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Intergranular fracture in polycrystals is often simulated by finite elements coupled to a cohesive zone model for the interfaces, requiring cohesive laws for grain boundaries as a function of their geometry. We discuss three challenges in understanding intergranular fracture in polycrystals. First, 3D grain boundary geometries comprise a five-dimensional space. Second, the energy and peak stress of grain boundaries have singularities for all commensurate grain boundaries, especially those with short repeat distances. Thirdly, fracture nucleation and growth depend not only upon the properties of grain boundaries, but also in crucial ways on edges, corners and triple junctions of even greater geometrical complexity. To address the first two challenges, we explore the physical underpinnings for creating functional forms to capture the hierarchical commensurability structure in the grain boundary properties. To address the last challenge, we demonstrate a method for atomistically extracting the fracture properties of geometrically complex local regions on the fly from within a finite element simulation.

Surveillance RADARs normally share time for detecting fixed and moving targets. With modern digital waveform synthesis and high performance computing, it is feasible to collect signals that use space, time and frequency encoding to simultaneously operate in GMTI and SAR modes. This paper summarizes a new waveform and adaptive RADAR architecture with the potential for simultaneous SAR and GMTI operation, along with a detailed simulation of target detections.

Fusion cross sections have been measured for the system $^{16}\mathrm{O}$${+}^{144}$Nd at bombarding energies in the range 67 MeV\ensuremath{\le}${\mathit{E}}_{\mathrm{lab}}$\ensuremath{\le}90 MeV by detecting directly evaporation residues in Si detectors and in the range 60 MeV\ensuremath{\le}${\mathit{E}}_{\mathrm{lab}}$\ensuremath{\le}75 MeV by off-line detection of the K x rays emitted by the radioactive evaporation residues and daughters. In order to obtain the spin distributions in the compound system gamma-ray multiplicity distributions for the most important neutron evaporation channels were also measured using a 4\ensuremath{\pi} ${\mathrm{BaF}}_{2}$ array, in conjunction with Ge detectors. Results are compared with calculations based on models that consider fluctuations in barrier height due to ground-state zero-point vibrations as well as couplings to various inelastic and transfer channels.

The fatigue resistance of metallic structures is inherently random due to environmental and boundary conditions, and microstructural geometry, including discontinuities, and material properties. A new methodology for fatigue life prediction is under development to account for these sources of randomness. One essential aspect of the methodology is the ability to perform truly multiscale simulations: simulations that directly link the boundary conditions on the structural length scale to the damage mechanisms of the microstructural length scale. This presentation compares and contrasts two multiscale methods suitable for fatigue life prediction. The first is a brute force method employing the widely-used multipoint constraint technique which couples a finite element model of the microstructure within the finite element model of the structural component. The second is a more subtle, modified multi-grid method which alternates analyses between the two finite element models while representing the evolving microstructural damage. Examples and comparisons are made for several geometries and preliminary validation is achieved with comparison to experimental tests conducted by the Northrop Grumman Corporation on a wing-panel structural geometry.Copyright © 2009 by ASME

The integral production probability of atomic positrons associated with elastic heavy ion collisions at beam energies from 5.9 to 10 MeV/u has been investigated as a function of the characteristic collision time t and with respect to its dependence on the combined nuclear charge Z united . Recent data from the systems Pb+Pb at 8.6 MeV/u and Pb+U at 8.4 MeV/u exhibit deviations from coupled-channels calculations: The experimental yield increases stronger towards shorter collision times than predicted.

