The basic assumptions and hypotheses construction of the computational model studies, taking into account the peculiarities of the work force imperfections of materials under dynamic loading, based on the phenomenological laws of nonlinear rheology and deformable elastic-creeping body. The values for the coefficient vibrocreep computational model under triaxial stress-strain state on the basis of the hypothesis of central symmetry of the hysteresis loop. The basic phenomenological equations allow the calculation of concrete elements in the conditions of triaxial stress-strain state under dynamic impacts and taking into account the non-linearity of the rheology of deformation.
The paper concentrates on investigation results dealing with ultra high performance concrete (UHPC) dies for the use in sheet metal hydroforming. A special mixture of UHPC for these tools with its mechanical properties and its manufacturing is presented. The developed UHPC features a compressive strength of approx. 250 MPa and a Young's modulus of approx. 50 GPa. Applying internal pressure, the UHPC die (without confinement or reinforcement) failed between 80 and 96 MPa. The forming test proved a good shape accuracy of the sheet metal parts but revealed critical tribological conditions. Drawing foil and already formed sheet metal parts were tested successfully as interlayer and led to an appropriate material flow and surface quality of the sheet metal parts.
Treten mehraxiale Spannungszustände im Beton auf, haben diese einen signifikanten Einfluss auf das Materialverhalten. Neben der festigkeitssteigernden bzw. festigkeitsmindernden Wirkung beeinflussen derartige Spannungszustände auch das Spannungs‐Dehnungs‐Verhalten maßgeblich. Zur Ermittlung des Materialverhaltens von Beton unter mehraxialer Beanspruchung ist es daher notwendig, experimentelle Untersuchungen durchzuführen. Im Rahmen von mehraxialen Zug‐Druck‐Druck‐Versuchen an Ultrahochleistungsbeton wurde festgestellt, dass es bei Spannungsverhältnissen mit kleinen Zug‐ und großen Druckbeanspruchungen zu einem randnahen Versagen der Probekörper bzw. zu einem Ablösen der Zugkrafteinleitung an der Probekörperoberfläche kommt. Durch theoretische Überlegungen konnte gezeigt werden, dass die dabei erzielten Festigkeiten nicht den tatsächlichen Festigkeitswerten des Materials entsprechen können und eine Verbesserung der Ausbildung der Zugkraftübertragung vorgenommen werden muss. Nach der Entwicklung einer neuen Zugkrafteinleitungsmethode ergaben sich signifikant höhere mehraxiale Festigkeitswerte, die auch die theoretisch festgelegte Grenze überschreiten, welche anhand von Versuchswerten, bei denen eindeutig ein Materialversagen vorlag, definiert wurde.Tension load transfer to concrete at multiaxial loading testsMultiaxial stress states have a significant influence on the material behaviour of concrete. Besides a strength increasing or decreasing effect, such stress states lead to changes of the stress‐strain‐behaviour. For the investigation of the material behaviour of concrete under multiaxial loading it is necessary to conduct experimental studies. Within the scope of multiaxial tests with ultra high performance concrete with small tension in the one and high compression loads in the other two directions it was observed, that the concrete failed close to the surface where the tensile force was initiated or, respectively, that the load transfer device detach from the specimen's surface. Because of theoretical consideration it was clear, that the achieved strength values at the tests couldn't be the “real” material strength values. Therefore, the method of tension load transfer had to be improved. After the development of a new tension load transfer method, significant higher strength values could be obtained at the tests. These values also pass the theoretically prescribed limit for “real” material strength, which was determined on experimental strength values where clearly material failure occurred.
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