DESIGN, CONSTRUCTION, AND OUTFITTING 71 



Fortunately the shipyard maintains the Navy's West Coast Shock Testing Facility and thus 

 has had a fair amount of experience in underwater explosions. The use of the energy of an 

 underwater explosion to form metal is a noval idea, used sparingly in the past to form rela- 

 tively small and simple pieces. The energy of explosion is transmitted as a pressure pulse 

 through the water, forming the steel against a female die. The forming process lasts only a 

 few milliseconds. The employment of this process to form large and complex sections like 

 these dished heads was hitherto never attempted anywhere. Expedience and necessity being 

 the parents of invention, the decision was made to attempt this quantum jump in the technology 

 of metal forming. 



Immediately several problems became apparent: die design and construction, including 

 curing of the concrete, handling and rigging, and configuration and size of the explosive charge. 

 Briefly, a large die, 14-1/2 ft in diameter and 5-1/2 ft high, filled with a special-formula quick- 

 curing concrete, was designed and built. A blank of steel was placed over the die and a vacuum 

 drawn under the blank. This vacuum is extremely important, since any entrapped gas would have 

 to vent, wrinkling the edges of the piece. One hundred pounds of C-4 plastic explosive were 

 distributed in two concentric rings and a lumped central charge. The calculations for charge 

 configuration, size, and standoff distance were extremely complex and important, as was the 

 depth of water at detonation. 



The entire assembly, weighing 60 tons, was lowered 30 ft beneath the surface of San Fran- 

 cisco Bay using the shipyard's large gunning crane. There the explosive was detonated, and in 

 approximately 0.004 sec the first dished head for Sealab II was formed. 



The results were phenomenally good, and only minor straightening in certain areas was 

 required. The heads checked dimensionally within 1/16 in. on the diameter and within 1/4 in. 

 on the contour, well within specifications. The metal did not thin at all, and thickening by ap- 

 proximately 0.075 in. occurred at the rim where stresses were highest. 



A detailed metallurgical analysis was conducted, comparing the stock plate, the place after 

 forming, and the plate after stress relieving. As expected, the severe cold working of the ex- 

 plosive forces embrittled and toughened the plate. Stress relieving restored most of the orig- 

 inal metallurgical properties. 



The expense of die fabrication was considerable, but once made it can be reused. Die life 

 can be made excellent, and once eight heads are formed the process becomes attractively com- 

 petitive. 



The significance of this feat can best be illustrated by excerpts from a UPI story in the 

 Berkeley, California, Gazette, dated Nov 18, 1965. 



"Denver (UPI) - A metal shaping process ... is being studied by Martin Co. and Denver 

 University scientists for possible use in forming missile domes, side plates for ships, and 

 other large structures. 



The technique was demonstrated Wednesday with the production of. . .ash trays. 



It involved the placing of a sheet of metal across a die or mold, then submerging the 

 mold and metal in water. An explosive charge was detonated a few inches away, beneath the 

 water, causing a shock wave to blast the metal into the mold. 



The experiment is being conducted under a one million dollar government grant by 

 DU's Denver Research Institute and the Denver division of the Martin Company. It is ex- 

 pected to take three years to prove or disprove the process." 



Figures 33 through 44 on the succeeding pages illustrate the process. 



