Increase In Specimen Temperature Research Articles

Ultrasonic Fatigue Device and Behavior of High-Temperature Superalloy Inconel 718 with Self-Heating Phenomenon

Ultrasonic resonance fatigue test method at 20 kHz related to the very high cycle fatigue (VHCF) aims to accelerate a time-consuming experiment. In this paper, an ultrasonic fatigue device with a data acquisition system was improved for monitoring and recording the data from fatigue tests in which self-heating phenomenon exists. Symmetric tension-compression sinusoidal vibrating mode (R = −1) was observed in this study. VHCF behavior and mechanism of Inconel 718 were carried out using this device. It was concluded that more than 99% of fatigue life is consumed in initiation duration. Specimen temperature increase was not a decisive factor in VHCF strength for Inconel 718, as long as it was far less than the design temperature limitation. A single initiation site existed at the subsurface facet or grain cluster, observed from scanning electron microscope (SEM) micrographs. Quasi-cleavage fracture in transgranular ductile mode emerged and then tended to trace grain boundaries in an intergranular manner by cleavage-dominated mixed mode.

Experimental method and failure mechanisms investigation for out-of-plane shear fatigue behavior of 3D woven composites

A set of experimental method is proposed for the out-of-plane shear fatigue performance and damage mechanisms of a type of three-dimensional (3D) woven composites. The S-N curves based on the ultimate failure cycles and the shear fatigue stiffness degradation curve of specimens are obtained. Simultaneously, the relation between the system stiffness degradation and the specimen temperature increase is discussed. Moreover, the damage evolution is captured and analyzed. Microscopic observation of the damage zone on the specimen surface is carried out. Based on the images of infrared thermography and the corresponding crack distribution, the mechanism of heat generation are discussed. It can be found that the temperature distribution of the specimen is positively correlated with the shear strain distribution and crack distribution. The stiffnesses and temperature changes of the specimen show certain consistency. The out-of-plane failure mechanisms under shear fatigue loading for 3D woven composites are revealed on the basis of observation and analysis.

Thermal Stress Relaxation and High-Temperature Corrosion of Cr-Mo Steel Processed Using Multifunction Cavitation.

This research investigated high-temperature corrosion (500 °C) of Cr-Mo steel processed using water jet peening or multifunction cavitation (MFC), and the suitability of such steel for high-temperature boilers and reaction vessels. High-temperature corrosion was induced using an embedment test and a coating test using sulfide-type K2SO4-Na2SO4 powder. To measure the relaxation of the residual stress due to the decrease in work hardening caused by an increase in specimen temperature and the difference in thermal shrinkage between the surface and interior of the specimen, a thermal cycling test was conducted. For the MFC-processed specimen, the oxide film that formed on the surface suppressed mass loss, prevented crack formation, and reduced the compressive residual stress caused by high-temperature corrosion. MFC-processed Cr-Mo steel is thus suitable for a high-temperature corrosion environment.

Direct Observation of Inner-Layer Inward Contractions of Multiwalled Boron Nitride Nanotubes upon in Situ Heating

In situ heating transmission electron microscopy observations clearly reveal remarkable interlayer expansion and inner-layer inward contraction in multi-walled boron nitride nanotubes (BNNTs) as the specimen temperature increases. We interpreted the observed inward contraction as being due to the presence of the strong constraints of the outer layers on radial expansion in the tubular structure upon in situ heating. The increase in specimen temperature upon heating can create pressure and stress toward the tubular center, which drive the lattice motion and yield inner diameter contraction for the multi-walled BNNTs. Using a simple model involving a wave-like pattern of layer-wise distortion, we discuss these peculiar structural alterations and the anisotropic thermal expansion properties of the tubular structures. Moreover, our in situ atomic images also reveal Russian-doll-type BN nanotubes, which show anisotropic thermal expansion behaviors.

