Photoelastic Investigation of Fragmentation Mechanisms. Part II. Flaw Initiated Network,

摘要

By using the techniques of dynamic photoelasticity and high speed photography,the instantaneous state of stress as well as the location of crack tips were simultaneously recorded in explosively loaded models. This permitted crack initiating stress systems to be identified and the propagation behavior of the cracks to be studied in detail. Considered here is the initiation of cracks at sites other than the borehole wall. Crack initiation from flaws are examined in detail and the stress systems that cause this initiation and provide the energy necessary for continued crack propagation are determined. Crack behavior is discussed in terms of fracture mechanics--a relatively new engineering discipline that can accurately predict the initiation and propagation behavior of cracks in brittle elastic materials. In order to simplify the discussion that describes the behavior of these stress wave initiated cracks, initiations are treated in two distinct categories:Crack initiation that occurs due to outwardly traveling waves and crack initiations that occur as a result of reflected waves. However,once initiated,the action of the crack is followed to completion regardless of the wave which provides the driving source. Cracks initiated by outgoing waves and cracks initiated by reflected waves are discussed. The overall fragmentation pattern is detailed including flaw induced fracture as well as fragmentation due to the borehole crack network.