Plotted in fig 9 is the weld energy expressed as

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Plotted in Fig. 9 is the weld energy, expressed as counts detected in a series of chromium and zinc spot welds versus nugget diameters. The curve would approach a straight line if the counts were plotted against the nugget area. This curve is typical of the relation between weld energy count and spot weld size and is the basis for controlling the welding process with acoustic emission instrumentation. 234 Acoustic Emission Testing F IGURE 9. Acoustic emission control of chromium and zinc spot welds. Acoustic emission energy (counts) Nugget diameter (arbitrary unit) 5000 4000 3000 2000 0.1 0.15 0.2
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Because of its low density, the Unified Numbering System A92195 aluminum lithium alloy was developed for the space shuttle external tank. The external tank is the single largest component of the space shuttle. It is 47 m (154 ft) long and 8.4 m (27.6 ft) in diameter and serves as the structural backbone for the shuttle during launch, absorbing most of the more than 31 MN (7 × 10 6 lb f ) thrust produced. The almost four percent decrease in density between the two materials provides an extra 34 kN (7500 lb f ) of payload capacity necessary to put the International Space Station components into orbit. The external tank is an entirely welded structure; hence, the requirement is for up to five rewelds without hot cracking. Unfortunately, hot cracking during rewelding or repair operations was occurring and had to be remedied before the new super lightweight tank could be used. Weld metal porosity formation was also of concern because it leads to hot cracking during weld repairs. Accordingly, acoustic emission testing was used to monitor the formation of porosity and hot cracks to select the best filler metal and optimize the weld schedule. The purpose of this work was to determine the feasibility of detecting hot cracking in welded aluminum lithium structures through the analysis of acoustic emission data. By acoustically characterizing the effects of reheating during a repair, the potential for hidden discontinuities coalescing and becoming unstable as the panel is repaired could be reduced. The term unstable refers to the tendency of microcracks present from a previous weld pass to join together, forming a critically sized discontinuity upon rewelding. Identifying of regions where microcrack growth is likely to occur and the locating of active discontinuity growth in the repair weld, as it is performed, will provide the welder with direct feedback on the current weld quality and permit adjustments to the repair process in the field. An acoustic emission analysis of the source mechanisms present during welding has been conducted with the goals of locating regions in the weld line that are susceptible to damage from a repair operation, identifying the formation of critically sized discontinuities and providing accept/reject criteria for a weld in progress.
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