Uwe Mühlich

Ductile crack propagation by plastic collapse of the intervoid ligaments

In the present study ductile crack initiation and propagation is investigated by means of a micro-mechanical model under small-scale yielding conditions. Voids are resolved discretely in the fracture process zone where steep gradients occur during the loading history and are taken into accounted by a homogenized porous plasticity law elsewhere. The size of the region of discrete voids is not set a priori but is determined consistently. The results show that effective crack growth occurs by plastic collapse, i.e. purely geometric softening of the intervoid ligaments without incorporating material separation. Due to this mechanism a limit load exists coinciding with the maximum fracture toughness. In addition, it turns out that the shielding due to the growth of voids around the crack plane has a considerable influence on the computed R-curves compared to models neglecting this effect. Depending on the void arrangement a diffuse softening zone or even crack branching is observed. A comparison with experimental data from literature indicates that plastic collapse and the formation of diffuse zones of void growth are realistic mechanisms of ductile crack propagation.

The Influence of Varying Cell Sizes on the Tensile Behaviour of Open-Cell Foams

The aim of this paper is to study the mechanical tensile properties of open-cell foam structures on the cell size variation by numerical simulations. For this purpose, random Laguerre tessellations were used, which allow the creation of foam structures with strongly varying cell sizes. First, a model was fitted to real open-cell foams based on x-ray computed tomography (XCT) scans by parameter optimization. Two more virtual foam models with different cell size variations were produced according to the first fitted one. Tensile properties of the model realizations were computed by finite-element analysis using beam elements. The numerical results were presented and discussed.

Modified Key-Curve-Method for Determination of Dynamic Crack Resistance Curves

Dynamic J-†a curves might be constructed by means of the results of various low-blow tests, where the experimental settings of the instrumented Charpy-V test are chosen in order to assure significant crack growth due to impact loading on the one hand and in order to avoid complete rupture on the other hand. While the final crack advance †a f can be measured for every low-blow test, the corresponding values for J have to be estimated in general from the load-deflection curve using standard concepts of conventional elastic plastic fracture mechanics. The experimental effort could be significantly reduced if a single specimen test method were used instead of this multi-specimen testing. The aim of the present work is to qualify the Key-Curve-Method (KCM), originally proposed by Ernst et al. [1] for quasi-static fracture tests, for this purpose.