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1
Content available remote Detection of strain localization in numerical simulation of sheet metal forming
EN
This paper presents an investigation on the detection of strain localization in numerical simulation of sheet metal forming. Two methods to determine the onset of localized necking have been compared. The first criterion, newly implemented in this work, is based on the analysis of the through-thickness thinning (through-thickness strain) and its first time derivative in the most strained zone. The limit strain in the second method, studied in the authors’ earlier works, is determined by the maximum of the strain acceleration. The limit strains have been determined for different specimens undergoing deformation at different strain paths covering the whole range of the strain paths typical for sheet forming processes. This has allowed to construct numerical forming limit curves (FLCs). The numerical FLCs have been compared with the experimental one. Mesh sensitivity analysis for these criteria has been performed for the selected specimens. It has been shown that the numerical FLC obtained with the new criterion predicts formability limits close to the experimental results so this method can be used as a potential alternative tool to determine formability in standard finite element simulations of sheet forming processes.
PL
W artykule przedstawiono stanowisko laboratoryjne do badań tłoczności blach umożliwiające prowadzenie badań zarówno w temperaturze otoczenia, jak i w warunkach izotermicznych, w zakresie do 600 °C. Omówiono podstawowe założenia konstrukcyjne oprzyrządowania oraz wyjaśniono metodykę prowadzenia badań. Jako przykład zastosowania stanowiska zaprezentowano wyznaczone charakterystyki krzywych odkształceń granicznych dla blach ze stopu magnezu AZ31.
EN
In the paper the die set for formability testing, enabling investigation both in a room temperature and warm isothermal condition up to 600°C was presented. The basic design targets of the die set was discussed and the method of formability testing was explained. Determination of forming limit curves of AZ31 magnesium alloy sheet was given as the example of the die set application.
PL
W artykule przedstawiono wyniki badań właściwości wytrzymałościowych dla stopu aluminium AA 6061 wraz z symulacją numeryczną cienkiej struktury użebrowanej. Wyznaczono doświadczalnie podstawowe parametry wytrzymałościowe (E, Rm, Rp02, Agt), stałą materiałową K, wykładnik umocnienia n oraz krzywą odkształcalności granicznej. Wyznaczone właściwości zostały użyte do budowy modelu materiałowego w symulacjach numerycznych. Analizowano proces tłoczenia cienkiej struktury z dwoma żebrami, uwzględniając różne parametry technologiczne. Wyznaczono rozkłady odkształceń plastycznych na powierzchni struktury oraz grubości ścianek. Rozważano wpływ współ- czynnika tarcia oraz zastosowanej siły docisku na rezultaty tłoczenia.
EN
In the paper the strength test results for AA 6061 are presented together with numerical simulation of thin sheet panels with stiffening ribs. The basic strength parameters (E, Rm, Rp02, Agt), material constant K, strain-hardening coefficient n and forming limit curve were determined experimentally. The determined properties were used for definition of material model in numerical simulations. Forming process of thin sheet panels with two ribs taking into account different technological parameters was analysed. The plastic strain and thickness distributions were determined. The influence of friction condition and holding force on the forming process was investigated.
EN
Purpose: The aim of this work is to develop a method for evaluating forming limit curves measured at high strain rates using drop tower while specimens are being captured by high-speed camera (2D). Design/methodology/approach: This article describes the first step to establishing such method – verification of compatibility between 3D and 2D capturing by ARAMIS system. Findings: Within this work, FLD recorded in 3D and 2D mode were determined. After correction of the angle α between projection onto the normal plane to direction of loading and tangent in the point where crack occurred, the obtained FLD diagram were almost identical. Research limitations/implications: Optical methods, such as digital image correlation used by ARAMIS measuring system, offer very detailed information of material’s surface at high resolution while significantly reducing the preparation and evaluation time for specimens testing. Originality/value: The analysis of true plane major strains in the forming limit diagram (FLD) is still the most established method for failure detection of sheet metal forming processes in industrial praxis. With the prerequisite of linear strain paths it is very simple to predict the start of necking by using the FLD.
5
Content available remote General overview of sheet incremental forming
EN
Purpose: The aim of this research paper is to give a general overview on sheet incremental forming as an emerging field in small and batch production. Design/methodology/approach: First the historical and literature background of sheet incremental forming will be given, and then some theoretical and practical issues of the incremental forming processes will be described including the research work done by the author at the University of Miskolc. This research is part of an international EUREKA project with the main focus on formability and accuracy in incremental sheet forming. Findings: In this research paper some important findings on the critical wall angle which is a characteristic formability feature in incremental sheet forming will be discussed. New specimen geometry was elaborated to reduce the great amount of experimental work to determine the formability limits. The main conclusions are that in incremental forming the formability is significantly higher compared to conventional sheet forming. The process is very flexible and economic due to the low tool costs. Research limitations/implications: One of the main target areas of further research work is the determination of technological window for sheet incremental forming of various materials and to introduce this novel process into industrial practice. Practical implications: In practical applications besides the economy of the process due to its very low tool expenses, the flexibility should be mentioned which is very important in small batch production and particularly in rapid prototyping. Originality/value: The results achieved within this research work are equally important both from the point of view of theoretical and practical aspects of sheet incremental forming.
