Low Temperature Creep Research Articles

A model for time dependent strain accumulation and damage at low temperatures in Ti–6Al–4V

Dwell fatigue effects at low temperatures in a range of titanium alloys have been well documented since the 1970s. However, attempts to model dwell fatigue phenomena are limited. The time dependent effects have been shown to be due to the inherent strain accumulation behaviour at ambient or low temperatures, loosely termed ‘cold creep’ in alloys such as Ti–6Al–4V. Periods of dwell in a fatigue cycle or the application of a high mean stress not only cause stress relaxation, but also can lead to premature failures characterised by quasi-cleavage facets under certain conditions. The conditions under which these facets form are not fully understood.This paper introduces a method for predicting cold creep in Ti–6Al–4V by attempting to quantify time dependent deformation and rupture caused by the formation of quasi-cleavage facets observed in this titanium alloy. A logarithmic creep method has been used to describe low temperature creep curves. However this method fails to predict the curve shape at higher stresses. An additional term is used to quantify creep rates at all conditions. A finite element model has been developed, based on this method, which accurately predicts stress relaxation at ambient temperature and quantifies time dependent failure in a notched specimen under cyclic loading.

Low-temperature creep of SnPb and SnAgCu solder alloys and reliability prediction in electronic packaging modules

Creep tests covering a broad temperature range (−40 to 120 °C) were systematically performed on Pb5Sn and Sn3Ag0.5Cu solder alloys. Experimental results showed that both solder alloys creep significantly within the temperatures and stress levels tested. A single set of constitutive equations was constructed to describe creep deformation over a wide range of stress and temperature. Numerical analyses revealed that, when evaluating creep failure, significant errors may result from ignoring creep at low temperatures, especially for the lead-free solder alloy Sn3Ag0.5Cu.

Grain Size Variation during Low Temperature Creep and Tensile Deformation of Ultrafine-Grained Copper

Grain Size Variation during Low Temperature Creep and Tensile Deformation of Ultrafine-Grained Copper

Long-term strength of thin-walled structural elements with low-temperature creep cracks

We formulate a criterion for the evaluation of the long-term strength of structural elements with low-temperature creep cracks under long-term static loading. The application of the proposed criterion is illustrated by an example of problems posed for beams of open profile containing cracks subjected to long-term tension and bending.

Grain Boundary Sliding during Low Temperature Creep of Ultrafine and Coarse Grained Aluminum

HCP metals show new dislocation creep at temperatures below 0.3 Tm with stresses below σ0.2, while FCC metals show it above σ0.2. In the former, grain boundaries absorb the dislocations through slip-induced grain-boundary sliding, while in the latter dislocations are accommodated by cross slip at cell walls. The difference comes from the difference in the crystal symmetry. In UFG-Al at low temperatures, it is anticipated that grains without cell structure lead creep deformation similar to CG HCP metals rather than CG Al. UFG Al specimens were fabricated by ARB method. They showed remarkable creep behavior at less than σ0.2 similary to CG HCP metals. It posseses stress exponent of about three, grain-size exponent of almost zero, and very low apparent activation energy of 20 kJ/mol, and also grain boundary sliding behavior is obserbed by AFM.

Use of the effect of stress relaxation for changing the shape of articles from nonplastic steels and alloys

It is shown that it is possible in principle to change the shape of articles from steels and alloys with low ductility without changing their mechanical properties due to the relaxation of stresses occurring in the range of low-temperature creep. A process for commercial production of springs from cold-worked high-hardness (up to 600 HV) austenitic wire is developed for making articles of specified shape and sizes without worsening the initial high hardness.

Crystallization behavior and mechanical properties of nano-CaCO3/β-nucleated ethylene-propylene random copolymer composites

To provide ethylene-propylene random copolymer (PPR) with balanced mechanical properties, !-nucleating agent and CaCO3 nanoparticles are incorporated into PPR matrix by melt blending. It is found that crystallization rate and relative content of !-crystal increase with the addition of !-nucleating agent together with nanoparticles. Size of PPR spherulite is greatly reduced, and a specific morphology appears, in which #-crystal lamella is grown upon the !-nucleus. The results suggest that both !-nucleating agent and nano-CaCO3 have heterogeneous nucleation and synergistic effects on !-nucleation of PPR. Mechanical characterization shows that mechanical properties of PPR can be tuned by incorporation of !-nucleating agent and nano-CaCO3 particles. Under suitable compositions, low temperature impact strength and high temperature creep resistance of PPR, the bottlenecks of application of such material, can be simultaneously improved with- out sacrificing the Youngs'modulus and tensile strength.

