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Porosity in sintered materials negatively affects its fatigue properties. In investigating its influence, the application of numerical simulations reduces experimental testing, but they are computation-ally very expensive. In this work, the application of a relatively simple numerical Phase Field (PF) model for fatigue fracture is proposed for the estimation of fatigue life of sintered steels by the analysis of microcrack evolution. The model for brittle fracture and a new cycle skipping algorithm are used to reduce computational costs. A multiphase sintered steel, consisting of bainite and fer-rite, is examined. Detailed finite element models of microstructure are generated from high-resolution metallography images. Microstructural elastic material parameters are obtained using instrumented indentation, while fracture model parameters are estimated from experimental S-N curves. Numerical results for monotonous and fatigue fracture obtained are compared with data from experimental measurements. The proposed methodology is able to capture some im-portant fracture phenomena in the considered material, such as the initiation of first damage in microstructure, the forming of larger cracks at the macroscopic level, and the total life in the high cycle fatigue regime. However, due to the adopted simplifications, to model is not suitable for predicting accurate realistic crack patterns of microcracks.

In this investigation a high-strength low-alloyed (HSLA) steel of the 700-MPa strength class was used as a base material. A butt-welded joint with X grooves was produced with an overmatched weld metal that had a yield strength 23 % greater than that of the base material. Three-point bending B x 2B test specimens (thickness B = 36 mm) were extracted from the welded joints. The straight crack front (a0 = 35,571 mm) crosses different microstructures through the thickness of the specimen. Both fracture-mechanics tests and a 3D finite-element analysis were performed. The CTOD parameter of the fracture toughness was measured and calculated numerically. The loading level at which stable crack growth occurred was also determined. The comparison of the experimental and numerical values of the CTOD (bs) displacements showed good agreement. The principal stress, ▫$/delta_{y}$▫, the Mises equivalent stress, ▫$/delta_{eq}$▫. and the plastic equivalent strain, ▫$/epsilon_{pl, eq}$▫, at the moment of crack initiation were studied for six equidistant layers from the surface to the mid-thickness of the specimen. The dependence of the crack-opening stress (denoted as ax in this paper) on the local fracture-toughness value was considered as the parameter which determines the direction of the crack-front propagation. The results show that the lower strength of the base metal contributes to the crack-path deviation in the mid-thickness of the specimen. Both the crack-path deviation and the higher toughness of the base metal increase the critical fracture toughness value of the welded joint. V tej raziskavi je bilo kot osnovni material uporabljeno visokotrdno malolegirano jeklo (HSLA) s trdnostjo 700 MPa za čelni zvar z X-žlebovi je bil pripravljen in uporabljen deponirani material z mejo plastičnosti, ki je bila za 23 % večja kot pri osnovnem materialu. Tri-točkovni upogibni preizkušanci B x 213 (debelina B = 36 mm) so bili izrezani iz zvarjenega spoja. Čelo ravne razpoke (a0 = 35,571 mm) seka skozi debelino preizkušanca različne mikrostrukture. Opravljeni so bili lomnomehanski preizkusi in analiza 3D končnih elementov. CTOD-parameter mehanike loma je bil izmerjen in numerično izračunan. Določen je bil tudi nivo obremenitve, pri katerem je nastala stabilna rast razpoke. Primerjava eksperimentalnih in numeričnih vrednosti za CTOD (65)-premike je pokazala dobro ujemanje. Glavna napetost /delta_y, ekvivalentna Mises-napetost /delta_eq in ekvivalentna plastična deformacija /epsilon_ok-e] so bile opredeljene za 6 med seboj enako oddaljenih plasti od površine do sredine preizkušanca. Odvisnost med napetostjo odprtja razpoke (označeno z /delta_x v tem članku) in lokalno vrednostjo za žilavost loma je upoštevana kot parameter, ki določa smer propagacije čela razpoke. Rezultati kažejo, da manjša trdnost osnovnega materiala povzroči deviacijo poteka razpoke v sredini preizkušanca. Deviacija poti razpoke in večja žilavost osnovnega materiala povečata žilavost loma zvarnega spoja.

