János Lukács

Dimensions of Lifetime Management

The lifetime management of different engineering appliances, structures and structure elements is one of the important technical-economic problems of nowadays. The problem is basically appeared in technical, economic and political dimensions and the solution can be imagined according to many approaches: microstructural approach, application of new design conceptions and operational approaches (conventional methods, fitness for purpose, structural integrity, integrity management, risk management). The methods, applied in the different approaches have basically two types: engineering and fracture mechanical methods. The time of operation, the frequency data of fractures and the fracture causing failure statistics of the engineering structures having great importance shows, that the significance of cyclic loadings, fatigue and fatigue crack propagation is emphasized in general. The sketched possibilities are shown by two examples in this paper. One of them is the system of lifetime management for cyclic loaded structures having cracks or crack like defects; the other is the building-up and purport of the integrated management system for hydrocarbon transporting pipelines.

Fatigue Curves for Aluminium Alloys and their Welded Joints Used in Automotive Industry

Our research work has focused from the materials world on the aluminium alloys, from the manufacturing processes on the welding technologies, and from different loads on the cyclic loads. The article aimed (i) to demonstrate the behaviour of the friction stir welded (FSW) joints under cyclic loading conditions; (ii) to determine fatigue (limit) curves for two aluminium alloys and their welded joints. Experiments were performed on 5754-H22 and 6082-T6 aluminium alloys and their FSW joints. Both high cycle fatigue (HCF) and fatigue crack propagation (FCG) tests were executed on both base materials and their welded joints. Statistical behaviour of the base materials and welded joints was represented by the cutting of the specimens and the using of different crack paths. HCF limit curves were determined based on staircase method and FCG limit curves can be determined by own developed six step method. The investigations and their results were compared with each other and with the results can be found in the literature.

The Properties of Welded Joints Made by 6082-T6 Aluminium Alloy and their Behaviour under Cyclic Loading Conditions

The magnitude of different aluminium alloys, especially the ones with higher strength, are increasing in the structural engineering, not just the usual applications (like the aerospace industry) but more likely in the automotive industry. There are more assumptions of the effective use of aluminium; we should highlight two important factors, the technological and the applicability criterions. The technological criterion is the joining of structural elements, frequently with welding thus the technological criterion ultimately is the weldability. The assumption of applicability comes from the loading capability of these structures, which is typically cyclic loading so the key issue from the point of view of applicability is the resistance to fatigue. This article represents physical simulation and fatigue test results both on the base material and on the welded joints.

Characteristics of Fatigue Cracks Propagating in Different Directions of FSW Joints Made of 5754-H22 and 6082-T6 Alloys

The Friction Stir Welding (FSW) is a dynamically developing version of the pressure welding processes. High-quality welded joints can be created using this process for different engineering applications (e. g. automotive parts). Nowadays, the knowing of the properties and the behaviour of the welded joints is an important direction of the investigations, especially under cyclic loading. The research work aimed to demonstrate the behaviour of the friction stir welded joints under cyclic loading conditions. Fatigue Crack Propagation (FCG) experiments were performed on 5754-H22 and 6082-T6 aluminium alloys and their friction stir welded joints. The CT type specimens were cut parallel and perpendicular to the characteristic directions of the base materials and the welded joints, and the notch locations in the specimens of welded joints were different, too. Therefore, the propagating cracks represent the possible directions of the fatigue cracks both on the base materials and on the welded joints. The results of the fatigue crack propagation tests on the welded joints clearly demonstrate the different characteristics of the thermo-mechanically affected zone (TMAZ), the heat affected zone (HAZ), and the advancing (AS) and retreating sides (RS) of the weld nugget (WN). The investigations and their results were compared with each other and with the results can be found in the literature.

Residual Stress Behavior in Hardened Shot Peened 42CrMo4 Specimens during Fatigue Load

