Damage process of a fiber bundle with a strain gradient

We study the damage process of fiber bundles in a wedge-shape geometry which ensures a constant strain gradient. To obtain the wedge geometry we consider the three-point bending of a bar, which is modelled as two rigid blocks glued together by a thin elastic interface. The interface is discretized by parallel fibers with random failure thresholds, which get elongated when the bar is bent. Analyzing the progressive damage of the system we show that the strain gradient results in a rich spectrum of novel behavior of fiber bundles. We find that for weak disorder an interface crack is formed as a continuous region of failed fibers. Ahead the crack a process zone develops which proved to shrink with increasing deformation making the crack tip sharper as the crack advances. For strong disorder, failure of the system occurs as a spatially random sequence of breakings. Damage of the fiber bundle proceeds in bursts whose size distribution shows a power law behavior with a crossover from an exponent 2.5 to 2.0 as the disorder is weakened. The size of the largest burst increases as a power law of the strength of disorder with an exponent 2/3 and saturates for strongly disordered bundles.Comment: 8 pages, 7 figures, accepted by PR

Correlated noise analysis and breakdown voltage distribution measurement, modelling of silicon photomultipliers

A 80-as évek végén Golovin és Sadygov által javasolt pixelizált Geiger-módban működő lavina fotodiódák, más néven a szilícium alapú fotoelektron-sokszorozók (silicon photomultiplier, SiPM), számos olyan tulajdonsággal bírnak, amelyek alkalmassá teszik őket a részecskefizikai, magfizikai, nukleáris képalkotó és asztrofizikai detektorokban való alkalmazásra. A SiPM-eket a nagy erősítésük és egy-foton (single photon) detektálási képességük miatt a fotoelektron-sokszorozó csövek (photomultiplier tube, PMT) egyik lehetséges helyettesítőjének tekintik. Értekezésemben két főbb területtel foglalkoztam: a SiPM korrelált-zajainak analízise és a SiPM letörési karakterisztikájának egyenáramú analízise.The pixelated Geiger-mode avalanche photodiode or silicon photomultiplier (SiPM) was proposed by Golovin and Sadygov in the late ’80s. This type of sensor is a promising candidate for applications in the field of high energy physics, nuclear physics, nuclear imagining and astrophysics. Due to their high gain and single photon detection capability SiPMs are considered as a possible substitution of photomultiplier tubes (PMT). I paid attention to the following two areas in my work: the analysis of the SiPM’s correlated noises and the DC analysis of the SiPM’s breakdown characteristics.N

Graph Transformations and Game Theory: A Generative Mechanism for Network Formation

Many systems can be described in terms of networks with characteristic structural properties. To better understand the formation and the dynamics of complex networks one can develop generative models. We propose here a generative model (named dynamic spatial game) that combines graph transformations and game theory. The idea is that a complex network is obtained by a sequence of node-based transformations determined by the interactions of nodes present in the network. We model the node-based transformations by using graph grammars and the interactions between the nodes by using game theory. We illustrate dynamic spatial games on a couple of examples: the role of cooperation in tissue formation and tumor development and the emergence of patterns during the formation of ecological networks

A model based DC analysis of SiPM breakdown voltages

A new method to determine the breakdown voltage of SiPMs is presented. It is backed up by a DC model which describes the breakdown phenomenon by distinct avalanche turn-on ($V_{01}$) and turn off ($V_{10}$) voltages. It is shown that $V_{01}$ is related to the 'breakdown voltage' that previous DC methods derive from simple reverse current-voltage measurements, while $V_{10}$ is the 'real' breakdown voltage commonly obtained from complex gain-voltage measurements. The proposed method reveals how the microcell population distributes around $V_{01}$ and $V_{10}$. It is found that if this distribution is assumed to be normal, then both voltages and even their standard deviation can readily be extracted from current-voltage curves. Measurements are in good agreement with the theoretical model

Efficiently parallelised algorithm to find isoptic surface of polyhedral meshes

The isoptic surface of a three-dimensional shape is defined in [1] as the generalization of isoptics of curves. The authors of the paper also presented an algorithm to determine isoptic surfaces of convex meshes. In [9] new searching algorithms are provided to find points of the isoptic surface of a triangulated model in E³. The new algorithms work for concave shapes as well. In this paper, we present a faster, simpler, and efficiently parallelised version of the algorithm of [9] that can be used to search for the points of the isoptic surface of a given closed polyhedral mesh, taking advantage of the computing capabilities of the high-performance graphics cards and using the benefits of nested parallelism. For the simultaneous computations, the NVIDIA’s Compute Unified Device Architecture (CUDA) was used. Our experiments show speedups up to 100 times using the new parallel algorithm