Crack Initiation Process Research Articles

Plastic-strain-dependent strength model to simulate the cracking process of brittle rocks with an existing non-persistent joint

Brittle rock failure has been extensively studied for decades and is recognized as a process of crack initiation, propagation, and coalescence. However, the cracking mechanism is still not fully understood. Numerical approaches are generally used to study the cracking process of rocks with pre-existing joints. The strength model of the rock matrix is crucial and should be adopted appropriately in numerical studies. This paper presents a plastic-strain-dependent strength model of brittle rocks based on a series of cyclic loading–unloading compression tests and Brazilian tension tests in the laboratory. The detailed steps used to obtain the mechanical parameters quantitatively during progressive failure are presented. It is shown experimentally that both the shear strength parameters and tensile strength vary during the failure process, i.e., cohesion weakening, frictional strength mobilization, and tensile strength loss (CWFS-TL). The model is then implemented in Fast Lagrangian Analysis of Continua in 3 Dimensions (FLAC3D) to represent the properties of the rock matrix, and the interface element in the software is adopted to simulate the non-persistent joint. Two types of numerical experiments are conducted on specimens with a pre-existing non-persistent joint: a direct tension experiment on a specimen with a horizontal joint and a uniaxial compression experiment on a specimen with an inclined joint. It is found that the cracking process in the numerical experiments can capture nearly all the features of physical experiments under the same conditions in the laboratory, and the numerical results also agree quantitatively with the theoretical solution. It seems that the approach can be widely used to study brittle rock masses with a non-persistent joint.

Fatigue tests of welded connections between longitudinal stringer and deck plate in railway bridge orthotropic steel decks

This research presented the fatigue tests of longitudinal stringer-to-deck (SD) welded connections, which have been identified as the locations most sensitive to fatigue damage in the orthotropic steel decks (OSDs) of railway bridges. Four full-scale SD connections were fabricated, and two loading patches were considered. Static loading was first carried out to obtain the structural hot spot stresses at weld toes as well as stress concentration factors (SCFs), by which the hot spots providing the highest stresses were identified. Cyclic loading was then implemented next to the static loading, and the behaviors including fatigue crack initiation and propagation process, fatigue failure mode, characteristic fatigue life, as well as degradation of vertical rigidity, were all obtained from the test. The crack growing process can be totally divided into four stages, and the fatigue lives after the crack arrived at the deck edge were very short. Variations of crack dimensions were also obtained, and the simplified formulae of crack growth rate were numerically fitted so that the crack propagation lives can be predicted by using the crack dimensions. Comparisons also show that the FAT 100 curve in IIW fatigue design recommendation could overestimate the fatigue resistance of such connections where double-sided fillet welds were used to connect the stringer web and the deck plate, and therefore double-sided groove welds with partial or full penetrations are recommended for the stringer-to-deck connections in railway bridge decks.

Multiple Fatigue Failure Behaviors and Long-Life Prediction Approach of Carburized Cr-Ni Steel with Variable Stress Ratio.

Axial loading tests with stress ratios R of −1, 0 and 0.3 were performed to examine the fatigue failure behavior of a carburized Cr-Ni steel in the long-life regime from 104 to 108 cycles. Results show that this steel represents continuously descending S-N characteristics with interior inclusion-induced failure under R = −1, whereas it shows duplex S-N characteristics with surface defect-induced failure and interior inclusion-induced failure under R = 0 and 0.3. The increasing tension eliminates the effect of compressive residual stress and promotes crack initiation from the surface or interior defects in the carburized layer. The FGA (fine granular area) formation greatly depends on the number of loading cycles, but can be inhibited by decreasing the compressive stress. Based on the evaluation of the stress intensity factor at the crack tip, the surface and interior failures in the short life regime can be characterized by the crack growth process, while the interior failure with the FGA in the long life regime can be characterized by the crack initiation process. In view of the good agreement between predicted and experimental results, the proposed approach can be well utilized to predict fatigue lives associated with interior inclusion-FGA-fisheye induced failure, interior inclusion-fisheye induced failure, and surface defect induced failure.

