Crack Initiation Process Research Articles

Measurement of ceramics cracking during water quenching by digital image correlation

Measuring the thermal shock crack growth process is crucial for revealing ceramic materials and structures’ thermal shock failure mechanisms and evaluating their reliability. We used a self-made water quenching system to conduct thermal shock tests on alumina and zirconia ceramics. The thermal shock process was recorded by high-speed digital image correlation (DIC) during the test. The process of thermal shock crack initiation and propagation in two kinds of ceramics was determined by analyzing the speckle image change on the sample’s surface. It is found that the crack growth rate of alumina is faster than that of zirconia, which is caused by different material parameters. This paper presents an in-situ measurement method for the initiation and propagation of thermal shock cracking in ceramic materials. It can provide a measurement method to identify and predict the thermal shock damage of ceramic components.

A framework to model thermomechanical coupled of fracture and martensite transformation in austenitic microstructures

A fully thermomechanical coupled phase-field (PF) model is presented to investigate the mechanism of austenite-to-martensite phase transformation (MPT) and crack initiation as well as its propagation in pure austenitic microstructures. The latent heat release and absorption involved in the MPT are explicitly taken into account by coupling the PF model with transient latent heat transfer. In order to consider temperature dependency in the PF model for MPT, a temperature-dependent Landau polynomial function, whose parameters are identified using molecular dynamics (MD) simulations, is proposed. Furthermore, the fracture surface energy is approximated based on the second-order PF model and then, the temporal evolution of the damage variable is given by the variational derivative of the total potential free energy of the system with respect to the damage variable. The achieved numerical results demonstrate that the model can be employed to predict the fracture mechanism of austenitic microstructures under a thermomechanical field in a multiphysics environment. The results reveal that the temperature has a tremendous impact on the growth rate of both martensitic variants and consequently on the crack growth path. The key contributions of this work are to shed light on the impact of thermal boundary conditions on the coupled process of MPT, crack initiation and growth.

Very high cycle fatigue properties at 973 K of additively manufactured and conventionally processed intermetallic TiAl 48-2-2 alloy

The fatigue lives of 2nd generation γ-TiAl alloy Ti–48Al–2Cr–2Nb manufactured both by additive manufacturing using electron beam powder bed fusion (EB-PBF) as well as conventional casting process by vacuum arc remelting for comparison (REF) were investigated. Both batches (EB-PBF and REF) were subject to the common treatment process, i.e. hot isostatic pressing and heat treatment to achieve a duplex microstructure. As a result, a fine- and a coarse-grained microstructure consisting of γ-grains and colonies of α2/γ lamellae have evolved for the EB-PBF and conventionally cast material, respectively. Uniaxial fatigue tests were performed in the very high cycle fatigue (VHCF) range up to 109 cycles at a load ratio of R = −1 using ultrasonic fatigue testing equipment at an elevated temperature of 973 K. The SN data of both batches were discussed with respect to the influence of microstructure and temperature on the processes of crack initiation and propagation. The additive manufactured material showed superior fatigue strength compared to the conventional material due to its smaller grain size. It was shown that the fine-grained duplex microstructure of the batch EB-PBF is particularly appropriate to decrease short crack propagation at elevated temperatures due to its microstructural characteristics.

Deformation behavior and fracture mechanism of Ti-22Al-20Nb-7Ta alloy

Three different typical microstructures (lath, equiaxed and duplex microstructures) of Ti-22Al-20Nb-7Ta alloy were obtained by thermomechanical processing. In order to analyze the deformation behavior, the typical microstructures were studied by tensile test and the fracture mechanism was also discussed. The results showed that for equiaxed microstructure, there are three slip transmission modes between O phase and B2 phase, indicating that the two-phases had good deformation coordination relationship. The O phase and B2 phase formed different type dislocation configurations, which is an important deformation strengthening mechanism of the alloy. For duplex microstructure, the fine lath O phase and α2/O phase precipitated in B2 matrix, which enhanced the hindrance effect on dislocation motion. The crack initiation and crack propagation process were observed by in-situ tensile test. For lath microstructure, the coarse lath O phase precipitated near B2 grain boundary, and the crack was easy to generate at grain boundary and rapidly fracture. For equiaxed microstructure, the microcracks were easy to generate in the slip lines and were connected to each other to generate the maincrack by the plastic deformation of B2 phase. For duplex microstructure, the microcracks generated in the lath O phase, the internal of α2/O phase and the phase interface of α2/O phase and B2 phase and were connected to each other to generate the maincrack by the plastic deformation of B2 phase. Because the disordered distribution of α2/O phase and lath O phase, the direction of crack propagation is tortuous.