The advantages of applied compressive residual stress on fatigue properties of materials is a well-described topic, but not in all respects. Compressive macro residual stresses in the surface region with medium and high hardness increase the fatigue life and the fatigue limit compared to materials that are free from designed compressive residual stresses because of their increased resistance against crack initiation and propagation. For this aim various surface compressing methods such as burnishing, shot peening, rolling have been developed. The monitoring of residual stress variation during fatigue is important. All properties, which exert lifetime, should be analyzed. The residual stress state of machine elements can change during application, therefore it is necessary to describe how these changes are related to the operational parameters. The surface residual stress state evolution of hardened (quenched and tempered)-and shot peened-42CrMo4 steel during fatigue tests was investigated nondestructively by X-ray diffraction. Four fatigue stress levels were applied. The residual stress state was recorded in shot peened state and monitored during the fatigue tests. The fatigue test was stopped after certain cycles until the specimens fractured. The stress state was measured after each fatigue test stops and the stress relaxation is given in the percentage of the initial stress state in function of cycle number. Introduction Shot peening is a widely used surface compressing method to create compressive residual stress in machined elements [1, 2]. The compressive residual stress state has a beneficial effect on the fatigue durability of metals, especially on fatigue strength [3, 4, 5]. For the assessment of the influence of residual stresses on fatigue behavior, the stability of the residual stress state is very important. During fatigue, the macro and micro residual stress states interact with the cyclic loading stresses and the work hardening and softening processes. For example the surface compressive residual stress improves the fatigue resistance properties of all materials. The stability of the surface compressive residual stress during low and high cycle fatigue in annealed micro-alloyed, medium carbon, perlitic and austenitic steels has been already investigated [6,-9]. The authors have shown that the residual stress relaxation is not monotonic in these alloys. However, in general, machine elements, which are tipically exposed to high cycle fatigue loads; (gears, shafts) are mainly made of high strength hardened steels, therefore it is worth to investigate the stability of residual stress in such material. The stability of surcafe compressive residual stress on hardened, shot peened CM45MV specimens during fatigue was investigated in one of our eralier project [10]. Residual stress relaxation was not observable in that steel, because of the effect of microstructure defects and specimen geometry. In this paper, Residual Stresses 2016: ICRS-10 Materials Research Forum LLC Materials Research Proceedings 2 (2016) 491-496 doi: http://dx.doi.org/10.21741/9781945291173-83 492 our aim was to analyze the variation of compressive residual stress state in shot peened hardened 42CrMo4 steel specimens during fatigue load. This is a widely used steel in the vehicle industry especially in the case of surface compressed machined elements. These measurements are based on our earlier project. Since then, the specimen geometry was improved and the material selection has changed. Materials The specimens were made of EN:42CrMo4 type steel. The chemical composition is given in Table 1. Table 1. Chemical composition of the examined steel

Design Curves for High-Cycle Fatigue Loaded Structural Elements

Frequently, the cause of the failure of different structures or structural elements is the cyclic loading. Both fatigue design curves and methods for determination of these curves can be found in the literature. Even so, there are structural details whereabouts executing of examinations is necessary. The aims of the study are as follows: to give a short summary of important design curves can be found in different standards or specifications; and to demonstrate of own high cycle fatigue tests on a soldered structural element and the comparing of our results and the results of an empirical method.

Residual Stress Evolution during Fatigue Test of a Shoot Peened Steel Sample

Directly-generated compressive residual stress has become a widely used surface modification technique. Intentional residual stress in solid materials can be produced by different surface compression methods. It is well known how residual stress field is influenced by the parameters of inducing technology, but during the operation of a surface compressed machine element the problem of compressive residual stress relaxation can occur. Only few articles address this phenomenon. The literature on residual stress decrease caused by fatigue load is scarce and the kinetics of this mechanism is not yet investigated. The present work reports on the examination of the effect of the fatigue load on compressive residual stress strengthened specimens. Investigations have been conducted at three stress levels. Residual stress data have been measured before fatigue load and measurements have been repeated after defined fatigue cycles.

Integrity Assessment of Power-Plant Structural Elements Using Fracture Mechanics

The research work aimed to present the usability of the cracked round bar (CRB) specimen type for the determination of the linear-elastic plain strain fracture toughness values (KIc or KQ) on steels; and to demonstrate the applicability of the testing results for the reliability assessment of structural elements having cracks or crack like defects. Micro-alloyed structural steel and heat resistant steel were tested at elevated temperatures, at 260 °C and 410 °C, respectively, using small-sized specimens with small diameters. Four different equations were used for the evaluation of the tests, and the determined fracture toughness values were compared with one another. Reliability assessment calculations were realised on two structural element models, critical crack sizes and safety factors were determined for all cases.

Application of Risk-Informed Inspection Strategy to Improve the Lifetime and Efficiency of Cleaning Pigs

The aim of the research work was to analyze the operation of cleaning pigs, in natural gas transportation pipelines from point of view of risk-informed in-service inspection. During the analysis, the main factors considered were the stresses (mechanical, thermal and environmental) and loads, which have effect on cleaning pigs during its operation. The possible damage mechanisms were analyzed which can occur due to their consequences and qualitative probability analysis was performed on the basis of the available information.

Two Methods for Determination of Fatigue Crack Propagation Limit Curves and their Application for Different Materials

There are different documents containing fatigue crack propagation limit or design curves and rules for the prediction of crack growth. The research work aimed to characterise the fatigue crack propagation resistance of different materials using limit curves and determination of limit curves under different loading conditions, based on statistical analysis of test results and the Paris-Erdogan law. With the help of the characteristic values of threshold stress intensity factor range (Kth), two constants of Paris-Erdogan law (C and n), fatigue fracture toughness (Kfc) or fracture toughness (KIc) two reliable method can be proposed. The limit curves calculated by both methods represent a compromise of rational risk (not the most disadvantageous case is considered) and striving for safety (uncertainty is known).