Tensile Creep Rupture Research Articles

Tensile and flexural creep rupture tests on partially-damaged concrete specimens

Three series of novel tensile and flexural creep tests on partially-damaged concrete specimens were carried out in order to gain some insight into creep crack growth and failure of strain-softening materials. In the tests, each specimen was initially loaded to a given point in the descending branch and thus had a lower load-carrying capacity than that at the peak-point. Then, the specimen was unloaded and reloaded to sustain a load which was from 70% to 95% of its current load-carrying capacity. Experimental creep curves display a three-stage process, consisting of primary, secondary and tertiary stages, with a decreasing, constant and increasing creep rate, respectively. The secondary stage dominates the whole failure lifetime, whereas both the secondary and tertiary stages are important in terms of creep deformation. Failure life-time seems to be more sensitive to the change of load level in flexural tests rather than in tensile tests. The decrease in load-carrying capacity due to damage tends to result in a shorter failure lifetime and a lower critical load level for creep rupture. The descending branch of the static load-deflection or load-CMOD curve may be used as an envelope criterion for creep fracture.

Effect of yttrium addition on mechanical behavior of powder metallurgy TP304 at elevated temperatures

It is well known that oxide dispersion strengthened (ODS) alloys, which are mechanically alloyed with yttria, possess excellent mechanical properties in comparison with conventional ingot metallurgy (IM) materials at elevated temperatures even higher than 1,000 C. However, mechanical alloying may not be desired in view of production cost for the high temperature application where such high strength is not necessarily required. There is not an extensive literature dealing with mechanical properties at above 800 C of the fully dense powder metallurgy (PM) materials without mechanical alloying. In the present study, the melt of TP304 containing yttrium was atomized to form yttria on and in the powder. The effects of such oxide inclusions on high temperature tensile properties and creep rupture strengths are investigated, using TP304 fully dense materials consolidated by hot extrusion.

REMAINING LIFE PREDICTION BY THE ISO-STRESS METHOD OF BOILER TUBES AFTER PROLONGED SERVICE

The prediction of remaining creep life by the Iso-stress method was investigated for 153500h service-exposed boiler tubes of 1.25Cr-0.5Mo-Si, 2.25Cr-1Mo and 9Cr-1Mo steels. At first, the internal pressure creep rupture strength of the service-exposed tube specimens of the steels was shown to be the same as the tensile creep rupture strength of the sheet specimens taken from the tubes. Then the temperature-accelerated creep rupture tests under iso-stress were carried out by the tensile creep rupture tests, because of simplicity of the tests. The Iso-stress method was verified for the virgin specimens of the steels by comparing the predicted long-term rupture times as long as 105h with the data in the NRIM Creep Data Sheets. The predicted rupture times agreed with the observed data within a factor of 2 or 3. The temperature-accelerated tests of the sheet specimens taken from the service-exposed tubes showed that the linear extrapolation of the rupture times to the service temperature gave the remaining creep rupture life of about 8×105h for the 1.25Cr-0.5Mo-Si steel and about 1.6×106h for the 2.25Cr-1Mo steel. However, the Iso-stress method could not be applied for the 9Cr-1Mo steel, because the logtr versus T curve was not linear, where tr is the creep rupture time and T is the testing temperature. It is concluded that the Iso-stress method is useful when the significant microstructural change does not take place during accelerated creep rupture tests at high temperatures.

The balance of mechanical and environmental properties of a multielement niobium-niobium silicide-basedIn Situ composite

This article describes room-temperature and high-temperature mechanical properties, as well as oxidation behavior, of a niobium-niobium silicide basedin situ composite directionally solidified from a Nb-Ti-Hf-Cr-Al-Si alloy. Room-temperature fracture toughness, high-temperature tensile strength (up to 1200 °C), and tensile creep rupture (1100 °C) data are described. The composite shows an excellent balance of high- and low-temperature mechanical properties with promising environmental resistance at temperatures above 1000 °C. The composite microstructures and phase chemistries are also described. Samples were prepared using directional solidification in order to generate an aligned composite of a Nb-based solid solution with Nb3Si- and Nb5Si3-type silicides. The high-temperature mechanical properties and oxidation behavior are also compared with the most recent Ni-based superalloys. This composite represents an excellent basis for the development of advanced Nb-based intermetallic matrix composites that offer improved properties over Ni-based superalloys at temperatures in excess of 1000 °C.

