Creep lifetime

The creep lifetimes obtained at 1000 °C in steam are comparable to those produced in air. All tests conducted in steam at 1000 °C achieved a 100-h run-out. However, the steam environment has a... 

Stress-rupture behavior is summarized in Fig. 7, where results obtained at 1200 °C from prior woric are also included. In air, an increase in temperature from 1000 to 1100 °C has no effect on creep lifetime up to 100 h. With the creep run-out condition defined as 100 h, 150 MPa is the run-out stress in air at both 1000 and 1100 C. However, a further temperature increase to 1200 °C significantly degrades creep lifetime. For the applied stress of ISO MPa, creep life at 1200 °C was reduced by 99% compared to that at 1100 °C. At 1200 "C in air, the creep run-out stress was only 80 MPa. At 1000 C the presence of steam has no influence on creep lifetimes (up to 100 h). In steam at 1000 C creep run-out was achieved at 160 MPa. At 1100 °C the presence of steam dramatically reduced creep lifetimes. In steam at 1100 °C creep run-out was achieved only at 100 MPa. For the applied stress of 150 MPa, the reduction in creep life due to steam was = 88%. An even greater degradation of creep life due to the presence of steam is seen at 1200 °C. At 1200 °C in steam creep run-out was not achieved. The loss of creep life due to steam at 1200 °C was at least 90% for applied... 

There are parametric methods for determining the creep lifetime of materials. Such methods are based on evaluating the stress-rupture behavior. In essence, the results of short-duration, high-temperature tests are correlated with the performance of long-term tests at lower temperatures. The most popular parametric methods are (a) Larson-Miller (b) Manson-Haferd (c) Orr-Sherby-Dom and, (d) Monkman-Grant. Of these methods, the following is a discussion on the Larson-Miller and the Monkman-Grant methods to the evaluation of ceramic-material lifetimes. 

Originally, the MG relation was developed for alloys, but it also has the ability to predict the rupture life of ceramic materials. This MG relation was experimentally applied to various ceramics at various temperatures and stresses and an example of its use to predict their creep lifetime is shown for advanced silicon nitrides. Certain departures from the uniqueness of the MG relation, in both metals and ceramics, have been noted by previous investigators. Consequently, some improvements were suggested by Mamballykalathil et al. [62]. in order to achieve a modified MG relation for ceramics ... 

Keywords physical aging, crystallization, mechanics, stabilizers, glass transition, morphology, creep, lifetime prediction, postcrystallization, Voigt-Kelvin model, comphance, molecular mobUity, Hencky strain, relaxation. 

The two-phase O + alloy Ti-22AI-27Nb had a 0.2% creep resistance (based on measurements at 380 MPa) that was comparable to the creep-resistant alloys Ti-25A-8Nb-2Ta-2Mo and TI-24AI-17Nb-1 Mo. The high volume fraction O phase alloy Tl-24.6AI-23.4Nb had the highest 0.2% creep lifetime of all of the titanium aluminides. 

Yang et al. [35] investigated the creep behavior of nanocomposites with 1 vol% of two types of MWCNTs having different aspect ratios. The creep strain and strain rate of the resulting PP-MWCNTs nanocomposites were markedly lower than that of pme PP matrix, this positive effect being more enhanced at higher (20 MPa) in comparison to lower (14 MPa) applied stress values. Moreover, the reinforcing effect resulted to be more evident as the aspect ratio of the MWCNTs mcreased. Additionally, the creep lifetime of the nanocomposites, i.e., the time to failure imder creep conditions, resulted to be considerably extended by 1000% compared to that of neat PP. 

Figure 6.21 (a) Creep lifetimes, measured experimentally on grades at different temperatures. Comparison with predictions of the necking model (lower and upper bounds). Temperature range 500—700°C (b) comparison between measured and predicted necking cross-sections during a test interrupted several times carried out on a grade 91 steel (350 MPa, 550°C,... 

