Short Propagation Research Articles

Fatigue propagation of short and long cracks in gaseous hydrogen environment in 3.5NiCrMoV steel

Abstract Turbo generators for nuclear plants are mostly equipped with hydrogen cooling systems. Current practice of characterizing the growth of fatigue cracks on the basis of fracture mechanics primarily relies on fatigue tests for long cracks which are typically of several millimeters in length. However, in view of extended life for the plants, the damage tolerance evaluation of such fatigue-critical engineering components requires understanding of the propagation of cracks of significantly smaller dimensions. Then the near threshold of short cracks is investigated and compared to the behavior of long crack by experiments under 4 bar hydrogen atmosphere. The short crack fatigue propagation in hydrogen atmosphere is shown similar to that in air, growing faster than the long crack and at ΔK ranging below the long crack threshold; this effect is related to a reduced crack closure shielding. The propagation behavior of long crack under hydrogen atmosphere is shown similar to that obtained in air in the low rate range, i.e. when the maximum of the stress intensity factor K max is lower than a critical level of about 16 MPa m 1/ 2 with higher crack growth rate than in high vacuum. This environment effect is related to the presence of residual water vapor in both gases. For higher K max , much faster growth rates under hydrogen atmosphere in comparison to air and vacuum are observed and related to hydrogen assisted intergranular propagation combining fatigue and sustained loading damage. The results are discussed on the basis of micrographic observations supporting the involved mechanisms.

Fatigue Microcrack Nucleation Modeling: a Survey of the State of the Art

Many different approaches have been proposed for modeling of fatigue crack nucleation and microstructurally short fatigue crack propagation in polycrystalline metals. The mechanisms governing dislocations and slip at micro- and nanoscale have to be taken into account to be able to modify the microstructure of metals and develop materials with improved fatigue resistance. This paper reviews modeling approaches that take the microstructure into the consideration specifically from the fatigue crack nucleation point of view. Key concepts are explained, challenges described, and uncertainties discussed, followed by a comprehensive list of articles on the subject published mainly in the past decade.

Extremely Short Optical Pulses and Ads/CFT Compliance

Dynamics of few cycle optical pulses in non-Fermi liquid was considered. Energy spectrum of non-Fermi liquid was taken from the AdS/CFT compliance. Conditions of quasiparticle excitation existence were defined. Non-Fermi liquid parameters impact on the shape of few cycle pulses were estimated. It was shown that extremely short optical pulse propagation in the non-Fermi liquid is a stable pattern. The value of chemical potential has a significant impact on extremely short pulse shape. An increase in initial pulse amplitude does not result in pulse-shape distortions under its propagation in considered medium that is why the non-Fermi liquid can be used in applications inherent in extremely short pulse processing.

Laboratory model of magnetosphere created by strong plasma perturbation with frozen-in magnetic field

Transient interactions of a magnetic dipole with plasma flow carrying a southward magnetic field has been studied in laboratory experiments. Flow with a transverse frozen-in field was generated by means of laser-produced plasma cross-field expansion into background plasma, which filled a vacuum chamber, along with the external application of a magnetic field prior to interaction. Probe measurements showed that at the plasma parameters realized, effective collisionless Larmor coupling takes place resulting in the formation of strong compressive perturbations, which propagate in the background with super-Alfvenic velocity and generate a frame-transverse electric field comparable in value to the expected induction one in the laboratory. Compression pulses with a southward field interact with a dipole and create a well-defined magnetosphere after a short propagation. Comparison of the magnetospheres created by laser-produced plasma expanding in a vacuum field and in a magnetized background revealed fundamental differences in the structure and behaviour of the electric potential in plasma. In the presence of a frozen-in southward field direct «sub-solar» penetration of outside electric potential deep inside of the magnetosphere was observed to take place, with a velocity close to the upstream Alfven speed.

Conditions for the Transition from Nonlocalized to Localized Damage in Metals and Alloys. Part 3. Determination of the Transition Conditions by the Analysis of Crack Propagation Kinetics

Special features of initiation and growth of short cracks under stresses above the fatigue limit are studied for a wide range of metals and alloys. The authors analyze the growth kinetics of short fatigue cracks, which is assessed by the dependence of crack length or crack growth rate on the number of loading cycles. The transition from the stage of short crack propagation (nonlocalized fatigue damage) to the stage of a main crack propagation (localized fatigue damage) is shown to be characterized by a more intensive increase of the crack growth rate. A procedure for determination of the main crack sizes corresponding to the transition from nonlocalized to localized fatigue damage has been substantiated. It has been found out that the fatigue crack sizes corresponding to this transition at stresses above the fatigue limit decrease with increasing stresses and remain smaller than those at stresses equal to the fatigue limit.

