Acoustic emission

About: Acoustic emission is a research topic. Over the lifetime, 16293 publications have been published within this topic receiving 211456 citations.

Experimental rock mechanics

Abstract: Part 1: Deformation and Fracture of Rocks 1. Precise Measurements of Fracture Strength of Rocks under Uniform Compressive Stress 2. Deformation and Failure or Rocks under Confining Pressure 3. Deformation and Fracture of Rocks under Triaxal Compression: The Effect of the Intermediate Principle Part 2: Acoustic Emission 4. Acoustic Emission Activity 5. Source Location of Acoustic Emission 6. Magnitude-Frequency Relation of Acoustic Emission Events 7. Acoustic Emission under Cyclic Loading Part 3: Rock Friction 8. Design of a New Apparatus for Friction Experiments 9. Laboratory Experimental Results 10. Stick-Slip Events in the Natural Field and Some Features in the Occurrence of Recent Great Earthquakes

A modified griffith criterion for the evolution of damage with a fractal distribution of crack lengths - application to seismic event rates and b-values

Abstract: SUMMARY The Griffith criterion for dynamic crack growth results from a calculation of a minimum in the Gibbs free energy of an infinite medium containing a single elliptical crack. However rock failure in the laboratory or during large earthquakes is usually preceded by the evolution of an aureole of damage in the form of subsidiary microcracks or faults. A characteristic of such precursory damage is that it is fractal, having a power-law crack length distribution, and also a power-law spatial and temporal correlation. Such fractal damage is consistent with the concept of faulting or cracking as a self-organized critical phenomenon, and has been widely confirmed by empirical observation of faults, laboratory fractures and indirect seismic monitoring in the laboratory and field. Here we consider the free energy change ΔF associated with an ensemble of NT weakly interacting, aligned, elliptical cracks of different lengths, with semilength expectation value 〈c〉, under a constant tensile stress s applied at the boundary of each element. The crack ensemble represents a state of damage which may evolve in a quasi-static way due to subcritical crack growth. By considering ∂(ΔF)/∂〈c〉= o, a modified strain energy release rate G′=∂U/∂Ad=f(〈c〉, 〈c2〉) is defined, where U is the potential strain energy, and Ad is the total surface area of the array of cracks. For a given NT, G′ is proportional to the rate of change of the total volume of damage with respect to the total area of damage Ad. This reflects the fact that mechanical energy is stored in a volume, and released on a surface. As the number of cracks tends to 1, G′ tends naturally to the strain energy release rate G for a single crack. For a fractal distribution of crack lengths Nc(c) =NT(c/co)-D, limited to a range (co, c1), it can be shown that G′ is negatively correlated to D for a given constant value of NT. Also, the curves for higher NT are associated with higher G′. A similar result can be obtained more simply by using the expectation value 〈G〉 directly as an appropriate parameter, with the advantage that both NT and D can be allowed to vary independently. These predictions are respectively consistent with the positive (negative) correlation established between acoustic emission event rates (seismic b-values) and G in the laboratory for quasi-static tensile failure by mode I subcritical crack growth due to stress corrosion reactions in double torsion loading. The theory also correctly predicts the order of magnitude of the stress corrosion index for these experiments, and the observation that more heterogeneous materials have higher stress corrosion indices. However the correlations established between event rate, D and G' (or 〈G〉) are completely general, and can apply in principle to other forms of fault development or crack growth with weak long-range interactions. Most studies of the statistics of damage evolution are by their very nature indirect or posthumous, unless the material of interest is optically transparent. Seismic monitoring of damage in the form of small earthquakes or acoustic emissions can be used to measure the parameters a and b of the earthquake frequency-magnitude relation log N, =a - bm, where a = log N, b = CD/3. C is the slope of the scaling relation between magnitude and the common algorithm of seismic moment, and the event rate N is assumed proportional to NT. Thus changes in a and b can in principle be used to infer G′ (or 〈G〉) during the evolution of damage in the intermediate term prior to dynamic failure of laboratory rock samples or the Earth.

Behavior of oxide scales on 2.25Cr-1Mo steel during thermal cycling. I. Scales formed in oxygen and air

Abstract: The acoustic-emission (AE) technique has been applied to study scale-damage processes during thermal cycling of a tube, preferentially between 600 and 300°C in air, oxygen, and air + 0.5% SO2. The AE measurements were accompanied by optical and electron-optical investigations on tube rings exposed to the same cycling conditions. During the first period of cycling, a scale rich in hematite is formed. It suffers compressive stresses during cooling. The result is a buckled multilayered scale with separated lamellae. The scaling rate is lower than under isothermal conditions. AE signals start after 175°C cooling. After longer exposure times, the scale contains an increasing amount of magnetite and becomes more compact. The scaling rate increases and is comparable to that under isothermal conditions. AE signals are already observed after 50°C cooling and are correlated with crack formation in the magnetite caused by tensile stresses there. The addition of SO2 to air enhances the crack-healing process due to higher Fe diffusion in FeS. The scale is more compact.