A simple representation of crack tip plasticity: the inclined strip yield superdislocation model

The basic equations for the representation of crack tip plasticity by an inclined strip yield superdislocation model are derived. The results are put into a form suitable for the analysis of fatigue and other stable crack growth processes. Specialization of the general results for small-scale yielding under a fixed load is also given. The crack tip crack-opening displacement obtained for this situation is compared with finite element and other more rigorously obtained results and found to be in quite good agreement.

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Résumé On établit des équations de base pour la représentation de la plasticité à l'extrémité d'une fissure par un modèle de superdislocations associées à des bandes de glissement inclinées. Les résultats sont présentés sous une forme compatible avec l'analyse de la fatigue et d'autres processus de croissance stable d'une fissure. Une spécialisation des résultats généraux est également donnée pour le cas de glissements plastiques de faible étendue sous une charge déterminée. Le COD de l'extrémité de la fissure relatif à cette situation est comparé avec les résultats obtenus par éléments finis ainsi que par des méthodes plus rigoureuses. On constate un très bon accord avec ces mesures.

     

Post Correspondence Problem for short words

We prove that the generalized Post Correspondence Problem is undecidable for instances where the lengths of the image words are at most 2 and the number of pairs of words is at most 30. The proof uses undecidability of the word problem of the Tzeitin semigroup. We also transform our constructions in order to achieve a proof for the undecidability of the (not generalized) Post Correspondence Problem with image words of length at most 2.     

Analyses of PACTEL passive safety injection experiments GDE-21 through GDE-25

In advanced light water reactors (ALWR), gravity-driven passive safety injection systems (PSIS) replace pump-driven emergency core cooling systems. PSISs often rely on small density differences and driving forces for natural circulation. In a typical loss-of-coolant accident (LOCA), interactions between different parts of the emergency core cooling system also take place. VTT Energy in Finland, in co-operation with the Lappeenranta University of Technology (LUT), performed five experiments in the PACTEL loop to study PSIS performance during SBLOCAs. The purpose of the PSIS, a passive core make-up tank (CMT), was to provide high-pressure safety injection water to the primary circuit. The purpose of these experiments was to produce data to validate the current thermal-hydraulic safety codes, and to study the effects of break size on the PSIS behaviour. In all experiments the CMT ran as planned. No problems with rapid condensation in the CMT, as seen in earlier passive safety injection experiments in PACTEL. The main reason was the new CMT arrangement, with a flow distributor (sparger) installed. The analyses of the test data supported the use of McAdams correlation for calculating the heat transfer from the hot liquid layer to the CMT wall. The use of Nusselt film condensation correlation for condensation at the CMT walls seems correct. The APROS code simulated successfully the overall primary system behaviour in the GDE-24 experiment, such as timing of the core heat-up at the end of the experiment. The code had some problems, in the simulation of thermal stratification in the CMT.     

Dynamic fracture toughness parameters for HY-80 and HY-130 steels and their weldments

Lower bound dynamic fracture toughness parameters for HY-80 and HY-130 steel and their weld metals are identified. Specific values of the parameters K_Id and K_Im obtained from direct measurements are reported together with estimates inferred from the large body of Charpy energy, nil ductility transition temperature and dynamic tear energy measurements. The emphasis is on reasonable lower bound values at 30°F, the lowest anticipated service temperature, for use in elastodynamic analyses of crack growth initiation, propagation, and arrest in ship structures. For these conditions, it has been found that the ratio K_Id/ σ_Y is approximately equal to 2 in^1/2 for HY-80 steel. For HY-130 steel and the HY-80 and HY-130 weld metals under these same conditions, K_Id/ σ_Y is approximately 1 in^1/2. Consequently, HY-80 plate appears to be substantially more resistant to fracture under dynamic loading than are the other three grades examined.     

Growth of immobilized DNA by polymerase: bridging nanoelectrodes with individual dsDNA molecules

We present a method for controlled connection of gold electrodes with dsDNA molecules (locally on a chip) by utilizing polymerase to elongate single-stranded DNA primers attached to the electrodes. Thiol-modified oligonucleotides are directed and immobilized to nanoscale electrodes by means of dielectrophoretic trapping, and extended in a procedure mimicking PCR, finally forming a complete dsDNA molecule bridging the gap between the electrodes. The technique opens up opportunities for building from the bottom-up, for detection and sensing applications, and also for molecular electronics.     

Determining electrical efficiency of permanent magnet synchronous machines with different control methods

An algorithm to calculate the electrical efficiency of permanent magnet synchronous machines with four different control methods is presented. The direct- and quadrature-axis stator current components are optimized to find the control method producing the maximum electrical efficiency. The developed computation algorithm was tested with three different permanent magnet synchronous machine applications, i.e., a low-speed high-torque direct-driven wind power generator with rotor surface magnets, a hybrid drive for an energy recovery system and a geared wind power generator with rotor buried magnets. The performance of the developed computation algorithm was verified by comparing the calculated electrical efficiencies with the measurements in the case of the hybrid drive machine.     

Environmental protection of inkjet-printed Ag conductors

Electrically conductive silver nanoparticle ink patterns were fabricated using the inkjet printing method. Two different polymer films were used as the substrate materials. The patterns were exposed to humidity and salt fog and the electrical performance (sheet resistance and RF performance) as well as mechanical endurance (adhesion) were measured before and after the environmental tests. The electrical properties of the printed structures remained good in all the measurable samples. The adhesion between the ink and a substrate material appeared to be a greater challenge in harsh environments. Protection capabilities of one dip coated and one hot laminated barrier materials were evaluated during the environmental tests. The results showed that there is a need for environmental protection in printed electronics. Especially the laminated barrier films can offer a potential solution for shielding printed electronics in harsh environments as they can provide good mechanical protection, and can easily be integrated in roll-to-roll process.     

A viscoelastic fracture mechanics assessment of slow crack growth in polyethylene gas distribution pipe materials

A substantial amount of the plastic gas distribution pipe currently in service, as well as that anticipated for future use, is polyethylene (PE). While this material has an excellent record, due to a variety of abnormal loadings that can arise in long time service, some slow crack growth (SCG) related field failures have occurred. Accelerated test procedures and analysis methodologies to predict accurately the long term performance of PE gas pipes are therefore required. One such procedure is the three-point bend test conducted on a pre-cracked pipe segment that is known as the SCG test. The research reported here examines the validity of this test through the application of advanced fracture mechanics principles. Specifically, considering the viscoelastic behavior of PE materials and using the region of dominance concept, the delimitation of a linear elastic fracture mechanics (LEFM) interpretation of the SCG test was developed. Excluding the time spent in crack initiation, the long-term performance of the category of materials for which the assumptions of LEFM are valid can therefore be effectively predicted from SCG test results. Through illustrative examples, this paper demonstrates the veracity of the approach by comparing time to failure predictions for abnormal service loadings with actual gas distribution service experiences.