Sides Of Crack Research Articles

Interior crystallographic plane induced cracking behavior of Ni-based superalloy in high-temperature and vacuum environment

Tension-tension fatigue tests at 750 °C were performed to state on fatigue structure-properties of Ni-based superalloy in long-life regime under high temperature by means of ultra-depth-of-field overlay three-dimensional, scanning electron microscopy, energy dispersive spectrometer, electron back scattered diffraction, focused ion beam and transmission electron microscope. Results show that interior crystallographic plane-fisheye failures are divided into the single plane & multiple planes cracking. The formation of plane with micro-crack initiation and growth is mainly under Mode Ⅱ cracking from the Copper grain with large size and the highest Schmid factor or the Goss grain with small size and high Schmid factor. Moreover, owing to high thermal activation energy, grain-related interior cracking is mainly induced by climbing and bypassing mechanisms with bowing of dislocations and zig-zag dislocations, and assisted by shearing mechanism with tangled dislocations. Finally, the interior plane-fisheye cracking behavior with microstructure in high-temperature and vacuum environment is elucidated.

Concrete failure simulation method based on discrete element method

This paper improves the method of high-efficiency calibrating the meso-parameters considering the concrete aggregate gradation, and carries out numerical simulations of four-point bending strength test and direct shear strength test on concrete beams. Firstly, unconfined uniaxial compression test carried out to calibrate the meso-parameters of C40 concrete based on DEM. Secondly, the accuracy of the numerical model is verified according to the stress-strain curve and failure morphology of concrete prism. Finally, the crack propagation and load-displacement curves of concrete beams under four-point bending strength test and direct shear strength test were analyzed. The result show as follow: ① The DEM numerical model which take into account the concrete as a three-phase composite material can well realize the mechanical property of concrete. The display of force chains and micro-cracks greatly contributes to the concrete failure mechanism. ② The numerical model is generated by the random distribution of particles, which shows the characteristics of concrete heterogeneity, and defines the contact model between particles with parallel bond model (PBM). Compared with the numerical simulation and experiment results, the error is within 10.4%; ③ In four-point bending strength test, the crack initiates from the bottom edge of the beam and propagates upward. With the increase and coalescence of micro-cracks, only one concentrated, wide and long macro-crack is formed; In direct shear strength test, however, the high shear stress of two shear surfaces resulted in the generation of cracks. With the one side crack coalescence, the other side of crack initiate follow, and finally two coalescence macro-cracks are formed.

Effect of Microstructure on Fatigue-Crack Propagation of 18CrNiMo7-6 High-Strength Steel

The effect of microstructure on fatigue crack propagation of 18CrNiMo7-6 high-strength steel was studied by scanning electron microscopy and electron backscatter diffraction techniques. The crack propagation path was related to the microstructure of the original austenite grain boundary, lath martensite and crystal orientation. The crack path was tortuous and many small cracks or extrusions appeared on both sides of the main crack, with most of the extrusions parallel to the martensite lath. When the crack propagation encountered a high-angle boundary or passed through different crystal orientations, the crack was deflected, and when it encountered a low-angle boundary, the crack expanded.

High-Cycle Fatigue Life and Strength Prediction for Medium-Carbon Bainitic Steels

High-cycle fatigue (HCF) behaviors of medium-carbon bainitic steels with various inclusion sizes and microstructural features were studied using the rotating–bending fatigue test. Here, the medium-carbon bainitic steels with different melting processes were treated by three heat treatment routes incorporating bainite formation, namely bainite-based quenching plus partitioning (BQ&P), bainite austempering (BAT) and “disturbed bainite austempering, DBAT”. The interior inclusion-induced crack initiation (IICI) and noninclusion-induced crack initiation (NIICI) modes were found after fatigue failure. The fracture surface of IICI is characterized by a “fish-eye” surrounding a “fine granular area, FGA” in the vicinity of an inclusion. In contrast, a microfacet, instead of an inclusion, is found at the center of FGA for the NIICI fracture surface. The predications of fatigue strength and life were performed on the two crack initiation modes based on fracture surface analysis. The results showed that a majority of fatigue life is consumed within the FGA for both the IICI and NIICI failure modes. The fatigue strength of the NIICI-fatigued samples can be conveniently predicted via the two parameters of the hardness of the sample and the size of the microfacet.

