Rupture Lifetime Research Articles

Experimental Study of Biaxial Creep Damage for Type 304 Stainless Steel

This paper studies the biaxial creep damage of type 304 stainless steel at 923 K. Biaxial tension creep tests were carried out using cruciform specimens and the effect of stress biaxiality on rupture lifetime and creep voiding was discussed. Mises equivalent stress and the equivalent stress based on crack opening displacement were a suitable parameter to assess the biaxial creep damage. The equivalent stress proposed by Huddleston overestimated the biaxial creep damage by more than a factor of two. Stress biaxiality had almost no influence on the orientation of voided grain boundaries and the critical value ofparameter A. Tests of alternative loading direction significantly dispersed the biaxial creep damage resulting in larger creep lifetime.

Failure of Crossply Ceramic-Matrix Composites

The fast-fracture and stress-rupture of a crossply ceramic-matrix composite with a matrix through-crack are examined numerically to assess the importance of fiber architecture and the associated stress concentrations at the 0/90 ply interface on failure. Fiber bridging in the cracked 0 ply is modeled using a line-spring bridging model that incorporates stochastic and time-dependent fiber fracture. A finite-element model is used to determine the stresses throughout the crossply in the presence of the bridged crack. For both SiC/SiC and a typical oxide/oxide, the fast-fracture simulations show that as global failure is approached, a significant fraction of fibers near the 0/90 interface are broken, greatly reducing the stress concentration. For fibers with low Weibull moduli (m textless 10), the tensile strength is thus nearly identical to that of a unidirectional composite scaled by the appropriate fiber volume fraction, while for fibers with larger Weibull moduli (m greater than or equal to 10), there are modest (10-17%) reductions in tensile strength. Stress-rupture simulations show that initially high stress concentrations are relieved as fibers fail with evolving time near the 0/90 interface and shed load away from the interface. For a wide range of fiber properties, efficient load redistribution occurs such that the crossply rupture lifetime is generally within an order of magnitude of the unidirectional lifetime, when the applied stress is normalized by the relevant fast-fracture strength. Overall, stress concentrations at the 0/90 inter-face are largely relieved with increasing load or time due to the nonlinear bridging response and preferential fiber failure near the interface, resulting in crossplies that respond very similarly to unidirectional composites.

Approximate creep rupture lifetimes for butt welded ferritic steel pressurised pipes

The creep rupture of butt welded ferritic steel pipes composed of a range of weld and heat affected zone materials has been previously analysed in detail (Leckie and Hayhurst, 1994). These analyses required substantial computational resources which cannot be justified during the preliminary phases of the design process. To reduce cost and improve speed an approximate method for the analysis of kinematically determinate structures, known as the modal method (Leckie and Hayhurst, 1974), has been developed to compute creep rupture lifetimes. This paper reports an extension of the method for the analysis of multi-material structures, such as weldments, and its implementation as a post processor to a stationary state finite element creep analysis. Histories of stress and damage have been determined using the modal method for weldments with a range of heat affected zone and weld material combinations. The lifetimes determined in this way are compared with those determined using complete continuum damage mechanics analyses, and are shown to be conservative; in addition the regions of intense damage have been shown to be accurately predicted. The modal method is also shown to be superior to more approximate reference stress methods.

Tensile Creep and Rupture of Silicon Nitride

We have characterized the tensile creep, rupture lifetime, and cavitation behavior of a commercial, gas‐pressure‐sintered silicon nitride in the temperature range 1150° to 1400°C and stress range 70 to 400 MPa. Individual creep curves generally show primary, secondary, and tertiary creep. The majority of the primary creep is not recoverable. The best representation of the data is one where the creep rate depends exponentially on stress, rather than on the traditional power law. This representation also removes the need to break the data into high and low stress regimes. Cavitation of the interstitial silicate phase accompanies creep under all conditions, and accounts for nearly all of the measured strain. These observations are consistent with a model where creep proceeds by the redistribution of silicate phase from cavitating interstitial pockets, accommodated by grain‐boundary sliding of silicon nitride.

