Estimation of rotary equipment integrity using fracture mechanics parameters

Abstract

Integritet konstrukcija kao naučna i inženjerska disciplina kao deo mehanike loma obuhvata analizu stanja i dijagnostiku ponašanja i otkaza, procenu veka i revitalizaciju konstrukcija, kao što je poznato od ranije. Primenom metoda definisanih ovom naučnom disciplinom mogu se odrediti „slaba“ mesta u konstrukciji, pre nastanka prsline kao i u prisustvu prsline kod konstrukcija koje su izložene razlitičitim tipovima opterećenja. Procena integriteta rotacione opreme, kao što je vratilo turbine, predstavlja poseban izazov zbog prirode njegovog opterećenja. Kada se na u ceo problem doda i uticaj prsline problem postaje još složeniji i zahteva uključivanje više naučnih disciplina i alata, poput softverskih paketa kojima je moguće simulirati radne uslove opreme, tačnije, dinimačko opterećenje koje može izazvati zamor materijala. Tema ove doktorske disertacije je istraživanje ponašanja rotacione opreme, konkretno vratila turbine, Hidroelektrane „Đerdap II“ u prisustvu prslina sa ciljem određivanja integriteta i preostalog radnog veka vratila turbine. Opasnost u slučaju prisustvu prsline kod ovog tipa opreme je mogućnost pojave krtog loma koji je, po prirodi, nepredvidiv i često sa fatalnim posledicama. Posebna pažnja u ovoj studiji posvećena je mestima koja izazivaju koncentraciju napona. U rešavanju problema procene integriteta rotacione opreme vratila hidroelektrane korišćeni su softverski paketi Abaqus i MORFEO, odnosno principi proširene metode konačnih elemenata (eng. XFEM). Kao ulazni podaci u primeni XFEM metode primenom ovih paketa korišćeni si eksperimentalni rezultati primenom koncepta elasto-plastične mehanike loma kao i primena poznatog Parisovog zakona sa ciljem određivanja brzine rasta zamorne prsline. Dobijeni podaci eksperimentalnim ispitivanjima, konkretno vrednosti parametara C i m poznatog Parisovog zakona, poslužili su kao osnova za urađene numeričke simulacije rasta zamorne prsline u samom vratilu trubine primenom zakona linealno-elastične mehanike loma. Pored toga, u ovoj studiji urađena je naponska analiza vratila turbine kao i neposredno poređenje analitičkih i numeričkih rezultata. Osnovni cilj celokupne studije može se svesti na objašnjenje kompleksnog stanja i generalno ponašanje rotacione opreme, odnosno vratila turbine u prisustvu prslina nastalih zamornim opterećenjem i u prisustvu koncetratora napona. Može se istaći da je i akcenat ove studije utvrđivanje zavisnosti brzine rasta prsline od veličine prsline za određenu geometriju vratila na osnovu čije je moguće proceniti integritet i vek vratila, pored svih poznatih osobina materijala. Indirektno, razvijeni i predstavljeni numerički modeli u ovoj disertaciji, kao i eksperimentalne procedure koji imaju veliku primenljivost u oblasti procene integriteta generalno kod opreme ovog tipa takođe su imali za cilj davanje naučnom ivi inženjerskom doprinosu analize ove studije sa ciljem utvrđivanja integriteta i preostalog radnog veka opreme izložene zamarajućem opterećenju u prisustvu prsline.As it is known, structural integrity as a scientific and engineering discipline as part of fracture mechanics includes structure analysis, its behavior and failure diagnostics, life assessment as well as structural revitalization. By applying the methods defined by this scientific discipline, it is possible to determine "weak" places in the structure, before crack occure and in the presence of a crack in structures that are exposed to a different types of loads. Assessing the integrity of rotary equipment, such as turbine shafts, is a particular challenge due to the nature of its load. Presence the crack makes the whole problem becoming even more complex and requires the inclusion of more scientific disciplines and tools, such as software packages that can simulate the working conditions of equipment, more precisely, dynamic load that can cause material fatigue of the structure. The topic of this doctoral dissertation is the research of the behavior of rotary equipment, specifically turbine shafts, Hydroelectric Power Plant "Đerdap II" in the presence of cracks with the aim of determining the integrity and remaining service life of turbine shafts. The danger in the presence of a crack with this type of equipment is the possibility of a brittle fracture which is, by its nature, unpredictable, and often with fatal consequences. Special attention in this study is dedicated to the places on the structure that cause stress concentration. The Abaqus and MORFEO software packages, i.e. the principles of the extended finite element method (eng. XFEM), were used to solve the problem of assessing the integrity of the rotary equipment of the hydroelectric shaft. As input data in application of XFEM method using aforementioned packages, experimental results obtained by using concept of elastic-plastic fracture mechanics i.e. application of the well-known Paris law with the aim of determining the fatigue crack growth rate are used. The data obtained by experimental tests, specifically the values of parameters C and m of the well-known Paris law, were the basis for numerical simulations of fatigue crack growth in the turbine shaft using the law of linear-elastic fracture mechanics. In addition, in this study, a stress analysis of the turbine shaft was performed, as well as a direct comparison of analytical and numerical results. The main goal of the entire study can be reduced to the explanation of the complex state and general behavior of the rotary equipment, i.e. the turbine shaft in the presence of cracks caused by fatigue load and in the presence of a stress concentrator. It can be pointed out that the main objective of this study is to determine the dependence of crack growth rate on crack size for a certain shaft geometry on the basis of which it is possible to estimate the integrity and life of the shaft, in addition to all known material properties. Indirectly, the developed and presented numerical models in this dissertation, as well as experimental procedures that have great applicability in integrity assessment of this type equipment,viii also provide scientific and engineering contributions of this study analysis, like determining integrity and remaining life of the equipment exposed to a fatigue load in the presence of a crack