Microscopic damage evolution during very‐high‐cycle fatigue (VHCF) of tempered martensitic steel

AbstractDimensioning with high‐strength steels relies on the knowledge of Wöhler‐type S/N data and the assumption that no failure occurs for load levels below the fatigue limit for applications where the number of load cycles exceeds 107. Very‐high‐cycle fatigue (VHCF) experiments applied to a 0.5C‐1.0Cr‐Mo tempered steel (German designation: 50CrMo4) revealed surface crack initiation at prior austenite grain boundaries in medium strength condition (37HRC) and internal crack initiation at nonmetallic inclusions at high strength condition (57HRC). Despite the formation of small cracks during cycling up to 109 cycles, it seems that these are nonpropagating cracks in the case of the medium strength condition and therefore a real fatigue limit exists. Application of automated electron back‐scatter diffraction (EBSD) within the shallow‐notched area of electro‐polished fatigue specimens had shown that prior austenite grain boundaries act as effective obstacles to crack propagation. High‐resolution thermography during cycling of the specimens allowed the identification of local plasticity, which led to crack initiation at a later stage of the fatigue life. It was found that Cr segregation rows play a decisive role in the crack initiation process. By means of high‐resolution electron microscopy in combination with focused ion beam milling (FIB), evolution of cyclic plasticity and crack initiation was correlated with the material's microstructure. The results are discussed in terms of the completely different crack initiation mechanisms of medium and high strength variants of the same steel. EBSD microstructure and crack propagation data are used to adapt a numerical modeling tool to predict microcrack propagation in the VHCF regime.

Nonlinear numerical study of crack initiation and propagation in a reactor pressure vessel under pressurized thermal shock using XFEM

AbstractUnder pressurized thermal shock (PTS), once crack initiation occurs in a reactor pressure vessel (RPV), the stress concentration around the crack tips may result in local instability and crack propagation. The temperature‐dependent material properties are introduced into the finite element analysis model. According to the response of the transient temperature field and stress field near the crack tip region, the influence of PTS on the carrying capacity of the structure is demonstrated. Also, the process of crack initiation and propagation is simulated by using the extended finite element method (XFEM). The results show that the crack of mode I is easy to be initiated on the nozzle in the initial high temperature and high pressure working state. The effect of cladding on the RPV integrity is enhanced with the increase of crack size, and it mainly depends on the crack length. During the PTS transient, a discontinuity of stress exists near the cladding‐base interface, and the cladding bears more loading than the base of the same size. As the unloading of the internal pressure, the stress does not decline due to the strong thermal shock. In the late stage of the PTS process, the internal pressure caused by repressurization poses a challenge to the strength of the structure. With the decrease of the base wall thickness, the allowable repressure value also gradually decreases. However, the increase of the base wall thickness causes the rise of thermal stress, and the allowable repressure load is not significantly improved.

Crack Initiation and Coalescence Behavior of Two Non-parallel Flaws

Crack initiation and coalescence behavior of rock or rock-like specimens containing artificial flaws under uniaxial compression have been subjects of intensive investigation in the past. Most of these investigations however focused on crack initiation and coalescence between two or more parallel flaws. Although there have been few experimental studies on non-parallel flaws, these studies did not address the influence of geometrical factors such as ligament length and ligament angle on the crack initiation and coalescence behavior of non-parallel flaws. To investigate whether the individual geometrical factors have similar effects on the crack initiation and coalescence behavior of both parallel and non-parallel flaws, we conducted uniaxial compression tests to investigate crack cracking and coalescence processes in rock like material containing two non-parallel flaws. The paper presents the influence of individual geometrical factors on the crack initiation process and coalescence pattern of non-parallel flaws. Initiation of primary first cracks from all the tips of the two flaws did not occur simultaneously in all the flaw configurations. The flaw configuration of the non-parallel flaws influences the crack initiation, crack trajectories and coalescence behavior. The crack coalescence pattern changes with an increasing ligament angle from indirect to shear crack or mixed tensile-shear crack to tensile crack coalescence. The chance of direct coalescence is reduced with an increase in ligament length. In conclusion, the crack initiation and coalescence behavior of prismatic rock-like specimens with non-parallel flaws, as influenced by the geometrical factors, are analogous to the cracking and coalescence pattern observed in specimens with parallel flaws.