A transition scheme from diffusive to discrete crack topologies at finite strain during the course of a staggered iterative procedure

AbstractThis study presents a novel transition scheme that can trace an actual crack path as closely as possible and stably update its explicit crack tip even in a large deformation regime. The crack initiation and propagation processes are determined from an energy minimization problem with respect to the displacement field and crack phase‐field, and the predicted path represented by a diffuse crack topology is replaced by a discrete path by applying the finite cover method. By developing a technique for determining explicit crack tips, the crack topology is updated from diffusive to discrete intermittently during the course of the staggered iterative procedure. Therefore, even a curved crack path that evolves significantly within a single time increment can be explicitly captured. Additionally, to stably update an explicit crack tip within the finite strain framework, we introduce a stabilization technique. Specifically, pseudo‐stiffness is applied to severely damaged elements around the discrete crack path to prevent excessively large deformations, and the corrector of the displacement increment in the global Newton–Raphson iterative procedure is intentionally diminished so that the discrete crack opens in a gradual and stable manner. After describing the individual techniques devised in the proposed scheme for achieving these features based on their algorithmic aspects, we present several representative numerical examples to demonstrate the performance and capability of the developed approach.

Propagation and intensity of blast wave from hydrogen pipe rupture due to internal detonation

The coupled fluid-structure-rupture model was developed to study the propagation and intensity of blast wave from hydrogen pipe rupture due to internal detonation. The dynamic rupture of pipe and propagation of blast wave were well coupled together in every timestep during the simulation. The numerical model was validated with experiments in terms of both typical rupture profiles and blast overpressures. Results reveal that crack branching of pipe can dramatically increase the rupture opening rate which controls the intensity and shape of the resultant blast wave. Due to the process of crack initiation and extension, the blast wave out of the pipe first forms and then is strengthened by the subsequent compression waves. This makes the maximum peak overpressure appears at a certain standoff distance above the rupture. Despite consuming some percentages of energy, the dynamic rupture of pipe generally presents positive effects (up to 2–3 times) on the blast wave intensity along the jetting direction due to the convergence effect of rupture opening on the release of internal high-pressure gas. Finally, through defining normalized overpressure and impulse based on the same hydrogen detonation in open spaces, the quantitative influences of pipe rupture on the blast wave intensity in cases of different detonation pressures and standoff distances are clarified.

Research Progress of Self-Healing Thermal Barrier Coatings: A Review

Reliability and durability are two important performance indicators for thermal barrier coatings (TBCs). The reliability of TBCs usually includes high adhesive strength, low thermal conductivity and high thermal shock resistance. The high reliability of TBCs ensures basic usage requirements. Durability demands TBCs have a long service lifetime before their eventual failure. The lifetimes of TBCs under actual service conditions are strongly dependent on crack initiation and propagation. Controlling and delaying the dynamic process of crack initiation and propagation is a direct approach to prolonging the service lifetime of TBCs. Self-healing TBCs usually have the specific function of inhibiting crack propagation, and thus promote the self-healing process of TBCs. The research progress of self-healing TBCs was reviewed. Firstly, the concept of self-healing or self-healing materials is clarified. Secondly, the research progress about some self-healing ceramic materials as well as self-healing TBCs is reviewed. Based on the review, the micro-structure design, propagation patterns of the crack and self-healing mechanism are discussed systematically. Additionally, the future development trend of the self-healing TBCs is also overviewed in this paper.