Finite Element Modeling of Diamond Indentation Creep in MgO

The modified dislocation creep model and the power law breakdown creep model were proposed to be used in the indentation creep deformation analyses for single crystal MgO at low temperature varying from 293K to 873K. A FE indentation creep modeling procedure was proposed and implemented. The activation energy and the shear flow stress for low temperature creep in single crystal MgO were predicted based on the analytical indentation creep analyses.

Modeling interstitial diffusion controlled twinning in alpha titanium during low-temperature creep

Structures of all sizes are subject to stresses over extended periods of time. Many structures consist of crystalline materials that can creep at room temperature. A recent discovery reveals twinning to be a major creep mechanism in metals. Traditionally, twins in metals are believed to grow rapidly, but this discovery shows that twins can grow very slowly. Crystallographic models are presented to explain slow twin growth. The models will have broad applications for understanding and developing materials for improved creep resistance.

Triaxial Low-Temperature Creep Tests of Artificially Frozen Soil

With the increasing in freezing shaft sinking depth of the mine，Mine permanent derrick was located into the frozen wall. The internal force distribution of the shaft-tower foundation was obtained by field measurement shaft-tower foundation basal pressure, foundation reinforced strain and the strain of concrete and foundation deformation of mechanical quantity. The numerical simulation of the interaction between coal derrick and foundation in freezing and thawing process shows that: the field measurement and numerical simulation laws were consistent and the values were in good agreement. It has very important theoretical and practical significance for the safe production of derrick and future derrick foundation design in freezing method construction.

Low-Temperature Creep at Ultra-Low Strain Rates in Pure Aluminum Studied by a Helicoid Spring Specimen Technique

The creep behavior in pure aluminum has been investigated by helicoid spring creep tests at strain rates, _ , lower than 10 � 10 s � 1 and low temperature ranging from 0:32Tm to 0:43Tm. It was found that the creep behavior in this region depends strongly on grain sizes and impurity concentrations. For high-purity aluminum (5 N Al) with an average grain size, dg > 1600 mm, nearly the wire diameter of the spring sample, where the role of grain boundary during creep deformation can be negligible, the stress exponent was n � 5 and the activation energy was Qc ¼ 32 kJ/mol. Microstructural observation showed the formation of large dislocation cells (� 10 mm) and tangled dislocations at the cell walls. For high-purity aluminum (5 N Al) with dg ¼ 24 mm, the stress exponent was n � 1 and the activation energy was Qc ¼ 15 kJ/mol. On the other hand, for commercial low-purity aluminum (2 N Al) with dg ¼ 25 mm, the stress exponent was n ¼ 2 and the activation energy was Qc ¼ 25 kJ/mol. Microstructural observations revealed dislocations emitted from grain boundaries, those dislocations interacting with intragranular dislocations and the formation of dislocation cells in the grains. Based on those experimental results, the low-temperature creep mechanisms in pure aluminum at _ 10 � 10 s � 1 have been discussed. [doi:10.2320/matertrans.M2010405]