It has been observed during fatigue cracking of AA 7075-T651 that a small percentage of Al7Cu2Fe particles crack during manufacturing or very early in their life. Some of the cracked particles eventually nucleate cracks into the surrounding microstructure, and among these the number of cycles required for nucleation varies widely. It is important to comprehend the mechanics underpinning the observed variation so that the subsequent propagation stage can be accurately modeled. To this end, finite element models of replicated grain and particle geometry are used to compute mechanical fields near monitored cracked particles using an elastic–viscoplastic crystal plasticity model that captures the effect of the orientation of the grains near each monitored particle. Nonlocal, slip-based metrics are used to study the localization and cyclic accumulation of slip near the cracked particles providing mechanics-based insight into the actuation of the nucleation event. A high slip localization and cyclic accumulation rate are found to be a necessary, but not sufficient, condition for nucleation from cracked particles. A sufficient local driving stress must also be present, which is strongly dependent on the local microstructure and accumulated slip. Furthermore, the simulation results elucidate a quantitative relationship between the slip accumulated during fatigue loading and a consequential reduction of the critical local driving stress for nucleation, providing a physical basis for the fatigue damage concept. The observed nucleation direction is orthogonal to the computed local maximum tangential stress direction, as expected for this alloy. The main result is a semi-empirical model for the number of cycles required for nucleation, which is dependent on the maximum tangential stress and cyclic slip-accumulation rate near a cracked particle.

The objective of this paper is to develop further a framework for computationally modeling microstructurally small fatigue crack growth in AA 7075-T651 (Bozek et al 2008 Modelling Simul. Mater. Sci. 16 065007). The focus is on the nucleation event, when a crack extends from within a second-phase particle into a surrounding grain, since this has been observed to be an initiating mechanism for fatigue crack growth in this alloy. It is hypothesized that nucleation can be predicted by computing a non-local nucleation metric near the crack front. The hypothesis is tested by employing a combination of experimentation and finite element modeling in which various slip-based and energy-based nucleation metrics are tested for validity, where each metric is derived from a continuum crystal plasticity formulation. To investigate each metric, a non-local procedure is developed for the calculation of nucleation metrics in the neighborhood of a crack front. Initially, an idealized baseline model consisting of a single grain containing a semi-ellipsoidal surface particle is studied to investigate the dependence of each nucleation metric on lattice orientation, number of load cycles and non-local regularization method. This is followed by a comparison of experimental observations and computational results for microstructural models constructed by replicating the observed microstructural geometry near second-phase particles in fatigue specimens. It is found that orientation strongly influences the direction of slip localization and, as a result, influences the nucleation mechanism. Also, the baseline models, replication models and past experimental observation consistently suggest that a set of particular grain orientations is most likely to nucleate fatigue cracks. It is found that a continuum crystal plasticity model and a non-local nucleation metric can be used to predict the nucleation event in AA 7075-T651. However, nucleation metric threshold values that correspond to various nucleation governing mechanisms must be calibrated.

Microstructurally small fatigue crack (MSFC) formation includes stages of incubation, nucleation and microstructurally small propagation. In AA 7075-T651, the fracture of Al7Cu2Fe constituent particles is the major incubation source. In experiments, it has been observed that only a small percentage of these Fe-bearing particles crack in a highly stressed volume. The work presented here addresses the identification of the particles prone to cracking and the prediction of particle cracking frequency, given a distribution of particles and crystallographic texture in such a volume. Three-dimensional elasto-viscoplastic finite element analyses are performed to develop a response surface for the tensile stress in the particle as a function of the strain level surrounding the particle, parent grain orientation and particle aspect ratio. A technique for estimating particle strength from fracture toughness, particle size and intrinsic flaw size is developed. Particle cracking is then determined by comparing particle stress and strength. The frequency of particle cracking is then predicted from sampling measured distributions of grain orientation, particle aspect ratio and size. Good agreement is found between the predicted frequency of particle cracking and two preliminary validation experiments. An estimate of particle cracking frequency is important for simulating the next stages of MSFC formation: inserting all particles into a microstructural model for these stages is computationally intractable and physically unnecessary.

