**1. Introduction**

Concerning the estimation of mechanical performance on creep and creep-fatigue interaction, the law on the relationship between applied stress and fracture life [1], the Larson-Miller parameter [2], the Manson-Coffin law [3, 4] and the Ω method [5] were proposed for a smooth specimen. Furthermore, from the point of the application to actual engineering structure, the prediction of the crack growth life originated from a site of stress concentration is important and the establishment of the law of creep crack growth rate to predict creep crack growth life has been conducted [6, 7]. Especially, the establishment of the quantitative estimation method on the mechanical performance for a weld joint specimen under creep condition is important, because the effect of the morphology of heat affected zone (HAZ) line and the difference of material structure, between the weld metal, HAZ and the base metal, on the creep fracture life are significant factors to predict the creep fracture life [8].

Furthermore, from the point of actual application under recent social demand, such as the short experimental period and economical situation, it is necessary to establish the predicting method of creep fracture life by using small number of specimens and by conducting the short life creep test.

In this study, under these social demands mentioned above, by using a notched specimen of creep ductile material, the methods of estimating the quantitative mechanical performance on creep strength (MPCS) and of predicting creep fracture life of the weld joint by using small number of specimens and by conducting the short life creep test were proposed.

## **2. Theoretical foundation and background**

#### **2.1 Similarity law of creep deformation**

For creep ductile materials, the time sequential characteristic of creep deformation plotted against non-dimensional time controlled by each fracture life, *tf* was found to be independent of applied stress and temperature, that is, similarity law of creep deformation [9]. This behavior was illustrated as shown in **Figure 1a** and **b**. For such case, fracture life is possible to be predicted by the value of *RNOD* at the current

#### **Figure 1.**

*Similarity law of creep deformation for a notched specimen. (a) The relationship between RNOD and loading time, (b) The relationship between RNOD and non-dimensional loading time controlled by each creep fracture life [9].*

*The Quantitative Estimation of Mechanical Performance on the Creep Strength… DOI: http://dx.doi.org/10.5772/intechopen.106419*

$$RNOD(t) = \frac{\phi(t) - \phi\_0}{\phi\_0}$$

**Figure 2.** *Definition of relative notch opening displacement (RNOD) [10].*

loading time, *t* as shown in **Figure 1b**. Where, as a measure of deformation for a notched specimen, the concept of the Relative Notch Opening Displacement (RNOD, a representative value of deformation of a DEN specimen) [10] given by Eq. (1) was used.

$$RNOD(t) = \frac{\phi(t) - \phi\_0}{\phi\_0} \tag{1}$$

where *ϕ*ð Þ*t* is the notch opening value at the time of *t* and *ϕ*<sup>0</sup> is the initial notch opening value as shown in **Figure 2**.
