Estudo de mecanismos de deformação lenta da gipsita bandada da Chapada do Araripe em ensaios de fluência monitorados por emissão acústica / Study of slow deformation mechanisms of the banded gypsum rock of the Chapada do Araripe in creep tests monitored by acoustic emission

AUTOR(ES)
DATA DE PUBLICAÇÃO

2009

RESUMO

Experimental studies by several authors have shown some of the key deformation mechanisms in gypsum, such as plastic flow and pressure dissolution. Even so, there is still sparse knowledge about the mechanical behavior of natural gypsum rocks. Creep mechanisms of the banded gypsum rock of Santana Formation from the Chapada do Araripe (Santana Formation) in creep tests were studied with acoustic emission technique correlating the stress levels related to microcracking to the energy release associated with these mechanisms. This procedure was adopted as an indirect technique of detection of the microcracking process, to prevent changes that occur in the microstructure during unloading and in sample preparation for its observations with direct techniques. Short and long-term creep tests were carried out in an MTS 815 servo-controlled testing system and in modular sets (single towers) in a specially equipped creep laboratory. It was found that the microcrack mechanism is strongly influenced by healing processes during long-term creep. Due to the microcrack healing process, tertiary creep was never reached, even in specimens loaded with up to 95% of the uniaxial compressive strength. In this thesis other important results were reached as: to identify that the microcracking phenomenon for the rock in question is detected by acoustic emission phenomena of intermediate frequencies (from 100 to 400 kHz) and to evidence that the Kaiser effect unequivocally manifests in this type of rock when subjected to quick loads, while after long-term creep tests, it is no longer evidenced on account of healing. In other words, the specimens subjected once again to uniaxial compression after unloading, again show acoustic emission events and therefore the generation of new microcracks. This fact can only be explained by a microcrack healing process during long-term creep. This healing, due to physical-chemical phenomena, presented other indications as an increase in the modulus of elasticity after creep tests, when compared to the values before the test, and even a hardening (increase of the uniaxial compressive strength) also after the creep. It was also clarified in unequivocal way that the unloading provokes microcracking progress. In addition to the microcracking characteristic indications, it was found that the modulus of elasticity, when measured at the same stress levels, is higher before unloading than after the unloading. The stiffness decrease is indication of the microcracking damage process.

ASSUNTO(S)

creep mechanisms microfissuramento formação santana microcracking acoustic emission fluência gipsita bandada emissão acústica santana formation banded gypsum rock mecanismos de deformação lenta creep efeito kaiser kaiser effect

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