Micromechanical fracture analysis of high strength steel weldments

Industrijska primena celika povišene cvrstoce u elementima zavarenih konstrukcija cini 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, imajuci u vidu uticaj ogranicenog deformisanja i heterogenosti na ponašanje pri lomu kriticnih zona spoja: zone uticaja toplote (ZUT) i metala šava (MŠ), koji cesto imaju nižu žilavost i višu prelaznu temperaturu. Takode, veoma je važno da procena integriteta bude uradena realno i da ne bude previše konzervativna, kako bi se sprecilo povecanje mase strukture i obezbedilo ekonomicno korišcenje materijala. U ovoj disertaciji, mikromehanicki pristup je korišcen za analizu uticaja mehanicke heterogenosti i ogranicenog deformisanja na žilav lom zavarenih spojeva celika povišene cvrstoce. Ovaj pristup je korišcen kao rešenje za problem prenosivosti parametara klasicne mehanike loma. Takode, motiv za njegovu primenu je i to što standardni parametri mehanike loma: faktor intenziteta napona, otvaranje prsline i konturni Jintegral, ne mogu na odgovarajuci nacin opisati odgovor materijala sa prslinom na dejstvo spoljnog opterecenja u svim uslovima, kao što su izraženo plasticno tecenje (large scale yielding), razliciti uticaji heterogenosti, obika i geometrije zavarenih konstrukcija u eksploataciji. Parametri mehanike loma, odredeni laboratorijskim ispitivanjem epruveta, nisu direktno prenosivi na komponente i stoga se moraju uzeti u obzir dodatni faktori (kao što je uticaj ogranicenog deformisanja). Takode, cilj ove disertacije je odredivanje mehanickih osobina zona zavarenog spoja korišcenjem kombinovanog eksperimentalnonumerickog postupka, što je narocito važno kod zona male širine u okviru ZUT koje su izložene opterecenju u transverzalnom pravcu. Rad na disertaciji je podrazumevao primenu metode konacnih elemenata i eksperimentalna ispitivanja. Eksperimentalna ispitivanja su uradena na zavarenim glatkim epruvetama (uz korišcenje ARAMIS stereometrijskog mernog sistema) za odredivanje mehanickih osobina, kao i epruvetama za savijanje u tri tacke i epruvetama za zatezanje sa pocetnom prslinom u metalu šava i zoni uticaja toplote za analizu ponašanja pri lomu. J-R krive i vrednosti parametra mehanike loma koje odgovaraju pocetku rasta prsline odredene su eksperimentalno i numericki na epruvetama sa pocetnom prslinom u ZUT i MŠ. Numericka analiza elasto-plasticnih modela konacnih elemenata (2D i 3D) je uradena u programskom paketu Abaqus, a mikromehanicki kompletni Gursonov model (CGM) je primenjen preko korisnickog potprograma UMAT (autor Z.L. Zhang). Ograniceno deformisanje oko vrha prsline i promena troosnosti u ligamentu su numericki analizirani na epruvetama za savijanje u tri tacke i onim za zatezanje, da bi se analizirala prenosivost mikromehanickih parametara oštecenja sa jedne epruvete na drugu. Takode, mikromehanickim pristupom je odreden uticaj geometrije i rezultati su uporedeni sa eksperimentalnim podacima. Najveci deo numerickih rezultata dobijenih korišcenjem CGM modela pokazuje dobro slaganje sa eksperimentalnim rezultatima...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 iii 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..

Micromechanical modelling of ductile fracture - local approach

Micromechanical modelling of ductile fracture by using the local approach has been presented as the review of previously obtained results for welded joints made of low alloyed high strength steel. Experimental work was performed on 3PB specimens and tensile panels, which were then modelled by finite element method, using two- and three-dimensional meshes, respectively. The local approach was used to simulate both for crack initiation and growth during ductile fracture process

Calibrating a ductile damage model for two pipeline steels : method and challenges

This work is part of a project that aims to develop a micromechanics based damage law taking into account hydrogen assisted degradation. A 'vintage' API 5L X56N and a 'modern' API 5L X70M pipeline steel have been selected for this purpose. The paper focuses on an experimental calibration of ductile damage properties of the well known complete Gurson model for the two steels in absence of hydrogen. A basic microstructural characterization is provided, showing a banded ferrite-pearlite microstructure for both steels. Charpy impact tests showed splits at the fracture surface for the X70 steel. Double-notched round bar tensile tests are performed, aiming to provide the appropriate input for damage model calibration. The double-notched nature of the specimens allows to examine the material state at maximum load in the unfailed notch, and the final material state in the failed notch. Different notch radii are used, capturing a broad range of positive stress triaxialities. The notches are optically monitored for transverse necking in two perpendicular directions (transverse to rolling and through thickness) to reveal any anisotropy in plastic deformation and/or damage. It is explained how the occurrence of splits at the segregation zone, and anisotropy complicate the calibration procedure. Calibration is done for each steel and acceptable results are obtained. However, the occurrence of splits did not allow to evaluate the damage model for the highest levels of tested stress triaxiality

Numerical analysis of constraint effect on ductile tearing in strength mismatched welded cct specimens using micromechanical approach

