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

Long Wave Ultrasonic Characterization of Inclusions in Silicon Nitride

The size and material content of Fe and Si inclusions in Si3N4 have been measured by means of pulse-echo scattering of elastic waves in the frequency range 5 to 100 MHz. The inclusions were of 100 μm and 400 μm nominal diameters and were located 3 mm deep in the Si3N4. The electronic noise was reduced by signal averaging and, in some cases, the noise due to grain scattering was reduced by averaging over transducer position. The scattering amplitude A(ω) and the impulse function R(t) were obtained by a desensitized deconvolution of the reference waveform obtained by reflecting the transmitted pulse from the back surface of the sample in a defect-free region. Comparison of theory and experiment are given for A(ω) and estimates of flaw size and material content are presented