Fracture of Elastomers by Gas Decompression

A study has been made of gas decompression failures in elastomeric seals using a fracture mechanics approach and considerations of gas permeation. An equation is proposed for the tearing energy associated with crack growth from internal gas bubbles in a finite thickness elastic media. When applied to a model experiment where an internal flaw of known size was pressurized up to failure, the equation agreed with experimental results. A series of seals were then subjected to high gas pressures (up to 69 MPa or 10,000 psi) for a range of temperatures between 20 and 230°C and the amount of crack growth was measured after decompression. The trends in crack growth were correctly accounted for when values for tearing energy and modulus were used that were appropriate for the temperature of decompression. The effect of mechanical boundary constraint was studied by varying the compression on the seal in specially designed test holders. The effect of decompression rate is also considered. The conditions under which failure will occur are created by a complex balance of the available tearing energy and the gas permeation kinetics. A fundamental problem is the unknown size of the initial flaw from which crack growth and rupture occurs. This is discussed in the light of the experimental results and it is proposed that effective surfaces of weakness form in the elastomer phase of size to provide initiation sites for crack growth.

Stability of structural aggregates of dry soil

The stability of natural aggregates of dry soil has been examined by a drop-shatter method similar to that used in coal technology, The median size was determined after dropping from heights of 30, 60, 120, and 240 cm. The dropping procedure subdivides aggregates along surfaces of weakness into smaller natural units. The method has been used to help define the structure and consistence of various soils. The size distribution of fractured dry soil is in general given satisfactorily by the Rosin-Rammler relation for fractured brittle materials. The possibility of using Atterberg's steel wedge to measure the strength of natural aggregates was also examined.