Several specimens leaked during the test. Most of the leaks 

 occurred between the concrete and steel end-closures. However, some 

 of the specimens had cracks through the wall and water slowly leaked 

 through these cracks until the pressure load exceeded approximately 

 200 psi (1.4 MPa). Leaks did not affect the test results. 



For the long-term loading tests, a digital comparator was used to 

 control the operation of an auxHiary pressure pump and maintain the 

 pressure load at ±2 psi (0.014 MPa). 



Failure of the specimens was instantaneous, with literally a fraction 

 of a second of advanced warning. High-speed motion picture film of 

 failure was not obtained during eight attempts. In several of the 

 cases, the fOm was exposed before implosion occurred. In the other 

 cases, implosion occurred while the operator was waiting for an indica- 

 tion of failure. Methods used to indicate failure are described below. 



(1) Radial Displacement - A real time signal from a poten- 

 tiometer was displayed on an oscilloscope. During 

 pressurization periods, the potentiometer was placed on 

 the worst flat-spot (probable failure zone), and the 

 rate of inward radial displacement with pressure was 

 monitored. It was beheved that an increase in the 

 rate of change of radial displacements would indicate 

 implosion. This was the case, but warning time was 

 not sufficient to trigger a toggle switch to operate the 

 camera. 



(2) Acoustic Emissions - Acoustic emission transducers 

 were placed on the pressure vessel head or on the top 

 stiff ener to record cracking activity of the concrete. 

 It was hoped that the concrete would show consider- 

 ably more cracking activity just before failure. This 

 method was not successful because the acoustic emis- 

 sion activity of concrete is high and erratic in the 



49 



