Creep Rupture Behavior Research Articles

Creep deformation and rupture behaviour of directionally solidified GTD 111 superalloy

ABSTRACTThe creep behaviour of directionally solidified (DS) Ni‐base superalloy GTD 111 has been investigated at various temperatures (649 °C to 982 °C) and stresses (124 MPa to 896 MPa). Specimens machined in longitudinal and transverse directions with respect to the grain orientation from three batches of the material were tested. The specimens in the longitudinal direction consistently exhibited higher creep rupture life and creep ductility than specimens from the transverse direction. There were some systematic variations in creep deformation and rupture behaviour among specimens from different batches. Optical and scanning electron microscopy investigations were conducted to understand the creep rupture behaviour. Various deformation and rupture models were evaluated for representing the creep behaviour of the alloy and a neural network model was applied to creep rupture data to assess its predictive capability.

Long-term creep rupture behavior of smoothed and notched bar specimens of low-carbon nitrogen-controlled 316 stainless steel (316FR) and their evaluation

Low-carbon, nitrogen-controlled 316 stainless steel is regarded as a principal candidate for a main structural material of future fast breeder reactor plants in Japan. To grasp creep deformation and rupture behavior of this steel whose modeling is indispensable in the design of high-temperature components, a number of uniaxial tensile creep tests have been conducted for four products of this steel at 550 °C and higher temperatures. Long-term creep rupture data up to about 94,000 h were obtained and used to examine the applicability of rupture and deformation estimation methods developed earlier. In addition, two tests were conducted using round-bar specimens with circumferential notches to make investigation of the effect of stress multiaxiality on creep damage.

Creep-rupture behavior of 3Cr–3W–V bainitic steels

A nominally Fe–3.0Cr–3.0W–0.25 V (3Cr–3WV) steel and this composition with 0.07% Ta (3Cr–3WVTa) were developed for elevated-temperature service in the power-generation and petrochemical industries. Creep-rupture strengths of the new steels to 600 °C exceeded those of the two advanced commercial 2.25Cr steels T23 (Fe–2.25Cr–1.6W–0.25V–0.05Nb–0.07C) and T24 (Fe–2.25Cr–1.0Mo–0.25V–0.07Ti–0.005B–0.07C). Moreover, the strength of 3Cr–3WVTa approached that of modified 9Cr–1Mo (T91) at 650 °C. Elevated-temperature strength in the new steels is obtained from a bainitic microstructure with a high number density of fine needle-like MX precipitates in the matrix. The presence of tantalum promotes a finer MX precipitate in the 3Cr–3WVTa than in the 3Cr–3WV, and it suppresses the coarsening of these fine precipitates during creep.

New Automated Long-term Extrapolation Method for Plastic Pipes under Hydrostatic Pressure

In this article, an automated method for prediction of the long-term (creep rupture) behavior of multilayer pipes under hydrostatic pressure is presented. For long-term extrapolation, a combined quadratic and linear regression analysis is used. This procedure is automated in the pipe software called automated design and analysis of pipes (ADAP) for design and analysis of pipelines. Two examples showing this procedure using the software ADAP are presented and the results are compared with the existing international standards. Through this study, a new automated procedure for the extrapolation of the long-term hydrostatic strength of thermoplastic and fiber reinforced pipes is presented. Hence the arbitrary action of defining a sharp knee becomes redundant. Moreover, instead of a computational but physically non-plausible sharp knee, a transition zone between the ductile and the brittle behavior at a higher temperature is obtained. This automated method and the inherent novel features may be of use in the revisions of the existing international standards. The proposed method can be used for the prediction of the long-term internal hydrostatic behavior of thermoplastics pipes, glass fiber reinforced laminate pipes, and multilayer plastics pipes.