Effects of strain rate on mechanical properties and deformation behavior of an austenitic Fe-25Mn-3Al-3Si TWIP-TRIP steel

The effects of quasi-static and low-dynamic strain rate (ε̇ = 10−4 /s to ε̇ = 102 /s) on tensile properties and deformation mechanisms were studied in a Fe-25Mn-3Al-3Si (wt%) twinning and transformation-induced plasticity [TWIP-TRIP] steel. The fully austenitic microstructure deforms primarily by dislocation glide but due to the room temperature stacking fault energy [SFE] of 21 ± 3mJ/m2 for this alloy, secondary deformation mechanisms such as mechanical twinning (TWIP) and epsilon martensite formation (TRIP) also play an important role in the deformation behavior. The mechanical twins and epsilon-martensite platelets act as planar obstacles to subsequent dislocation motion on non-coplanar glide planes and reduce the dislocation mean free path. A high-speed thermal camera was used to measure the increase in specimen temperature as a function of strain, which enabled the use of a thermodynamic model to predict the increase in SFE. The influence of strain rate and strain on microstructural parameters such as the thickness and spacing of mechanical twins and epsilon-martensite laths was quantified using dark field transmission electron microscopy, electron channeling contrast imaging, and electron backscattered diffraction. The effect of sheet thickness on mechanical properties was also investigated. Increasing the tensile specimen thickness increased the product of ultimate tensile strength and total elongation, but had no significant effect on uniform elongation or yield strength. The yield strength exhibited a significant increase with increasing strain rate, indicating that dislocation glide becomes more difficult with increasing strain rate due to thermally-activated short-range barriers. A modest increase in ultimate tensile strength and minimal decrease in uniform elongation were noted at higher strain rates, suggesting adiabatic heating, slight changes in strain-hardening rate and observed strain localizations as root causes, rather than a significant change in the underlying TWIP-TRIP mechanisms at low values of strain.

Frequency effect in short-beam shear fatigue of a glass fiber reinforced polyester composite

The effect of loading frequency on short-beam shear fatigue behavior of a glass fiber reinforced polyester laminated was studied. Surface specimen temperature increase and macroscopic specimen damage were also evaluated. The tests were performed employing a short-beam device with three amplitude stress levels, four frequencies between 1 and 10Hz and a shear stress ratio R=0.1. Specimens with the smallest stress amplitude showed a fatigue life between 105 and 106 cycles and the largest stress amplitude endured approximately 104 cycles. The high stress amplitude tests produced increases in specimen temperature up to 7°C, 4°C and 1°C for 10Hz, 6Hz and both 3 and 1Hz respectively, while no temperature changes were measured in low amplitude tests. According to the statistical analysis performed, the changes in fatigue life were not significant for the frequency range employed in this study.

Experimental investigation of residual stresses in thin-walled welded H-sections after fire exposure

Residual stresses can have a significant effect on the stiffness of steel members. Residual stresses in welded H-shaped sections at room temperature have been widely studied. However, little information is available on residual stresses in steel members after fire exposure. Fire exposure affects the magnitude and distribution of residual stresses due to plastic deformation and creep in steel at elevated temperatures. This paper presents the results of an experimental investigation on post-fire residual stresses in thin-walled welded H-shaped sections, both for mild Q235 steel with a nominal yield stress of 235MPa and high strength Q460 steel with a nominal yield stress of 460MPa. Five Q235 steel H-sections and five Q460 steel H-sections were investigated. Residual stresses at ambient condition were also measured as a baseline. Specimens were heated to 200°C, 400°C, 600°C and 800°C and residual stresses were measured after the temperature of specimens reached the target temperature and cooled down to room temperature. It was found that the residual stresses after fire exposure decrease significantly with increase of specimen temperature. Based on these experiments, recommendations are provided for the distribution and reduction in magnitude of residual stresses after fire exposure.

Influence of curing temperature and stress conditions on mechanical properties of cementing paste backfill

Cemented paste backfill (CPB) has been observed to achieve greater cemented strength when cured in situ compared with equivalent mixes cured and tested in a laboratory environment. This is in part due to the development of effective stress and generation of elevated temperatures by exothermic cement hydration reactions occurring during curing in a typical underground stope environment. This differs from curing in typical laboratory environments, where little or no effective stresses are generated and curing occurs under constant-temperature conditions. This paper outlines the development, calibration, and testing of a temperature-controlled hydration cell that provides closer representation of in situ conditions by controlling the rate and final amount of specimen temperature increase, in addition to curing under effective stress. The temperature-controlled hydration cell was used to examine the effect of curing under combined effective stress and temperature conditions on the development of small-strain stiffness over a 7 day curing period and the unconfined compressive strength at the end of this period. Curing with both elevated temperature and effective stress was found to significantly increase the mechanical properties of CPB compared with curing at elevated effective stress or ambient temperatures alone.

