Transgranular Crack Propagation Research Articles

A thermodynamic framework for ductile phase-field fracture and gradient-enhanced crystal plasticity

This study addresses ductile fracture of single grains in metals by modeling of the formation and propagation of transgranular cracks. A proposed model integrates gradient-extended hardening, phase-field modeling for fracture, and crystal plasticity. It is presented in a thermodynamical framework in large deformation kinematics and accounts for damage irreversibility. A micromorphic approach for variationally and thermodynamically consistent damage irreversibility is adopted. The main objective of this work is to analyze the capability of the proposed model to describe transgranular crack propagation. Further, the micromorphic approach for damage irreversibility is evaluated in the context of the presented ductile phase-field model. This is done by analyzing the impact of gradient-enhanced hardening considering micro-free and micro-hard boundary conditions, studying the effect of the micromorphic regularization parameter, evaluating the performance of the model in ratcheting loading and testing its capability to predict three-dimensional crack propagation. In order to solve the fully coupled global and local equation systems, a staggered solution scheme that extends to the local level is presented.

Phase-field modeling of anisotropic crack propagation based on higher-order nonlocal operator theory

This paper presents a novel higher-order nonlocal operator theory for the phase-field modeling of brittle fracture in anisotropic materials. Incorporating higher order nonlocal operators can enhance the accuracy of the phase-field model by effectively capturing long-range interactions that hold significance in numerous materials. The reproducing kernel particle method is employed to derive a nonlocal differential operator to enhance computational stability and accuracy. Moreover, the proposed method eliminates the need for direct computation of derivatives of the modified kernel function, which avoids the calculation of moment matrix derivatives and improves computational efficiency. The phase-field modeling of polycrystalline materials, considering the anisotropic fracture resistance of each grain, is implemented using this numerical framework. The present method is able to capture different scenarios intergranular and transgranular crack propagation patterns in polycrystalline materials. The proposed method involves a detailed representation of the complex process of crack initiation and propagation in 2D and 3D models of polycrystalline materials.

Study on the effect of grain size on the microstructure evolution of Q345 steel plate under high-speed impact

This study used a two-stage light gas gun to launch reactive fragments at high speeds into steel plates with different grain sizes (large and small). In addition, utilizing material characterization technologies like SEM, EBSD, and TEM, the microstructure evolution of steel plates with varying grain sizes was measured and analyzed both before and after impact. The results indicate that small-grain steel plates are more prone to intergranular or transgranular crack propagation, and they produce more macroscopic and microscopic cracks on the perforated surface under high-speed impact than large-grain steel plates. At the grain boundaries of both large and small grains of steel plate, as well as high-density dislocations, the impact process produces a significant number of minor angles with local misorientation ranging from 2 to 10°. A small-grain steel plate has an obvious grain boundary-strengthening effect. Small grains are more likely than large grains to cause changes in the crystal structure of BCC-HCP close to the perforation, and some grains form a lot of nanoscale elongated structures with the same crystal structure as the matrix. The interaction between dislocations or the emission of a large number of dislocation cells from grain boundaries may occur within some of the larger grains. The study's findings, which are illustrated in this article, can be used as a reference to understand how grain size influences the steel plates' microstructure changes after impacts.

The Tensile Properties and Fracture Toughness of a Cast Mg-9Gd-4Y-0.5Zr Alloy

Low fracture toughness has been a major barrier for the structural applications of cast Mg-Gd-Y-Zr alloys. In this work, the tensile properties and fracture toughness of a direct-chill-cast Mg-9Gd-4Y-0.5Zr (VW94K) alloy were investigated in different conditions, including its as-cast and as-homogenized states. The results show that the tensile properties of the as-cast VW94K alloy are greatly improved after the homogenization treatment due to the strengthening of the solid solution. The plane strain fracture toughness values KIc of the as-cast and as-homogenized VW94K alloys are 10.6 ± 0.5 and 13.8 ± 0.6 MPa·m1/2, respectively, i.e., an improvement of 30.2% in KIc is achieved via the dissolution of the Mg24(Gd, Y)5 eutectic phases. The initiation and propagation of microcracks in an interrupted fracture test are observed via an optical microscope (OM) and scanning electron microscope (SEM). The fracture surfaces of the failed samples after the fracture toughness tests are examined via an SEM. The electron backscatter diffraction (EBSD) technique is adopted to determine the failure mechanism. The results show that the microcracks are initiated and propagated across the Mg24(Gd, Y)5 eutectic compounds in the as-cast VW94K alloy. The propagation of the main cracks exhibits an intergranular fracture pattern and the whole crack propagation path displays a zigzag style. The microcracks in the as-homogenized alloy are initiated and propagated along the basal plane of the grains. The main crack in the as-homogenized alloy shows a more tortuous fracture characteristic and a trans-granular crack propagation behavior, leading to the improvement of the fracture toughness.

