Random Shots Research Articles

Studies on residual stress and surface topography after pneumatic shot peening using discrete element method–finite element method coupled model

After pneumatic shot peening, compressive residual stress is induced on the surface of the target, which will improve the fatigue life. During the process, surface roughness is also induced, which may reduce the fatigue life. To achieve the optimum compressive residual stress with the smallest surface roughness, the formation mechanism of the residual stress field and the surface topography are revealed. Then, pneumatic shot peening is simulated by using a discrete element method–finite element method coupled model. Based on the effects of four variables (the incident angle θ, the initial shot velocity v0, the shot radius R) and the mass flow rate rm on five parameters (the equivalent plastic strain, the equivalent stress, the surface compressive residual stress, the maximum compressive residual stress), the surface roughness is investigated. The results show that it induces a residual elastic–plastic strain after the impact of a single shot, which can form a crater and a hemispherical residual stress field. After the impact of many random shots, scattered craters connect with each other, therewith a rough and continuous surface topography is formed. Scattered residual stress fields couple with each other, and a constant residual stress layer with the compressive residual stress in the surface and the tensile residual stress in the subsurface are formed. All five parameters are determined by the residual elastic–plastic strain, which increases as the augment of the normal impact velocity, the shot mass, and the coverage rate. Along with the increase of θ and rm, these five parameters firstly increase and then decrease; with the increase of v0 and R, these five parameters increase. Therefore, reasonable values of θ, v0, R, and rm should be chosen to obtain the optimum compressive residual stress with as small surface roughness as possible.

A simulation modeling methodology considering random multiple shots for shot peening process

Abstract Shot peening (SP) process is a typical surface strengthening process for metal and metal matrix composites, which can significantly improve the fatigue life and strength. The traditional SP simulation model falls short as it only takes into account one or a few shots, proving insufficient for accurately simulating the entire impact process involving hundreds of shots. In this study, a random multiple shots simulation modeling methodology with hundreds of random shots is proposed to simulate the impact process of SP. In order to reduce the simulation error, the random function Rand of MATLAB is used to generate the shot distributions many times, and the shot distribution closest to the average number is selected and the three-dimension parametric explicit dynamics numerical simulation model is built using ABAQUS software. Orthogonal experiments are carried out to investigate the influences of shot diameter, incident impact velocity, and angle on the residual stress distribution, roughness, and specimen deformation. Results showed that the average relative errors of maximum residual compressive stress, roughness, and deformation of specimen between simulation model and experimental value are 30.99, 16.14, and 16.73%, respectively. The primary factors affecting residual stress and deformation is shot diameter, and the main factor affecting roughness is impact velocity.

DEM-FEM coupling analysis of shot′s energy distribution and target′s residual stress of pneumatic shot peening

Pneumatic shot peening is a widely used surface strengthening method. During the peening process, shots often collide with each other, resulting in large energy loss and small compressive residual stress. In order to achieve the optimum compressive residual stress with as little energy loss as possible, firstly the collision mechanism of shots and the forming and coupling mechanism of the target’s residual stress are revealed, and then pneumatic shot peening is simulated by using DEM-FEM coupling model. Then, the effects of impact angle θ, initial shot velocity v 0, shot diameter d p, and mass flow rate r m on the percentage η of shots with different ratios of the impact velocity to initial shot velocity v m/v 0, the energy loss (EL), the energy transferred from shots to the target (ET), the residual energy (ER) and the compressive residual stress (RS) are investigated. The results show that as many random shots successively impact the target, the RS field induced by each shot couples with some adjacent RS fields induced by other shots, so that disperse RS fields are gradually transformed into a continuous RS layer with the compressive RS in the surface and the tensile RS in the subsurface. With the increase of θ and r m and with the decrease of v 0 and d p, the collision probability of shots increases, so EL also increases and η of shots with a large v m/v 0 decreases. While, ET increases with the increase of v 0 and d p, decreases with the increase of r m, and first increases and then decreases with the increase of θ. ET does not entirely determine but greatly affects the compressive RS field. So, the surface compressive RS and the maximum compressive RS first increase and then decrease with the increase of θ and r m, while the two parameters increase with the increase of v 0 and d p. The optimum parameters of shots are θ = 75°, v 0 = 60 m s−1, d p = 0.25 mm and r m = 2 kg min−1, in which ET reaches 45%, the surface compressive RS of S11 and S33 reach 512 MPa and 510 MPa respectively, and the maximum compressive RS of S11 and S33 reach 665 MPa and 746 MPa respectively.