Narrowband Short Range Directive Channel Propagation Loss in Indoor Environment at Three Frequency Bands

Propagation loss models for indoor environment are presented. The narrowband directive channel propagation loss in indoor environment at three frequency bands namely (2.4, 3.3 and 5.5) GHz is measured. Sets of directive panel antennas are used in the measurements. RF signal generator and a spectrum analyzer are used in the measurement campaign. It is found that propagation loss is sensitive to the scenario configuration. It is noticed that the propagation loss can be modelled by a single slope propagation model or two slopes propagation model.

Partition of Cyclic Plasticity in an Austenitic-Ferritic Duplex Stainless Steel and its Significance for Fatigue Crack Initiation under High and Very High Cycle Fatigue Loading Conditions

The present study documents that at loading amplitudes close to the fatigue limit, cyclic irreversible plastic deformation in form of slip band generation in the austenitic-ferritic duplex stainless steel X2CrNiMoN22-5-3 (318 LN) mainly takes place in few austenite grains without any microcrack initiation in these grains. This was shown by means of focused ion beam (FIB) cutting in combination with high resolution scanning electron microscopy (SEM) at pronounced extrusion-intrusion-pairs in several austenite grains. Investigations by means of confocal laser scanning microscopy (CLSM) revealed that the slip band density in these grains increases with the number of loading cycles and remains constant in the very high cycle fatigue (VHCF) regime. Under such loading conditions, fatigue cracks frequently initiate in the ferrite phase due to anisotropy stresses which are strongly superimposed by stress intensifications at the tip of austenite slip bands. TEM investigations revealed that austenite slip bands, which are piling up against phase boundaries, cause localized dislocation generation and motion in neighboring ferrite grains. The cyclic irreversible motion of these dislocations on several parallel slip planes is correlated with the stage of fatigue crack initiation. A crystal plasticity model based on a finite element program, which considers anisotropic elasticity, allows for the determination of crack initiation sites in real microstructures according to the above mentioned mechanisms. Crystallographic orientations, measured by means of the electron back scatter diffraction (EBSD) technique, serve as input parameters for the calculations regarding microcrack initiation as well as for the analysis of the subsequent short fatigue crack propagation, which is strongly affected by microstructural barriers such as grain and phase boundaries.

Fatigue Short Crack Growth and Mechanism of GH4133B Superalloy Used in Turbine Disk of Aero-Engine

The short fatigue crack growth tests for circular notched compact tension specimens of GH4133B superalloy used in turbine disk of aero-engine are carried out at ambient temperature and atmospheric pressure. The stress intensity factor ranges and the fatigue crack growth rates at various stress ratios are measured, and the corresponding effective stress intensity factor ranges considering the crack closure effect are calculated. It is shown that the effective stress intensity factor range ΔKeff, can be applied to describe the deceleration and acceleration of crack growth rate during the short crack propagation. The fatigue fracture surface morphologies in the short crack growth region are investigated using a scanning electron microscopy. It is found that there is a cleavage step between two adjacent radial striations, a series of early fatigue striations exist on the cleavage step, and some secondary cracks perpendicular to the direction of main crack propagation emerge on the fracture surface, the superalloy exhibits a mixed fracture mode in the short crack growth region, which reveals the microscopic mechanism of short crack propagation that the fatigue crack growth rate is primarily higher, and then gradually decreases with the propagation of short crack.

The study of pulse laser propagation through breast tissues by means of the Green function method

Propagation of a short pulse through human breast tissues is studied by numerically solving the diffuse equation. Different numerical methods, such as Monte Carlo and finite difference time domain, have been used to study the short pulse laser propagation through biological tissues. In this paper, we use boundary integral method to study the laser pulse–tissue interaction. The diffuse equation is used to examine the propagation of laser into biological tissues, and boundary integral method is used to alter this equation to the integral form and the result is solved by using boundary element method. To verify the precision of a boundary element method code, we compared the obtained results with those obtained by finite difference time domain method. In addition, the effects of different optical parameters of breast tissues, i.e., reduced scattering and absorption coefficients, on time evolution of a diffusely reflected pulse are studied.