Isothermal low-cycle fatigue, fatigue–creep and thermo-mechanical fatigue of SiMo 4.06 cast iron: Damage mechanisms and life prediction

A series of strain-controlled Low-Cycle Fatigue tests (LCF) and Thermo-Mechanical Fatigue tests (TMF) were performed on silicon-molybdenum ductile cast iron SiMo 4.06. LCF tests were performed for various mechanical strain amplitudes at a high strain rate of 3×10−3/s for various temperatures between 20 °C and 750 °C. In addition, several LCF tests were performed for a low strain rate of 1×10−4/s and as fatigue–creep tests with a tensile strain dwell. TMF tests were performed as out-of-phase and fully constrained between 100 °C and 650 °C. The investigation of the damage mechanisms revealed that the predominant failure mode is a combination of graphite-matrix debonding and transgranular crack propagation through ferritic matrix. Finally, a novel damage model based on the dissipated hysteresis energy is proposed in order to effectively predict the lifetime of SiMo 4.06 under LCF and TMF loading. Creep and fatigue damage are each calculated separately and the oxidation effect is taken into account indirectly.

Different augmentations of absorbed hydrogen under elastic straining in high-pressure gaseous hydrogen environment by as-quenched and as-tempered martensitic steels: combined experimental and simulation study

Hydrogen embrittlement (HE) behavior of as-quenched and as-tempered martensitic steels under elastic straining in a high-pressure gaseous hydrogen environment was studied for the first time. The difference in the total content of defects acting as H trap sites has the highest contribution to the increase in absorbed hydrogen content under the elastic loading. The hydrogen-induced fracture occurred in the as-quenched specimen during elastic straining in hydrogen environment, while the tempered specimen did not fracture under the elastic loading. Higher accumulation of the dislocations near the main crack initiation sites (prior austenite grain boundaries (PAGBs)) in the as-quenched specimen will be the main source of the hydrogen to the potential flaw to crack to initiate. H accumulated near PAGBs enhances dislocation slip along {011} planes and helps transgranular crack propagation. Due to less possibility to provide the critical local amount of hydrogen at PAGBs for the crack to initiate, the as-tempered specimen remained unfractured.

A comparison on isothermal and thermomechanical fatigue behavior of 316LN stainless steel with various tension dwell time

Isothermal and thermomechanical fatigue tests with various tension dwell time (designated as creep-IF and creep-TMF respectively) were conducted. The cyclic stress response and stress relaxation behavior were qualitatively and quantitatively discussed based on TEM and EBSD characterization. The fatigue dominated transgranular crack initiation and propagation led to specimen fracture in all creep-IF tests. The transition of dominant damage mechanism from fatigue to creep-fatigue interaction occurred when dwell time increased to 1800 s in creep-TMF test, which caused the intergranular fracture mode and a much shorter life than creep-IF. Moreover, the life prediction model based on tension strain energy was proposed.

Influence of build orientation on high temperature fatigue crack growth mechanisms in Inconel 718 fabricated by laser powder bed fusion: Effects of temperature and hold time

Inconel 718 manufactured by Laser Powder Bed Fusionhas been characterized in terms of the microstructure, effect of post-processing heat treatment, and hence dependence of fatigue crack growth (FCG) on build orientation. Long crack fatigue tests were conductedat different temperatures and frequencies. It was found that at 350 ⁰C (transgranular crack propagation) build orientation had no effect on FCG rates. However,at higher temperatures (time-dependent crack propagation)build orientation had a significant effect on FCG rates.This was primarily related to microstructural differences caused by the printing strategy in grain size distribution, grain shape and grain boundary character.

