Accelerated Crack Propagation Research Articles

Changes in retained fracturing fluid properties and their effect on shale mechanical properties

To understand the fluid-shale interactions and the mechanisms that produce cracks during the retention of the fracturing fluid. This paper details experiments performed in shale particle immersion and core plug imbibition, and it investigates changes in fluid properties during a shale well shut-in period at the Longmaxi formation in Southeast Chongqing, China. The results indicate that soluble salts in a shale formation are dissolved during fracturing fluid retention in a shale reservoir, causing the salinity and pH of the fracturing fluid to increase significantly and eventually resulting in the formation of a weakly alkaline solution. Furthermore, fracturing fluid retention significantly reduces the mechanical strength of shale. In the experiments, the elasticity modulus and Poisson's ratio of the shale decreased by 85% and 54%, respectively, which was immersed in the fracturing fluid for seven days. Two types of forces, repulsive hydration and electric double-layer repulsion, are shown to be the main factors that lead to the formation of a large number of microcracks. The repulsive hydration pressure force is in a short range, operating over only a few nanometers, while the electric double-layer repulsion force operates over a long distance and can act between clay mineral slices. As the concentration of the solution increases, the double-layer repulsive force increases, though the range over which it acts becomes shorter. The increase in the OH− concentration of the solution also accelerates crack propagation. These conclusions are conducive to developing reasonable rules for fracturing fluid flowback in shale gas wells.

Experimental Study on Mechanical Properties of Prefabricated Single‐Cracked Red Sandstone under Uniaxial Compression

This paper aims at the phenomenon of the fractured rock column in underground engineering which is prone to collapse when subjected to ground pressure. Uniaxial compression test and particle flow code (PFC2D) are used to analyze the influence of crack dip angle on the mechanical properties, crack propagation, and the failure mode of red sandstone. From the results, it is observed that the stress–strain curve of the precracked red sandstone can be divided into five stages, and there is a critical stress value in the stage of accelerated crack propagation and unstable fracture of the rock sample. Further, the peak strength, maximum strain, and the elastic modulus of the precracked red sandstone increase with the increase of crack dip angle, and the ultimate failure mode of rock sample changes from the “ladder” type failure to slope uneven failure. Furthermore, from PFC2D simulation, it is found that the tensile microcracks contribute more towards the failure of rock samples than the shear cracks. The contact force chain is very weak at the places where the precracks and macroshear planes are formed. This indicates that the original contact force is weakened due to particle fracture. Therefore, the bearing capacity of the precracked rock samples decreases with the increase in load. From the simulation results, it is found that the displacement at the shear plane of the rock sample is large, and the shear dilatation occurs. With the increase in load, the specimen falls off and is ejected. This is due to the weakening of the contact force between the internal particles. Thereafter, it fractures to produce microcracks, which gradually converge, thus providing a prerequisite for the transformation of elastic strain energy into kinetic energy.

Effect of copper content on the tensile elongation of Al–Cu–Mn–Zr alloys: Experiments and finite element simulations

Microstructures of cast aluminum alloys used in automotive engine applications often consist of intermetallic particles that can impact the tensile elongation of these alloys. Here, we investigate the effect of intermetallic grain boundary particles on tensile elongation by fabricating a series of alloys with Cu content varying between 6.0 - 9.0 wt% in cast Al–Cu–Mn–Zr (ACMZ) type compositions. The tensile elongation of as-aged ACMZ alloys decreases monotonically with increase in Cu content. While the microstructure within the grains and yield stress of the alloy remains invariant with Cu content, the decrease in tensile elongation correlates well with increase in the size and volume fraction of grain boundary particles. Microstructural observations are combined with finite element simulations to explain the trend in tensile elongation with changing Cu content. Crack initiation is found to occur by brittle fracture of the grain boundary particles. Increase in particle size promotes crack initiation by reduction in size dependent particle fracture strength. Lower inter-particle spacing at higher particle volume fraction further facilitates crack initiation by increasing stress within the particles caused by the interaction between stress fields of neighboring particles. Increase in particle volume fraction also accelerates crack propagation through the formation of macro shear zones in the microstructure. The increase in Cu content of cast ACMZ alloys, therefore, decreases tensile elongation by promoting both crack initiation and crack propagation.

