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Suction and shear wave velocity measurements for assessing sample quality.

100%

Tanaka H. , Nishida K.

2007

tom Vol. 29, nr 1-2

163-175

Sample quality is very important for obtaining reliable geotechnical parameters, especially because the reliability design method has been recently introduced into geotechnical engineering (for example, Eurocode 7). The sample quality is usually assessed by comparing the values of strength, Young's modulus and strain at failure from unconfined compression test as well as the values of pre-consolidation pressure, compression index, and volumetric strain at in situ stresses from oedometer test. However, since the assessment criteria are based on past experience, the above conventional methods should be examined carefully in order to explain whether they can or cannot be applied to the layers tested. In addition, it should be noted that they are destructive, i.e., after assessing the specimen quality, it is cannot be used for testing anymore. In this paper, the possibility of assessing a sample quality by suction (p'r) and shear wave velocity (vv) is examined.

Prediction of forming limit in stretch flanging by finite element simulation combined with ductile fracture criterion

70%

Hirohiko T. , Hama T. , Nishida K. , Yoshida T. , Nitta J.

2009

tom Vol. 9, No. 1

137-142

The use of high-strength steel sheets to reduce car weight is drawing much attention from the viewpoint of environmental preservation. However, high-strength steel sheets are much inferior to ordinary steel sheets in formability. In order to find the forming method and conditions suitable for high-strength steel sheets, the forming limit in sheet forming processes has to be correctly predicted. The authors have recently proposed an approach to predict the forming limit by introducing criteria for ductile fracture into the finite element simulation of sheet metal forming processes. In the criteria the occurrence of ductile fracture is estimated by the macroscopic stress and strain during forming which are calculated by the finite element simulation. In the previous studies, the possibility of the application of some criteria for ductile fracture to the high-strength steel sheets was examined. The forming limits of a few types of high-strength steel sheets under various strain paths from balanced biaxial to uniaxial tension were examined by the Marciniak-type in-plane biaxial stretching test, and they were compared with those derived from the ductile fracture criteria. As a result, it turned out that the fracture strains derived from the criterion by Cockcroft and Latham gave the best fit to the experimental results. Then, the forming limit predictions were successfully carried out for 3-dimensional deep-drawing and stretching processes by the finite element simulation combined with the ductile fracture criterion. Stretch flanging is also one of the fundamental press forming processes like deep-drawing and stretching, but is different from others in the point that fracture occurs at the edge of the sheets. The sheet edge suffers influences of the pre-forming, i.e. blanking. In this study, stretch flanging tests of a 780 MPa grade high-strength steel sheet are carried out for various blank geometries, and they are analysed by the finite element method combined with the ductile fracture criterion. The dynamic explicit finite element program LS-DYNA ver.970 with thin shell is used. The comparison with the experimental results demonstrates the possibility of forming limit prediction via the present approach in which we take the work-hardening and damage in the blanking process into consideration.

Prowadzone wcześniej przez autorów badania miały na celu określenie odkształceń granicznych w kilku typach stali o podwyższonej granicy plastyczności podczas różnych schematów obciążenia, np. dla stanu dwuosiowego lub stanu jednoosiowego. Wyniki tych badań porównano z rezultatami uzyskanymi w drodze analizy teoretycznej bazującej na kryteriach plastycznego pękania. Porównanie to wykazało, iż graniczne wartości odkształcenia określone w oparciu o kryterium Cockcrofta i Lathama są najbliższe wynikom badań laboratoryjnych. W niniejszej pracy analogiczną analizę zastosowano do symulacji próby tłoczności. Przeprowadzona analiza numeryczna wykorzystuje pakiet komercyjny metody elementów skończonych LS-DYNA w wersji 971. W pracy wykorzystano elementy skończone typu powłokowego. Wyniki symulacji wykazały dużą zgodność uzyskanych wartości odkształcalności granicznej z wynikami badań laboratoryjnych. Taka zgodność była możliwa poprzez uwzględnienie w modelu numerycznym procesu umocnienia oraz pękania występującego na brzegu wsadu.