A Summary of a Steel Formability Assessment Tool through the use of Electromagnetic Acoustic Spectroscopy and the monitoring of Spectral Damping
Properties. The direct measurement of Internal Friction The testing of the samples sets provided by Charter provided enough information to establish the magnitude and quality of the acoustic responses to narrowband spectral excitation. More importantly a roadmap for how to develop the test protocol was
researched and verified. There has been a significant body of work in anisotropy measurement, lifespan determination and morphological changes in steels through fatigue cycling and failure using EMAS and confirmed through TEmicroscopy analysis. The test procedure deviated from the original concepts laid out in the proposal. One of the critical differences came with the observations of the damping characteristics that were consistent across the sample populations. The measurement of the decay phenomena through the measurement of the exponential decay time of the resonance (α) became the primary focus, not the Rayleigh scattering properties which are ideally suited to the identification of inhomogeneity’s of precipitates, defects such as cracks and voids, inclusions or localized residual stress anomalies. The scattering properties are the predominant phenomenon monitored in the current Resonic ARIS systems.

There is some evidence of this in the data across frequency in some of the sample sets. However modal selection becomes the critical issue in this measurement scheme a critically important observation here is that the low carbon steel did not support resonance behavior beyond 2.5MHz. As the testing progressed we realized that the time required for the precision damping characteristization testing at multiple frequencies did not add any additional insight into the obvious and consistent differences in the internal friction/exponential decay information. This phenomenon is the overwhelmingly predominant acoustic feature in this test sequence. The influence of the of his information details the dislocation mobility, thermal history and grain size in
polycrystalline materials. Defined as

Where W is a description of the elastic energy either stored or absorbed during cyclic stress loading. One of the complicating issues was the measurements were made with a combination of test instruments due to the highly damped response of the low alloy wire to the resonance stimulus particularly at high frequencies. Thus the ability to measure arrival time or frequency of the resonant event became too complex in this test regime. The curvature(s) of the cut wire segments made taking reliable repeatable measurements at multiple positions on each bar impractical. So measurements were made as close to the
inside center of the segment as possible. Of note there was a fair amount of variance observed in the spectral characteristics along the segment length probably attributed to the non-concentricity of the samples and the transducer fit against the varying curved surfaces.

Reviewing the relevant papers on this topic, Johnson, Granato, Birnbaum, Ogi et al. it has become apparent that the issues of dislocation mobility and pinning become the predominant phenomena in the general trends common to the alloy families with differing thermal histories.

The increasing anelastic behavior or inhomogeneity in the lattice is certainly related to the damping characteristics. Not having any of the metallurgical morphology is a significant hindrance in making anything more than generalized statements about the common alloy properties. Following are three charts containing the Exponential decay times ΔT of the resonance event and the highest frequency component (reverberation) within the damped sine wave decay.

These charts are representative of the data consistency in ultrasonic attenuation, within each alloy group at select frequencies or vibrational modes. The amplitude versus time and or the reverberation content has quite similar slopes in the thermal differing thermal stages. Thus by inspection of the data the obvious conclusion is that the constituent elements within an alloy group do affect the attenuation loss at different modal frequencies. The testing has proven the following:

  • Internal friction measurements can be accurately made independent of diameter
  • Dislocation mobility is the predominant background feature in the resonance
    exponential decay patterns (α
  • The lower alloy steels will not support the higher frequency modes this is directly
    related to dislocation pinning sites

The deltas in the different thermal states between green and the 1-A states demonstrate a remarkable yet tight difference of approximately 300 percent.

Physical Properties/Strength

With the need to modify the measurement platform in order to take quantitative data on the low carbon steel the precise measurement of frequency needed to be put aside due to resource constraints and the primary objective of thermal history differentiation was precisely achieved through the α decay measurements. There were significant differences in the observed frequencies across the individual samples within the sample sets approaching 1% in variance. Precise diameter measurements would need to be taken in order to fully attribute the differences in the residual stresses, including anisotropy. The obvious impact of such would be to modify the slopes of the acoustoelastic responses to applied stresses these should be reflected in the physical
strength measurements. Certainly the α decay data which contains dislocation information will be reflected in the strength tests, this has already been observed in the independent tests conducted on Wire-Tech processed 52100 steel. Establishing any function is not within the scope of this study.

The samples will next be tested for tensile and compressive strength properties. While not a specific objective of the acoustics study the measurement of initial frequencies over a number of select modes would be the method of determining the third order shear elastic constants. A study of frequencies, of both static (initial) and acoustoelastic (dynamic) stress behaviors would be advisable. However the acoustoelastic effect is well established in the art, what is of interest is the slope and inflection points in the classical first order stress/strain behavior vs that of the acoustoelastic behavior.

The further discussion of determining nonlinear elastic and acoustoelastic properties is possibly part of future investigation and analysis.

In conclusion the data submitted serves as the final analysis submitted by Resonic, without detailed physical analytical information about the actual lattice it is not possible to posit a function whether it is power law, inverse iterative, etc.