Studies on the environmental stress corrosion cracking of model epoxy-glass composites.

The stress corrosion cracking of model epoxy-glass 0°/90°/0° crossply and 0 unidirectional composites in aqueous sulphuric acid has been studied. Specimens, in the form of coupons cut from laminates were tested under conditions of uniaxial tension at constant load, whilst partially immersed in an acidic environment. Two different modes of failure, which resulted in fracture were observed. In the "so-called" Mode I failure, fracture occurred within that part of the composite immersed in the acid,whereas in Mode II, it occurred in the unimmersed part. Both failure modes were observed for crossply and unidirectional composites. In unstressed 0°/90°/0° specimens a third failure mode (Mode III) was observed, in which the damage took the form of transverse and longitudinal cracking of the unimmersed part. The failure mode depended upon the magnitude of the initial applied strain, the nature of the environment, and the type of environment cell. At initial applied strains of greater than about 0.15% only Mode I failure was observed. The similarity between the times-to-failure of laminates with those recorded for single E-glass fibres showed that the resin was not providing significant protection from the acidic environment. To account for this result it is postulated that the acid rapidly permeates these composites through environmental microcracks, which form parallel to the axis of the glass reinforcement. in the 0° plies. The formation of these microcracks is due to a reduction in the resin/glass interfacial strength in the presence of acids. Confirmation of this phenomenon was obtained from experiments on the transverse cracking behaviour of crossply laminates, immersed in aqueous acid and tensile tested at constant strain rate. At initial applied strains of less than about 0.15% and depending upon the experimental conditions, failure was by either Mode I or II. Mode II, which occurs in shorter times than Mode I was observed in acids (e. g. sulphuric acid) giving rise to relatively insoluble glass degradation products, providing evaporation of moisture from the unimmersed part of the specimen was possible. The Mode II failure mechanism has been identified with the transport of the aqueous acid along the glass resin interface from the immersed to the unimmersed part of the composite. Here precipitation of the less soluble glass degradation products causes a localised stress sufficient to initiate and propagate a stress corrosion crack. At these strains both Mode I and II failures occur at times significantly greater than those observed for glass fibres. Mode III failure is similarly attributed to the precipitation of glass degradation products within the composite. Microscopical examination of the stress corrosion fracture surfaces did not reveal any morphological differences between Mode I and II failure, apart from the appearance of more crystalline products in the Mode II fractures. Although glass fibre fracture is the ultimate failure mechanism in these composites, stress corrosion of the fibre matrix interface was found to be a necessary precursor

Bedrest in healthy women: Effects of menstrual function and oral contraceptives

With the development of the space shuttle program, space flight for the first time is available to individuals who have not been specially selected and trained to be astronauts. In addition, women are being actively recruited into the space program, both as mission specialists and as career astronauts. One purpose of this project was to examine some of the physiological responses of women to a simulated weightlessness program (12 day horizontal bedrest), to compare their responses to those reported in men during similar programs, and to test whether menstrual function might alter some of the physiological changes which occur during bedrest, specifically changes in the plasma volume, exercise tolerance, and venous compliance before and after bedrest

Human Mesenchymal Stem Cells Self-Renew and Differentiate According to a Deterministic Hierarchy

BACKGROUND:Mesenchymal progenitor cells (MPCs) have been isolated from a variety of connective tissues, and are commonly called "mesenchymal stem cells" (MSCs). A stem cell is defined as having robust clonal self-renewal and multilineage differentiation potential. Accordingly, the term "MSC" has been criticised, as there is little data demonstrating self-renewal of definitive single-cell-derived (SCD) clonal populations from a mesenchymal cell source. METHODOLOGY/PRINCIPAL FINDINGS:Here we show that a tractable MPC population, human umbilical cord perivascular cells (HUCPVCs), was capable of multilineage differentiation in vitro and, more importantly, contributed to rapid connective tissue healing in vivo by producing bone, cartilage and fibrous stroma. Furthermore, HUCPVCs exhibit a high clonogenic frequency, allowing us to isolate definitive SCD parent and daughter clones from mixed gender suspensions as determined by Y-chromosome fluorescent in situ hybridization. CONCLUSIONS/SIGNIFICANCE:Analysis of the multilineage differentiation capacity of SCD parent clones and daughter clones enabled us to formulate a new hierarchical schema for MSC self-renewal and differentiation in which a self-renewing multipotent MSC gives rise to more restricted self-renewing progenitors that gradually lose differentiation potential until a state of complete restriction to the fibroblast is reached