Crack initiation and short crack propagation of friction stir welded TC17 alloy joint

Heterogeneities of microstructure, tensile and fatigue behavior of FSWed titanium alloy joint were in-situ studied by utilizing replica, DIC and electron back-scattered diffraction methods. Banded TMAZ consisting of needle-like α precipitates and elongated αp within DRXed β grains bridges the HAZ and SZ. Fine β grains in SZ contribute to the highest tensile strength in the 3rd layer. Fatigue cracks prefer to nucleate from HAGBs and {112} 〈111〉 slip deformation of β structure in the 2nd layer of SZ below 210 MPa and 3rd layer of HAZ otherwise. Transgranular crack propagation along the {112} planes exhibits a higher crack growth rate in HAZ than SZ.

Creep-fatigue of P92 in service-like tests with combined stress- and strain-controlled dwell times

Complex service-like relaxation- and creep-fatigue tests with strain- and stress-controlled dwells and fatigue cycle durations of approx. 2200 s were performed exemplarily on a grade P92 steel at 620 °C in this study. The results indicate deviations in the prevailing creep mechanisms of long-term relaxation and creep dwells, affecting subsequent dwells, load shifts, and the macroscopic softening behavior quite differently. In addition, fracture surfaces and longitudinal metallographic sections reveal intergranular crack growth for complex loading with stress-controlled dwells, whereas complex strain-controlled tests enhance oxidation and transgranular crack propagation. These findings substantiate the limited transferability of relaxation-fatigue to creep-fatigue conditions.

Fracture in lightweight magnesia spinel refractory using heterogeneous simulation approach with a multiscale framework

The fracture behaviour of lightweight magnesia spinel refractory is simulated in this work and compared with common one with dense aggregates to demonstrate the influence of porous composite aggregate. With the consideration of aggregate-matrix microstructure as well as the high heterogeneity of refractory and porous composite aggregate, a microstructure-based heterogeneous continuum modelling strategy is proposed for establishing respectively both the meso-scale interfacial model and refractory multiscale wedge splitting test model. Due to the existence of fine pores and two mineral phases of periclase and spinel, the porous composite aggregate has rougher surface and localized microcracks caused by the thermal misfit of phases. The better interlocking of aggregate/matrix interface and diminishment of wall effect promote the propagation of transgranular cracks. Although the main failure is still in matrix and along interface, the sub-fracture process zone development within the porous composite aggregate further dissipates the energy, which results in the higher fracture energy compared with common magnesia spinel refractory. Two kind of magnesia spinel refractory present very different fracture mechanism.

Effects of Grain Boundary Engineering on the Microstructure and Corrosion Fatigue Properties of 316L Austenitic Stainless Steel

Grain boundary engineering (GBE) treatment was performed through thermomechanical processing (TMP) to optimize the grain boundary character distribution (GBCD) of 316L austenitic stainless steel. The effects of TMP on the GBCD and corrosion fatigue properties in high-temperature and high-pressure water were investigated. The results indicated that a high fraction (about 74%) of special boundaries as well as the interrupted network of random high-angle grain boundaries were obtained through 5% strain followed by annealing at 1,273 K for 90 min. The Σ9 and Σ27 boundaries were generated by the reaction of special boundaries. The highest corrosion fatigue life for 3,187 cycles was obtained when the TMP parameters of the 316L ASS were of 5% strain, annealing temperature of 1,273 K, and annealing time of 45 min. The low-energy special boundaries had strong intergranular corrosion resistance, but the strength of these boundaries was not enough to resist the propagation of transgranular fatigue cracks.

Crystallographic effects on transgranular chloride-induced stress corrosion crack propagation of arc welded austenitic stainless steel

The effect of crystallography on transgranular chloride-induced stress corrosion cracking (TGCISCC) of arc welded 304L austenitic stainless steel is studied on >300 grains along crack paths. Schmid and Taylor factor mismatches across grain boundaries (GBs) reveal that cracks propagate either from a hard to soft grain, which can be explained merely by mechanical arguments, or soft to hard grain. In the latter case, finite element analysis reveals that TGCISCC will arrest at GBs without sufficient mechanical stress, favorable crystallographic orientations, or crack tip corrosion. GB type does not play a significant role in determining TGCISCC cracking behavior nor susceptibility. TGCISCC crack behaviors at GBs are discussed in the context of the competition between mechanical, crystallographic, and corrosion factors.

