Constant Tensile Stress Research Articles

Effect of the intermediate action of hydrostatic pressure on the high-temperature creep and durability of copper

The intermediate action of hydrostatic pressure on the high-temperature creep of copper is studied at various creep stages. Tests performed at a constant tensile stress of 12.5 MPa at 773 K show that the application of a pressure at the creep third stage decreases the steady-state creep rate and extends the time to failure. At the steady-state stage of creep, the effect of the pressure may be ignored. At pressures of up to 1 GPa, this effect is found to be only related to healing of grain-boundary porosity. At higher pressures, the steady-state creep rate is governed by porosity healing and structural changes.

Microstructural Interactions during Stress Ageing a 7475 Aerospace Alloy

In industrial process, like creep-ageforming, materials are aged under load. To investigate the influence of an applied stress on the ageing behaviour of Cu bearing Al-7xxx series alloys, a 7475 alloy was aged under a constant tensile stress and analysed by means of Small Angle X-ray Scattering (SAXS) and TEM. Mechanical testing was also employed, to determine if there was any effect on the materials strength. The results show that during the early stages of ageing significant interactions takes place, which preferentially aligns one type of GP zone, as well as affecting their size and volume fraction. During the second stage ageing treatment, the applied stress was observed to cause more rapid over-ageing, by promoting the formation of the η phase.

A method to enhance cyclic reversibility of NiTiHf high temperature shape memory alloys

Abstract The effect of severe plastic deformation via equal channel angular extrusion (ECAE) at 650 °C on the thermal cyclic response of the 49.8Ni–42.2Ti–8Hf (in at.%) high temperature shape memory alloy was investigated. The improved thermal cyclic stability under constant tensile stresses in the ECAEd samples was attributed to the increase in critical shear stress levels for dislocation slip, which would accompany the martensitic phase transformation, due to the microstructural refinement and increase in favorable dislocation density.

Shape memory thin round wires produced by the in rotating water melt-spinning technique

Rapid solidification techniques have been shown to be suitable for producing semi-finished shape memory alloys, as thin ribbons with good functional properties obtained by the planar flow melt-spinning method. We applied the in rotating water melt-spinning technique to produce thin round wires directly from the melt. Good results have been obtained for Cu–Al–Ni and Cu–Al–Ni–Ti–Cr alloys in the diameter range from 100 to 250μm. However, these wires can still present some surface irregularities, depending on the parameters of the process. All wires including those that are not well optimized exhibit good shape memory properties, both for uniaxial elongation and bending. When biased by a constant uniaxial tensile stress, an elongation up to 2% (at 70MPa) is attained on cooling below the martensitic transformation, completely recovered on heating. The transformation temperatures of the as-prepared wires are depressed in relation to those of the bulk alloys annealed at 850°C and quenched, as is observed for melt-spun shape memory ribbons. The temperatures of the wires and bulk alloys evolve in rather parallel ways upon ageing in air at moderate temperatures (250°C); thus a difference of about 20 or 50°C for the ternary and quinternary alloys, respectively, persists in the transformation temperatures between wires and bulk, due to the finer microstructure of the wires. The ageing treatment also causes a degradation of the shape memory properties; after a treatment for 700h at 250°C, the recoverable elongation under 70MPa is reduced to 1.2%.

Mechanisms of short crack growth at constant stress in bone

This paper describes an experimental study of the growth of small (i.e. sub-millimetre) cracks in samples of cortical bone subjected to a constant tensile stress. Slow, stable crack growth occurred at a rate and angle which were dependent on the orientation of the sample: tests were conducted with the loading axis both parallel and perpendicular to the longitudinal axis of the bone. All cracks showed intermittent growth in which periods of relatively rapid propagation alternated with periods of temporary crack arrest or relatively slow growth. In some cases crack arrest could be clearly linked to microstructural features such as osteons or Volkmann's canals, which acted as barriers to crack growth. Crack-opening displacement increased over time during the arrest periods. These observations suggest a mechanism for the growth of small cracks in bone at constant stress, involving microstructural barriers, time-dependent deformation of material near the crack tip and strain-controlled propagation.