Low Cycle Fatigue of Single Crystal $$\gamma ^{\prime }$$-containing Co-based Superalloys at $$750\,^{\circ }\hbox {C}$$

A new class of $$\gamma ^{\prime }$$-containing Co-based superalloys that are promising for high temperature applications has been investigated under cyclic loading conditions. A series of single crystal variants of these Co-based superalloys have been cyclically loaded above their elastic limit at $$750\,^{\circ }\hbox {C}$$ in air to study their behavior in the low cycle regime. Interrupted testing was performed to observe the early stages of fatigue failure. Optical microscopy, scanning electron microscopy, and transmission electron microscopy were used to characterize post-mortem specimens. Fatigue failure occurred due to surface cracks that developed during the early stages of cycling in the aluminide-coated sample gauge sections. Growth of these surface cracks into the substrate was associated with extensive oxidation and intermetallic phase precipitation, which accelerated crack propagation as compared to Ni-based superalloys. These observations suggest that improvements in the oxidation resistance and high temperature strength of $$\gamma ^{\prime }$$-containing Co-based superalloys will enhance their fatigue behavior.

The stress intensity factor calculation for combined sliding wear and fatigue of GH4169 superalloy based on three-dimensional simulation

In this paper, a sliding fatigue-wear methodology based on conventional rotating bending fatigue testing device is proposed. The crack propagation characteristics of GH4169 superalloy under plain fatigue and sliding fatigue-wear conditions are investigated by fatigue tests and SEM. In addition, the crack propagation and wear analysis are carried out with the finite element program ABAQUS and the three-dimensional fracture analysis software FRANC3D. The influence factors of crack growth rate including bending stress, contact stress and wear depth are investigated based on the stress intensity factor calculation and experimental measurements. The results show that the fatigue strength is severely decreased from 780 MPa to 200 MPa due to wear, although both contact stress state and material removal mechanism have positive effects on restraining the crack propagation acceleration. This competition between fatigue and wear is discussed and it is concluded that the fatigue defeats the resisting effect of wear and occupies the dominate position after the nominal crack length reaches a critical value. However, the wear volume and depth would increase the fatigue stress and accelerate crack propagation. Therefore, the positive and negative influence of wear on material fatigue lives also have a competitive mechanism and a correction model is carried out based on the classic Paris law.

Effect of corrosion on the fracture properties of steel plates

Corroded steel structures show significant degradation in fracture behaviors. This research is aimed to investigate the degradation laws of corroded steel fracture toughness, and reveal the mechanism of accelerated crack propagation and premature unstable fracture caused by corrosion. The tensile tests of center notched corroded steel plates and numerical analysis were conducted. Research results show that the residual fracture toughness of corroded steel decreases as the uniform and pitting corrosion damage get worse. Empirical models were established to indicate relationships between corrosion degree and residual fracture toughness. The degraded fracture toughness and accelerated crack propagation can be attributed to the more intense crack tip field or increased driving force of crack propagation induced by corrosion. The calculated residual fracture toughness can be utilized to predict fracture in corroded steel joints with macroscopic fatigue crack.

Cracking mechanism and susceptibility of laser melting deposited Inconel 738 superalloy

Laser Melting Deposition (LMD) of Inconel 738 (IN738) superalloy is a promising process for the remanufacturing of gas turbines and aerospace engines, but the cracking has not been thoroughly understood and controlled. This paper conducts a comprehensive study on the cracking behavior by using optical microscope (OM), scanning electron microscopy (SEM), energy dispersion spectrum (EDS), electron backscatter diffraction (EBSD), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The results indicate that liquation cracking which follows the penetration mechanism is the major origin to those cracks. Whereas, at the propagation stage, ductility-dip cracking (DDC) is an important supplement to liquation cracking, especially at the triple junction points of grain boundaries (GBs) or the bottom of already formed liquation cracks. The cracks are very sensitive to GB morphology, the long-straight GBs which result from higher heat input or unidirectional scanning strategy are very vulnerable to cracking. Moreover, GB oxidation always plays an important role in accelerating crack propagation, the local protection from the cladding head is not enough for the LMD of IN738. For these factors, the test specimens which are bidirectionally deposited in an Argon (Ar) chamber with lower heat input achieve better tensile strengthen reaching 1093 to 1116 MPa.

