Crack Initiation Mechanism Research Articles

Dependence of material removal on crystal orientation of sapphire under cross scratching

The cross scratching experiments were carried out on C- and M-planes of sapphire to investigate the crystal-orientation dependence of crack initiation and damage mechanism. The observed scratching patterns showed that the shallow lateral cracks were dominant along [1¯100] direction, while both lateral and radial cracks occurred along [011¯0] direction on C-plane. However, less surface fractures appeared on M-plane, but with greater scratching affected zone caused by deep lateral cracks in subsurface. The anisotropic scratching patterns were closely related to the activated slip/twinning systems, which accommodate dislocation and provide fracture nucleation during scratching. In addition, the acoustic emission (AE) signals were processed based on Fast Fourier Transform (FFT) and Wavelet Packet Decomposition (WPD), and the results indicate that the anisotropic material deformation under scratching can be identified by the frequency bandwidth and the intensity of special frequency components of the AE signals for the C- and M-planes of sapphire.

Phase-field-crystal study on the evolution behavior of microcracks initiated on grain boundaries under constant strain

The initiation and propagation behavior of microcracks is closely associated with the cleavage of crystalline materials. We use phase-field-crystal method to explore the evolution process and mechanism of microcracks initiated on grain boundaries (GBs). Constant biaxial strain is applied to the (0001) crystal plane of hexagonal close-packed bicrystal. Under applied strain, cracks are initiated at the end of semi-boundaries between grains. Following this crack initiation (CIN) mechanism, grain boundary misorientation angle (GBMA) can greatly affect the process of crack initiation and propagation (CINPR) in normal GB system. Along with the increase in GBMA, CIN is promoted and the type of subsequent crack propagation changes from trans-granular propagation into inter-granular propagation. System temperature and applied strain also have influence on CINPR. Associated with fracture mechanics theory, the effects of GBMA, temperature and strain on the behavior of CINPR are investigated. Additionally, we give curves of change in free energy to illustrate these effects from energy perspective. Crack arrestment mechanism, especially the plastic relaxation mechanism, is also discussed. The simulation results acquired in this work are in good agreement with theoretical analyses and experimental results.

Very High Cycle Fatigue Crack Initiation Mechanism in Nugget Zone of AA 7075 Friction Stir Welded Joint

Very high cycle fatigue behavior of nugget zone in AA 7075 friction stir welded joint was experimentally investigated using ultrasonic fatigue testing system (20 kHz) to clarify the crack initiation mechanism. It was found that the fatigue strength of nugget zone decreased continuously even beyond 107 cycles with no traditional fatigue limits. Fatigue cracks initiated from the welding defects located at the bottom side of the friction stir weld. Moreover, a special semicircular zone could be characterized around the crack initiation site, of which the stress intensity factor approximately equaled the threshold of fatigue crack propagation rate. Finally, a simplified model was proposed to estimate the fatigue life by correlating the welding defect size and applied stress. The predicted results are in good agreement with the experimental results.

High-Cycle Fatigue Properties and Damage Mechanism of Q345B Structural Steel

The high-cycle fatigue behavior of Q345B structural steel was investigated experimentally. Highfrequency vibration fatigue testing machine and scanning electron microscopy were used to study the high-cycle fatigue S–N curve characteristics and crack initiation mechanism at ambient temperature. The surface temperature of the specimens was monitored. The relation between the fatigue limit and the amount of heat dissipation was also investigated. It was found that the fatigue life changed inversely with the stress amplitude in the high-cycle range. The fatigue limit in high cycle range was obtained from heat dissipation in the specimen and found to have good agreement with the S–N curve. The crack initiation was attributed to the surface defects and the persistent slip bands due to the cycle slip in fatigue loading.

Effect of Grain Size on Corrosion Fatigue Behaviors of Primary Coolant Pipes of Nuclear Power Plants

Corrosion fatigue is one of the most important failure modes of primary coolant pipe used in nuclear power plant, due to the thermal cyclic stress caused by a variety of start-up and shutdown as well as transients when severed in the high temperature and pressure water environment. And 316LN stainless steel is one of the main materials used in pressurized water reactor in nuclear power plant. The mechanisms of fatigue crack initiation and propagation were elucidated by investigating the composition properties of oxide films formed in simulated service environment. The effects of grain size on the fatigue life and crack initiation mechanism of 316LN stainless steel in 320°C water environment were also investigated. The results indicated that the specimens with fine grains (30μm) own the longest fatigue lives than those with intermediate (80μm) and coarse grains (210μm). The fatigue stress amplitudes of the specimens increased with the grain refinement, although the fatigue lives of the specimens with intermediate and coarse grains were close. Fatigue cracks were often initiated at the persistent slip bands on the surfaces of the specimens with intermediate and coarse grains, while it was not easy to be initiated on the fine grain specimens. Finally, combined with the experimental results and the actual production of industry, a suggestion for the production of the primary coolant pipe is put forward.

