Multiscale aspects of monitoring geo-engineering structural systems: theory and methodology

摘要

The physical meaning of monitored field data is one of the uncertainties that prevails in non-destructive detection (NDD) of defects. This is especially true for the surveillance of geo-engineering systems that are expected to function over an extended period of time such as millions of cycles in fatigue where small defects may grow into critical sizes that will cause damage and render the structure out of commission. A critical issue involved in the development of NDD model is the gradual degradation of the material properties in time. This effect is demonstrated by the dual scale micro/macro cracking model employing the parameter ko that varies statistically in a random manner with normal distribution because material microsctructure interacts with the changing geometry of the growing defect. The mean and deviation of the normal distribution are shown to alter the range of the crack growth range up to the design life. Adjustment of the governing parameters may appear to be unmanageable at first.However, they can be determined after a few trials based on the prescribed design conditions for the tolerable crack size and the elapsed time or cycles that can be pre-set as required by inspection. The procedure is illustrated using the fracture control methodology applied to assure safety of the commercial aircrafts. The inspection period is set by the crack length versus time record that is not so readily and clearly identifiable in geo-engineering systems where the fatigue crack growth behavior is considerably more irregular than that in high strength metal alloys. To this end, monitoring of the local compliance or stiffness may be more appropriate. A dual scale micro/macro crack growth model is then used to translate the data into a form that can define the damage threshold based on the local compliance. The results are valid for any shape of the representative block that contains the defect(s) under sinvestigation. Verification of the proposed procedure canbe demonstrated in the laboratory.