Analysis of stresses during the freezing of solid spherical foods

An analytical model is presented to calculate thermal stresses and strains during the freezing of a spherical food, taking into account both the expansion during phase change and subsequent thermal contraction due to temperature decrease. The Young modulus and Poisson ratio are assumed to undergo a step change at the freezing point. The expansion due to phase change cause a uniform and virtually constant isotropic tensile stress in the unfrozen core. In the frozen shell, this expansion gives rise to tensile radial stress and compressive tangential stress. The thermal contraction subsequent to phase change causes reverse effects, i.e. uniform compressive stress in the unfrozen core and compressive radial stress in the frozen shell, while tangential stress is tensile on the outside and compressive on the inside of the frozen shell. The effect of thermal contraction is noticeable only at cryogenic temperatures.

Growth stresses and cracking in GaN films on (111) Si grown by metal-organic chemical-vapor deposition. I. AlN buffer layers

Intrinsic stress evolution during the growth of GaN by metal-organic chemical-vapor deposition on (111) Si, using an AlN buffer layer, was monitored in situ with a multiple-beam optical stress sensor. Data show that stress evolution takes place in two stages: an initial compressive regime up to about 100nm in thickness followed by a transition to a constant tensile stress, ∼0.3GPa, in films up to 1μm thick. Correlation of the stress evolution with surface morphological evolution by sequential atomic force microscopy images clearly shows that the incremental stress remains compressive in spite of grain coalescence, which is generally considered to be the dominant source of tensile stress in GaN films on sapphire. Rather, the most dominant feature accompanying the transition in stress from compressive to tensile, which takes place after grain coalescence, is an increase in the lateral size of individual islands. It is shown that this incremental tensile stress accompanied by an increase in lateral grain size can be accounted for by the annihilation of free volume associated with the grain boundaries. On samples cooled to room temperature, surface cracks mainly on the (1010) planes are observed to have channeled in films thicker than 250nm. Analysis of cracking using the theory of brittle fracture, using the measured growth stress profile and value for the critical thickness, yields a thermal-expansion mismatch stress off 1.1GPa for GaN films deposited at 1100°C and cooled to room temperature.

A simple apparatus for measuring long-term extension of plant cell walls subjected to tensile stress

Investigations of the control of cell expansion through cell wall-tightening and -loosening require replicate measurements of long-term irreversible wall extension. A simple, inexpensive, high-throughput apparatus is described for monitoring long-term wall extension in elongating plant organs [e.g., silks (carpellary styles) from maize (Zea mays), or phloem fibre bundles from celery (Apium graveolens) petioles] subjected to a constant tensile stress. The specimen was methanol-killed, and a suitable zone of the (relaxed) specimen was marked with ink. The specimen was then hung within a glass tube filled with buffer (2 ml) plus any additives of interest, a suitable tension being applied by weights. Total creep (i.e. plastic plus elastic increase in taut length) was estimated approximately. After 16 h, the specimen was taken down and its relaxed length re-measured. Marked zones (initially 75 mm) of celery petiole fibre bundles, when stretched at pH 4.0 for 16 h with a 25-g weight, underwent an irreversible increase in length of about 20 mm, easily measured with a ruler. The capacity to undergo irreversible extension was strongly inhibited by neutral buffers, or by pre-incubation in formaldehyde or boiling water (but not in boiling methanol), suggesting a role for endogenous expansins. Irreversible extension was slightly inhibited by 1 mM Al3 + , whereas total (reversible plus irreversible) creep was strongly inhibited. Thus, total creep did not always correctly predict the concomitant irreversible extension. We propose that irreversible extension is more closely related (than is total creep) to the mechanism of plant growth.