In this paper, constraint effect on ductile fracture initiation and propagation has been studied. Three-dimensional finite element analyses of mismatched welded joints made of high strength steel, have been performed for centre-cracked tensile (CCT) specimens. Different weld widths and material mismatching ratios have been considered. Ductile fracture parameter crack tip opening displacement at crack growth initiation (CTODi), has been obtained for centre-cracked tensile (CCT) specimens using local approach to fracture and compared with experimental results of single edge bend specimens. Micromechanical complete Gurson model has been applied to investigate fracture behaviour of cracked welded joints. Crack tip constraint has been analysed through stress triaxiality in order to study transferability of fracture parameters from one geometry to anothe

Numerical prediction of ductile fracture resistance of welded joint zones

This study deals with the numerical prediction of ductile fracture initiation and development in welded joints of a high strength low alloyed steel. Having in mind the material heterogeneity in the joint zone, a combined experimental-numerical procedure is applied for determination of properties of the weld metal and heat affected zone - HAZ (both coarse-grained and fine-grained portion). Single smooth tensile specimen is tested, and the surface strains are determined during this test using stereometric measurement. Combined with numerical analysis, this enabled determination of stress-strain curves, which are subsequently used in numerical analysis of fracture of pre-cracked specimens. Two different geometries are considered: standard single-edge notched bend (SENB) specimens and surface-cracked tensile specimens. In each of them, the crack is positioned either in weld metal or between the coarse-grained and fine-grained HAZ. Micromechanical model (complete Gurson model, by Z.L. Zhang) is applied in numerical analysis. Higher resistance to ductile fracture initiation and crack growth in HAZ is successfully predicted, as well as constraint effect caused by different crack shapes

Numerical prediction of ductile fracture resistance of welded joint zones

This study deals with the numerical prediction of ductile fracture initiation and development in welded joints of a high strength low alloyed steel. Having in mind the material heterogeneity in the joint zone, a combined experimental-numerical procedure is applied for determination of properties of the weld metal and heat affected zone - HAZ (both coarse-grained and fine-grained portion). Single smooth tensile specimen is tested, and the surface strains are determined during this test using stereometric measurement. Combined with numerical analysis, this enabled determination of stress-strain curves, which are subsequently used in numerical analysis of fracture of pre-cracked specimens. Two different geometries are considered: standard single-edge notched bend (SENB) specimens and surface-cracked tensile specimens. In each of them, the crack is positioned either in weld metal or between the coarse-grained and fine-grained HAZ. Micromechanical model (complete Gurson model, by Z.L. Zhang) is applied in numerical analysis. Higher resistance to ductile fracture initiation and crack growth in HAZ is successfully predicted, as well as constraint effect caused by different crack shapes

True Stress-strain Curves for HSLA Steel Weldment - Iteration Procedure Based on DIC and FEM

True stress-strain curves of HSLA steel welded joint regions were estimated by using numerical simulation of strains measured by DIC and special iteration procedure to match the results. Strains were measured at certain level of loading in all regions of welded joint with different tensile properties, namely base metal (BM), weld metal (WM) and two subzones of Heat-Affected-Zone (fine grain, FGHAZ, and coarse grain CGHAZ). By converting engineering stress (load divided by initial cross section area) into true stress, experimental true stress-true strain curves are obtained and used as initial iteration for the Finite Element Method (FEM) simulation, after being fitted by the power law relation for stress-strain curves. Obtained results are then compared with the experimental results to fmd the differences and make appropriate correction in power law curves and make as many iterations as needed before the differences become small enough. This procedure has been verified by an example of welded tensile panel

Micromechanical modelling of ductile fracture – local approach

Micromechanical modelling of ductile fracture by using the local approach has been presented as the review of previously obtained results for welded joints made of low alloyed high strength steel. Experimental work was performed on 3PB specimens and tensile panels, which were then modelled by finite element method, using two- and three-dimensional meshes, respectively. The local approach was used to simulate both for crack initiation and growth during ductile fracture process

Numerical analysis of constraint effect on ductile tearing in strength mismatched welded cct specimens using micromechanical approach

In this paper, constraint effect on ductile fracture initiation and propagation has been studied. Three-dimensional finite element analyses of mismatched welded joints made of high strength steel, have been performed for centre-cracked tensile (CCT) specimens. Different weld widths and material mismatching ratios have been considered. Ductile fracture parameter crack tip opening displacement at crack growth initiation (CTODi), has been obtained for centre-cracked tensile (CCT) specimens using local approach to fracture and compared with experimental results of single edge bend specimens. Micromechanical complete Gurson model has been applied to investigate fracture behaviour of cracked welded joints. Crack tip constraint has been analysed through stress triaxiality in order to study transferability of fracture parameters from one geometry to anothe

True Stress-strain Curves for HSLA Steel Weldment - Iteration Procedure Based on DIC and FEM

True stress-strain curves of HSLA steel welded joint regions were estimated by using numerical simulation of strains measured by DIC and special iteration procedure to match the results. Strains were measured at certain level of loading in all regions of welded joint with different tensile properties, namely base metal (BM), weld metal (WM) and two subzones of Heat-Affected-Zone (fine grain, FGHAZ, and coarse grain CGHAZ). By converting engineering stress (load divided by initial cross section area) into true stress, experimental true stress-true strain curves are obtained and used as initial iteration for the Finite Element Method (FEM) simulation, after being fitted by the power law relation for stress-strain curves. Obtained results are then compared with the experimental results to fmd the differences and make appropriate correction in power law curves and make as many iterations as needed before the differences become small enough. This procedure has been verified by an example of welded tensile panel