Search

46-3364 HD1698 2007-45423 CIP Ray, Binayak. Water: the looming crisis in India. Lexington Books, 2008. 231p bibl index afp ISBN 0739126016, $80.00; ISBN 9780739126011, $80.00 This highly specialized study focuses [...]

A frequency stable self-sustained low-pressure TE CO2 laser oscillator incorporating an intracavity telescope has been developed with a variable pulse duration of between 30 and 10 μs. Heterodyne measurements show that the intrapulse frequency is constant to within 50 kHz for a 20 mJ output pulse for the first 8 μs of the optical pulse. The laser output has been used for pulsed injection into a dispersionless TEA CO2 laser cavity to obtain single line, single mode selection and the results obtained are compared with those obtained from a c.w. injection source.

A resonantly pumped three-level laser is analysed in a high-Q cavity under conditions of negligible and high pump depletion to obtain expressions for the emission characteristics in analytic form. Under high pump depletion our analysis predicts bistable emission in its steady state behaviour.

The use of cavity length control to obtain multiline emission from a hybrid CO2 laser has been investigated. We have obtained simultaneous laser oscillation on up to four lines in the P and R branches of the 10 μm band by operating the c.w. section above the lasing threshold. For sub-threshold operation of the c.w. section the hybrid laser emission is restricted to the stronger 10P lines.

Synopsis A simple mathematical model is proposed in which the failure of concrete under uniaxial compression is attributed to the instability which develops in the macro-elemental structure formed by cracking. The model predicts that there is a critical fracture length which is, with the ultimate stress, dependent upon factors such as size of specimen and longitudinal stress gradient which have previously only been explained by statistical strength theories. The model predictions are compared with the experimental evidence available.

Synopsis The results of an experimental programme on the failure of concrete when subjected to longitudinal gradients of compressive stress are reported. The test specimens are point-loaded reinforced concrete beams. beams of tapering width under constant moment and tapering prisms under axial compression. The results confirm that the compressive strength of concrete is significantly increased in the presence of such non-uniform stress states. Existing theories of failure, which are usually derived from and tested under uniform stress states, do not appear to be capable of explaining this effect. The fracture mechanism observed involves a finite volume of material rather than a section as is usually implied, and appears to be due to the formation of a post-cracking macro-structure of longitudinal strips which fails owing to flexural instability. Such a fracture model could explain the observed increases in the compressive strength of concrete due to longitudinal stress gradients.