The creep rupture behaviour and steam oxidation resistance of P92 weldments

The creep rupture behaviour and steam oxidation resistance of P92 weldments

508 一方向炭素繊維強化複合材料の高温における非主軸クリープ破断挙動(OS1(2) 微視構造を有する材料の変形と破壊複合材料の特性評価と解析)

Creep rupture behavior of a unidirectional T800H/Epoxy composite under constant load conditions at different temperatures is examined. Tensile creep rupture tests are performed at different stress levels on plain coupon specimens with various fiber orientations θ=0, 10, 30, 45, 90°. Transient creep behavior is dominant for all the fiber orientations, and ultimate creep rupture takes place along reinforcing fibers regardless of fiber orientation and test temperature. A creep damage mechanics model is then developed for describing anisotropic creep rupture behavior at different temperatures. The proposed model can adequately describe the directional and temperature-dependent off-axis creep rupture behavior of the unidirectional composite. Another creep rupture model that is formulated using the Larson and Miller parameter also succeeds in predicting the off-axis creep rupture behavior.

The creep rupture behaviour and steam oxidation resistance of P92 weldments

The creep rupture properties of P92 autogenous welds have been investigated using cross-weld specimens and weld simulation specimens. The influence of post-weld heat treatment was also examined. The results have shown that the weldments in creep rupture tests failed in the softened heat affected zone (Type IV failure). Transmission electron microscopy of thin foils taken from the heat affected zone revealed a low dislocation density, which is associated with the low creep rupture strength. The stress rupture strength reduction factor of the weldments compared with the base material is, however, within acceptable limits. Steam oxidation tests were carried out on the weld deposit and on cross-weld specimens and showed that the weldments exhibited lower oxidation rates than the parent material, due to the higher Si and Mn contents of the filler metal.

Fracture and Viscoelastic Behavior of Some Polymers at High Temperatures

Polymers are vital materials in better performance of specific strength. However their application can be restricted by the lower glass transition temperature, Tg. Some polymers have been developed as engineering plastics for the high temperature applications. We examined the high temperature strength of polymers at constant applied stress. The creep rupture and viscoelastic behavior were scrutinized for PC (polycarbonate) and PMMA (polymethyl methacrylate), which were quite different in the molecular structures. The former contains benzene rings and the latter is a single polymer. Tg is 423 K for PC and 378 K for PMMA. The large difference in the creep behavior was observed near Tg. The creep life strongly depends upon the applied stress just below Tg. The creep life is a function of the applied stress as follows. n life t − µ s . The stress exponent, n depends upon the temperature. Mechanical models were applied to evaluate the viscoelastic properties of the polymers at high temperatures. The viscosity rapidly decreased near Tg , regardless of the smaller decrease in the elastic constant. The results would be due to the difference in the molecular structures. The benzene ring could contribute to the higher resistance against the creep deformation through the higher viscosity.

Off-axis tensile creep rupture of unidirectional CFRP laminates at elevated temperature

Abstract Off-axis tensile creep fracture behavior of unidirectional carbon/epoxy T800H/Epoxy laminates is studied at 100 °C under constant load conditions. Off-axis creep rupture data on plain coupon specimens are obtained within the time range up to 10 h for four kinds of off-axis fiber orientations. The log–log plots of the creep stress against the rupture time can approximately be described by straight lines with negative slopes over the range of creep life for all the fiber orientations. The static tensile strengths extrapolated from those straight lines almost agree with the experimental results for respective fiber orientations. The creep rupture data normalized with respect to the static strength approximately falls on a single creep rupture curve. These observations suggest that the fiber orientation dependence of creep rupture strength is similar to that of static tensile strength. The creep fracture occurs along reinforcing fibers in a brittle manner without accompanying the appreciable secondary and tertiary creep stages, regardless of the fiber orientations, and thus the off-axis creep deformation prior to fracture is characterized by the primary creep stage for all the fiber orientations. Then, a phenomenological model for the creep deformation and rupture behaviors of the unidirectional composite is developed, with a view to making preliminary predictions of creep life from a limited amount of data. It is demonstrated that the proposed model can moderately describe the observed features of the creep deformation and creep rupture behaviors under off-axis loading conditions.