Influence of secondary precipitates and crystallographic orientation on the strength of single crystals of a Ni-based superalloy

The effect of crystallographic orientation and aging heat treatment at 850 °C on the creep rupture strength of single crystals of a nickel-based superalloy was examined at 700 °C in detail. Initial tensile orientations were selected over a wide range on the standard stereographic triangle. The {111}〈112〉-type slip systems were found to be operative during the creep deformation. The creep behavior was found to be greatly influenced by the additional aging at 850 °C for 20 hours. It was found that the effect of the aging at 850 °C was quite different between orientations favored for the $$(\bar 111)[1\bar 12]$$ slip system and those favored for the (111) $$[\bar 211]$$ slip system and that the creep deformation mechanisms of these two slip systems were different. In the orientations favored for $$(\bar 111)[1\bar 12]$$ slip systems, in the single-aged specimens, a small mean surface-to-surface spacing due to hyperfine γ′ precipitates in the matrix channel promoted the $$(\bar 111)[1\bar 12]$$ slip and the primary creep. As a result of the additional aging at 850 °C, the hyperfine γ′ precipitates were dissolved into the matrix, and the resultant large mean surface-to-surface spacing between the cuboidal precipitates inhibited extensive shearing of the γ-γ′ structure by the $$(\bar 111)[1\bar 12]$$ slip system. As a result, the creep strengths of these orientations were increased in double-aged specimens; however, the low ductility associated with the difficulty of secondary noncoplanar slip did not enlarge rupture lifetime in the double-aged [001] specimen. In the orientations favored for the (111) $$[\bar 211]$$ slip system, creep deformation occurred by twinning shear through γ and γ′ precipitates, and a distinct effect of the aging at 850 °C was not observed. In the multiple orientation of the {111} $$\left\langle {\bar 211} \right\rangle $$ -type slip systems, i.e., the $$[\bar 112]$$ and $$[\bar 111]$$ orientations, hyperfine precipitates improved creep strength because they prevented dislocations from gliding in the matrix channel in the single-aged specimens.

Application of a visco-elastic tension-softening constitutive model to cracked and ageing concrete

The development of a non-linear visco-elastic tension-softening constitutive model for cracked and ageing concrete is outlined. It combines a micro-mechanical model for tension softening with a Kelvin chain rheological model. The ageing phenomenon is included through the concept of consolidation theory and the introduction of an ageing function—the same for all Kelvin chain units. The constitutive model is then used with the fictitious crack model for studying the creep-induced sub-critical growth of a crack in a large concrete dam. The rate of crack growth is determined in order to calculate the rupture lifetime.

High temperature rupture lifetime of 304 stainless steel under multiaxial stress states

High temperature rupture lifetime of 304 stainless steel under multiaxial stress states

Static and cyclic creep behaviour of 2024/SiCp and its matrix alloy at high temperature

The tensile creep and tensile-tensile cyclic creep behaviour of 2024/SiCp composite and its matrix alloy have been investigated and analysed at high temperature. It was found that the creep threshold stress of the composite may not be caused by SiCp alone: the matrix alloy also contributes to the threshold stress. The higher threshold stress of the composite compared with that of the matrix alloy can be explained in terms of load transfer in the composite and the value of threshold stress for the matrix alloy. A direct comparison between the composite and its matrix alloy indicates that only below a critical stress does the composite show a creep resistance higher than that of its matrix alloy. The two materials shown cyclic creep retardation in the tested stress range, cyclic creep showing a higher stress exponent and higher apparent activation energies in comparison with static creep. An analysis based on anelasticity is introduced to explain this result. The relationships between rupture lifetimes and applied stress, creep rate, and unloading amount show that the creep fracture mechanism is dominant in the present test condition.

Air-environment-creep interaction in a nickel base superalloy

A Nimonic 263 alloy, widely used for combustion chamber applications in aeroengines, was exposed to air at 1020 °C to study the oxidation-creep interaction at 780°C in the stress range of 160–275 MPa. Prior exposure of the alloy led to degradation of creep properties, resulting in enhanced creep rate, poor rupture lifetime and creep ductility embrittlement. Degradation of creep properties in the air-exposed alloy has been discussed in terms of oxygen-induced damage in the grain boundary due to the formation of CO gas bubbles and the depletion of γ' in the near-surface region.

The effects of physical aging on the brittle fracture behavior of polymers

AbstractThe effects of physical aging on the failure behavior of a typical brittle polymer, polystyrene, have been studied. Properties examined were creep rupture lifetimes, fatigue lifetimes, and environmental stress cracking in ethanol. Fractured samples were examined both optically and by scanning electron microscopy to determine the degree of crazing. It was found that a longer physical aging time produced shorter lifetimes in all cases. The main reason for this is the reduction in craze strength caused by a reduced toughness due to physical aging. A long aging time was found to delay craze formation, but once formed, these crazes were much less stable than those formed with a short aging time. The effects of aging are important on failure prediction criteria and on testing methodologies, and the implications are discussed.