Mechanical behaviors and acoustic emission fractal characteristics of coal specimens with a pre-existing flaw of various inclinations under uniaxial compression

To study the influence of hydraulic slotting inclination on the mechanical behaviors of coal seam during mining process, uniaxial compression experiments on coal specimens with a single pre-existing flaw inclined at 0°, 15°, 30°, 45°, 60°, 75°, 90° and intact specimens were conducted. Acoustic emission (AE) signals in the loading process were monitored, and fractal analysis method was introduced to investigate the AE characteristics. Additionally, the laboratory experiments were simulated by a finite element code. Both the experimental and numerical results show that the existence of a flaw reduces the mechanical properties of coal. The uniaxial compressive strength and modulus of elasticity increase polynomially and linearly with the increase of inclination angle, respectively. When the coal specimen ruptures finally, the fewer the surface secondary cracks or the more the sudden drops of stress, the smaller the peak value of AE count. According to the stress–strain curve, the loading process is divided into five stages: (I) compaction stage; (II) linear elastic stage; (III) stable crack propagation stage; (IV) accelerating crack propagation stage; (V) post peak and residual stage. AE fractal characteristics in various stages of each specimen were determined by Grassberger and Procaccia algorithm based on phase space reconstruction theory. AE count show fractal characteristics from stage III. The fractal dimension declines rapidly in stage IV, and continues to decline further or rise slightly in stage V, but both are lower than that in stage III. Therefore, the changing rule of AE fractal dimension in different loading stages can be used as a precursor to coal and rock dynamic disasters.

Evaluation of thermal crack propagation for low alloy steel brake disk

Very high-speed trains are usually equipped with railway brake disks. During braking, frictional heat is generated by rubbing on the disk surface. Because of non-uniform contact, the frictional heat is localized, and the disk surface temperature rapidly increases in that local area, which is known as the hot spot. This hot spot causes damage such as cracking, which affects the disk life. Therefore, to enhance the disk life or to optimize maintenance intervals, accurately predicting crack propagation behaviours is necessary. This study aims to clarify the relationship between the disk crack propagation rate and braking conditions. Braking tests were conducted by using a reduced-sized brake dynamometer under various braking conditions. The tests revealed that the crack propagation rate decreased under two higher braking energy conditions. Fewer crack propagations were considered because surface cracks were removed as a result of higher wear rates. Furthermore, wear rates could be described by equations as functions of the power with the braking energy. The crack propagation behaviour was predicted using a developed equation that includes the rates of both crack propagation and wear. The crack propagation rates were assessed based on the fracture mechanics using the residual and thermal stress distributions calculated by finite element analysis. The test and predicted results were in close agreement. Thus, the braking conditions that accelerate crack propagation were clarified. This method contributes to the development of long-life disks or provides reasonable maintenance intervals.

Corrosion and corrosion fatigue performances of micro‐arc oxidation coating on AZ31B cast magnesium alloy

In this work, the corrosion performances of micro‐arc oxidation (MAO) coating on AZ31B cast alloy were examined. MAO coating synthesized at a low current density of 34 mA/cm2 has a uniform and compact microstructure with very fine micro‐pores and few structural defects; thus, robust protection is provided for the AZ31B substrate in corrosive solutions. The long‐term corrosion and corrosion fatigue behaviors of the MAO coatings formed at the optimal current density for 5 and 10 min were characterized. The results show that the MAO coating formed for 10 min provides greater protection in a salt fog environment than the coating formed for 5 min does because it is thicker. The fatigue life and fracture analysis of bare, MAO‐coated and E‐paint MAO‐coated AZ31B indicate that the MAO coating reduces corrosion fatigue life by 55% due to the micro‐pores and micro‐cracks on its surface providing initiation sites under stress conditions and accelerating crack propagation. However, E‐paint MAO‐coated specimens show improved corrosion fatigue strength in 107 cycles at ∼60 and ∼50 MPa under the same conditions due to the sealing of the micro‐pores and micro‐cracks in the MAO coatings.

High-temperature low-cycle fatigue behavior of a 9Cr-ODS steel: Part 2 - hold time influence, microstructural evolution and damage characteristics

Creep-fatigue (CF) interaction in a tempered martensitic Fe-9%Cr-based oxide dispersion strengthened (ODS) steel was studied at 650 °C by introducing hold-time of up to 30 min at peak tensile strain of 0.7%. The symmetrical loops under pure fatigue/continuous cycling (PF/CC) became asymmetrical due to stress relaxation during hold-time. Moreover, this also resulted in a reduction of cyclic life. For the investigated hold-time durations, the increase in tensile hold (TH) period had a negligible effect on peak stresses, but led to a further reduction in lifetime. Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) were used to visualize microstructural evolution under both PF/CC and TH waveforms. In general, PF/CC resulted in: (1) rearrangement and/or annihilation of dislocations, (2) partial elimination of the original sub-grain structures, (3) grain growth, (4) M23C6 carbides coarsening and (5) Cr-W enriched Laves phase precipitation. Nevertheless, upon introducing tensile hold-time, no substantial additional microstructural changes were identified. To uncover reasons for specimens premature failure under TH waveforms, detailed investigations on their surfaces, cross-sections and fracture surfaces were carried out. These investigations led to two important conclusions. Firstly, due to comparatively longer high-temperature exposure, the extent of oxidation increased upon introducing TH which expedited damage progression. Secondly, TH induced intergranular damage in the form of creep cavities does not only provide additional crack initiation sites but also their growth/coalescence under tensile stresses act as a bridging link for accelerated crack propagation. These two findings are associated with a reduction of cyclic life due to introduction of hold-time. Hence, the effect of hold-times is primarily due to (1) oxidation-fatigue interaction and (2) creep-fatigue interaction.

