The three low density indications indicate varying

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density indications and (4) delaminations. The three low density indications indicate varying degrees of resin richness or starvation — areas in which absence of fibers shows up as dark spots on the radiographic image. These low density indications are normally present where the skirt and dome meet the cylindrical portion of the case (Fig. 20) because of the bridging gaps produced there by changes in material thickness. Low density indications are also common in the loosely woven helical layers of the domes. The case was radiographed twice, once before hydrostatic proof testing and once afterward. Neither test revealed any serious damage. The predominant indications were least severe low density indications in the first helical layers. This condition is typical of all cases. There were a few moderate low density indications in the third helical layer plus the standard bridging gap indications near the domes but there were no most severe low density indications and no visible delaminations. The only correlation found between the radiographic and acoustic emission test data was that both seemed to indicate activity primarily in the helical layers. The acoustically active areas did not appear anomalous in the radiographic images. Conclusions The most important acoustic emission quantification parameters for characterizing discontinuity types are event amplitude, energy and duration. Posttest discontinuity criticality is best described by the curve of cumulative events versus time and of the events during load hold. The energy distribution is the best real time indicator of the onset of delaminations or fiber breaks. The acoustic emission amplitude and energy data during pressurization indicated localized delaminations and fiber breaks near transducers 4, 5, 6, 8, 10, 11, 14 and 26. There was also a possibility of damage near transducer 29. These areas were active during both the leak check and the hydrostatic proof testing. However, none of this activity was confirmed by the radiographs. This is not surprising in that radiographs reveal only macroscopic discontinuities whereas acoustic emission testing detects microscopic activity such as individual fiber breaks. Hence, the acoustic emission test indicated discontinuity growth activity was undoubtedly well below the threshold of detectability of the radiographic equipment. Another reason for the lack of confirmation between the two techniques was the sampling interval. Where the acoustic emission test covered the entire case, with some overlap, the tangential radiographs were taken at 0.52 rad (30 deg) intervals, meaning that only 12 discrete longitudinal slices (six cross sections in Fig. 20) were taken along the length of the case. Many of the acoustically active areas could well have been between these cross sections.
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  • Fall '19
  • Fighter aircraft, Nondestructive testing, Acoustic Emission, Acoustic Emission Testing

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