6
EN
This paper highlights the basic characteristics of magnesium related to forming at elevated temperatures. The paper is divided into three sections. In the first section basic characteristics and applications of magnesium alloys are discussed, after then the focus is diverted to pre-processing and post-processing parameters including punch force, blank holder force, texture conditions, thickness and temperature distributions during forming, and springback effects. In the last section improvements in formability are highlighted by referring to forming limit diagrams to compare magnesium’s performances with other alloys. By summarizing all these pre-processing and post-processing parameters directions are established to improve the formability of magnesium and guidance is provided for future research in this area.
7
Content available remote Numerical simulation of formability tests of pre-deformed steel blanks
EN
This paper presents the results of numerical simulations of the formability tests carried out for a pre-stretched 1 mm thick DC04 steel sheet. Simulation consisted of the subsequent stages as follows: uniaxial stretching of the sheet, unloading and stress relaxation, cutting specimens out of the pre-stretched sheet and bulging the blank with a hemispherical punch. Numerical modeling has been verified by comparison of the simulation results with the experimental ones. Good concordance of the results indicates correct performance of the numerical model and possibility to use it in further theoretical studies.
8
Content available remote Numerical determination of the forming limit diagrams
EN
Purpose: At present the industrial practice demands a reliable determination of forming limits which assures the prediction of properly selecting the forming process in a digital environment. Therefore, technological limits defined with the forming limit diagrams (FLDs) have to be known. The experimental evaluation of FLDs for sheet metal is time consuming and demands expensive equipment. The experimental work could be omitted by predicting the FLD with numerical simulations. Design/methodology/approach: The paper presents a methodology to determine the entire range of the FLD for sheet metal in a digital environment. The Marciniak testing procedure simulated with the FEM program ABAQUS was selected to determine the FLD. To assure the reliability of the developed method, different materials were analysed: two types of deep drawing steel, an aluminium 3000 alloy, and a Ti-alloy. The selected materials have different mechanical properties and sheet thicknesses ranging from 0.5 mm to 1.23 mm. For the verification of numerically obtained results parallel experimental determinations of the FLDs were performed showing a good correlation between the FLDs obtained by both approaches. Findings: A specially developed method for the evaluation of the thickness strain as a function of time as well as the first and the second time derivation of the thickness strain enable the determination of the onset of necking. Research limitations/implications: The presented method of the digital evaluation of the FLDs is still in a developmental phase and needs further improvements for industrial practice. However, in some cases the numerical approach had already been used for a fast prediction of the FLD prior to performing the experiments. At the current level the developed program still needs an expert to support it in some critical decisions. Originality/value: Considering some methological improvements and automation procedures the developed method could be used in everyday practice.
EN
In order to determine the formability of sheets and obtain the optimum process parameters for the processes, various experiments have to be performed. Some of these experiments are Limiting Dome Height (LDH) and Marciniak tests which are traditional biaxial tests and sheet hydroforming tests. For the purpose of determining process limitations and estimation of stamping characteristics in sheet metal forming, the forming limit diagram (FLD) is used. In this study, a test unit has designed for simulating the processes. In this test unit, traditional biaxial tests and hydroforming tests can be conducted. Detailed system shame for the unit is given. The punch has 35 ton punch force, 300 mm punch stroke and 5-250 mm/min punch velocity. The blank holder has 40 ton force. The hydraulic system's properties are; the fluid medium capable to have 10-700 bar, the pressurized fluid has 10 lt/min volume of flow. The electronic system is composed of a computer and an electronic circuit which are used for adjusting the blank holder force, pressure of fluid medium and punch velocity as using convenient hydraulic elements.