Dynamic damage and fracture of a conductive glass under high-rate compression: A synchrotron based study

Dynamic damage and fracture of conductive glass are investigated using a split Hopkinson pressure bar, implemented with in situ X-ray phase contrast imaging (XPCI) and optical imaging for comparison. Quantitative comparison between X-ray and optical images demonstrates that XPCI exhibits much higher resolution in resolving micro cracks and dynamic fracture modes. Multiple predamage and fracture modes of glass samples under dynamic loading are revealed with XPCI to depend on competing nucleation of initial flaws across scales, which give rise to a scattered fracture strength distribution. The fracture strengths increase with increasing strain rates due to accelerated crack propagation and damage growth. Quantitative gray-scale statistical analysis of XPCI and optical images yields spatial and temporal evolutions of damage. Unexpected plateaus essentially without damage growth are observed on the damage curves of glass, deviating from conventional theoretical predictions. The damage plateaus are attributed to growth, closure and re-expansion of inclined main cracks, due to interactions of stress waves with crack tips. The current results also demonstrate a reliable experimental technique for dynamic damage characterization of brittle materials.

A phase‐field approach to fracture coupled with mass transport for the simulation of environmentally‐assisted damage

AbstractWith the introduction of a mass transport mechanism the entire problem is subjected to a time frame that dictates the time‐dependent action of soluted species on mechanical properties. A numerical framework within the phase‐field approach is presented with an embrittlement‐based coupling mechanism. The underlying functionals are expressed in terms of the displacement, mass concentration and crack phase‐field. Within the phase‐field approach the modelling of sharp crack discontinuities is replaced by a diffusive crack model facilitating crack initiation and complex crack topologies without the requirement of a predefined crack path. The isotropic hardening of the elasto‐plastic deformation model and the local fracture criterion are affected by the species concentration. This allows for embrittlement and leads to an accelerated crack propagation. An extended mass transport equation for hydrogen embrittlement, accounting for mechanical stresses and deformations, is implemented. For stabilisation purposes a staggered scheme is applied to solve the system of partial differential equations by a multi‐field finite‐element method. A thermodynamically consistent coupling relation that accommodates the required mechanisms is presented. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The influence of partial surface shot peening on fatigue crack growth behaviour of a high‐strength ferritic steel

AbstractThe effects of partial surface shot peening on the fatigue crack growth behaviour of a ferritic steel have been experimentally investigated in this paper. Dog‐bone specimens fabricated from Optim700QL were tested under tension‐tension fatigue loads. Three distinct extents of partial shot peening, with respect to the crack tip and specimen symmetry line, were tested. The fatigue crack growth results from these experiments have been compared with those obtained from the same specimen geometry but with no peening. The results show that the residual stress fields formed ahead of the initial notch tip due to the partial peening process play a significant role in the fatigue crack growth behaviour of the material and effectively result in accelerated crack propagation at the midwidth of the specimens. It has been shown in this study that partial peening can lead to a fatigue crack growth rate around twice as fast as that of the unpeened specimen.

Hydrogen embrittlement revealed via novel in situ fracture experiments using notched micro-cantilever specimens

The susceptibility of the FeAl intermetallic alloy to hydrogen-assisted cracking was investigated by in situ fracture experiments using notched micrometre-sized specimens using an Environmental Scanning Electron Microscope (ESEM). The notched beams were loaded under two different environmental conditions: one in high vacuum (5 × 10−4 Pa) to avoid hydrogen effects and one under a certain water vapor pressure (450 Pa) to promote hydrogen uptake. The fracture behaviour on a non-ASTM-standard micro-sized specimen was successfully studied by the experimental approach, and the microstructure of the whole crack area was analysed by Transmission Kikuchi Diffraction (TKD) and Transmission Electron Microscopy (TEM) techniques. Three crack growth stages were observed in all the specimens: i) elastic regime, ii) notch blunting and micro-crack formation; and iii) stable crack growth. We observed an accelerated crack propagation rate in specimens under hydrogen exposure. The hydrogen embrittlement phenomenon was found to occur because of the strong hydrogen-dislocation interactions. The combined effect of hydrogen-enhanced dislocation nucleation and hydrogen-restricted dislocation mobility is responsible for the hydrogen-enhanced cracking behaviour.

