This standard outlines the laboratory procedure for evaluating the dynamic modulus of rock core samples through ultrasonic pulse velocity methods. It provides detailed guidance for geotechnical and civil engineering experts to determine compressional and shear wave speeds in rock specimens, enabling calculation of dynamic elastic properties vital for rock mechanics analyses.
Overview
This standard outlines the laboratory procedure for evaluating the dynamic modulus of rock core samples through ultrasonic pulse velocity methods. It provides detailed guidance for geotechnical and civil engineering experts to determine compressional and shear wave speeds in rock specimens, enabling calculation of dynamic elastic properties vital for rock mechanics analyses.
Audience
Contents
Structure
This section defines the scope of the standard, focusing on ultrasonic pulse velocity testing of rock cores to determine dynamic properties.
Specimen dimensions are preferably cylindrical with tolerances as outlined. Other regular shapes are permissible, with preparation following specified guidelines.
Oscilloscope requirements include a double beam unit with a bandwidth from DC to 15 MHz, rise time ≤ 0.02 µs, built-in or external delay markers, time sweep rate not exceeding 0.1 µs/cm, and minimum X-axis amplification of 10×.
Results rounding adheres to prescribed standards ensuring consistency.
The standard details the essential parameters for rock core samples, recommending cylindrical specimens with diameters typically 38 or 50 mm and lengths two to three times the diameter.
Ends must be smooth and parallel for precise wave transmission.
Formulas for dynamic Young’s modulus, Poisson’s ratio, and dynamic shear modulus are provided, utilizing measured densities and wave velocities.
Typical ranges for these parameters are tabulated for reference.
Guidelines for apparatus specify specimen shape and dimensional tolerances.
Acoustic coupling media such as light oil, soft grease, phenolic jelly, resin, salol, or epoxy compounds should be used to ensure effective contact between transducers and specimens.
A nominal contact pressure around 10 N/cm² is maintained using a jig.
Rounding of results follows established standards.
Typical specimen dimensions and coupling stress calculations are included.
This section describes suitable coupling agents and the importance of a nominal contact pressure approximately 10 N/cm².
Matched pairs of piezoelectric transducers, polarized for thickness mode (for compressional waves) or shear mode (for shear waves), operating between 100 kHz and 10 MHz are recommended.
Specimen faces must be polished and parallel to ensure effective acoustic transmission.
Oscilloscope specifications for capturing waveforms accurately are reiterated.
The procedure emphasizes specimen preparation with preferred geometries and tolerances.
Use of coupling media and consistent application of contact pressure is critical.
Final data rounding is according to standards.
Sample test specimen dimension examples and a schematic of the test setup are included for clarity.
Accurate measurement of specimen dimensions is necessary for valid results.
Shear wave travel time is measured with direct contact to avoid errors in first arrival time.
Formulas for dynamic moduli are stated clearly, including Young’s modulus, shear modulus, Poisson’s ratio, bulk modulus, and compressibility.
Results rounding guidelines are presented.
Reporting requirements include specimen details such as origin, geometry, and collection methods.
Measurement parameters like initial delays, wave velocities, and stress values must be included.
Pulse travel distance should exceed ten times the average grain size or wavelength.
Reports must document equipment settings, measurement procedures, and elastic modulus results.
Laboratory and observer identification are mandatory.
Frequently Asked
Specimen preparation involves carefully drilling rock cores to prevent damage, followed by cutting, grinding, lapping, and polishing to achieve smooth, flat, and parallel faces. Specimen dimensions must conform to specified tolerances. For saturated samples, immersion in water for at least 72 hours at ambient temperature is necessary, maintaining saturation until testing. Proper face preparation is essential to avoid non-parallelism and ensure accurate acoustic contact.
While the standard does not specify exact frequencies, it recommends selecting appropriate piezoelectric transducers for P-wave and S-wave measurements. Typically, P-wave transducers operate between 0.5 and 2 MHz, and S-wave transducers between 0.5 and 1 MHz. These frequency ranges provide a balance between penetration depth and resolution for rock specimens.
Acoustic coupling is achieved by applying coupling agents such as light oil, soft grease, phenolic jelly, resin, salol, or epoxy compounds between the transducers and specimen surfaces. A nominal contact pressure of approximately 10 N/cm² is maintained using a jig to ensure uniform contact. Specimen faces must be polished and parallel, and matched pairs of piezoelectric transducers operating in the 100 kHz to 10 MHz range are used to transmit and receive ultrasonic pulses effectively.
The oscilloscope should be a double beam type with a bandwidth from DC to 15 MHz and a rise time of 0.02 microseconds or less. It must have built-in or external movable delay time markers, a time base sweep rate not exceeding 0.1 microseconds per cm, and X-axis amplification of at least 10×. Proper acoustic coupling media and pulse adjustments ensure sharp pulses, and accurate measurement of initial delays and transit times are essential.
The dynamic modulus is calculated using the measured compressional (Vp) and shear (Vs) wave velocities along with the rock density (ρ). The rigidity modulus (G) is ρ times Vs squared. Poisson’s ratio (ν) is derived from the difference between squares of Vp and Vs. The dynamic Young’s modulus (E) is then computed as twice G multiplied by (1 + ν). Bulk modulus (K) and compressibility (β) are also calculated from these velocities, linking ultrasonic measurements to the rock’s mechanical properties.
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