Appendix B 

 FAILURE MODES OF FLAT ACRYLIC WINDOWS 



DISCUSSION 



In nearly all cases for all sizes of windows tested, failure began with radial 

 cracking on the window's low-pressure face. Radiating outward from near the 

 center, the cracks commonly formed a nonsymmetrical, three- or four-pointed 

 figure. This form of cracking preceded failure in nearly all cases and Is assumed to 

 be the beginning of failure (Figures B-1 and B-2). Depth of cracking was found to 

 be a function of the thickness, t/D| ratio of the window, and the pressure of the 

 fluid. Since audible cracking was noted during testing, it Is postulated that these 

 radial cracks were rapidly formed, terminating at the window's D;. Depth of 

 cracking in the low-pressure face in most cases was found to be a small fraction of 

 the window's thickness. 



With additional pressurization, a second stage of failure began to develop. 

 A conchoidal or "cupped cone" fracture was established, emanating from the 

 base of the radial cracks and proceeding radially inward and circumferentially 

 (Figures B-3 and B-4). The formation of a conchoidal fracture surface preceded 

 failure in all cases observed. 



Simultaneously, as the conchoidal fracture surface was formed, the radial 

 cracks increased slightly in depth (Figures B-5 and B-6). Cracks did not deepen 

 uniformly and new cracks developed with further pressurization. The additional 

 cracking gave rise to formation of new and deeper conchoidal fracture surfaces. 

 Additional pressurization caused the circumferential expansion and coalescence 

 of the conchoidal fracture surfaces into one conical fracture surface as well as an 

 increase in fracture depth (Figures B-7 and B-8). Cracking and formation of con- 

 choidal fracture surfaces continued (Figures B-9 and B-10) deeper into the window's 

 thickness until the critical pressure was finally reached resulting in the fragmentation 

 and expulsion of the window's low-pressure face (Figures B-11 and B-12). The size 

 of the central hole was a function of t/D; ratio and D|. The conical cavity resulting 

 from the expulsion of the center portion of the window consistently assumed an 

 approximate angle of 30 degrees with the high-pressure face. 



Cracking between the window's Dj and Dq occurred concentrically with the 

 window's circumference, nearly perpendicular to and emanating from the low-pressure 

 face. This cracking was sometimes accompanied by small radial intersecting cracks 

 (Figure B-9). This form occurred with larger t/Dj ratios, failure still assuming the 

 conical surface form. The circumferential cracks sometimes penetrated the window's 

 thickness but still did not constitute a plane of failure. 



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