Dynamic characterization of rock using an instrumented drop-weight test system: Impact compressive strength of Rock
(2016) , 79 pages
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Dynamiccharacterizationofrockusinganinstrumenteddrop-weighttestsystem-Impactcompressivestrengthofrock.pdf
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- Abstract
- In recent years, offshore wind turbines are emerging in seas. Most of offshore wind turbines are founded on monopiles, which are driven into sea bed. In such projects, soil resistance to driving (SRD) is a key technical, schedule and financial constraint in planning operations of foundation installation. Driving pile foundations in indurated formations has become a major challenge of the industry since not much is known about their behavior to dynamic efforts. Pile driving involves high strain rates comparing to those of quasi-static strength tests. In order to characterize dynamic resistance of rock and its intrinsic response to dynamic efforts, an instrumented drop-weight test system is investigated using a synthetic rock. This testing system is usually used for puncture test on composite materials. Use of this machine for dynamic characterization of rock requires an experiment design involving a study of its applicability, some modifications to the machine in addition to developing consistent testing procedures. Firstly, a mechanical model is developed for the impact loading of a specimen made of synthetic rock with an arbitrary geometry and dimensions behaving according to a material model proposed by European Committee for Structural Concrete (CEB 1990). This mechanical model is used to ensure that impact testing can reveal the true constitutive behavior of the specimen material. Furthermore, it is utilized to obtain the necessary impact testing parameters such as necessary striker impact velocity and mass. Next, specimen geometry is varied considering multiple scenarios in order to account for machine limitations such as available impactor masses, measurable load and striker impact velocity limits. Impact compressive strength tests were conducted using the assumed mass-velocity combinations that would result in specimen failure if the synthetic rock behaved similar to concrete. The obtained force time-histories were normalized by cross-sectional areas of specimens and treated with signal processing techniques such as FFT. A linear relationship between impact strength and stress-rate is observed. The results are compared with those of CEB (1990). The results are carefully studied from several perspectives in order to account for effects of various factors, which may lead to false interpretations. These factors include inertial effects, possible limitation of system frequency response, system ringing and equilibrium of stresses. Based on this analysis, bending resonance of test system is, primarily, suspected for resulting in sharp spikes in the signal causing higher strain rates and peaks. Besides, it is doubted if stress equilibrium is achieved since impact durations are very short. Finally, some recommendations are made for improving the experiment design in order to avoid resonance, to ensure equilibrium of stresses and minimize the experiment errors associated with the fabrication and preparations of specimens.
- Affiliations
University of Nantes Polytech Nantes
Citations
Mustafa Jafari. (2016). Dynamic characterization of rock using an instrumented drop-weight test system: Impact compressive strength of Rock. https://hdl.handle.net/2078.5/124291