Interior crack initiation during the very‐high‐cycle fatigue of railway wheel steel under axial loading and rolling contact loading

AbstractIn this paper, a comparative study of the very‐high‐cycle fatigue (VHCF) behavior of railway wheel steel under axial loading and rolling contact loading was conducted. Fatigue tests were performed with an ultrasonic fatigue test machine under axial loading, and the fracture surfaces from the fatigue tests and shattered rims taken from the failed railway wheels were observed. The wheel steel under axial loading presents a VHCF behavior with Mode I crack, and that under rolling contact loading is a VHCF behavior with mix Mode II–III crack. For the VHCF behavior with Mode I crack, surface and interior crack initiation occurred with equal probability at both low and high stress levels and produced a dual linear S–N curve since the value of fatigue limits for the surface and interior crack initiation are close. For the VHCF behavior with mix Mode II–III crack, cracks were initiated from the interior Al2O3 inclusion, and the fatigue life was beyond 107 cycles. Fatigue bands were observed on the fracture surface under rolling contact loading. The ferrite nanograins formed due to the stress state of shear plastic strain with a large compressive stress. The formed nanograins were softer than the matrix caused by the redistribution of the carbon.

A generalized solution to the combo-crack problem—II. Remote load

In Part I, we studied the combo-crack problem, which contains an interior penny-shaped crack (PSC) and a coplanar and concentric externally circular crack (ECC), under pressure load applied to the surface of the interior PSC. In fact, a more common type of loading experienced by cracked media might be remote tension normal to the crack plane. For a combo crack under remote tension, a reliable solution remains deficient. In this paper, we extend our discussion to the case of remote tension in the theoretical framework developed in Part I. We for the first time formulate the concerned mixed boundary value problem into a single inhomogeneous Fredholm integral equation, which is then solved numerically with a Gauss-Lobatto quadrature-based algorithm. The normal stress in the ligament (bonded) region on the plane of the crack, the stress intensity factors at the tips of PSC and ECC, and the critical force load for trigging crack propagation are determined, showing an excellent agreement with the finite element results. Our work provides a high-precision numerical solution to the combo-crack problem for the most basic loading case. The obtained results should be of general value to the resolution of a bunch of fracture and adhesion problems in combo-crack configuration in engineering and deserve to be absorbed into the updated handbook for basic crack problems.

The Effect of Temperature, Cooling Rate and Casting Speed on Quality of Continuous Cast Steel Billets

In continuous casting, the cooling rate, casting speed, and molten metal temperature significantly affect the quality of cast steel billets. Appropriate casting parameters can minimize quality problems such as surface, subsurface and interior crack, rhomboidity, oscillation mark depth, and central porosity. This research determines the relationship between defects and three significant factors temperature, cooling rate, and casting speed. The work has been performed on the two-strand continuous casting machine to investigate the billet (BS-4449 steel grade) cross-section of 150 x 150 cm2. The defects analysis through macro examination at different tundish temperatures (1510 °C to 1560 °C), varying cooling rates, and casting speeds (0.9 to 1.6 m/min). The present study provides detailed insight into the three parameters mentioned earlier, which directly affect the quality of cast steel billets.

Defect induced cracking and modeling of fatigue strength for an additively manufactured Ti-6Al-4V alloy in very high cycle fatigue regime

Additively manufactured (AM) alloy usually inevitably contains defects during the manufacturing process or in service. Defect, as a harmful factor, could significantly reduce the fatigue performance of materials. This paper shows that the location and introduced form of defects play an important role in high cycle fatigue and very high cycle fatigue (VHCF) behavior of selective laser melting Ti-6Al-4V alloys. The S-N curve descends approximately linearly for internal defect induced failure. While for artificial surface defect induced failure, the S-N curve descends at first and then exhibits a plateau region feature. The competition of interior crack initiation with fine granular area feature is also observed in VHCF regime. The paper indicates that only the size or the stress intensity factor range of the defect is not an appropriate parameter for describing the effect of defect on fatigue crack initiation. Finally, the effect of artificial surface defect on high cycle fatigue and VHCF strength is modeled, i.e., the fatigue strength σ, fatigue life N and defect size area (square root of the projection area of defect perpendicular to principal stress direction) is expressed as σ=CNaarean,N<N0CN0aarean,N≥N0, where C, a and n are constants, and N0 is the number of cycles at the knee point of the curve.

