PVC (Poly-vinyl chloride) pipes of a small diameter are frequently used for distribution of water in farmland irrigation pipelines. In Japan, more than half of the existing 300 mm caliber pipelines are PVC pipes. The pipe have the highest number of rupture accidents of all pipe species. Therefore, reducing the number of rupture accidents in PVC pipes is very important to keep facilities secure and reduce maintenance costs.
?This report discusses the cause of the breakage to the PVC pipe in the high-pressure pipeline for farmland irrigation. We have made apparent that the breakage of PVC pipe is caused by fatigue fracture through the method of the fracture surface analysis. Therefore, we tried to clear the cause of the fatigue fracture through the following procedure from (1) to (3). (1) Measurement of strain of the pipe affected by pipe pressure hydrocephalus and road traffic load, (2) Examination of pipe pressure fluctuation due to water hammer analysis, (3) Reconsideration of fatigue failure curve of PVC pipe. At first, we tested that the strain change which is the cause of the fatigue fracture was affected mainly by whether the changes of the pressure head in the pipe or the changes of the road traffic load at the field for research.?We revealed that the change of the pressure head influence on the variation of strain of the pipe is larger than the change of road traffic load. In (2), because the dendritic pipes in farmland has more many reflected waves of water hammer than the single pipe, we abstracted the feature of the waves and proposed the optimal time for closing valve. Finally, in (3), we examined how the amplitude and frequency of pulsation of the pressure head is affecting the fatigue failure.