Absolute measurements of elastic-wave phase and group velocities in lossy materials

Ogi

Hirao

2005

We studied the microstructural evolution of 2.25Cr-1Mo steels subjected to tensile creep at 923 K through monitoring of shear-wave attenuation and velocity, using electromagnetic acoustic resonance (EMAR). Contactless transduction based on the magnetostrictive mechanism is the key to establishing a monitor for microstructural change in the bulk of the metals with a high sensitivity. In the short interval, 50 to 60 pct of the creep life, attenuation experiences a peak, being independent of the applied stress. This novel phenomenon is interpreted in terms of the drastic change in dislocation mobility and rearrangement, which is supported by transmission electron microscopy (TEM) observations for dislocation structure. At this particular period, the dense dislocation structure starts to transform to subgrain boundaries, which temporally accompanies long, free dislocation, absorbing much ultrasonic energy to produce the attenuation peak. The EMAR has the potential to assess the damage advance and to predict the remaining creep life of metals.way of evaluating the internal damage state with comparatively simple and easy instrumentation, while many others, including X-ray, replication, hardness, and magnetic properties, merely inspect the surface or near-surface region of the materials.Ultrasonics have been studied for estimating the creep damage. [5][6][7][8][9][10][11] In these applications, the ultrasonic attenuation and velocity have been measured with a contacting piezoelectric transducer. In some cases, the velocity decreased with damage accumulation; in others, this tendency was not observed. The velocity was often more sensitive to the damage than the attenuation. The creep mechanism is dominated by dislocations, which also dissipate acoustic energy causing ultrasonic attenuation (or internal friction). However, the attenuation changes measured using the conventional transducer cannot exactly reflect the creep damage because the as-measured attenuation includes not only the attenuation of the sample, but also the damping through the transducer, the couplant and the buffer, the reflection and transmission losses at the interface, and the energy leakage into the transducer. Because the attenuation within the sample has to be considered in evaluating the creep damage, these extra losses are much greater than the attenuation within the sample [12]

Section: B the Measurement Of Attenuation With Emarmentioning

confidence: 99%

Dislocation damping and microstructural evolutions during creep of 2.25Cr-1Mo steels

Ogi

Hirao

2005

“…[45] In measuring the pure attenuation within the material, we took several steps to insure that the measurements were independent of certain conditions, including the EMAT used, the sample thickness, the surface condition, the liftoff, and the operator. [46] First, a resonant spectrum was obtained by sweeping the frequency of the RF burst.…”

Section: The Measurement Of Attenuation With Emarmentioning

confidence: 99%

Acoustic damping characterization and microstructure evolution during high-temperature creep of an austenitic stainless steel

2005

We studied the microstructure evolution of an austenitic stainless steel, Type 316L, subjected to tensile creep at 973 K through the monitoring of shear-wave attenuation and velocity using electromagnetic acoustic resonance (EMAR). Contactless transduction based on the Lorentz force mechanism is the key to establishing a monitor for microstructural change in the bulk of metals with high sensitivity. In the short interval, 60 to 70 pct of the creep life, attenuation experiences a peak, independent of the applied stress. A drastic change in dislocation mobility and rearrangement interrupted this novel phenomenon, as is supported by observations using a scanning electron microscope (SEM) and a transmission electron microscope (TEM). The EMAR exhibited the potential for the assessment of damage advance and the prediction of the remaining creep life of metals.

“…[32] We measured resonant frequencies and attenuation coefficients for resonant modes with a superheterodyne spectrometer. [33] The resonant peaks appear at equal frequency intervals and at each resonant frequency; the attenuation coefficient is measured by the free-decay method, leading to the frequency dependence of attenuation. [34] …”

Section: B the Ematmentioning

confidence: 99%

Creep-Induced Microstructural Evolution and Acoustic Characterization in Tempered Martensitic Stainless Steel

2007

The microstructural evolution of a tempered martensitic stainless steel (a 12 pct chromium steel, type-403), during creep test, subjected to a tensile creep test at 873 K, was studied by monitoring the shear-wave attenuation and velocity using electromagnetic acoustic resonance (EMAR). The study disclosed an attenuation peak at around 20 to 30 pct of the creep life, and a minimum value at 50 pct of the creep life, and a local minimum velocity at the attenuation peak, independent of the applied stress. These novel phenomena are interpreted as a result of microstructural changes, especially dislocation recovery. This interpretation is supported by transmission electron microscopy observations of the dislocation structure. The relationship between the changes in attenuation, velocity, and microstructure evolution can be explained with the string model. The study results have suggested that EMAR possesses the potential to assess the progress of creep damage and to predict the remaining creep life of metals.