Tensile Testing of Nitinol Tubes and Wires with Higher Strain Rates

During tensile testing of superelastic Nitinol material, the specimen temperature increases as result of the exothermic Austenite-to-Martensite phase transformation. The increase in specimen temperature has great influence on the stress-strain response—in particular, upper and lower plateau values—and limits the strain rate of the tensile test, so that for larger specimen dimension, the strain rate has to be reduced. A special setup of the tensile testing equipment has been developed using a fan to improve the heat exchange between the specimen and the ambiance to allow much higher strain rates as well as even gradient of the upper and lower plateau. It could be shown that the strain rate of the first loading and unloading cycle could be two to four times higher as recommended in ASTM F 2516-07 without any negative impact on the determined values. The needed time for tensile testing of Nitinol products could be reduced considerably. The improved heat exchange gives a better comparability and reproducibility of the tensile test data.

Effect of frequency upon fatigue strength of a short glass fiber reinforced polyamide 6: A superposition method based on cyclic creep parameters

AbstractThe fatigue behavior of a conditioned short glass‐fiber reinforced polyamide 6 was studied and the effect of the cyclic frequency investigated. Load controlled fatigue tests were performed, and the strains and surface temperature of specimens were recorded continuously. The number of cycles to failure was found to be dependent upon cyclic creep rate, as is typical for short glass fiber reinforced polyamides in the conditioned state. A strong reduction of fatigue strength was observed for increasing cyclic load frequency. This was mainly related to the specimen temperature increase due to hysteretic self heating and its effect on the cyclic creep speed. A frequency superposition method is proposed, expressing the relationship between temperature rise, applied stress, and cyclic creep speed in terms of a parameter derived from the Larson–Miller steady creep parameter. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers

Influence of High-Pressure Torsion Straining Conditions on Microstructure Evolution in Commercial Purity Aluminum

The influence of heat generation during severe plastic deformation on microstructure evolution was investigated in commercial purity aluminum (Al 1050, CP-Al) by using high-pressure torsion (HPT) process. The microstructure was characterized by the observations of the torsion and the longitudinal planes. CP-Al disks were deformed by HPT-straining up to 20 turns (equivalent strain, eq ,o f� 600) at 0.2 or 5 rpm at room temperature. To prevent the increase in specimen temperature, HPT-straining was also carried out in liquid nitrogen. In the all conditions, the value of Vicker’s microhardness, Hv, was saturated around 0.65 GPa and the microstructure consisted of the equiaxed grains of about 500 nm with quite low dislocation density. The microstructure in the early stage of HPT-straining showed the deformed (sub)structure, and then the equiaxed grain structure with high-angle boundaries formed by grain subdivision, recovery, continuous recrystallization and grain growth with increase in strain amounts and specimen temperature. [doi:10.2320/matertrans.ME200713]

On the ability of some cyclic plasticity models to predict the evolution of stored energy in a type 304L stainless steel submitted to high cycle fatigue

Fatigue analyses of materials are generally based on a so-called stabilized cycle, on which plastic strain amplitude, plastic energy, maximum shear stress and so on are determined. The part of plastic energy which is dissipated in heat cannot be used to accumulate damage and it should be worthwhile extracting only the part of plastic energy which is stored in material microstructure in order to build a consistent damage model. In this paper, some cyclic plasticity models including a polycrystalline model are reformulated in the thermodynamic framework in order to test their capacity to predict both the stress-strain behaviour and the partition of plastic energy for a high cycle fatigue test on a type 304L stainless steel. For an equivalent description of stress-strain loops, the number of kinematic hardening variables chosen in a model may qualitatively alter the prediction of plastic energy partition due to the modification of the isotropic hardening variable. Measurements of the specimen temperature increase due to plastic dissipation is therefore proposed as a convenient complementary experimental data to identify the constitutive equation of the isotropic hardening variable of a cyclic plasticity model.