Intrinsic stresses in AlN layers grown by metal organic chemical vapor deposition on (0001) sapphire and (111) Si substrates

Stress evolution during metal organic chemical vapor deposition growth of AlN layers on (111) Si and (0001) sapphire substrates was investigated using in situ wafer curvature measurements in order to understand the origin of growth stresses. AlN layers 170±30nm thick were deposited over a temperature range of 600–1100°C at a growth rate of 0.2±0.05nm∕s. On (111) Si, AlN films grow under a constant tensile stress right from the beginning of growth in the temperature range investigated. In contrast, above 900°C on sapphire, an initial compressive growth stress is observed followed by a transition to a final tensile stress, while below 800°C the stress is tensile right from the beginning of growth as observed on Si. The origin of this stress behavior is explained in terms of a combination of epitaxial stress and grain coalescence stress. Calculation of the grain coalescence stresses show that the value is higher for growth on sapphire substrates than on Si substrates. Also, for growth on both substrates, a sharp drop in the value of the coalescence related tensile stress is observed between 900°C and 800°C. Both these features are correlated to the variation in the integral breadth of the (0002) and (101¯1) AlN x-ray rocking curves. It is shown that both the lower tensile stress on Si substrates and the drop in stress between 800°C and 900°C can be attributed to the reduction in grain coalescence stresses due to increasing grain boundary energies in the film resulting from increasing grain misorientation with respect to the substrate/film interface.

Evolution of surface morphology and film stress during MOCVD growth of InN on sapphire substrates

The evolution of surface morphology and film stress during metalorganic chemical vapor deposition (MOCVD) growth of InN on sapphire substrates was investigated. In situ stress measurements were carried out using a multi-beam optical stress sensor system, which measures changes in sample curvature during film deposition. Growth of InN on a low temperature AlN buffer layer on sapphire was observed to occur via island nucleation and coalescence. A constant tensile growth stress of approximately 0.2 GPa was measured in the InN films for thickness < 200 nm. For films thicker than approximately 200 nm, growth continued at almost zero stress. This corresponded to the point at which film delamination was observed to occur at growth temperature. This undesirable stress relief mechanism limits the thickness of InN layers that were grown by MOCVD on sapphire using an AlN buffer layer.

Abnormal acceleration of creep deformation rate above 700 °C in the orthorhombic based Ti–22Al–27Nb alloy

For the past decade, Ti 2AlNb-based alloys have been investigated for being applied to the structural materials in aerospace engine. In this study, creep tests of Ti–22Al–27Nb (at.%) alloys are conducted at a constant tensile stress of 200 MPa at the various temperatures in the range from 650 to 750 °C. It is observed that the creep deformation rate of the O+bcc dual phase alloys is abnormally increased at temperatures above 700 °C. The results of TEM investigation show that the active slip systems are varied and the density of dislocation is increased with increasing temperature above 700 °C. In this temperature range, prismatic dislocations in the orthorhombic phase are newly generated from O–bcc interfaces by the result of phase transformation and the movement of the prismatic dislocations is very active compared to the basal slip system. Therefore, it is suggested that the phase transformation of bcc to O phase activates another slip system by the abundant supply of actively moving dislocations, thereby accelerating creep deformation rate at the elevated temperature above 700 °C.

Shape memory properties of Cu-based thin wires obtained by the “in rotating water spinning” technique

Cu-Al-Ni-based cylindrical wires with diameters ranging between 100 and 250 μm have been successfully produced directly from the melt using the In Rotating Water Melt-Spinning (INROWASP) technique. In such a method, a cylindrical jet of molten alloy is ejected into a liquid cooling layer (water) disposed in the inner surface of a rotating drum. This soft cooling medium allows to preserve the cylindrical shape after solidification. Due to the rapid solidification, the martensitic transformation temperatures of the as-prepared wires are depressed in relation to the bulk alloy. Ageing at moderate temperatures (250°C) increases the transformation temperatures of the wires and of the same alloys in bulk form. However, the decrement in the transformation temperatures of the wires still persists, due to their finer grain size. They exhibit superelasticity as well as shape memory properties, both for uniaxial elongation and bending. When biased by a constant uniaxial tensile stress, an elongation up to 2% (at 70 MPa) is attained, in the as-prepared form, upon cooling down the transformation, completely recovered on heating. The recoverable elongation decreases to 1.2% for wires aged 700 h at 520K.

