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Title: Geomechanics of penetration :laboratory analog experiments using a modified split hopkinson pressure bar/impact testing procedure.

Abstract

This research continues previous efforts to re-focus the question of penetrability away from the behavior of the penetrator itself and toward understanding the dynamic, possibly strain-rate dependent, behavior of the affected materials. A modified split Hopkinson pressure bar technique is prototyped to determine the value of reproducing the stress states, and mechanical responses, of geomaterials observed in actual penetrator tests within a laboratory setting. Conceptually, this technique simulates the passage of the penetrator surface past any fixed point in the penetrator trajectory by allowing for a controlled stress-time function to be transmitted into a sample, thereby mimicking the 1D radial projection inherent to analyses of the cavity expansion problem. Test results from a suite of weak (unconfined compressive strength, or UCS, of 22 MPa) concrete samples, with incident strain rates of 100-250 s{sup -1}, show that the complex mechanical response includes both plastic and anelastic wave propagation, and is critically dependent on incident particle velocity and saturation state. For instance, examination of the transmitted stress-time data, and post-test volumetric measurements of pulverized material, provide independent estimates of the plasticized zone length (1-2 cm) formed for incident particle velocity of {approx}16.7 m/s. The results also shed light on the elastic ormore » energy propagation property changes that occur in the concrete. For example, the pre- and post-test zero-stress elastic wave propagation velocities show that the Young's modulus drops from {approx}19 GPa to <8 GPa for material within the first centimeter from the plastic transition front, while the Young's modulus of the dynamically confined, axially-stressed (in 6-18 MPa range) plasticized material drops to 0.5-0.6 GPa. The data also suggest that the critical particle velocity for formation of a plastic zone in the weak concrete is 13-15 m/s, with increased saturation tending to increase the critical particle velocity limit. Overall, the data produced from these experiments suggests that further pursuit of this approach is warranted for penetration research but also as a potential new method for dynamic testing of materials.« less

Authors:
; ;
Publication Date:
Research Org.:
Sandia National Laboratories
Sponsoring Org.:
USDOE
OSTI Identifier:
877715
Report Number(s):
SAND2005-7335
TRN: US200608%%556
DOE Contract Number:
AC04-94AL85000
Resource Type:
Technical Report
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; PENETRATORS; CONCRETES; STRESS ANALYSIS; STRAIN RATE; WAVE PROPAGATION; MECHANICAL PROPERTIES; MATERIALS TESTING; Mechanics.; Penetration mechanics.; Rock mechanics.; Penetration tests.

Citation Formats

Holcomb, David Joseph, Gettemy, Glen L., and Bronowski, David R. Geomechanics of penetration :laboratory analog experiments using a modified split hopkinson pressure bar/impact testing procedure.. United States: N. p., 2005. Web. doi:10.2172/877715.
Holcomb, David Joseph, Gettemy, Glen L., & Bronowski, David R. Geomechanics of penetration :laboratory analog experiments using a modified split hopkinson pressure bar/impact testing procedure.. United States. doi:10.2172/877715.
Holcomb, David Joseph, Gettemy, Glen L., and Bronowski, David R. Tue . "Geomechanics of penetration :laboratory analog experiments using a modified split hopkinson pressure bar/impact testing procedure.". United States. doi:10.2172/877715. https://www.osti.gov/servlets/purl/877715.
@article{osti_877715,
title = {Geomechanics of penetration :laboratory analog experiments using a modified split hopkinson pressure bar/impact testing procedure.},
author = {Holcomb, David Joseph and Gettemy, Glen L. and Bronowski, David R.},
abstractNote = {This research continues previous efforts to re-focus the question of penetrability away from the behavior of the penetrator itself and toward understanding the dynamic, possibly strain-rate dependent, behavior of the affected materials. A modified split Hopkinson pressure bar technique is prototyped to determine the value of reproducing the stress states, and mechanical responses, of geomaterials observed in actual penetrator tests within a laboratory setting. Conceptually, this technique simulates the passage of the penetrator surface past any fixed point in the penetrator trajectory by allowing for a controlled stress-time function to be transmitted into a sample, thereby mimicking the 1D radial projection inherent to analyses of the cavity expansion problem. Test results from a suite of weak (unconfined compressive strength, or UCS, of 22 MPa) concrete samples, with incident strain rates of 100-250 s{sup -1}, show that the complex mechanical response includes both plastic and anelastic wave propagation, and is critically dependent on incident particle velocity and saturation state. For instance, examination of the transmitted stress-time data, and post-test volumetric measurements of pulverized material, provide independent estimates of the plasticized zone length (1-2 cm) formed for incident particle velocity of {approx}16.7 m/s. The results also shed light on the elastic or energy propagation property changes that occur in the concrete. For example, the pre- and post-test zero-stress elastic wave propagation velocities show that the Young's modulus drops from {approx}19 GPa to <8 GPa for material within the first centimeter from the plastic transition front, while the Young's modulus of the dynamically confined, axially-stressed (in 6-18 MPa range) plasticized material drops to 0.5-0.6 GPa. The data also suggest that the critical particle velocity for formation of a plastic zone in the weak concrete is 13-15 m/s, with increased saturation tending to increase the critical particle velocity limit. Overall, the data produced from these experiments suggests that further pursuit of this approach is warranted for penetration research but also as a potential new method for dynamic testing of materials.},
doi = {10.2172/877715},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Tue Nov 01 00:00:00 EST 2005},
month = {Tue Nov 01 00:00:00 EST 2005}
}

