Materials Selection, Testing, and Design Considerations

Materials Selection, Testing, and Design Considerations 445 11.3.3 Prioritization... 

A basic aim of fracture mechanics is to identify fracture criteria such as G,c and KI(. and others discussed later which are independent of the geometry of the cracked body and, ideally, even of the test conditions, e.g. test temperature. Thus fracture mechanics parameters should greatly assist in developing a more fundamental understanding of the fracture process and should be of considerable use in the practical areas of material selection and development, engineering design and life prediction. 

Vapor—aerosol interactions which are assumed to take place in the primary system were studied in detail in the British Falcon test facility. This facility, which is schematically shown in Fig. 7.19. (according to Beard et al., 1991), has been specifically designed to investigate the transport and deposition behavior of fission products under severe accident conditions. To produce representative aerosols, fuel pellets containing simulant fission products as well as trace-irradiated fuel pellets, both cladded in Zircaloy, were heated up to 2000 K in a steam—helium atmosphere in the presence of bulk-core materials. Fission product transport could be studied along a pathway which was designed to represent the upper plenum, hot-leg structures and containment. Considerable efforts were made to ensure that, as far as possible, the thermal-hydraulic conditions represented those of a selected accident. 

There are a considerable number of sources of material testing data that have been accumulated over the years and are available to process plant designers to assess the most suitable materials selection for a particular plant and operating conditions. However, many of the factors involved in the selection of an optimum choice are subject to variability, and this makes the selection process more complex. 

Refer to Sections II and III for information on designing tests and for detailed descriptions of common laboratory tests. Considerable skill is required to develop a corrosion test program that adequately defines materials requirements. First, we must identify those conditions that will limit the materials selections. This includes the conditions that will be encountered under nonstandard conditions such as startup, shutdown, and process upsets. In many cases we also need to consider the conditions encountered when the unit is in standby status that is, when the unit is not operating but is being maintained so that it can be put back into operation when it is needed. The best materials selections are made when the full range of these conditions is identified and considered before the test program is started. 

Tables and isocorrosion charts are available to document experience with general, or uniform, corrosion in relatively pure chemicals at various temperatures and concentrations. An example is shown in Fig. 2. The selection of a material, or the proper test to evaluate a material, often includes more than consideration of uniform attack in pure chemicals. The presence of impurities can accelerate or retard general corrosion, e.g., chlorides (and some other halides) can cause pitting or SCC. Crevices due to the design or adherent deposits can be sites of attack, particularly in the presence of chlorides. Higher velocity of the corrodent can (a) reduce attack if deposits and the resulting crevice attack...

This chapter will outline the procedure for the design of plastics parts and give the methods of sizing, material selection, cost consideration, design life, performance evaluation, and testing. The monitoring of quality will be indicated to insure performance. All the basic information and approaches have been covered, thus this chapter is a synthesis of previous discussions. 

However, there is an important feature in designing cement-based materials it is carried on with numerous uncertainties in relation to the properties of the components, their effective volume fractions, future conditions of execution and cure, etc. The result is subject to random variations of all properties. That is why corrections and adjustments of composition, selection of components and technologies are necessary before the final result is obtained. Then the realised design of a material is again verified in local conditions. The personal knowledge and experience of the designer are considerably valued in that process. The verifications are executed by calculation and testing and more often by a combination of both. 

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