Ductile cracking simulation of uncracked high strength steel using an energy approach

Ductile crack initiation and propagation simulation is of great importance in metal structures, which is one of main failure modes. Research objects of common fracture mechanics are solids with existing cracks. However, ductile fracture of uncracked solids is also one of the main concerns in structural engineering, especially for strong seismic loading. There is a great difficulty in simulating both crack initiation and propagation of uncracked solids using conventional fracture mechanics. A simple unified method to simulate the two stages with acceptable accuracy is thus required. For ductile crack initiation, a number of models based on the void growth concept have been proposed. However, research on ductile crack propagation is still limited, and there is still a lack of simple approaches to simulate both ductile crack initiation and propagation of uncracked solids. This paper aims to investigate ductile crack initiation and propagation of high strength steel under high stress triaxiality by combination of the void growth model for ductile crack initiation and an energy balance approach for ductile crack propagation. In this paper, a straightforward approach to obtain the ductile fracture energy only using smooth tension coupon tests is proposed. Two series of single edge notched specimens with different notch depths and notch configurations are manufactured. Both experimental and numerical studies under monotonic tension are conducted. The ductile crack initiation and propagation processes of notched high strength steel specimens are simulated with acceptable accuracy using the proposed approach.

Fracture Failure Mechanisms of Long Single PA6 Fibers.

The present study investigates the failure mechanisms of industrial fiber materials, using a custom designed fiber cutting performance test bench. The fracture morphologies of single PA6 fibers are examined by scanning electron microscopy. The analysis reveals that fiber cutting can be distinguished according to four distinct stages of fiber failure represented by shearing, cutting, brittle fracture, and tensile failure, which are the result of different mechanisms active during the processes of crack initiation, extension, and fracture. The results of fractographic analysis are further verified by an analysis of the blade assembly speed with respect to time over the entire fracture failure process based on high-speed camera data. The results of fractographic analysis and blade assembly speed are fully consistent.

Charpy impact test on A508-3 steel after neutron irradiation

Reactor pressure vessel (RPV) is the critical un-changeable component of the reactor during its service lifetime, which prevents the radioactive leak of the nuclear power plant core. The irradiation test (about 10 × 1019 cm− 2, E > 1 MeV) in research reactor of the pressure vessel material was carried out, and the charpy impact test has been carried out before and after the neutron irradiation. Analysis of the impact energy and the fracture morphology has been done to estimate the embrittlement due to neutron irradiation. It shows that the main effects of neutron irradiation to fracture are the crack initiation and stable expansion process. And there also are a small amount of intergranular fracture in the unstable crack expansion after neutron irradiated which aware us pay more attention to the increasing intergranular fracture behavior of higher neutron fluence level after 60a nuclear power plant operation.

Fracture analysis of marble specimens with a hole under uniaxial compression by digital image correlation