Lifetime Predictions For High-Temperature Low-Alloy Ferritic Steel Weldments

The paper reports the finite element creep continuum damage mechanics (CDM) analysis of the creep rupture behaviour of thin-section butt-welded, internally pressurized pipes, and of uniaxial cross-welded tension testpieces made of Cr-Mo-V steels. The CDM analyses are shown to predict accurately lifetimes and failure mechanisms observed from metallographic analyses of sectioned components. The paper then addresses the lifetime prediction of components by comparing predictions made using British Standards BSi PD 5500 with the results of the experiments. Predictions have been based on uniaxial plain bar data for both parent and Type IV materials; both of these have been obtained by integration of constitutive equations. Comparison of predicted and experimental weldment lifetimes has been made using a weld strength reduction factor. It is found that the required factors are very dependent on temperature and the ratio of axial to hoop pipe stresses. Finally, the paper examines the British Energy Generation Limited R5 assessment procedure and compares lifetime predictions with those made using the British Standards BSi PD 5500 approach. It is noted that the stresses in both approaches are in good agreement for the external/internal radius ratio of 1.25 considered in the butt-welded pipe experiments.

Determination of the long-term hydrostatic strength of multilayer pipes

In this paper, a new methodology for prediction of the long-term (creep rupture) behavior of multilayer pipes under internal hydrostatic pressure is presented. For this purpose, a procedure using the three dimensional theory of thick-walled multilayer pipes together with a combined quadratic/linear regression analysis is used. The theory of thick walled tubes is used to assess the role of each layer in carrying the internal pressure and also the onset of the creep rupture in the composite pipe. For long-term extrapolation, a combined quadratic and linear regression analysis was used and the contribution of each component to the long-term strength of the composite pipe was quantitatively assessed. Using this procedure, the layer at which the creep rupture under internal pressure first initiates would be identified and the additional capacity of the remaining layer would be quantitatively assessed. This procedure is already incorporated in the pipe software called ADAP dealing with the automated design and analysis of pipelines. An example showing this procedure using ADAP is presented. The proposed methodology overcomes the limitations in the existing testing and extrapolation standards and, thus, can be used as a new extrapolative procedure for prediction of the service life of multilayer plastic pipes and pipe fittings. As particular applications, the procedure can be used for the estimation of the long-term behavior of multilayer pipes with metallic inter-layers and also single layer as well as structured pipes.

Effect of Repair Welding on the Creep Rupture Strength of an Aged Weldment

Weldments in high-energy piping of fossil power plants are known to suffer extensive creep damage over the course of long-term operations. Repair welding after the removal of the deteriorated areas is considered as a potential method for improving the life of such weldments. Very few papers, however, are reported concerning the influence on creep rupture behaviour of repair welding performed at the end of the creep life of the weldments. In this paper, creep test of large weld specimens was carried out to create substantial numbers of creep voids, and these specimens were then repair-welded using various methods. On the basis of the results of subsequent creep rupture tests and stress analyses of these test specimens, the influence of repair welding on the creep rupture strength of deteriorated weldments was discussed.

Creep rupture of lead-free Sn-3.5Ag and Sn-3.5Ag-0.5Cu solders

The aim of this study is to investigate the creep rupture behavior of lead-free Sn-3.5Ag and Sn-3.5Ag-0.5Cu solders at three temperatures ranging from room temperature (RT) to 90 °C, under a tensile stress range of σ/E=10−4 to 10−3. The ultimate tensile strength (UTS) and creep resistance were found to be decreased with increasing temperature for each given lead-free solder. Both the binary and ternary Ag-containing alloys exhibited superior UTS and creep strength to the conventional Sn-37Pb solder at a similar temperature. Due to a more uniform distribution of eutectic phases and a larger volume fraction of intermetallic compounds (IMCs), the Sn-3.5Ag-0.5Cu alloy had greater UTS and creep strength than did the eutectic Sn-3.5Ag solder at each testing temperature. The stress exponents (n) of minimum strain rate (˙emin) were decreased from 7 and 9 at RT to 5 and 6 at 60 and 90 °C, for the binary and ternary lead-free alloys, respectively. Fractography analyses revealed typical rupture by the nucleation and growth of voids/microcracks at IMCs on the grain boundaries. Both Monkman-Grant and Larson-Miller relationships showed good results in estimating the rupture times under various combinations of applied stress and temperature. A model, using a term of applied stress normalized by Young’s modulus, was proposed to correlate the rupture times at various temperatures and could explain the rupture time data reasonably well for the given two lead-free solders.