High temperature deformation and fracture mechanisms in a nickel aluminide alloy

The mechanisms that control high temperature deformation and fracture were studied in a nickel aluminide(Ni3Al) alloy that was thermo-mechanically treated to produce a non-porous microstructure. Comparisons of data corresponding to the dendritic morphology with that for the equiaxed grain structures indicated that the dendritic morphology results in a significantly lower creep rates as well as substantially greater times to rupture. From the microstructural observations, isolated interdendritic cavitation in th absence of grain boundary sliding was found to lead to rupture lifetime that are longer than those observed for the other microstructures.

Performance enhancement of Inconel alloy X-750 during creep via optimal solution treatment and control of morphology of grain boundary carbide

AbstractThe effect of solution treatment temperature on both the morphology of grain boundary carbides and the creep rupture behaviour of Inconel alloy X-750 has been investigated. Solution treatment at 1050°C led to precipitation of Ti/Nb carbide along grain boundaries, and as a result the formation of chromium carbide during subsequent aging treatment was considerably affected. This led to changes in the morphology of the grain boundary carbide and reduction in the width of ‘clear’ zone. This modification in the morphology of grain boundary carbide reduced the creep resistance of the alloy, leading to a reduction in rupture lifetimes, whereas strains tofracture were unaffected.MST/1981

Performance enhancement of Inconel alloy X-750 during creep via optimal solution treatment and control of morphology of grain boundary carbide

The effect of solution treatment temperature on both the morphology of grain boundary carbides and the creep rupture behaviour of Inconel alloy X-750 has been investigated. Solution treatment at 1050°C led to precipitation of Ti/Nb carbide along grain boundaries, and as a result the formation of chromium carbide during subsequent aging treatment was considerably affected. This led to changes in the morphology of the grain boundary carbide and reduction in the width of ‘clear’ zone. This modification in the morphology of grain boundary carbide reduced the creep resistance of the alloy, leading to a reduction in rupture lifetimes, whereas strains tofracture were unaffected.MST/1981

Prediction of rupture lifetime in thin sections of a nickel base superalloy

The prediction of rupture lifetime of a component can be based on the data generated from round, flat and tubular specimens. An investigation on the influence of specimen geometry on the creep behavior of Inconel alloy X-750 showed that the tubular specimen exhibited better creep performance when rupture lifetime data were compared on the basis of section size. However, the time to rupture data of all three specimen geometries merged together when compared on the basis of the volume to surface area ratio (V/S) indicating that there is a definite relationship between V/S and the rupture lifetime. This analysis is now extended in another gamma prime strengthened nickel base superalloy. The chemical composition of the alloy in wt%, is as follows: 0.07C-0.66Si-0.07Cu-0.79Fe-19.1Cr-2.35Ti-1.52Al and remainder nickel. Analysis of the rupture lifetime data of a gamma prime strengthened nickel base superalloy shows that time-to-rupture is controlled by the volume to surface area ratio. Furthermore, the normalization of the volume to surface area ratio with the grain size shows that the time-to-rupture data of two grain sizes, 55 and 250 [mu]m, can be represented by a single curve.

The use of supercomputer modelling of high-temperature failure in pipe weldments to optimize weld and heat affected zone materials property selection

The creep deformation and damage evolution in a pipe weldment has been mod­elled by using the finite-element continuum damage mechanics (CDM) method. The finite-element CDM computer program Damage xx has been adapted to run with increased speed on a Cray XMP/416 supercomputer. Run times are suffi­ciently short (20 min) to permit many parametric studies to be carried out on vessel lifetimes for different weld and heat affected zone (HAZ) materials. Finite-element mesh sensitivity was studied first in order to select a mesh capable of correctly predicting experimentally observed results using the least possible com­puter time. A study was then made of the effect on the lifetime of a butt welded vessel of each of the commonly measured material parameters for the weld and HAZ materials. Forty different ferritic steel welded vessels were analysed for a constant internal pressure of 45.5 MPa at a temperature of 565 °C; each ves­sel having the same parent pipe material but different weld and HAZ materials. From these studies it has been concluded that good creep rupture ductility and strength of the weld material are necessary to achieve long vessel lifetimes. Vessel lifetimes are not strongly influenced by the HAZ material properties provided that the rupture strength is maintained above that of the parent pipe material, and that the ductility is greater than one half of that for the parent pipe material. Also it is found that the optimum design can be achieved with both the weld and the HAZ materials having properties of high primary creep strain, mean sec­ondary creep strain, mean rupture strain, and long rupture lifetime, relative to the base or parent pipe material. A lifetime improvement has been demonstrated of 30% over that obtained for the initial materials property data. A methodology for weldment design has been established which uses supercomputer-based CDM analysis techniques; it is quick to use, provides accurate results, and is a viable design tool.

