Study on the Dynamic Evolution of Through-Crack in the Double Hole of Elliptical Bipolar Linear-Shaped Charge Blasting

Seeking the law of through-crack in the double hole of shaped charge can help reveal the rock failure mechanism of directional controlled blasting. Using LS-DYNA numerical simulation analysis, the dynamic mechanical behaviors of double-hole crack development and double-hole crack penetration in elliptical bipolar linear-shaped charge blasting and ordinary blasting were compared and studied. The results showed that it was difficult to form a straight line through the double holes under ordinary blasting, while easy to cause over-under-excavation problems. The blasting of the elliptical bipolar linear-shaped charge had a significant effect on the formation of directional crack. The crack penetrated along the connecting center line of the two holes. The main crack growth form was tensile fracture mode, and the explosion gas was the important driving force for continuous crack growth. The elliptical bipolar linear-shaped charge blasting produced fewer cracks in the nonenergy-accumulating direction, which could effectively reduce the damage of the retained rock mass.

Comparative Study on Mineral-Scale Microcrack Propagation of Shale under Different Loading Methods

Producing a sufficient volume of multiscale crack networks is key to enhancing recovery of shale gas. The formation of crack network largely depends on initiation and propagation of microcracks. To reveal the influence of different loading methods on the propagation of mineral-scale microcracks, this study used the Voronoi tessellation technique to establish a cohesive zone model of shale mineral distribution and applied six different boundary conditions to represent different loading methods. Crack path characteristics, rupture characteristics, continuous crack propagation and turning, and en echelon intermittent crack propagation under different loading methods were compared and analyzed. The essence of different loading methods affecting the length and complexity of cracks was the spreading range of tensile microcracks. The mechanical properties of minerals led to dissimilarities in continuous crack propagation and turning. The formation and propagation of en echelon intermittent fractures of different scales were mainly impacted by the heterogeneity of minerals and mineral aggregates. The spreading direction and connection form of en echelon intermittent fractures were mainly affected by the loading method. Conclusions arising from mineral-scale simulations contribute to understanding the mechanism of microcrack propagation resulting from different loading methods, and these conclusions have a guiding significance to enhanced shale gas recovery.

Analysis of the Crack Initiation and Growth in Crystalline Materials Using Discrete Dislocations and the Modified Kitagawa–Takahashi Diagram

Crack growth kinetics in crystalline materials is examined both from the point of continuum mechanics and discrete dislocation dynamics. Kinetics ranging from the Griffith crack to continuous elastic-plastic cracks are analyzed. Initiation and propagation of incipient cracks require very high stresses and appropriate stress gradients. These can be obtained either by pre-existing notches, as is done in a typical American Society of Testing and Materials (ASTM) fatigue and fracture tests, or by in situ generated stress concentrations via dislocation pile-ups. Crack growth kinetics are also examined using the modified Kitagawa–Takahashi diagram to show the role of internal stresses and their gradients needed to sustain continuous crack growth. Incipient crack initiation and growth are also examined using discrete dislocation modeling. The analysis is supported by the experimental data available in the literature.

Effect of Z-pinning on fatigue crack propagation in composite skin/stiffener structures

The skin/stiffener interface debonding has been a longstanding problem for composite stiffened panels. Proper crack-arresting reinforcements become a necessity for the wide application on large-scale framed structures. Z-pinning was employed to strengthen composite skin/stiffener bond in this study. Herein, static and fatigue tensile tests were conducted on a generic configuration to characterize the improvement of Z-pinning on skin/stiffener debonding resistance to skin stretching. Results show that the improvements on ultimate debonding strength and fatigue life are significant, even though the effect on crack onset is marginal under either monotonic or cyclic loading. Z-pinning changes the unstable continuous crack growth into a propagation-suspension-propagation evolution pattern. The crack growth rate is decreased by up to three orders of magnitude due to Z-pinning. Effects of pin distribution were experimentally studied. A locally densified distribution is found to be more effective than the traditional uniform distribution.