XFEM Potential Cracks Investigation using Two Classical Tests

Extended Finite Element Method (XFEM) is used in this work, first to perform the simulation of crack initiation and propagation mechanisms in plane models and then to determine the stress distribution singularities in the closest surroundings of a front fracture inserted in three-dimensional models. The essentials of XFEM is the well-known Finite Element Method (FEM) adding to degrees of freedom and enrichment functions, which serve to describe local discontinuities in the model. In XFEM, the fracture geometry is developed independent of the mesh, allowing it to move freely through the domain, without the need to adapt the mesh to discontinuity. In other words, the XFEM reproduces the discontinuity of the displacement field along the fracture, without discretizing this feature directly in the mesh. XFEM carry out the spatial discretization of two classic models in Fracture Mechanics: the single-edge-notch bending test (SEN (B)); and the disck-shaped compact tension test (CDT). The propagation criterion is based on the proportion of energy released and the stress intensity factors (SIF). The solutions provided by the XFEM numerical model indicated an excellent agreement with the results obtained from the experimental data.

Simulation of Mesofracture Process of Asphalt Mixture Using Digital Image Processing and Extended Finite-Element Method

Abstract This study combines the digital image processing (DIP) technique and extended finite-element method (XFEM) to analyze the crack initiation and propagation mechanism of an asphalt mixture in the indirect tensile test (IDT) at a mesoscopic level. Digital camera images (DCIs) of an asphalt mixture were used to generate mesoscale geometric information of the mixture structure, which was then processed by the free mesh generation technique to build a mesh model of the mixture mesostructure. By introducing a bilinear cohesive zone model (CZM) and the XFEM, the mesh model was used to simulate the complete failure process of the asphalt mixture in the IDT. After validation with laboratory test results and theoretical solutions, the model was used to investigate the crack initiation and propagation process and the stress distribution in the asphalt mixture. It was found that the developed finite-element model cannot only produce simulation results that are consistent with laboratory test results and theoretical solutions, but also provide detailed information, such as the stress distribution in between the aggregate and mastic. In the IDT, crack initiates in weak areas around the interface between asphalt mortar and aggregates. Multiple cracks would occur in the stress concentration areas, and then propagate along both ends at various rates. During crack propagation, crack width plays a leading role. Results of this study showed that the adopted mesostructure analysis method can overcome some shortcomings of conventional finite-element methods and, hence, may provide an effective way for improving the design of flexible pavements.

Effects of Inclusions on the Very High Cycle Fatigue Properties of a High Strength Martensitic Steel within the Transition Area

In this paper fatigue tests of a high strength steel are presented. Focus was put on the transition area (TA) from 105 to 2 x 107 cycles. A change of crack initiation mechanism could be observed. Different specimen sizes D4=4mm, D7.5=7.5 mm were tested at a stress ratio of R = -1 and T = 20°C. D7.5 were additionally tested at T = 350°C. D4 results showed a pronounced TA with defects mainly consisting of non-metallic inclusions (Al2O3, MgO and CaO). The average defect size (area, D4) was measured to be 28.98 µm. D7.5 specimens indicated a less pronounced gap between surface and subsurface crack origins. Same type of non-metallic inclusions could be found by SEM analyses. The average defect size (area, D7.5) was measured to be 10.98 µm. Murakami’s area approach showed a negligible dependance of the fatigue strength according to the different defect sizes combined with a large scatter.

