Creep Larson-Miller

Tube wall temperature is an important parameter in the design and operation of steam reformers. The tubes are exposed to an extreme thermal environment. Creep of the tube material is inevitable, leading to failure of the tubes, which is exacerbated if the tube temperature is not adequately controlled. The effects of tube temperature on the strength of a tube are considered by use of the Larson-Miller parameter, P (Ridler and Twigg, 1996) ... 

The creep rupture data are derived from laboratory tests on material samples having a standardized geometrical form. The convenient way to express these data is to plot the stress versus the Larson-Miller parameter P ... 

Comparable tedmiques developed for metal creep are, due to Dom, the temperature-oompeiisated tune parameter, and the related Larson-Miller parameter. 

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. 

The Larson-Miller parameter, P, in Eq. (6.102), is one of the useful parameters used for predicting creep life in metallic materials, but it is useful for ceramics as well. The LMP may be used to describe the stress-temperature-life relation in a SiC/SiC composite by means of the following expression ... 

With a required life time of the shaft of 200 000 h, it is not sensible to wait for creep data measured experimentally at comparable times, for this would require about twenty years. To avoid this, data at shorter testing times, which are partially measured at temperatures beyond the service temperature, are extrapolated to larger service times. This is frequently done using the so-called Larson-Miller parameter, moti-... 

Table 11.2. Creep rupture strength of several alloys (after [39,125,129]). The creep rupture strength iim/iooooo/T i- e., the stress needed to cause fracture in a specimen at temperature T after 10 hours (creep rupture time), is stated. The creep resistance of the ferritic steels with large amounts of vanadium and chromium is significantly larger than that of simpler steels because vanadium and chrome carbides have a better temperature stabihty. Due to their close-packed face-centred cubic structure, the creep resistance of austenitic steels is larger. The creep strength of the nickel-base superalloys IN 738 (polycrystaUine) and SC 16 (single crystalline) were estimated from Larson-Miller data...

Fig. 11.19 Application of Larson-Miller parameter method to creep of a [90/60/-60/90J2S graphite/epoxy laminate. (Data from Dillard, (1981)). Line is a fit of the data using the Larson Miller equation.

Larson-Miller comparison of creep behavior tor beta forged and annealed TI-11 Ti-6AI-2Sn-4Zr-2Mo, and Ti-6AI-2Sn-4Zr-6Mo. 0.2% creep deformation. Annealed, AC condition for Ti-11. 

The Larson—Miller relationship [100,101] is a parametric equation which is used to extrapolate creep experimental data. It is widely used even when no clear and convincing physical basis of this curve has been proposed, at least from our knowledge. The specific material constant is approximately equal to 20 for most materials. As can be observed in Fig. 6.31(a,b), the combined model based on necking for high stress and intergranular damage for low stress allows us to plot a master curve... 

Figure 10,24 Creep properties of the CEA ODS 14Cr alloy compared to hteratme with a Larson—Miller parameter. The red triangles are the CEA data [59]. In this graph, all the data are coming from samples obtained in the longitudinal direction.

The need often arises for engineering creep data that are impractical to collect from normal laboratory tests. This is especially true for prolonged exposures (on the order of years). One solution to this problem involves performing creep and/or creep rupture tests at temperatures in excess of those required, for shorter time periods, and at a comparable stress level, and then making a suitable extrapolation to the in-service condition. A commonly used extrapolation procedure employs the Larson-Miller parameter, m, defined as... 

Steady-state creep rate (constant temperature) Steady-state creep rate Larson-Miller parameter... 

Larson-Miller Parameter (Prediction of Long-Term Creep Properties)... 

Engineering design often requires engineers to predict material properties at high temperatures where no experimental data are available. The creep deformation rate can be so slow that it might require 10 years test time to reach 1% deformation. Reliable predictions based on accelerated test data obtained over a shorter period of time are essential. Several theoretical parameters were proposed to predict long-term metal creep or stress rupture life based on short-term test data. One of the most utilized parameters is the Larson-Miller parameter, as defined by Equation 4.20 ... 

Larson, F. R., and Miller, J. A. A Time-Temperature Relationship for Rupture and Creep Stresses. Transactions, ASME, Vol. 175, 1952. 

F.R. Larson, J. Miller, A time-temperature relationship for rupture and creep stresses, Trans. ASME (1952) 765-775. 

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