Residual stresses in squeeze-cast aluminum rods

The longitudinal slitting technique has been applied to determining and comparing the residual stresses in as-cast and squeeze-cast aluminum rods. Residual stresses in the squeeze-cast aluminum alloy rods are found to increase with applied punch pressures under a constant die-base thermocouple reference temperature. For the variations of residual stresses with varying die-base thermocouple reference temperatures, a peak residual stress is found to occur at a die-base thermocouple reference temperature of 100° C. A semi-empirical formula is derived for the determination of the maximum longitudinal residual stress in the tapered cylindrical as-cast aluminum alloy, from which the maximum longitudinal residual stresses for squeeze cast can be determined, using the residual-stress ratios obtained experimentally     

Development of High Temperature Nanoindentation Methodology and its Application in the Nanoindentation of Polycrystalline Tungsten in Vacuum to 950 °C

The capability for high temperature nanoindentation measurements to 950 °C in high vacuum has been demonstrated on polycrystalline tungsten, a material of great importance for nuclear fusion and spallation applications and as a potential high temperature nanomechanics reference sample. It was possible to produce measurements with minimal thermal drift (typically ~0.05 nm/s at 750–950 °C) and no visible oxidative damage. The temperature dependence of the hardness, elastic modulus, plasticity index, creep, creep strain, and creep recovery were investigated over the temperature range, testing at 25, 750, 800, 850, 900 and 950 °C. The nanoindentation hardness measurements were found to be consistent with previous determinations by hot microhardness. Above 800 °C the hardness changes relatively little but more pronounced time-dependent deformation and plasticity were observed from 850 °C. Plasticity index, indentation creep and creep recovery all increase with temperature. The importance of increased time-dependent deformation and pile-up on the accuracy of the elastic modulus measurements are discussed. Elastic modulus measurements determined from elastic analysis of the unloading curves at 750–800 °C are close to literature bulk values (to within ~11 %). The high temperature modulus measurements deviate more from bulk values determined taking account of the high temperature properties of the indenter material at the point (850 °C) at which more significant time-dependent deformation is observed. This is thought to be due to the dual influence of increased time-dependency and pile-up that are not being accounted for in the elastic unloading analysis. Accounting for this time-dependency by applying a viscoelastic compliance correction developed by G. Feng and A.H.W. Ngan (J. Mater. Res. (2002) 17:660–668) greatly reduces the values of the elastic modulus, so they are agree to within 6 % of literature values at 950 °C.     

Experimental creep buckling of aluminum cylinders in axial compression

Creep-buckling tests were conducted on aluminum alloy 2024-0 circular cylinders in axial compression at 500° F having nominalR/t values of 90 and 50. Creep-buckling times for a variety of applied creep-stress values were compared with theoretical predictions of Gerard's unified theory of creep buckling of columns, plates and shells. In this theory, creep-buckling solutions are analogous to plastic-buckling solutions, provided that the material parameters used in the theoretical relation are developed from constant-strain-rate stress-strain data derived by a graphical process from compressive-creep data. The theoretical data were evaluated using appropriate classical plastic-buckling theory and previously obtained creep data on the 2024-0 aluminum material at 500° F. End shortening of the cylinders was autographically recorded during the tests and creep-buckling times were obtained from an analysis of the end-shortening record. A comparison of theory and test data indicated that the theory was somewhat conservative in predicting creep-buckling times. The discrepancy may have been due, in part, to the uncertainty in determining the precise time at which the experimental cylinders buckled. The cylinders withR/t～90 buckled in the axisymmetric mode for the lower creep stresses while, forR/t～50, all buckling occurred in the axisymmetric mode.     

Finite-strain pressure-tension loading of thick-walled cylinders including temperature effects

Experimental data were obtained from thick-walled cylinders made of hot-rolled SAE 1045 steel at room temperature, annealed OFHC copper at room temperature and at 500°F (260°C), and annealed aluminum alloy 1100 at room temperature and at 305° F (152° C). Experimental pressure-strain curves were compared with curves predicted by two different analytical solutions. One solution is a finite-strain, incremental, compressible analytical solution. The second solution is a finite-strain, total-strain, incompressible analytical solution which was corrected to make it applicable for compressible materials. With both solutions, the material is assumed to be an isotropic-hardening material that obeys the von Mises yield condition. The loading function for the material was obtained from tension specimens tested at the some temperature and loading rate as the thick-walled cylinders. Good agreement was found between each solution and experimental data.     

