D. E. Bray

Bio: D. E. Bray is an academic researcher. The author has contributed to research in topics: Isotropy & Acoustoelastic effect. The author has an hindex of 1, co-authored 1 publications receiving 261 citations.

Measurement of acoustoelastic and third-order elastic constants for rail steel

Abstract: Measurements of the stress‐induced changes in ultrasonic wave speeds in several samples of steels typically used in railroad rails are presented. All of the five possible relative changes in wave speeds for an initially isotropic material subjected to a uniaxial state of stress have been determined and agree to within the limits of accuracy of the measurement with the second‐order theory of Hughes and Kelly. The third‐order elastic constants are calculated from the acoustoelastic data.

Monitoring stress related velocity variation in concrete with a 2×10−5 relative resolution using diffuse ultrasound.

Abstract: Ultrasonic waves propagating in solids have stress-dependent velocities. The relation between stress (or strain) and velocity forms the basis of non-linear acoustics. In homogeneous solids, conventional time-of-flight techniques have measured this dependence with spectacular precision. In heterogeneous media like concrete, the direct (ballistic) wave around 500~kHz is strongly attenuated and conventional techniques are less efficient. In this manuscript, the effect of weak stress changes on the late arrivals constituting the acoustic diffuse coda is tracked. A resolution of $2.10^{-5}$ in relative velocity change is attained which corresponds to a sensitivity to stress change of better than 50 kPa. Therefore the technique described here provides an original way to measure the non-linear parameter with stress variations on the order of tens of kPa.

A model for seasonal changes in GPS positions and seismic wave speeds due to thermoelastic and hydrologic variations

Abstract: It is known that GPS time series contain a seasonal variation that is not due to tectonic motions, and it has recently been shown that crustal seismic velocities may also vary seasonally. In order to explain these changes, a number of hypotheses have been given, among which thermoelastic and hydrology-induced stresses and strains are leading candidates. Unfortunately, though, since a general framework does not exist for understanding such seasonal variations, it is currently not possible to quickly evaluate the plausibility of these hypotheses. To fill this gap in the literature, I generalize a two-dimensional thermoelastic strain model to provide an analytic solution for the displacements and wave speed changes due to either thermoelastic stresses or hydrologic loading, which consists of poroelastic stresses and purely elastic stresses. The thermoelastic model assumes a periodic surface temperature, and the hydrologic models similarly assume a periodic near-surface water load. Since all three models are two-dimensional and periodic, they are expected to only approximate any realistic scenario; but the models nonetheless provide a quantitative framework for estimating the effects of thermoelastic and hydrologic variations. Quantitative comparison between the models and observations is further complicated by the large uncertainty in some of the relevant parameters. Despite this uncertainty, though, I find that maximum realistic thermoelastic effects are unlikely to explain a large fraction of the observed annual variation in a typical GPS displacement time series or of the observed annual variations in seismic wave speeds in southern California. Hydrologic loading, on the other hand, may be able to explain a larger fraction of both the annual variations in displacements and seismic wave speeds. Neither model is likely to explain all of the seismic wave speed variations inferred from observations. However, more definitive conclusions cannot be made until the model parameters are better constrained.

Monitoring stress related velocity variation in concrete with a 2.10 -5 relative resolution using diffuse ultrasound

Abstract: Ultrasonic waves propagating in solids have stress-dependent velocities. The relation between stress (or strain) and velocity forms the basis of non-linear acoustics. In homogeneous solids, conventional time-of-flight techniques have measured this dependence with spectacular precision. In heterogeneous media like concrete, the direct (ballistic) wave around 500~kHz is strongly attenuated and conventional techniques are less efficient. In this manuscript, the effect of weak stress changes on the late arrivals constituting the acoustic diffuse coda is tracked. A resolution of $2.10^{-5}$ in relative velocity change is attained which corresponds to a sensitivity to stress change of better than 50 kPa. Therefore the technique described here provides an original way to measure the non-linear parameter with stress variations on the order of tens of kPa.

Damage detection in concrete using coda wave interferometry

Abstract: Coda wave interferometry (CWI) is a nondestructive evaluation technique for monitoring wave velocity changes in a strongly heterogeneous medium as demonstrated in previous seismic and acoustic experiments. The multiple-scattering effect in such a medium promotes the rapid formation of a diffuse field, and waves can travel much longer than the direct path, and thus are more sensitive to small changes occurring in the medium. This research applies the CWI technique in conjunction with acoustoelastic measurements to characterize two different types of damage in concrete: damage due to thermal shock and dynamic cyclic loading. The diffuse ultrasonic signals are taken at different levels of compressive stress and then relative velocity changes are extracted using the CWI technique. The relative velocity change (or the material nonlinearity) increases considerably with increasing damage level in most samples for both types of damage. The feasibility and sensitivity of this CWI-based technique in characterizing damage in cement-based materials are demonstrated.