Continuous Depth Sizing of ILI Ultrasonic Crack Detection

Since the market launch of Ultrasonic crack detection tools, the conventional crack depth sizing is based on four depth classes or buckets. A more differentiated, continuous depth sizing is becoming increasingly relevant for the pipeline operators and especially for pipelines with large populations of planar anomalies (SCC colonies, lack-of-fusion in ERW seam-welds, etc.). The ILI industry is introducing a continuous crack depth sizing. Next to the better differentiation and the linearity of the depth reporting, the main advantage of the continuous depth sizing is the direct comparability to the results of the field verifications. The continuous depth sizing improves the ability to assess the performance validation of the depth sizing and thus, contributes to a general improvement of the crack depth sizing. This paper describes the development and implementation of a continuous crack depth sizing approach and shows its advantages in comparison to the conventional depth classes. A sizing model is introduced, making use of an empirically derived function, that relates the amplitude measurement to the defect depth. The continuous depth sizing applies to crack-like defects with depths ranging from 1mm to 4mm. The parameters of the model are derived from performance tests based on artificial flaws. In addition, the model is validated by means of field verification results. The depth sizing accuracy and confidence levels are obtained from the performance test data in accordance to API 1163 [1] and POF 2009 [2]. In addition, the paper discusses the extraction of the crack depth profiles from inspection data, making use of the newly developed continuous depth sizing model. In comparison to standard reporting of maximum depth and length, crack depth profiles deliver more accurate and more valuable input to the integrity assessment for pipeline operators. Examples of a direct comparison of these crack depth profiles to field verification data are included.

A Computational Study of Fracture in the Calcaneus Under Variable Impact Conditions

This preliminary study aims to computationally model and study the fracture patterns in the human calcaneus during variable impact loading conditions. A finite element model of the foot and ankle is used to understand the effect of loading rates and orientation of the foot on fracture patterns. Simulations are carried out by applying varying impact velocities of steel plate to the foot & ankle model in accordance with data regarding underbody blasts. These impact velocities are applied to reach a peak in 1.5 ms. Fracture of bone is represented using the plastic kinematic constitutive model with element erosion method, where elements are removed from the simulation after an inelastic failure strain is exceeded. The simulations last for 5 ms to observe the extent of fracture in the calcaneus. Following simulations, the resulting fracture patterns are compared to available images from experimental impact tests to qualitatively assess the simufutions. A mesh convergence study is performed to determine the level of refinement of mesh necessary to represent this problem. The mesh appears to converge at the refinement level of the medium coarse mesh. The effect of impact velocities on fracture is studied on unjlexed and flexed foot models. At lower velocities, fracture is observed in the form of a single continuous crack, and a pronounced branched type of network is observed at higher velocities. Finally, variation in fracture networks due to variability in strength of the bone is studied. For lower values of failure strain, significantly larger and branched networks of fracture are observed.

Some Remarks on the Numerical Modelling of Localized Failure

An efficient numerical framework suitable for three-dimensional analyses of brittle material failure is presented. The proposed model is based on an (embedded) Strong Discontinuity Approach (SDA). Hence, the deformation mapping is elementwise additively decomposed into a conforming, continuous part and an enhanced part associated with the kinematics induced by material failure. To overcome locking effects and to provide a continuous crack path approximation, the approach is extended and combined with advantages known from classical interface elements. More precisely, several discontinuities each of them being parallel to a facet of the respective finite element are considered. By doing so, crack path continuity is automatically fulfilled and no tracking algorithm is necessary. However, though this idea is similar to that of interface elements, the novel SDA is strictly local (finite element level) and thus, it does not require any modification of the global data structure, e.g., no duplication of nodes. An additional positive feature of the advocated finite element formulation is that it leads to a symmetric tangent matrix. It is shown that several simultaneously active discontinuities in each finite element are required to capture highly localized material failure. The performance and predictive ability of the model are demonstrated by means of two benchmark examples.