Rapid Stress Drop Research Articles

Electric potential changes associated with slip failure of granite: Preseismic and coseismic signals

Electric potential changes were measured for stick‐slip events in granite samples with a three‐block direct shear arrangement at 8 MPa normal stress. Two electrodes were mounted on the left‐ and right‐hand blocks, and the electric potential difference between each electrode and the ground was measured with a high input impedance recording system of frequency range from DC to 100 Hz. As well as coseismic electric signals of about 1.5 V which appeared the moment of the dynamic slip event, preseismic signals were detected just before the slip event. The coseismic signal rises stepwise with opposite polarities at the two electrodes and exponentially decays with a time constant of ε/s, where ε is the permittivity and s is the conductivity of the rock sample. We conducted a simple test of rapid stress drop without slipping and observed almost the same electric signal as the coseismic signal. This suggests that the electric signal is generated by the piezoelectric effect. We proposed a generation model based on the piezoelectric effect and the resultant relaxation process and obtained a theoretical frequency response, which is in agreement with experimental data. The preseismic signal appears about 2–3 s before the dynamic event with an amplitude of about 50 mV. The local strains along two sliding surfaces were also measured to monitor the growth of the rupture nucleation zone. When the growth of the rupture nucleation zone occurred on the left sliding surface, a clear preseismic signal was detected at the electrode mounted on the left granite block. When the growth occurred on the right‐hand surface, a signal was detected at the electrode on the right block. This shows that the preseismic electric signal is caused by stress change in the rupture nucleation zone. These preseismic and coseismic signals were also detected with an antenna, which was placed away from the sample surface.

Analyses of damage localization at crack tip in a brittle damaged material

Using the analysis method of the Dugdale-Barenblatt model and considering the rapid stress drop and strain softening of material, the fracture process zone at the near tip of a mode-I crack in a brittle damaged material is studied. It is pointed out that under external loads a narrow strip zone of damage localization, i.e. a damage localization band, will form ahead of the crack and it is impossible that a complete damage zone with a finite width occurs around the crack tip. A central crack and semi-infinite crack in infinite sheets have been taken as examples; the relations between the length of the damage localization band and external loads are obtained. The structures of crack tip fields in a brittle damaged material are shown.

Micromechanical modelling of tensile response of elastic-brittle materials

The stress-strain relation of elastic-brittle materials like concrete under tensile loading includes some of the stages of linear elasticity, pre-peak non-linear hardening, rapid stress drop and tension softening. These stages, which correspond to different mechanisms of damage, are modelled through micromechanical analyses. The behaviours of rapid stress drop and tension softening are the results of localization of damage and deformation. Based on the damage model of domain of microcrack growth proposed by Feng and Yu [Acta Mech. Sinica9, 251–260 (1993)], this paper gives the complete constitutive relation including the four stages of an elastic-brittle material subjected to uniaxial and triaxial tensile loads. As an illustration, the theoretical stress-strain curve in uniaxial tension is compared with an experimental result.

Estimates of the rates of microstructural changes in mylonites

Abstract Microstructures developed during dislocation creep such as the dynamically recrystallized grain-size and the subgrain-size are modified in response to increasing differential stresses and shear-strain rates adjacent to rigid minerals in mylonites. In two specimens variations in quartz subgrain-size around rigid minerals has been quantified from orientation contrast SEM images. In a deformed granite from the Pogallo ductile fault zone, quartz two-dimensional subgrain-size is reduced from background values of about 30 μm to 8 μm and then 4 μm in progressively narrower channels between rigid feldspars. In pelitic mylonites from the Alpine fault zone, New Zealand, quartz bands are wrapped around rigid garnets and the subgrain-size is reduced from background values of about 20 μm to 14 μm within 2000 μm of the garnet to 11 μm adjacent to the garnet. In both specimens examined the microstructure responds to both increases and decreases in differential stresses and coupled strain-rates. The analysis of the distribution of subgrain sizes provides information for the assessment of microstructural stability and the estimation of the rates of microstructural changes. The New Zealand specimens have good regional shear strain-rate controls and minimum subgrain boundary migration rates (during reduction in differential stresses and strain-rates) of 1.2 × 10 −9 μm s −1 to 1.2 × 10 −11 μm s −1 are estimated, for bulk shear strain rates of 10 −10 s −1 and 10 12 s −1 respectively. Palaeopiezometry coupled with rheological considerations suggests that these microstructural changes correspond to minimum differential stress-rates of 5 × 10 −9 MPa s −1 to 5 × 10 −11 MPa s −1 . Similar or slower regional stress-rates during fault zone evolution, will allow continuous re-equilibration of the microstructure. Thus the time at which any given microstructure is frozen in will be critically dependent upon fault zone deformation history. In the Alpine fault zone rapid uplift and high near surface temperatures combine to generate rapid stress drops as mylonitic deformation gives way to cataclastic faulting during uplift. Mylonite microstructure is frozen in during this rapid stress drop and represents the shallowest level of major crystal-plastic deformation.

