Creep-rupture lifetimes

Carburisation Carbon deposition can induce the precipitation of coarse internal carbides that embrittle the Ni-Cr-Mo alloys at low temperature [10], Moreover, the creep strain is strongly affected by carburisation [11, 12]. However, it is worth noting that the creep rupture lifetime of Hastelloy X under carburising helium is not far below that in air [13]. 

O have a creep rupture lifetime of at least 20 days at 650°C (923 K). Compute the maximum allowable stress level. 

Hua] Tensile, creep rupture tests Yield strength, creep properties (creep rupture lifetime, grain size, ductility)... 

The proposed model for creep rupture based on the condition of maximum shear strain and the Eyring reduced time model explain the observed relations concerning the lifetime of aramid, polyamide 66 and polyacrylonitrile fibres. However, with increasing temperatures, in particular above 300 °C, chemical degradation of PpPTA also determines the lifetime. Furthermore, the model... 

Creep leads ultimately to rupture, referred to as creep-rupture, stress-rupture or static fatigue. Creep-rupture of thermoplastics can take three different forms brittle failure at low temperatures and high strain rates ductile failure at intermediate loads and temperatures and slow, low energy brittle failure at long lifetimes. It is this transition back to brittle failure that is critical in the prediction of lifetime, and it is always prudent to assume that such a transition will occur [1], A notch or stress concentration will help to initiate failure. 

Time-temperature shifting is used widely in the prediction of creep and creep-rupture in polyester geosynthetics. Creep of oriented polyesters is expressed by a linear or quadratic increase of strain with log (lifetime). The lifetime under constant load is expressed by the semilogarithmic formula ... 

Despite the large number of creep and creep rupture mechanisms,91,92 the lifetime of structural materials at elevated temperatures is often a simple function of the creep rate. This relationship was first noted by Monkman and Grant114 who presented an empirical relationship between rupture life and... 

Two important conclusions are reached from the Monkman-Grant equation. First, the most important engineering requirement to improve the lifetime of materials that fail by creep rupture is to improve creep behavior. Over the years, this has been the goal of much of the research on high temperature alloys. Second, theoretical treatments of creep rupture must be consistent with the Monkman-Grant equation, as has been shown by Ashby and Dyson.111... 

In Fig. 4.21, creep rupture data from a number of different grades of silicon nitride are plotted in a Monkman-Grant format.30,31,34,115 116 For purposes of comparison with metallic alloys, the temperature dependence of the Monkman-Grant curves has been ignored. As with the metallic alloys, the curves for all of the grades of material tend to plot within a relatively narrow band. These results imply that lifetime can be improved merely by improving creep rate the lower the creep rate, the longer the lifetime. 

Above the threshold, deformation occurs as a consequence of direct particle interaction. Several mechanisms of interaction have been suggested solution-precipitation flow of fluid between particles and cavity formation at the particle matrix interface. These theories of creep suggest several rules to improve creep behavior (1) increase the viscosity of the matrix phase in multiphase materials (2) decrease the volume fraction of the intergranular phase (3) increase the grain size (4) use fiber or whisker reinforcement when possible. As the creep rupture life is inversely proportional to creep rate, lifetime can be improved by improving creep resistance. 

S. L. Phoenix, P. Schwartz, and H. H. Robinson, IV, Statistics for the Strength and Lifetime in Creep-Rupture of Model Carbon/Epoxy Composites, Composites Science and Technology, 32, 81-120 (1988). 

H. Otani, S. L. Phoenix, and P. Petrina, Matrix Effects on Lifetime Statistics for Carbon Fibre-Epoxy Microcomposites in Creep Rupture, Journal of Materials Science, 26, 1955-1970 (1991). 

To construct such a map (see Worked Example 12.4), SCG and creep rupture data must be known for various temperatures. The temperature dependence of the stress levels required to result in a given lifetime are then calculated from Eqs. (12.45) and (12.46). The mechanism that results in the lowest failure stress at a given temperature thus defines the threshold stress or highest applicable stress for the survival of a part for a given time. In other words, the lifetime of the part is determined by the fastest possible path. Such maps are best understood by actually plotting them. 

