Acoustic emission frequency discrimination

In acoustic emission nondestructive testing, broadband frequency noise is distinguished from narrow banded acoustic emission signals, since the latter are valid events indicative of structural flaws in the material being examined. This is accomplished by separating out those signals which contain frequency components both within and beyond (either above or below) the range of valid acoustic emission events. Application to acoustic emission monitoring during nondestructive bond verification and proof loading of undensified tiles on the Space Shuttle Orbiter is considered

Acoustic Emission in Composites Using MPA

The purpose of this study is to try to determine the current mechanical state of a composite specimen (or structure) and predict its remaining lifetime from the characteristics of the acoustic emission (AE) signals it emits under load. In previous studies of the characteristics of AE generated in graphite-epoxy composites, empirical observations were made relating the frequency content and the amplitude distributions of the AE signals to singular points on the loading curve of a specimen and to the composite\u27s, moisture content. Up to now, these relationships have been difficult to study systematically because of limitations in efficiently handling the large amount of data contained in the emission signals. With the Acoustic Emission Multi-Parameter Analyzer (AEMPA) (developed under a Science Center IR&D program), pertinent information is abstracted from each emission signal as it occurs during a test and is stored in compact digital form for subsequent data processing. Multi-parameter correlation and pattern recognition techniques among the 23 abstracted parameters are then used to identify distinct types of AE events, and various observations used ,to \u27identify the microscopic mechanisms of flaw growth in the material which generate these different\u27types. For those geometries and load conditions which product failure by a well defined series of mechanistic steps (e.g., matrix crazing, fiber-matrix interface debonding, fiber fracture, interlaminar fracture), it may be possible to predict specimen·failure by determining the relative amounts of the various mechanisms occurring at a given time in the life of the specimen from the AE signals. Progress along these lines using MPA is described

Sources of Acoustic Emission in Aluminum Alloys

I\u27d like to start the talk with a brief description of this task on acoustic emission (AE) source identification in terms of its immediate aims and ultimate goals. The _immediate aims were to, first, identify the sources of AE in a variety of materials, making a survey of just where they originate, and their dependence on changes in microstructure. The second aim was to identify characteristics of the AE signals which might be related to these sources and, therefore, indirectly to the microstructural effects. These aims were realized for the materials studied. The ultimate goals of such a study would be the extrapolation of AE data from one test situation or from one material to another, and, ideally, to relate the emissions to a determination of flaw criticality

Statistical Evaluation of Sources of Acoustic Emission in Composites

Acoustic emission (AE) signals that are generated by different microscopic processes during flaw growth in graphite-epoxy specimens have measurably different characteristics. In particular, the amplitudes of the emissions and a parameter that describes their frequency spectral content seem to give the most information about the processes. These parameters have a range of values for a given process which can be described by certain types of analytical distribution functions. When several processes occur simultaneously during flaw growth, such as epoxy crazing, fiber fracture, fiber-matrix disband and interlaminar cleavage, the distributions in the values of the AE parameters generally overlap so that identification of an individual AE signal as being caused by a particular process is not possible. However, statistical evaluation of the data for a few hundred events in terr.ls of the analytical distributions, once the shape and modal value of these distributions are defined for each process, should provide a quantitative measure of the relative amounts of the various processes which occurred. Analyses of many data sets are required to develop confidence in the decomposed distributions as being descriptive of the individual processes. The ultimate purpose for this determination is to provide a description of the stage of flaw growth from the quantitative knowledge of the types and the amounts of the microscopic processes which occurred

Characterization of Acoustic Emission Signals and Application to Composite Structures Monitoring

The objectives of this study were first, to identify characteristics of the acoustic emission signals from graphite-epoxy composites which could be related to the various fracture mechanisms, and second, to determine how these are related to the history of the flaw growth and to the degree of degradation of the strength of the composites due to moisture

Effects of rising temperatures and [CO2] on the physiology of tropical forest trees

Using a mixture of observations and climate model outputs and a simple parametrization of leaf-level photosynthesis incorporating known temperature sensitivities, we find no evidence for tropical forests currently existing ‘dangerously close’ to their optimum temperature range. Our model suggests that although reductions in photosynthetic rate at leaf temperatures (TL) above 30°C may occur, these are almost entirely accountable for in terms of reductions in stomatal conductance in response to higher leaf-to-air vapour pressure deficits D. This is as opposed to direct effects of TL on photosynthetic metabolism. We also find that increases in photosynthetic rates associated with increases in ambient [CO2] over forthcoming decades should more than offset any decline in photosynthetic productivity due to higher D or TL or increased autotrophic respiration rates as a consequence of higher tissue temperatures. We also find little direct evidence that tropical forests should not be able to respond to increases in [CO2] and argue that the magnitude and pattern of increases in forest dynamics across Amazonia observed over the last few decades are consistent with a [CO2]-induced stimulation of tree growth

Book Reviews

Book Reviews The American Lawyer Albert P. Blaustein Charles O. Porter with Charles T. Duncan Chicago: The University of Chicago Press,1954. Pp. xiii, 360. $5.50. reviewer: Lloyd Paul Stryker =================================== American Business Corporations Until 1860; with Special Reference to Massachusetts By Edwin Merrick Dodd. Cambridge: Harvard University Press, 1954. Pp. xix, 524.$7.50 reviewer: Howard Jay Graha

Th/K and Th/U ratios from spectral gamma-ray surveys improve the mapped definition of subsurface structures

Spectral gamma-ray data can be obtained by non-destructive, automated, rapid and inexpensive survey methods. Previous studies utilise only total count, or total count as well as K, U and Th data. This work examines the use of Th/K and Th/U ratios to define the subsurface extent of partially-buried features at the centimeter to meter scale. On-site results are presented from two case studies as two-dimensional cross-sections. Changes in Th/K and Th/U ratios coincide with the known location of buried structures to within 10cm horizontal resolution. Gradual changes in total count, K, U and Th measurements give a lower horizontal accuracy of 30cmto1m. Grids of data were manipulated in ArcGIS™ using a thin plate spline function to maximise information use and provide ‘easy to interpret’ maps of the survey areas. Unlike total count or individual element maps, Th/K and Th/U ratio maps can be compared to the known location of subsurface structures, vindicating the use of ratio cross-sections and maps in archaeological, geotechnical and forensic applications. It is concluded that the capacity to observe sub-surface features is enhanced through the use of Th/K and Th/U ratios