Experimental Study On Damage Evolution Of Quasi-brittle Materials

The mechanical property and failure mechanism of quasi-brittle materials, such asrocks, are important science issues in underground space construction, hydraulicengineering, tunnel excavation and mineral resource exploration. Measurement andcharacterization of damage from experiment, is a basic and key work to study thefailure process of rock. This dissertation mainly focuses on damage and failure ofquasi-brittle materials. Based on the full-field measurement of digital imagecorrelation and experimental data analysis, the damage evolution and failuremechanism of quasi-brittle materials are investigated; this study has importantengineering background and scientific significance.In this thesis, the damage evolution and localization of rock under compression issystematically investigated by digital image correlation (DIC) technique. Thedisplacement and strain fields have also been obtained by DIC. Based on theexperimental results, the apparent strain1is used to describe the rock damage, andthe experimental data have been analyzed statistically. The average value of strain ofall points increases linearly with the load which show that most of the points are in astate of uniform deformation. The average value of top10%apparent strain pointsbehaves rapid increment in the damage localization and crack initiation stage. Itindicates that the top10%apparent strain points dominate the damage evolution andcrack propagation.On the basis of the experimental results and statistical analysis, a new method forcharacterizing the damage evolution by using two key factors, has been firstlypresented. The principal strain1has been used as a key parameter which can beobtained by real-time measurement. The localization factorLfis presented to reflectthe damage localization and the damage degree factorDfis proposed to characterizethe evolution of damage. The two curves ofDfandLfare also given. The twofactors method is applied to investigate the damage evolution in different rockexperiments, such uniaxial compression tests, Brazilian disc, indentation tests andcyclic compression. The experimental results show that the two curves ofDfandLfcross each other at the damage localization stage. Subsequently the curveDfshowsa turning point, that is the formation of the crack which is induced by the damagelocalization. The feasibility of two factors method for characterizing damageevolution in quasi-brittle materials has been proved.The damage and fracture process of rock in three loading conditions, indentationtests, cyclic indentation and three point bending tests, are also discussed. The failuremechanisms in rock under indentation have been analyzed by splitting fracture model and the relationship between indentation load and crack length has been given. It isfound that dissipated energy exists in the cyclic indentation and the residualdisplacements increase during the loading–unloading cycles. DIC and acousticemission (AE) techniques have been applied jointly to study the deformation andfracture process during three point bending tests on sandstone specimens. Byanalyzing AE parameters and deformation pattern around the notch tip, it has beenrevealed that the onset of crack propagation occurs at about80%of the peak load. Byanalyzing deformation pattern around the notch tip shown by DIC observation, thecrack opening displacement (COD) and the whole process of crack propagation havebeen obtained.