Alternative Procedure for Determination of Hot Mix Asphalt Creep Compliance

Abstract Creep compliance function is one of the fundamental properties of viscoelastic materials. In asphalt research, creep compliance is used in the prediction of low-temperature cracking, which is a prevalent distress in asphalt pavements in northern parts of the United States and Canada. The thermal cracking prediction model included in the current version of the Mechanistic-Empirical Pavement Design Guide requires the creep compliance of hot mix asphalt (HMA) materials as a primary input. The current AASHTO procedure for determining creep compliance of HMA is based on the indirect tension (IDT) test. The IDT test is performed on relatively thick cylindrical specimens, which makes this test unsuitable for field cores from thin layers and construction lifts. In addition, thick specimens do not allow for investigation of the gradual aging that occurs in asphalt layers due to oxidation and volatilization. This paper outlines the procedure that uses the bending beam rheometer (BBR) as an alternative to the IDT test to determine the low-temperature creep compliance of HMA using thin beam specimens. Twenty different HMA mixtures are tested at three temperature levels using both the IDT and the BBR instruments. The analysis of test results is presented in two parts. First, the variability in creep compliance values obtained from the IDT and the BBR tests is discussed. Next, the artificial neural networks (ANNs) are trained to (1) predict IDT results from BBR measurements using design parameters of HMA mixture and testing temperature as the model inputs and (2) backcalculate HMA creep compliance from the binder creep compliance and vice versa. It is concluded that testing HMA beams can be used instead of IDT specimens for the low-temperature characterization of HMA mixtures, and the ANN can be successfully used for the backcalculation of the creep compliance measured by both IDT and BBR devices.

Fatigue of steel in air at low cyclic loading frequency

This investigation was aimed at studying crack growth behaviour at extremely low loading frequencies of steels exhibiting low temperature creep. It was found that crack dormancy at low loading frequency is an intrinsic characteristic of the steels. Frequency can be an independent driving force, like the stress intensity factor range (ΔK) and the maximum stress intensity factor (Kmax), in low cyclic loading frequency regimes. A dormant state or threshold condition can be achieved by reducing loading frequency.

Low-temperature creep behavior of ultrafine-grained 5083 Al alloy processed by equal-channel angular pressing

Low temperature creep behavior of ECAPed Al 5083 alloy with grain sizes of approximately 300 nm was investigated at temperatures of 498, 523 and 548 K. The value of the stress exponent was found to be 3.5 at a low stress level and increased to 5.0 at a high stress level. At the low stress level, the creep curve exhibits typical class II behavior due to the accumulated strain during the ECAP process, even though the creep is controlled by solute-drag processes with a stress exponent of 3.5. The average value of Q obtained from the analysis of the data is close to that for dislocation pipe diffusion. Therefore, on the basis of the activation energy in a temperature range of 498K to 548K at low and high stress level, the creep deformation is controlled by dislocation glide and climb processes, respectively, and the rate-controlling diffusion step might be dislocation pipe diffusion.

Research on Cryogenic Properties of Different Fiber Asphalts and Mixtures

From microstructure and technical performance of different fibers, the variation and influence of the different fibers’ properties and parameters on cryogenic properties of matrix asphalt and asphalt mixture were researched and evaluated through BBR and cryogenic trabecular bending tests. Research shows that: the addition of fibers makes the matrix asphalt’s low temperature creep stiffness S increases, creep slope m decreases, namely the matrix asphalt cryogenic flexibility and stress relaxation ability are weakened; cryogenic anti-cracking ability of asphalt mixture is significantly improved through adding different fibers, the order of improving effectiveness: Dolanit AS fiber> ARBOCEL lignin fiber> Basalt fiber; while asphalt content is constant, with the increase of fiber content, the mixture maximum bending strain, strength, deformation of mixture firstly increase and then decrease, the optimal asphalt content of asphalt mixtures and optimal fiber content of three fibers are identified according to the above three indicators’ maximum. Research points out the existed problems that are scope of application and evaluation indicators in BBR test of SHARP, optimal asphalt and fiber content are fully recommended, it provides scientific guidance and practical foundation for actual project.

The effect of prestrain on low and high temperature creep in Ti834

The low and high temperature creep properties of the α + β titanium alloy Ti834 have been studied for various levels of applied plastic prestrain. Evidence is shown that the prestrain at room temperature causes the formation of quasi-cleavage facets in a manner described by the Evans–Bache model. These facets are also produced when the material is creep tested at room temperature, although larger areas of faceting are found. The applied prestrain causes a drop in the strain at failure at room temperature due to a high dislocation density inhibiting further movement of dislocations. At high temperature this increase in applied prestrain results in an accelerated creep rate. This is attributed to a number of issues, including increased thermal energy allowing greater dislocation recovery and mobility. Swaged Ti834 material is also considered and is shown to behave similar to highly prestrained material with evidence of strain hardening and reduced cold creep rate.

