Crack Initiation Mechanism Research Articles

Crack initiation mechanism of laser powder bed fusion additive manufactured Al-Zn-Mg-Cu alloy

Crack is a serious defect limiting the additive manufacturing of metal materials. The morphologies and crystallographic orientation of cracks for Al-Zn-Mg-Cu alloy prepared by laser powder bed fusion (LPBF) were clarified. The mechanisms of crack initiation and propagation were expounded. Results show that the cracks were crisscrossed horizontally and propagated through multilayers along the building direction in the as-printed specimens. The cracks were classified as solidification cracks, which occurred in the final stage during solidification when the liquid fraction was <20%. Al-Zn-Mg-Cu alloy has a wide temperature range between the liquidus and solidus, leading to high cracking susceptibility. Solidification cracks were prone to initiate at high-angle grain boundaries (HAGBs) and terminate at low-angle grain boundaries (LAGBs) or within grains. LPBF process could affect crack initiation and propagation by refining the grain size and inhibiting the growth orientation. Solidification and thermal shrinkage of adjacent grains during solidification provided subjective factors for cracking initiation. Directional preferential growth of dendrites along the 〈001〉 crystal orientation and the untimely filling of the liquid phase provided secondary factors. This study can provide theoretical guidance for crack suppression in additive manufacturing of 7075 aluminum alloy and other metals with high cracking susceptibility. • Crack initiation and propagation mechanisms of Al-Zn-Mg-Cu alloy in LPBF were investigated. • Internal cracks of as-printed specimens are solidification cracks. • Cracks occur in the final stage during solidification. • Oriented crystallization and texture affect the initiation and propagation of cracks. • Cracks initiate at straight HAGBs and terminate at LAGBs or within grains.

Mechanisms of intergranular fatigue crack initiation in polycrystalline materials

Fatigue failure in polycrystalline materials occurs by initiation and propagation of fatigue cracks. Fatigue cracks can initiate transgranularly or intergranularly. While considerable progress has been achieved in the explanation of the mechanism of transgranular fatigue crack initiation intergranular initiation is still the subject of controversy. A recent study of the crack initiation in superaustenitic Sanicro 25 steel [1] revealed the important role of persistent slip markings (PSMs) in the grains adjoining the grain boundary. A novel mechanism of intergranular fatigue crack initiation has been proposed. Provided the cyclic slip cannot proceed from one grain to the neighbour one, extrusions and intrusions arise not only on the surface but also on the grain boundary. Growing intrusions represent void-like defects and growing intrusions push neighbour grain apart. A systematic study of intergranular cracking has been performed on polycrystalline copper cycled with high and low strain amplitudes. SEM observations of the early stages of fatigue crack initiation on the grain boundaries, EBSD study of grains orientation, FIB cutting of the grain boundaries and simultaneous documentation of the effect of extrusions and intrusions on the grain boundary cohesion are reported simultaneously with the conditions favourable for the initiation of the fatigue cracks.

The effect of loading mode on fatigue crack initiation of IN718 superalloy

The article deals with the effect of applied fatigue load with different cycle asymmetry parameters R, test temperature and loading frequency on fatigue crack initiation in IN718 superalloy. For this purpose, four sets of experimental samples were produced. The first and second sets of samples with cycle asymmetry parameter R = -1 were subjected to fatigue tests at loading frequency f ≈ 20,000 kHz and ambient temperature, respectively f ≈ 64 Hz and a temperature of 700°C. The third and fourth sets of samples with cycle asymmetry parameter R < 1 with loading frequency f ≈ 150 Hz and ambient temperature, respectively after applying annealing 800°C/72 hours. SEM fractography analysis was performed on all samples to define the fatigue crack initiation mechanism.

