Study and characterization of localization and failure behaviour of ultra high strength steel
Abstract (Summary)In the vehicle industry there is a constant struggle to develop cars with high passive safety without increasing the fuel consumption. High passive safety requires a very rigid behaviour of the crash protecting components. Accomplishing this often leads to an increase in the weight of the components. An increase in weight results in a higher fuel consumption which is bad for the environment as well as for the economy of the car owner, therefore the manufacturers turn to new materials. One of these new materials is ultra high strength steel which is the material in focus for the present thesis. To be able to utilize all the advantages of ultra high strength steel the material behaviour must be investigated in detail. In this thesis, sheets of ultra high strength steel, which are produced by press-hardening, are investigated using a method called digital speckle photography. When using the method of digital speckle photography (DSP) a series of photographs are taken of a deforming specimen. Prior to the experiment a random pattern (speckles) has been applied to the specimen and by studying the deforming speckle pattern on the images, the deformation fields through time can be established. Within the present thesis the deformation fields up to the point of fracture have been investigated on a length scale of the order of 10e-4 meters. With length scales of this magnitude the deformation inside a localized neck can be investigated. This is done, both for a specimen shape that induces a fracture initiation at an inner point of the specimen and a specimen shape where fracture starts from the edge of a hole. These investigations show that there is a strong localization of the strain before fracture is initiated. The local strain values inside a localized neck are significantly higher than the strain values that can be observed with conventional experimental techniques involving extensometers. It is also noticed that the method used to make holes play an important role for the onset of fracture. Some methods hardly affect the material at all while others can decrease the level of local strain at the onset of fracture down to about a third of the value for unaffected material. Furthermore, a method for characterizing the material based on full-field measurements is presented. The method is a fast and simple alternative to previously used inverse modelling procedures where the material model of a finite element simulation is updated iteratively to make the simulation produce the same results as the experiment.
School:Luleå tekniska universitet
Source Type:Master's Thesis
Date of Publication:01/01/2007