Hoop-stress levels

So = Maximum operating hoop stress level as determined by the Barlow formula using the maximum operating pressure. 

Repair or Removal of Defective Welds in Piping Intended to Operate at Hoop Stress Levels of 20% or More of the Specified Minimum Yield... 

GR-5.21 REPAIR, TESTING, AND EXAMINATION OF MAINS OPERATING AT HOOP STRESS LEVELS AT OR ABOVE 30% OF THE SPECIFIED MINIMUM YIELD STRENGTH... 

C-2.8.3 For control of maximum test pressure when hoop stress levels exceed 100% SMYS within a test section, one of the following measures may be used ... 

So = Maximum operating hoop stress level as determined by the Barlow formula using the maximum operating pressure, Ksi R = O.D. of the pipe, in. t = Pipe W.T., in. 

The testing process involves subjecting pipe samples to various hoop stress levels, and then recording the time to rupture. For some samples at some pressures, rupture will occur in as little as 10 hours. As hoop stress is reduced, the time-to- failure increases. At some hoop stress level, at least one of the tested specimens will not rupture until at least 10,000 hours (slightly more than 1 year). After the rupture data points (hoop stresses and times-to-failure) for this material have been recorded, the data points are plotted on log-log coordinates as the relationship between hoop stress and time-to-failure. (see Figure 13.3.) A mathematically developed best-fit straight line is correlated with the data points to represent the material s resistance to rapturing at various hoop stress levels. 

Once the best-fit straight hue is calculated to 10,000 hours, it is extrapolated to 100,000 hours (about 11 years). The hoop stress level that coincides with the point at which the line intersects the 100,000-hour time line represents the calculated long-term hydrostatic strength of that particular material. 

In the present study we analyze the results of HDPE pipe sustain pressure test at elevated temperature and various hoop stress level (80 C 4.7 5.5 MPa). Some material properties are shown in Table 1. The failure time for each stress level is recorded, when the leak is observed. After the specimen failure, the fracture surface was inspected by optical and SEM microscopy to observe and characterize the fracture mechanism. All brittle fractures observed resulted from through cracks. A number of various size small cracks have also been developed at the time, when the largest crack(s) caused the le. In many cases, multiple through cracks were detected. It offers for analysis a set of cracks grown under the same controlled tenqierature and stress level. All the observed cracks have been originatod from inclusions. The size and location of an inclusion play an important role in failure time. A combination of random size and location of the inclusions results in a large scatter in failure time at the same applied stress, commonly reported for pipe failures [9]. A summary of plied stresses, failure time, inclusion size and location observed on the fracture surface is presented Table 2. The size of the inclusion is reported in wall thickness direction. The normalized apparent depth of inclusion location indicated in the Table 2, was measured as the ratio between the distance from the inclusion center to outer surface and... 

A study was made with ellipsoidal heads to determine the effect of the ratio a/ on the stress level at the head-to-shell junction for a constant ratio 32 of head thickness t to shell radius r. The study indicated that the point of maximum stress in the head changes with a change of a/b. For heads shallower than a/b of 2.5, the maximum stress is in the hoop direction at the outside surface of the knuckle region and is in compression as shown in Fig. 9.4. For ratios of a/b between 2.5 and 1.2, the maximum stress occurs at the junction and is a hoop tensile stress. The stress magnitude for various ratios of a/b is shown in Fig. 9.4. A simplified equation used by the ASME Code, VIII-1, approximates the theoretical stress ratios of Fig. 9.4 for values of a/b between 2.6, which is the maximum allowed by the code, and 1.0 for a spherical head ... 

The pressure rating of thermoplastic pipe is mathematically calculated from the SDR and the allowable hoop-stress. The allowable hoop-stress is commonly known as the long-term hydrostatic design stress. This is the stress level that can exist in the pipe wall continuously with a high degree of confidence that the pipe wiU operate under pressure for at least 50 years with safety. The American Society for Testing and Materials (ASTM) and the Plastics Pipe Institute (PPl) has adopted... 

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