Effect of ageing on rheological properties of storage-stable SBS/sulfur-modified asphalts

Oxidative ageing as an inevitable process in practical road paving has a great effect on the properties of polymer-modified asphalts (PMAs). In this article, the effect of short-term and long-term oxidative ageing on the rheological, physical properties and the morphology of the styrene-butadiene-styrene (SBS)- and storage-stable SBS/sulfur-modified asphalts was studied, respectively. The analysis on the rheological and physical properties of the PMAs before and after ageing showed the two major effects of ageing. On one hand, ageing prompted the degradation of polymer and increased the viscous behaviour of the modified binders, on the other, ageing changed the asphalt compositions and improved the elastic behaviour of the modified binders. The final performance of the aged binders depended on the combined effect. After ageing, the storage-stable SBS/sulfur-modified asphalts showed an obvious viscous behaviour compare with the SBS-modified asphalts and this led to an improved low-temperature creep property. The rutting resistance of the SBS-modified asphalts declined by the addition of sulfur due to the structural instability of the SBS/sulfur-modified asphalts. The rheological properties of the modified binders before and after ageing also depended strongly on the structural characteristics of SBS. The observation by using optical microscopy showed the compatibility between asphalt and SBS was improved with further ageing, especially for the storage-stable SBS/sulfur-modified asphalts.

Fitness for service assessment of degraded CANDU feeder piping—Canadian regulatory expectations

Allowance for the continued operation of feeder piping at some Canadian CANDU stations, which is experiencing active degradation mechanisms, has been based primarily on augmented inspection practices and conservative fitness for service assessments. The major degradation mechanisms identified to date are: pipe wall thinning due to Flow Accelerated Corrosion (FAC) and service induced cracking due to Intergranular Cracking due to Stress Corrosion Cracking (SCC) and potentially Low Temperature Creep Cracking (LTCC) mechanisms. Given that currently available industry codes and standards do not provide sufficient guidelines/criteria for assessing the degradation of feeder pipes, the Canadian Nuclear Safety Commission (CNSC) has asked the utilities to establish feeder pipe specific procedures to provide reasonable assurance that the risk associated with the feeder degradation is maintained at an acceptably low level. In response to this requirement, the Canadian CANDU industry has developed and continued to update feeder fitness for service guidelines to provide evaluation procedures and industry standard acceptance criteria for assessing the structural integrity of the feeder pipes. The scope and frequency of inspections are determined based on the results of the fitness for service assessments taking into account the relative susceptibility of feeder pipes to each specific degradation mechanism. While industry practices for the management of degraded feeder pipes have, in general, been complied with the regulatory expectations, outstanding issues still remain. Major regulatory concerns include uncertainties associated with limitations in both the inspection techniques and the mechanistic understanding of the degradation processes, which can impede inspection planning and fitness for service assessments.This paper presents the regulator's view of the current situation with respect to degradation of feeder piping, its implications for nuclear safety and the regulatory expectations on industry's management of the critical ageing phenomena.

Effect of hydrogen on the low-temperature creep of a submicrocrystalline Ti-6Al-4V alloy

The effect of alloying with 0.002–0.24 wt % H on the creep of the Ti-6Al-4V alloy at a temperature of ∼0.15Tm(Tm is the melting temperature) is studied. The formation of a submicrocrystalline structure in the alloy is found to increase the stress-rupture strength and the hydrogen embrittlement resistance. Possible causes of the increase in the deformation localization resistance of the alloy in the presence of hydrogen in a solid solution are discussed.

Microstructural changes during creep of CMSX‐4 single crystal Ni base superalloy at 750°C

TEM studies of creep tested CMSX-4 nickel-base single crystal superalloy were performed to analyse a microstructure evolution during creep at temperature 750 degrees C, and uniaxial tensile stress of 675 MPa. Microstructural analyses were focused mainly on examination of dislocation configurations during primary and secondary creep stages of high temperature deformation. At such low temperature and high stress creep deformation proceed by cutting of gamma' particles by dislocations. It was found that primary creep is initiated by movement of dislocations with Burgers vector a/2 <110> in the gamma phase. The second type of dislocations active at primary creep stage are extended dislocation ribbons with overall a<112> Burgers vector, separated by superlattice stacking faults, cutting both the gamma and gamma' phases. The movement of the dislocation ribbons is inhibited at secondary creep stage by dislocation networks formed at gamma-gamma' interfaces.