Comparisons of experimental measurements and a theoretical model for specimen self-heating during fatigue of type 316 LN stainless steel

The Type 316 stainless steel is being considered as a candidate target-container material for the spallation neutron source (SNS) being built at the Oak Ridge National Laboratory. Satisfactory behavior under fatigue loading is a requirement for the target container. Stress-controlled fatigue experiments were performed on the 316 stainless steel at 0.2 and 10 Hz with an R ratio of −1, where R=σ min./σ max.; σ min. and σ max. are the minimum and maximum applied stresses, respectively. At R=−1, a large specimen-temperature increase at 10 Hz was observed, which approached approximately 350 °C at a stress amplitude of 263 MPa, and affected fatigue lives. The specimen temperature at 0.2 Hz was about room temperature. The fatigue lives at 10 Hz were found to be shorter than those at 0.2 Hz. Different specimen temperatures were achieved by varying test frequencies. Significant differences in fatigue lives as a function of test frequency were observed with shorter fatigue lives at higher frequencies. The higher specimen temperature at 10 than at 0.2 Hz reduced the fatigue life at 10 Hz. A model based on the dissipation energy of the specimen during fatigue tests was developed to explain the fatigue-life result and predict the specimen-temperature evolution.

Some basic aspects of electromagnetic radiation during crack propagation in metals

Measurements of the electromagnetic radiation (EMR) emitted during crack propagation and fracture and the effect of modes of fracture, physical properties and high temperature on the characteristics of emitted EMR from metals have been discussed. It has been observed that all the three modes of fracture give rise to EMR emission; however, the relative amplitude in tearing mode is very low. A linear variation of EMR peak voltage with bond energy has been observed while frequency varies parabolically with bond energy. Both these curves indicate that no EMR emission or negligible EMR emission is expected in metals having bond energy < 270 kJ/mole. EMR characteristics decrease with increase in lattice parameter. Higher tensile strength metals emit stronger EMR signals. Experiments conducted at high temperatures validate the prediction of Molotskii that an increase in specimen temperature should decrease the EMR frequency. One additional but important observation has been that while the EMR peak amplitude decreases with increase in temperature in steel, it increases with increase in temperature in copper.

The effect of mercury on the fatigue behavior of 316 LN stainless steel

Design of the mercury target system components for the Spallation Neutron Source (SNS) requires data on high- and low-cycle fatigue behavior, and the program in progress includes determining the effects of mercury on the fatigue behavior of type 316 LN stainless steel, the primary material of choice for the target vessel. Uniaxial, load-controlled fatigue tests with R=−1 and (minimum stress/maximum stress) have been conducted in air and mercury at room temperature employing constant amplitude, sinusoidal loading at frequencies from 0.1 to 700 Hz. Stress amplitude versus fatigue life ( S– N curves) data at 10 Hz for both air and mercury show a sharp knee at approximately 1 million cycles indicating a fatigue endurance limit in either air or mercury around 240 Mpa. Mean stress ( R=0.1) lowers the endurance limit to 160 MPa. At relatively low frequency, both frequency and environment (mercury) had some impact on fatigue life of type 316 LN stainless steel at high-stress levels (i.e., stresses considerably above the apparent fatigue limit). Although testing at a high frequency of 700 Hz, showed a decrease in fatigue life in air compared with that at 10 Hz, a significant increase in specimen temperature was observed in air due to self-heating. No pronounced effects of waveform have yet been found, but data are limited.

Intrinsic ‘soft-start’ polymerisation shrinkage-kinetics in an acrylate-based resin-composite

