Propagation and aperture of staged hydraulic fractures in unconventional resources in toughness-dominated regimes

Simultaneous multistage hydraulic fracturing of unconventional gas shale in parallel multilateral wells is an effective technique to raise the connectivity of the reservoir to the wellbore and improve reservoir permeability for an economical production. However, this technique should be accompanied with some optimization procedures to obtain an efficiently fractured reservoir with the highest production and the lowest cost. In unconventional hydraulic fracturing, fracture deviation/collapse and trapping are familiar phenomena which occur when a non-optimized fracturing pattern is used. These problems occur respectively when stress shadow size has not been considered in optimization and fracturing pressure is higher than the available pressure in the sealed section. Therefore, in an optimized hydraulic fracturing, having straight fractures with no deviation or collapse needs consideration of stress shadow effect (SSE). Apart from that, having efficiently propagated fractures to the extent of the reservoir without any fracture trap requires consideration of stress intensity factor (SIF) and aperture. SSE was studied and published by the authors in 2014. For the case of SIF, investigating any change in mode I SIF and aperture with different influencing variables such as fracture geometry and pattern are studied in the current research work. Three different fracturing techniques are assumed as multistage fracturing, simultaneous single-stage fracturing, and simultaneous multistage fracturing techniques. Since obtaining SIF for three-dimensional fractures is a challenging issue, a stress ratio technique is used for calculation of SIF ratios of different fracturing scenarios compared to the case of a single fracture. Therefore, changes of SIF for different fracturing schemes are estimated and analyzed to understand whether or not a fracturing scheme is efficient and all the spaced perforations are activated and change to hydraulic fractures.     

用碳/环氧和石墨/环氧片修复半圆凹槽边缘裂缝有效性的比较

粘结复合材料片修复技术可以有效地用于加固或延长带裂缝铝结构的使用寿命。采用有限元方法分析在半圆形侧向凹槽处粘结不同复合材料片用于裂缝修复的性能。相邻裂缝处的应力分布对于根据片材几何尺寸进行的修复分析是很重要的。力学和几何性能对裂缝尖端处应力集中系数变化的影响显著。粘结剂性能和片材尺寸对I型模型中裂缝尖端处应力集中系数变化的影响也很显著。并进行了双片和单片修复的比较。结果表明双片修复时的裂缝尖端应力集中系数比单片修复时的裂缝尖端应力集中系数减少一半,具有高刚度的纤维走向垂直于裂缝走向时显著地降低应力集中系数。为了提高片材修复或加固性能,必须进行粘结剂性能的优化。     

碳纤维增强复合材料加固钢板的边界元分析

采用边界元分析对碳纤维增强复合材料加固钢板的疲劳性能进行扩展研究。采用BEASY软件计算应力强度因子、裂缝扩展和疲劳周期。应用面单元模拟组合增强片和带裂缝的钢板,将粘结层作为连接增强片和钢板的界面单元。数值模拟结果与试验结果具有一致性,从而验证了本文方法的有效性。采用边界元方法,对影响应力强度因子和疲劳寿命的因素进行评估,这些因素包括粘结长度、粘结宽度、增强片配置、碳纤维增强复合材料的层数组合增强片模量、粘结组合模量。     

Fatigue life prediction of heavy mining equipment. Part 2: Behaviour of corner crack in steel welded box section and remaining fatigue life determination

Frequent fatigue failure of mining equipment has created a need for fatigue life prediction of several components that suffer from frequent recurrence of fatigue cracking. The focus of this paper is the frequent occurrence of fatigue cracks in the boom of cable shovels, which consists of a twin box girder joining together near both extremities. After the determination of load spectrum and the material fatigue properties by field monitoring and material testing, the fatigue behaviour of a cracked shovel boom was investigated. Two-tip corner cracks were of particular interest since they are commonly observed in the field. These cracks typically initiate in the flange at the toe of web to flange or flange to diaphragm welds and propagate into the web. Detailed finite element models with corner cracks were developed for the boom and stress intensity factors for various crack configurations were evaluated from the finite element analysis results. The growth behaviour of corner cracks was studied and a simplified method is presented for the fatigue life prediction. Fracture toughness tests were conducted on single edge notch bend specimens at room temperature and low temperature (−50 ^°C). The remaining life of a cracked boom is then determined based on brittle fracture or unstable crack growth.