Hoop stress prevention

The design of the threads requires control, to prevent excessive shear, resulting in stripping the threads when torqued, and also to limit hoop stresses that can result in tensile failure. Although the mechanics of stress analysis for screw threads are readily available, the equations for them can be rather complicated. 

When establishing test pressures for a test section, the maximum test pressure shall be determined by the operator to prevent damage to the pipeline and its components. Consideration must be given to the effect of test section elevation differences on the test pressure. Whenever test pressure will cause a hoop stress in excess of 100% of the specified minimum yield strength (SMYS), refer to Nonmandatory Appendix C for guidance on yield monitoring. The minimum test pressure shall be as required by (a) through (c) below. 

Molded plastic parts acquire internal stresses during the molding process, and these increase when fasteners are installed. As a resulf bulk material is generally added to area of the joint in the form of a boss. The added material of a boss is intended to prevent cracking of the plastic when the fastener is installed and tightened. The boss outside diameter should also provide enough strength to withstand possible hoop stresses developed by screw insertion. 

Weaker anaerobics should be used with soft alloy nuts and other threaded fasteners to prevent thread stripping during disassembly. Soft alloy and plastics fasteners exhibit a lower removal torque than their steel equivalents, because less hoop stress is generated as the adhesive is crushed during removal. Despite this apparently lower performance (remember removal is only a test method ranking) the parts will still be effectively locked and resistant to vibration. 

All bridgework will contain one or more pontics. If the porcelain is completely covering the pon-tic then the heat within the alloy cannot escape. Within hours of being fabricated the porcelain will continue to develop stress around its circumference until the interior of the alloy reaches room temperature. Occasionally the stress may be sufficient to produce a crack in the porcelain. This time-delay type of fracture can be alleviated by leaving a collar of metal on one side of the pontic. This band will prevent the formation of hoop stresses in the porcelain that cause the cracks. Similar to the glass-to-metal seals, adherence of the porcelain to the alloy is via a chemical bond between the porcelain and alloy. The critical sequence is as follows ... 

The stresses in a pressurised pipe, free to expand in length, were analysed in Section C.3 of Appendix C. However, buried gas pipe is connected at both ends to immovable objects such as houses, and the surrounding soil prevents the pipe moving laterally. Consequently, the longitudinal strain in the pipe is zero. In this elastic plane strain situation, the hoop strain is given by Eq. (C.21) as... 

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