NONADIABATICALLY DRIVEN SUBCRITICAL CRACK NUCLEATION IN SOLIDS

This paper discusses a subcritical crack initiation mechanism in a brittle solid within a real range of applied stress. A medium deformed by uniaxial tension is considered as an open nonequilibrium system of nuclei and electrons. Structural relaxation of the medium begins with the excitation of dynamic displacements during nonadiabatic Landau-Zener transitions. Dynamic displacements induce the instability of the medium to the longitudinal displacement wave. The kinetics of structural relaxation is described by two nonlinear parabolic kinetic equations for dynamic order parameters. Conditions are derived for the existence of localized solutions (autosolitons). The excitation of autosolitons leads to a local elongation and cross-sectional reduction of the specimen. The resulting neck is a subcritical crack.

Microstructural and oxidation effects on fatigue crack initiation mechanisms in a turbine disc alloy

Effects of microstructure and oxidation on fatigue crack initiation and early propagation processes were investigated in RR1000 turbine disc alloy with different γ′ distributions and carbide distributions on the grain boundary. Fatigue tests were carried out under three-point bending and trapezoidal waveform loading (with a 90 s dwell) at 650 °C in air. The failure mode in both γ′ variants is clearly characterised by intergranular features. A number of fatigue cracks are seen to initiate at grain boundaries with bulged Co-rich oxides at the surface and/or interfaces between carbides and grain boundaries, resulting from oxidation damage assisted by applied loading. Reduced lifetime is closely linked to significant intergranular crack initiation and frequent consequent crack coalescence events, which results in enhanced fatigue crack growth (FCG) rates. The extent of intergranular features and enhanced FCG are more marked where more continuous carbides exist at the grain boundary.

Microscopic mechanisms of coral reef limestone crack propagation

Coral reef limestone (CRL) is a marine sedimentary rock dominated by biogenesis, and its composition as well as internal structure characteristics are different from traditional terrigenous rocks. This article used CT scanning and petrographic thin section observations to visually and quantitatively analyze crack morphological characteristics after triaxial shear failure at a microscopic scale. The analysis revealed the mechanism of crack initiation and propagation from the perspectives of geological genesis and fracture mechanics. Results show that the crack propagation mechanism in the bio-cemented framework includes propagation along crystal boundaries and across crystals. Cracks can be classified as intergranular cracks, transgranular cracks, and bifurcation cracks based on crack morphology. The destruction mode of the specimen is controlled by the pore structure and cemented framework. Based on the pore distribution, the destruction mode can be divided into local tension failure and monoclinic shear failure. Understanding CRL crack propagation mechanisms has important scientific significance and practical engineering value for enriching rock damage theory and long-term stability analysis of island reef engineering foundations.

Effects of microstructure on crack initiation in super martensitic stainless steel under very‐high‐cycle fatigue at elevated temperature

AbstractThe present study envisaged the behavior and relevant crack initiation mechanism of super martensitic stainless steel (0Cr13Ni5Mo) at elevated temperature (ET) under very‐high‐cycle fatigue (VHCF). Fractographic analysis revealed that all the VHCF cracks of the failure specimens initiated from the surface and formed fine granular area (FGA) morphology at ET in an absence of vacuum. The initiation characteristic area (ICA) consisted of an initiation site, a relatively smooth area, and the annular continuous FGA. FGA was formed in the peaks and valleys of local microstructures, and the smooth regions connected these peaks and valleys dominated by shear stress. It was observed that the FGA was unevenly distributed on the crack surface related to the local microstructure and below the grain surface relevant to local strain distribution. The stress intensity factor (SIF) and the crack tip plastic zone of the ICA slightly changed with temperature.

In situ observation of microscopic damage and crack initiation mechanisms in a filled EPDM

The paper presents a precise 3D quantification of damage evolution and eventual crack initiation due to metallic oxide particles and filler agglomerates in a peroxide crosslinked filled EPDM during uniaxial solicitation, thanks to synchrotron radiation X-ray Tomography. An in-situ tensile study using this technique reveals polymer debonding at the poles of all metallic oxide particles upon stretching. The cavities caused by this decohesion do not lead to crack initiation since they grow parallel to the applied stress direction. Conversely, crack always initiates from carbon black agglomerates (CBaggl). The crack initiation mechanism is a three step process that begins with the nucleation of cavities inside the CBaggl upon stretching. This is followed by the growth of these cavities which brings about the fracture of the agglomerates. Finally, this fracture can lead to the creation of a matrix crack at the origin of the material rupture. It is also confirmed that CBaggl that initiate the critical crack in the material are the biggest sized CBaggl and are located close to the edges of the sample.