The feeding pattern of the superdeformed yrast band in 149 Gd has been found to remain essentially the same, using the 124 Sn+ 30 Si reaction at beam energies of 150, 155 and 160 MeV leading to different input angular momenta and excitation energies in the compound nuclear system. This similarity as well as the relative yields of the superdeformed states as a function of beam energy may be understood in terms of the nuclear level density dependence on deformation and the onset of fission. In addition, it has been shown that, at all three beam energies, the selection of high-energy gamma rays does not enhance the yield of the superdeformed yrast states.

Fusion excitation functions for the systems 12 C + 46, 48, 50 Ti, 28, 30 Si + 30 Si and 18 O + 44 Ca have been determined at energies near and below the Coulomb barrier. Inelastic excitation functions for the targets 48, 50 Ti and 44 Ca have been also measured in the same energy range. The absolute cross sections were obtained by in-beam γ-ray spectroscopy technique using a rotating target. Fusion cross sections ranging in magnitude from ∼ 0.3 mb to ∼ 1300 mb were determined with an accuracy of 10–20%. The fusion excitation functions are analysed in the frame of a semiclassical barrier-penetration model. From the analysis, the height, the radius and the curvature of the fusion barrier for the different systems are extracted. The fusion cross sections are compared with the calculations performed using different heavy-ion potentials. The enhancement of the cross sections at sub-Coulomb energies can be reproduced with a one-dimensional barrier-penetration model taking into account the zero-point motion of the surface of the reaction partners. The fusion cross section of the system 18 O + 44 Ca is well reproduced by quantum-mechanical calculations, introducing a new degree of freedom taking into account the formation of a neck during the fusion process.

Nuclear g -factors have been measured for the I π = 10 + isomers in 138 Ce and 140 Nd by the pulsed-beam time-differential spin-rotation method. The 3538 keV level in 138 Ce and 3621 keV level in 140 Nd were excited by the 130 Te( 12 C, 4nγ) and 128 Te( 16 O, 4nγ) reactions, respectively. The nuclear g -factors of these 10 + states were measured to be g = −0.176(10) and g = −0.192(12) for 138 Ce and 140 Nd respectively. These values are well explained by a two neutron-hole h 11 2 −2 configuration for these states.

Yrast states of $^{160}\mathrm{Yb}$ with spins up to $30\ensuremath{\hbar}$ were identified in the reaction $^{148}\mathrm{Sm}$($^{16}\mathrm{O}$,$4n\ensuremath{\gamma}$). In addition to an initial backbend at $I=12\ensuremath{\hbar}$, a second backbend occurs at $I=28\ensuremath{\hbar}$. This result is discussed in terms of recently proposed explanations of the backbending effect.

Excitation energies, spins, and parities have been determined for /sup 152/Dy in (HI,xn) reactions up to I/sup ..pi../ = 27/sup +/. Three isomers with T/sub 1/2/ = 49.5 ns (E/sub x/ = 5035 keV, I/sub ..pi../ = 16/sup +/), 9.9 ns (E/sub x/ = 6076 keV, I/sup ..pi../ = 20/sup -/), and 1.6 ns (E/sub x/ = 7828 keV, I/sup ..pi../ = 26/sup -/) have been found. The g factor of the second isomeric state was measured to be g = 0.55 +- 0.06. The present experimental data compares well with microscopic calculations which imply an oblate shape for /sup 152/Dy at high angular momenta (I/sup ..pi../ > or = 16/sup +/).

The δ-electron spectra from binary dissipative PbPb collisions at 8.6 MeV/u incident energy exhibit the influence of an interference pattern due to time delay caused by nuclear contact. A model-independent analysis of these δ-electron spectra is developed to yield the mean trajectories of the colliding nuclei.

The rotation of the angular distribution pattern of the 524 keV γ-radiation from the 6+, 3200 keV state in 50Ti was measured in an external magnetic field using the 48Ca(α, 2n) reaction. The IPAD method was applied. From the experimental precession angle, the value of the g-factor, g = 1.57 ± 0.17, has been derived.