Experimental and theoretical quantification of the development of damage in fatigue tests of bone and antler

This study concerns the development of damage (as measured by a reduction in elastic modulus) in two kinds of bones differing considerably in their degrees of mineralisation: laminar bone from bovine femur and osteonal bone from red deer antler. Antler bone is much tougher than ‘ordinary’ bone and its failure properties have been investigated in: (i) monotonic tensile tests and (ii) creep rupture experiments. Tensile fatigue is another way of examining how damage develops in bone. The development of damage in the present fatigue tests was non-linear with the cycle number, the degree of non-linearity was dependent on the level of stress and followed a clearly different course for bone and antler. Antler was a more damage-tolerant material, being able to achieve a reduction in the final modulus of elasticity, just prior to failure, three tkmes greater than ‘ordinary’ bone. The evolution of damage is quantified by an empirical and a graphical method and by the use of Continuum Damage Mechanics (CDM) expressions. The CDM method shows important conditions, found in antler, but not in bone, that seem necessary for achieving stable fractures and consequently producing very tough materials.

Effect of Oxide Inclusions on Mechanical Behavior of Powder Metallurgy SUS304 at Elevated Temperatures.

Powder metallurgy (PM) SUS304 stainless steels consolidated by hot extrusion process with different oxide inclusions are investigated in terms of tensile properties and creep rupture properties at elevated temperatures. Formation of oxides at powder surfaces and within powders is dependent on the chemical composition: An addition of Al, Mn and Si generated Al, Cr-Mn and Si oxides, respectively, while Cr oxides form without additions of those elements. The ductility of PM material is reduced by Al oxides in the high temperature tensile tests above 800°C even though the material with Al oxides has a low level of oxygen content. Furthermore, Al and Si oxides in the PM materials are more detrimental to creep rupture strength at 950°C than Cr or Cr-Mn oxides.

High temperature deformation and rupture in SiC-C composites

Creep studies have been performed on an SiC-C composite under flexural, compressive and tensile loading conditions. It has been found that the creep rates in all loadings are similar and consistent with deformation dominated by the fibers. The implication is that the C matrix is susceptible to extensive microcrack damage, causing all the stress to be borne by the fibers. The elevated stress on the fibers causes tensile creep rupture at small creep ductilities. This behavior has been attributed to slow crack growth in the fibers.

Elevated temperature creep properties of NiAl eryomilled with and without Y2O3

The creep properties of lots of NiAl eryomilled with and without Y2O3 have been determined in compression and tension. Although identical cryomilling procedures were used, differences in composition were found between the lot ground with 0.5 vol % yttria and the lot ground without Y2O3. Compression testing between 1000 and 1300 K yielded similar crecp strengths for both materials, while tensile creep rupture testing indicated that the yttria-containing alloy was slightly stronger than the Y2O3-free version. Both compression and tensile testing showed two deformation regimes; whereas the stress state did not affect the high stress exponent (n ≍ 10) mechanism, the low stress exponent regime n was ∼6 in tension and ∼2 in compression. The strengths in tension were somewhat less than those measured in compression, but the estimated activation energies (Q) of ∼600 kJ/mol for tensile testing were closer to the previously measured values (∼700 kJ/mol) for NiAl-AlN and very different from the Q's of 400 and 200 kJ/mol for compression tests in the high and low stress exponent regimes, respectively. A Larson-Miller comparison indicated that cyromilling can produce an alloy with long-term, high-temperature strength at least equal to conventional superalloys.

Effect of Grain Boundary Morphology on Tensile Creep and Torsional Creep Rupture Properties of Austenitic Heat-Resistant Steel.

Tensile creep and torsional creep rupture tests of a commercial austenitic heat-resistant steel, JIS SUH 38, were conducted at 973 K, using two types of specimens : one with a weak rectilinear grain boundary (WQ) and the other with a strong zigzag grain boundary (FA). It was found that the tensile creep rupture strength of FA specimens was considerably higher than that of WQ specimens, and that all specimens tested showed brittle grain boundary fracture. On the other hand, in torsional creep rupture tests, WQ specimens had comparatively higher strength than FA specimens at a higher stress level, while at lower stress levels, WQ specimens became weaker than FA specimens. In the present test, WQ specimens displayed ductile transgranular fracture at higher stresses, but with decreasing stress, they showed brittle intergranular fracture. In all FA specimens, however, the fracture modes were relatively ductile transgranular irrespective of the stress level. These results were explained by the difference in mean normal stress between tensile creep and torsional creep rupture tests, as well as the difference in grain boundary strength between WQ and FA specimens.

Transient Liquid Phase Bonding of Ni-base Single Crystal Superalloy, CMSX-2.