Influence of Moisture on Ultra‐High‐Temperature Tensile Creep Behavior of in Situ Single‐Crystal Oxide Ceramic Alumina/Yttrium Aluminum Garnet Eutectic Composite

Tensile creep tests were conducted for an in situ single‐crystal alumina/yttrium aluminum garnet (Al2O3/Y3Al5O12 (YAG)) binary system eutectic composite at temperatures between 1773 and 1873 K in air and in a moist environment having a water‐vapor pressure range of 0.06–0.6 MPa, under a constant tensile stress range of 100–160 MPa. The Al2O3/YAG eutectic composite exhibited a stress exponent of 8–13, indicative of tensile creep behavior characterized by a dislocation back‐stress mechanism. Water‐vapor pressures ≤0.4 MPa led to a significant acceleration of creep rates as a result of enhanced dislocation mobility in the Al2O3 and YAG phases.

Correlation between cyclic deformation behaviour and microstructure in a duplex steel between 300 and 773 K

ABSTRACTCyclic tests performed in the temperature range 300–773 K on duplex stainless steel DIN 1.4460 show that the cyclic stress–strain behaviour of this steel is strongly temperature dependent. At 300 and 473 K an almost constant peak tensile stress stage, is followed by a slight softening that continues up to failure in the case of 300 K, but by a secondary hardening at 473 K. Pronounced initial cyclic hardening followed by secondary hardening was the main feature of the temperature range between 573 and 723 K. At 773 K, after a weak hardening stage, a strong softening continues up to failure. The mechanical behaviour and the evolution of the microstructure were analysed, and the internal and the effective stresses were studied. It was found that the internal stress is responsible for the strong hardening that occurs in the intermediate temperature range and for the softening at 773 K.

Stress Evolution with Annealing Methods in SOI Wafer Pairs

It is of importance to know that the bonding strength and interfacial stress of SOI wafer pairs to meet with mechanical and thermal stresses during process. We fabricated Si/2000<TEX>$\AA$</TEX>-SiO<TEX>$_2$</TEX> ∥ 2000<TEX>$\AA$</TEX>-SiO<TEX>$_2$</TEX>/Si SOI wafer pairs with electric furnace annealing, rapid thermal annealing (RTA), and fast linear annealing (FLA), respectively, by varying the annealing temperatures at a given annealing process. Bonding strength and interfacial stress were measured by a razor blade crack opening method and a laser curvature characterization method, respectively. All the annealing process induced the tensile thermal stresses. Electrical furnace annealing achieved the maximum bonding strength at <TEX>$1000^{\circ}C$</TEX>-2 hr anneal, while it produced constant thermal tensile stress by <TEX>$1000^{\circ}C$</TEX>. RTA showed very small bonding strength due to premating failure during annealing. FLA showed enough bonding strength at <TEX>$500^{\circ}C$</TEX>, however large thermal tensile stress were induced. We confirmed that premated wafer pairs should have appropriate compressive interfacial stress to compensate the thermal tensile stress during a given annealing process.

Electrical resistivity and strain recovery studies on the effect of thermal cycling under constant stress on R-phase in NiTi shape memory alloy

In this paper, the results of electrical resistivity and strain recovery measurements involved in the study of the stability of R-phase in NiTi shape memory alloy upon thermal cycling under a constant tensile stress of 100 and 200 MPa are presented. Two wire samples are chosen such that the one heat-treated at 560°C exhibits a pure martensitic phase and the other heat-treated at 380°C consists of a mixture of R-phase and martensitic phase with residual austenites at ambient temperature. In both cases, the applied stress has been found to promote the R→A transformation during the heating part of the thermal cycling unlike the case of stress free condition in which R-phase is found to exist only in the cooling part of thermal cycling. R-phase is found to become more prominent with increasing applied stress. It is also found that the applied tensile stress enables the development of R-phase in the cooling part of the first thermal cycle itself in the sample heat-treated at 560°C whereas under stress-free condition it requires about 15 thermal cycles to develop R-phase. The NiTi wire heat-treated at 560°C exhibits more recoverable strain in the initial cycles than the wire heat-treated at 380°C, but after a large number of thermal cycles of the order of 1000 the recoverable strain in both samples is found to be almost the same. Under tensile stress, the sample heat-treated at 380°C is found to be more stable against plastic deformation with thermal cycling and hence can be preferred over the sample heat-treated at 560°C for two-way applications of SMA.

