Larson-Miller parameters

The creep properties of a newly developed high-temperature material are to be investigated experimentally. Several experiments are performed. 

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FIG. 29-41 Larson-Miller parameters for turbine blade alloys. 

Figure 4-118. Larson-Miller parameter curves allow estimation of bucket life.

Stress-rupture data are often presented in a Larson-Miller eurve, whieh indieates the performanee of an alloy in a eomplete and eompaet graphieal style. While widely used to deseribe an alloy s stress-rupture eharaeteristies over a wide temperature, life, and stress range, it is also useful in eomparing the elevated temperature eapabilities of many alloys. The Larson-Miller parameter is... 

Plm = Larson-Miller parameter T = temperature, °R t = rupture time, hr... 

The Larson-Miller parameters are plotted in Figure 11-4 for the speeified turbine blade alloys. A comparison of A-286 and Udimet 700 alloy curves reveals the difference in capabilities. The operational life (hrs) of the alloys can be compared for similar stress and temperature conditions. 

Figure 11-4. Larson-Miller parameter for various types of biades.

Blade life eomparison is provided in the form of the stress required for rupture as a funetion of a parameter that relates time and temperature (the Larson-Miller Parameter). The Larson-Miller parameter is a funetion of... 

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) ... 

C Material constant used in computing Larson-Miller parameter ... 

LMP Larson-Miller parameter, used to estimate design life ... 

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 ... 

More recently, DiCarlo (1997) has compared ceramic composites to the best superalloy using the strength-to-density ratio versus the Larson-Miller parameter, q. In this analysis (Figure 3-25), the Nextel 720 fiber-reinforced composite is inferior to the metal in short-term fracture but comparable to the superalloy in long-term rupture. On this basis, the Hi-Nicalon fiber is superior to the superalloy except at short times and high stress levels, when fiber failure is dominated by slow... 

Boe] Mechanical tests with Larson-Miller parameters applying, conductivity measurements Stress relaxation behavior, hardness, electrical conductivity... 

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

P = T(C -I- Intf) is the Larson-Miller parameter [henceforth LMP] and is f(activation energy is independent of applied stress, this equation may be used to relate the difference in rupture life to differences in temperature for a given stress. In many cases, C is indeed 20, which is obtained from the intercept with the logt axis of a logt, versus 1/T plott (Fig. 6.111). The slope of such a plot, namely Q/k (=P), is a function of stress, as seen in the schematic Fig. 6.111. For a graphic presentation, Eq. (6.102) may be written as ... 

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-... 

The left-hand side of this equation is an unknown function of the stress, called the Larson-Miller parameter P. The quantities ff and 9 are divided by their unit h to render them unit-free. The equation can be re-written as... 

This factor is usually absorbed into the (now modified) Larson-Miller parameter, resulting in the equation... 

A further example of using the Larson-Miller parameter can be found in exercise 32. 

In the discussion of the Larson-Miller parameter in section 11.1, we used a stress-dependent energy in the Boltzmann factor. Here, we found the reason for this dependence. 

P is Larson-Miller parameter, dimensionless T is Tube wall temperature, °K t is tube life, hours... 

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.

Discuss the relative merits of the Larson Miller parameter method and the Zurkov method compared to the Reiner-Weissenberg crite-... 

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