Propagation Of Single Crack Research Articles

Effect of Fatigue and Precorrosion Fatigue on Fatigue Crack Propagation and Fracture Failure Behavior of 5B70 Aluminum Alloy

Aluminum alloys are ideal lightweight structural materials for spacecraft, but they are susceptible to local corrosion during service. Herein, a systematic investigation is carried out to determine the fatigue behavior of 5B70 aluminum alloy under salt spray precorrosion. The mechanism of salt spray corrosion, fatigue life, crack propagation, and failure fracture mechanism of 5B70 aluminum alloy are studied. Under the salt spray corrosion, 5B70 aluminum alloy mainly undergoes pitting corrosion via a galvanic corrosion mechanism. After precorrosion, the formation of macroscopic fatigue cracks is accelerated, which significantly reduces its fatigue life. Fracture occurs due to single crack initiation and propagation under fatigue conditions. The fatigue crack tip undergoes a mixture of intergranular and transgranular modes, but mainly transgranular mode. Multiple strain localization phenomena occur during precorrosion fatigue, and corrosion pits on the surface provide a tortuous crack propagation path; the fatigue crack tip is mainly transgranular mode.

Single fluid-driven crack propagation in analogue rock assisted by chemical environment

During the operation of hydraulic fracturing as used in many geo-energy and geo-environment applications, chemical stimulation is often incorporated for cracking enhancement in low-permeability geological formations for the purpose of an optimization of energy recovery. The mechanism of subcritical crack propagation in a chemically reactive environment is essential for understanding of the involved coupled chemo-mechanical process and a better control of acid-assisted hydraulic fracturing. It has been postulated that the rate of crack propagation under environmental loads is inherited from the chemical processes involved including the reaction and the diffusive transport. However, laboratory explorations focusing on the evolving interplay between the propagation of a fluid-pressurizing individual crack and the environment it is subject to via a variable chemical intensity imposed have been rare. Here we present an experimental investigation on a single tensile crack propagation in alginate hydrogel as an analogue material for brittle rocks, driven by the injection of a chemically reactive fluid kept at constant pressure using a Hele-Shaw cell setup. We show that an intensified chemical environment can accelerate tensile crack propagation in both subcritical crack growth and fracturing regimes, while leading to the Region III fracturing of less brittle characteristics. During the experiment, crack-tip blunting upon injection of reactive solutions was observed, suggesting a competing mechanism between the crack-tip geometry induced toughening and the chemically induced softening within the process zone as the crack advances. Our results provide quantitative insights into how a chemically reactive environment facilitates the growth of a single macroscopic crack of mode I opening in a low-permeability matrix through coupled chemo-mechanical feedback. The imposed chemical environment promotes crack propagation while alleviating the stress concentration at the advancing crack tip, suggesting a more energy-efficient method compared to pure water fracturing. We anticipate our experimental investigation presented here to be a starting point of sound laboratory support for future studies towards a more controllable technique of chemical stimulation in geomaterials as well as complementing the ongoing modeling efforts in reactive chemo-mechanics.

In situ measurement of plasticity accompanying hydrogen induced cracking in a polycrystalline AlZnMg alloy

Hydrogen induced single crack propagation is studied in an embrittled aluminum alloy. Hydrogen is introduced into the system by electrochemical reactions in an acidic aqueous medium. After hydrogen charging, tensile tests are performed in air, on notched samples, with a microtensile machine under an optical microscope. A high magnification of × 2000 is used to follow the single crack initiation and propagation. Digital Image Correlation gives the displacement field on the surface with a spatial resolution of approximately 1 μm. It enables the determination of the position of the crack tip and the local velocity at a sub-grain scale. The von Mises strain is calculated and provides a precise measure of the local plastic field that accompanies crack propagation. In addition to the primary plasticity which is emitted from the crack tip or its immediate neighborhood in the form of two intense slip bands, a secondary plastic zone that spreads over several microns ahead of the tip is sytematically found. The characteristics of the plastic zone are measured, together with the velocity and the applied stress intensity factor. In addition, different fracture mechanisms are found on the fracture surface. In particular there are transitions in the fracture mode from intergranular smooth to transgranular parallel to the grain boundary plane. The local fracture mechanisms, in the vicinity of the surface, are linked to the local velocities and plastic deformations. Surprisingly no strong velocity/plasticity correlations are found while the velocities are scattered over a wide range, which is interpreted as a strong polycrystalline effect.