Microstructure based cracking behavior and life assessment of titanium alloy under very-high-cycle fatigue with elevated temperatures

High and very high cycle fatigue tests for TC11 titanium alloy were performed to clarify the interior cracking behavior under fully reversed and pulsating tension at three service temperatures. According to the estimation of threshold values for small and long crack growth, the fatigue design curves are established. Combined with electron backscatter diffraction and fracture mechanics analysis, the interior cracking is closely related to grain strength and size, and microtexture. The interior failure is attributed to the facet formation caused by larger αp phase fracture. Based on FIP model, a temperature-based method is proposed to predict the crack nucleation life.

Interior long‐life‐fatigue cracking behavior and life prediction of a selective laser melted GH4169 superalloy at different temperatures and stress ratios

AbstractThe axial fatigue tests at elevated (650°C) temperature with stress ratios of 0.1 and −1 were conducted under the loading cycle range of 104–108 cycles to investigate interior cracking mechanism at elevated temperature of one selective laser melted GH4169 superalloy. Results showed that the interior fatigue crack nucleation was the main failure mode with both stress ratios at 650°C in the long‐life regime. The interior crack nucleation region consisting of many facets was formed under shear stress and strain according to the 3D morphology restructure of the fracture surface. Combined with the evaluation of the threshold stress intensity factor, the transition lengths from a small crack to a long crack for interior failure at both stress ratios were evaluated. Based on the above analysis, the microstructure‐related interior failure mechanism was summarized. Finally, crack nucleation and propagation lives were predicted to obtain the total fatigue life, which showed that the prediction and experimental data scattered between the factor‐of‐three boundaries.

Single-side J-integral method for interfacial cracks

To evaluate the debonding failure behavior of the interface between two materials, such as a fiber reinforced polymer (FRP)–concrete interface, accurately and conveniently, a single-side J-integral method was proposed in which a path is only established on one side (concrete side) of an interfacial crack. A ΔJ criterion was established to identify the unstable propagation of an interfacial crack considering the influence of the nearby concrete cracks and interface mismatch phenomenon on the single-side J-integral. For validation, double-shear tests of fully bonded FRP–concrete specimens were carried out, with different pre-crack lengths (0, 20, 40, and 60 mm). The displacement field of the concrete surface was obtained by the digital image correlation (DIC) method. The single-side J-integral of the interfacial crack was calculated by a self-designed calculation program, and the J-integral–displacement curves were obtained. The calculation results of 14 integral paths for four kinds of specimens showed that the relative error was 15.8% between the mean absolute value of the critical ΔJ and the critical energy release rate Gside-C of the interfacial crack on the concrete side. Therefore, with the proposed method, not only the debonding failure of the interface and local crack propagation direction can be predicted, but the concrete cracks inside the J-integral paths can also be identified.

Bonding enhancement of cold rolling Al/steel composite plates via self-nano film modification

Al/steel composite plates have wide application prospects. However, obtaining high bonding strength using the roll bonding method is difficult because the properties of aluminum and steel are quite different. In this study, an innovative process of producing high bonding strength Al/steel composite plates by using an oxidation treatment on steel surface and cold rolling is proposed. Results reveal that the steel side oxidation treatment can effectively improve the bonding strength of Al/steel composite plates. Compared with the surface treatment without oxidation, the shear strength of “wire brush grinding + steel side oxidation” and “belt grinding + steel side oxidation” can be increased by at least 3 times, and the shear strength is 95 and 100 MPa, respectively, at the reduction rate of 60 % in single pass of cold rolling. The mechanism analysis shows the brittle oxide on the surface of the steel plate forms a crack during the rolling process. A fresh metal bonding zone forms under rolling pressure because as the fresh metal inside the aluminum is squeezed into the side cracks of the steel, a high-strength bond is formed with the fresh metal inside the steel. Although the crack in the oxide-bonding area is small, it also undergoes mechanical interlock and element diffusion. This situation is conducive to the improvement of the bonding strength of Al/steel composite plates.