Initiation and propagation behaviour of cohesive cracks at high temperatures: A dynamic experimental study

Temperature, as an important factor, affects the fracturing effect in deep reservoirs, so it is urgent to carry out high-temperature crack propagation experiments. Here, by combining a high-speed camera and DIC technology, Brazilian splitting tests of three types of rocks at different temperatures are carried out to study the crack initiation and propagation characteristics from a mesoscopic perspective. A new way to identify the cohesive crack tip is proposed based on the high-temperature cohesive zone model and opening displacement patterns. In addition, the dynamic propagation process and changes in the characteristics of cohesive cracks at different temperatures are analysed. The experimental results demonstrate that dynamic process of high-temperature cohesive crack initiation and propagation can be subdivided into three stages: stress concentration, cohesive microcrack initiation and propagation and cohesive macrocrack penetration. Interestingly, multiple interconnected rather than single cohesive microcracks develop in the centre region of sandstone and limestone specimen surfaces at 260 ℃. Notably, the duration of the stress concentration stage of sandstone and limestone shows an increasing trend with temperature. The average propagation speed and the opening displacement change rate of the cohesive crack in sandstone, limestone and shale specimens show a declining trend with increasing temperature. The results can provide some insights into the high-temperature crack propagation mechanism in deep reservoirs.

Characteristics and micromechanisms of fish-eye crack initiation of a Ti-6Al-4V alloy in very high cycle fatigue regime

The fracture surface characteristics and the associated micromechanisms governing the fish-eye crack initiation of a Ti-6Al-4V alloy under very high cycle fatigue (VHCF) loading are investigated. Microscopic observation revealed that the crack initiation area of fish-eye center is covered by ragged-like mountains and dimples containing various irregular rods/particles, grooves and microvoids. These micro-ductile characteristics primarily result from the nucleation and coalescence of microvoids and grooves at αs/βr interface inside βtrans. This could be due to the incompatibility of both micro-plastic deformation and slips in αs and βr. The βtrans plays a dominant role in the fish-eye crack initiation and early propagation process of the Ti-6Al-4V alloy in the VHCF regime.

Revealing impact failure behavior of tungsten composite using instrumented Charpy impact testing

The impact toughness of W-Ni-Fe alloys with different W content (90, 93 and 95 wt%) were studied by means of instrumented Charpy impact testing, and the relationship between impact toughness and microstructural parameters was discussed, with an emphasis on impact failure behavior. With increasing of W content, the elongation decreases from 31% for 90 W to 21% for 95 W, but the tensile strength slightly increases from 940 to 974 MPa and the yield strength increases from 655 to 688 MPa. The impact load-displacement curves reveal that the impact toughness is predominantly controlled by the crack initiation energy and decreased with increasing of W content, whereas the crack propagation energy is relatively low and remains constant. The decrease in the crack initiation energy attributed to a decrease in matrix volume fraction and an increase in W contiguity. 90 W shows the highest crack initiation energy due to the severe plastic deformation during crack initiation process, which contains multiple deformation characteristics such as dislocation tangles, dislocation forest, stacking faults, nano-scaled twins and subgrains. The formation of those features is discussed, and it is found that they are expected to provide a toughening effect. From the perspective of crack nucleation and growth, it is also found that the high crack initiation energy of 90 W profited from the high energy-consuming processes of W cleavage events, micro-crack arrest, blunting and bridging mechanisms. Although these three alloys display different failure features of crack propagation paths, they present similar crack propagation energies. The current study provides a deep insight into the impact failure behavior of WHAs.