Deformation mechanisms of selective laser melted 316L austenitic stainless steel in high temperature low cycle fatigue

To investigate the low cycle fatigue (LCF) properties of SLM (Selective laser melting) 316L at 550 °C high temperature, a series of LCF tests at different strain amplitudes is conducted on SLM 316L and traditional 316L. In contrast to the cyclic hardening behavior of traditional 316L, SLM 316L is shown to have a stable cyclic softening behavior and higher fatigue life due to its higher strength (yield strength is 1.9 times of traditional 316L). The coarsening of cellular sub-structure, evolution of geometrically necessary dislocations (GND) and texture direction contribute to the cyclic softening behavior of SLM 316L. Fractographic observations illustrate that strain amplitude has a significant influence on initiation and propagation of transgranular fatigue cracks, and lack-of-fusion defects near the surface are key initiation sites of fatigue cracks. Additionally, the life prediction method of the non-Masing model is suitable for SLM 316L.

A comparative fracture analysis on as-received and electrochemically hydrogen charged API X60 and API X60SS pipeline steels subjected to tensile testing

In this study, a comparative analysis of fracture surfaces in API X60 and API X60SS pipeline steels is carried out after hydrogen charging followed by tensile testing. The JIS test setup is also utilized in order to measure the amount of discharged hydrogen through the microstructure of both steels. EBSD measurements are also done on as-received and hydrogen charged specimens in order to observed the HIC propagation. The results show that a considerable amount of hydrogen is able to diffuse through the microstructure of both steels. The tensile testing after hydrogen charging proves that hydrogen embrittlement occurs significantly in both steels. At the center of cross section in both steels, the fracture occurs by micro-voids formation, their growth and coalescence around the different types of inclusions. In charged specimens, dimples are shallower in both specimens compared with the dimples in the fracture surfaces of as-received specimens. Comparatively, hydrogen embrittlement in API X60 steel is more noticeable and its surface is more flat than the API X60SS steel. In hydrogen-charged API X60 steel, EBSD map shows a crack stepwise jumping with 45° from one crack traction line to another, with both intergranular and transgranular crack propagation mechanisms. Moreover, the crack propagates intergranularly along gains with mismatch Taylor values, while transgranular propagation mostly observed within the hard grains with high amount of dislocation density and internal energy associated with KAM map. In the case of API X60SS steel, no evidence of HIC crack was observed.

Investigation of the fatigue damage mechanism of Inconel 617 at elevated temperatures obtained by in situ SEM fatigue tests

In the present study, fatigue damage evolution of Inconel 617 at 600 °C and 700 °C was investigated by using an in situ scanning electron microscope (SEM) fatigue test. The fatigue crack initiation and propagation processes were directly observed and recorded online. Fractography and microstructure analyses were performed to further reveal the mechanism of high-temperature failure. Experimental results showed that Inconel 617 was not sensitive to an artificial notch even if the curvature reached approximately a hundred microns, and it was more vulnerable to intergranular fracture. This alloy showed multiple crack initiation and propagation behavior at high temperatures. Multiple crack initiation sites did not occur in the same planes, and the number of crack initiation sites increased with increasing environmental temperature. Intergranular cracking behavior was observed in the near surface region, and transgranular crack initiation and propagation were mainly observed in the interior. Coarsening and discrete M23C6-type carbides formed at grain boundaries were the main factors responsible for intergranular cracking. The twin boundary (TB) was directly observed as the crack initiation site, and it retarded crack propagation along the TB and changed the crack propagation mode.

The evolution of the microstructure and mechanical properties of coral aggregate mortar under uniaxial compression using ultrasonic analysis

This study investigated the compression behavior of coral aggregate mortar (CAM) under uniaxial loading using ultrasonic pulse velocity measurements. Silica sand mortar (SSM) was used for comparison. The results indicated that the compressive strength of the CAM was slightly higher than that of the SSM prepared with similar-sized silica sand. Numerous open pores in the porous structure of the coral aggregate provided water reservoirs for internal curing, enhancing the solid structure and improving the compressive strength. Since the grain strength is related to the grain size, which decreased with decreasing grain size, the compressive strength of the CAM decreased with an increase in the size of the coral aggregate. The ultrasonic wave velocities were stable around 4200 m/s for the P-wave and 2100 m/s for the S-wave before fracturing and dropping sharply due to the brittle failure of the SSM and the CAM. The compression behavior of the CAM and the SSM could be divided into five stages. The cumulative voltage energy indicated that the microcrack evolution of the two mortars was significantly different. Intergranular and transgranular crack propagation occurred in the CAM and depended on the particle strength and number of internal pores of the coral aggregate. We discuss in detail the mechanism between the cementitious materials and the pores of the coral sand mortar. The results can provide a reference for improving the strength of coral sand cement-based materials.

