A MICRO-MECHANICAL AND MACRO-MECHANICAL INVESTIGATION OF CREEP MECHANISMS FOR THE WIPP ROCK-SALT

This report covers the first two phases of the investigation on mechan ical mechanisms beyond the secondary stage of creep in WIPP rock salt. With the use of macro- and micro- experimental techniques, the creep mechanisms of the rock salt specimens have been investigated under sim ple loading paths with an emphasis on the transition between the secon dary and tertiary stages of creep. A reliable experimental procedure h as been developed based on the lessons learned. It appears that the es sential feature of creep damage associated with microcracking is manif ested by the formation of a localized damage zone and the moving bound ary between localized and non-localized deformation fields. Based on a n energy point of view, a partitioned-modeling approach has been propo sed to predict nonlocal creep damage. To establish a sound mathematica l foundation for the proposed procedure, a set of moving jump forms of conservation laws have been used to define the moving boundary. Since large deformations often occur inside the localized damage zone, a fi nite particle method, combined with a dynamic relaxation scheme, is be ing modified to accommodate the partitioned-modeling approach. Prelimi nary numerical results look quite promising for sample problems under quasi-static loading conditions. The next phase of the project will sy nthesize experimental, theoretical and computational efforts and devel op a complete analysis tool to predict the essential feature of rock s alt creep.