Initial plastic deformation and residual stress influencing the welding joint behavior in the presence of cracks

Abstract

The full-scale model of penstock was produced using weldable high strength low alloyed steel (HSLA) Sumiten 80P (SM 80P). Steel SM 80P belongs to HT80 steel with tensile strength above 800 MPa and yield strength above 700MPa. Tensile properties were achieved by quenching and tempered technology which requires strong obeying of welding procedures. Finite element analysis has been carried out by using ABAQUS software to simulate the hydrostatic test of the full-scale model of penstock. A finite element model of penstock is a little bit different than the experimental test model in welding the shape and geometry (the third segment of the cylindrical mental of the experimental model has been neglected for the finite element model). In the first portion of the analysis the von Misses stress distribution will be investigated in two steps, the first load-unload and the second load-unload, and to focus on where the yielding initiates and spreads. For the second portion of the numerical study, the behavior of the model with initial residual stresses in weld joints have been analyzed for von Misses stresses distribution. The von Misses stress-strain relationship has been calculated in three ways: using linear elastic formulas, when the experimental model was treated as an ideal cylindrical vessel (without a 5º angle). The second relationship is obtained from strain guage measurements, and the third behavior is the stress-strain curve obtained from a numerical calculation (ABAQUS software). For the residual strength prediction and structural integrity assessment of penstock, a study of fracture mechanics behavior of an under-matched weld joints with small and large surface cracks for high strength low alloy steel of penstock structures has been performed by the J-R curve approach. Sumiten 80P (SM 80P) grade steel plate was butt welded by submerged arc welding. Three tensile panels with surface cracks positioned in the base metal (BM), weld metal (WM) and the heat affected zone (HAZ) were tested at room temperature. And continuous measurement of force versus crack mouth opening displacement and crack extension was monitored during the test by the compliance method. In addition, J-R curves were built for three parts of the weld joint. Crack driving force is obtained for various values of applied stresses ratio and it plotted as a function of crack depth ratio.

Model cevovoda pune razmere je proizveden od zavarljivog niskolegiranog čelika visoke čvrstoće Sumiten 80P (SM 80P). Čelik SM 80P pripada grupi HT80 čelika koji imaju zateznu čvrstoću iznad 800 MPa i napon tečenja iznad 700 MPa. Zatezne karakteristike su posledica kaljenja i otpuštanja, stoga se prilikom procesa zavarivanja mora ispoštovati odgovarajuća procedura. Uz pomoć softeva ABAQUS, metodom konačnih elemenata na modelu je izvršena simulacija hidrostatičkog testa. Model koji je korišćen za proračun metodom konačnih elemenata razlikuje se od eksperimentalnog modela u izgledu zavarenog šava i gemetrije šava (treći segment cilindričnog ekperimentalnog modela je prilikom proračuna metodom konalnih elemenata zanemaren). U prvom delu analize promena von Mizesovih n napona će se ispitati u dva koraka, prvi korak opterećenja-rasterećenja i drugi korak opterećenja-rasterećenja i fokus će biti pozicije početka tečenja materijala i njegova širenje. U drugom delu numeričkog proračuna analizirano je ponašanje modela u kome su generisani inicijalni zaostali naponi. Relacija von Mizesovih napona i deformacija je računata na tri načina: linearno-elastična analiza, kada se model tretira kao idealno cilindrična posuda (bez ugla od 5º). Druga relacija je dobijena uz pomoć eksperimentalnog merenja deformacija i treće, ponašanje napon-deformacija krive dobijene numeričkim proračunom (ABAQUS softver). Za pretpostavke procene preostalog veka i proceneintegriteta konstrukcije, korišćena je studija mehanike loma ispitivanjem J-R krive metala šava niže čvrstoće sa malom i velikom prslinom za niskolegirane čelike visoke čvrstoće. EPP postupak je korišćen za čeono zavarivanjeSuminet 80P (SM 80P) ploče. Kontinualno merenje sile i usta otvaranja kao i širenje prsline je posmatrano tokom testa odgovarajućim metodama. Pored toga, J-R krive su dobijene za tri različite pozicije zavarenog spoja. Sila rasta prsline je dobijena za različite vrednosti upotrebljenog napona i grafički je predstavljen u funkciji dubine prsline.