Modeling of Creep Deformation and Creep Fracture

Qiang Xu and Zhongyu Lu

### Abstract

This chapter reports the recent progresses in (1) the development of a modified hyperbolic sine law able to depict the minimum creep strain rate over a wider range of stress levels; (2) the development of the creep fracture criterion and model based on the cavity area fraction along grain boundary calibrated with the most representative and comprehensive cavitation data obtained from X-ray synchrotron investigation; and (3) the development of mesoscopic composite approach modeling of creep deformation and creep damage. The first progress facilitates to overcome the difficulty in creep deformation modeling caused by stress breakdown phenomenon; the second progress is of a really scientifically sound and fundamental new approach, first in the world; the third progress provides the concept and tool, at the appropriate size scale, for the modeling of the creep deformation and creep fracture. They all contribute to the specific knowledge and new methodology to the topic area. Furthermore, it is expected that cavitation fracture modeling methodology reported here will find use in the analysis and modeling of other types of failure such as ductile and fatigue failure. This chapter presents an excellent example of interdisciplinary collaborative research and it advocates further such collaboration in its conclusion.

Keywords: creep stress breakdown, creep strain and stress law, Xu's modified hyperbolic sine law, creep cavitation damage and fracture model, X-ray synchrotron cavitation; mesoscopic composite type modeling

#### 1. Introduction

#### 1.1 General

Creep damage is one of the life-limiting factors for high-temperature components. A sound scientific understanding and an accurate mathematical description of the creep deformation and creep fracture are of great interest to and a challenge for the materials and structural integrity research communities and hightemperature industries.

It is generally understood and accepted that for the majority of metals and alloys, creep cavitation at grain boundary is the cause for the creep fracture [1, 2].

Creep continuum damage mechanics (CDM) has been developed to model creep deformation and creep fracture, where internal variables were introduced to depict the macroscopic behavior and the cavitation is incorporated in an average, smearedout manner.

High Cr alloys is one of the alloys developed for and utilized in power generation industry. So, they have been chosen for the research, and progress made with them will be reported here. Additionally, copper-antimony alloy is used in the illustrative example of the mesoscopic composite modeling of creep deformation and fracture due to the unavailability of the micro-mechanical constitutive equation for high Cr alloys. Although the examples of progress reported here are based on the specific material, the methodology is generic and not material dependent.

#### 1.2 The problem

In the development of and particularly in the characterization of hightemperature structural materials, the accelerated creep testing (short-term) may be conducted; then the result might be extrapolated for long-term service condition (say 100,000 hours). Normally, this approach is not reliable, as this microstructure may progressively degrade with often unexpected consequences for long-term creep performance under lower applied stress. Such phenomenon is called stress breakdown; it profoundly exists in high Cr alloys [3–7].

Ennis et al. [3] found that the Norton stress exponent n was found to be 16 for the stresses of above 150 MPa at 600°C and above 110 MPa for 650°C, and an n value was 6 below these stresses. A selection of quantitative presentation of the dependence of the minimum creep strain rate, creep lifetime, and strain at failure on the stress level can be found in [3]. Furthermore, Lee et al. [4] found (1) the stress exponent for rupture life to be decreased from 17 in short-term creep to 8 in long-term creep for the ASTM grade 92 steel crept at 550–650°C for up to 63,151 h, (2) the change of fracture mechanisms with stress level, and (3) creep cavities nucleated at coarse precipitates of Laves phase along grain boundaries.

It can be seen that (1) a wider range of stresses, particularly the lower stress level, must be considered; (2) there is a change of creep deformation mechanism and possible creep damage mechanism under the different stress level. Currently, there is no adequate and accurate data to characterize the creep cavity nucleation, growth, and coalescence under lower stress.

#### 1.3 Current creep cavity damage modeling

In an attempt to model the long-term creep behavior, Yin et al. [8] have proposed a phenomenological relationship between the creep cavity damage and creep strain, which departed from the firm and well-known mechanism-based relationship of Dyson [9]. The relevant equations are listed below for completeness:

Dyson [9]:

$$
\dot{D}\_n = \frac{k\_N}{\varepsilon\_{f\_u}} \dot{\varepsilon} \tag{1}
$$

Yin et al. [8]:

$$
\dot{D}\_n = A \epsilon^{\beta'} \dot{\epsilon} \tag{2}
$$

where A is the creep cavity damage coefficient, and it is assumed that it does not change with stress, it changes with temperature.

Yin's approach cannot be extrapolated into a lower stress level than it has been calibrated according to Yang et al. [10] as a constant value of A is not able to depict the stress breakdown phenomenon.

Modeling of Creep Deformation and Creep Fracture DOI: http://dx.doi.org/10.5772/intechopen.89009

Basirat et al. followed Yin's approach but allowed the cavity damage coefficient to be stress level dependent, in the following form [11]:

$$
\dot{D}\_n = A \dot{e} e^{0.9} \tag{3}
$$

However, an unexpected abnormal variation of the value of A with stress level occurred which is shown in Table 1 and graphically in Figure 1 [12]. Due to the lack of a trend with stress level, it is hard to use them in prediction with confidence. The concept of creep cavity damage coefficient to be stress level dependent had been introduced by the first author in 2003 [13] for low Cr alloy creep damage modeling where no such abnormality occurred.

Hence, the phenomenological modeling of creep cavity damage for high Cr alloy is not satisfactory.

Furthermore, the methodology, based on the isochronous surface concept only, for the generalization of a set of uniaxial creep damage constitutive equations into a set of multiaxial version is conceptually flawed [14–16]. Though the creep deformation consistence has to be included, this has not been very well appreciated by the majority of research community, even in the published review type of articles. Progress can be found only in very limited publications, for example, the original one [16, 17] and the more recent one [18].

#### 1.4 Opportunity and research progress



Table 1. The variation of cavitation coefficient A [11].

Figure 1. The variation of creep cavity damage coefficient A with different stress and temperature [12].

In 2013, the first author noted that the available creep cavitation data is produced with X-ray synchrotron technique by materials scientists. The X-ray synchrotron is a very advanced technique, and it is able to produce a detail and un-destructive, hence more presentative, information over a small volume. This is a very significant advance for the characterization of creep cavitation. Hence, it was the author' view [19] that such information about cavitation should be very valuable for the modeling of creep cavity damage and fracture. Such breakthrough in modeling of creep cavity damage and fracture will be reported with selected examples.

3.The mesoscopic composite approach modeling of creep deformation and damage

Furthermore, the authors [15] have observed that that the current creep continuum damage mechanics operates at macroscopic level is of phenomenological suffering an ambiguity in the depicting of the creep deformation and creep cavity damage and fracture [15], hence, concluded that a mesoscopic composite approach modeling is necessary and better. In this new approach, the grain and grain boundary will be separately presented in space and in property (constitutive equations) to specifically reflect the creep deformation processes and creep damage processes and the ultimate creep fracture. The initial progress will be reported in this chapter.

4.This chapter ends with discussion, conclusion, and suggestion for future work which advocates closer interdisciplinary collaboration.