This study investigated bonding and the crystallization behavior of Ni-base single crystal superalloy, CMSX-2, base material during transient liquid phase (TLP) bonding using MBF-80 insert metal. Joint strength was evaluated using tensile and creep rupture testing at elevated temperature. TLP-bonding of CMSX-2 was carried out at 1373-1548 K for 0-19.6 ks in vacuum and the (001) plane of each test specimen was always aligned perpendicular to the joint interface. The dissolution width at the bonding temperature increased when the bonding temperature and holding time increased. The eutectic width decreased linearly with the square root of holding time during isothermal solidification. Electron back scattering patterns of completed joints revealed that the bonded layer had single-crystallized during the TLP-bonding process and matched the crystallographic orientation of the bonded substrates. The elevated temperature tensile strength and creep rupture strength of the joints were the identical ones of the base metal.

Tensile Creep Behavior of Alumina/Silicon Carbide Nanocomposite

Tensile creep and creep rupture behaviors of alumina/17 vol% silicon carbide nanocomposite and monolithic alumina Were investigated at 1200° to 1300°C and at 50 to 150 MPa. Compared to the monolithic alumina, the nanocomposite exhibited excellent creep resistance. The minimum creep rate of the nanocomposite was about three orders of magnitude lower and the creep life was 10 times longer than those of the monolith. The nanocomposite demonstrated transient creep until failure, while accelerated creep was observed in the monolith. It was revealed that rotating and plunging of intergranular silicon carbide nanoparticles into the alumina matrix increased the creep resistance with grain boundary sliding.

Comparison of the Creep and Creep Rupture Performance of Two HIPed Silicon Nitride Ceramics

Measurements of the tensile creep and creep rupture behavior were used to evaluate the long‐term mechanical reliability of a commercially available and a developmental hot isostatically pressed (HIPed) silicon nitride. Measurements were conducted at 1260° and 1370°C utilizing button–head tensile specimens. The stress and temperature sensitivities of the secondary creep rates were used to estimate the stress exponent and activation energy associated with the dominant creep mechanism. The stress and temperature dependencies of creep rupture life were determined by continuing individual creep tests to specimen failure. Creep deformation in both materials was associated with cavitation at multigrain junctions. Two‐grain cavitation was also observed in the commercial material. Failure in both materials resulted from the evolution of an extensive damage zone. The failure times were uniquely related to the creep rates, suggesting that the zone growth was constrained by the bulk creep response. The fact that the creep and creep rupture behaviors of the developmental silicon nitride were significantly improved compared to those of the commercial material was attributed to the absence of cavitation along two‐grain junctions in the developmental material.

Tensile Creep and Creep Rupture Behavior of Monolithic and SiC‐Whisker‐Reinforced Silicon Nitride Ceramics

The tensile creep and creep rupture behavior of silicon nitride was investigated at 1200° to 1350°C using hotpressed materials with and without SiC whiskers. Stable steady‐state creep was observed under low applied stresses at 1200°C. Accelerated creep regimes, which were absent below 1300°C, were identified above that temperature. The appearance of accelerated creep at the higher temperatures is attributable to formation of microcracks throughout a specimen. The whisker‐reinforced material exhibited better creep resistance than the monolith at 1200°C; however, the superiority disappeared above 1300°C. Considerably high values, 3 to 5, were obtained for the creep exponent in the overall temperature range. The exponent tended to decrease with decreasing applied stress at 1200°C. The primary creep mechanism was considered cavitationenhanced creep. Specimen lifetimes followed the Monkman–Grant relationship except for fractures with large accelerated creep regimes. The creep rupture behavior is discussed in association with cavity formation and crack coalescence.

Damage Resistance of In Situ Reinforced Silicon Nitride

ABSTRACTThe room temperature fracture behavior for in situ reinforced (ISR) silicon nitride is correlated to its microstructure and R-curve behavior. The relation of strength to fracture origin suggests that stable growth of the intrinsic flaw precedes catastrophic fracture. Grainbridging that generates a rising bridging stress behind the crack-tip has been proposed as the cause for stable crack growth, which in turn reduces the strength dependency on initial flaw size. As a result of strong bridging by the acicular β-Si3N4 grains, ISR Si3N4 is characterized for high Weibull modulus. At elevated temperatures, the material's tensile creep rupture behavior follows the Monkman-Grant type plot. A tensile creep rate of -10−9s−1 at 1260°C/250 MPa, 1300°C/180 MPa, and 1350°C/90 MPa has been recorded. This relatively strong creep resistance is related to the sliding-resistance of the acicular grains and the properties of the amorphous film between the grains in ISR Si3N4.

