•A numerical model based on the inserted CZM method is developed.•Interface crack evolution in layered disc is investigation.•The conditions for crack evolution are discussed.
Crack initiation and ...propagation from a pre-existing interface crack are of great importance in assessing the integrity and stability of layered rocks, rock/concrete systems and other layered structures. In this study, a numerical model, based on the inserted cohesive zone model (CZM) method, was developed to simulate the complex crack behaviours in layered discs with a pre-existing interface crack, which were further validated by Brazilian tests. Different typical crack patterns were identified: interface cracks, wing cracks and secondary cracks. The results indicate that the interface cracks can be observed for disc samples under smaller loading angles while the secondary cracks can be observed for disc samples under larger loading angles. The formation of the typical cracks is also controlled by the interface strength. If 0° < loading angle < 60°, interface cracks can form for disc samples with lower interface strength. By increasing the interface strength of disc samples, interface cracks disappear while wing and boundary cracks can be found. If the interface strength further increases to a higher level, the secondary cracks will disappear. If the loading angle >60°, no interface crack can be found even with a very small interface strength, and the secondary cracks remain, regardless of the interface strength. The material match and confining pressure are also important factors that influence the crack patterns. The stress intensity factors of both tips of the pre-existing interface crack are also discussed. The energy release ratios and the corresponding phase angles for both crack tips are quite different, while the moduli ratio and the crack length-to-diameter ratios are noticeably larger.
A fully integrated system for the automatic detection and characterization of cracks in road flexible pavement surfaces, which does not require manually labeled samples, is proposed to minimize the ...human subjectivity resulting from traditional visual surveys. The first task addressed, i.e., crack detection, is based on a learning from samples paradigm, where a subset of the available image database is automatically selected and used for unsupervised training of the system. The system classifies nonoverlapping image blocks as either containing crack pixels or not. The second task deals with crack type characterization, for which another classification system is constructed, to characterize the detected cracks' connect components. Cracks are labeled according to the types defined in the Portuguese Distress Catalog, with each different crack present in a given image receiving the appropriate label. Moreover, a novel methodology for the assignment of crack severity levels is introduced, computing an estimate for the width of each detected crack. Experimental crack detection and characterization results are presented based on images captured during a visual road pavement surface survey over Portuguese roads, with promising results. This is shown by the quantitative evaluation methodology introduced for the evaluation of this type of system, including a comparison with human experts' manual labeling results.
Delayed hydride cracking (DHC) of Zircaloy-2 is one possible mechanism for the failure of boiling water reactor fuel rods in ramp tests at high burnup. Analyses were made for hydrogen diffusion ...around a crack tip to estimate the crack velocity of DHC in zirconium alloys, placing importance on effects of precipitation of 8-hydride. The stress distribution around the crack tip is significantly altered by precipitation of hydride, which was strictly analyzed using a finite element computer code. Then, stress-driven hydrogen diffusion under the altered stress distribution was analyzed by a differential method. Overlapping of external stress and hydride precipitation at a crack tip induces two stress peaks; one at a crack tip and the other at the front end of the hydride precipitate. Since the latter is larger than the former, more hydrogen diffuses to the front end of the hydride precipitate, thereby accelerating hydride growth compared with that in the absence of the hydride. These results indicated that, after hydride was formed in front of the crack tip, it grew almost steadily accompanying the interaction of hydrogen diffusion, hydride growth and the stress alteration by hydride precipitation. Finally, crack velocity was estimated from the calculated hydrogen flux into the crack tip as a function of temperature, stress intensity factor and material strength. There was qualitatively good agreement between calculation results and experimental data.
•A low cycle fatigue based FCG model has been proposed for both long and physically short crack.•A proper crack opening function is used for physically short cracks.•Provides an appealing alternate ...to standard NASGRO.
This work proposes an improved and simplified fatigue life equation called LAPS, formulated from low cycle fatigue data of smooth specimens and the Rice-Kujawski-Ellyin asymptotic field, with proper crack opening functions for closure effects. The model captures both long and physically short fatigue crack growth behavior, but the emphasis of this contribution is on the modifications for physically short cracks based on empirical models found in the literature. The predictions for physically short cracks from this model coincide well with experimental data for the railway axle used steel 25CrMo4. The predictions for long cracks match well with data from a variety of different metals. This makes the model a suitable alternative, e.g. to NASGRO, for engineering applications.
The effect of aging by the relaxation of the initial thermal stresses related to the processing on the initiation and propagation of the inter-splat and the intra-spat cracks in plasma sprayed ...zirconia is analyzed using finite element model and a description of failure with cohesive surfaces. A multi-scale approach is adopted in which the inter-splat and intra-splat crack growth is described with a rate-temperature and humidity dependent cohesive zone model that mechanically represents the reaction-rupture mechanism underlying stress and environmentally assisted sub-critical failure. It is found that the relaxation of the initial thermal stresses generates a significant initial damage at the inter-splat scale by the nucleation of inter-splat cracks and a minor initial damage at the intra-splat scale. The results show that the rate of inter-splat cracks increases with the relative humidity and especially with the temperature at which the relaxation occurs. The effect of the initial damage generated by the thermal aging on the resistance of the polycrystal of plasma sprayed zirconia against intra-splat slow crack growth under static fatigue loading is investigated. The results show that the initial damage at the intra-splat scale does not affect its resistance against intra-splat slow crack growth. However, the initial damage at the inter-splat scale leads to an increase in the slow cracking rate for a loading level KI and a reduction in the threshold load K0 below which no slow crack growth occurs as the individual splat is embedded in a damaged equivalent continuum representing the overall splat structure. The aim of this work is to provide a reliable predictions and insight in long lasting applications of plasma sprayed ceramic materials.
