Determining the Creep function of viscoelastic pipes is one of the challenges of modeling these pipes to calibrate or determine defects. The present research aims to determine the Creep function of viscoelastic pipes using transient flow pressure in the time and frequency domains. For this purpose, the proposed method is first implemented using a numerical example. The numerical part investigated the effect of signal sample size, the number of Kelvin-Voigt (K-V) elements, repeatability, and decision variables. Then, using an experimental test, the desired methodology has been evaluated. In this research, the K-V mechanical model was used to define the Creep function, and its parameters, including elastic pressure wave speed, retardation times, and Creep complaint coefficients, were calibrated. The results showed that using pressure signals in both time and frequency domains provides stable results for the investigated pipeline. Examining the effect of signal size showed that the Creep function can be estimated with reasonable accuracy in the time domain with a few initial cycles. Also, 14.33 dimensionless frequency for a simple reservoir-pipe-valve system can provide accurate results in the frequency domain. The results of this research can be used as a suitable pre-processing to reduce the dimensions of inputs in models based on artificial intelligence.

There is an increasing need to assess the service life of components containing defect which operate at high temperature. This paper describes the current fracture mechanics concepts that are employed to predict cracking of engineering materials at high temperatures under static and cyclic loading. The relationship between these concepts and those of high temperature life assessment methods is also discussed.A model for predicting Creep crack growth initiation and growth in terms of C* and the Creep uniaxial ductility is presented and it is shown that this model gives good agreement with the experimental results. The effects of cyclic loading on crack growth behavior are considered and fractography evidence is shown to back a simple cumulative damage concept when dealing with Creep/fatigue interaction. Finally a discussion is presented which highlights the important aspect of life assessment methodology for high temperature plant.

One of the yet unresolved engineering problems is forecasting the Creep lives of weldment in a pragmatic way with sufficient accuracy. There are number of obstacles to circumvent including: complex material behavior, lack of accurate knowledge about the Creep material behavior specially about the heat affected zones (HAZ), accurate and multi-axial Creep damage models, etc.In general, Creep life forecasting may be categorized into two groups, viz., those that are based on microscopic modeling and others that are based on macroscopic (phemenological) concepts. Many different micro-structural processes may cause Creep damage. The micro-structural processes highlight the fact that the Creep damages can be due to cavity nucleation and growth. Dislocation Creep is another mechanism with micro-structural features such as sub-grain formation and growth, new phase formation, such as the Z phase, coarsening leading to the dissolution of the MX phase. This leads to the removal of pinning precipitates, which allow local heterogeneous subgrain growth, weakening due to this growth and also to the dissolution of the MX. These features normally lead to the earlier formation of tertiary Creep and reduced life. Considering welded joints , the development of models for practical yet sufficiently accurate Creep life forecasting based on micro-structural modeling becomes even more complicated due to variation of material in the base, weld and heat-affected-zone (HAZ) and variation of the micro-structure within HAZ and their interactions. So far, and until this date, none of the micro-structural models can forecast the Creep life of industrial components with sufficient accuracy in an economic manner. There are several macroscopic (phemenological) models for Creep life forecasting, including: time-fraction rule, strain-fraction rule, the reference stress and skeletal stress method, continuum damage model, etc. Each of which has their own limitations. This paper gauges to a multi-axial yet pragmatic and simple model for Creep life forecasting weldment operating at high temperature and subjected to an elastic-plastic-Creep deformation.