Soil can be probed relatively gently to a depth of several feet.
Ultrasonic/sonic impacting penetrators (USIPs) are recent additions to the series of apparatuses based on ultrasonic/sonic drill corers (USDCs). A USIP enables a rod probe to penetrate packed soil or another substance of similar consistency, without need to apply a large axial force that could result in buckling of the probe or in damage to some buried objects. USIPs were conceived for use in probing and analyzing soil to depths of tens of centimeters in the vicinity of buried barrels containing toxic waste, without causing rupture of the barrels. USIPs could also be used for other purposes, including, for example, searching for pipes, barrels, or other hard objects buried in soil; and detecting land mines.
USDCs and other apparatuses based on USDCs have been described in numerous previous NASA Tech Briefs articles. The ones reported previously were designed, variously, for boring into, and/or acquiring samples of, rock or other hard, brittle materials of geological interest. To recapitulate: A USDC can be characterized as a light-weight, low-power, piezoelectrically driven jackhammer in which ultrasonic and sonic vibrations are generated and coupled to a tool bit. As shown in the figure, a basic USDC includes a piezoelectric stack, a backing and a horn connected to the stack, a free mass (“free” in the sense that it can slide axially a short distance between the horn and the shoulder of tool bit), and a tool bit, i.e., probe for USIP. The piezoelectric stack is driven at the resonance frequency of the stack/horn/backing assembly to create ultrasonic vibrations that are mechanically amplified by the horn. To prevent fracture during operation, the piezoelectric stack is held in compression by a bolt. The bouncing of the free mass between the horn and the tool bit at sonic frequencies generates hammering actions to the bit that are more effective for drilling than is the microhammering action of ultrasonic vibrations in ordinary ultrasonic drills. The hammering actions are so effective that the axial force needed to make the tool bit advance into the material of interest is much smaller than in ordinary twist drilling, ultrasonic drilling, or ordinary steady pushing.
The differences between a USIP and a USDC-based apparatus described above lie in design details that make a USIP more suitable for penetrating packed soil. The piezoelectric stack in an experimental prototype USIP had a diameter of 1.0 in. (≈25 mm) and could be made to resonate at a frequency between 12 and 20 kHz, the exact value depending on the specific design and operating conditions. The probe rod had a diameter of 1/8 in. (≈3 mm) and a length sufficient to enable penetration to a depth of 3 ft (≈91 cm). The piezoelectric stack was driven at a 20-percent duty cycle, with a combination of automatic and manual adjustments of the frequency of the driving signal to compensate for changes in the resonance frequency induced by changes in mechanical loading and by temperature rise during operation.
The design of the horn and a piezoelectric-stack-backing structure was optimized for coupling power from the stack to the horn and for amplification of the longitudinal displacement. The optimization was accomplished with the help of a computer program that numerically solved the governing equations to perform impact and vibration-mode analyses. The modal analysis was used to determine the dimensions of the horn and backing for a resonance frequency in the required range and to further adjust the dimensions of the horn so that the neutral plane matched the mounting plane to minimize adverse effects of transducer vibration on a supporting structure. The impact analysis, in which the focus was on the interaction between the free mass and the horn, was used to derive an optimal weight of the free mass.
In experiments, an axial force of 7 lb (≈31 N)] was found to be sufficient to cause the probe tip to reach a depth of 3 ft (≈91 cm) in a packed soil sample. In contrast, the axial force that would be needed to make an equivalent probe tip penetrate to the same depth by ordinary steady pushing has been estimated to be about 200 lb (≈890 N), which is large enough to easily cause buckling of the probe without a holding mechanism and to damage a buried barrel.
This work was done by Xiaoqi Bao, Yoseph Bar-Cohen, Zensheu Chang, Stewart Sherrit, and Randall A. Stark of Caltech for NASA’s Jet Propulsion Laboratory.
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