Proposal of New Pipe-Ring Specimen for Fracture Mechanics

Thin - walled pipes are not suitable for measuring fracture toughness parameters which are of vital importance in pipes because the crack failure is the most common in pipes. They are not suitable because it is impossible to make standard specimens for measuring the fracture toughness like SENB or CT specimens from the thin wall of the pipe. Lot of researches noticed this problem, but only few are found the good and cheap alternative solution for measuring the fracture toughness. Gubeljak and Matvienko proposed so - called Pipe - Ring specimen (PRS) or Pipe - Ring Notched Bend specimen (PRNB) as alternative solution to the SENB specimen. Until now, only the idealized geometry of PRS specimen is analysed, so specimen which is not cut out from the real pipe but made from plate. On this way, the residual stresses are neglected. The aim of this work is to estimate the hoop residual stresses in the real pipes used in boiler industry, produced by hot - rolling technique, so seamless pipes. These kinds of pipes are delivered only normalized, but not stress relieved too. Therefore, there are surly residual stresses present in pipes which are result of manufacturing technique, but also of uneven cooling after the production process. Within this work, hoop stress as the most relevant for pipes is estimated by two methods: incremental hole drilling method and splitting method. Although residual stresses are cause of collapse of many constructions, still in most cases of analysis and design of individual parts, structures and plants residual stresses are not taken into consideration. Residual stresses surly have some effect on the fatigue behaviour of materials as well as on the fracture toughness of material, therefore it is recommended to take them into account while calculating the lifetime of component or structure.