Creep rupture tests of plastic pipe, described in Chapter 14, are typical of whole-lifetime testing. It is difficult to manufacture plastic products without incorporating foreign particles, of size about 0.1mm, such as undispersed pigment or stabiliser, or metal wear fragments from extruder screws. The... 

Grades of PE have been specially developed for the gas pressure pipe market. Eirst-generation HDPEs were not used in the UK. The second generation of MDPE copolymers has superior creep rupture resistance at the pipe design lifetime of 50 years (Fig. 14.5). The data falls on one or more lines lines of shallow slope are associated with ductile failure and lines of steeper slope with brittle plane strain crack growth (Fig. 14.3). The... 

P(2) The creep rupture behaviour of the composite selected should be checked against Figure 4.14 to ensure that the structure wlU not fail via a stress rupture mechanism during the required lifetime. 

Equation (13.6) was used to develop a set of creep rupture envelopes to predict attainable creep rupture time at given load L for three mega-coupled composites at 80°C as shown in Fig. 13.18. With measured data only at creep rupture times <100 hr for accelerated tensile creep determinations, we can make estimates for anticipated service lifetime as a function of applied stress load that is indicated by the weight L. 

To improve the performance of SiC and to increase its resistance against creep failure, generally various constituents are added to monolithic SiC ceramics. Additives in various shapes and sizes are usually added to SiC to achieve a better material for structural use and to extend its service lifetime. An evaluation of creep failure, commonly referred to as creep rupture or stress rupture , is a critical step in evaluating the suitability of a certain ceramic for use in the desired application. The stress rupture and creep properties of a SiC matrix reinforced with SiC fiber (i.e., a SiC/SiC composite) has been evaluated by tests conducted in order to assess the propensity of SiC/SiC for high-temperature appfications over an extended lifetime. In Fig. 6.105, plots of stress versus time-to-rupture are shown for several temperatures. As commonly done, these plots are on a log-log scale. Each curve can be fitted by means of an empirical relation, similar to the earlier exponential equation expressing the time-to-rupture, h, to a stress exponent for stress rupture as ... 

Most creep analyses for long-term applications involve the determination of a steady-state or minimum creep rate and its examination as a function of applied stress and temperature. The steady-state or minimum, compressive creep rate (dc/ dtmin) may be related to the applied stress and temperature by an empirical Arrhenius power law or the familiar Norton-Badey creep equation . Often in creep testing, a minimal 1 % (of the expected lifetime) criterion is used. However, a 10 % criterion is preferable in order to obtain a meaningtid prediction of the usable lifetime for creep applications. In critical applications, such as for turbine components, an 25 % criterion is recommended, even preferred, so as to avoid failure by creep rupture. 

Creep rupture data minimum jreep rate as a function of the time to failure. Although the data set is limited, time-to-failure seems to be determined uniquely bye the rate of creep in this material, suggesting that improvement of creep rate is of paramount importance to improving lifetime. 

Given a creep plot for some material, determine (a) the steady-state creep rate and (b) the rupture lifetime. 

Both temperature and the level of the applied stress influence the creep characteristics (Figure 8.30). At a temperature substantially below 0.4r , and after the initial deformation, the strain is virtually independent of time. With either increasing stress or temperature, the following will be noted (1) the instantaneous strain at the time of stress application increases, (2) the steady-state creep rate increases, and (3) the rupture lifetime decreases. 

Figure 8.29 Typical creep curve of strain versus time at constant load and constant elevated temperature. The minimum creep rate Ae/Af is the slope of the linear segment in the secondary region. Rupture lifetime is the total time to rupture.

The results of creep rupture tests are most commonly presented as the logarithm of stress versus the logarithm of rupture lifetime. Figure 8.31 is one such plot for an S-590 alloy in which a set of linear relationships can be seen to exist at each temperature. For some alloys and over relatively large stress ranges, nonlinearity in these curves is observed. 

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