Creep and rupture behavior of a commercial aluminum–magnesium alloy A5083 at constant applied stress

Commercial aluminum–magnesium alloy was tested to examine the creep-rupture behavior at constant applied stress. At the initial stage, the alloy showed the inverse transition creep with the incubation time. The incubation time would be induced through the release of the free dislocations from the magnesium atmosphere or/and second particles. Minimum strain rate was observed instead of steady state creep rate. The minimum strain rate had the dependence of the applied stress and temperature. The stress exponent was about 5.4 and the activation energy was about 210 kJ/mol. They were independent of the applied stress and temperature. The high activation energy was for the diffusion of magnesium atoms and of vacancies as the additional energy to continue the further creep deformation at constant applied stress. The Monkman–Grant relationship was observed between the minimum strain rate and the creep life.

Two new criteria for the service life prediction of plastics pipes

In this contribution, two new criteria and related methodologies for the prediction of the long-term (creep rupture) behavior of single layer and multilayer plastics pipes under hydrostatic pressure are presented. One of these is the ultimate strain extrapolation method (USEM) and the other is called the distortion energy extrapolation method (DEEM). The strain concept is based on the use of the failure strain criteria instead of the normally employed stress concept. A related long-term extrapolation methodology is presented that employs the ultimate strain instead of the rupture stress. The strain energy concept is based on the use of the distortion energy corresponding to the failure stress. For both of these two criteria, related extrapolation methodologies are introduced. An example is presented that compares the classical standard extrapolation method (SEM) with the ultimate strain and the energy methods. For correlation of various models, an example of a PVC-U pipe under internal hydrostatic pressure at T=20 °C was studied. The three models employed were the stress-based, the strain-based, and the energy-based regression analyses. Direct regression analysis was performed for all three failure criteria. However, for comparison, the modified version of the SEM was also used. In all cases, a complete match between the independent model and the modified SEM analysis was obtained. A backward calculation of failure stress from the long-term failure distortion energy gave a 50-year failure stress equal to 18.59 MPa. This value was lower than the stress-based extrapolation (25.37 MPa) and higher than the strain-based extrapolation. The proposed USEM is suitable for materials which fail due to the ultimate strain state and not necessarily due to the maximum stresses. Thus, the proposed strain extrapolation criteria may prove to be especially suitable for brittle and fiber reinforced materials. The strain-based extrapolation can be used in connection with rupture data in internal hydrostatic tests or creep rupture of pipe samples under other loading conditions. The DEEM, on the other hand, is believed to be applicable to a broad range of material types. The proposed methodologies can be used as a new guideline for prediction of the service life of single layer brittle thermoplastics pipes, glass fiber reinforced laminate pipes, and multilayer plastics pipes with fiber reinforced layers.

Finite element analysis of notched bar skeletal point stresses and dimension changes due to creep

ABSTRACTIn this investigation, finite element calculations have been performed to obtain the creep stress distributions generated in circumferentially notched bar test‐pieces. They have also been made to determine the relation between axial extension and notch throat diameter changes. It has been found that an approximate skeletal point can be identified where the stress state is insensitive to the power law stress dependence of creep. Consistent trends in skeletal point stress ratios to those given in an existing Code of Practice for notch bar creep testing have been obtained. Nevertheless updated values, particularly for sharp notches, are proposed and these have now been inserted into a new version of the Code of Practice. In contrast, the link between extension and notch throat diameter changes has been found to depend on the creep stress index as well as the notch geometry. It is anticipated that the analysis can be used to establish the multi‐axial creep stress deformation and rupture behaviour of materials.