Investigation of extremely low cycle fatigue behavior of low yield strength steel LY225 under different stress states

• Monotonic tensile and ELCF tests were conducted for LY225 low yield point steel. • ELCF fracture behavior between LY225 steel and Q235 steel was compared. • The LCVGM parameters of LY225 steel were calibrated based on the experimental results. • Buckling and ELCF behavior of LY225 steel dog bone specimen were experimental studied. • The LCVGM can well simulate the post buckling ELCF fracture of dog bone specimen. Low yield point steel is frequently utilized in the energy dissipation components of structures because of its good seismic performance. A low yield point steel LY225 subjected to different stress states is experimental investigated in the study by the monotonic tensile tests and extremely low cycle fatigue tests. The test results demonstrate that LY225 steel shows cyclic hardening characteristic as well as dimple fracture failure mode. In addition, LY225 steel exhibits higher deformability and energy dissipation capacity than Q235 low carbon steel, especially in pure shear stress state, the deformation capacity and extremely low cycle fatigue life of LY225 steel are more than twice that of Q235 steel. The Lode parameter enhanced cyclic void growth model (LCVGM) comprehensively considering the influence of stress triaxiality and Lode parameter can be employed to predict the extremely low cycle fatigue fracture. The LCVGM parameters of LY225 steel were determined based on the experimental results. It indicates that the fracture failure of LY225 steel is free from the influence of Lode parameter due to its good shear performance. The monotonic tensile test and extremely low cycle fatigue test were carried out on the LY225 dog bone specimens, and the fracture failure phenomenon after buckling occurred in the test. The post buckling crack initiation and growth process were numerically simulated by the LCVGM of LY225 steel. The prediction accuracy of the LCVGM was verified by comparing the test results and simulations. This study may provide valuable information about the fracture behavior of LY225 steel under various stress conditions, as well as an effective approach to analyze the post buckling fracture process on the member level.

Heat dissipation and fatigue crack kinetic features of titanium alloy Grade 2 after laser shock peening

The work is devoted to experimental investigation of the laser shock peening (LSP) effect on fatigue crack propagation rate and heat dissipation at the crack tip in specimens made of titanium alloy Grade 2 with a stress concentrator. It is shown that the LSP can leads both to positive and negative effect on fatigue lifetime. The effective processing scheme, which includes stress concentrator zone, was proposed. This type of treatment forms an optimal residual stress field, which slows down the crack initiation and propagation processes. The effective LSP processing scheme reduces the value of the stress intensity factor and, as a consequence, effects on an intensity of plastic deformation at the crack tip. This effect can be visualised by measurement of heat flux from the crack tip area. Both heat flux from the crack tip and crack rate are less in the LSP processed specimens. Structural investigations of LSP treated material near fatigue crack path have shown that structural defects (twins) that appear on the surface of the material as a result of LSP do not have a significant effect on the fatigue crack propagation, and the configuration of the residual stresses field created by LSP plays a decisive role.

Experimental study on mechanical properties and failure characteristics of sandstone containing a pre-existing surface flaw under dynamic loading

Dynamic loading tests were conducted on sandstone specimens containing a three-dimensional surface flaw with different angles and depths using a modified split Hopkinson pressure bar (SHPB) device. A high-speed camera with digital image correlation method was used to record the crack initiation, propagation and failure process of specimens and to monitor the surface strain field. The test results show that both the angle and depth of the surface flaw have an important influence on the dynamic mechanical properties and energy dissipation of the specimens. The specimen is least weakened at a surface flaw angle of 90°, with the angle having a greater effect on dynamic strength and energy absorption at greater depths. Analysis of the x-axis direction strain, shear strain and y-axis direction strain fields by DIC shows that the crack initiation is due to a combination of tensile and shear stresses. Both the angle and depth of the surface flaw have a significant effect on the failure patterns. Specimens show shear failure at surface flaw angles less than or equal to 60° and tensile failure at surface flaw angles of 75° and 90°. When the angle of surface flaw is less than or equal to 60°, there are more shear cracks at greater depths. The complexity of three-dimensional crack propagation under dynamic loading was confirmed by the observation of traces of internal shell-like crack growth in a partial fragmentation of specimen after failure.

