Papers by Author: János Lukács

Abstract: 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.

Abstract: The material quality, the deformation rate, the temperature and the stress state influence mechanical behaviour and properties of different materials. Due to this great variety of the influencing factors we do not have one model of general validity describing the behaviour of materials, but we have to use a great number of material constants in order to characterize the properties. The exponents of the Manson-Coffin, the Basquin and the Paris-Erdogan laws were applied for the verification of the connection among the fatigue fracture types. Own measured values and test results can be found in the literature were used for the illustration of the connections. “Fracture surface”-s were determined for characterizing of different steel grades and their welded joints. It can be concluded that “fracture surface”-s are suitable for the describing of the fracture behaviour and the conversion of different fracture parameters of steels.

Abstract: 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).

Abstract: 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.

Abstract: This paper presents two basic methods for the assessment of failed girth welds of steel hydrocarbon transporting pipelines. One of them is based on the principles of linear elastic fracture mechanics (LEFM) and stress intensity factor conception for planar material discontinuities, and the other can be used for the complex assessment of all kinds of occurring defects. The results of the presented methods are compared to the results of burst test of pipeline sections containing a failed girth weld and cut from a Hungarian gas pipeline.

Abstract: E-glass fibre reinforced polyester matrix composite was investigated, which was made by pullwinding process. Round three point bending (RTPB) specimens were tested under quasi-static and mode I cyclic loading conditions. Load vs. displacement (F-f), load vs. crack opening displacement (F-v) and crack opening displacement range vs. number of cycles (ΔCOD-N) curves were registered and analysed. Interfacial cracks were caused the final longitudinal fracture of the specimens under quasi-static and cyclic loading conditions.

Abstract: Composite materials combine the advantages of their components. Carbon fibre reinforced composites are used in construction where reduced weight is critical. To produce carbon fibre reinforced composites, aluminium alloys can be the matrix. Advantageous properties of aluminium matrix composites – good toughness, low weight – are applied in aerospace and automotive industry. Because aluminium alloys are not reactive to carbon, therefore the coating of the fibres can solve the problem. Nickel coated and chemically treated carbon fibres were used to producing of aluminium matrix composites. The investigated composite materials were prepared by pressure infiltration. The influence of treating of carbon fibres was examined on the fracture mechanical properties of aluminium matrix composites. Three types of matrix materials, three types of carbon fibres and four types of surface treatment were studied. Fatigue crack growth tests were performed under mode I loading condition and the failure mechanisms of the composite materials were investigated. Test results belonging to different coated fibres were compared, and our results were compared with the results from the literature, too.