Failure analysis of aircraft tubular form-fit joints in rotating-bending fatigue tests

Tubular form-fit joints play a key role of connecting and sealing in the pipeline system of aircraft. However, they experience the complex fatigue behavior in operating conditions of compound excitation with internal fluid pulse pressure and external structural high-frequency vibration. To investigate the above fatigue behavior, the tubing component composed of tubular form-fit joints is fabricated by elastomeric swaging firstly; then the rotating-bending experimental platform for the tubing component is built to simulate the operating conditions in the above actual service period, and the rotating-bending fatigue (RBF) tests of tubing components under different deflections are carried out, so that the relationship between the fatigue strength and life is obtained; then, the bending-torsion multiaxial mechanical behavior of tubing component is analyzed when it is filled with constant internal pressure, bent at a deflection and rotated at a high speed in the RBF test. It can be found that the area near the sharp edge of sleeve groove is the most prone to crack initiation. Furthermore, the mechanism causing the fatigue fracture of tubing component is explored at the certain position of the joined tube by means of numerical simulation. Eventually, with the aid of scanning electron microscope, the mechanism of bending-torsion multiaxial fatigue crack initiation and propagation is revealed by fracture morphology observation and analysis. It can be found that the fatigue crack source on the outer wall of the joined tube is various, and the crack propagation direction is from the outer wall to the inner wall along the radial direction.

Crack initiation mechanisms in Ti-6Al-4V subjected to cold dwell-fatigue, low-cycle fatigue and high-cycle fatigue loadings

Although Ti-6Al-4V is the most commonly used titanium alloy in the aerospace industry, the mechanisms governing crack initiation in the different fatigue regimes are not well understood yet. This situation partly pertains to a competition between multiple crack initiation mechanisms. In particular, applied loading conditions were identified as a key parameter governing the transition between mechanisms. The fatigue behavior of Ti-6Al-4V with a bi-modal microstructure was investigated in the present study using different waveforms, load ratios and frequencies to clarify this feature. A detailed characterization of the main crack initiation sites was carried out to identify microstructural configurations governing crack nucleation in low-cycle dwell-fatigue, low-cycle fatigue and high-cycle fatigue regimes. While lifetimes and fracture surfaces showed a good consistency with data from prior studies, a single microstructural configuration was found involved with the formation of crack initiation facets. All investigated cracks leading to specimen failure were nucleated along (0001) twist grain boundaries exhibiting similar features. Based on this finding, a criterion is proposed to identify candidates for crack initiation. This also demonstrates no major sensitivity of the critical microstructural configurations to environment, free surface, loading conditions and, as shown in previous studies, microstructure and composition. However, conventional fatigue failure results from one, or a few, surface or subsurface crack initiation sites while dwell-fatigue failure involves the formation of multiple internal cracks. This is accompanied by a significant dwell-fatigue life debit at peak stress magnitudes close to the yield strength. Features of microstructural configurations prone to crack nucleation are finally compared with data reported in existing literature to propose a mechanism of crack formation in Ti alloys.

Planar-Biaxial Thermo-mechanical Fatigue Behavior of Nickel-Base Superalloy Inconel 718 Under Proportional Loading

The planar-biaxial thermo-mechanical fatigue behavior of nickel-base superalloy Inconel 718 was studied for selected proportional loading conditions, in particular biaxial strain ratios of 1.0, 0.6, and − 1.0. The cyclic temperature loading with minimum and maximum temperatures of 400 °C and 630 °C and a duration of 250 seconds was either In-Phase or Out-of-Phase to the mechanical axes. Besides the multiaxial tests, uniaxial thermo-mechanical fatigue tests were conducted In-Phase and Out-of-Phase with the same temperature cycle and cycle duration. The performed thermo-mechanical fatigue tests were analyzed regarding the deformation and lifetime behavior and compared with high-temperature isothermal low-cycle fatigue tests from a previous work of the authors. On the side of the lifetime description, a strain- and a stress-based approach were presented. For the planar-biaxial tests, the crack initiation mechanism and crack paths were shown.