Strain-rate-dependent compressive and compression-shear response of an alumina ceramic

This paper assessed the microstructure and properties of CeramTec ALOTEC 98 SB alumina ceramic through microscopic characterization and mechanical experiments. The rate-dependent strength and failure response of an alumina ceramic were studied under both uniaxial compression and compression-shear loading. Under quasi-static uniaxial compression at rates of 10−5 to 103 s−1, the strength had an average of 3393 ± 306 MPa, and at dynamic strain rates of 102 to 103 s−1, the strength ranged from 3763 to 4645 MPa. The CeramTec ALOTEC 98 SB alumina ceramic was found to have greater mechanical properties than other commercial alumina ceramics from the literature (i.e., AD-995). To monitor the strain field and the failure process of the alumina ceramic during testing, an ultra-high-speed camera coupled with digital image correlation (DIC) was used to visualize crack initiation and propagation processes, and obtain quantitative stress-strain information. A new data processing method was then proposed in this study to calculate the shear components for the compression-shear tests. Validation of the proposed method was confirmed by the shear strain obtained from the DIC analysis with the ultra-high-speed camera. Using the results obtained by the proposed model and the DIC analysis, new observations and understandings of failure mechanisms are obtained. (1) In compression-shear tests, the shear failure happens before complete failure, and shear behavior plays an important role during the failure process. (2) The equivalent peak stress (strength) of compression-shear test is smaller than the uniaxial compression one. (3) The directional cracks have weak influence on the compressive stiffness, but have a strong influence on the shear response.

Numerical simulation of stress wave propagation in joint rock specimens with cavity defects

The process of crack initiation, propagation, and coalescence is the essential cause of rock failure. A three-dimensional numerical model based on microscopic damage mechanics is adopted to simulate the failure process and acoustic emissions (AEs) of a jointed rock mass containing a pre-existing hole subjected to stress waves. The numerically simulated results demonstrate that transmission energy plays an important role in the failure process of specimens. The greater the energy of joint transmission is, the greater the damage to the joint transmission area of the rock mass is. Furthermore, the joint width could significantly influence crack propagation patterns and the damage of the joint transmission area of rock specimens. Moreover, the degree of damage to the local joint transmission area of the rock mass is small but then becomes more obvious when the joint angle grows larger. In addition, the wavelength of the stress wave can also affect the failure modes of the rock when stress waves are applied. As the wavelength of the stress wave reduces, the larger the damage of the rock mass is and the smaller the effect of the joint on crack propagation is. Finally, the numerical results demonstrate that the width of the specimen has a significant effect on its dynamic failure mode and degree, showing an obvious size effect. This finding could explain the lateral growth of an existing flaw in its own plane, which is a phenomenon that has not been observed in laboratory experiments.

Effects of biochar particle size and dosage on the desiccation cracking behavior of a silty clay.

Desiccation cracking can significantly change the integrity of soils, and potentially result in the instability of infrastructure as well as the migration of contaminants. Biochar is regarded as a promising low-carbon material for geotechnical applications, including cracking prevention. This study investigates the effects of biochar particle size and dosage on the desiccation cracking characteristics of a silty clay. For samples with fine biochar particles (<0.25 mm), coarser primary cracks initiate first, followed by finer secondary cracks regardless of biochar dosage. Quantitative analysis of the cracking characteristics at the stable stage shows that the surface crack ratio, the number of crack segments, the total length of cracks and the average width of cracks decreased by 31.29%, 30.78%, 14.18%, and 20.45% after 10% biochar addition. For samples with coarse biochar particles (>0.25 mm), cracks initiate simultaneously on the soil surface, and primary and secondary cracks are difficult to distinguish after drying, especially in high dosage samples. In the presence of 10% biochar, the surface crack ratio and average width of cracks decreased by 28.64% and 62.84%, but the number of crack segments and total length of cracks increased by 163.39% and 42.13%. Microstructure and image processing analysis of soil cracks indicate that biochar affects the crack initiation and propagation process by altering the soil microstructure and thereby the crack parameters. The contact between biochar and soil particles transitions from close contact to loose contact as the size of the biochar particles increases. In general, the application of 10% biochar with fine particle size had the best performance in inhibiting soil cracking.