In addition to the known (f/sub 7/2/)/sup n/ multiplets, states with spins ranging up to 10/sup +/ (11/sup +/) have been identified in /sup 5/0Ti, /sup 5/2Cr, and /sup 5/4Fe via (..cap alpha..,2n..gamma..) reactions. Shell-model calculations including excitations of one particle from the N = 28 core successfully account for the newly observed states and indicate their 2p/sub 3/2/ neutron character in /sup 5/0Ti and /sup 5/2Cr. The experimental results also require configuration mixing of about 20% for the low-lying f/sub 7/2/ states.

The 10.23 and 10.29 MeV 32 S levels were observed through the reaction 28 Si( α , γ ) 32 S ( E α = 3.753 and 3.820 MeV). The decay scheme has been studied using NaI(Tl) and Ge(Li) detectors. Angular correlation measurements determine the spin of the 4.465 MeV level as J = 4. Branching ratios and multiple mixing ratios are given. The 10.23 and 10.29 MeV levels of 32 S might be a split isobaric analogue state, since both levels have the same spin ( J π = 3 − ), and very similar branching ratios with strong branches to the relatively highly excited state with J π = 3 − at 5.012 MeV. Moreover, the 3 − → 3 − transitions are strong (≧ 0.3 W.u. and ≧ 0.1 W.u. ) and have pure M1 character. The parent state might be the 3.324 MeV level of 32 P.

Sixteen resonances were observed in the reaction 28 Si(α, γ) 32 S in the range E α = 4.2–6.0 MeV. Spins and parities have been determined by angular distribution and correlation measurements. The lifetime of the 4.29 MeV levels of 32 S was deduced from a Doppler-shift attenuation measurement as τ m = 94±30 fs. The transition strengths, deduced from measured resonance strengths and branching ratios, exhibit the expected inhibition of dipole transitions in self-conjugated nuclei.

Eighteen resonances were observed in the reaction 34S(α,γ)38Ar, in the range Eα = 3.6–4.8 MeV. Spins and parities have been determined by angular distribution and correlation measurements. The measured transition strengths and branching ratios are discussed.

Measurements of the time dependence of the angular correlation anisotropy for 133–482 keV γ-γ cascade in 181Ta were carried out. An exponential attenuation of the anisotropy was found in the case of Ta ions in concentrated sulphuric acid. The experimental values of the attenuation constant λ2 for different temperatures are compared with the theory.

The hyperfine field at Ta nuclei diffused into a nickel foil was studied by the PAC method both with and without and external polarizing magnetic field. The value Hhf = (−82.6 ± 1.1) KG is smaller than the reported one of 90 kG, which could be explained by the large field distribution observed. For assumed Lorentzian shape of the field distribution FWHM (δ) = 22%. Es wurde das Hyperfeinfeld an Ta-Kernen, die in eine Nickelschicht eindiffundiert wurden, mittels der PAC-Methode ohne Feld und mit aserem, polarisierendem Magnetfeld untersucht. Der Wert Hhf = (−82,6 ± 1,1) kG ist kleiner als der berichtete von 90 kG, was durch die grose beobachtete Feldverteilung erklart werden kann. Fur eine angenommene Lorentzform der Feldverteilung ergibt sich FWHM (δ) = 22%.

To study the lowest levels of 58 Co, nγ angular correlations (method II) and g -factor and lifetime measurements have been performed using the 58 Fe(p, nγ). 58 Co reaction. The lifetime of the 111.5 keV level was found to be T 1 2 = 0.18±0.03 ns . The lifetime of the 53.2 keV level was remeasured and found to be T 1 2 = 10.4±0.3 μ s . The g -factor of 111.5 keV state was determined to be g = +0.74±0.13 using the IPAD method. Branching and mixing ratios have also been obtained. Values of J π = 4 + and 3 + were assigned to the levels at 53.2 keV and 111.5 keV, respectively.

