Co-detection of acoustic emissions during failure of heterogeneous media: new perspectives for natural hazard early warning

A promising method for real time early warning of gravity driven rupture that considers both the heterogeneity of natural media and characteristics of acoustic emissions attenuation is proposed. The method capitalizes on co-detection of elastic waves emanating from micro-cracks by multiple and spatially separated sensors. Event co-detection is considered as surrogate for large event size with more frequent co-detected events marking imminence of catastrophic failure. Using a spatially explicit fiber bundle numerical model with spatially correlated mechanical strength and two load redistribution rules, we constructed a range of mechanical failure scenarios and associated failure events (mapped into AE) in space and time. Analysis considering hypothetical arrays of sensors and consideration of signal attenuation demonstrate the potential of the co-detection principles even for insensitive sensors to provide early warning for imminent global failure

Weak layer fracture: facets and depth hoar

Understanding failure initiation within weak snow layers is essential for modeling and predicting dry-snow slab avalanches. We therefore performed laboratory experiments with snow samples containing a weak layer consisting of either faceted crystals or depth hoar. During these experiments the samples were loaded with different loading rates and at various tilt angles until fracture. The strength of the samples decreased with increasing loading rate and increasing tilt angle. Additionally, we took pictures of the side of four samples with a high-speed video camera and calculated the displacement using a particle image velocimetry (PIV) algorithm. The fracture process within the weak layer could thus be observed in detail. Catastrophic failure started due to a shear fracture just above the interface between the depth hoar layer and the underlying crust

Konfliktprävention und Krieg

Die vorliegende Arbeit beschäftigt sich mit Annahmen und Prämissen in Bezug auf Krieg, Konflikt und deren Prävention bzw. dem Umgang mit dem Phänomen. Im theoretischen Teil der Arbeit werden grundlegende Theorien und Thesen der Sozialwissenschaften eruiert, analysiert und miteinander verglichen. Aufgrund der weitreichenden wissenschaftlichen Landschaft, die zu dieser Problematik existiert wurde versucht, eine möglichst breite Palette von unterschiedlichen Theorien aufzuarbeiten. In einem Exkurs werden auch Theorien und Untersuchungen aus dem Bereich der Psychologie untersucht und fließen in die Überlegungen mit ein. Der praktische Teil der Arbeit bezieht sich auf jene Annahmen, die in offiziellen Dokumenten der Österreichischen Entwicklungszusammenarbeit (OEZA) und in entsprechenden Dokumenten des Development Assistance Committee (DAC), welches u.a. entwicklungspolitische Empfehlungen und Richtlinien für Geberstaaten wie Österreich, herausgibt, getroffen werden. Im letzten Teil der Arbeit werden diese beiden Teile zu einem Ganzen zusammengefügt und es wird analysiert, inwiefern sich die untersuchten Theorien in den Annahmen der OEZA und des DAC wiederspiegeln.The following master thesis deals with the assumptions and premises concerning war, conflict and how to prevent, respectively manage these phenomena. The theoretical part identifies, analyzes and compares theories and assumptions of social sciences. Due the wide range of publications and authors dealing with the problem, this work attempts to present and work with a broad spectrum of different theories. Theories and ideas from the field of psychology are also being examined and will be used in the analysis. The praxis part of this thesis focuses on the assumptions which are presented in the official documents of the Austrian Development Cooperation (OEZA) and in the relevant documents of the Development Assistance Committee (DAC), which gives advice and guidelines concerning development policy to donor states like Austria. The final part of the master thesis constitutes a synthesis between theory and praxis and thus analyzes if, how and which theory can be found in the assumptions of the OEZA and the DAC

Stress measurements in the weak layer during snow stability tests

The snow compression test is a snow stability test where an isolated column of snow is progressively loaded by tapping on it to induce failure in a possible weak layer. The test result provides valuable information about the propensity of failure initiation within the snowpack. However, different persons might tap with different force and thus reduce the reproducibility of the test results. The aim of this work was to quantify the influence of different test persons and different snowpacks on snow compression test results. We therefore let 62 persons tap on a stress measurement plate during a workshop of the European Avalanche Warning Services. Moreover, in the field, we performed stress measurements during 116 snow compression tests with 13 persons at eight different locations in the Alps. Data on persons’ body features and snow properties were also collected. Our results show that the stresses that reach a weak snow layer due to tapping are influenced by both the snowpack as well as different persons. Still, the data’s scattering is surprisingly small for lower loading steps and decreases with depth. Therefore, we can deduce that, especially when avalanche conditions are particularly dangerous, snow compression test results are quite reproducible

A new mixed-mode failure criterion for weak snowpack layers

The failure of a weak snow layer is the first in a series of processes involved in dry‐snow slab avalanche release. The nature of the initial failure within the weak layer is not yet fully understood but widely debated. The knowledge of the failure criterion is essential for developing avalanche release models and hence for avalanche hazard assessment. Yet different release models assume contradictory criteria as input parameters. We analyzed loading experiments on snow failure performed in a cold laboratory with samples containing a persistent weak snow layer of either faceted crystal, depth hoar, or buried surface hoar. The failure behavior of these layers can be described well with a modified Mohr‐Coulomb model accounting for the possible compressive failure of snow. We consequently propose a new mixed‐mode shear‐compression failure criterion that can be used in avalanche release models

Fiber-bundle model with time-dependent healing mechanisms to simulate progressive failure of snow

Snow is a heterogeneous material with strain- and/or load-rate-dependent strength. In particular, a transition from ductile-to-brittle failure behavior with increasing load rate is observed. The rate-dependent behavior can partly be explained with the existence of a unique healing mechanism in snow that stems from its high homologous temperature (temperature close to melting point). As soon as broken elements in the ice matrix get in contact, they start sintering and the structure may regain strength. Moreover, the ice matrix is subjected to viscous deformation, inducing a relaxation of local load concentrations and, therefore, further counteracting the damage process. Ideal tools for studying the failure process of heterogeneous materials are the fiber-bundle models (FBMs), which allow investigating the effects of basic microstructural characteristics on the general macroscopic failure behavior. We present an FBM with two concurrent time-dependent healing mechanisms: sintering of broken fibers and relaxation of load inhomogeneities. Sintering compensates damage by creating additional intact, load-supporting fibers which lead to an increase of the bundle strength. However, the character of the failure is not changed by sintering alone. With combined sintering and load relaxation, load is distributed from old stronger fibers to new fibers that carry fewer load. So as we additionally incorporated load redistribution to the FBM, the failure occurred suddenly without decrease of the order parameter—describing the amount of damage in the bundle—and without divergence of the fiber failure rate. Moreover, the b value, i.e., the power-law exponent of frequency-magnitude statistics of fibers breaking in load redistribution steps, at failure converged to b≈2, a value higher than that of a classical FBM without healing (b=32). These results indicate that healing, as the combined effect of sintering and load relaxation, changes the type of the phase transition at failure. This change of the phase transition is important for quantifying or predicting the failure (e.g., by monitoring acoustic emissions) of snow or other materials for which healing plays an important role.ISSN:1539-3755ISSN:1063-651XISSN:1095-3787ISSN:1550-237