Ductile damage development in welded joints of high strength low alloyed (HSLA) steel NIOMOL 490K was examined experimentally and analyzed using the micromechanical approach. Single-edge notched bend (SENB) specimens, cut from the welded plates and subsequently pre-cracked in HAZ and WM, were tested. True stress - true strain curves of the heat-affected zone (HAZ) and weld metal (WM) were determined on the smooth tensile plate specimen, by application of a combined experimental-numerical procedure which included stereometric strain measurement and finite element analysis. The complete Gurson model (CGM) was applied to estimate the fracture behavior of the analyzed zones of the joint. An important property of this model is that critical damage parameter value (used as failure criterion) is not a material constant-it is calculated in every load step, based on the stress and strain state in the structure. This feature is especially important in the examined joints, since the material heterogeneity causes pronounced stress and strain gradients. Finite element software Abaqus was used for numerical analysis. The crack growth initiation was predicted at the moment when damage parameter reaches its critical value in the element nearest to the crack tip. The advance of damage (i.e. the crack growth) was modeled by tracking the loss of load carrying capacity in the ligament (local stiffness reduction in the elements). It is concluded that the micromechanical approach can be used to assess the effects of constraint and heterogeneity on the fracture of the examined HSLA steel welded joints. HAZ fracture properties concerning both crack growth initiation and stable growth are slightly better in comparison with WM, as obtained by testing the SENB specimens with pre-cracks in these two weld zones. The influence of the finite element (FE) size on the results was also analyzed. It is found that the finite element size near the crack tip strongly influences the prediction of crack initiation and growth, and that there is a very significant difference in appropriate FE size between the WM and HAZ. For the analyzed materials, this size corresponds to the mean free path between the non-metallic inclusions in the examined welded joint zones.

The fracture of a turbine shaft in the case of overloading can exhibit brittle or plastic failure, depending on the material properties, the turbine-shaft geometry and the form of loading. Usually, when the toughness increases, the stiffness of the shaft material is reduced, which can lead to the plastic twist of the shaft. If the fatigue crack appears in the critical region of the shaft then the low impact-toughness value may induce a brittle fracture. During the retrofit of a hydro-power plant a new turbine shaft was produced by quenching-and-tempering technology. Charpy impact-toughness tests showed lower values for the shaft material than those prescribed by the project documentation. Since the turbine shaft for a hydro-power plant is a massive and expensive component, it is necessary to determine a sufficient impact toughness for the material in terms of the geometry and the manner of loading for the turbine shaft. Since only the yield strength and the impact toughness of the material were prescribed, the level 0 of the SINTAP should be applied. The minimum impact toughness values that ensure the ductile fracture of the shaft, cracked circumferentially was also estimated. We also analyzed the variation of the shaft’s carrying capacity resulting from a reduction of the non-cracked ligament in the transversal direction.

Highly precise measuring of 3D displacements in the vicinity of the crack tip by using object grating method enables the visualization of deformation and characterization of the crack propagation process. Modified CTOD concept based on deformations analysis along the set of lines shifted in direction of the crack propagation is introduced. Through the visualization it provides comprehension of specimen's behavior in conditions of high stress concentration and large displacements. Object grating method is advantageously used to analyze two specimens with initial crack in a macroscopic heterogeneous welded joint. The crack propagates from the over-match (OM) material towards the under-match (UM) material or vice versa, when the crack propagates in the opposite direction. The results of the experiments show the potential of object grating method for acquiring deformation field around the crack tip because is enabled to observe the fracture behavior in heterogeneous material structure during the crack propagation.

The integrity of welded structures depends on the load-carrying capacities of their welded joints. Having in mind that welding process produces a large microstructural gradient through the weld, resulting in variation of mechanical properties, micromechanical analysis that correlates the local stresses and strains with fracture toughness seems to be appropriate to ensure integrity. In this paper, a micromechanical model for ductile fracture has been applied for assessment of the crack growth onset in the strength-mismatched welded joints of various widths. High-strength low-alloyed (HSLA) steel is used as the base metal in quenched and tempered condition. The flux-cored arc-welding process in shielding gas is used. Two different fillers are selected to obtain over- and undermatched weld metal. Precracked (single-edged notch bend - SENB) specimens are experimentally and numerically analysed in static conditions of external loading. Computations of the void volume fraction ahead of the crack tip are performed using the FE method. Volume fraction of non-metallic inclusions in the materials forming the joint was determined using the quantitative microstructural analysis. Based on the critical value, fc, determined on round tensile specimens, CTOD at ductile crack growth initiation on SENB specimens is determined. It has been found that the micromechanical model enables determination of the constraint effect induced by different widths of the welded joints at the onset of the crack growth in tested specimens.

Integrity of cylindrical structures, such as pipes and small-scale pressure vessels, is typically assessed by testing fracture mechanics specimens, like compact tensile CT or single-edge notched bending, SENB. However, their fabrication is mainly not possible for thin-walled pressurised elements, commonly used in structures of process and energetic facilities, such as hydrogen and hydrogen carriers storage and delivery systems. Recently proposed nonstandard ring-shaped specimen is used to test the fracture resistance of pressurised cylinders with defects (cracks or notches) in the most critical direction - axial. Some of the advantages of the pipe-ring notch bend specimen, PRNB, are simple fabrication and same material history as actual exploited structure. A combined experimental-numerical procedure is applied for prediction of fracture behaviour of specimens (applicable to ductile metals, mainly steels and Al alloys), with the aim to further validate the new specimen. The results obtained so far lead to conclusion that PRNB specimen can be applied in integrity assessment of cylindrical structures.