Mitigation of hydrogen embrittlement in alloy custom age 625 PLUS® via grain boundary engineering

Hydrogen embrittlement is the deterioration of mechanical properties in a metal exposed to hydrogen, often characterized by brittle, intergranular fracture at low applied stresses. While grain boundary engineering has been applied to mitigate this issue, ambiguity in the mechanisms behind hydrogen embrittlement leads to ambiguity in the mechanism by which grain boundary engineering helps to mitigate this problem. In this study, grain boundary engineering was applied to improve resistance to hydrogen embrittlement in Custom Age 625 PLUS®, an alloy frequently used in corrosive environments where hydrogen embrittlement is of particular concern. Iterative low strain cold rolling followed by annealing at intermediate temperature successfully produced a grain boundary engineered microstructure with large twin-related domains and a high fraction of interconnected coincident site lattice (CSL) boundaries. Rising step load testing demonstrated that grain boundary engineering increased the stress intensity at which failure from hydrogen embrittlement occurred and caused a shift from intergranular to transgranular crack propagation. Evidence of localized plasticity on fracture surfaces suggest that hydrogen-enhanced localized plasticity (HELP) is the dominant mechanism of hydrogen embrittlement.

Analysis of the fully-reversed creep-fatigue behavior with tensile-dwell periods of superalloy DZ445 at 900 °C

High total strain-controlled creep-fatigue tests of 1.8% were performed for a directionally-solidified Ni-based superalloy, DZ445, at 900 °C in air. Dwell times of 0, 2, and 5 min. were introduced at the tensile peak strain for each cycle. Experimental results show a decrease in fatigue life with increasing dwell time. The significant cyclic softening is observed after the application of the dwell time. The mean compressive stress and large plastic-strain accumulation are produced due to the stress relaxation in the tensile-dwell period, resulting in the increase of the hysteresis loop area. The typical single-source crack-initiation and transgranular crack-propagation are transformed into multi-source crack-initiation and mixed transgranular and intergranular crack-propagation mode by these characteristics. Dislocation features are transformed from cutting of γ' precipitates by multiple stacking faults to the mixed mode of dislocation climbing and slipping. The reduction in the fatigue life with increasing the dwell time is well explained on the basis of the mechanical response and damage mechanism. The fatigue life is also predicted by the energy-based life-prediction model.

Evaluation of the surface fatigue behavior of amorphous carbon coatings through cyclic nanoindentation

Diamond-like carbon (DLC) coatings, frequently used to reduce wear and friction in machine components as well as on forming tools, are often subjected to cyclic loading. Doping of DLC coatings with metals or metal carbides as well as the usage of multilayer architectures represent promising approaches to enhance toughness, which is beneficial for the coatings' behavior under cyclic loading. In this study, we utilized cyclic nanoindentation to characterize the tribologically induced surface fatigue behavior of single-layer tungsten-doped (a-C:H:W) and multilayer silicon oxide containing (a-C:H:Si:O/a-C:H)25 amorphous carbon coatings under cyclic loading. Columnar growth was observed for both coatings by focused ion beam microscopy and scanning electron microscopy, while the multilayer architecture of the (a-C:H:Si:O/a-C:H)25 coating was verified by the silicon content using glow-discharge optical emission spectroscopy. In cyclic nanoindentation of the (a-C:H:Si:O/a-C:H)25 multilayer coating, stepwise small changes in indentation depth were observed over several indentation cycles. The surface fatigue process of the single-layer a-C:H:W covered a smaller number of indentation cycles and was characterized by an early steep increase of the static displacement signal. Microscopical analyses hint at grain deformation, sliding at columnar boundaries, and grain detachment as underlying fatigue mechanisms of the a-C:H:W coating, while the (a-C:H:Si:O/a-C:H)25 multilayer coating showed transgranular crack propagation and gradual fracturing. In case of the (a-C:H:Si:O/a-C:H)25 multilayer coating, superior indentation hardness (HIT) and indentation modulus (EIT) as well as a higher HIT3/EIT2 ratio suggest a higher resistance to plastic deformation. A high HIT3/EIT2 ratio, being an indicator for hindered crack initiation, combined with the capability of stress relaxation in soft layers contributed to the favorable surface fatigue behavior of the (a-C:H:Si:O/a-C:H)25 multilayer coating observed in this cyclic nanoindentation studies.