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

地球上巨大的构造板块的运动通常以每年毫米到几十厘米为单位进行测量，这似乎证实了人们普遍持有的观点，即构造过程是缓慢的，并且一直贯穿整个地球。年代的历史。根据这一观点，大多数专注于岩石破坏的实验室研究仅限于使用慢加载速率的实验。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）以及与人类活动有关的风险（爆炸、地雷失效、炮弹穿透）的发生速度比实验室通常模拟的速度快数百至数千倍。因此，很少有实验数据可以证实或校准理论模型，以解释这些戏剧性事件与在断层带、撞击或爆炸地点发现的粉状岩石之间的联系。因此，提出了试验与现场调查相结合的方法来研究地震和冲击环境下脆性岩石的破坏。假定不同岩石类型在快速加载速率下的力学行为取决于岩石内单个矿物的微观组成和结构。如果这是真的，这将使科学家能够更好地预测地震和冲击事件的后果，基于个别地区的岩石结构，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。与这项研究计划相结合的是与“为美国而教”（Teach for America，简称TFA）合作创建“为美国而教”地球队（TFA georcorps）。“为美国而教”是一个由大学毕业生和专业人士组成的全国性教师队伍，他们承诺在公立学校教书两年，提高学生的成绩。TFA地球队将由高水平的中学教师组成，他们将参与与该项目有关的夏季研究活动，他们还将与首席研究员合作，设计以地球物理学为基础的主题课程单元，在他们自己的课堂上教授。获此计划资助的研究生，将与参与计划的TFA地球服务队教师积极合作，以补充他们的学术训练。脆性损伤在地球上积累？美国地壳经过了从非常缓慢（断层蠕变）到非常快速（地外撞击）的许多过程。岩石中脆性损伤形成的微观力学，特别是在约束条件下，受到的约束很少，但人们普遍认为，岩石在更高的应变速率下变得更强，并且在临界应变速率阈值以上，压缩破坏从沿离散裂缝的局部损伤转变为非局部（分布）断裂损伤（即破碎或粉碎化）。最近一系列关于断层破坏带岩石粉状的研究表明，与破裂尖端传播和/或超剪切地震破裂相关的超高应变速率（100/s）是造成这种粉状的原因。从离散断裂到破碎的转变取决于约束（埋藏深度）；其机理虽然约束不严格，但可能受岩石晶粒结构和微裂纹扩展动力学的控制。本文提出通过综合现场、实验和理论研究来表征岩石的高应变率非弹性响应，研究重点是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，并与现场观察结果进行比较，以确定应变率的微观结构特征。所提出的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）制度下形成的损伤区域。预期结果将为基于观察裂缝网络和地震各向异性来区分导致岩石损伤的应变速率和应力条件提供依据。虽然这项工作的重点将放在地震和冲击造成的破坏上，但高应变率下岩石的强度和破坏特征也对采矿、石油和与爆破、岩爆、地下爆炸和防护设计相关的军事应用具有根本的兴趣。

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.