Tatjana Haramina, Lovro Fulanović, Jelena Macan: Effect of Weathering on Dynamic Mechanical Properties of the Multilayer Polyamide//Ethylene Vinyl Alcohol Copolymer//Polyamide//Polyethylene Film

Commercial co-extruded multilayer film of 25 micron thickness was tested for use as an airborne module. The module in a form of a cylindrical balloon rotates at high altitudes in order to convert wind energy to electrical energy. The film consists of low-density polyethylene, ethylene vinyl alcohol copolymer, two layers of polyamide 6 (PA6//EVOH//PA6//LDPE) and tie layers between them.
The asymmetric film was aged in a weathering chamber at room temperature, with either the PA6 surface or the LDPE surface directly exposed to the xenon-arc light. The exposure of 1000 hours to a rainy and a sunny cycle ruined the film; therefore, the film was exposed to shorter sunny cycles only, for 3, 6, 16, 44, and 67 hours. The effect of solar radiation on tensile properties was tested in the machining and transverse directions by means of tensile testing at room temperature and dynamic mechanical analysis (DMA) at temperatures ranging from 83 to 373 K. In order to get a better insight into structural changes, the Fourier transform infrared spectroscopy (FTIR) analysis is performed. Water vapour sorption and the melting temperature range of the unexposed film were determined by means of thermogravimetric analysis (TGA) and optical microscopy, respectively.
The tensile tests indicate a high anisotropy of the coextruded film. After irradiation, there is an increase in crystallinity degree. Kinetics of crystallization and hence the mechanical properties depend on the exposed surface.

Key words: Multilayer film; Weathering; Mechanical properties; Viscoelasticity; FTIR spectroscopy

Stoja Rešković, Branko Grizelj, Tin Brlić, Marijan Balenović : The Application of a Factorial Experiment with Repeated Measurements in the Investigation of Parameters Influencing the Properties of Niobium Microalloyed Cold Drawn Welded Steel Tubes

It has been observed that inhomogeneous deformations occur at the beginning of the plastic material flow during the cold drawing of welded tubes and after the proportionality limit Rp has been reached. Therefore, it is important to determine the most influential factor at the beginning of the plastic material flow. Tests were carried out on a welded joint and on the base material. The study was performed on two steels with different contents of the microalloying element niobium. Tubes were drawn with two different degrees of reduction. The factorial experiment consisting of three factors at two levels 2^3 with repeated measurements showed that the degree of reduction has the greatest influence on the proportionality limit.

Key words: niobium microalloyed steel; cold drawn tubes; proportionality limit; factorial experiment

Goran Tepić, Bojan Lalić, Ilija Tanackov, Siniša Sremac, Stevan Milisavljević, Milan Kostelac: Numerical Model of Fragmentation Hazards Caused by a Tank Explosion

The paper analyses the fragmentation of a horizontal cylindrical tank caused by the effect of boiling liquid expanding vapour explosion (BLEVE). A fragmentation model for identification of kinematic parameters is proposed. The originality of the model lies in the introduction of initial acceleration. Using this model, the initial velocity can be assessed without knowing the values of explosion energy and the mass of fragments. The application of this model reduces the uncertainty in assessing the range of fragments and the risk of fragmentation. The initial acceleration of fragments generated in an explosion is assessed according to the geometry and type of the tank material. The initial acceleration, which does not depend on the kinematic parameters of the constant wall thickness of the tank, allows a reliable assessment of the launch angle of a fragment. Characteristic forms of the fragment trajectory are identified, depending on the aerodynamic and thrust acceleration coefficients, and probability distributions of the fragment ranges are given. Relevant factors in the assessment of fragmentation hazards include the trajectory of a fragment, the height of a target and its distance from the tank. It was concluded that aerodynamic fragments at distances of up to 50 m are not a danger to targets of up to 15 m high. Fragments with high air resistance and low thrust can endanger targets of the same height at distances of over 200 m. The presented fragmentation model includes the effect of heating due to the BLEVE effect and can be applied to all types of tanks.

Key words: fragmentation; tank; explosion; target; hazard

Miho Klaić, Danko Brezak, Tomislav Staroveški, Zrinka Murat: On-Line Workpiece Hardness Monitoring in Stone Machining

The application of four types of process signals in the indirect on-line monitoring of stone hardness has been analysed in this paper. Cutting forces, servomotor currents, vibration and acoustic emission signals were measured during the drilling of three types of stones characterised by different hardness and heterogeneity values. A group of features were extracted from each signal from the time and frequency domain. Their capacity to correctly classify stone hardness was analysed using an artificial neural network classifier. Stone samples were drilled with new drill bits and drill bits worn to three different wear levels in order to analyse the influence of tool wear on the hardness classification process. Nine combinations of cutting parameters were applied for each drill wear level and stone type. Features extracted from the vibration signals obtained the best results in the stone hardness classification. The results indicate their potential industrial application, since they have achieved a high classification precision regardless of the drill bit wear level.

Key words: stone drilling; hardness classification; process monitoring; signal analysis

Lesław Kyzioł: Dynamic Properties of 40HM Steels at High Strain Rates

When designing ballistic shields, it is of significance to know how materials behave at high strain rates. The paper presents results of the experimental investigation into the influence of the strain rate on the dynamic yield point of 40HM chromium-molybdenum steel. The behaviour of a material differs depending on impact speed, even if the impact energy remains the same. Dynamic tests were conducted on two specimens using two testing stations: a station equipped with a rotary hammer for testing at a speed reaching 40 m/s and a station accelerating the specimen to a high speed (160 to 270 m/s) and launching it to impact a hard, non-deformable shield. The dynamic yield point was determined by using the Taylor theory. To determine the impact speed, an optical measurement method and a digital high-speed camera were used. The impact speed was determined on the basis of the analysis of recorded images using a specialized software program. The impact speed was adjusted experimentally depending on the behaviour of the specimen material. The basic criterion for selecting the impact speed was that the requirement was fulfilled that the cylinder – as a result of the impact – succumbs to an evident plastic deformation without a visible material integrity infringement. The questions of plastic deformation, temperature influence, strain rate and stress–strain relation require an analysis of the formation and effects of dislocations. The unified constitutive law called Johnson-Cook’s law describes the relations between the material’s tension and strengthening ratio, and between strain rate and temperature. The tests indicated that a strain rate increase considerably improves strength properties of 40HM steel.

