that were already present in the joints because the surface of the preceding 

 pour was not removed were eliminated in some cases by drilling or milling 

 out (trepanning) the separation space and refilling it with fresh PS-18. No 

 reliable method was found to prevent formation of small dispersed cavities 

 in the set adhesive, although it can be hypothesized that a too-rapid setting 

 rate (triggered by high ambient atmospheric temperature) was probably their 

 cause. This hypothesis is substantiated by the fact that some of the pours 

 resulted in completely cavity-free joints, while others had many cavities even 

 though the chemical composition of the mix was identical in all cases. Most 

 of the cavities were too small to merit a trepanning operation, particularly 

 since it was felt that in joints subjected to operational tensile stresses less 

 than 200 psi, the debilitating effect of cavities on the 4,000-to-5,000-psi 

 ability of the joint to withstand 200-psi stresses was negligible. Only in very 

 few cases where the cavities appeared to be interconnected was a trepanning 

 operation performed and the cut refilled with fresh PS-18 adhesive. The tre- 

 panning and refilling of separation spaces in the joint, however, generally did 

 not decrease the peak residual stress level at that location, and in most cases 

 introduced many incipient fine cracks (Figures 53 and B-2b). For this reason, 

 it was used only when the separation space penetrated the whole joint width 

 creating a leak (three locations). 



Figure 49. Bonded hemisphere after removal from assembly jig. 



78 



