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

职业：广泛应变率下岩石的损伤和断裂特征

• 批准号: 1831126
• 负责人: WIlliam Griffith
• 金额: $ 18.47万
• 依托单位: Ohio State University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2017
• 资助国家: 美国
• 起止时间: 2017-08-17 至 2021-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1831126&HistoricalAwards=false
• 关键词: CAREER; Damage; Fracture; Characteristics; Rocks

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.

地球巨大构造板块的运动通常以每年毫米到数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，并且贯穿了地球的历史。 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利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（例如地震和外星撞击）以及与人类活动相关的风险（爆炸、地雷故障、射弹穿透）的发生速度比自然过程快数百至数千倍。通常在实验室中模拟。因此，几乎没有实验数据来确认或校准理论模型来解释这些戏剧性事件与断层带、撞击或爆炸地点发现的粉状岩石之间的联系。因此，建议结合实验和现场调查来研究地震和冲击环境下的脆性岩石破坏。 不同岩石类型在快速加载速率下的机械行为被认为取决于岩石内单个矿物的微观组成和结构。 如果属实，这将使科学家能够根据各个区域的岩石结构更好地预测地震和冲击事件的后果，并进一步使工程师能够设计更有效的结构来承受采矿、石油和军事环境的压力。 该研究计划融入了与 Teach for America (TFA) 的合作伙伴关系，TFA Geocorps 是一个由大学毕业生和专业人士组成的全国教师团队，致力于在公立学校任教两年并提高学生的成绩，从而创建 TFA Geocorps。 TFA Geocorps 将由参与与该项目相关的夏季研究活动的优秀中学教师组成，他们还将与首席研究员合作设计基于地球物理学的主题课程单元，以便在自己的课堂上进行教学。 受该项目支持的研究生将通过与参与的 TFA Geocorps 教师的积极合作来补充他们的学术培训。 脆性损伤通过从非常慢（断层蠕变）到非常快（地外撞击）的众多过程在地壳中积累。 岩石中脆性损伤形成的依赖于应变率的微观力学，特别是在限制条件下，受到的约束很差，但人们普遍认为，岩石在较高应变率下会变得更强，而在临界应变率阈值之上，压缩转变会失效从沿离散裂缝的局部损伤到离域（分布式）裂缝损伤（即碎裂或粉碎）。 最近一系列针对断层损伤区粉碎岩石的研究提供了证据，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率（100/s）是造成这种粉碎的原因。从离散断裂到碎裂的转变取决于限制（埋藏深度）；尽管限制很少，但该机制很可能是由岩石的晶粒结构和微裂纹扩展的动力学控制的。 这里建议通过综合现场、实验和理论研究来表征岩石的高应变率非弹性响应，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）依赖性以及比较通过现场观察来确定应变率的微观结构特征。 所提出的工作预计将校准高应变率岩石破坏的损伤力学模型，并表征不同应变率（和约束）状态下形成的损伤区域。 预期结果应为基于观察裂缝网络和地震各向异性区分导致岩石损伤的应变率和应力条件提供基础。 虽然这项工作的重点是地震和冲击期间造成的损坏，但高应变率下岩石的强度和破坏特性对于与爆破、岩爆、地下爆炸相关的采矿、石油和军事应用也具有根本意义。和防护设计。