A New Combination Shell Strength and Permeability Test

摘要

The standard shell MOR / MOE test requires considerable sample preparation, in particular a dusty grinding step. The potential exists for introducing stress-raising defects that might affect the results. Because it is a bend test, the specimen has one side in tension while the other is in compression. This is a major difference from dewax where the entire shell thickness is in tension. A new test has been developed using PVC water pipe. The water pipe is invested in the conventional manner. After final dry the shelled pipe is placed in a freezer. An hour later the plastic pipe can be easily pulled from the shell. The tubular shell is then sawed to roughly a seven-inch length. Cylindrical aluminum pieces, with double O-rings, are inserted in each end. They are held in position with an aluminum tool or bar clamps. The shell pipe can be burst tested with pressurized water. Water pressure is slowly increased until the pipe fractures. Maximum tensile hoop stress can be computed using the formula for stresses in a thick walled pipe. Cold water can be used, but hot water is preferred as it simulates what happens during autoclave dewax. Testing can also be done with square pipe specimens having rounded or sharp corners. Double wedge specimens are also possible. The is expected to totally replace the current MOR test in the industry. Current permeability test methods are experimentally time consuming. In this test a known flow rate of air or nitrogen is passed through the shell pipe from the inside to the outside and the pressure drop noted. This can be done prior to the burst test on the same specimen. A number of shelling variables were investigated using this combination test.