Ictal near-infrared spectroscopy and electrocorticography study of supplementary motor area seizures

The seizure onset zone (SOZ) and propagation pathways in supplementary motor area (SMA) seizures are difficult to identify because of the short duration and swift propagations of this seizure type. Herein, we studied ictal cerebral blood flow changes, seizure electrical activities, and clinical signs employing simultaneous near-infrared spectroscopy (NIRS) and electrocorticography (ECoG) recordings in a brain tumor patient with SMA seizures. Increased cerebral blood flow was observed from the SOZ in the ipsilateral SMA and spread to the ipsilateral premotor cortex, ipsilateral sensorimotor cortex, and also the contralateral hemisphere. These propagation patterns were concordant with ictal ECoG seizure activities and clinical signs. Removal of the SOZ and surrounding areas, where the higher blood flow and higher frequency oscillations had been identified, achieved a good outcome for this patient. Our ictal NIRS-ECoG technique is robust and useful for detecting the SOZ and seizure propagations.

Transition of crack propagation from a transgranular to an intergranular path in an overaged Al-Zn-Mg-Cu alloy during cyclic loading

The fatigue crack propagation behavior in the overaged Al-Zn-Mg-Cu alloy was characterized by optical microscopy, scanning electron microscopy, transmission electron microscopy and electron backscatter diffraction. The results revealed that a fatigue crack tended to transgranularly propagate in the near-threshold regime, whereas intergranular crack propagation was dominant at the high ΔK regime. The transition of crack propagation from a transgranular to an intergranular path that occurred in the Paris regime was strongly influenced by the misorientation of adjacent grains and precipitate free zones. In addition, a crystallographic model of crack propagation was proposed to interpret the transition. The fatigue short crack propagation on a single slip plane was responsible for the formation of a transgranular propagation path in the near-threshold regime. The fatigue long crack propagation, which was conducted by a duplex slip mechanism in the Paris regime, led to the formation of fatigue striations. The formation of a zigzag crack in the near-threshold regime was ascribed to the high misorientation of adjacent grains.

Hybrid Real-Time and Preset Processing for Indoor Radio Geo-Location in Dense Multipath Environments

The existence of various geo-location applications and their accuracy requirements enhance the necessities for suitable processing techniques to solve the indoor geo-location problems. Since, Impulse Radio Ultra-Wideband (IR-UWB) signals have very short duration pulses; they can provide very accurate ranging and geo-location capability in short range indoor radio propagation environments. Our research puts emphasis on indoor geo-location using UWB signaling considering both of non line-of-sight (NLOS) and LOS radio propagation environments. In this paper, we introduce and investigate a noble approach which makes a hybrid combination of Channel Impulse Response (CIR)-based finger-printing (FP) method with polygonal arrangement of reference nodes (or tags) and an iterative-TOA based real-time geo-location method using UWB signaling for wireless ad hoc networks. The proposed hybrid approach assures significant improvement in positioning accuracy compared to TOA only, FP only and conventional iterative-TOA geo-location methods by mitigating NLOS errors effectively in the indoor environment. Besides, this hybrid approach minimizes the calculation complexity of the FP method while maintaining improved geo-location accuracy in the dense multipath propagation environment.

Laser-plasma interaction physics for shock ignition

In the shock ignition scheme, the ICF target is first compressed with a long (nanosecond) pulse before creating a convergent shock with a short (∼100ps) pulse to ignite thermonuclear reactions. This short pulse is typically (∼2.10 15 -10 16 W/cm 2 ) above LPI (Laser Plasma Instabilities) thresholds. The plasma is in a regime where the electron temperature is expected to be very high (2-4keV) and the laser coupling to the plasma is not well understood. Emulating LPI in the corona requires large and hot plasmas produced by high-energy lasers. We conducted experiments on the LIL (Ligne d'Integration Laser, 10kJ at 3) and the LULI2000 (0.4kJ at 2) facilities, to approach these conditions and study absorption and LPI produced by a high intensity beam in preformed plasmas. After introducing the main risks associated with the short pulse propagation, we present the latest experiment we conducted on LPI in relevant conditions for shock ignition.

Analysis of the Crack Initiation at Non-Metallic Inclusions in High-Strength Steels

Abstract High-strength steels exhibit different fracture surface characteristics under cyclic load when failure is initiated by non-metallic inclusions depending on the lifespan. These can be attributed to different crack propagation mechanisms. These characteristics are correlated with the short and long crack propagation. As far as the short crack is concerned, the mechanism of the discontinous decohesion of carbides in the immediate vicinity of the inclusions and the merging of the resulting microcracks to form a macrocrack is discussed. This postulated mechanism is investigated by means of a combination of scanning electron microscope (SEM) analyses of the fracture surface and target preparation by means of focused ion beam (FIB) by a detailed resolution of the local microstructure around the crack-initiating inclusion and the crack path.

