Search

For recovering the block-sparse signal with unknown block structures, this paper presents a Block Stage-wise Weak Orthogonal Matching Pursuit (BSWOMP) algorithm. Which partitions the original signal into different groups or segments and conducts the selection for these groups separately using weak selection method to update reconstructing support set. And it proves that the Restricted Isometry Property (RIP) of BSWOMP algorithm is slack than Stage-wise Weak Orthogonal Matching Pursuit (SWOMP) algorithm and Block Orthogonal Matching Pursuit (BOMP) algorithm, this means BSWOMP algorithm generates better convergence than traditional sparse signal recovery algorithm. To further analysis of the performance of BSWOMP, the effectiveness of this algorithm is demonstrated by simulations. Experiments results show that the proposed algorithm effectively selects the nonzero elements of the block-sparse signal by a threshold, and compared with the traditional algorithms, BSWOMP algorithm has higher recovering stability, expedites the approximate conjugate gradient update step and brings an evident performance improvement and application prospect.

MicroRNAs (miRNAs) are potent regulators of gene transcription and posttranscriptional processes. The majority of miRNAs are localized within intronic regions of protein-coding genes (host genes) and have diverse functions in regulating important cellular processes in animals. To date, few plant intronic miRNAs have been studied functionally. Here we report a comprehensive computational analysis to characterize intronic miRNAs in rice and Arabidopsis. RT-PCR analysis confirmed that the identified intronic miRNAs were derived from the real introns of host genes. Interestingly, 13 intronic miRNAs in rice and two in Arabidopsis were located within seven clusters, of which four polycistronic clusters contain miRNAs derived from different families, suggesting that these clustered intronic miRNAs might be involved in extremely complex regulation in rice. Length analysis of miRNA-carrying introns, promoter prediction and qRT-PCR analysis results indicated that intronic miRNAs are coexpressed with their host genes. Expression pattern analysis demonstrated that host genes had a very broad expression spectrum in different stages of development, suggesting the intronic miRNAs might play an important role in plant development. This comparative genomics analysis of intronic miRNAs in rice and Arabidopsis provides new insight into the functions and regulatory mechanisms of intronic miRNAs in monocots and dicots.

Fatigue life prediction and optimization is becoming a critical issue affecting the structural applications of cast aluminum-silicon alloys in the aerospace and automobile industries. In this study, a range of microstructure and porosity populations in A356 alloy was created by controlling the casting conditions and by applying a subsequent hot isostatic pressing (“hipping”) treatment. The microstructure and defects introduced during the processing were then quantitatively characterized, and their effects on the fatigue performance were examined through both experiment and modeling. The results indicated that whenever a pore is present at or near the surface, it initiates fatigue failure. In the absence of large pores, a microcell consisting of α-Al dendrites and associated Si particles was found to be responsible for crack initiation. Crack initiation life was quantitatively assessed using a local plastic strain accumulation model. Moreover, the subsequent crack growth from either a pore or a microcell was found to follow a small-crack propagation law. Based on experimental observation and finite-element analysis, a unified model incorporating both the initiation and small crack growth stages was developed to quantitatively predict the dependency of fatigue life on the microstructure and porosity. Good agreement was obtained between the model and experiment.

The demand for low-cost, high strength-to-weight ratio components continues to drive the development of aluminum alloy castings for new applications. The automotive brake caliper, which has traditionally been made from cast iron, is one such component where aluminum alloys are being considered. The fatigue performance of the brake caliper is an important consideration in evaluating new designs. The use of aluminum alloys in cast components under cyclic loading conditions necessitates a design approach that, in addition to in-service loading, incorporates the impact of microstructural features on fatigue life. In this investigation, a through-process modeling approach has been employed to link a series of mathematical models describing the processing steps of (1) casting, (2) heat treatment, (3) machining, and (4) in-service performance for a preliminary design of a brake caliper made of aluminum alloy A356, Step (1) includes microstructural predictions (secondary dendrite arm spacing and maximum pore size), which are tracked through to the final component. The final lifing of the component combines the effects of these microstructural features with the complex stress state arising from the combined service loading and residual stresses. It was found that all three factors have a strong influence upon the component’s fatigue performance.

Porosity is well known to be a potent initiator of fatigue cracks in cast aluminum alloys. This article addresses the observed scatter in fatigue life of a cast A356-T6 aluminum-silicon alloy due to the presence of porosity. Specimens containing a controlled amount of porosity were prepared by employing a wedge-shaped casting mold and adjusting the degassing process during casting. High-cycle fatigue tests were conducted under fixed stress conditions on a series of specimens with controlled microstructures (especially, the secondary dendrite-arm spacing), and the degree of scatter in the results was assessed. Stochastically, such scatter was found to be adequately characterized by a three-parameter Weibull distribution function. Large pores at or close to the specimen surface were found to be responsible for crack initiation in all fatigue-test specimens, and the resultant fatigue life was related to the initiating pore size through a relationship based on the rate of small-fatigue-crack propagation. With respect to the probabilities for the pores of various sizes and locations to initiate a fatigue crack, a statistical model was developed to establish the relationship between the porosity population and the resultant scatter in fatigue life. The modeling predictions are in agreement with the experimental results. Moreover, Monte-Carlo simulation based on this model demonstrated that the average pore size, pore density, and standard deviation of the pore sizes, together with the specimen size and geometry, are all of consequence regarding scatter in fatigue life.

A micro-scale analytical model for predicting size-dependent microhardness is presented. The indentation size effect is explained by dissipation energy associated with the contact surfaces. Micro-mechanics mechanism of this energy dissipation is studied via a plastic deformation analysis of micro-scale asperities on the contact surface under indentation. The analysis shows that the microhardness depends not only on the properties of the bulk material under test, but also the properties of the contact surface, such as the plastic behavior of the micro-asperities on contact surfaces. The dependence of microhardness on the roughness parameters of the contact surfaces is also revealed from the analysis.