Measurement of irradiation creep of zirconium alloys using stress relaxation

Bent-beam stress relaxation tests provide a simple means of assessing the in-reactor creep behaviour of zirconium alloys with different prior thermo-mechanical treatments. By assuming that stress relaxation is equivalent to creep under decreasing stress, the irradiation-induced creep rate can be derived from tests in which small elastically constrained beams are exposed to fast neutron irradiation. In-reactor creep rates for stresses below about 150 MPa have been observed to depend linearly on applied stress, namely ge = Cσφ, hence the stress relaxation behaviour can be described by the relation σ/σ 0 = D exp(−CEφt) where σ/σ 0 is the ratio of the unrelaxed stress (derived from the change in curvature of the unconstrained beam) to the initial stress to bend the beam to the shape of the jig, D describes an initial rapid stress drop, C is a material constant, E is Young's modulus, φ is fast neutron flux and t is time. We review stress relaxation test results that have been used to characterize the effects of operating parameters (such as temperature, fast neutron flux and fluence) and microstructural properties (such as anisotropy, dislocation structure, grain shape, thermo-mechanical treatments and alloy content) on the in-reactor creep behaviour of zirconium alloys. Correlations between stress relaxation and creep results are also presented with support the use of the relaxation test for gathering in-reactor creep information. Strain recovery in in-reactor and out-reactor annealing experiments suggest that in addition to anelastic strain, a significant part of the in-reactor relaxation strain is due to irradiation damage-related creep.

Low cycle fatigue of aluminum at elevated temperatures

High temperature low cycle fatigue data are reported for polycrystalline aluminum of 99.988% purity. Experiments were carried out at a constant strain rate over a range of temperatures from ambient temperature to 400°C. Strain rate change tests were employed to determine the rate-sensitive behavior. Cyclic hardening curves and saturated hysteresis loops undergo a gradual transition in their characteristic shapes with increasing temperature, suggesting a change in the mechanisms controlling deformation. These changes in behavior are not reflected in the cyclic stress-strain curve which shows a gradual decrease in work-hardening rate with increasing temperature. The temperature dependence of the cyclic saturation stress appears to be linear but does not exhibit a region of rapid stress drop. The discussion focuses on the role of vacancies in determining the high temperature deformation behavior of a cellular microstructure.

異方性岩石における選択的破壊について

For the purpose of investigating the fracture process of earthquake which will be affected by various kind of anisotropy in the earth's crust, we carried out triaxial test up to 500MPa (5 kbar) of confining pressure for the suitable anisotropic rocks for the present study, that is, gneiss and graywacke sandstone.For gneiss with the anisotropy oriented at 45° to the principal stress axis, the strength was widely distributed and the small rapid stress drop occured frequently below 240MPa of confining pressure. Above 240MPa, on the other hand, the strengh was fitted well with a single curve, which was lower than that for gneiss with the anisotropy oriented at 90° to the pricipal stress axis, and the large stress drop occured.These results means that at low confining pressures (less than 240MPa) the effect of anisotropy on the fracture is small and local, but at higher confining pressure (more than 240MPa), this effect spreads all over the sample.It is interesting for earthquake prediction to study the complex strain variation and the phenomena of multiple stress drop, because they result from the fault formed in the inhomogeous distribution of strengh obtained for the moderate anisotropic rocks. It seems important for speculating micro-earthquake distribution for depth that the behaviour of stress drop and strength for gneiss are stongly affected by confining pressure.

大理石と方解石の圧縮強度に関する研究 (第1報)

Uni-axial, bi-axial and confined compression tests were performed on Akiyoshi Marble. Specimens were unloaded on several stages of these experiments, and optical micro-scopic observations were performed about thin sections made from these specimens. On the other hand, uni-axial compressive test was also performed on calcite specimens.Based on these experimental results, some considerations about strength failure of these materials were discussed and following conclusions were obtained.1) The fracture behavior of calcite was anisotropic. In c-axis direction it behaved as brittle, on the other hand in a-axis direction it behaved as elasto-plastic. And, the strength failure and yielding point was about 1500 and 40kg/cm2 respectively.2) The linearity at low stress level and non-linearity at high stress level of the stress-strain relation of marble could be explained by the mobilization of the hindered cleavage sliding of calcite grain due to micro-crackings surrounding the grain.3) The extended Von Mises fracture criterion well explained experimental results qualitatively, but quantitatively insufficient.4) The anisotropy of lateral strain under bi-axial compression could be explained by the directional development of cracking from pre-existent cracks.5) The formation of ultimate failure plane did not correspond to the point of strength failure, but rather corresponded to the rapid stress drop after the strength failure.

Plastic Instability of Tantalum Single Crystals Compressed at 4.2 K

Serrations due to slip were observed on the stress-strain curves of zone-refined tantalum single crystals compressed at 4.2 K. It was found on an oscillograph that each rapid stress drop was accompanied by a rapid temperature rise of about 40 K. This suggests that a discontinuous slip is caused by the plastic instability due to the heat generated during the deformation. Using a deformation model involving a temperature rise due to the work done, calculations of the stress-strain curve and the corresponding temperature change were made for the present crystal, which gave a rapid stress drop and a rapid temperature rise in agreement with the experiments within a factor of 1.5. The present calculation shows that shows that no triggering action is necessary for the initiation of a discontinuous stress drop. A criterion for the thermal instability to occur was also discussed.