To investigate the effects of pre-fabricated holes with different geometries, including size, shape and inclination angle, on the strength and fracture behavior of rock materials, a series of uniaxial compressive tests were conducted on prismatic marble specimens containing a circular or an elliptical hole using a servo-hydraulic machine synchronized with a charge-coupled device (CCD) camera. Digital image correlation (DIC) was applied to record and analyze the deformation and fracturing process of marble. Experimental results combined with theoretical analysis indicate that the geometries of the internal hole are important factors affecting the strength of marble and also the boundary tangential stress distribution. The specimen with a higher tangential compressive stress concentration coefficient will have a lower UCS values. Observations show that the evolution of deformation and fracturing process of marble specimens can be visually displayed by the apparent displacement and strain fields. The propagation of cracks is a progressive development of high major principal strain zones, and the process of damage accumulation, crack initiation and propagation of rock at different stress stages can be well reflected by DIC. When the stress increases to 78.2% of peak value, critical cracks initiate in the tensile stress concentration zones. The progressive process of “open-close-reopen” of tensile cracks was quantitatively studied by the tensile strain of the specimens. It was found that the propagation of tensile cracks surrounding the hole in marble specimens was mainly affected by the nucleation and propagation of strain localization zones.

A numerical study of the effects of shot peening on the short crack growth behaviour in notched geometries under bending fatigue tests

The current paper presents a numerical analysis of the effects of shot peening on short crack growth in a low pressure (LP) steam turbine material, FV448. The fatigue behaviour of this material has been experimentally evaluated using a U-notched specimen (representing the fir tree root geometry of the turbine blade) under 3-point bend tests. Two different shot peening intensities were considered in this study: an industrially applied shot peening process and a less intense shot peening process.In the modelling work, a 2-D finite element (FE) model with static short cracks has been developed, incorporating both compressive residual stress and strain hardening distribution effects caused by shot peening. Both linear-elastic (LEFM) and elasto-plastic (EPFM) fracture mechanics were used to characterise the crack driving force in the un-peened and shot-peened conditions, taking into account the effects of stress redistribution caused by residual stress relaxation and crack opening. The stress intensity factor used in the LEFM approach was calculated using the weight function method, and the equivalent stress intensity factor used in the EPFM approach was calculated from the J-integral, which was evaluated using the cracked FE model. These results could explain the mechanism of (experimentally observed) retardation of crack growth through the shot-peening-affected layer and also quantified this influence on fatigue life. The relative contributions of compressive residual stresses and strain hardening were assessed by investigating them separately. The sub-surface compressive residual stress distribution produced by shot peening could effectively reduce crack propagation but the strain hardening distribution, in contrast, can accelerate it. However, strain hardening is expected to hinder the crack initiation process by restricting the plastic deformation during cyclic loading. Predictions of the fatigue life of the shot-peened notched specimens were made based on this numerical analysis. Acceptable results were obtained using both the LEFM and EPFM approaches and the difference between them is discussed.

Experimental study on fatigue failure of rib-to-deck welded connections in orthotropic steel bridge decks

The rib-to-deck (RD) welded connections are the most sensitive locations to encounter the fatigue failure in orthotropic steel decks (OSDs), and numbers of fatigue cracks arising from these areas have been found in existing OSD bridges. This research focus on the fatigue cracking process, fatigue characteristics as well as failure mechanics of RD connections under cyclic loading. Six full-scale RD welded joints were fabricated, and two load cases of centric and eccentric loading were considered. Static loading was first conducted with aims of measuring elastic strain distributions at potential hot spots. Structural hot spot stresses at weld toes as well as stress concentration factors (SCFs) were linearly extrapolated by using the recorded strains, based on which critical locations were identified. High-cycle repeated loading was subsequently implemented, from which the fatigue crack initiation and propagation process, fatigue failure mode, characteristic fatigue life, as well as degradation of vertical rigidity, were obtained. Four stages of crack propagation were mainly observed, and the remaining fatigue lives after the crack reached the deck edge were short to be neglected. Variations of crack dimensions including the longitudinal length and the depth in the plate thickness were also revealed. Comparison between experimental numbers of cycles and standard S-N formulae indicates that the FAT 100 curve provided in the IIW fatigue recommendation could be conservatively used to estimate the fatigue resistance of such rib-to-deck welded connections composed of 16mm thick deck plates and 80% PJP welds.