Simultaneous source separation by shot collocation and strength variation

Simultaneous shooting offers opportunities for significant cost savings in seismic data acquisitions. The most common strategy uses random delay shots where source separation is achieved during the processing stage, thereby doubling source densities. We have determined that the creation of a collocated source survey, where shots are repeated simultaneously at multiple positions, is a viable alternative strategy, with the additional benefit that it may increase source density even further while keeping the acquisition duration unchanged. Source separation is achieved using overcomplete independent component analysis by first by applying a directional wavelet transform to separate source signals with different slowness, then estimating the mixing matrix, followed by solving an optimization problem with an energy constraint combined with a sparseness inducing prior and obtaining the required waveforms. Synthetic tests find average reconstruction quality on the order of 22.1, 15.4, and 8.4 dB if, respectively, three, four, or five shots are acquired in two mixtures. Examination of the true versus obtained zero-offset sections also demonstrates the robustness of our signal recovery strategy. The advantage of shot collocation over conventional acquisitions is that it may triple or even quadruple the source density for unchanged acquisition durations with superior reconstruction results compared with dithered acquisitions using similar blending factors.

Ionization phase retrieval by angular streaking from random shots of XUV radiation

We demonstrate an accurate phase retrieval of extreme ultraviolet (XUV) atomic ionization by streaking the photoelectron in a circularly polarized infrared (IR) laser field. Our demonstration is based on a numerical solution of the time-dependent Schr\"odinger equation. We test this technique using the hydrogen atom ionized by isolated attosecond XUV pulses across a wide range of the photon energies. Importantly, the proposed method works in a random shot mode when the time delay between the ionizing and streaking XUV and IR pulses may vary from shot to shot. This is a significant advantage over the existing interferometric techniques which require a systematic and controllable scan of the XUV and IR delay in one set of measurements. Such a scan may not be possible to take because of the arrival time jitter inherent to the stochastic nature of self-amplified free-electron laser radiation.

Coupled finite and discrete element shot peening simulation based on Johnson–Cook material model

Shot peening is an essential treatment that produces a beneficial compressive layer on the material’s surface, which significantly improves its fatigue life. To minimizes the cost and resources used in determining the finest shot peening parameters based on the experimental approach, a numerical model capable of computing the induced compressive residual stress accurately is required. Hence, the numerical simulation of the shot peening process with multiple random shots that depict the actual shot peening is presented in this paper. The model is developed using the coupled finite and discrete element methods. The two numerical tools were coupled via code in ABAQUS, whereby shot–shot and shot–target interaction behaviors were accurately included. The induced compressive residual stress was computed due to the multiple random shots impact based on the Johnson–Cook material model. The model was experimentally validated and applied to evaluated the influence of shot velocity, shot size, and angle of impact on the final compressive residual stress.

A Comprehensive Numerical Approach for Analyzing the Residual Stresses in AISI 301LN Stainless Steel Induced by Shot Peening.

Shot peening is one of the most famous mechanical surface treatments to improve fatigue performance of metallic components, which is attributed to high amplitude compressive residual stresses. A numerical approach is developed to analyze the residual stresses in 301LN metastable austenitic stainless steel by shot peening. The material behavior is described by a proposed constitutive model in which strain-induced martensitic transformation, isotropic hardening and kinematic hardening effects are taken into account properly. Both single shot and random multiple shots peening were simulated and analyzed. A numerical method is presented with the Python programming language to make the multiple shots follow a random probability distribution. Results demonstrate that the simulated equivalent plastic strains and martensitic volume fractions agree well with the experimental ones, which verify the validity of the constitutive model. Besides, the numerical method is effective at achieving a realistic surface coverage. The maximum compressive residual stress by the Johnson–Cook model is 12% higher than that of the proposed model. Additionally, each hardening effect has an effect on the simulated residual stress. The developed numerical approach can provide a feasible simulation of the shot-peening process and makes an accurate prediction of the residual stress field in 301LN steel.

