Marko Rakin

Prediction of ductile fracture initiation using micromechanical analysis

Abstract In the paper ductile fracture initiation analysis of low-alloyed ferritic steel has been made by application of two micromechanical models: the Rice–Tracey void growth model and the Gurson–Tvergaard–Needleman (GTN) model. The aim of the study was to analyse transferability of micromechanical parameters determined on specimens without initial crack to pre-cracked specimens. A significant part of the research has been carried out through participation in the round robin project organised by the European Structural Integrity Society (ESIS). Tensile tests have been performed on cylindrical smooth specimens and CT specimens. Critical values of micromechanical parameters determined on smooth specimen for both applied models, have been used for prediction of the crack growth initiation in CT specimen. Modelling of the first phase of ductile fracture––void nucleation––has been carried out using quantitative metallographic analysis of non-metallic inclusion content in tested steel. For determination of critical values of model parameters corresponding to ductile fracture initiation a simple procedure has been applied based on a combination of experimental and numerical results. Evaluated J -integral values corresponding to onset of crack growth, J i , are in good agreement with experimental result and both models have proved to be suitable for determination of the ductile fracture initiation in tested steel. The effect of FE size at a crack tip on J i -value has been particularly analysed: it has been established that the calculation with FE size corresponding to the mean free path λ between inclusions in steel gives results that are in accordance with the experimental ones.

API J55 Steel Casing Pipe with an Initial Surface Crack under Internal Pressure - Determination of Fracture Parameters

Seam casing pipe used in an oil drilling rig, manufactured by high frequency (HF) contact welding of API J55 steel, is tested. The influence of an initial defect (machined surface crack) is analysed, by performing pressure test of a pipe segment closed at both ends. Besides the damages at the internal surface, casing pipes are exposed to damage at the external surface, which is why such configuration is analysed here. Measurement of strains and crack mouth opening displacement (CMOD) enabled the application of direct method for J integral evaluation. This procedure is based on the path independence of the J integral and can be applied both in laboratory conditions (on specimens) and on structures. However, it requires a demanding experimental - computational procedure, which is accomplished here using the developed routine. Additionally, the behaviour of the pipe under internal pressure, including fracture mechanics parameters determination, is modelled numerically (by finite element method) in software package Abaqus. The pressure is applied as distributed load acting on the inner surface of the three-dimensional model, and axial tension is applied at the end of the pipe to simulate the closed end. J integral values determined numerically and using direct method are used for estimation of the critical pressure corresponding to the crack growth initiation. Additionally, plastic limit load, i.e. pressure which causes yielding of the ligament, is determined. Based on the results, criteria for pipe integrity assessment are discussed

Influence of Chitosan Coating on Mechanical Stability of Biopolymer Carriers with Probiotic Starter Culture in Fermented Whey Beverages

The aim of this study was to improve the mechanical stability of biopolymer carriers and cell viability with addition of chitosan coating during fermentation process and product storage. Dairy starter culture (1% (w/v)) was diluted in whey and mixed with sodium alginate solution and the beads were made using extrusion technique. The mechanical stability of coated and uncoated beads, the release behavior, and the viability of encapsulated probiotic dairy starter culture in fermented whey beverages were analyzed. The mechanical properties of the beads were determined according to force-displacement and engineering stress-strain curves obtained after compression testing. It was observed that addition of chitosan as a coating on the beads as well as the fermentation process increased the elastic modulus of the calcium alginate-whey beads and cell survival. The current study revealed that the coating did not significantly improve the viability of probiotics during the fermentation but had an important influence on preservation of the strength of the carrier during storage. Our results indicate that whey-based substrate has positive effect on the mechanical stability of biopolymer beads with encapsulated probiotics.

Micromechanical model for fracture toughness prediction in Al–Zn–Mg–Cu alloy forgings

An attempt has been made to model the plane-strain fracture toughness, K Ic, in Al–Zn–Mg–Cu alloy forgings subjected to overageing. The proposed model, based on the multiple micromechanisms, reveals the quantitative relations between fracture toughness, fraction of all fracture modes and microstructural parameters associated with multiscale-sized second-phase particles and precipitate-free zones. The new model is validated by the present quantitative data of microstructural and fractographic analysis performed along with mechanical tests on hot-forged plates in T73 condition. The relevant parameters changed by the compositional variations were determined in two orientations. It was found that the predicted K Ic values represent the tendency of fracture toughness change well. The new model provides better agreement for the case of dominant transgranular fracture mode.