Characterization on Crack Initiation and Early Propagation Region of Nickel-Based Alloys in Very High Cycle Fatigue

As nickel-based alloys are more and more widely used in engineering fields for bearing cyclic loadings, it is necessary to study their very-high-cycle fatigue (VHCF) properties. In this paper, the fatigue properties of nickel-based alloy 625 were investigated using an ultrasonic fatigue test apparatus. The fracture microscopy shows that around the crack initiation site there are two characteristic zones, a rough area (RA) and a fine granular area (FGA). Inclusions caused the interior fatigue crack initiation, and the coalescence of neighboring micro cracks was strongly influenced by the local microstructure, resulting in the RA morphology. Subsequently, the contact and compressing of the crack surfaces contributed to the formation of the FGA. Finally, the stress intensity factors of the RA and FGA were quantitatively evaluated for further discussion of the crack initiation and propagation processes.

Mechanism of artificial surface defect induced cracking for very high cycle fatigue of Ti alloys

In-situ observation of crack growth and electron backscatter diffraction observation of microstructure around artificial surface defect and at mated crack surfaces are conducted for very high cycle fatigue (VHCF) of TC17 alloy. The artificial surface defect induced VHCF failure presents no slow process of crack initiation and early growth accompanied with nanograin formation commonly observed for internal cracking induced VHCF. It is attributed to the synergistic effect of cyclic stress with time dependent process (e.g. the influence of moisture, action of hydrogen, etc.). The nanograins and deformation twins are also observed in some α grains at or near crack surfaces.

An innovative and flexible approach to fabricate Mg/Al composite plates: Cold spraying and hot rolling post-treatment

In this work, an innovative and flexible approach was used to fabricate Mg/Al composite plates. Instead of using ready-made Al alloy sheets, 7075 Al powder was directly deposited on AZ31B Mg plates by cold spraying (CS), resulting in well-bonded Mg/Al composite plates (As-Sprayed). The As-Sprayed composite plates were then subjected to hot rolling (HR) to further enhance the mechanical properties (As-Rolled). The interfacial microstructures, texture evolution, intermetallic compounds (IMCs) formation, mechanical properties and fracture behavior of the As-Sprayed and As-Rolled Mg/Al composite plates were systematically investigated both in the rolling direction (RD) and transverse direction (TD). The results showed that the mechanical properties of the Mg/Al composite plates (prepared by CS + HR) are comparable to or even better than those of their counterparts prepared by traditional roll bonding, explosive welding and diffusion bonding processes, etc. The As-Sprayed composite plates exhibited the highest shear bond strength due to the strong mechanical interlocking at the interface, which was induced by the CS process. The HR strengthened the bonding between Al–Al particles and resulted in grain refinement of AZ31B Mg layer. In addition, strip-distributed IMCs were formed at the interface of the HR sample. Consequently, the ultimate tensile strength (UTS) of the constituent Al and Mg sheets as well as the composite plate were increased, while the shear bond strength was slightly reduced. Moreover, the process of crack initiation and propagation in the composite plates was studied in detail to understand the fracture mechanism.

Fatigue behaviors of a titanium alloy in the VHCF regime based on a vibration‐based bending fatigue test

AbstractA vibration‐based bending fatigue test based on electrodynamic shaker is conducted in order to study the high cycle and very high cycle fatigue (VHCF) behaviors of a titanium alloy TC17 with a sort of novel specimens. After obtaining a full S‐N curve covering the lifetime up to 109 cycles, several fatigue crack initiation and failure process modes are revealed based on the observation of the fracture surfaces. Especially, the statistical distribution characteristics of the fatigue lifetime and some relevant factors are discussed. The results indicate that the obtained S‐N curve does not exhibit a plateau region in the VHCF regime. And the present crack initiation mechanism involves the facets formed by the primary α‐phase cleavage and the development of microcracks. Moreover, the scatter of the fatigue lifetime can be obviously influenced by the resonance frequency range of the specimens.