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Rapid crack propagation in polyethylene gas pipes

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  1. Tez No: 400458
  2. Yazar: PAŞA YAYLA
  3. Danışmanlar: DR. P.S. LEEVERS
  4. Tez Türü: Doktora
  5. Konular: Makine Mühendisliği, Mechanical Engineering
  6. Anahtar Kelimeler: plastic pipes, gas pipelines, rapid crack propagation, critical pressure
  7. Yıl: 1991
  8. Dil: İngilizce
  9. Üniversite: University of London
  10. Enstitü: Yurtdışı Enstitü
  11. Ana Bilim Dalı: Makine Mühendisliği Ana Bilim Dalı
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 289

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Özet (Çeviri)

Steady-state axial Rapid Crack Propagation (RCP) in pressurised plastic pipelines is a major concern of water and fuel gas distribution industries. Fast cracks are generally difficult to initiate, but once initiated, they may run through the entire length of pipeline at velocities of 70-370 ms- 1 if the operating pressure exceeds a critical value. This critical pressure (pc) is shown to be a function of material properties, pipe dimensions, and the properties of the pressurising fluid. Although, for a given pipe system, the most widely trusted assessment of Pc is by the use of full-scale (FS) tests, these are expensive and laborious. A new Small-Scale-Steady-State (S4) test has been developed in order to define pc for any piping system at a required temperature. The main features of the technique are: a fast crack is initiated by radial impact onto a well supported initiation zone. This crack penetrates a propagation zone along which internal gas flow baffles and an external containment cage against flaring suppress decompression and control the crack tip environment. With this technique, pc values for various low and high density PE pipes are well defined for a wide range of temperatures. Crack velocity remains constant during propagation, or decelerates abruptly in the arrest case. However, the pc defined by this technique is extremely low, but on the basis of a gas dynamics model of RCP, a scaling factor is derived to correlate the S4 and FS test results. The effects of some important parameters (pipe wall thickness, pipe diameter, test temperature, and pipe material grade) upon RCP performance are studied. It is shown that the effect of each parameter defined from S4 test is in good agreement with the results obtained from FS tests Charpy impact energy results, on two different PE pipe grades for a wide range of temperatures, are presented and the relevance of these and the dynamic fracture resistance results of the High Speed Double Torsion tests are discussed. It is concluded that pc is more sensitive to temperature than the Charpy results. The influence of the tear lips taking place at each free boundary of the pipe walls (particularly at the inner surfaces) is shown to dominate RCP resistance. The elimination of this highly ductile region by internal notching along the prospective crack significantly reduces pc. Quasi-static and specially developed dynamic finite element codes are employed to analyse RCP in PE pipes, focussing on the stress state around the crack tip. The axial stresses around the crack tip are generally higher than the circumferential stresses and stresses at the inner surface are higher than those at the outside.

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