Effect of treatment mode on microstructure evolution and mechanical properties of nickel-based superalloy fabricated by selective laser melting

Combined with scanning electron microscopy, transmission electron microscope, X-ray diffraction and electron backscatter diffraction, the effects of three treatment modes consisting of as-deposited, solid solution and solution aging state on mechanical structure-properties of a nickel-based superalloy fabricated by selective laser melting were studied. Grains have preferred orientation, i.e. strong texture along {100}. The phase transformation processes during post-heat treatments include dissolution of Laves phase, precipitation of strengthening phases γ′, γ′′ and δ, and size reduction of δ phase. Especially for solution aging treatment, the refined grains, high dislocation density and added high-angle grain boundaries cause a significant increase in strength and a decrease in ductility, which is consistent with the results of the tensile tests. The interior cracking behavior related to the micro-voids and microcracks formed by dislocation pile-ups and uncoordinated deformation around harder δ phase in a vacuum environment becomes a typical ductile failure mode.

A fatigue life prediction approach to interior cracking induced high cycle and very high cycle fatigue for surface‐carburized steels

AbstractHigh cycle fatigue and very high cycle fatigue tests with pulsating tension for three surface‐carburized steels were performed to investigate interior failure behavior and life prediction method. As a result, all cracks with fine granular area and fisheye characteristics are induced from inclusions in matrix or in carburized layer. However, the later cracking morphology is smoother, which is due to the difference of plastic deformation and microstructure of the two regions. Based on estimation of threshold values for small and long crack growth, fatigue design curves are established. The maximum inclusion sizes of three carburizing steels were estimated by four statistical analysis methods. Combined with material microstructure characteristics and the critical plane theory, a failure‐based life prediction model is proposed by using fatigue indicator parameter, whose validity is verified based on good agreement between predicted and experimental ones.

Retracted] Effect of Aluminium Tetrahydrate Nanofiller Addition on the Mechanical and Thermal Behaviour of Luffa Fibre‐Based Polyester Composites under Cryogenic Environment

In recent times, research has shifted away from conventional materials and alloys and more towards nanocomposites to create lighter, more efficient materials for specific applications. The major goal of this research is to see how successful adding aluminium tetrahydride (ATH) filler to a luffa fibre/polyester‐based hybrid composite is. The compression moulding process was used to create the nanocomposite. The following limitations were used to achieve the goals mentioned above: (i) weight percent of ATH, (ii) weight percent of luffa fibres, and (iii) cryogenic treatment hours. The mechanical properties of the materials, such as flexural, tensile, and impact, were examined. The scanning electron microscope observed the morphology pictures, revealing flaws such as interface behaviour, fibre pullouts, voids, and interior cracks. As a result, the current study found that adding nanofiller to a natural fibre composite can improve its mechanical properties, because it established a strong link between the matrix and its reinforcements, which would aid in the effective transmission of stress in the hybrid system. It also improved moisture resistance, which might be useful in construction and commercial industries. The composite with 1 wt.% of ATH, 24 wt.% of luffa fibres, and 30 minutes of cryogenic treatment showed better mechanical strength. Cryogenic treatment reduces compressive interface stresses, which helps maintain fibre and matrix in contact and improve adhesion, resulting in superior results. TGA analysis was used to confirm it.

Modelling the cracking of fresh concrete

The cracking of fresh concrete, while still in a plastic state, includes both plastic settlement and plastic shrinkage cracking, which starts once the concrete is cast to around the final setting time. The cracking process is complex and is influenced by numerous factors which include the climate, mix proportions, element geometry and construction procedures. Preventing these cracks therefore remains a problem in practice. One of the reasons for this is the lack of a model that can be used to determine the location, timing and severity of the cracking before the cracking occurs. The main challenges with such a model are the testing of the fresh concrete to determine the tensile material properties, the appropriate constitutive law needed, and the time dependency of material properties as well as the anisotropic volume change. This paper presents a finite element model that uses a total strain smeared cracking approach and accounts for both the time dependency of material properties and the anisotropic volume change. The model gives an adequate representation of the cracking behaviour of fresh concrete for extreme climates but not for normal to moderate climates, mainly due to the size discrepancy between the interior and surface cracks during experiments as well as the relaxation of stresses that are not accounted for in the model. A parameter study showed that both the settlement and shrinkage strains significantly influence and therefore govern the size of the final plastic crack, while the material mechanical properties only influence the time of crack onset and rate of crack widening.