Tensile creep rupture at cyclic load variation

Assuming the validity of the life fraction rule the life time is calculated for creep rupture of material subjected to tensile cyclic stress variation. Two cases are considered: a saw tooth stress-time profile with zero and non-zero holding period. The life times and hence the number of cycles at fracture can be predicted by means of values from the static stress-rupture diagram ( n, τ o, τ max ) and values characterizing the stress cycle ( a, ν, t H ). The calculations for zero holding period are compared to experiments performed on the stainless steel type: DIN 1.4970 at 700°C. Very good agreement is obtained for loading conditions leading to shorter rupture times.

Deterioration of 21/4Cr-1Mo steel ofter long-term service at elevated temperatures. 2.

An outstanding decrease in low temperature toughness was observed from the examination of 21/4 Cr-1 Mo steel tubes after long-term service at elevated temperatures. Various forms of heat treatments were conducted on the samples to investigate the effect of thermal history of raw material on embrittlement, the relation between the accelerated short-time embrittlement tested in the laboratory and that encountered in the actual plant, and the effect of heat treatment for restoration of toughness.At the same time, a large decrease in high temperature strength was found in the materials embrittled after long-term service at elevated temperatures.Moreover, the restoration of not only toughness but also high temperature strength was found after de-embrittlement heat treatment. The strength then decreased again after reembrittlement heat treatment.In our previous paper, it was pointed out that some restrictions should be introduced with regard to the temperature range when the degree of embrittlement was evaluated by the time-temperature equivalent parameter method.The results of this investigation suggest the necessity of similar restrictions with regard to the time-temperature equivalent method in evaluating high temperature tensile strength and creep rupture strength.Up to this time, a comparison of creep strengths of used and unused material was made using the Larson-Miller parameter, and it was found that there existed some biased phenomena in many cases which could not be explained by means of conventional data.The unconditional parameter method was considered to be one of the reasons for this. Namely, as is evident from this study, the reversibility of high temperature tensile strength of a material, which decreased in a difinite range of temperatures, indicates that it is essential to give sufficient consideration to the application of the parameter method, which has been used as an indication related to the deterioration of the material.

Tensile creep of beta-phase zircaloy-2

The tensile creep and creep rupture properties of beta-phase Zircaloy-2 are studied under vacuum in the temperature and stress range 1300–1550 K and 0.5–2 MN/m 2. The new results are compared with previously reported uniaxial and biaxial data. A small, but systematic difference is noted between the uniaxial and biaxial creep data and reasons for this discrepancy are discussed.

Determination of the multi-axial stress creep fracture criterion using a modified tensile creep unit

AbstractIdentification of the complex-stress criterion governing creep fracture in metals has been shown to be of fundamental significance in the assessment of component life. In the present paper the authors consider the problems encountered when deriving the criterion without access to sophisticated complex-stress testing equipment. These have been solved by comparing the results of direct double-shear creep fracture tests with predictions of those results, made on the basis of tensile creep rupture data in association with either an octahedral shear stress criterion or a maximum principal tensile stress criterion. An Al alloy, Cu, a Ni alloy, a Ti alloy, cast iron, and two steels have been investigated at temperatures where creep conditions are operative,. the results have shown that the method may be used to predict, with sufficient accuracy, the complex-stress creep fracture criterion for the metals studied.

Study on Creep Rupture of Notched Cylindrical Specimens of 1Cr-1Mo-1/4V Steel at Elevated Temperature

From the tensile creep rupture tests for unnotched and round notched bar specimens of 1Cr-1Mo-1/4V steel at 600°C and the numerical calculation results by using finite element method, the following are concluded: (1) The material tested has the transition phenomena from notch-strengthening to notch-weakening at elapsed time of about 120 hrs. The rupture ductility of the material remarkably decreases after elapsed time of this transition. (2) In the region of notch-strengthening, the levels of the equivalent stress and the strain at the notch root of the notched specimen are lower than those of the notched specimen under the same nominal stress. This holds also in the case of hydrostatic stress at the notch root. On the contrary, the quantitative relation mentioned above is reversed in the notch-weakening. The former is one of the important factors determining the notch-strengthening, while the latter is a factor for the notch-weakening.

The validity of various fracture mechanics methods at creep temperatures

The application of stress intensity factors derived from linear elastic fracture mechanics (LEFM) to fracture at creep temperatures has been considered. From tensile creep rupture tests on single edge notched and notched centre hole specimens of solution treated A.I.S.I. type 316 stainless steel, it is shown that a LEFM approach is inapplicable to predicting creep crack growth rates, whilst the net section stress is found to correlate well with the crack growth rates. These observations have been explained by considering the creep relaxation that takes place at the notch root, smoothing out the local stresses and thus making the LEFM stress distribution inapplicable. The resulting stress distribution supports the observation that the net section stress is a successful criterion on which to predict creep rupture in stainless steel.