Creep and Creep Rupture of an Advanced Silicon Nitride Ceramic

Creep and creep rupture behavior of an advanced silicon nitride ceramic were systematically characterized in the temperature range 1150° to 1300°C using uniaxial tensile creep tests. Absence of tertiary creep and the order‐of‐magnitude breaks in both creep rate and rupture lifetime at certain threshold combinations of stress and temperature were two characteristic features of the creep behavior observed. Thermal annealing was found to have enhanced both subsequent creep resistance and creep rupture life. The stress exponent (n) and the activation energy (Q) defined in the Norton relation were found to be 12.6 and 1645 kJ/mol for the material investigated. Both values appear to fall in the general range of those reported for other but similar types of Si3N4 ceramic materials. The stress exponent, m, equivalent to the slope of the Larson–Miller equation was found to be in the range 13 to 14.4, and that defined as p in the Monkman–Grant relation to be 0.91, based on the available experimental data. The values of m, n, and p obtained above approximately support the interrelationship of the three exponents given by p=m/n.

On the influence of grain morphology on creep rupture times of cavitating materials

Scatter in mechanical data, especially creep rupture lifetimes, can be attributed to three sources: (1) testing related parameters, which reflect the quality of control of temperature and stress during the experiments; (2) sampling related parameters, where a specimen only samples a limited amount of material, which can then result in scatter when the material is heterogeneous; (3) the nature of creep damage accumulation can be inherently stochastic. The present work is concerned with this third source of scatter. The authors consider microstructural scenarios in which creep damage accumulates by the cavitation of grain boundaries (gbs), which are perpendicular to the axis of the applied stress. They further assume that creep life can be divided into two phases. In the first phase, creep damage accumulation occurs homogeneously in the microstructure. The term homogeneously is used to describe a situation where each gb perpendicular to the stress axis has the same probability of failing by cavitation, irrespectively of its location in the microstructure and, irrespectively of whether or not neighboring gbs are damaged.

An experimental study of behavior of low alloy steels in the initial growth stage of creep cracks

In the early stage of creep crack growth, low alloy steels exhibit a transitory behavior by which the expression for the crack growth rate ( da dt or dot a ) as a function either of the C ∗ integral or of the load line displacement rate (δ) becomes anomalous: the data deviate from the linearity describing a nose, thereby forming a part called the tail. In empirically analyzing this phenomenon, which has been taken to indicate the extent of degradation of the material, the following observations are made for its formation mechanism and chronological progress for 1CrMoV steel and 2.25Cr-1Mo steel. The electrical potential method overestimates the crack length while it is small. The times to attain the minimum da dt ( T dot a min ) and that to the minimum δ ( T δ ̇ min ) remain invariable within the scope study, irrespective of the species and state of steel, temperature, specimen configuration, and load at T dot a min Tf = 0.1−0.2 and T δ ̇ min Tf = 0.2−0.35 . The time increase in transition from small-scale creep to extensive creep ( Ttr), is less than T dot a min , so that the transition is completed before T a ̇ min . The creep rupture lifetime ( Tf) can be correlated quite well to dot δ min in a Monkman-Grant expression as log(Tf) + c · log( dot δ min) = m . This observation is taken to indicate that prediction of Tf an early stage should be possible.

Comparison of creep rupture lifetimes of single and double notched tensile bars

Creep rupture behaviour and lifetimes have been studied for two notched bars—one having a single notch and the other a double notch—by use of the creep finite element solver, Damage XX, based on Continuum Damage Mechanics (C.D.M.). A physically-based constitutive equation which utilises two damage parameters was used to model tertiary creep behaviour. Its versatility allows a wide spectrum of material parameters to be used which simulated a range of material behaviour. All computations showed that lifetimes for the double-notch testpiece within 95% of those for the equivalent single notch, and therefore validated the double notch geometry being studied. A consequence of the calculations has been the verification of a previous suggestion that the lifetimes of materials which exhibit tertiary creep due to dislocation strain softening (suc as nickel-based superalloys) could be less affected by stress-state effects on ductility, than materials which do not exhibit such softening (such as pure metals and simple solid solution alloys).

Prediction of long-term creep curves

This paper aims at discussing how to predict long-term irradiation enhanced creep properties from shor-term tests. The predictive method based on the π concept was examined by using creep data of ferritic steels. The method was successful in predicting creep curves including the tertiary creep stage as well as rupture lifetimes. Some material constants involved in the method are insensitive to the irradiation environment, and their values obtained in thermal creep are applicable to irradiation enhanced creep. The creep mechanisms of most engineering materials definitely change at the athermal yield stress in the non-creep regime. One should be aware that short-term tests must be carried out at stresses lower than the athermal yield stress in order to predict the creep behavior of structural components correctly.