CAREER: Damage and Fracture Characteristics of Rocks Under a Broad Spectrum of Strain Rates

地球巨大的构造板块的运动通常以毫米到几十厘米为单位，这似乎证实了普遍持有的观点，即构造过程是缓慢的，并且贯穿了地球-S的整个历史。与这一观点一致，大多数专注于岩石破坏的实验室研究一直局限于使用低加载速率的实验。然而，许多对人类构成重大风险的自然过程(例如地震和地外撞击)以及与人类活动相关的风险(爆炸、地雷故障、弹丸侵彻)的发生速度比通常在实验室模拟的速度快数百至数千倍。因此，几乎没有实验数据来证实或校准解释这些戏剧性事件与在断裂带、撞击或爆炸地点发现的粉碎岩石之间的联系的理论模型。因此，本文提出了一种实验和现场相结合的方法来研究脆性岩石在地震和冲击环境中的破坏。不同岩石类型在快速加载速率下的力学行为假定取决于岩石中单个矿物的微观组成和结构。如果这是真的，这将使科学家能够根据个别地区的岩石结构更好地预测地震和撞击事件的后果，并进一步允许工程师设计更有效的结构，以承受采矿、石油和军事环境中的压力。在这项研究计划中加入了与Teach for America(TFA)的合作伙伴关系，TFA是一个由大学毕业生和专业人员组成的全国教师团队，他们承诺在公立学校任教两年，提高学生的成绩，创建TFA地理公司。TFA Geocorps将是参与与该项目有关的暑期研究活动的优秀中学教师，他们还将与首席调查员合作，设计以地球物理为基础的专题课程单元，在自己的教室里授课。该项目支持的研究生将通过与参与TFA Geocorps教师的合作发挥积极作用来补充他们的学术培训。脆性破坏在地球上积累-S地壳通过许多过程，从非常慢的(断层蠕变)到非常快的(外星撞击)。岩石中脆性损伤形成的应变率相关细观力学，特别是在约束条件下，约束较差，但人们普遍认为，岩石在较高应变率下变得更坚固，超过临界应变率阈值，压缩破坏从沿离散裂缝的局部损伤转变为非局部(分布)的断裂损伤(即破碎或粉化)。最近的一系列研究集中在断层破坏带中的粉碎岩，提供了与破裂尖端扩展和/或超剪切地震破裂有关的超高应变率(100/S)是这种粉碎化的原因的证据。从离散断裂到破碎的转变取决于约束(埋藏深度)；虽然约束较差，但机制可能受控于岩石的颗粒尺度结构和微裂纹扩展的动力学。本文提出了一种岩石高应变率非弹性响应的表征方法，它是一种综合的场、实验和理论研究，集中于断裂韧性的应变率依赖性、抗压强度和损伤的应变率(和约束)依赖性，并与现场观测进行比较，以确定应变率的微观结构特征。这项工作有望校准高应变率岩石破坏的损伤力学模型，并表征在不同应变率(和约束)下形成的损伤区。预期结果应该为根据观察裂隙网络和地震各向异性来区分引起岩石损伤的应变率和应力条件提供依据。虽然这项工作的重点将是地震和冲击造成的破坏，但高应变率下岩石的强度和破坏特征也是采矿、石油和军事应用中与爆破、岩爆、地下爆炸和防护设计相关的基本问题。

The motions of Earth's enormous tectonic plates are typically measured in millimeters to tens of centimeters per year, seemingly confirming the generally-held view that tectonic processes are slow, and have been throughout Earth?s history. In line with this perspective, most laboratory research focused on rock failure has been limited to experiments utilizing slow loading rates. However, many natural processes that pose significant risk for humans (e.g., earthquakes and extraterrestrial impacts), as well as risks associated with human activities (explosions, mine failures, projectile penetration), occur at rates which are hundreds to thousands of time faster than typically simulated in the laboratory. As a result, little experimental data exists to confirm or calibrate theoretical models explaining the connection between these dramatic events and the pulverized rocks found in fault zones, impact, or explosion sites. Therefore, a combined experimental and field investigation is proposed to study brittle rock failure in both earthquake and impact environments. The mechanical behavior of different rock types at fast loading rates is postulated to depend on the microscopic composition and structure of individual minerals within the rocks. If true, this will allow scientists to better predict the consequences of earthquakes and impact events based on the rock structure in individual areas and furthermore allow engineers to design more effective structures to withstand the pressures in mining, petroleum and military environments. Integrated into this research plan is a partnership with Teach for America (TFA), a national teacher corps of college graduates and professionals who commit to teach for two years and raise student achievement in public schools, to create the TFA Geocorps. The TFA Geocorps will be high-achieving secondary school teachers involved in summer research activities related to the project who will also work with the Principal Investigator to design Geophysics-based thematic curriculum units to teach in their own classrooms. Graduate students supported by this project will supplement their academic training by taking active roles in collaborations with the participating TFA Geocorps teachers. Brittle damage accumulates in the earth?s crust via numerous processes ranging from very slow (fault creep) to very fast (extraterrestrial impact). The strain-rate-dependent micromechanics of brittle damage formation in rocks, particularly under confinement, is poorly constrained, yet it is generally understood that rocks become stronger at higher strain rates, and that above a critical strain-rate threshold, failure in compression transitions from localized damage along discrete fractures to delocalized (distributed) fracture damage (i.e., fragmentation or pulverization). A recent series of studies focused on pulverized rocks in fault damage zones provide evidence that ultra-high strain rates (100/s) associated with rupture tip propagation and/or supershear earthquake rupture are responsible for this pulverization. The transition from discrete fracture to fragmentation depends on confinement (burial depth); and the mechanism, although poorly constrained, is likely controlled by the grain-scale structure of rocks and the dynamics of microcrack propagation. Here it is proposed to characterize the high strain rate inelastic response of rocks by following an integrated field, experimental, and theoretical study focused on the strain rate dependence of fracture toughness, strain rate (and confinement) dependence of compressive strength and damage, and comparison with field observations to determine a microstructural signature of strain rate. The proposed work is expected to calibrate damage mechanics models of high strain-rate rock failure, and to characterize damage zones formed in different strain rate (and confinement) regimes. Expected results should provide a basis for distinguishing strain rate and stress conditions responsible for rock damage based on observing fracture networks and seismic anisotropy. While the focus of this work will be on damage created during earthquakes and impacts, the strength and failure characteristics of rocks under high strain rates are also of fundamental interest in mining, petroleum, and military applications related to blasting, rock burst, underground explosions, and protective design.