Microstrain Level Research Articles

Modeling nonlinearity and slow dynamics for rock in resonance experiments

The presence of compliant features in rock causes nonlinear distortion of sound waves propagating through a sample, even at microstrain levels. When performing resonance experiments, one observes nonlinear behavior in different ways: resonant frequency shifts as function of drive amplitude, nonlinear dissipation including hysteresis, asymmetry of the resonance curves, and rich harmonic spectra. Recently, a new interesting feature has been added to these nonlinearity observations: the existence of a slow dynamics conditioning and recovering characteristic. Experimental evidence will be shown in a companion ASA contribution by TenCate et al. The focus is on the mathematical modeling of all of the nonlinear resonance observations, including the slow dynamics characteristics. The model uses a finite-difference time-domain solution of the one-dimensional wave equation with appropriate boundary conditions, and calculates waveforms along the sample. The key part of the model is the description of a modulus which depends on higher-order elastic constants, hysteresis strength, and its own weighted response over previous times. By integrating the history of the modulus using an exponential weighting function, interesting conditioning and recovering effects can be simulated which agree well with the nonlinear and slow time constant observations in rock. [Work supported by DOE/OBES/UCal.]

Correlation between the structural defects induced by ball-milling of Pb 3O 4 and the structure of PbO yielded from its thermal decomposition

The influence of grinding in a high planetary ball-mill on the texture and thermal stability of Pb 3O 4 has been studied. The results obtained show that the crystallite size decreases while the level of microstrains increases upon increasing the grinding time. The Pb 3O 4→PbO conversion temperature decreases by increasing the grinding time of the precursor when the orthorhombic phase of PbO (massicot) becomes stabilized with regards to the tetragonal modification (litharge). The analysis of the X-ray diffraction profiles of the lead monoxide obtained from the thermal decomposition of Pb 3O 4 demonstrates that PbO keeps memory of the lattice imperfections of Pb 3O 4. Moreover, higher the grinding time of Pb 3O 4, higher the reactivity of the resulting PbO sample towards reoxidation.

The effect of manganese doping on the mechanical behavior of cordierite glass-ceramics

Abstract The effect of manganese doping on the mechanical properties of cordierite-based glass-ceramics series was investigated. The fracture toughness dependence on microstrain level of cordierite crystals was established. Two regions of increased fracture toughness were due to the crack front interactions with the microcracks and with the residual stress fields, respectively. The effect of doping on the thermal shock fracture resistance is associated with the decrease in the thermal coefficient of linear expansion of glass-ceramics.

Reversible Transformation Plasticity in Uniaxial Tensioncompression Cycling of Mg-PSZ

ABSTRACTPreviously, the pressure, temperature and strain rate sensitivities of transformation plasticity have been investigated for monotonic loading of Mg- PSZ. Research in this area has been extended to fully reversed cyclic loading of the type used in plastic strain control fatigue. Cyclic deformation experiments were performed to permit investigation of constitutive behavior under stable deformation conditions at microstrain levels. It was found that cyclic microstrains over a range of temperatures and strain rates were associated with reversible transformation plasticity in the strongly thermally-activated regime. These results are compared to the constitutive relations of transformation plasticity which have been previously developed to explain macrostrain observations.

Micromechanical oscillators and techniques for determining the dynamic moduli of microsamples of human cortical bone at microstrains.

Two micromechanical oscillators are described along with methods and techniques which have been successfully used to determine the elastic, dynamic, shear and Young's moduli at microstrain levels, and which with proper design modifications might, under restricted experimental conditions, also be employed to determine viscous moduli. Although these systems can be used with any microsamples, they were designed originally for dynamic studies of the unit microstructural element of human cortical bone, the secondary osteon (Haversian system).

Microplasticity and fatigue of some magnesium-lithium alloys

Cyclic stress-strain curves have been obtained for a series of magnesium-lithium alloys with lithium contents up to 12. 5wt%. The results show that the cyclic strain hardening exponents for stresses leading to failure in excess of 104 cycles are similar for all the alloys, even though the structure changes from h c p to b c c with increasing lithium content. This was attributed to similar deformation processes being involved in the materials at the microstrain levels involved in high endurance fatigue. There is a tendency for the plastic strain amplitude leading to failure at a given life to increase with lithium content. The accumulated work to failure for all the materials studied increases with fatigue life.