Analysis of interfacial crack initiation mechanism of thermal barrier coatings in isothermal oxidation process based on interfacial stress state

In this paper, the interfacial stress state is used to analyze the interfacial crack initiation mechanism of the thermal barrier coatings (TBCs) during isothermal oxidation. The influence of thermal growth stress, initial residual stress, and creep behavior on the stress distribution is considered to have an accurate simulation result. A parameter that integrates the effects of interfacial normal and tangential stress is modified for evaluating interfacial crack initiation. It is found that, in the cooling stage, the interfacial cracks sprout at the top coat (TC)/thermally grown oxide (TGO) interface valley region and the TGO/bond coat (BC) interface peak region, which agrees with the experimental results. Furthermore, the influence of interfacial roughness on crack initiation is investigated. The result shows that different interfacial roughness affects the sprouting region of interfacial cracks and cracks within the TC layer.

Microscale fatigue mechanisms in high silicon alloyed nodular cast iron

In high silicon alloyed nodular cast iron, the graphite morphology influences the fatigue strength in absence of large casting defects. However, the underlying microscale fatigue mechanisms are rarely discussed in the literature. This paper contributes to a deeper understanding of the microscale fatigue mechanisms. Flat bending tests of alloys with varying graphite morphologies were carried out. Several specimens in the transition region of the S–N curve were subjected to quasi in-situ optical and electron microscopic analyses. Results show a strong dependence of crack initiation on graphite morphology. Crack initiation and propagation mechanisms are highly grain orientation dependent.

Tensile properties of cement-stabilised clays and their contribution to seawall design

Deep cement mixing is a typical ground improvement technique, and has recently been introduced in some large-scale reclamation projects in Hong Kong. There is a lack of internationally recognised testing standard for determining the tensile strength of cement-stabilised soil. In this study, tensile properties of cement-stabilised clays have been investigated using direct tension test and Brazilian test. The interpretation of tensile strength in Brazilian test is not straightforward due to the formation of multiple cracks during loading. As such, focus is placed on the validation of the fundamental assumption on crack initiation mechanism. A consolidated database of tensile strengths of cement-stabilised soils is compiled. A constitutive model in the finite-element program Plaxis, namely concrete model, has been studied. This model, originally aiming to simulate the behaviour of concrete and shotcrete, duly considers the cracking and the strain-softening characteristics, and is able to reasonably simulate the fundamental tensile behaviour of cement-stabilised soil. Numerical simulations have been carried out using the concrete model to assess the stability of seawall founded on cement-stabilised clay. Tensile properties are found to have an important bearing in maintaining the stability of the seawall when column-pattern stabilisation is adopted.

Porosity-related high-cycle fatigue strength of nickel-base single crystals: Fatigue experiments and electron back-scattered diffraction analysis

The fatigue limit of nickel-base single crystal (NBSX) is experimentally studied at an elevated temperature of 980 °C. The crack initiation position is revealed as the casting pores in the single crystal under scanning electron microscope (SEM), and distinct “fish-eye” region is observed on the long cycle fatigue fracture surface. The failure mechanism is explored around the “fish-eye” by adopting the focused ion beam (FIB) and electron backscatter diffraction (EBSD) technique. The crystalline motion and the fatigue crack initiation mechanism around the initiation pore are revealed by conducting microscopic analyses.

