Plotted in Fig. 9 is the weld energy,expressed as counts detected in a series ofchromium and zinc spot welds versusnugget diameters. The curve wouldapproach a straight line if the counts wereplotted against the nugget area. This curveis typical of the relation between weldenergy count and spot weld size and isthe basis for controlling the weldingprocess with acoustic emissioninstrumentation.234Acoustic Emission TestingFIGURE9.Acoustic emission control ofchromium and zinc spot welds.Acoustic emission energy (counts)Nugget diameter (arbitrary unit)50004000300020000.10.150.2
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Because of its low density, the UnifiedNumbering System A92195 aluminumlithium alloy was developed for the spaceshuttle external tank. The external tank isthe single largest component of the spaceshuttle. It is 47 m (154 ft) long and 8.4 m(27.6 ft) in diameter and serves as thestructural backbone for the shuttle duringlaunch, absorbing most of the more than31 MN (7 ×106lbf) thrust produced. Thealmost four percent decrease in densitybetween the two materials provides anextra 34 kN (7500 lbf) of payload capacitynecessary to put the International SpaceStation components into orbit.The external tank is an entirely weldedstructure; hence, the requirement is for upto five rewelds without hot cracking.Unfortunately, hot cracking duringrewelding or repair operations wasoccurring and had to be remedied beforethe new super lightweight tank could beused. Weld metal porosity formation wasalso of concern because it leads to hotcracking during weld repairs. Accordingly,acoustic emission testing was used tomonitor the formation of porosity andhot cracks to select the best filler metaland optimize the weld schedule.The purpose of this work was todetermine the feasibility of detecting hotcracking in welded aluminum lithiumstructures through the analysis of acousticemission data. By acousticallycharacterizing the effects of reheatingduring a repair, the potential for hiddendiscontinuities coalescing and becomingunstable as the panel is repaired could bereduced. The term unstablerefers to thetendency of microcracks present from aprevious weld pass to join together,forming a critically sized discontinuityupon rewelding. Identifying of regionswhere microcrack growth is likely to occurand the locating of active discontinuitygrowth in the repair weld, as it isperformed, will provide the welder withdirect feedback on the current weldquality and permit adjustments to therepair process in the field.An acoustic emission analysis of thesource mechanisms present duringwelding has been conducted with thegoals of locating regions in the weld linethat are susceptible to damage from arepair operation, identifying theformation of critically sizeddiscontinuities and providingaccept/reject criteria for a weld inprogress.
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