Key words: steel; dynamic stretching and compression test; Taylor’s theory; rotary hammer; Johnson-Cook’s equation

Aurimas Ralys, Vadim Mokšin: Numerical Simulation of a Cavitating Pulsating Water Jet Used for Removing Contaminants from Metal Surfaces

The paper presents an environmentally friendly method for cleaning metal surfaces from contaminants. In this method, the contaminants are removed from surfaces by a cavitating pulsating water jet generated in the nozzle and directed at the surface. The cavitation-generating effectiveness of three various design nozzles was investigated numerically using the SolidWorks 2015 Flow Simulation software. The volume fraction of the vapour generated in the flow inside the nozzles was determined. Selected nozzle was studied experimentally in respect of efficiency of abrasive particle removal. After having carried out experiments to test the removal efficiency of the method, it was established that the water that has remained on the cleaned surface decreases its effectiveness. To solve this problem, a prototype of a cavitation-generating head with a water removal system was created and studied numerically.

Key words: cavitation; clean surface; microbubbles; nozzle

Yunhuan Sui, Xingzhong Zhang, Long Guo, Dianfeng Xu: Microstructural Study of High Temperature Creep in Q460E Steel Based on the Solidification Method

A tensile creep test has been carried out to study the high temperature creep mechanism of Q460E steel and thus develop a better understanding about how the creep phenomenon affects the performance of a cast slab. Because the heating process in the solidification method is more similar to the actual solidification process of casting a slab, the high temperature tensile creep test was conducted by using the solidification method. Further observation of the microstructure was carried out after the tensile creep test has been carried out. The microstructure of the Q460E steel after the high temperature tensile creep test and water quenching observed with a metallographic microscope revealed mainly martensite and retained austenite. From the observation with a transmission electron microscope (TEM) it could be found that dislocation and its substructure were the root cause which triggered high temperature creep deformation of the Q460E steel. In addition, the formation of a subboundary also provided the impetus to creep deformation.

Key words: Q460E steel; high temperature creep behaviour; solidification method; microstructure observation; continuous casting

Navakodi Gurusamy, Jawahar Natarajan, Palaniappan Kanagasabai Palaniappan: Parameter Optimization of the CNC Wire-Cut EDM Process for Machining Aluminium 6063-B4C Metal Matrix Composites

In the competitive manufacturing environment, conventional monolithic materials cannot compete with composite materials in the ever growing market because of their inherent limitations. Consequently, composite materials are preferred globally in major industries. CNC wire-cut electrical discharge machining is one of the non-traditional machining (NTM) processes, which are used to cut ferrous and non-ferrous metals with properties ranging from low hardness to high hardness and especially to cut high hardness materials and complicated profiles and shapes of all engineering and aerodynamic products. The process parameter setting is critical in the wire-cut electric discharge machining (WEDM) as it has a direct impact on performance characteristics. This paper addresses the preparation of aluminium metal matrix composites (AMMCs) and the optimization of the WEDM process parameters for machining AMMCs to improve the key performance characteristics, namely the metal removal rate (MRR), the surface roughness (SR), and the kerf width (KW). The optimization of the WEDM process parameters is multi-objective in nature. The prime objective of this study was to obtain optimal WEDM process parameters for machining AMMCs with the maximum MRR and the minimum SR and KW. The grey-based Taguchi method was applied to choose an optimal parameter combination to achieve the above said performance characteristics. AMMCs with the base metal Al6063 and the reinforcement of boron carbide (B4C) in three different percentages (3%, 6% and 9%) were obtained by using the stir casting method. WEDM experiments were conducted and the optimal process parameters were found to be as follows: servo voltage (SV)-26V, pulse on time (Ton)-122µs, pulse off time (Toff)-52 µs, B4C-6%, wire feed-3 m/min, and wire tension-49.05N. The key findings from this study reveal that the MRR was increased from 35.759 mm3/min to 42.229 mm3/min, the SR decreased from 4.500µm to 4.382µm and the KW decreased from 371 micron to 364 micron.

Key words: machining optimization; metal matrix composites; WEDM process; Taguchi; grey relational analysis

Dragan Đurđević, Srećko Manasijević, Momčilo Miljuš: Rationalization of a Core Warehouse in the Casting Plant: A Case Study

The warehouse is an important factor in the manufacturing process. At present, there are constant demands for the more efficient and effective operation of warehouse systems. For this reason, the rationalization of existing warehouses and/or storage systems is one of the ways to achieve the previously set targets. The possibilities of rationalization are numerous in all segments of the observed system. The most suitable solutions for a particular warehouse will be found based on the nature of the observed problem and the available investment funds. The paper defines the place and the role of the warehouse in a casting plant. Besides, the current situation in the warehouse segment of casting plants is analyzed to identify problems. Finally, the development of different concepts of rationalization to relevant constraints is discussed. This approach has enabled us to solve the problem of the rationalization task and to produce the desired effects.

Key words: Warehouse; rationalization; logistics processes; cores; casting

Ispravak: Article retraction

Vijest: Professor Hrvoje Jasak received the 2018 Annual Award for Science

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