Brittle fracture prevention

Supplemental requirements used by Esso Research Engineering Company for brittle fracture prevention, based on the foregoing, are given in Table 4-3. Since these requirements are more severe than the long-standing code requirements, a few words of substantiation are in order. 

API RP 920, Prevention of Brittle Fracture of Pressure Vessels, 1st ed., March 1990 (ANSI/API Publ 920-1992). 

P. G. Snyder, Brittle Fracture of a High Pressure Heat Exchanger, Paper presented at AIChE Loss Prevention Symposium, Minneapolis, Aug, 1987. 

Since the aggregate risks from Process Unit 2 are largely the result of single event—brittle fracture—the qualitative hazard assessment identified potential safeguards that could be put in place to prevent a brittle fracture occurrence. It was decided that the best option would be to install an emergency shutdown system in Process Unit 2 to prevent pressuring the nitrogen vapor vessel if a cold temperature situation was present. 

Cast iron is undesirable (and prohibited by certain flammable liquid codes) because a brittle fracture can lead to a massive leak. Some months after this mistake, the unit received a loss prevention audit and as a result the pump bowls were replaced with ductile iron units. This same audit prompted the installation of remotely operated emergency shut-off valves around these pumps. 

Adsorption-induced brittle fracture. This model is based on the hypothesis that adsorption of environmental species lowers the interatomic bond strength and the stress required for cleavage. This model of chemical adsorption can explain the fact that a certain alloy is susceptible to specific ions. An important factor in support of this mechanism is the existence of a critical potential below which the SCC does not occur in some systems, and this model underlines the relation between the potential value and the capacity of adsorption of the aggressive ion. It also explains the preventive action of SCC for some systems by cathodic protection. This model may interpret the rupture of plastic materials or glass. It is referred to as the stress-sorption model, and similar mechanisms have been proposed for HE and LME. In this model, the crack should propagate in a continuous way at a rate determined by the arrival of the embrittling species at the crack tip. The model does not explain how the crack maintains a sharp tip in a normally ductile material.156... 

A technique generally applied to characterize and prevent the capping and lamination of a material intended to be compacted is using the brittle fracture index (BFI). The BFI was designed by Hiestand et al. [31] and measures the ability of a material to relieve stress by plastic deformation around a defect. It is obtained by applying Equation (8) and compares the tensile strength of a tablet with a hole in its center (To), which acts as a built-in stress concentrator defect, with the tensile strength of a similar tablet without a hole (T), both at the same relative density ... 

This increase in stability, however, is accomplished at the sacrifice of mix flexibility. The data in Table IV indicate that the flexural strain at break of mixes No. 3 and 4 is reduced significantly below that of mixes No. 1 and 2 when the sulfur concentration in the binder is raised to 40 and 50 wt %, without increasing the total binder content. Flexibility is a measure of mix susceptibility to brittle fracture and is an important mix design consideration in rigid materials to prevent premature pavement cracking. 

The prevention of brittle fracture has challenged the ingenuity of polymer chemists, polymer physicists, and plastics engineers. They have made great progress in the past 20 years toward practical solutions of the problem and in the past 10 years toward theoretical understanding as well. 

ASME Section VIII, Division 1 requirements to prevent brittle fracture are 15 ft. -lb. Charpy Keyhole applied only below -20°F. Until recently, these requirements were thought to protect against brittle fracture. In the past few years, however, a considerable number of catastrophic brittle fractures in thick-wall pressure vessels have occurred throughout all industry. In each instance, the code impact values seemed to have been met or exceeded. 

Ductility is desirable in the high temperature and high pressure applications in reactor plants because of the added stresses on the metals. High ductility in these applications helps prevent brittle fracture, which is discussed in Module 4. 

MPT curves are based on reactor vessel and head stress limitations, and the need to prevent reactor vessel and head brittle fracture. 

At high temperatures (> 600°C) the composites degrade in strength and toughness due to oxidation of C fibers and/or these interface layers and prevention of fiber pullout (brittle fracture mode). Studies are in progress to increase oxidation resistance by use of a CVD-SiC overlayer which seals the surface of the porous composites. 

VI. 3. This appendix provides guidance for the evaluation of designs to prevent one such potential mode of failure, namely brittle fracture of structural components in radioactive materials transport packages. Three methods are discussed ... 

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