A Probabilistic Treatment of Creep Rupture Behavior for Polymers and Other Materials

A recent deterministic derivation for the creep rupture behavior ofpolymers is taken as the basis for the present work. The formulation isthat for the prediction of the lifetime of viscoelastic materials underconstant load, based upon kinetic crack growth from initial flaws. Thestatistical generalization involves using a Weibull type distributionfor the instantaneous static strengths. Then the resulting fullstatistical character for the lifetimes as a function of stress level isfound. The lifetime distributions are determined to be much broader thanare the static strength distributions. An unusual statisticalcharacteristic emerges from the derivation. When the lifetimes areviewed on log stress versus log lifetime scales the envelopes for thevarious probability levels of failure are found to have a common shape,as in a single master curve, but they are shifted relative to each otheralong the log stress axis. This shifting property could haveconsiderable utility in reducing the number of tests required to obtaina complete, statistical database. Finally, similar implications forfatigue characterization are considered.

Effect of Melt Treatment on the Microstructure and Creep-Rupture Behavior of M963 Cast Nickel-Base Superalloy

The effect of melt treatment on the microstructure and creep-rupture behavior of M963 superalloy at 1248K under 225MPa has been investigated. The microstructure of the as-cast superalloy without melt treatment consists of γ solid solution matrix, γ´precipitate, coarse blocky MC carbide and (γ+γ´)eutectic. The striking difference in microstructure is that the melt treatment changes the MC carbide from the coarse blocky morphology into the fine script-like morphology. After heat-treated at 1483K for 4h followed by air-cooling, both the creep life and rupture elongation of the melt-treated alloy are all doubled those of the alloy without melt treatment. The mechanism of the melt treatment on the creep-rupture behavior of the M963 superalloy is discussed.

Ｗ含有高Ｃｒフェライト系耐熱鋼の６５０℃における粒界クリープ強化

Creep strengthening mechanism of tungsten containing 9 to 12% chromium steels for high temperature application was experimentally extracted and discussed from the view point of grain boundary shielding by Fe2W type Laves phase and M23C6 type carbide during the creep exposure at 650°C. Increase of tungsten content from 1.8mass% for ASME Gr. 92 to 2.5mass% for model alloys resulted in the abrupt creep rupture strength deterioration for about 1000 hours creep time. Increase of chromium from 9mass% to 11mass% or more also affected the creep rupture curve. Excess tungsten and chromium almost precipitated grain boundary, and a grain boundary shielding ratio by the precipitates explained the creep rupture behavior, bending of the curves, in tungsten or chromium increased model alloys according to the analyze by thin foil observation through the Transmission Electron Microscope (TEM) with Energy Dispersive X-ray Spectroscopy. Grain boundary shielding possibly delayed the recovery of dislocation substructure by rigid grain structure. Grain boundary shieldng ratio f0/f expressed the creep curves of model alloys based on the dislocation density degradation at 650°C. Species of precipitates did not seem to affect the dynamic recrystalization of a grain boundary.

Mechanical properties of gas pressure sintered Si3N4/SiC nanocomposites

Abstract The mechanical properties of gas pressure sintered Si3N4/SiC nanocomposite materials were studied at room temperature and in the range of 1400–1600 °C. Fracture strength at room temperature was found to be between 640 and 840 MPa and thus within the range of data reported in the literature for other gas pressure sintered silicon nitride ceramics. At elevated temperature the creep and stress rupture behaviour were determined in four-point bending. The creep behaviour is characterised by primary and pronounced secondary creep, very high activation energies for creep in the range of 1050–1450 kJ/mol, and two different stress dependencies. At stresses below a characteristic transition stress in the range 170–200 MPa the stress dependency is described by a stress exponent of around two. Above the transition stress, stress rupture was observed to be controlled by subcritical crack growth with stress rupture exponents of 30–40. Annealing of the as-sintered materials promotes crystallisation of the intergranular glassy phase resulting in an increase in creep strength and resistance against subcritical crack growth. An extensive analysis of the available literature data revealed that the materials tested in this study belong to the most creep resistant silicon nitride ceramics prepared by gas pressure sintering.