Rheology of carbon black suspensions. V. Heat-induced gelation and its reversibility

Linear viscoelastic properties of carbon black (CB) suspensions in a mixture of a rosin-modified phenol resin-type varnish (Varnish-1)/an alkyd resin-type varnish (Varnish-2), which exhibited a sol?Cgel transition on an increase in CB concentration, were investigated from 30°C to 80°C. The viscoelastic properties were reversible from 30°C to 60°C. In contrast, at temperatures above 60°C, the storage (G??) and loss (G??) moduli were irreversible and increased significantly with increasing temperature. This increase in the moduli is due to a change of the dispersion state to agglomerated state by heating. The agglomerated state was held, when the suspensions were lowered at 30°C. However, the G?? and G?? recovered to the original values upon shearing. This heat-induced gelation should be a universal feature for suspensions of weakly attractive particles. The temperature and shearing histories of the suspensions were discussed in relation to adsorption of polymeric component in the varnish on the CB particles.     

On the propagation of elastic waves through composite media II

Summary The propagation of elastic waves (both longitudinal and transverse) through polyurethane rubbers filled with different amounts of sodium chloride particles was studied at 0.8 MHz and 5 MHz. At a constant filler concentration (∼10% by volume), the velocity of these waves appeared to be independent of filler size. On the other hand, both velocities were found to increase with filler content. From the wave velocities, the effective modulus for longitudinal waves, L, bulk modulus, K, and shear modulus, G, were calculated according to the relations for a homogeneous isotropic material. All three moduli appear to be monotonically increasing functions of filler content, c, over the whole experimentally accessible temperature range (−80°C to +80°C for L and K; −80°C to about −30°C for G) and they, moreover, reflect the glass-rubber transition of the binder. Poissons ratio, μ, was found to decrease with increasing filler content and shows a rise at about −30°C as a result of the approach of the glass-rubber transition. The attenuation of the elastic waves was also measured in the temperature ranges mentioned. For filler particles beyond a critical size both tan δ_L and tan δ_G in the hard region are independent of the filler content within the accuracy of the measurements. The critical size depends on the type of wave and on its frequency. In the rubbery region, however, tan δ_L increases with particle size (at a constant content of 10% by volume) and even shows an enhancement with the smallest particles (1–5 μ) at 0.8 MHz. Moreover, it is found that for the same filler size tan δ_L increases with filler content. In some cases an anomalous damping behaviour was found, such that in the rubbery region the attenuation rises indefinitely with temperature. For filler particles larger than the above-mentioned critical size, tan δ_G and tan δ_L increase in the hard region as well. Finally, the experimental results are compared with existing theories on the elastic properties of and wave propagation through composite media.     

Rheological properties of poly(vinyl chloride)/plasticizer systems—relation between sol–gel transition and elongational viscosity

Poly(vinyl chloride) (PVC)/di-isononyl phthalate systems with PVC content of 45.5 (PVC8) and 70.4 wt% (PVC6) were prepared by a hot roller at 150 °C and press molded at 180 °C. The dynamic viscoelasticity and elongational viscosity of PVC8 and PVC6 were measured in the temperature range from 150 to 220 °C. We have found that the storage and loss shear moduli, G′ and G″, of PVC8 and PVC6 exhibited the power-law dependence on the angular frequency ω at 190 and 210 °C, respectively. Correspondingly, the tan δ values did not depend on ω. These temperatures indicate the critical gel temperature T _gel of each system. The critical relaxation exponent n obtained from these data was 0.75 irrespective of PVC content, which was in agreement with the n values reported previously for the low PVC concentration samples. These results suggest that the PVC gels of different plasticizer content have a similar fractal structure. Below T _gel, the gradual melting of the PVC crystallites takes place with elevating temperature, and above T _gel, a densely connected network throughout the whole system disappears. Correspondingly, the elongational viscosity behavior of PVC8 and PVC6 exhibited strong strain hardening below T _gel, although it did not show any strain hardening above T _gel. These changes in rheological behavior are attributed to the gradual melting of the PVC crystallites worked as the cross-linking domains in this physical gel, thereby inapplicability of the of time–temperature superposition for PVC/plasticizer systems.     