•Environmentally assisted failure initiation in splats and inter-splats•Rate-temperature and humidity dependent cohesive model•Predicting slow crack growth in thermal barrier coatings•Influence of aging on slow crack growth
Determination on underlying mechanisms of crack initiation is of vital importance to understand the failure processes of geomaterials in practical engineering. In this study, uniaxial compression ...experiments of granitic samples containing a single pre-existing flaw were conducted and the failure processes were recorded by using the high-speed camera. To quantitatively determine the crack initiation mechanism, a novel method was first proposed based on digital image correlation (DIC) analysis and then its validity was confirmed. By utilizing this method, three types of cracks with different initiation mechanisms were identified and the effect of flaw inclination angle on crack initiation mechanisms was discussed from the viewpoint of theoretical analysis. With the increase of inclination angles, wing cracks change from mixed mode I/II cracks to mode I cracks, while anti-wing cracks have no evident changes and are dominated by mode II cracks. Under compressive pressure, the upper and bottom surfaces of pre-existing flaw deform to each other and the distributions of full-field tangential stress around flaw are different, which might induce the variation of crack initiation mechanisms with regard to the inclination angle.
Due to high cost of full-scale experimental setup, this study presents a numerical model on fatigue behaviours of offshore pipeline with multiple coplanar cracks under cyclic tensile loadings. The ...validation on numerical results is made by other researchers' experimental results, and significant parameters affecting fatigue crack growth are studied.
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•Fatigue investigation is performed on pipeline with multiple interacting cracks.•Extended finite element method (XFEM) is adopted for numerical simulations.•The fatigue results obtained are similar to those from experiments.•Surface crack growth rate is highly expedited as the interacting cracks propagate.•A user-friendly approach to fatigue life estimation of pipelines is provided.
Cracking processes have been extensively studied in brittle rock and rock-like materials. Due to the experimental limitations and the complexity of rock texture, details of the cracking processes ...could not always be observed and assessed comprehensively. To contribute to this field of research, a numerical approach based on the particle element model was used in present study. It would give us insights into what is happening to crack initiation, propagation and coalescence. Parallel bond model, a type of bonded-particle model, was used to numerically simulate the cracking process in rock-like material containing a single flaw under uniaxial vertical compression. The single flaw’s inclinations varied from 0° to 75° measured from the horizontal. As the uniaxial compression load was increased, multiple new microcracks initiated in the rock, which later propagated and eventually coalesced into longer macrocracks. The inclination of the pre-existing flaw was found to have a strong influence on the crack initiation and propagation patterns. The simulations replicated most of the phenomena observed in the physical experiments, such as the type, the initiation location and the initiate angle of the first cracks, as well as the development of hair-line cracks, which later evolved to macrocracks. Analyses of the parallel bond forces and displacement fields revealed some important mechanisms of the cracking processes. The first cracks typically initiated from the tensile stress concentration regions, in which the tensile stress was partially released after their initiation. The tensile stress concentration regions subsequently shifted outwards close to the propagating tips of the first cracks. The initiation and propagation of the first cracks would not significantly influence the compressive stress singularity at the flaw tips, which was the driving force of the initiation of secondary cracks. The initiation of microcracking zone consisting almost exclusively of micro-tensile cracks, and that of microcracking zone consisting of micro-tensile cracks and mixed micro-tensile and shear cracks, were found to be correlated with two distinct types of displacement fields, namely type I (DF_I) and type II (DF_II), respectively.
•Extension of phase field to interface damage is extended to saturated porous media.•The fluid can flow within both bulk cracks and interfacial cracks.•The formulation is adapted to voxel-based ...models of realistic heterogeneous media.•3D simulations of hydraulic fracture with interfacial damage induced by the fluid.
In this work, we propose an extension of the phase field model for the modeling of hydraulic fracturing or cracking in heterogeneous saturated porous media. The involved extensions comprise: (a) taking into account interfacial damage between the inclusions and the matrix; (b) modeling fluid flow within both matrix cracks and interfacial cracks; (c) the possibility to handle geometries of the heterogeneous media in the form of regular grids of voxels e.g. as obtained from experimental imaging techniques. The developed numerical framework is based on the phase field method with a regularized description of both bulk and interface discontinuities, extended to a fully coupled hydro-mechanical framework. Both 2D and 3D examples are presented for hydro-mechanical microcracking initiation and propagation in voxel-based models of complex heterogeneous media with interfacial damage between the inclusions and the matrix.
Microscale fatigue damage evolution of off-axis tunnelling cracks was investigated for non-crimp fabric composites used in wind turbine blades. Test specimens fabricated from a multidirectional ...laminate of the layup sequence 0b/60b/0b/−60bs (with subscript ’b’ representing backing) with glass fibres and epoxy resin, were subjected to cyclic tension-tension loads. Following the testing, small-sized off-axis samples with sides parallel to the major −60° off-axis fibre bundle were cut from the gauge zone of the tested specimens and were investigated by X-ray computed tomography using a dye penetrant. The height of the tunnelling cracks was found to vary in both the one-bundle +60° and the two-bundle −60°2 plies of the laminate. In the presence of the resin rich pockets above or below crack-tips in the thickness direction, the cracks were observed to penetrate with twisted crack planes aligned perpendicular to the overall loading direction. In the presence of backing, the crack-tips either continued to penetrate between fibres along fibre/matrix interface or deflected at near right angles on the fibre, depending on the orientation of backing fibres. The experimental observations of crack penetration and crack deflection mechanisms were analysed with respect to a crack deflection criterion.
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