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A Numeric Predictive Failure Model for Percussive Excavation


NASA is currently developing technology for future human space exploration missions. One of these technologies is percussive excavation. The presented research examines how percussion affects soil behavior during the excavation process.

The purpose of this research was to develop a numeric code for the prediction of reaction forces associated with soil failure during percussive excavation. In order to achieve this objective a variety of different excavation variables were tested. Those variables include: percussive frequency, percussive impact energy, excavation speed, excavation attack angle, excavation depth, and soil relative density.

The results of the experimental testing showed that through percussion the effects of dilatancy along a soil's failure boundary layer were mitigated. This result was seen both in the reduction of the soil draft force as well as the soil's ability to continuously create shear planes during excavation. In relation to the draft force, the effects of percussion resulted in an exponential decay in the soil's internal friction angle.

Within this report a numeric code is proposed and tested which predicts an excavation draft force given input parameters of: percussive frequency, excavation speed, percussive impact energy, geometric dimensions of a flat-backed excavation implement, excavation attack angle, gravitational constant, and in situ soil internal friction angle.

The theoretical basis of the numeric code is the upper limit analysis method which uses virtual work to back-calculate an unknown applied traction force at incipient soil failure. In order to use the upper limit analysis approach the soil is idealized as perfectly plastic, stable, and obeying the flow rule.

To incorporate the effects of percussion into the theoretical model the upper limit analysis is used, but changes are made to the defining control volume geometry of the failure volume based on percussive, soil, and speed parameters. Those changes are implemented through the internal friction angle. The internal friction angle is characterized as having an exponentially decaying relationship in terms of applied percussive energy. The exponential decay factor is given as a function of the in situ internal friction angle of the soil, and the applied excavation velocity.

Results from the theoretical model show agreement between predicted and experimentally measured excavation reaction forces. Results are provided using a wide array of different input values from the following variables: percussive frequency, percussive impact energy, excavation speed, excavation attack angle, excavation depth, and soil relative density.

In addition, the theoretical model is used to predict percussive excavation forces when gravity is changed to 1/6 the magnitude of earth's gravitational force. By comparing those results with ones taken using earth's gravity, it is found that the asymptotic limit of the internal friction angle is achieved through lower percussive frequencies and lower power requirements in lunar gravity than in earth gravity. Furthermore, it is shown that the reduction in excavation force between a lunar gravity environment and an earth gravity environment is not a constant, rather a value dependent on applied percussion and excavation parameters.

It is concluded that this work provides an adequate first generation numeric model which can be used for approximating percussive excavation reaction forces in terms of a wide array of different input variables. It is recommended that future work be done to continue refinement and calibration of the code, as well as further validation.

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