Characterization of Carbon Fibre Reinforced Polyphenylene Sulfide Composite Under Interlaminar Shear Strength

Aerospace industry is seeking thermoplastic composites materials due to several advantages over thermoset material and one of them is mechanical properties. Mechanical properties of thermoplastic composite materials are found to be superior compared to thermoset materials. High strength fibre composites parts are made of several plies with different fibre orientation with a designed balanced of lay-up sequence. Interlaminar Shear Strength (ILSS) is an essential consideration when designing laminated structures exposed to transverse stresses. It is to predict quality and strength of laminate levels of composite due to, composite failure cannot be observed directly to the propagation of single macroscopic crack when subjected to cyclic loading. This test is employed to evaluate both the interfacial adhesion of the matrix and the influence of the reinforcement on the mechanical characteristics of the composite. This study provides the understanding of ILSS test of 2 different thickness for 6 and 8 plies Carbon Fibre Reinforced Polyphenylene Sulfide (CF/PPS) according to the testing standard EN 2563. The results show that 8 plies have higher ILSS properties compared to 6 plies. The fractography of the CF/PPS composite under Scanning Electron Microscope (SEM) shows good agreement with the experimental values which was recorded 8 plies can withstand to shear strength of 79 MPa with maximum load applied was 2.738kN while, 6 plies recorded 75 MPa on shear strength with load of 1.971kN. More damage/cracks on 6 plies specimen compared to 8 plies.

Micromechanics-based model of single crack propagation in Engineered cementitious composites (ECC)

Engineered Cementitious Composites (ECC) are composites exhibiting strain-hardening behavior accompanied by the formation of multiple cracks. A special feature of ECC is the ability to tune the material composition to meet the different needs of various structural components, which benefits from the special micromechanics-based design theory of ECC. However, these existing micromechanics-based design theories are specific to the case of direct tension and has not been fully discussed and developed for the case where the ECC is subjected to bending load. In this study, a micromechanics-based crack propagation model for ECC under bending load is developed to guide the design of flexural performance of ECC. The entire crack propagation process is simulated by considering the microstructure of ECC and the crack propagation criterion. For ECCs with different types of fiber, the simulated load-crack mouth opening displacement curves agree well with experimental results and the dispersion of the ECC specimens can be captured by imparting randomness to the model. Based on this model, parametric studies of several representative parameters are performed to illustrate the applicability of the model. The results indicate that the material has a higher ultimate flexural bearing capacity near a fiber aspect ratio of 448 for the given material property parameters; and the fiber/matrix bonding should not be excessively strong. Insights from this model can facilitate the design of ECC members with excellent bending properties through fiber tailoring and surface treatment.

Fracture of brittle material with two 3D parallel internal cracks under thermal stress: Experimental and numerical analysis

Many defects such as cracks are often present inside brittle solids, and most of the cracks do not appear in a single form. The fracture and failure of brittle solids are basically caused by the interaction and expansion of multiple cracks. Most of the traditional studies on this topic have focused on surface or penetration crack interaction under tensile and compressive loads, while there have been very few studies on the expansion characteristics and interaction of parallel internal cracks under temperature loading. This study applies the 3D internal laser-engraved crack (3D-ILC) method to fabricate parallel internal cracks inside the 3D cube. Physical tests and numerical simulations of parallel internal cracks of different sizes and single crack propagation under temperature fields are carried out, and the propagation characteristics of internal cracks and the interaction between parallel internal cracks of unequal size under temperature loading are investigated. The study results show that the big crack will inhibit and shield the expansion of the small crack. In addition, there are typical “anti-binary tree” mode III features on both sides of the parallel large crack and single large and small crack propagation surface, which are classified as mixed I-II-III crack, while the parallel small cracks are mode I-II. Compared with the quasi-static crack growth criterion, the simulation results of the subcritical crack growth criterion are more in line with the physical test results, and there is a subcritical growth phenomenon in the parallel crack propagation under the temperature load. The results provide experimental and theoretical references for the study of 3D bi-parallel cracks interaction of different sizes and the fracture of mode I-II-III crack under the temperature field.

In situ X-ray imaging of fatigue crack growth from multiple defects in additively manufactured AlSi10Mg alloy