Influence of H2S Corrosion on Rotating Bending Fatigue Properties of S135 Drill Pipe Steel

Fatigue properties of S135 drill pipe steel 27CrMo under different conditions were investigated by rotating bending fatigue test. The results show that the fatigue limit at 106 cycles of the tested steel is 540 MPa. However, the fatigue cycles of the specimens after H2S corrosion shows a sharp decline from 106 to 104, when the stress is also 540 MPa. SEM observation of fatigue fracture surfaces reveal that the fatigue fracture has the characteristics of ductile fracture before H2S corrosion. After H2S corrosion, its shows the characteristics of brittle fracture. The main effect of H2S corrosion is that the hydrogen atoms gathered at grain boundaries, inclusions and defects, which promoted the process of fatigue crack initiation and propagation, reducing the fatigue properties.

Effect of stress relieving treatment on low cycle fatigue behavior of USSP treated 7075 aluminium alloy

The effect of ultrasonic shot peening (USSP) on low cycle fatigue (LCF) behavior of the 7075 aluminium alloy was studied at room temperature. There was grain refinement approximately to 20 nm size, appreciable increase in micro hardness, and inducement of residual compressive stress in the surface region, due to the USSP treatment. The modified microstructure in surface region of the specimens subjected to USSP was characterized by X-ray diffraction and transmission electron microscopy. There was marked increase in LCF life of the specimens due to USSP, however, LCF life of the USSP specimens was reduced due to subsequent treatment of stress relieving. The results are discussed in terms of the process of crack initiation and propagation in the different conditions.

Simulation of Low Velocity Impact Induced Inter- and Intra-Laminar Damage of Composite Beams Based on XFEM

In this paper, the Inter-Fiber Fracture (IFF) criterion of Puck failure theory based on the eXtended Finite Element Method (XFEM) was implemented in ABAQUS code to predict the intra-laminar crack initiation of unidirectional (UD) composite laminate. The transverse crack path in the matrix can be simulated accurately by the presented method. After the crack initiation, the propagation of the crack is simulated by Cohesive Zoom Model (CZM), in which the displacement discontinuities and stress concentration caused by matrix crack is introduced into the finite element (FE) model. Combined with the usage of the enriched element interface, which can be used to simulate the inter-laminar delamination crack, the Low Velocity Impact (LVI) induced damage of UD composite laminate beam with a typical stacking of composite laminates [05/903]S is studied. A complete crack initiation and propagation process was simulated and the numerical results obtained by the XFEM are consistent with the experimental results.

Digital features of main constituents in granite during crack initiation and propagation

The purpose of this study is to investigate the digital features of various constituents on granite specimen in the crack initiation and propagation process. The test video image photographed during laboratory uniaxial compression test and digital image processing techniques were used to conduct this study. The video image was first converted into the format easily recognized in programming. A multiple-object tracking technique was then developed to detect the locations of the individual cracks at any time. The initiation of each crack was determined by the occurring instant of the bounding box including the cracks. The length, width and area of each crack at any time were thereafter recorded. The relation of the propagation process with digital features of the constituents in various locations was furthermore investigated. It shows that two cracks propagated respectively along the vertical and horizontal directions as uniaxial compression test was performed; a limit box technique can be used to determinate the initiation instants of the multi cracks on the specimen surfaces; during propagation, the width of the cracks increased quickly just after initiation, and then fluctuated around a magnitude until the completely failure of the specimen; the constituents near the crack would split as the crack initiated; the shape features of biotites, feldspar and quartz changed in different manners during crack's initiation and propagation. The technique presented herein may be referable in investigating the failure mechanism of rock materials.

