Papers by Author: Nenad Gubeljak

Abstract: Components and structures exposed to elastic dynamic loading respond with different surface or bulk material changes which could be monitored on the through surface deformation measurements. The measurements could be performed with electronic devices used for on-line computerized measurement systems. Fatigue induced flaw growth was monitored on a 4-point specimen, loaded by cyclic dynamic bend forces. The flaw growth was monitored by strain gauges and analyzed to determine the shape, propagation and cross sections of the crack. To determine the stress intensity factor a numerical model was developed based on measured crack shapes, material properties and cyclic loading data of the actual tested specimen. The analyses results showed that derived calibration curve could be used to predict surface deformations as a result of crack propagation and growth. With the determination of surface deformation, one could follow the crack transition from surface crack to through thickness crack. The goal of this paper is to describe methodology and results based on experimental analysis during crack propagation and potential use of this technique for online monitoring purposes.

Abstract: The differences in fracture behavior between the compact tension C(T) and the middle tensile M(T) specimens make structure integrity assessment uncertain. Two different types of specimens C(T) and M(T) specimens made from stainless steel have been used for fracture toughness testing at the room temperature by the principles of the ASTM 1820-05 standard procedure. Stable crack initiation and crack propagation occurred for the C(T) specimens at lower values of crack driving force than for the M(T) specimens. Crack tip opening displacement-CTOD has been directly measured on the surface of specimens by using a stereo-optical grading method. The critical crack tip opening displacement at crack initiation CTODi has been measured as a plastic Stretch Zone Width (SZW) during a post test fractographic inspection. Comparison between the CTOD-R curves of both types of specimens shows some difference between the C(T) and the M(T) specimens, but a more significant difference appeared in the crack driving force, as consequence of different constraint (triaxiality) of the C(T) versus the M(T) specimens. Therefore, the result obtained by test on laboratory C(T) specimens cannot be directly used as fracture toughness material properties in a structure integrity assessment, except as a conservative lower bound estimate.

Abstract: Since welded constructions are widely used in engineering, a certain flaws in welded joints may occur either in process of welding or in exploitation period. Easiest way to prolong working life of such welded construction is to repair welded joint to eliminate possibility of construction failure. Process of repair welding usually gives heterogeneous welded joints because during process of repair additional material is introduced into welded joint, resulting in heterogeneity from the presence of materials in welded joint point of view. Such difference in materials usually results in yield strength difference between materials, represented with mismatch ratio, and it is commonly present in welds where high strength low-alloyed (HSLA) steels were welded. Since I butt welded joints are very common in welding, a systematic investigation of such welds is performed and presented in this paper. Therefore in this investigation the influence of present material in heterogeneous weld and geometry of weld is investigated in context of fracture resistance of welded joint represented as yield load solutions in the first place. A flaw in form of crack was implemented in such heterogeneous weld and using finite element method yield load solutions for different combinations of weld geometry and material strength are obtained and presented in form of diagrams.

Abstract: Among various micromechanical models for ductile fracture analysis using local approach, those based on the Gurson plastic flow criterion have been intensively developed in the past three decades. However, their application is still subject of many researches, with the aim to improve the assessment of ductile fracture in various, mainly metallic, materials. In this work, Gurson criterion is applied to welded joints produced with one or two different weld metals (one of these is overmatched, while the other is undermatched). Welded single-edge notched bend (SENB) specimens are examined, with an initial pre-crack located in the symmetry plane of each joint. Local approach is chosen for assessment of behaviour of the joints under the external loading, to capture the effect of material inhomogeneity and initial crack length on deformation and ductile fracture initiation in analysed specimens.

Abstract: For designing machines and devices the dimensioning with respect to service life is increasingly taken into account. This applies also for gearing which are still today one of very important components of almost all machines. The problem of determination of the service life of gearing is directly related to geometry of gears, multiaxial loadings, materials and appropriate models for prediction of the crack propagation.Obviously gears and gearing belong to the real complex structure, by correctly selected and carefully planned experiments we obtained results with which we can confirm and justified the mathematical model for calculating different parameters, i.e. service life.

Abstract: Ductile fracture process includes three stages: void nucleation, their growth and coalescence. The voids nucleate due to the fracture or separation of non-metallic inclusions and secondary-phase particles from the material matrix. Micromechanical models based on the Gurson plastic flow criterion are often used for analysis of ductile fracture. They consider the material as a porous medium in which the effect of voids on the stress-strain state and plastic flow cannot be neglected. Another important property of the Gurson criterion is that the hydrostatic stress component influences the plastic flow of the material.

Abstract: The fatigue behavior assessment of high strength low alloy steel has main importance for reliable and safe service of welded structure. The mechanisms of fatigue crack initiation and propagation always represent a potential danger of crack propagation during the exploitation of a structure, which can lead to catastrophic failure. If the fatigue’s crack appear at the welded structure it is important to reliable find loading regime which can reduce risk of failure for still operating structure.

Abstract: 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.