Assessing age-dependent susceptibility to measles in Japan

BackgroundRoutine vaccination against measles in Japan started in 1978. Whereas measles elimination was verified in 2015, multiple chains of measles transmission were observed in 2016. We aimed to reconstruct the age-dependent susceptibility to measles in Japan so that future vaccination strategies can be elucidated. MethodsAn epidemiological model was used to quantify the age-dependent immune fraction using datasets of vaccination coverage and seroepidemiological survey. The second dose was interpreted in two different scenarios, i.e., booster and random shots. The effective reproduction number, the average number of secondary cases generated by a single infected individual, and the age at infection were explored using the age-dependent transmission model and the next generation matrix. ResultsWhile the herd immunity threshold of measles likely ranges from 90% to 95%, assuming that the basic reproductive number ranges from 10 to 20, the estimated immune fraction in Japan was below those thresholds in 2016, despite the fact that the estimates were above 80% for all ages. If the second dose completely acted as the booster shot, a proportion immune above 90% was achieved only among those aged 5years or below in 2016. Alternatively, if the second dose was randomly distributed regardless of primary vaccination status, a proportion immune over 90% was achieved among those aged below 25years. The effective reproduction number was estimated to range from 1.50 to 3.01 and from 1.50 to 3.00, respectively, for scenarios 1 and 2 in 2016; if the current vaccination schedule were continued, the reproduction number is projected to range from 1.50 to 3.01 and 1.39 to 2.78, respectively, in 2025. ConclusionJapan continues to be prone to imported cases of measles. Supplementary vaccination among adults aged 20–49years would be effective if the chains of transmission continue to be observed in that age group.

Predicting the Peen Forming Effectiveness of Ti-6Al-4V Strips With Different Thicknesses Using Realistic Finite Element Simulations

This paper is intended to quantify the relationship between the peen forming effectiveness and various involved parameters through a realistic numerical study. For this purpose, a new finite element (FE) model is proposed with full geometry representation, random shots generation, and rate-dependent material law of kinematic strain-hardening. The mesh sensitivity and effects of boundary conditions are carefully examined. The FE model is validated by comparing the results with the experimental measurements. The proposed model is then used to investigate the effects of the peening intensity (represented as the shot velocity) and the strip thickness on the peen-formed deflection and the residual stress distribution for strips made of Ti-6Al-4V. Our results indicate the existence of a maximum convex deflection for different strip thicknesses. In addition, a reversed deflection (i.e., concaved curvature) is observed for severe peening conditions (i.e., thin strip under high peening intensity). Our simulations verify the previous proposition that a concaved curvature can be generated only when the whole cross section is plastically deformed.

Which CT Set for Lung SBRT Planning: The Effect of Varying Lung Density on Monte Carlo Dose Calculation