The James Webb Space Telescope Mission

Twenty-six years ago a small committee report, building on earlier studies, expounded a compelling and poetic vision for the future of astronomy, calling for an infrared-optimized space telescope with an aperture of at least $4m$. With the support of their governments in the US, Europe, and Canada, 20,000 people realized that vision as the $6.5m$ James Webb Space Telescope. A generation of astronomers will celebrate their accomplishments for the life of the mission, potentially as long as 20 years, and beyond. This report and the scientific discoveries that follow are extended thank-you notes to the 20,000 team members. The telescope is working perfectly, with much better image quality than expected. In this and accompanying papers, we give a brief history, describe the observatory, outline its objectives and current observing program, and discuss the inventions and people who made it possible. We cite detailed reports on the design and the measured performance on orbit.Comment: Accepted by PASP for the special issue on The James Webb Space Telescope Overview, 29 pages, 4 figure

The Science Performance of JWST as Characterized in Commissioning

This paper characterizes the actual science performance of the James Webb Space Telescope (JWST), as determined from the six month commissioning period. We summarize the performance of the spacecraft, telescope, science instruments, and ground system, with an emphasis on differences from pre-launch expectations. Commissioning has made clear that JWST is fully capable of achieving the discoveries for which it was built. Moreover, almost across the board, the science performance of JWST is better than expected; in most cases, JWST will go deeper faster than expected. The telescope and instrument suite have demonstrated the sensitivity, stability, image quality, and spectral range that are necessary to transform our understanding of the cosmos through observations spanning from near-earth asteroids to the most distant galaxies.Comment: 5th version as accepted to PASP; 31 pages, 18 figures; https://iopscience.iop.org/article/10.1088/1538-3873/acb29

Finishing the euchromatic sequence of the human genome

The sequence of the human genome encodes the genetic instructions for human physiology, as well as rich information about human evolution. In 2001, the International Human Genome Sequencing Consortium reported a draft sequence of the euchromatic portion of the human genome. Since then, the international collaboration has worked to convert this draft into a genome sequence with high accuracy and nearly complete coverage. Here, we report the result of this finishing process. The current genome sequence (Build 35) contains 2.85 billion nucleotides interrupted by only 341 gaps. It covers ∼99% of the euchromatic genome and is accurate to an error rate of ∼1 event per 100,000 bases. Many of the remaining euchromatic gaps are associated with segmental duplications and will require focused work with new methods. The near-complete sequence, the first for a vertebrate, greatly improves the precision of biological analyses of the human genome including studies of gene number, birth and death. Notably, the human enome seems to encode only 20,000-25,000 protein-coding genes. The genome sequence reported here should serve as a firm foundation for biomedical research in the decades ahead

Initial Public Offerings and the Firm Location

The firm geographic location matters in IPOs because investors have a strong preference for newly issued local stocks and provide abnormal demand in local offerings. Using equity holdings data for more than 53,000 households, we show the probability to participate to the stock market and the proportion of the equity wealth is abnormally increasing with the volume of the IPOs inside the investor region. Upon nearly the universe of the 167,515 going public and private domestic manufacturing firms, we provide consistent evidence that the isolated private firms have higher probability to go public, larger IPO underpricing cross-sectional average and volatility, and less pronounced long-run under-performance. Similar but opposite evidence holds for the local concentration of the investor wealth. These effects are economically relevant and robust to local delistings, IPO market timing, agglomeration economies, firm location endogeneity, self-selection bias, and information asymmetries, among others. Findings suggest IPO waves have a strong geographic component, highlight that underwriters significantly under-estimate the local demand component thus leaving unexpected money on the table, and support state-contingent but constant investor propensity for risk