Micromechanical fracture analysis of high strength steel weldments

Abstract

The industrial application of high strength steel in structural welded components has increased the demand for understanding the ductile failure behavior of this type of welded materials. Therefore, integrity assessment of those welded structures is required in order to ensure a certain level of safety and reliability, having in mind the effects of constraints and heterogeneity on fracture behavior of crucial regions such as: heat affected zone (HAZ) and weld metal (WM) which usually have low toughness and higher transition temperature. It is also essential that the assessment is done in a realistic and not too conservative way in order not to increase the mass of the structure or impair the economic efficiency too much. In this thesis, micromechanical approach has been used to study the effect of mechanical heterogeneity and constraints on ductile fracture of high strength steel weldments. This approach has been used as a solution for the transferability problem of conventional fracture mechanics parameters. It has also been used on basis of that fracture mechanics parameters recommended by standard, such as: stress intensity factor, crack opening displacement and contour J-integral, cannot reliably describe the reaction of a pre-cracked material to the effects of external loading under all conditions such as: large scale yielding, various effects of heterogeneity, shape and geometry of real welded structures. The fracture mechanics parameters, determined from laboratory scale experiments are not also directly transferable to components and hence additional considerations (like constraint effects etc.) need to be taken care of. In addition, the aim of the thesis was to estimate precise mechanical properties using a combined experimental and numerical procedure for various welded joint regions, especially for narrow HAZ regions, when they are subjected to transversally applied load. The study was carried out using finite element method and experiments. Experimental analysis was carried out on: welded smooth tensile specimen with ARAMIS measuring system for estimation mechanical properties, welded single-edge notched bend and flat tensile specimens with pre-cracks in weld metal (WM) and heat-affected zone (HAZ) for studying the ductile fracture behavior. J-R curves and crack growth initiation values of fracture mechanics parameter were experimentally and numerically obtained for specimens with a pre-crack in HAZ and WM. Numerical analysis of elastic-plastic finite element models (2D and 3D) was performed in software package Abaqus, with micromechanical complete Gurson model (CGM) applied through user subroutine, UMAT (author: Z.L. Zhang). The crack tip constraint and variation of stress triaxiality in ligament were numerically analyzed on single-edge notched bend and tensile specimens to analyze the transferability of micromechanical damage parameters from one specimen to another. In addition, the geometry effects were also studied by the micromechanical approach and the results were compared with those of the experiments. Most of numerical results obtained with CGM model are in good agreement with the experimental results. Analysis of results shows that micromechanical approach is reliable technique to describe ductile fracture behavior in high strength steel weldments, including the effect of constraints and heterogeneity. It was also observed that the initiation toughness and the crack growth resistance are greatly affected by the mechanical heterogeneity of the weldment and the crack tip constraint. Additionally, the numerical values of the initiation toughness and the crack growth resistance are also affected greatly by the size of finite element. Moreover, micromechanical damage parameters could be transferred from one component to another without depending on constraint effect etc. The results also show that estimation of mechanical properties for various welded regions, especially for narrow HAZ regions, could be considered as alternative technique to the standard one, which may not be applied to estimate their mechanical properties when welded joint is transversally subjected to applied load.

Industrijska primena celika povišene čvrstoće u elementima zavarenih konstrukcija čini neophodnim poznavanje ponašanja spojeva ovih materijala pri žilavom lomu. Stoga, procena integriteta zavarenih struktura je potrebna da bi se obezbedio potrebni nivo sigurnsti i pouzdanosti, imajući u vidu uticaj ograničenog deformisanja i heterogenosti na ponašanje pri lomu kritičnih zona spoja: zone uticaja toplote (ZUT) i metala šava (MŠ), koji često imaju nižu žilavost i višu prelaznu temperaturu. Takođe, veoma je važno da procena integriteta bude urađena realno i da ne bude previše konzervativna, kako bi se sprečilo povećanje mase strukture i obezbedilo ekonomično korišćenje materijala. U ovoj disertaciji, mikromehanički pristup je korišćen za analizu uticaja mehaničke heterogenosti i ograničenog deformisanja na žilav lom zavarenih spojeva čelika povišene čvrstoće. Ovaj pristup je korišćen kao rešenje za problem prenosivosti parametara klasične mehanike loma. Takođe, motiv za njegovu primenu je i to što standardni parametri mehanike loma: faktor intenziteta napona, otvaranje prsline i konturni J integral, ne mogu na odgovarajući način opisati odgovor materijala sa prslinom na dejstvo spoljnog opterećenja u svim uslovima, kao što su izraženo plastično tečenje (large scale yielding), različiti uticaji heterogenosti, obika i geometrije zavarenih konstrukcija u eksploataciji. Parametri mehanike loma, određeni laboratorijskim ispitivanjem epruveta, nisu direktno prenosivi na komponente i stoga se moraju uzeti u obzir dodatni faktori (kao što je uticaj ograničenog deformisanja). Takođe, cilj ove disertacije je određivanje mehaničkih osobina zona zavarenog spoja korišćenjem kombinovanog eksperimentalno numeričkog postupka, što je naročito važno kod zona male širine u okviru ZUT koje su izložene opterećenju u transverzalnom pravcu. Rad na disertaciji je podrazumevao primenu metode konačnih elemenata i eksperimentalna ispitivanja. Eksperimentalna ispitivanja su urađena na zavarenim glatkim epruvetama (uz korišćenje ARAMIS stereometrijskog mernog sistema) za određivanje mehaničkih osobina, kao i epruvetama za savijanje u tri tačke i epruvetama za zatezanje sa početnom prslinom u metalu šava i zoni uticaja toplote za analizu ponašanja pri lomu. J-R krive i vrednosti parametra mehanike loma koje odgovaraju početku rasta prsline određene su eksperimentalno i numericki na epruvetama sa početnom prslinom u ZUT i MŠ. Numerička analiza elasto-plastičnih modela konačnih elemenata (2D i 3D) je urađena u programskom paketu Abaqus, a mikromehanički kompletni Gursonov model (CGM) je primenjen preko korisničkog potprograma UMAT (autor Z.L. Zhang). Ograničeno deformisanje oko vrha prsline i promena troosnosti u ligamentu su numerički analizirani na epruvetama za savijanje u tri tačke i onim za zatezanje, da bi se analizirala prenosivost mikromehaničkih parametara oštećenja sa jedne epruvete na drugu. Takođe, mikromehaničkim pristupom je određen uticaj geometrije i rezultati su upoređeni sa eksperimentalnim podacima. Najveći deo numeričkih rezultata dobijenih korišćenjem CGM modela pokazuje dobro slaganje sa eksperimentalnim rezultatima. Analiza rezultata je pokazala da je mikromehanički pristup pouzdana tehnika za predviđanje žilavog loma u zavarenim spojevima čelika povišene čvrstoće, uključujući određivanje uticaja ograničenog deformisanja i heterogenosti. Pokazano je da mehanička heterogenost spoja i ograničeno deformisanje oko vrha prsline imaju veliki uticaj na otpornost prema nastanku i rastu prsline. Takođe, numerički određena otpornost prema nastanku i rastu prsline zavise od veličine konačnih elemenata. Štaviše, mikromehanički parametri oštećenja preneti su sa jedne na drugu komponentu, bez obzira na uticaj ograničenog deformisanja. Pokazano je da se određivanje mehaničkih osobina zona zavarenog spoja, naročito za delove ZUT male debljine, može koristiti kao alternativna tehnika standardnoj, koja se ne može primeniti za određivanje osobina zona spoja u slučaju dejstva opterećenja u transverzalnom pravcu (u odnosu na zavareni spoj).