Free-breathing (FB) and average CTs are common CT images used for SBRT lung planning. The FB CT, a fast 3D scan, consists of random snap shots at different breathing phases for different CT bed positions. The average CT, often created as a mean intensity projection from a slow 4D CT, represents a time-averaged anatomy. While the average CT has been shown to be advantageous for GTV contouring and target localization, both are believed to be equivalent for dose calculation with conventional algorithms (pencil beam, convolution superposition). Since previous studies demonstrated considerable MC dose dependence on varying lung densities, whether in phantoms or between patients with different lung densities, we aimed to study the lung density variation between FB and average CTs, and its effect on MC dose calculation increasingly used in lung SBRT planning. With IRB approval, 10 previously-treated lung SBRT patients were randomly chosen. For each patient, both a FB CT and an 8-phase 4D CT were acquired. The 4D CT was used for contouring an ITV while the FB CT was used for treatment planning with dynamic conformal arcs in clinical treatments. Lungs were contoured on the FB CT, on each phase of the 4D CT, and on an average CT created from the 4D CT. Lung density was compared for each patient among these 10 different data sets. Dose distribution was calculated with MC (iPlan, BrainLab) on FB and average CTs for dose comparison using D5, Dmean and D95 of PTV and GTV, as well as RTOG 0813 protocol compliance. Wilcoxon’s signed rank test was used. For the 10 studied patients, the median (range) of the minimum, mean, and maximum lung density from the 4D CT was 0.26 (0.10∼0.34), 0.31 (0.12∼0.43), 0.35 (0.22∼0.51) g/cc, respectively. Using the mean lung density from the 4D CT as the gold standard, the median (range) deviation of that on the FB and average CT was -0.03 (-0.09∼0.02) and 0.00 (-0.01∼0.01) g/cc, respectively. For the PTV, the median (range) D5, Dmean and D95 percent difference between the FB and average CT dose calculation was -1.9 (-5.1∼1.2%), -0.2 (-1.1∼0.4%) and -1.1 (-4.3∼0.7%), and for the GTV, -1.0 (-2.9∼0.8%), -0.2 (-0.4∼0.3%) and -0.8 (-2.4∼0.5%), respectively. There was no dose index difference that was statistically significant (P< 0.05) for the studied population. RTOG compliance was not different between the two CT sets for any patient. Lung density on the average CT accurately represents the mean density from the 4D CT. Varying density differences were observed between the average and FB CTs, hence leading to varying dosimetric differences in MC dose calculated. However, for the 10 studied patients, the dosimetric differences were not statistically significant. This suggests that negligible difference between FB and average CT is expected for MC calculation, similar to what previously believed true for conventional dose algorithms.

Realistic Finite Element Simulations of Arc-Height Development in Shot-Peened Almen Strips

It is the objective of this study to conduct realistic simulations of the arc-height development in shot-peened Almen strips using the finite element (FE) method. Unlike our earlier work which is devoted to relaxation of shot peening induced residual stress, in this paper, the focus is on peen forming as a result of repeated spherical impingement. Specifically, a 3D FE model with 1500 randomly distributed shots bombarding an Almen strip was developed. Strain rate dependent plasticity was considered and an artificial material damping was applied to control the undesired high-frequency oscillations. The solution further adopts both explicit dynamic and implicit quasi-static analyses to simulate the entire arc-height development in the Almen strips. Quantitative relationships between the resulting equivalent plastic strain and the associated residual stress distribution for a given shot velocity and shot numbers are established and discussed. The work also considers the effect of repeated impacts upon the induced residual stress field using a large number of random shots. Attention was further devoted to the effect of the strip constraint upon the outcome of the impingement. Our results indicate that the proposed FE model is a powerful tool in investigating the underlying mechanisms of the peening treatment.

Nonparametric estimation of surface integrals

The estimation of surface integrals on the boundary of an unknown body is a challenge for nonparametric methods in statistics, with powerful applications to physics and image analysis, among other fields. Provided that one can determine whether random shots hit the body, Cuevas et al. [Ann. Statist. 35 (2007) 1031--1051] estimate the boundary measure (the boundary length for planar sets and the surface area for 3-dimensional objects) via the consideration of shots at a box containing the body. The statistics considered by these authors, as well as those in subsequent papers, are based on the estimation of Minkowski content and depend on a smoothing parameter which must be carefully chosen. For the same sampling scheme, we introduce a new approach which bypasses this issue, providing strongly consistent estimators of both the boundary measure and the surface integrals of scalar functions, provided one can collect the function values at the sample points. Examples arise in experiments in which the density of the body can be measured by physical properties of the impacts, or in situations where such quantities as temperature and humidity are observed by randomly distributed sensors. Our method is based on random Delaunay triangulations and involves a simple procedure for surface reconstruction from a dense cloud of points inside and outside the body. We obtain basic asymptotics of the estimator, perform simulations and discuss, via Google Earth's data, an application to the image analysis of the Aral Sea coast and its cliffs.