Defects introduced during additive manufacturing currently control fatigue resistance and lead to a large scatter in lifetime, with pancake shaped lack of fusion (LOF) defects being particularly potent. In this study the fatigue crack propagation life of selective laser-melted (SLM) AlSi10Mg alloy is considered in cases where single cracks and multiple cracks can initiate from LOF defects under high cycle fatigue (HCF). Firstly, the aspect ratios of initially long fatigue cracks were determined for critical LOF defects obtained from X-ray CT renderings using the critical defect regularization method, and the response surface method used to obtain the stress intensity factor of the crack front quickly and continuously. Then a single crack propagation model considering the evolution of the crack aspect ratio established to predict the crack propagation life which is in good agreement within in situ X-ray CT imaging of the crack front when a single crack is dominant. The crack propagation phase was predicted to represent 35–60% of the total fatigue life representing a larger fraction at high stress amplitudes. Multiple cracks were found to initiate cracks at the larger stress amplitudes. In cases where multiple cracks arise this is non-conservative and so a synergistic multiple fatigue crack growth (smFCG) model was developed based on multiple defects measured a priori by X-ray CT to depict the competitive cracking effect. Compared with the single crack model, the smFCG model predicts a shorter propagation life (by 5–10%) when multiple defects are involved since it considers all the initial defects within the crack initiation region. Given the propensity of large numbers of defects in AM material this approach may be more appropriate in many cases.

Three-dimensional modeling of fracture in quasi-brittle materials using plasticity and cohesive finite elements

This paper presents a three-dimensional discrete approach based on the use of interface cohesive elements with zero thickness to model the crack formation and propagation in quasi-brittle materials. The interface constitutive model incorporates concepts of nonlinear fracture mechanics and plasticity. A Coulomb-based surface is used as yield function, and an exponential traction–separation curve as softening law. The model features a non-associated flow rule with an implicit integration scheme and is able to simulate the propagation of single and multiple cracks in mode I and mixed-mode. The proposed model was tested through extension and shear paths and applied in the simulation of experimental tests in concrete specimens available in the literature. Also, a study on the effect of the use of different bulk elements was performed. Numerical results, including peak loads and load-CMOD curves showing softening behavior, are in good agreement with the experimental data.

Simulating three dimensional thermal cracking with TOUGH-FEMM

Temperature change often generates thermal stress, which leads to crack propagation in rocks. In this paper, the TOUGH-FEMM simulator, which links the TOUGH2 thermal-hydraulic simulator and a mechanical simulator based on hybrid the finite-element meshfree method (FEMM), is developed to model three-dimensional cracking induced by thermal stress. The temperature distribution is solved using TOUGH2, which is an established software for modeling fluid flow and heat transfer in porous and fractured media. The thermal stress, mechanical deformation and cracking simulated with FEMM does not require any remeshing during cracking simulation, which greatly reduces the complexity and computational cost. Three benchmark examples are carried out to verify and validate the performance of the TOUGH-FEMM simulator, including thermal-mechanical stress under steady-state heat transfer, transient heat transfer and single crack propagation influenced by a heat source. Finally, the TOUGH-FEMM simulator is applied to model propagation of multiple cracks during cryogenic (low temperature) fracturing, in which the numerical results provide an effective prediction of fracture distribution after thermal cooling shock.

Studies of dynamic fracture in functionally graded materials using peridynamic modeling with composite weighted bond

In this paper, based on peridynamics (PD) modeling with composite weighted bonds, the crack propagation of functionally graded materials (FGMs) subjected to dynamic loads under a triangular micro-modulus function were studied. Both single crack and double cracks propagation in FGM plate under tensile load were studied. Furthermore, the influence of material gradient forms, crack position and length on crack propagation behavior was discussed. The results show that the position of the crack significantly affect the crack propagation, while the effect of the material gradient form on the crack propagation in FGMs is limit. In addition, the influence of the initial single crack length on the crack growth path was also studied. It was found that the change of the initial single crack length did not affect the final growth path of the crack.

A scaled boundary finite element method for modelling wing crack propagation problems

In this paper, a new shape function based on the scaled boundary finite element method (SBFEM) with side-face loading is used to study the problem of wing crack propagation. Crack contact is modelled by introducing the contact interface constraint condition by the Lagrange multiplier method. In the crack propagation process, the contact state of the crack surface at each step is obtained by an iterative method of determination and validation, and an accurate simulation for the frictional contact expansion process of compressed wing cracks is carried out. Polygon SBFEM remeshing technology is used to simulate the propagation of single and multiple wing cracks. The correctness and effectiveness of the new SBFEM shape function method in modelling wing crack propagation are verified using related experimental and numerical simulation results. The results indicate that under pressure, the wing crack propagates along the direction of the maximum circumferential stress during initial expansion, and the crack propagation angle is −70°32′, which is a pure type II crack. The crack then opens and gradually expands towards the pressure. In the considered wing crack problems, contact phenomena only occur on the initial crack surface during crack propagation. The contact force gradually decreases from the centre to the tips of the initial crack. As the crack expands, the contact force decreases, and the contact area of the crack gradually decreases. The friction coefficient has a strong influence on the crack propagation direction. The larger the friction coefficient, the closer the propagation direction is to the linear extension in the direction of the applied load.

