Experimental results from Experimental and numerical study of drill bit drop tests on Kuru granite

This paper presents an experimental and numerical study of Kuru grey granite impacted with a sevenbuttons drill bit mounted on an instrumented drop test machine. The force versus displacement curves during the impact, so-called bit-rock interaction (BRI) curves, were obtained using strain gauge measurements for two levels of impact energy. Moreover, the volume of removed rock after each drop test was evaluated by stereo-lithography (three-dimensional surface reconstruction). A modified version of the Holmquist-Johnson-Cook (MHJC) material model was calibrated using Kuru granite test results available from the literature. Numerical simulations of the single drop tests were carried out using the MHJC model available in the LS-DYNA explicit finite-element solver. The influence of the impact energy and additional confining pressure on the BRI curves and the volume of the removed rock is discussed. In addition, the influence of the rock surface shape before impact was evaluated using two different mesh geometries: a flat surface and a hyperbolic surface. The experimental and numerical results are compared and discussed in terms of drilling efficiency through the mechanical specific energy.This article is part of the themed issue ‘Experimental testing and modelling of brittlematerials at high strain rates’.本文针对搭载于配备传感测试系统的落锤试验机（instrumented drop test machine）上的七齿钻头（sevenbuttons drill bit）冲击库鲁花岗岩的过程，开展了试验与数值研究。试验中通过应变片测量，获取了两级冲击能量下的冲击过程力-位移曲线，即所谓的钻齿-岩石相互作用（bit-rock interaction, BRI）曲线。此外，通过立体光刻（stereo-lithography，三维表面重构）技术，评估了每次落锤试验后被切削岩石的体积。研究利用公开文献中的库鲁花岗岩试验数据，对改进型Holmquist-Johnson-Cook（MHJC）材料本构模型进行了参数标定；并采用LS-DYNA显式有限元求解器内置的MHJC模型，开展了单次落锤试验的数值模拟。本文探讨了冲击能量与附加围压对BRI曲线及被切削岩石体积的影响规律。此外，通过两种不同的网格几何构型——平面与双曲面，评估了冲击前岩石表面形貌的影响。最后从比机械能（mechanical specific energy）的视角，对试验与数值模拟结果的钻进效率开展了对比分析。本文属于"高应变率下脆性材料的试验测试与建模"专题栏目。