Papers by Author: Jožef Predan

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: 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: 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.