An extensive study of the decay of the compound nuclei 58,60,62 Ni formed in the reactions ( 12 C + 46 Ti), ( 28 Si + 30 Si), ( 12 C + 48 Ti), ( 30 Si + 30 Si), ( 12 C + 50 Ti), ( 18 O + 44 Ca) was undertaken in the energy range ~ 0.7 to ~ 1.6 Coulomb barrier in about 20 steps. The fusion cross sections are fragmented in a large number of exit channels, with cross sections from 0.1 to 430 mb. By means of in-beam γ-ray measurements, the evaporation residues were identified both in A and Z . This large data base was compared with fusion-evaporation calculations through individual residue excitation functions and summed evaporation-residue cross sections (resulting from nucleons or α-emission). The model, with a standard set of parameters, does account for the general features of the experimental data, especially for the stronger exit channels. However, these data are not sensitive enough to the presence of other types of mechanisms and must be complemented by other observables to boost the predictive power of the model.

The multipolarity of the continuum transitions in 152 Dy and 160 Er has been deduced from γ-ray linear polarisation and angular distribution measurements. The yrast part for 160 Er agrees with the predicted stretched E2 cascade. The statistical parts for both 152 Dy and 160 Er consist of a mixture of dipolar stretched and non-stretched transitions, predominantly of electrical character. Below 1.5 MeV, the data of 152 Dy exhibit a different structure, and reveal a stretched M1 component in the low-energy region.

The lifetime of the 5.14 MeV excited state of 22 Ne was investigated by use of the electronic timing, recoil distance and Doppler shift attenuation techniques. A mean life τ = 1.2±0.3 ps was obtained, corresponding to a transition strenght of 9.8 × 10 −6 W.u. for the El decay of this state.

The perturbed integral angular distribution technique has been used to study hyperfine interactions of light nuclei recoil-implanted into iron. Heavy- ion-induced fusion-evaporation reactions populated the excited states of $sup 37$Ar, $sup 39$,$sup 40$,$sup 41$K, and $sup 41$Ca, whose deexciting $gamma$ rays were observed in a Ge(Li) detector. In all cases the measured shifts of the angular distributions were found much smaller than the ones calculated using the known values of g factors, lifetimes, and strengths of the internal hyperfine fields. This can be explained under the assumption that only a fraction of the nuclei feel the full internal magnetic field, while the rest of them do not feel any magnetic interaction at all. Different fractions obtained for $sup 40$K and $sup 41$K suggest a migration process on a ns time scale of the implanted ions in the lattice. (AIP)

The main research line of the Tori group is the study of the reaction dynamics of dissipative collisions between heavy ions ia positron and electron spectroscopy. The last five years since the Lahnstein-Conference1 are marked for our group by the installation of a new experimental device for detecting positrons and electrons emitted in these collisions, the so-called Tori spectrometer2. The first part of this report is devoted therefore to describe the main characteristics of this apparatus.

This report describes the design and implementation of the Interactive Digital Receiver Simulator (IDRS). The IDRS is a computer program that is capable of processing predetected signals that have been converted to digital format and recorded. It allows a user to computer interactively produce (1) desired hardcopy patterns of signals for a detailed analysis of modulation characteristics and (2) processed signal segments for processing by pattern analysis/recognition programs for feature extraction. In essence, the IDRS program provides the user with the capability of interactively creating a digital receiver to his specifications. (Author)

Recent measurements of positrons in coincidence with electrons emitted in U+Th collisions near the Coulomb barrier show sharp line structures in the sum and difference energy spectra of the two leptons (ref. 1). A possible method to clarify the origin of these lines is to study the angular correlation of the leptons. In this short contribution we want to report on a positron-electron coincidence test measurement performed with the TORI spectrometer. This apparatus offers the possibility to measure positrons in coincidence with electrons emitted into the same and opposite hemisphere as well.

The nuclear g-factor of the 2543 keV, 7+ state 40K has been measured as g = 0.59 ± 0.10. Time-integral perturbed angular distributions were measured in an external magnetic field and, after implantation, in the hyperfine field at potassium in nickel. The experimental value is discussed within the f 7 2 2 and d 3 2 −2 f 7 2 2 , configurations.