Advances on creep–fatigue damage assessment in notched components

AbstractIn this paper, the extended Direct Steady Cyclic Analysis method (eDSCA) within the Linear Matching Method Framework (LMMF) is combined with the Stress Modified Ductility Exhaustion method and the modified Cavity Growth Factor (CGF) for the first time. This new procedure is used to systematically investigate the effect of several load parameters including load level, load type and creep dwell duration on the creep–fatigue crack initiation process in a notched specimen. The results obtained are verified through a direct comparison with experimental results available in the literature demonstrating great accuracy in predicting the crack initiation life and the driving mechanisms. Furthermore, this extensive numerical study highlighted the possible detrimental effect of the creep–ratchetting mechanism on the crack growth process. This work has a significant impact on structural integrity assessments of complex industrial components and for the better understanding of creep–fatigue lab scale tests.

Study of the fracture process in heterogeneous materials around boreholes filled with expansion cement

A numerical model is proposed to study crack initiation, propagation and coalescence in heterogeneous materials subjected to expansion pressure in boreholes using the finite element method and damage mechanics. The strength heterogeneity of materials is modeled at the mesoscopic level using the Weibull distribution. Using this numerical model, the initiation and propagation of cracks generated by expansion cement in boreholes were simulated. The numerical simulations qualitatively reproduced the crack patterns in samples tested in laboratory experiments. Moreover, the results clarified the failure mechanisms and fracture processes, thereby contributing to a better understanding of fracture mechanisms in heterogeneous materials subjected to expansion pressure in boreholes. The numerical model allows direct observation of the entire process of crack initiation and growth to a degree that would be difficult to obtain in experimental studies.

High cycle fatigue testing of thermosonic ball bonds

During operation, miniaturized thermosonic Cu ball bond interconnects on Al pads occurring in microelectronic devices experience thermomechanical cyclic stresses, which lead to a degradation and subsequent fatigue fracture at the bond interface. Standard static tests, however, ignore the performance of such bonds under cyclic loads.Therefore, a new mechanical fatigue testing method tailored for such interconnects has been introduced, allowing to study their high cycle fatigue behavior in reasonable time. By means of a vibrating system and a special specimen setup cyclic stresses are mechanically induced at the bond interface causing fatigue lift off, where the bond is separated at its weakest site.For this purpose, two distinct specimen preparation methods basing on industrially applicable soldering techniques are suggested and can be used equally, depending on the focus of the investigation and the availability of the required testing structures.The first method - “single bond testing” - allows to test each bond individually regardless of the chip layout. In contrast, the second test method - “multiple bond testing” - allows to test several bonds simultaneously. To interpret and analyze the stresses occurring at the bond interface during these tests, finite element analyses were conducted.In the present study both methods are applied to CuAl ball bonds of the same quality and chip layout. It is shown that the aluminum is responsible for the fatigue crack initiation and propagation processes as confirmed by fractographic analyses of the fatigued bond interfaces.It can be concluded that the proposed fatigue test method is a powerful alternative screening method for such miniaturized bond interfaces, which allows to reveal their mechanical fatigue behavior in reasonable time and to identify the weakest link of the tested bond interface.

Quasicontinuum simulation of brittle cracking in single‐crystal material

Multiscale modeling has received unprecedented attention in various fields recently, with bottom‐up and top‐down transitions between neighboring subscales considered as important strategies for understanding defect initiation and evolution on the atomistic, nano, and submicron scales. The multiscale approach to the simultaneous modeling of multiple scales has been applied to the strong coupling between variable groups of different scales. In addition, the quasicontinuum (QC) method uses non‐local and local repatom regions for seamless bridging between the atomic and continuum regions. With specific application to the classical plate‐with‐a‐hole problem, a quantitative evaluation of the precision of the QC model was undertaken in this study. The analytical displacement field was solved using the specific regions as the atomic displacement comparison criteria. A certain amount of “ghost” interference was observed at the interface between the atomic and continuum mediums, and the crystal orientation effect of the α‐Fe crack tip atoms was used to analyze the dislocation emission, twinning, void nucleation, and crack propagation in specific grains. The defect evolution under tension loading was also analyzed with the aid of a typical crystal, and the strain‐force response relationship was used to elucidate the processes of crack initiation, brittle cracking, blunting, void nucleation, and complete rupture.