Short Fatigue Crack Propagation Threshold of Railway LZ50 Axle Steel

Collective short fatigue behaviour nearby material micro-structural barrier threshold is experimentally observed for the smooth specimens of Chinese railway LZ50 axle steel. Effective micro-cracks which result in the specimens’ failure were initiated from the material ferrite particles on the surfaces of the specimens. Four elements i.e. crack size, orientation angle, and the two crack tip orientation angles should be contained for each micro-crack. The collective micro-crack behaviour should be described by an equivalent dominant effective short fatigue crack (ESFC) concept using an ESFC theory. And an equivalent method is then developed with an equivalent growth driving energy concept. A random modeling of the equivalent dominant ESFC growth rates is further constructed for describing the random behaviour of the collective ESFCs nearby the material micro-structure barrier threshold. And the kinetic threshold is subsequently conducted from the modeling.

Effects of intermodal dispersion on short pulse propagation in multi-core fibers

The effects of intermodal dispersion on ultrashort optical pulse propagation through multi-core fibers are analyzed theoretically, which has been ignored in previous studies. A three-core fiber with collinear and triangular configuration and a four-core fiber are considered. We demonstrate with numerical examples that the intermodal dispersion can cause pulse breakup effect in multi-core fibers.

Gigacycle fatigue of bearing steels

For a practical point of view, the bearing steels are loaded up to the very high cycle fatigue regime, in service, far away from the Wöhler regime. In the present paper, several approaches are discussed in order to open some ways of investigation, in the range of 109 cycles: accelerated fatigue testing, thermal dissipation, fish-eye formation, inclusion effect, short crack propagation and stability of the microstructure.

Target time smearing with short transmissions and multipath propagation

In active sonar the target echo level is often estimated with a propagation model that adds all multipath arrivals. If the (post-correlator) transmitted pulse is short compared to the multipath time spread then there is effectively an extra loss (which may be substantial) since only a few of the paths contribute to the target echo at any one instant. This well known "time-smearing" loss is treated in a self-consistent manner with previous calculations of reverberation [Harrison, J. Acoust. Soc. Am. 114, 2744-2756 (2003)] to estimate the target response and the signal-to-reverberation-ratio. Again isovelocity water, Lambert's law, and reflection loss proportional to angle are assumed. In this important short pulse regime the target response becomes independent of boundary reflection properties but proportional to transmitted pulse length. Thus the signal-to-reverberation-ratio becomes independent of pulse length. The effect on signal-to-ambient-noise is also investigated and the resulting formulas presented in a table.

Saturation performance analysis of directional CSMA/CA in mmWave WPANs

The millimeter-wave (mmWave) band offers the potential for multi-gigabit indoor wireless personal area networks (WPANs). However, there are problems such as the short wavelength and high propagation losses. In order to compensate for these, it is highly recommended to use directional antennas at the physical layer. In this paper, directional carrier sense multiple access/collision avoidance (CSMA/CA) is analyzed in IEEE 802.15.3c, a standard for mmWave WPANs, under a saturated environment. A Markov chain model is presented and analyzed for the no-acknowledgment (No-ACK) mode. For the analysis, the device interaction is considered using directional antennas over a shared channel. A Markov chain model is presented in which the features of IEEE 802.15.3c such as backoff counter freezing and the effects of using directional antennas are incorporated. The maximal number of frames that can be transmitted concurrently and successfully is derived by an algorithm, and the system throughput and the average transmission delay are obtained in the closed forms. Numerical results show that the effects of using directional antennas in CSMA/CA and the overall analysis are verified by the simulation. The obtained results illustrate the physical layer impact on the CSMA/CA medium access control protocol, and these insights can be helpful in developing a medium access control protocol for enhancing the performance of mmWave WPANs. Copyright © 2011 John Wiley & Sons, Ltd.

Efficient Iterative Time-Domain Beam Propagation Methods for Ultra Short Pulse Propagation: Analysis and Assessment

The time-domain beam propagation method (TD-BPM) has been implemented and analyzed using several iterative numerical techniques to model the propagation of ultra short pulses in optical structures. The methods depend on one-way non-paraxial time domain propagation that use Pade approximant formulation. Several numerical tests showed that the iterative TD-BPM techniques are very stable and converge using few iterations. From accuracy assessment compared to the FDTD, it has been observed that the longitudinal and the temporal steps sizes can be a number of orders of magnitude larger than the FDTD step sizes with little percentage difference. Computer performance analysis showed the TD-BPM is well suited for long dielectric structures interaction of short and ultra short pulse propagation.