Objectives: A resin-composite based on multi-acrylate monomers was to be evaluated for any differences in the setting shrinkage-strain kinetics relative to more conventional formulations based on di-methacrylate monomers. Methods: Four resin-composites were examined for shrinkage-strain over time periods up to 60 min, from initial irradiation, using a ‘bonded disk’ measurement device. One material was evaluated with a standard and also a two-level (low to high) blue light irradiation regime. The test material ( Solitaire), and the other two controls were studied under fixed-(normal) level irradiation by blue light. Measurements were made at 23 and 37°C. Results: The low–high light irradiation applied to one di-methacrylate composite led to a highly significant ( p<0.001) reduction in shrinkage-strain to a minimal level for the initial 0.2 min. The fixed-level irradiation with the acrylate-composite also led to a formally identical, but intrinsic ‘soft-start’ shrinkage-delay regime; significant at p<0.001. Di-methacryate composites under the same fixed-level light regime did not exhibit shrinkage delay. Both di-methacrylate and multi-acrylate composites exhibited increases in the range 18–29% in final equilibrium shrinkage-strain by increase of specimen temperature. Significance: Reductions in the rate of initial shrinkage of light-cured restoratives may have clinical benefits for restoration bond-integrity nearly as important as reductions in the final equilibrium shrinkage-strain. This may be achieved either by special light irradiation regimes (low to high, or ramped) or in favourable cases by novel monomer-composite formulations and setting chemistry (e.g. Solitaire). The bonded disk shrinkage-strain measurement technique is suitable for the elucidation of such rapid kinetic and temperature-dependent events during photo-polymerisation setting processes.

Effects of target polycrystalline structure and surface gas coverage on magnetron I–V characteristics

The effects of Al target surface grain structure and surface gas coverage on sputtering magnetron I–V characteristics have been investigated to reveal the net effect of variations in the ion-induced electron emission yield (IIEEY) on I–V characteristics. One (110) single-crystal and three polycrystalline Al–1 wt % Cu targets with the similar (110) crystallographic orientations and grain sizes of 0.25, 0.95, and 5.5 mm were studied. The partial pressures were monitored before, during, and after sputtering. The direct IIEEY was measured under 600–900 eV Ar+ and He+ beam bombardment as a function of grain size and gas coverage at specimen temperatures of 19 and 74 °C. The IIEEY measurements demonstrated that both the surface grain structure and the gas coverage affect the electron emission. The direct IIEEY was highest in the small grain size and lowest in the large grain size specimens. An increase in specimen temperature raised the emission yield due to increased chemisorption of residual gas species. In contrast magnetron I–V characteristics suggested the highest IIEEY for medium grain size target. This difference is explained by the combined effect of residual gas adsorption and plasma sheath distortion around surface relief which results in IIEEY increase.

Temperature Dependence of the Thermal Diffusivity/Conductivity of Aluminum Nitride

The temperature dependence of the thermal conductivity of a polycrystalline AIN was measured using the flash‐diffusivity technique over a range of experimental conditions. Thermal conductivity data from room temperature to 300°C, obtained with attenuated laser pulse energies to minimize the specimen temperature increase, were found to be inversely proportional to the absolute temperature, as expected from theory. For high pulse energies, the experimental data deviated significantly from expected behavior. This latter effect is offered as an explanation for the anomalously low temperature dependence for the thermal conductivity reported in the literature.

HVEM in situ deformation of AlLiX alloys

Lithium additions to aluminum alloys increase both the strength and elastic modulus while decreasing the density, thereby resulting in very attractive combinations of properties. The commercial utilization of these alloys, however, has been hindered by a lack of adequate ductility at peak strength. Recent investigations have attributed the low ductility to intense, localized deformation. This is considered to be due to the promotion of planar slip by coherent, shearable, delta' (Al/sub 3/Li) precipitates and the presence of precipitate free zones (PFZ's) at high angle grain boundaries. An Al-Cu-Li-Mg-Zr alloy, produced by rapidly solidified powder processing, was found to exhibit ductility improvements over comparable, lithium-containing alloys. Thin foils prepared from bulk tensile samples were examined by transmission electron microscopy (TEM), and no evidence of localized deformation was found. These, however, were only successfully produced from the region of uniform elongation below the neck and were thus limited to approximately 4% plastic strain. In order to observe the deformation behavior under severe strain, an in situ deformation study was conducted in a high voltage electron microscope (HVEM). Several investigators have used in situ HVEM techniques to study ductile fracture processes. The advantages of HVEM versus TEM for this purpose include: thicker specimensmore » (due to a lower energy exchange of the electrons), a lower specimen contamination rate and a negligible increase in specimen temperature. Two lithium-containing alloys which had been previously reported to demonstrate localized, planar slip were studied for comparison.« less