New universality class for the fragmentation of plastic materials

We present an experimental and theoretical study of the fragmentation of polymeric materials by impacting polypropylene particles of spherical shape against a hard wall. Experiments reveal a power law mass distribution of fragments with an exponent close to 1.2, which is significantly different from the known exponents of three-dimensional bulk materials. A 3D discrete element model is introduced which reproduces both the large permanent deformation of the polymer during impact, and the novel value of the mass distribution exponent. We demonstrate that the dominance of shear in the crack formation and the plastic response of the material are the key features which give rise to the emergence of the novel universality class of fragmentation phenomena.Comment: 4 pages, 4 figures, appearing in Phys. Rev. Let

Universality class of the fragmentation of plastic materials

We carry out an experimental and theoretical study of the fragmentation of polymeric materials by impacting polypropylene (PP) particles of spherical shape against a hard wall. Our experiments revealed that the mass distribution of fragments has a power law behavior with an exponent close to 1.2, which is significantly different from the known exponents of threedimensional bulk materials. To understand the fragmentation of plastic materials we developed a threedimensional discrete element model where the sample is represented as a random packing of spherical particles connected by elastic beams. The model reproduces both the large permanent deformation of the polymer during impact, and the novel value of the mass distribution exponent. Computer simulations revealed that the dominance of shear in the crack formation and the healing of compressed crack surfaces are the key features which give rise to the emergence of the novel universality class of fragmentation phenomena

Universality class of the fragmentation of plastic materials

We carry out an experimental and theoretical study of the fragmentation of polymeric materials by impacting polypropylene (PP) particles of spherical shape against a hard wall. Our experiments revealed that the mass distribution of fragments has a power law behavior with an exponent close to 1.2, which is significantly different from the known exponents of threedimensional bulk materials. To understand the fragmentation of plastic materials we developed a threedimensional discrete element model where the sample is represented as a random packing of spherical particles connected by elastic beams. The model reproduces both the large permanent deformation of the polymer during impact, and the novel value of the mass distribution exponent. Computer simulations revealed that the dominance of shear in the crack formation and the healing of compressed crack surfaces are the key features which give rise to the emergence of the novel universality class of fragmentation phenomena

Formal Security Analysis and Performance Evaluation of the Linkable Anonymous Access Protocol

Part 2: The 2014 Asian Conference on Availability, Reliability and Security, AsiaARES 2014International audienceThe introduction of e-Health applications has not only brought benefits, but also raised serious concerns regarding security and privacy of health data. The increasing demands of accessing health data, highlighted critical questions and challenges concerning the confidentiality of electronic patient records and the efficiency of accessing these records. Therefore, the aim of this paper is to provide secure and efficient access to electronic patient records. In this paper, we propose a novel protocol called the Linkable Anonymous Access protocol (LAA). We formally verify and analyse the protocol against security properties such as secrecy and authentication using the Casper/FDR2 verification tool. In addition, we have implemented the protocol using the Java technology to evaluate its performance. Our formal security analysis and performance evaluation proved that the LAA protocol supports secure access to electronic patient records without compromising performance

Anonymity and Rewards in Peer Rating Systems

When peers rate each other, they may choose to rate inaccurately in order to boost their own reputation or unfairly lower another’s. This could be successfully mitigated by having a reputation server incentivise accurate ratings with a reward. However, assigning rewards becomes a challenge when ratings are anonymous, since the reputation server cannot tell which peers to reward for rating accurately. To address this, we propose an anonymous peer rating system in which users can be rewarded for accurate ratings, and we formally define its model and security requirements. In our system ratings are rewarded in batches, so that users claiming their rewards only reveal they authored one in this batch of ratings. To ensure the anonymity set of rewarded users is not reduced, we also split the reputation server into two entities, the Rewarder, who knows which ratings are rewarded, and the Reputation Holder, who knows which users were rewarded. We give a provably secure construction satisfying all the security properties required. For our construction we use a modification of a Direct Anonymous Attestation scheme to ensure that peers can prove their own reputation when rating others, and that multiple feedback on the same subject can be detected. We then use Linkable Ring Signatures to enable peers to be rewarded for their accurate ratings, while still ensuring that ratings are anonymous. Our work results in a system which allows for accurate ratings to be rewarded, whilst still providing anonymity of ratings with respect to the central entities managing the system

Unemployment effects of the German minimum wage in an equilibrium job search model

We structurally estimate an equilibrium search model using German administrative data and use this for counterfactual analyses of a uniform minimum wage. The model with worker and firm heterogeneity does not restrict the sign of employment effects a priori and allows for different job offer arrival rates for the employed and the unemployed. We find that unemployment is a non-monotonic function of the minimum wage level. Effects differ strongly by labour market segment. Cross-segment variation of the estimated effects is mostly driven by firm productivity levels rather than by search frictions or the opportunity cost of employment

Who let the DOGS out: Anonymous but Auditable communications using Group Signature schemes with Distributed Opening

Over the past two decades, group signature schemes have been developed and used to enable authenticated and anonymous peer-to-peer communications. Initial protocols rely on two main authorities, Issuer and Opener, which are given substantial capabilities compared to (regular) participants, such as the ability to arbitrarily identify users. Building efficient, fast, and short group signature schemes has been the focus of a large number of research contributions. However, only a few dealt with the major privacy-preservation challenge of group signatures; this consists in providing user anonymity and action traceability while not necessarily relying on a central and fully trusted authority. In this paper, we present DOGS, a privacy-preserving Blockchain-supported group signature scheme with a distributed Opening functionality. In DOGS, participants no longer depend on the Opener entity to identify the signer of a potentially fraudulent message; they instead collaborate and perform this auditing process themselves. We provide a high-level description of the DOGS scheme and show that it provides both user anonymity and action traceability. Additionally, we prove how DOGS is secure against message forgery and anonymity attacks