Anisotropy and microcrack-induced failure precursor of shales under dynamic splitting

The dynamic anisotropy and failure mechanism of shales are greatly affected by bedding surfaces. To reveal the influence of beddings on anisotropic characteristics of shales under dynamic impact, the Brazilian splitting tests were conducted by the split Hopkinson pressure bar system. The fracturing process were monitored by the high-speed camera. Meanwhile, to understand crack initiation and propagation mechanism, the stress buildup, stress shadow and stress transfer were modelled based on the digital image processing and the rock failure process analysis method. The effect of dip angle and bedding spacing on crack initiation, propagation and coalescence was analyzed. Simultaneously, the spatial distribution and energy magnitude of crack-induced acoustic emissions were captured numerically. The results show that the shale discs continue to produce parallel cracks and cambered cracks induced by the high stresses at the tips of initial cracks; the tensile strength under dynamic splitting changes in the U-shaped trend with the bedding dip angle increasing; the cracking percentage of bedding surfaces decreases, and the cracking percentage of rock matrix increases with the bedding dip angle increasing. In addition, the acceleration of crack growth and the rapid growth of AE energy can be regarded as the effective precursors of shale failure.

Effect of temperature on low cycle fatigue behavior of T91 steel in liquid lead-bismuth eutectic environment at 150–550 °C

The low cycle fatigue behavior of T91 steel in a lead–bismuth eutectic and an air environment was investigated at 150–550 °C. The temperature dependent “fatigue endurance peak and trough” was found in liquid LBE. Quasi-cleavage cracking was observed on the fracture surface at 150 °C and 350 °C, which indicates the occurrence of liquid metal embrittlement (LME). The plasticity recovery reduced LME susceptibility of T91 steel at 250 °C and 550 °C. The effects of LME, liquid LBE corrosion and dynamic strain aging on the mechanisms of fatigue crack initiation and propagation are discussed.

Interior defect-induced crack initiation mechanism and initial growth behavior for Ti6Al4V alloy fabricated using laser powder bed fusion

The interior defect-induced crack initiation mechanism and early growth behavior of Ti6Al4V alloy fabricated by laser powder bed fusion (LPBF) has been investigated in very high cycle fatigue (VHCF) regime. P–S–N curves under 10% and 90% failure probabilities are obtained in VHCF regime. The cracks inside the early stages of fine granular area (FGA) formation are driven by the maximum shear stress and propagate as Mode II + III mixed cracks. It can be found that the FGA region is composed of many discontinuous nanograins for Ti6Al4V alloys manufactured by LPBF, which are responsible for grain refinement. Grain refinement is associated with dislocation movement within the martensite laths. Dislocation pileup and rearrangement in martensitic laths form dislocation cells, which further develop into nanograins and low angle boundaries. Besides, both the fatigue loading process and the LPBF process form their respective microvoids, which merge and aggregate with each other, thus accelerating the microcrack extension.

Application of Molecular Dynamics Calculations to Elucidation of the Mechanism of Hydrogen-Induced Crack Initiation in Fracture Toughness Tests Using Tempered Martensitic Steels

Application of Molecular Dynamics Calculations to Elucidation of the Mechanism of Hydrogen-Induced Crack Initiation in Fracture Toughness Tests Using Tempered Martensitic Steels

Insights into the characteristics and mechanism of vacuum drying technology for municipal sludge processing

Vacuum drying is an effective approach for sludge treatment and valorization. However, the vacuum drying of sludge has not been industrialized at present. The objective of this study was to elucidate the vacuum drying characteristics of static sludge and crack initiation mechanism. Our results indicate that crusting on the sludge surface under a high vacuum inhibited drying by reducing major cracks at sludge thicknesses of 13.6 and 10.2 mm. The inhibition effect weakened with decreasing sludge thickness. At 6.8 mm, the mean drying rate (VM) was the lowest at 0.08 MPa, while VM decreased with increasing vacuum degree at thicknesses of 13.6 and 10.2 mm. The decrease in drying rate could be attributed to rapid evaporation on the sludge surface under a high vacuum, leading to crusting, which inhibited crack initiation. VM was raised by 67.9–162.2% from 10.2 to 6.8 mm because the suction force of vacuum on water was much higher than the resistance to water diffusion of small isolation piles at 6.8 mm. Additionally, this study provided essential information to improve existing sludge treatment methods.