A Phase Field Approach to Fracture with Mass Transport Extension for the Simulation of Environmentally-Assisted Cracking

The fracture mechanics assessment of materials exposed to harmful environments requires the understanding of the interaction between the soluted species and the affected mechanical behaviour. With the introduction of a mass transport mechanism the entire problem is subjected to a time frame that dictates the time-dependent action of soluted species on mechanical properties. A numerical framework within the phase field approach is presented with an embrittlement-based coupling mechanism showing the influence on crack patterns and fracture toughness. Within the phase field approach the modeling of sharp crack discontinuities is replaced by a diffusive crack model facilitating crack initiation and complex crack topologies such as curvilinear crack patterns, without the requirement of a predefined crack path. The isotropic hardening of the elasto-plastic deformation model and the local fracture criterion are affected by the species concentration. This allows for embrittlement and leads to accelerated crack propagation. An extended mass transport equation for hydrogen embrittlement, accounting for mechanical stresses and deformations, is implemented. For stabilisation purposes a staggered scheme is applied to solve the system of partial differential equations. The simulation of showcases demonstrates crack initiation and crack propagation aiming for the determination of stress-intensity factors and crack-resistance curves.

A computational study of the dynamic propagation of two offset cracks using the phase field method

The study of the propagation of multiple cracks is essential to modeling and predicting structural integrity. The interaction between two cracks depends on a number of factors such as the domain geometry, the relative crack sizes and the separation between the two crack tips. In this paper, we study the interaction between two dynamically propagating cracks. We use the phase field method to track the crack paths, since this method can handle complex crack behavior such as crack branching, without any ad hoc criteria for crack evolution. The results from our dynamic simulations indicate that, unlike crack interaction under quasi-static or fatigue loading, the presence of another crack does not accelerate crack propagation when dynamic loads are applied. However, some similarities in the crack topologies are observed for both quasi-static and dynamic loading.

Effects of lamella size and connectivity on fatigue crack resistance of TRIP-maraging steel

The effect of austenitization time on the fatigue crack resistance of transformation-induced plasticity (TRIP)-maraging steel was investigated by observing the crack initiation site, propagation path and fracture surface. Our analyses show that austenitization for a longer time increases austenite/martensite lamella size and connectivity of austenite. Simultaneously, increasing lamella size leads to a reduction in austenite hardness; higher austenite connectivity accelerates crack propagation. In addition, remarkable roughness on the crack surface associated with the laminated structure was observed in both steels, which caused roughness-induced crack closure.

A study of long-term static load on degradation and mechanical integrity of Mg alloys-based biodegradable metals

Predicting degradation behavior of biodegradable metals in vivo is crucial for the clinical success of medical devices. This paper reports on the effect of long-term static stress on degradation of magnesium alloys and further changes in mechanical integrity. AZ31B (H24) and ZE41A (T5) alloys were tested to evaluate stress corrosion cracking (SCC) in a physiological solution for 30days and 90days (ASTM G39 testing standard). Scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDX) and micro-computed tomography (micro-CT) were used to characterize surface morphology and micro-structure of degraded alloys. The results show the different mechanisms of stress corrosion cracking for AZ31B (transgranular stress corrosion cracking, TGSCC) and ZE41A (intergranular stress corrosion cracking, IGSCC). AZ31B was more susceptible to stress corrosion cracking under a long term static load than ZE41A. In conclusion, we observed that long-term static loading accelerated crack propagation, leading to the loss of mechanical integrity.

Experimental verification of the hydrogen concentration around a crack tip using spot X-ray diffraction

We employed X-ray diffraction using collimated X-rays to quantitatively evaluate the local hydrogen concentration behavior in metals. Hydrogen concentrating around a crack tip significantly accelerates crack propagation, i.e., hydrogen embrittlement. In order to clarify the mechanism leading to this, the local hydrogen concentration behavior, i.e., at a crack tip, was evaluated by numerical analysis and experimental measurements. Although thermal desorption analysis can be used to evaluate the total hydrogen content in metals, it cannot be applied to local areas. Microprint methods, which use chemical reactions between hydrogen and coated elements cannot quantitatively evaluate the hydrogen content. The present study takes account of hydrogen-induced strain, and X-ray diffraction in a confined area was employed to detect variations in lattice spacing before and after hydrogen charging. Using X-ray diffraction applied to a small area, we demonstrate that the hydrogen concentrates in the vicinity of the crack, i.e., at the elastic–plastic boundary.