied to the specimen, elastic energy E _e stored in the specimen, and fracture energy E _f for creating a new fracture surface A _s. Energy release rate was then estimated using G _f = E _f/A _s. Values of G _f were correlated with fracture loads and mean crack velocity v _m determined from load and time relationships. We then examined the effect of particle size on G _f and v _m, and results indicated that particle size plays an important role in changing the values of G _f and v _m.     

Influence of combined preliminary loading on the brittle strength of refractory steel

We study the influence of combined preliminary loading (tensile straining combined with a low-amplitude cyclic component) on the tensile strength, crack-tip opening displacement, and brittle-fracture resistance of 15Kh2MFA refractory steel after thermal treatment simulating the process of embrittlement of the reactor vessel at the end of its service life. The tensile strength is determined under the conditions of uniaxial tension for cylindrical specimens with diameters of the working part of 5 and 8 mm in liquid nitrogen. The influence of combined thermomechanical loading on the brittle strength and kinetics of crack-tip opening displacement is investigated under the conditions of eccentric tension for compact specimens 19 mm in thickness. The basic regularities of the influence of temperature and the level and amplitude of the cyclic component of overloading on the tensile strength and critical stress intensity factor of steel are established. For a temperature of preloading of 623°K, which is much higher than the temperature of brittleness of steel (T _br = 390°K), the combined preliminary thermomechanical loading increases the critical stress intensity factor K _f of steel by up to 30% as compared with its value under static loading. At the same time, for a temperature of preloading of 423°K (close to the temperature of brittleness), the procedure of combined preliminary thermomechanical loading decreases the value of K _f for the analyzed steel as compared with the case of static loading. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 42, No. 3, pp. 100–106, May–June, 2006.     

Effect of temperature on the creep behavior of a Ni–Cr–Fe–Al alloy: a comparison of the experimental data and a model

In order to characterise the mechanical behaviour of sandwich structures, which combine an interlayer of a woven wire mesh between two thin walled sheet metals, creep tests at 650, 680 and 750 °C were carried out on sheet metals made of the nickel based alloy Nicrofer 6025 HT (2.4633). In addition to the tests the creep behaviour was simulated by a model, which considers the creep rate as a function of the applied stress σ and the internal deformation resistance including an internal back stress σ_i and a particle resistance σ_P. The damage is included by a damage parameter D, which converges to “one” with increasing damage. A concluding comparison with the creep test results shows that the model is able to describe the creep behaviour of the investigated sheet metals.