Detection Method of Fatigue Damage in Carbon Steel Using Laser Ultrasonics

Ultrasonic attenuation was measured by the laser ultrasonic technique among the carbon steel specimens which were subjected to five stages of fatigue damage by constant cyclic tensile stress amplitude. Irradiation of a Q-switched laser was used for ultrasonic generation, and a laser interferometer which consists of a frequency-doubled continuous wave laser and a Fabry-Perot etalon was used for detection of ultrasonic vibration on the specimen surface. From the comparison of the broadband ultrasonic waveforms among five stages of fatigue damage, the attenuation coefficient of the longitudinal wave by the intrinsic material attenuation increased progressively with increasing the number of fatigue cycle. Ultrasonic attenuation at 2.5, 5.0, 7.5 and 10.0MHz were also calculated by Fourier transformation in order to compare the decay of the longitudinal wave among these frequency components. From the results, ultrasonic attenuation increased as the number of fatigue cycle increased, in both broadband and narrowband frequency components. Therefore, the measurement of broadband and narrowband ultrasonic wave obtained by the laser ultrasonic technique would be effective as the detection method of fatigue damage in nuclear power plant components.

Creep behaviour of leaded brass

The creep behaviour of Cu–36wt.% Zn alloys containing 2.5 wt.% of lead has been investigated. Constant tensile stress creep experiments were carried out in the temperature range from 523 to 823 K and under constant stresses from 5 to 250 MPa. Scanning electron microscopy and energy dispersive X-ray analysis were performed on crept as well as uncrept parts of the specimens in order to examine the mechanisms of creep deformation in leaded brass. Attention has been paid to the role of lead on the creep behaviour of brass, the influence of temperature on creep curves, the stress sensitivity parameter and apparent activation energy of creep. The creep data obtained are examined by several methods of conventional analysis of steady-state creep.

On the Monkman–Grant relation for small punch test data

Creep behavior of chromium steel (X20CrMoV 12 1) and of the low-alloy steel (14MoV 6 3) was studied at elevated temperatures. The tests were performed (i) in small punch arrangement at constant force and (ii) in conventional uniaxial mode at constant tensile stress. Time to fracture and the minimum deflection rate in small punch tests are related in a similar manner as the corresponding quantities in conventional creep tests. A formula is established for re-calculating the minimum creep rate from the minimum deflection rate.

Detection Method of Fatigue Damage in Carbon Steel Using Laser Ultrasonics.

Ultrasonic attenuation was measured by the laser ultrasonic technique among the carbon steel specimens which were subjected to five stages of fatigue damage by constant cyclic tensile stress amplitude. Irradiation of a Q-switched laser was used for ultrasonic generation, and a laser interferometer which consists of a frequency-doubled continuous wave laser and a Fabry-Perot etalon was used for detection of ultrasonic vibration on the specimen surface. From the comparison of the broadband ultrasonic waveforms among five stages of fatigue damage, the attenuation coefficient of the longitudinal wave by the intrinsic material attenuation increased progressively with increasing the number of fatigue cycle. Ultrasonic attenuation at 2.5, 5.0, 7.5 and 10.0MHz were also calculated by Fourier transformation in order to compare the decay of the longitudinal wave among these frequency components. From the results, ultrasonic attenuation increased as the number of fatigue cycle increased, in both broadband and narrowband frequency components. Therefore, the measurement of broadband and narrowband ultrasonic wave obtained by the laser ultrasonic technique would be effective as the detection method of fatigue damage in nuclear power plant components.