Experimental and numerical analyses of an U-bend tube made of an output inter-heater tube after exploitation

One of the important tasks of evaluating the integrity of mechanical process elements and structures is to determine the local mechanical properties. In this paper, experimental and numerical analyses of the mechanical behavior of an output inter-heater tube, made of 12H1MF heat-resistant steel, was performed after 200,000 h of exploitation. During exploitation, the tube was exposed to various mechanisms of damage including gas corrosion. The tube was cut from a pipe system during reparations of a thermal power plant, and then cold-deformed by bending to obtain a U-bend tube, which was then used in the experiment. For this purpose, a specimen holder made of structural steel S235 was specifically designed to test such a sample. The U-bend tube was then exposed to the external compressive load during the experiment. Experimental research was based on the application of the 3D digital image correlation (DIC), while a finite element method (FEM) was applied in numerical simulation performed by using the Abaqus software package. The 3D DIC is an optical and contactless experimental method that allows measurements of displacement fields and deformations of geometrically complex structures. The Aramis system was used for the experimental analysis as well as for verification of the numerical model. During the experiment, the von Mises strain field was measured at the top of the U-bend tube, in the tightening zone, as it represents a critical place for crack initiation and propagation during the work of an inter-heater. Based on the obtained results and a comparative analysis of experimental and numerical values of the von Mises strain field at the U-bend tube, deviation of the model predictions of about 18 % was determined. The FEM predicted smaller values of the von Mises strain field compared to the DIC method. This is the result of an incomplete geometry applied in the model due to deformation that occurred in the bend zone of the U-bend tube, loss of material and the tube surface damage due to the influence of gas corrosion during 200,000 h of exploitation. Experimental analysis has confirmed that the U-bend tube, after 200,000 h of exploitation, can remain in service even if it is damaged due to the effect of gas corrosion.Jedan od važnih zadataka procene integriteta mašinskih elemenata i konstrukcija je određivanje lokalnih mehaničkih svojstava. U ovom radu izvršeno je eksperimentalno i numeričko ispitivanje mehaničkih svojstava cevi, izlaznog međupregrejača pare, izrađenog od toplotnopostojanog čelika 12H1MF, nakon 200.000 h rada. Cev izlaznog međupregrejača pare je isečena iz cevnog sistema tokom remonta termoelektrane, a potom hladno deformisana savijanjem, u cilju dobijanja cevnog luka, za potrebe eksperimenta. Namenski je konstruisan držač za ispitivanje takvog uzorka. Eksperimentalno istraživanje je bazirano na primeni 3D metode korelacije digitalnih slika, dok je metoda konačnih elemenata primenjena u numeričkoj simulaciji. Metoda 3D korelacije digitalnih slika predstavlja optičku i beskontaktnu eksperimentalnu metodu koja omogućava merenje polja pomeranja i deformacija geometrijski kompleksnih struktura. Za eksperimentalnu analizu kao i verifikaciju numeričkog modela korišćen je sistem Aramis, baziran na metodi korelacije digitalnih slika. Na osnovu dobijenih rezultata i uporedne analize eksperimentalnih i numeričkih vrednosti, koje se odnose na fon Mizesove deformacije na temenu cevnog luka, javlja se odstupanje od oko 18%. Ovo odstupanje nastaje kao rezultat korišćenih metoda koje imaju bitne razlike u njihovim početnim uslovima ispitivanja