Fear and humour in the art of cholera

Fear and humour in the art of cholera

Niño Fidencio: De Roma a Espinazo Directed by Juan Farré

Niño Fidencio: De Roma a Espinazo Directed by Juan Farré

Swift multi-wavelength observations of the bright flaring burst GRB 051117A

We report on the temporal and spectral characteristics of the early X-ray emission from the Gamma Ray Burst 051117A as observed by Swift . The superb quality of the early X-ray light-curve and spectra of this source, one of the brightest seen by the X-ray Telescope at such early times, allows an unprecedented look at the spectral and temporal evolution of the prompt and early afterglow emission for this GRB and allows us to place stringent limits on the detection of lines. GRB 051117A displays a highly complex light-curve, with an apparent initial slow decline of slope () dominated by numerous superposed flares of varying amplitude and duration. Between orbits 2 and 3, the X-ray light-curve drops abruptly, highlighting the dominance of flaring activity at early times, and indicating that the central engine for this burst remains active for several kiloseconds after the initial explosion. The late time slope ( s) also decays relatively slowly with a powerlaw index of , breaking to a steeper slope of 1.1, 170 ks after the BAT trigger. The X-ray light-curve at early times is characteristic of a noise process, consisting of random shots superposed on an underlying powerlaw decay, with individual shots well-modelled by a fast-rise and exponential decay spanning a broad range in rise-times and decay rates. A temporal spectral analysis of the early light-curve shows that the photon index and source intensity are highly correlated with the spectrum being significantly harder when brighter, consistent with the movement of the peak of the Band function to lower energies following individual flares. The high quality spectrum obtained from the first orbit of WT mode data, enables us to place a upper limit on the strength of any emission line features of eV, assuming a narrow emission-line of 100 eV at the peak of the effective area.

Cortical Activation While Watching Video Montage: An fMRI Study

Comprehending edited film or video that depicts visual action requires complex perceptual and cognitive activities to appreciate the flow of action through space and time across sequences of shots. We hypothesized that these complex events are associated with the coordinated activities of multiple brain areas that are not activated by random sequences of shots. Using functional magnetic resonance imaging (fMRI), a distributed cortical network was identified that is uniquely activated during viewing of normal video action sequences, but not by sequences of random video shots or by highly scrambled video image sequences. This cortical network includes extrastriate, inferotemporal, parietal, posterior cingulate, and frontal areas and are predominantly in the right hemisphere. Notably, though there was no activation of classical, left hemisphere language areas, there was activation in the right hemisphere homologues of left hemisphere language areas. In all anatomical areas but 1 in the identified network, there was nearby activation during the random shot sequences. This exception, activated only by normal, coherent shot sequences, was in the posterior cingulate (Brodmann area 31). The comprehension of edited visual action sequences that are typical of contemporary film and video formats appears to be based upon the coordinated activities of multiple brain areas that are bound together functionally in a high-level cognitive network.

