Slow and fast motion of cracks in inelastic solids. Part 1: Slow growth of cracks in a rate sensitive tresca solid. Part 2: Dynamic crack represented by the Dugdale model

An extension is proposed of the classical theory of fracture to viscoelastic and elastic-plastic materials in which the plasticity effects are confined to a narrow band encompassing the crack front. It is suggested that the Griffith-Irwin criterion of fracture, which requires that the energy release rate computed for a given boundary value problem equals the critical threshold, ought to be replaced by a differential equation governing the slow growth of a crack prior to the onset of rapid propagation. A new term which enters the equation of motion in the dissipative media is proportional to the energy lost within the end sections of the crack, and thus reflects the extent of inelastic behavior of a solid. A concept of apparent surface energy is introduced to account for the geometry dependent and the rate dependent phenomena which influence toughness of an inelastic solid. Three hypotheses regarding the condition for fracture in the subcritical range of load are compared. These are: (1) constant fracture energy (Cherepanov), (2) constant opening displacement at instability (Morozov) and (3) final stretch criterion (Wnuk)

Delayed fracture in viscoelastic-plastic solids

Delayed fracture in viscoelastic-plastic solids - assessment of load carrying ability of glass-like polymers having potential applications as structural material

Prior-to-failure extension of flaws under monotonic and pulsating loadings: Inelastic fatigue

An equation governing the prior to failure crack propagation is proposed. For a rate-sensitive solid containing two-dimensional crack and subject to the tensile mode of fracture, the differential equations are integrated numerically for the loads increasing monotonically in time. The resulting integral curves sigma = sigma(l) and l = l(t), i.e. load vs. crack length and length vs. time, indicate that the growth of cracks in the subcritical range is strongly rate dependent. The fatigue growth, viewed as a sequence of slow growth periods, is simulated on an EAI 380 analogue computer. The fourth power law proposed by Paris is confirmed only within a certain range of high-cycle fatigue propagation and for a rate-insensitive solid. For a more pronounced rate dependency, induced by the viscosity of a solid and/or in the proximity of the final instability point, the growth is markedly enhanced

Full 3D modelling of pulse propagation enables efficient nonlinear frequency conversion with low energy laser pulses in a single-element tripler

Although new optical materials continue to open up access to more and more wavelength bands where femtosecond laser pulses can be generated, light frequency conversion techniques are still indispensable in filling the gaps on the ultrafast spectral scale. With high repetition rate, low pulse energy laser sources (oscillators) tight focusing is necessary for a robust wave mixing and the efficiency of broadband nonlinear conversion is limited by diffraction as well as spatial and temporal walk-off. Here we demonstrate a miniature third harmonic generator (tripler) with conversion efficiency exceeding 30%, producing 246 fs UV pulses via cascaded second order processes within a single laser beam focus. Designing this highly efficient and ultra compact frequency converter was made possible by full 3-dimentional modelling of propagation of tightly focused, broadband light fields in nonlinear and birefringent media

Optimization of broadband semiconductor chirped mirrors with genetic algorithm

Genetic algorithm was applied for optimization of dispersion properties in semiconductor Bragg reflectors for applications in femtosecond lasers. Broadband, large negative group-delay dispersion was achieved in the optimized design: The group-delay dispersion (GDD) as large as −3500 fs2 was theoretically obtained over a 10-nm bandwidth. The designed structure was manufactured and tested, providing GDD −3320 fs2 over a 7-nm bandwidth. The mirror performance was verified in semiconductor structures grown with molecular beam epitaxy. The mirror was tested in a passively mode-locked Yb:KYW laser

Attosecond correlated electron dynamics at C₆₀ giant plasmon resonance

Fullerenes have unique physical and chemical properties that are associated with their delocalized conjugated electronic structure. Among them, there is a giant ultra-broadband - and therefore ultrafast - plasmon resonance, which for C60 is in the extreme-ultraviolet energy range. While this peculiar resonance has attracted considerable interest for the potential downscaling of nanoplasmonic applications such as sensing, drug delivery and photocatalysis at the atomic level, its electronic character has remained elusive. The ultrafast decay time of this collective excitation demands attosecond techniques for real-time access to the photoinduced dynamics. Here, we uncover the role of electron correlations in the giant plasmon resonance of C60 by employing attosecond photoemission chronoscopy. We find a characteristic photoemission delay of up to 200 attoseconds pertaining to the plasmon that is purely induced by coherent large-scale correlations. This result provides novel insight into the quantum nature of plasmonic resonances, and sets a benchmark for advancing nanoplasmonic applications

Investigation of reactions postulated to occur during inhibition of ribonucleotide reductases by 2 0 -azido-2 0 -deoxynucleotides

a b s t r a c t Model 3 0 -azido-3 0 -deoxynucleosides with thiol or vicinal dithiol substituents at C2 0 or C5 0 were synthesized to study reactions postulated to occur during inhibition of ribonucleotide reductases by 2 0 -azido-2 0 -deoxynucleotides. Esterification of 5 0 -(tert-butyldiphenylsilyl)-3 0 -azido-3 0 -deoxyadenosine and 3 0 -azido-3 0 -deoxythymidine (AZT) with 2,3-S-isopropylidene-2,3-dimercaptopropanoic acid or N-Boc-Strityl-L-cysteine and deprotection gave 3 0 -azido-3 0 -deoxy-2 0 -O-(2,3-dimercaptopropanoyl or cysteinyl) adenosine and the 3 0 -azido-3 0 -deoxy-5 0 -O-(2,3-dimercaptopropanoyl or cysteinyl)thymidine analogs. Density functional calculations predicted that intramolecular reactions between generated thiyl radicals and an azido group on such model compounds would be exothermic by 33.6e41.2 kcal/mol and have low energy barriers of 10.4e13.5 kcal/mol. Reduction of the azido group occurred to give 3 0 -amino-3 0 -deoxythymidine, which was postulated to occur with thiyl radicals generated by treatment of 3 0 -azido-3 0 -deoxy-5 0 -O-(2,3-dimercaptopropanoyl)thymidine with 2,2 0 -azobis-(2-methyl-2-propionamidine) dihydrochloride. Gamma radiolysis of N 2 O-saturated aqueous solutions of AZT and cysteine produced 3 0 -amino-3 0 -deoxythymidine and thymine most likely by both radical and ionic processes

Pressure-induced structural transformations in Si:V and Si:V, Mn

Semiconductors doped with magnetically active atoms are expected to find application in spintronics. Si samples implanted with Mn⁺ (Si:Mn) or with V⁺ (Si:V) can order magnetically after processing at high temperature (HT) and also under enhanced hydrostatic pressure (HP). This work presents new results on structure-related properties of single crystalline Si implanted at 200 keV with V⁺ as well as that co-implanted additionally with Mn⁺ ions (Si:V, Mn), with dosages DV⁺ ≤ 5·10¹⁵ cm⁻² and DMn⁺ = 1·10¹⁵ cm⁻². The samples were processed for 1–5 h at HT ≤ 1270 K under HP ≤ 1.1 GPa. Secondary Ion Mass Spectrometry, Transmission Electron Microscopy, X-ray and related methods were applied for sample characterization. The HT- (HP) treatment affects, among others, solid phase epitaxial re-growth (SPER) of amorphous silicon created at implantation and distribution of implanted species