Three-dimensional crack tip deformation measurement using combined Moiré-Sagnac interferometry

A combined Moiré-Sagnac interferometry method is developed for in-plane (u andv) and out-of-plane (w) surface deformation measurement. The combined optical setup is used to measure three-dimensional crack tip deformations of AI 2024-0 and AI 2024-T4 specimens at room temperature and an inconel 909 specimen at 570°C. Measured displacements near the crack tip region of the AI 2024-T4 specimen are used as input nodal displacements to determine stress intensity factors based on two-dimensional and three-dimensional Jacobian derivative method. The values compare favorably with theoretical calculations. The extent of the three-dimensional crack tip deformation zone is also discussed.     

Mechanical Properties of Transparent Polycrystalline Alumina Ceramics Processed Using an Environmentally Benign Thermal Gel Casting Process

Technological advancements in ceramic powder synthesis, shaping and sintering have made it possible to tailor the microstructural, mechanical and optical property relationships in the case of advanced transparent ceramic materials. Transparent polycrystalline alumina (TPCA) is the hardest known transparent ceramic and one of the emerging candidate materials for transparent armour applications. The prerequisites for obtaining transparency with the high hardness, is to achieve the sintered average grain sizes <1 μm in combination with density close to the theoretical value. This paper outlines the processing of TPCA by an environmentally benign methyl cellulose based thermal gel casting (MCTG) process, which is employed for the first time in shaping of the TPCA. The green specimens shaped through this technique were pressureless sintered (PLS) to >96 % density at an optimum temperature of 1350 °C. The post sintering by Hot Isostatic Pressing (HIP) at an optimum temperature of 1350 °C and a pressure of 195 MPa resulted in >99.5 % of the theoretical density and a grain size of 0.7 μm. For the sake of comparison, conventional polycrystalline alumina samples (non-transparent) were also processed by sintering at 1550 °C under PLS condition with nearly the same densities (designated as PCA). The TPCA thus developed exhibit a combination of high hardness of 21 GPa, flexural strength of 550 MPa and excellent fracture resistance properties as compared to conventional PCA samples.     

Methods and correlations for the prediction of quenching rates on hot surfaces

A substantial quantity of experimental data on rewetting, much of which has not been previously reported, is analysed using the results of calculations of the two-dimensional conduction processes taking place in the walls of tubes, which have been used to simulate the cladding of nuclear fuel elements. Correlations giving the quenching heat-transfer coefficient and sputtering temperature are proposed as a result of the analysis. These correlations may be combined with the previously reported conduction analysis to predict rewetting rates under a wide range of conditions. The new data include falling film rewetting rates measured for a range of system pressures (1–15 bars), initial wall temperatures (200–650°C), coolant mass flowrates (3–50 g sec^?1) and subcoolings (0–90°C). Measurements have also been made of rewetting rates by bottom flooding of both saturated and subcooled water at atmospheric pressure.     

Numerical homogenization of elastic and thermal material properties for metal matrix composites (MMC)

A two-scale material modeling approach is adopted in order to determine macroscopic thermal and elastic constitutive laws and the respective parameters for metal matrix composite (MMC). Since the common homogenization framework violates the thermodynamical consistency for non-constant temperature fields, i.e., the dissipation is not conserved through the scale transition, the respective error is calculated numerically in order to prove the applicability of the homogenization method. The thermomechanical homogenization is applied to compute the macroscopic mass density, thermal expansion, elasticity, heat capacity and thermal conductivity for two specific MMCs, i.e., aluminum alloy Al2024 reinforced with 17 or 30 % silicon carbide particles. The temperature dependency of the material properties has been considered in the range from 0 to \(500{\,}^\circ \mathrm {C}\), the melting temperature of the alloy. The numerically determined material properties are validated with experimental data from the literature as far as possible.     