Dispatch from Mali: A Soap Opera Education

Dispatch from Mali:A Soap Opera Education Anh Ly (bio) Click for larger view View full resolution Figure 1. Television in Mali. Photo by Anh Ly. [End Page 94] My first image of Malian TV occurred unexpectedly during my training as a Peace Corps volunteer. A teacher pulled out a TV one night and turned it on to watch the news. What I saw was clearly an amateur video presented as national TV. Stoic and stiff faces appeared next to random background shots that panned endlessly before focusing haphazardly on some inconsequential object such as a cup. Needless to say, the camera work interested me more than the content of the news. The year was 1996. Mali was still in its nascent stage of TV programming, having only been given the freedom to explore this medium in 1983, a period still under the dictatorial regime of Moussa Traoré. Traoré governed Mali from 1968 until 1991, when a coup under Amadou Toumani Touré, Mali's current president, successfully overthrew him and opened the first democratic elections a year later. Alpha Konaré became Mali's first democratically elected president in 1992. His presidency ushered in a democratic spirit that touched upon every sector of the government, including its radio and TV programming known as L'Office de Radio et Télévision Maliens (Malian Office of Radio and Television), or ORTM. The effects of this new democratic spirit were especially liberating for Malian TV. Ten years later, I returned to Mali to do research for my dissertation. This was not my first visit to Mali since my Peace Corps days. Earlier and shorter visits had already hinted at the bourgeoning presence of TV in Malian life. Walking around Bamako, the capital city, sights of Malians—men, women and children—gathering at sunset around a TV have become a normal albeit unusual scene—normal because group gatherings are ubiquitous in Mali; unusual because they are all gathering in front of a TV. Usually [End Page 95] a Malian who owns a TV will pull it outside in the evenings for himself and the neighbors to watch. TV viewing turns into a collective as well as a neighborhood event. Moreover, owning a TV is no longer quite the luxury item that it was in previous years. After radios, TVs are the most common electronic accessory for Malians (of course, cell phones will soon supercede the two). What has made this change so profound? Surely, it was not the amateur news program that I witnessed ten years earlier. What has captivated Malians, young and old and men and women alike, are the soap operas, otherwise known as feuilleton in French. Under Konaré's presidency, Malians engaged hesitantly but willingly in an unexplored artistic freedom coupled with a creative expression in TV programming. Of course, Malian artistic expression is not a new phenomenon since it has its precedence in the kotéba, a traditional theater group that travels from village to village to perform its varied satires and critiques of Malian society. Extending this artistic expression during the Konaré years to TV programming was a new and unexplored step in Mali's creative production. These years stood in stark contrast to earlier ones when the sole purpose of TV was to show les émissions pédagogiques (pedagogic shows). Today, Malians separate the development of Malian TV from the pre-dictatorial regime of Traoré to the post-democratic rule of Konaré. It was not until 1999 that the first Malian-created, -produced, and -acted soap opera appeared. Before the turn of the century, Malians viewed many imported shows that ranged from Brazilian telenovellas to the egregiously bad nighttime soap operas from the United States. Does anyone remember Dynasty (ABC, US, 1981–1989)? These imported shows provided a sense of escape, at the best, and a dream to imitate, at the worst. Not surprisingly, Malians questioned the values and morals these shows perpetuated. Mali, after all, is a strongly Muslim country. Not only were the values not coherent with their Islamic and cultural beliefs, but also the problems they addressed were foreign—not Islamic, Malian, or even African. The older generation also feared the influence the imported...

Diagnostics of Neutron Star and Black Hole X‐Ray Binaries with X‐Ray Shot Widths

A statistic, w, the differential coefficient of the mean absolute difference of an observed light curve, is proposed for timescale analysis of shot widths. The shortest width of random shots can be measured by the position of the lower cutoff in the timescale spectrum of w. We use the statistic to analyze X-ray light curves from a sample of neutron star and black hole binaries, and the results show that the timescale analysis can help us distinguish between neutron star binaries and black hole binaries. The analysis can further reveal the structure and dynamics of accretion disks around black holes.

Power Spectra of X‐Ray Binaries

The interpretation of Fourier spectra in the time domain is critically examined. Power density spectra defined and calculated in the time domain are compared with Fourier spectra in the frequency domain for three different types of variability: periodic signals, Markov processes, and random shots. The power density spectra for a sample of neutron stars and black hole binaries are analyzed in both the time and the frequency domains. For broadband noise, the two kinds of power spectra in accreting neutron stars are usually consistent with each other, but the time domain power spectra for black hole candidates are significantly higher than corresponding Fourier spectra in the high-frequency range (10-1000 Hz). Comparing the two kinds of power density spectra may help to probe the intrinsic nature of timing phenomena in compact objects.

Caviar and Cabbage: Selected Columns by Melvin B. Tolson from the Washington Tribune, 1937-1944.

Melvin B. Tolson is best known as the poet who wrote Harlem Gallery and Libretto for the Republic of Liberia. He received national acclaim only toward the end of his life, but early in his career he achieved considerable recognition as a challenging speaker and activist within the black American community.Tolson wrote a weekly column for the Washington Tribune from 9 October 1937 to 24 June 1944, entitled Caviar and Cabbage. As the title suggests, the subjects he treated were various. He perceived the problems of the black world of the late thirties and early forties with the insight of an intellectual and the verbal richness and rhythms of a poet heavily influenced by a strong pulpit tradition. This combination makes the columns valuable both as literature and as cultural history.Robert Farnsworth has selected and edited these columns. His introduction describes their cultural and biographical context. He has arranged the columns according to subject: Christ and Radicalism, Race and Class, World War II, Random Shots, Writers and Readings, and Reminiscences. The background material and the arrangement of the works underline their significance.