Short-term and long-term creep lifetime predictions... 

One of the main challenges for some reactor components in austenitic stainless steels at high-temperature in-service conditions is the demonstration of their behavior up to 60 years. The evaluation of creep lifetime of these stainless steels requires on the one hand to carry out very long-term creep tests and on the other hand to understand and model the damage mechanisms in order to propose physically based predictions toward 60 years of service. 

Experimental creep failure stress—lifetime curves of the steel 316L(N) are plotted for tests carried out at temperatures between 525 and 700°C (Fig. 6.3). The extrapolation of these curves based on high-stress data leads to overestimated lifetimes. For example, the extrapolation of a curve at 700° C differs by a factor of 10 at low stress with respect to available experimental data. Therefore, long-term creep lifetimes cannot be predicted by the extrapolations based on short-term data. Similar conclusions have been drawn for ferritic-martensitic steels. But it should be highlighted that this transition occurs much earlier in austenitic stainless steels. The comparison of Figs. 6.23 and 6.24 shows that the transition time is about 4 years in austenitic stainless steels but reaches at least 10 years in tempered martensite-ferritic steels. 

The transition from power-law creep with a stress exponent of about seven to a viscous creep regime occurs at a stress of about one below 30 MPa at 700°C. Any extrapolation from the power-law creep regime to stresses below 30 MPa may lead to serious underestimation of the creep rate and therefore overestimation of lifetime based on the Dyson nucleation law (Eq. (6.5)) which is accounted for in the lifetime prediction. As the strain rates measured at low stress are used as inputs of the Riedel model (Eq. (6.6)), the long-term creep lifetimes are more correctly predicted (Fig. 6.30(a,b)) and the experimental data are within the predicted scatter bounds. 

Interestingly, even if the microstructural evolutions are different, advanced austenitic stainless steels and the Incolloy 800 alloy, which is close to nickel-based alloys, are subjected to the same creep damage mechanisms as martensitic steels and conventional austenitic stainless steels. The same modelings may be applied and lead to creep lifetime predictions in agreement with experimental data up to the longest experimental lifetimes published in the literamre. 

A CONSTITUTIVE MODEL FOR CREEP LIFETIME OF PBO BRAIDED CORD... 

A constitutive model to describe the creep lifetime of PBO braided cord has been developed and fit to laboratory data. The model follows an approach proposed for p-aramid cord in similar applications, and has an Arrhenius-type representation that arises from consideration of the failure phenomenon mechanism. The data were obtained using a hydraulic-type universal testing machine, and were analyzed according to Weibull statistics using commercially-available software. The application of concern to the author is NASA s Ultra-Long Duration Balloon and other gossamer spacecraft, but the motivations for the related p-aramid works suggest broader interest. 

Fortunately, the literature discusses that creep lifetime for p-aramid (Kevlar ), a cousin to PBO, may be represented by a mechanistic mathematical model of the material failure phenomenon. Although the failure mechanism(s) may be different for PBO, the... 

The scission energy comes from two sources, heat and stress, which implies that at higher temperature less stress is needed for rupture, and further suggests that creep lifetime increases as temperature decreases. Recent publications strongly support the Zhurkov theory [14,17] and mention that this theory has been independently developed by others [10,17],... 

The creep lifetime data are presented in Figure 1 on Weibull axes. In an analogous paradigm to logarithmic axes, data which fit the Weibull distribution well appear... 

An Arrhenius-type phenomenological model for creep rupture, when coupled with a Weibull statistical analysis of laboratory data, provides a useful constitutive representation for the creep lifetime of braided PBO cord. This model will be useful to NASA s ULDB project, and also perhaps to other aerospace and civil engineering applications. 

Braided cord, creep lifetime, creep rupture, Kevlar , PBO, p-aramid, ULDB, Zylon . 

Table 1. Weibull fit parameters (Eq. 3) for creep lifetime data shown in Fig. 1.

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