Elastic waves in truncated cones

An experimental investigation of elastic waves produced by the axial collision of strikers with truncated 2024 aluminum cones with apex angles of 0.48, 5.38, 20, and 30 deg was performed. Wave propagation was initiated at the small end of all four cones and at the large end of the 0.48-deg and 5.38-deg cones. The striker consisted of a 1/2-in.-diam steel ball or a soft phenol-impregnated fiber cylinder. In most cases, impact was caused by firing the striker from an air gun at approximately 1300 ips; in an additional series of tests, a steel ball was dropped on the cone. The metamorphosis of the pulse at the surface of the target was recorded using both foil and semiconductor resistance strain gages. Data were obtained for periods ranging from 200 to 500 μsec; this permitted the observation of several reflections from the ends of the specimen. In several instances, cylindrical aluminum rods were glued to the cone to form a composite target; this permitted observation of the initial pulse incident on the conical section both from surface strain gage and sandwiched crystal records. Studies were also conduced to ascertain the stress distribution across the base of the 20-deg cone. Initial pulse records were employed to predict the surface response in the target using the one-dimensional equation of elastic wave propagation in a cone of infinite length. Reasonable agreement between the data and the results of calculations based on the analysis was obtained.     

Measurements of the gas-particle convective heat transfer coefficient in a packed bed for high-temperature energy storage

Experimental measurements of the forced convection gas-particle heat transfer coefficient in a packed bed, high-temperature, thermal energy storage system were performed using a custom-made experimental facility. Special attention was paid to the application of uncertainty analysis (a very important concept in experimentation). General and detailed uncertainty analyses were carried out, which identified the choices that were made in the experimental planning and procedure to ensure reliable final results. The experimental data reduction program used the governing equations and the results of the uncertainty analysis while making allowance for media property variations with temperature. Results were correlated in terms of Nusselt number, Prandtl number and Reynolds number and comparisons were made with existing correlations developed with similar storage media. The maximum temperature for the bed was about 1000°C (1830°F) with flue gas as the operating fluid in the storage mode and atmospheric air in the recovery mode. Because most related previous studies were not necessarily focused on high-temperature applications, the published gas-particle heat transfer correlations were obtained at relatively low temperature ranges, generally at room temperature or at temperatures slightly above room temperature. Moreover, only a few of the previously reported correlations associated the results with the corresponding uncertainty margins. The results from this study give a convective gas-particle heat transfer correlation for high-temperature thermal energy storage applications. Also, due to substantial uncertainties normally associated with the measurements of this heat transfer coefficient, it is significant to note that no firm conclusions can be reached on the validity or non-validity of previously reported related correlations for which the uncertainty margins were not reported.     

Characterization of mechanical properties of aluminum cast alloy at elevated temperature

The tensile response, the low cycle fatigue(LCF) resistance, and the creep behavior of an aluminum(Al) cast alloy are studied at ambient and elevated temperatures.A non-contact real-time optical extensometer based on the digital image correlation(DIC)is developed to achieve strain measurements without damage to the specimen. The optical extensometer is validated and used to monitor dynamic strains during the mechanical experiments. Results show that Young's modulus of the cast alloy decreases with the increasing temperature, and the percentage elongation to fracture at 100℃ is the lowest over the temperature range evaluated from 25℃ to 300℃. In the LCF test, the fatigue strength coefficient decreases, whereas the fatigue strength exponent increases with the rising temperature. The fatigue ductility coefficient and exponent reach maximum values at 100℃. As expected, the resistance to creep decreases with the increasing temperature and changes from 200℃ to 300℃.     

A viscoplastic constitutive model for nickel-base superalloy,part 2: modeling under anisothermal conditions

In Part 2 of this study, extensive deformation tests were carried out on the nickel-base polycrystalline superalloy IN738LC under isothermal and anisothermal conditions between 450 and 950 °C. Under the isothermal conditions, the material showed almost no rate/time-dependency below 700 °C, while it showed distinct rate/time-dependency above 800 °C. Regarding the cyclic deformation, slight cyclic hardening behavior was observed when the temperature was below 700 °C and the imposed strain rate was fast, whereas in the case of the temperature above 800 °C or under slower strain rate conditions, the cyclic hardening behavior was scarcely observed. Unique inelastic behavior was observed under in-phase and out-of-phase anisothermal conditions: with an increase in the number of cycles, the stress at higher temperatures became smaller and the stress at lower temperatures became larger in absolute value although the stress range was approximately constant during the cyclic loading. In other words, the mean stress continues to evolve cycle-by-cycle in the direction of the stress at lower temperatures. Based on the experimental results, it was assumed that evolution of the variable Y that had been incorporated into a kinematic hardening rule in Part 1 of this study is active under higher temperatures and is negligible under lower temperatures. The material constants used in the constitutive equations were determined with the isothermal data, and were expressed as functions of temperature empirically. The extended viscoplastic constitutive equations were applied to the anisothermal cyclic loading as well as the monotonic tension, stress relaxation, creep and cyclic loading under the isothermal conditions. It was demonstrated that the present viscoplastic constitutive model was successful in describing the inelastic behavior of the material adequately, including the anomalous inelastic behavior observed under the anisothermal conditions, owing to the consideration of the variable Y.     

