This is a project in the Materials Reliability Division to conduct research on acoustic emission directed toward, and funded in part, by the Federal Highway department. Acoustic emissions are elastic waves produced by defects in a material when that material is placed under stress. The goal of the research is to describe these emissions in terms of wave forms, rather than simply looking at their minimum amplitude or frequency band. Better sensors have recently been developed at NIST, which provides the capability to inspect the wave forms.
Over the past few years we have developed finite difference and finite element models of elastic waves in plates. These have been validated by comparison with waves generated in the laboratory by breaking pencil leads on a plate. The main models are a two-dimensional model which assumes cylindrical symmetry and a three dimensional code for anisotropic media.
This year numerical experiments were done on one-inch-thick plates in order to determine the resolution required to reproduce the Rayleigh wave as well as the Lamb waves. Comparisons were made with laboratory measurements. Lead breaks along the edge of the plate as well as the surface were compared. A paper has been submitted which describes this work. This work has attracted the attention of a consortium of oil companies which has started a three year project to develop acoustic emission techniques to check pipelines. They have offered to contribute $10000 to NIST to indicate their support for NIST research in this area, and also to receive data from the NIST work as soon as possible. The cylindrical model has been used to compare measurements done within a CRADA between NIST and Deci Consultants Inc. A message passing routine for the 3-d Wave code was written in MPL and timed on the IBM SP2.
Although two papers have been written concerning the validation of the model by comparison with laboratory measurements, the main justification for this effort is the analysis of numerical experiments on emissions from sources internal to the plate. These can not be done in a controlled manner in the laboratory. Such experiments will be run in the near future. The message passing routine will be combined with the 3-d wave code so that the latter can be run on a parallel machine.