Parametric features of an UIC60 rail at weld bottom crack in transverse direction under weld-wheel contact forces

In this study, the stress fields of a welded beam track under the influence of a wheel-rail weld force are explored using a finite element model assembly of a weld wheel contact system which has been built on ANSYS space-claim. Practical observations have been shown that semi-elliptical type of cracks are most of the reason to failure of a weld thus a 3-D semi-elliptical type of crack has been induced on the bottom of the rail having transverse direction propagation. Simulation was performed using singular elemental method with ANSYS mechanical workbench for stress field singularity and transverse pattern of crack propagation on the bottom of the rail weld has been observed. The crack analysis and simulation tool in ANSYS workbench provides three modes of stress intensity factors where Mode-I factor KI shows increase in its value with increase in wheel force on its rail-weld contact under each type of forces i.e. vertical, lateral and longitudinal, and if there is an increase in the lateral force the KI took its maximum value. Now if we increase the vertical force the rest of two intensity factors KII and KIII shows a decreasing tendency. It has been observed also that if the longitudinal and lateral forces raised simultaneously both KII and KIII increases in magnitude. The amplitude of KI at the tip side of crack on welds outer edge is greatest within the Stress Intensity Factor of the crack tip, whereas the peak of KII and KIII on the crack-tip of weld centerline seems the highest in the range of Stress Intensity Factor.

Construction of probabilistic model on interior crack initiation and growth behaviors for high strength steels in very high cycle fatigue

In very high cycle fatigue for high strength steels, one can usually observe the fish-eye on the fracture surface and an inclusion is observed at the center of the fish-eye. The fine granular area (FGA) is also found in the vicinity around the inclusion at the crack initiation site on the fracture surface. If the stress cycles required to form the FGA occupies the predominant part of the final fatigue life of these steels, the FGA formation mechanism is one of the most important subjects in the very high cycle fatigue. From this point of view, the authors have attempted to construct the FGA formation model based on the current experimental results obtained by the authors. Supposing four different probabilistic models, the FGA formation is interpreted as intermittent micro-debondings and their coalescence during the long sequence of the cyclic loadings. After the FGA was accomplished, the penny-shape crack tends to grow steadily following the well-known Paris’ law. Based on this analysis of the FGA formation, it was found that polygonization around the inclusion is more important behavior to govern the total fatigue life of the high strength steels.

On the growth of rolling contact fatigue cracks using weight functions

The present paper discusses the estimation of the stress state of Rolling Contact Fatigue (RCF) cracks according to linear elastic fracture mechanics, by making use of the weight function to obtain the Stress Intensity Factors. The weight functions are considered for both the mode I and mode II Stress Intensity Factor and are based on existing formulations for an inclined edge crack in a finite width plate for which Green’s functions are derived by means of a numerical methods. The Stress Intensity Factors been predicted for a side crack having an inclination angle typical for RCF defects in a plate of finite width supported by an elastic foundation and subjected to a moving wheel load, modelled as a patch load. The Stress Intensity Factors are obtained by superimposing the bending moment due to the moving load, and local stresses due to normal pressure and tangential tractions resulting from the contact with the wheel. These local stresses are determined using theoretical solutions partially available in the literature. The Stress Intensity Factors obtained are compared with the results of numerical models presented in the literature regarding the RCF crack growth.

Progressive destabilization and triggering mechanism analysis using multiple data for Chamoli rockslide of 7 February 2021

A catastrophic rockslide occurred on 7 February 2021 in Chamoli area in the high Himalaya. In the absence of field data, multiple satellites data of decade span have been used to investigate and understand the progressive destabilization of rockslide body. A 3D geometric model was developed using geospatial information about geology, terrain, and ice cover to understand the triggering mechanism. Several causes are uncovered as: the pronounced long-term change of land surface temperature facilitated local permafrost degradation and led to ice cover shrinking since 2010; the occurrence of ice avalanche nearby in 2016 accompanying with sidewall-to-bedrock fracturing enhanced the ice segregation beneath the rockslide body; and the development of side cracks in early February 2021 led to dropping of side support and percolating of surface water. Heavy precipitation several days before favoured the destabilization, top-corner cracks developing and top-side bergschrund breaking abruptly two days before, and ice strength reduction owing air temperature rising few hours before the event triggered finally the rockslide. The frequent disasters such as cloudburst, extreme precipitation, landslides, and snow avalanches responding to global warming and climate change in the Himalayan region needs immediate attention to the chain-like geohazards and collaborative observation with satellites and other platforms.