Determination of elastic and plastic characteristics of TiC–TiNi alloys by the ultrasonic resonance method

Elastic characteristics and propagation velocities of ultrasonic waves in a TiC–TiNi composite material are determined by the ultrasonic resonance method. The values of the elastic moduli of the solid composite obtained are used to estimate its plastic properties. The effect of various additives on the elastic and plastic properties of the composite is studied.     

2D temperature measurements in the wake of a heated cylinder using LIF

A technique is described to measure the instantaneous 2D temperature distribution in the wake of a heated cylinder using `laser-induced fluorescence'. Rhodamine B, a fluorescent dye, is used as a temperature indicator. The relation between fluorescence intensity and temperature is determined by means of calibration experiments in the temperature range of 20–35 °C with an accuracy of ±0.1 °C. The temperature distribution behind the heated cylinder is well visible and can be measured with a high spatial resolution. Corrections for variation in laser energy and intensity distribution in the laser sheet have to be made to further improve the accuracy of the measuring method. Received: 3 January 2001/Accepted: 18 May 2001     

A Measuring System for Bulk and Shear Characterization of Polymers

This paper describes a novel measuring system for investigating the influence of pressure and temperature on the mechanical properties of time-dependent polymer materials. The system can measure the volume and the shear relaxation moduli of solid polymer specimens simultaneously subjected to temperatures from −50 to +120°C with a precision of ±0.01°C, and pressures from atmospheric to 500 MPa with a precision of ±0.1 MPa. The paper demonstrates the measuring capabilities of the apparatus. For poly(vinyl) acetate (PVAc) are presented sample measurements of the shear relaxation modulus as function of time, pressure and temperature; specific volume; the bulk creep compliance; the coefficient of thermal expansion; the bulk modulus; and the pressure drop experiments which simulate conditions to which a material is exposed during the injection molding process. The shear moduli may be measured in the range from 1 to 4,000 MPa with the relative error of 3%.The error of volumetric measurements is 0.05%, which corresponds to 0.00005 cm³/g. In all cases results are shown as measured, no additional smoothing or filtering was employed. This paper is dedicated to Professor Nicholas W. Tschoegl on the occasion of his 87th birthday, for his contributions to the field of time-dependent bulk properties of polymeric materials.     

Investigation of heat transfer by means of pool film boiling on vertical cylinders in gravity

In this study, the heat transfer by means of pool film boiling on immersed vertical cylindrical rods was investigated. For this purpose, the rods with various dimensions, which have been heated up to 600°C, were immersed in a pure water pool in the different temperatures. The centre temperature and water temperature versus operation time were measured by K type thermocouples at the atmospheric pressure. After experimental studies, the surface temperatures of rods and heat transfer coefficients were calculated by means of Lumped method from the measured temperatures. Consequently, an empirical equation was developed between the Nusselt, Grashof, Prandtl and Jakob numbers. The experimental results showed that the specimens having the same characteristic lengths exhibited the same heat transfers performance although the specimen’s diameters and lengths differed considerably.     

Application of 2-D LIF temperature measurements in water using a Nd : YAG laser

 The application of Laser Induced Fluorescence (LIF) for temperature measurements in water using a Nd : YAG laser is investigated. A natural convection problem is used to test the applicability of LIF in the temperature range of 20–60°C. The measured temperature field is compared with numerical results and the influences of shadowgraph effects on the measured temperature field are investigated. An accuracy of 1.7°C is attained if shadowgraph effects can be neglected. This only holds if correction for photobleaching and variation of laser power output is applied. Received: 8 July 1998/Accepted: 3 February 1999