i the top polar acrylic plastic pentagon. Note that the beveled bearing surface 

 nor cracking at the location that corresponds to the O-ring groove on the 



Figure 1 19c. Mechanism of failure by spalling in the bottom polar acrylic plastic pentagon 

 reconstructed from observation of deformation in the bottom steel plate and 

 fracture in the bottom polar acrylic plastic pentagon. 



Figure 1 19b. Fragment from the bottom polar acrylic plastic pentagon. Note that serious spalling has 

 taken place on the beveled bearing surface initiated at the location that corresponds to 

 the O-ring groove on the bottom steel plate. Mechanism of failure is shown in Figure 1 19c. 



