Miles, Mutants and Microbes - Lois McMaster Bujold [15]
Leo rotated the image so that each student could see it from every angle. "Now let's magnify this part. You see the deep-V cross section from the high-energy-density beam, familiar from your basic welding courses, right? Note the small round porosities here . . ." The magnification jumped again. "Would you say this weld is defective or not?" He almost added, Raise your hand, before realizing what a particularly unintelligible directive that was here. Several of the red-clad students solved the dilemma for him by crossing their upper arms formally across their chests instead, looking properly hesitant. Leo nodded toward Tony.
"Those are gas bubbles, aren't they sir? It must be defective."
Leo smiled thanks for the desired straight line. "They are indeed gas porosities. Oddly enough, though, when we crunch the numbers through, they do not appear to be defects. Let us run the computer scan down this length, with an eye to the digital read-out. As you see," the numbers flickered at a corner of the display as the cross-section moved dizzyingly, "at no point do more than two porosities appear in a cross-section, and at all points the voids occupy less than five percent of the section. Also, spherical cavities like these are the least damaging of all potential shapes of discontinuities, the least likely to propagate cracks in service. A non-critical defect is called a discontinuity." Leo paused politely while two dozen heads bent in unison to highlight this pleasingly unambiguous fact on the autotranscription of their light boards, braced between lower hands for a portable recording surface. "When I add that this weld was in a fairly low-pressure liquid storage tank, and not, for example, in a thruster propulsion chamber with its massively greater stresses, the slipperiness of this definition becomes clearer. For in a thruster the particular degree of defect that shows up here would have been critical.
"Now." He switched the holovid display to one in red light. "This is a holovid of the same weld from data bits mapped by an ultrasonic pulse reflective scan. Looks quite different, doesn't it? Can anyone identify this discontinuity?" He zoomed in on a bright area.
Several sets of arms crossed again. Leo nodded toward another student, a striking boy with aquiline nose, brilliant black eyes, wiry muscles, and dark mahogany skin contrasting elegantly with his red T-shirt and shorts. "Yes, Pramod?"
"It's an unbonded lamination."
"Right!" Leo tapped his holovid controls. "But check down this scan—where have all our little bubbles gone? Anybody think they magically closed between tests? Thank you," he said to their knowing grins, "I'm glad you don't think that. Now let's put both maps together." Red and blue melded to purple at overlapping points as the computer integrated the two displays.
"And now we see the little bugger," said Leo, zooming in again. "These two porosities, plus this lamination, all in the same plane. You can see the fatal crack starting to propagate already, on this rotation—" The holovid turned, and Leo emphasized the crack with a bright pink light. "That, children, is a defect."
They oohed in gratifying fascination. Leo grinned and plunged on. "Now, here's the point. Both these test scans were valid pictures—as far as they went. But neither one was complete, neither alone sufficient. The maps were not the territories. You have to know that x-radiography is excellent for revealing voids and inclusions, but poor at finding cracks except at certain chance alignments, and ultrasound is optimum for just those laminar discontinuities x-rays are most likely to miss. Both maps, intelligently integrated, yielded a judgment."
"Now." Leo smiled a bit grimly, and replaced the gaudy image with another, monochrome green this time. "Look at this. What do you see?" He nodded at Tony again.