The even N = 28 isotones 50Ti, 52Cr and 54Fe have been investigated using (α, 2nγ) reactions. The experiments involved the measurement of γ-ray angular distributions and correlations, linear polarizations, excitation functions and γ-γ coincidences. Spin-parity values were established for new levels observed in 50Ti at 6135, 6539, 6769, 7570 and 8790 keV having, respectively, Jπ = 7+, 8+, 9+, 10+ and (11)+, and in 52Cr at 6454, 7238 and 8217 keV having, respectively, (9)+, (10)+ and (11+). Other new levels were also observed in 50Ti at 3975, 4148, 7539 and 8257 keV, in 52Cr at 6365 keV, and in 54Fe at 7507 and 8023 keV. In addition, definite Jπ values of 8+, 6+, 7+ and 8+ were assigned to the levels at 4750, 4806, 5397 and 5825 keV, respectively in 52Cr, while the mean lifetime of the (10+), 6529 keV level in 54Fe was measured to be 517 ± 45 ns, using a γ-γ delayed coincidence technique. Shell-model calculations, including excitations of one particle from the N = 28 core, successfully account for the newly observed states and indicate their 2p 3 2 neutron character in 50Ti and 52Cr. The interpretation of the experimental result is consistent with configuration mixing of about 20 % for the low-lying f 7 2 states.

The lifetime of the 738 keV level of 43K, excited by means of the 40Ar(α, p)43K reaction, has been deduced from the distribution of time delays between protons and decay γ-rays. The half-life, 205±10 ns, favors a J π = 7 2 − assignment.

A new magnetic transport system, the Tori spectrometer, which is able to measure positrons and electrons simultaneously, has been used to study the dynamics of collisions between very heavy ions via their influence on the positron and δ-electron spectra. While these spectra obtained in coincidence to elastic scattering agree well with theory based on pure Rutherford trajectories, those observed in coincidence to dissipative collisions can only be described by calculations taking into account a time delay in the collision process. A quantitative analysis gives an estimation for the nuclear contact time in the U + U collision.

The 62Cu nucleus has one proton and five neutrons outside the closed shells of Z, N = 28. The reported experimental value of the magnetic moment of the nucleus in the 1+ ground state is in fair agreement with theoretical predictions based on the Hamada — Johnston interaction potential, with which a very strong configuration mixing was obtained1). Similarly, for the 2+ ground state of 60Cu the agreement between the experimental and predicted values of the magnetic moment is quite satisfactory. As the low — lying first excited 2+ state of 62Cu has the same spin and parity as the ground state of 60Cu, it is interesting to measure its magnetic moment. The performed experiment includes directional correlation measurements on the (596-42) keV γ — γ cascade observed in the decay of 62Zn.

A detailed -γ-ray study of152Dy has been performed using the124Sn(32S,4n) reaction at low bombarding energy. Several γ rays deexciting non-Yrast states have been identified, among them transitions previously observed in coincidence with the superdeformed band. However we have no evidence for the lowest members of this band.

The half life of the 57-kev level in Pr/sup 143/ was measured. A transistorized fast-slow coincidence circuit with a timeto-pulse-height converter was used to measure the gammagamma cascade for Ce(NO/sub 3/)/sub 3/ dissolved in nitric acid and polycrystalline Ce/sub 2/O/sub 3/. The mean value obtained for the half life is 4.20 plus or minus 0.07 nsec. (C.E.S.)

Coincidence time spectra for the 133 to 482 kev gamma - gamma cascade were taken using two scintillation counters and having the source located in an external magnetic field perpendicular to the plane of successive gamma rays. Measurements were performed at theta /sub 1/ = (5 pi /8) and theta /sub 2/ = (7 pi /8) between counter axes and for the two opposite directions of the magnetic field. Four coincidence time spectra were obtained. It is indicated that perturbation of the angular correlation could be present. (L.N.N.)