Modeling and simulation of thermal fatigue crack in EB-PVD TBCs under non-uniform temperature

Abstract Demand for enhanced jet engine efficiencies has led to a significant increase in the combustion temperature. Thus protecting components against the combustion products is necessary and is possible by using thermal barrier coatings (TBCs). In this research, thermal fatigue and creep interaction are studied via analytical and numerical finite element methods. Thermal stress and crack propagation analyses in the ceramic top coat are carried out based on plane stress condition and under inhomogeneous temperature distribution across the layers. The crack is assumed as a penny-shaped crack in both vertical and inclined growth directions. The study proposed that the creep-plasticity results in thermal stress alleviation and the tensile stress transforms into a compressive stress of − 200 MPa in forwarding cycles that will induce crack closure. In addition, the results confirmed that vertical cracks grow much faster than oblique ones due to single mode crack propagation and about 0.14 MPa√m greater values in stress intensity factor. The modeling and simulation results match together, and the obtained crack behavior is in compliance with other researcher's output.

Effect of Skew Angle of Holes on the Thermal Fatigue Behavior of a Ni-based Single Crystal Superalloy

In the present work, holes of various skew angles were electrochemically machined in the middle of the plate specimens in a Ni-based single crystal superalloy and crack initiation and propagation around holes during thermal fatigue cycles (20–1100 °C) were investigated. It was demonstrated that the skew angles had a significant effect on the initiation and propagation of thermal fatigue cracks. During thermal fatigue process, stress concentration occurred at the edge of the holes. As for skew angles, the maximum stress concentration appeared at the acute side of holes. The maximum stress concentration resulted in plastic deformation at the acute side of the 30° hole, driving the thermal fatigue cracks to initiate after 220 cycles and propagate along \([0\bar{1}1]\) direction. However, the stresses concentrated at the edge of 90° or 60° holes were not large enough to initiate cracks even after 580 thermal cycles. This work will help to understand the local deformation behavior in the vicinity of cooling holes with various skew angles and have serious design implications for turbine blades.

Dynamic crack propagation with a variational phase-field model: limiting speed, crack branching and velocity-toughening mechanisms

We address the simulation of dynamic crack propagation in brittle materials using a regularized phase-field description, which can also be interpreted as a damage-gradient model. Benefiting from a variational framework, the dynamic evolution of the mechanical fields are obtained as a succession of energy minimizations. We investigate the capacity of such a simple model to reproduce specific experimental features of dynamic in-plane fracture. These include the crack branching phenomenon as well as the existence of a limiting crack velocity below the Rayleigh wave speed for mode I propagation. Numerical results show that, when a crack accelerates , the damaged band tends to widen in a direction perpendicular to the propagation direction, before forming two distinct macroscopic branches. This transition from a single crack propagation to a branched configuration is described by a well-defined master-curve of the apparent fracture energy Γ as an increasing function of the crack velocity. This Γ(v) relationship can be associated, from a macroscopic point of view, with the well-known velocity-toughening mechanism. These results also support the existence of a critical value of the energy release rate associated with branching: a critical value of approximately 2Gc is observed i.e. the fracture energy contribution of two crack tips. Finally, our work demonstrates the efficiency of the phase-field approach to simulate crack propagation dynamics interacting with heterogeneities, revealing the complex interplay between heterogeneity patterns and branching mechanisms.

Experimental and Numerical Study of Window Glass Breakage with Varying Shaded Widths under Thermal Loading

To investigate the effect of shaded width on the breaking behavior of window glass, a series of experiments was carried out on float glass with dimension of 600 mm × 600 mm × 6 mm in an enclosed compartment under radiant heat. The shaded width of glass pane ranged from 10 mm to 50 mm with an interval of 10 mm. Experimental results showed that crack patterns of the glass pane were influenced little by the shaded width, while the average value of the first breaking time of the glass pane decreased firstly and then increased with an increase in the shaded width. The average time to the first crack with the shaded width of 20 mm was shortest in experiments and the corresponding time was 572.5 s. In addition, the finite element method was also used to simulate the process of crack initiation and single crack propagation. Temperatures measured by thermocouples in experiments were employed as thermal loads for the problem of glass breakage. The first breaking time obtained by the program was in good agreement with experimental data.