10
PL
Magnez i jego stopy cieszą się coraz większym zainteresowaniem przemysłu metalowego. Materiały te łączą gęstość z właściwościami mechanicznymi i fizycznymi w sposób, który czyni je doskonale przydatnymi do zastosowań w lekkich konstrukcjach. Mają zatem wysoki potencjał innowacyjny. Zalety magnezu są już intensywnie wykorzystywane w wyrobach odlewanych. Dotychczas przerobione plastycznie stopy magnezu stosowane były na małą skalę. Głównymi powodami tego stanu rzeczy była mała osiągalność półproduktów ze stopów przerobionych plastycznie i wysoka cena. W niniejszej pracy omówiona została produkcja wyrobów z blach i taśm, z przedstawieniem stanu aktualnego i motywów intensywnego rozwoju. W części końcowej, na przykładzie stopów magnezu AZ31, rozważono właściwości materiału. Szczególnie ciekawy jest tu wpływ temperatury na zmianę właściwości. Celem lepszej oceny zachowania się materiału podczas kształtowania oraz granicy plastyczności w różnych stanach naprężenia i kształtowania, stosuje się wykresy odkształceń granicznych.
EN
Magnesium and its alloys are becoming more and more interesting for the metal working industry. These mate-rials combine density with mechanical and physical properties in a way that gives them excellent suitability for light-weight applications. Therefore, they offer a high innovation potential. The advantages of magnesium are already being used intensively in cast products. So far, wrought magnesium alloys have been used only on a small scale. The major reasons for this are the low availability of semi-finished products made of wrought alloys and the high price. This paper will discuss the production of sheet and strip products, looking into the current state of development and the motives for the intensive development. At the end, material properties will be considered using the magnesium alloys AZ31 as an example. Thereby the influence of temperature on the change of properties is of special interest. In addition, for better assessment of the forming behaviour as well as of the limit of forming at different stress- and forming states, forming limit diagrams are used.
EN
The forming limits of austenitic stainless steel sheets were studied. It was found that the observed limit of straining in stretch forming, when both of the principal stresses are positive, is not set by localized necking, but instead by shearing fracture in the through thickness direction. Thus, the Marciniak-Kuczynski type of analysis, which has recently been successfully applied to both low-carbon steels and aluminum, may not apply to austenitic stainless steels. It appears that the forming limits of austenitic stainless steels may be predicted fairly well by using the classical localized and diffuse necking criteria developed by Hill. The fracture criterion of Ritchie and Thompson seems to overestimate the fracture limit. Better models are needed for the work hardening and to develop better limit strain criteria, since the work hardening seems to depend strongly on both strain rate and temperature. The formability of austenitic stainless steels appears to remain good even when coated with hard TiN, although unavoidable cracks will appear with continuing straining.
PL
Przedmiotem pracy jest odkształcalność graniczna stali austenitycznych. Zaobserwowano, że odkształcalność przy dwuosiowym rozciąganiu, gdy dwa naprężenia główne są dodatnie, nie jest wyznaczana przez lokalne tworzenie się szyjki, ale przez pękanie wywołane przez ścinanie w kierunku grubości blachy. Dlatego metoda Marciniaka-Kuczynskiego, która jest z powodzeniem stosowana do stali nisko-węglowych i aluminium, może nie stosować się do stali austenitycznych. Okazuje się, że odkształcenia graniczne stali austenitycznych mogą być poprawnie przewidywane przez zastosowanie opracowanego przez Hilla klasycznego kryterium lokalizacji i rozprzestrzeniania się szyjki. Kryterium pękania Ritchie-Thompsona wydaje się przeszacowywać granicę pękania. Ponieważ umocnienie zależy mocno od prędkości odkształcenia i temperatury, lepsze modele są potrzebne do opisu umocnienia i opracowania poprawnego kryterium granicznych odkształceń. Odkształcalność graniczna stali austenitycznej utrzymuje się na dobrym poziomie nawet wtedy, gdy stal jest pokryta TiN, chociaż nieuniknione pęknięcia pojawią się w czasie odkształcenia.
12
Content available remote Wpływ funkcji umacniania plastycznego na ocenę odkształceń granicznych blach
PL
W artykule zanalizowano wpływ doboru funkcji opisującej umacnianie plastyczne materiału na ocenę wielkości odkształceń granicznych przy wyznaczaniu granicznej krzywej tłoczności blach. Oprócz znanych w analizach obróbki plastycznej funkcji umacniania wprowadzono - na podstawie wyników prób doświadczalnych rozciągania cienkich blach - opis uwzględniający obszar występowania stałej siły. Dla zaproponowanego modelu przedstawiono wzory do obliczeń i przykłady wykresów parametrycznych granicznych odkształceń.
EN
In this study there was performed the analysis of choice of the function describing strain hardening of raw materials on limit deformation size evaluation, during drawability limit curve tracing for metal sheets. Besides of strengthening functions analysis that are well known in plastic forming (Nadaj, Swift, Ludwik) influencing the thin steel sheets bumping test results, the descrip-tion of the occurrance of the constant force up to bittle state, has been also given. For proposed model the calculation formulas and examples of parameter diagrams of limit deformation with strain paths are presented.
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