Crack Growth Resistance of Overaged Al-Zn-Mg-Cu Alloys

The crack growth resistance of the Al-Zn-Mg-Cu alloy forgings in overaged condition was investigated with three industrially produced alloys, which showed differences in the microstructures governed by compositional variations. Fatigue-crack propagation experiments were conducted at ambient temperature and variations in crack growth rates (da/dN) as a function of applied stress intensity range (ΔK) were related to the characteristics of microstructures, including coarse intermetallic (IM) particles and precipitates. It appears that the crack growth rate increases systematically with an increase of the impurity level, which in turn increases the amount and size of large Fe- and Si-containing IM particles while decreases their spacing. That degradation in resistance to crack growth was attributed to the acceleration of the crack initiation and propagation by coarse IM particles were confirmed by in-situ SEM observation of the fracture process. The observed anisotropy in fatigue behavior was caused by the anisotropy in coarse IM particle orientation.

Fracture Toughness Modeling in High-Strength Al-Based Alloys

The fracture toughness of the commercial 7xxx alloy in T73 overaged temper is modeled to determine the influence of the microstructural parameters associated with the coarse constituent particles on the fracture process. To develop a quantitative relationship between the plane strain fracture toughness, KIc, and the microstructural attributes, an extensive stereological analysis and mechanical tests are performed. Fracture behavior of heat-treated forgings is characterized as a function of Fe and Si impurity levels. The data are compared to current fracture toughness models, with the results applied to improve the modeling of toughness using microstructural parameters and basic tensile properties. The appropriate modification of the existing model is proposed and a multiple micromechanism-based model is developed. This required the measure of parameters representing the relative contributions of different fracture micromechanisms to the fracture surface morphology.

Micromechanical Analysis of Constraint Effect on Fracture Initiation in Strength Mismatched Welded Joints

In this paper the micromechanical approach to ductile fracture was applied in a study of constraint effect on crack growth initiation in mismatched welded joints. The single-edged notched bend specimens (precrack length a0/W=0.32) were experimentally and numerically analyzed. The coupled micromechanical model proposed by Gurson, Tvergaard and Needleman was used. Constraint effect was tested by varying widths of the welded joints (6, 12 and 18mm). Highstrength low-alloyed (HSLA) steel was used as the base metal in a quenched and tempered condition. The flux-cored arc-welding process in shielding gas was used. Two different fillers were selected to obtain over- and undermatched weld metal. The micromechanical parameters used in prediction of the crack growth initiation on precracked specimen were calibrated on a round smooth specimen. The difference in fracture behavior between over- and undermatched welded joints obtained in experimental results was followed by numerical computations of void volume fraction in front of the crack tip.

In vitro biocompatibility assessment of Co-Cr-Mo dental cast alloy

Metallic materials, such as Co-Cr-Mo alloys, are exposed to aggres- sive conditions in the oral cavity that represents an ideal environment for metallic ion release and biodegradation. The metallic ions released from dental materials can cause local and/or systemic adverse effects in the human body. Therefore, dental materials are required to possess appropriate mechanical, physical, chemical and biological properties. The biocompatibility of metallic materials is very important for dental applications. Accordingly, the aim of this study was to examine metallic ion release and cytotoxicity of Co-30Cr-5Mo cast alloy as the initial phase of biocompatibility evaluation. Determination of the viability of human (MRC-5) and animal (L929) fibroblast cells were conducted using three in vitro test methods: the colorimetric methyl-thiazol- tetrazolium (MTT) test, the dye exclusion test (DET) and the agar diffusion test (ADT). Furthermore, the morphology and growth of the cells were analyzed using scanning electron microscopy (SEM). The obtained results indicated that Co-30Cr-5Mo alloy did not release harmful elements in concentrations high enough to have detrimental effects on human and animal fibroblasts under the given experimental conditions. Moreover, the fibroblast cells showed good adhesion on the surface of the Co-30Cr-5Mo alloy. Therefore, it could be con- cluded that Co-30Cr-5Mo alloy is a biocompatible material that could be safely used in dentistry.

Ductile fracture prediction of steam pipeline steel

Considering the conditions to which steels used for the manufacture of steam pipelines are exposed, the micromechanism of their destruction in exploitation is exclusively the ductile one. In order to make an estimation of the level of the damage that occurs in exploitation, in this paper a combined experimental and numerical procedure has been developed based on micromechanical or local approach to the fracture mechanics of metallic materials. After the analysis of the results obtained for micromechanical criterion of failure for virgin steel and that used in the steam pipeline, a proposal for prolongation of the working life of tested steel for steam pipelines until the next overhaul is given.

The Influence of Microstructure on Ductile Fracture Initiation in Low-Alloyed Steel

In this paper the quantitative microstructural analysis of low-alloyed pressure vessel steel has been discussed. The values of volume fraction and mean free path of non-metallic inclusions determined in that way were used in the numerical analysis. A series of elastic-plastic calculations of round smooth specimen and compact tension (CT) specimen was made using finite elements (FE) method under conditions of ductile fracture initiation of tested steel, followed by large plastic deformation. The influence of microstructure of tested ferritic steel on crack initiation and growth was considered using two micromechanical models.