Shearing of Materials with Intermittent Joints

The strength of fractures is much lower as a rule than that of intact rock. As a result they play a controlling part in the mechanical behaviour in general and the failure in particular of rock mass. Though a large volume of experimental data is available on the shear resistance of joints, as well as on the propagation of single cracks, the same is not true for the mechanical behaviour of intermittent joints. The experimental data available in this case are limited and the strength of rock mass with intermittent joints is usually modelled using averaged values of cohesion or assuming the fractures to be continuous. In the present work, the results of simple shear tests on a series of gypsum specimens with pre-existing cracks are presented. Twelve different crack orientations and two normal stresses were tested. The hypothesis of averaged cohesion and the theory of fracture mechanics are used to reproduce the results. It is found that fracture mechanics provides a more suitable model for the experimental results, especially when crack interaction is taken into account.

Bending behaviors and fracture characteristics of laminatedductile-tough composites under different modes

The bending load-deflection curves and fracture characteristics of a range of laminated Ti(TiBw/Ti) composites were investigated under five modes. The results show ductile-brittle transition with decreasing Ti layer thicknesses and increasing volume fractions of TiBw, which is attributed to the constrained plastic deformation mechanism accompanying with high stress traixiality and tensile stress. The laminated composites with weak interfaces display superior fracture toughness under notched crack arrested orientation, which is related to the delamination cracks and multiple tunnel cracks. Moreover, single tunnel crack propagation and periodic multiple tunnel cracks were observed in the laminated composites under mode IV, which depend on the thickness and ultimate strength ratio of ductile Ti layer and tough TiBw/Ti composite layer. There are many interfacial delamination cracks and multiple tunneling cracks presented under mode V, playing an effective role in toughening the laminated composites. In addition, with decreasing the Ti layer thickness, laminated composites reveal obvious size effect characterized by more tunnel cracks.

Some aspects of strength in sea ice mechanics

The paper provides estimates of the relation between the strength of sea ice and its porosity in the framework of a porous solid model proposed earlier. The influence of the ice capillary structure filled with brine on the ice strength is analyzed, showing that the rise of local stresses in the ice changes its limit equilibrium conditions. Comparison is made to elucidate how the sea ice strength responds to loading length- and crosswise the capillary axes. Patterns by which ice structure elements merge resulting mostly in main cracks or in multiple ordered crack systems under compression are discussed. The tendency toward generation of ordered crack systems or propagation of single main cracks in the structured material is determined by the activity of its structural elements involved in fracture. A method is proposed for estimating the average strength of sea ice cover relative to horizontal compression with regard to partial loss of the bearing capacity of ice layers during deformation.

α-Al2O3 nanoslab fracture and fatigue behavior

Strain rate effects and cyclic loading behavior, relevant for fatigue initiation processes, of single crystalline α-Al2O3 nanoslab structures in vacuum have been characterized and compared under finite temperature dynamic loading and incremental static loading conditions by reactive molecular dynamics and ionic relaxation simulations, respectively. Different failure mechanisms for each loading case are observed and compared in view of known applicable material failure mechanisms. In particular, structure size effects along with unit cell and bulk defect distribution based mechanisms are considered. The effects of lateral pre-strain are assessed. Conclusions are drawn regarding conditions which could lead to low cycle failure of the material.The results indicate that finite temperature and strain rate result in lower failure strains, as compared to static relaxation calculations, however low strength enhancement of ductility with kinematic strain hardening upon repeated loading may occur. We suggest that the latter facilitates a shakedown mechanism. Positive pre-straining results in increased stress triaxiality (hydrostatic vs. equivalent stress), which significantly reduces crack healing probability, due to single sharp crack propagation. Instead, volume pre-relaxation results in multiple crack branching and/or amorphous band formation, which facilitates crack healing and deformation induced transition to purely elastic response (shakedown) possibility. Amorphization, which manifests as small strain plasticity enhancement, is found to occur ahead of propagating cracks due to multiple dislocation mechanism as a low energy barrier partially reversible low density phase transition, both at static and finite temperature/strain rate conditions. The observed property changes and phase change related defect healing mechanisms have been investigated further by bulk unit cell simulations and validated against DFT calculations.

From fracture to fragmentation: Discrete element modeling

Discrete element modelling (DEM) is one of the most efficient computational approaches to the fracture processes of heterogeneous materials on mesoscopic scales. From the dynamics of single crack propagation through the statistics of crack ensembles to the rapid fragmentation of materials DEM had a substantial contribution to our understanding over the past decades. Recently, the combination of DEM with other simulation techniques like Finite Element Modelling further extended the field of applicability. In this paper we briefly review the motivations and basic idea behind the DEM approach to cohesive particulate matter and then we give an overview of on-going developments and applications of the method focusing on two fields where recent success has been achieved. We discuss current challenges of this rapidly evolving field and outline possible future perspectives and debates.