Technical Report:

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  • A technique for conducting instrumented Charpy impact tests using a Hopkinson bar is presented. Data for three grades of beryllium covering a range in impact velocities from 20 to 200 in/sec (.5 to 5 mm/s) are obtained in the form of load-deflection curves from which maximum load, maximum deflection, and total energy are obtained. Results show good agreement with data on identical materials obtained from an instrumented impact test at 54 in/sec (1.37 m/s) and from a standard Charpy impact machine at 135 in/sec (3.43 m/s). The advantages and limitations of the Hopkinson bar apparatus are discussed.
  • The split Hopkinson pressure bar or Kolsky bar has provided for many years a technique for performing compression tests at strain rates approaching 10/sup 4/ s/sup -1/. At these strain rates, the small dimensions possible in a compression test specimen give an advantage over a dynamic tensile test by allowing the stress within the specimen to equilibrate within the shortest possible time. The maximum strain rates possible with this technique are limited by stress wave propagation in the elastic pressure bars as well as in the deforming specimen. This subject is reviewed in this paper, and it is emphasized thatmore » a slowly rising excitation is preferred to one that rises steeply. Experimental techniques for pulse shaping and a numerical procedure for correcting the raw data for wave dispersion in the pressure bars are presented. For tests at elevated temperature a bar mover apparatus has been developed which effectively brings the cold pressure bars into contact with the specimen, which is heated with a specially designed furnace, shortly before the pressure wave arrives. This procedure has been used successfully in tests at temperatures as high as 1000/sup 0/C.« less
  • Loading on buried structures subjected to nuclear or conventional high explosive weapons is strongly influenced by the backfill adjacent to and overlying the structure. The relative stiffness of the structure and the surrounding soil and their interactions will determine the level and extent of damage due to blast loads. White soil response may be in the laboratory, this environment must be able to reflect the type of confinements, magnitude of stress change, and the time scale of loading expected in the problem. The split-Hopkinson pressure bar (SHPB) technique has been adapted to measure the dynamic response of soil to implulsemore » loads. This technique can significantly extend the range of stresses and strain rates that can be applied beyond the capabilities of conventional laboratory dynamic soil-testing equipment. Various assumptions and considerations involve in designing an SHPB experiment and evaluating the data with soil as a specimen are discussed in detail. Soils have low wave speeds, nonlinear hysteretic behavior, and low unconfined compressive strength which complicate SHPB testing. Insight is provided as to how these factors affect experimental accuracy and data reliability. The dynamic soil stress-strain response was found to be governed principally by the initial gas porosity of the specimen. Examples of stress-strain curves are present for specimens with applied stresses ans strain rates up to 520 MPa and 4000/s respectively.« less
  • Three-point impact bending tests, using the split Hopkinson pressure bar method, were performed to evaluate the fracture resistance of monolithic silicon nitride (SN) and carbon-fiber-reinforced silicon nitride (CFRSN) ceramics. By applying ramped incident-stress waves in the split Hopkinson pressure bar apparatus, relatively smooth stress-time curves could be recorded without using any artificial filtering process. The maximum load in the load-deflection curve of the CFRSN material increased, in comparison to its static value, when impact testing was applied. Such behavior was substantially different from that of the monolithic SN material, for which the maximum load values from impact and static testingmore » were almost the same. The time dependence of strength in the CFRSN ceramic was then investigated by using relaxation tests, and the impact strength behavior could be explained by these results. Also, the shear strength was significantly dependent on the deformation rate, whereas the tensile strength was almost independent of it. The experimental results were compared with the numerical predictions of the stress distribution that were obtained by using finite-element analysis.« less