Investigation of changing failure mechanisms in the VHCF regime caused by different strengths of a martensitic steel

The present study gives an overview of recent investigations dealing with the fatigue behaviour of the tempered martensitic steel 50CrMo4 (Fe-0.5wt%C-1wt%Cr) in the HCF and VHCF regime by taking into account a variation in material strength, by modifying the heat treatment parameters. The parameters for the tempering treatment were adapted to receive two material conditions with 37HRC and 57HRC, respectively. Subsequently, fatigue specimens were machined from the heat-treated bars for fatigue tests in an ultrasonic (f=20000Hz) and a resonance (f=95Hz) fatigue testing machine under fully reversed loading (R=-1) at laboratory air atmosphere. It was found that the dominant fatigue and fracture mechanisms change with increasing material strength. For 37HRC moderate-strength specimens crack initiation was shown to occur on the specimen surface within Cr depleted bands (segregation bands) as the dominant fatigue damage mechanism. Contrary to that, only internal crack initiation at non-metallic inclusions was observed for the high strength 57HRC condition. Furthermore, the completely different crack initiation mechanisms of the two heat treatment conditions were assessed by applying the Murakami approach relating the fatigue limit with the size of non-metallic inclusions.

Study on Very-High-Cycle-Fatigue Property of Aero-engine Blades Based on Subcomponent Specimen

The cyclic load number of aero-engine blade during its service life is very likely beyond 10 7 , which is regarded as the conventional fatigue limit. Moreover, surface strengthening is very often used in the manufacturing process of blade. The conventional testing method in the VHCF regime cannot exactly reflect the stress state of the blade, including the mechanism of crack initiation and propagation. To study the fatigue behavior and effects of laser shock peening, a kind of dissymmetrical bending fatigue subcomponent specimen was designed and the laser shock peening model was established. Experiment about TC17 was accomplished by the Ultra-High Cycle bend fatigue system. It is found that the fatigue damage occurs beneath the surface and the S-N curve is continuously rather than multi-step declining in the VHCF regime. Process of surface strengthening has a significant effect on fatigue performance of TC17 titanium alloy.

Effect of Mean Stress on Fatigue Crack Initiation in Magnesium Alloy and Crystallographic Analysis

Plane bending fatigue test had been conducted to investigate fatigue crack initiation mechanism in coarse-grained magnesium alloy, AZ31, under different mean stress. The initial crystallographic structure was analyzed by an electron backscatter diffraction (EBSD) method. When the test had been conducted at the load ratio R=-1, secondary twin operated in a primary twin and resulted in the fatigue crack initiation. On the other hand at R=0.1, dual twin was activated and crack has initiated along the grain boundary.

Crack Initiation in Austenitic Stainless Steel Sanicro 25 Subjected to Thermomechanical Fatigue

Thermomechanical fatigue experiments were performed with austenitic stainless Sanicro 25 steel. Several amplitudes of mechanical strain in a wide temperature interval (250-700 °C) were applied to the specimens. Mechanical response was recorded and fatigue lives were obtained. Scanning electron microscopy combined with FIB technique was used to study the mechanism of crack initiation in in-phase and in out-of-phase thermomechanical cycling. Different mechanisms of the crack initiation were found in these two types of loading. During in-phase loading fatigue cracks start in grain boundaries by cracking of the oxide. Cracks grew preferentially along grain boundaries which resulted in rapid crack initiation and low fatigue life. In out-of-phase loading multiple cracks perpendicular to the stress axis developed only after sufficiently thick oxide layer was formed and cracked in low temperature loading half-cycle. The cracks in oxide allowed localized repeated oxidation and finally also cracking. The cracks grow transgranularly and result in longer fatigue life.

Investigation on the fretting fatigue behaviors of Ti-6Al-4V dovetail joint specimens treated with shot-peening

In this study, Ti-6Al-4V dovetail joint specimens which highly simulate the attachment between the blade and disk of fan stages in a certain gas turbine engine were designed and used in fretting fatigue experiments. The fretting wear mechanism was characterized using a bench level experimental method. The effect of shot-peening on both fretting wear mechanism and fretting fatigue property was evaluated. The results indicated that shot-peening transformed the fretting wear behavior, altered the dominant crack initiation mechanism and dramatically promoted the fretting fatigue performance of Ti-6Al-4V dovetail joint specimens. Different reinforcement mechanisms of the multiple surface modifications created by shot-peening were also analyzed and summarized in this study.

Failure analysis of an aluminum extrusion aircraft wing component

Purpose The purpose of this paper is to deal with the failure analysis of a fractured spar stiffener, extruded from 7075-T6 aluminum alloy, which was found in the central wing, trailing edge structure of a military transport aircraft. The previous loading history and the dominant environmental factors (corrosive and humid atmosphere, water entrapment, etc.) suggest corrosion and fatigue as the principal failure modes, synergistically acting on the wing component. Design/methodology/approach This study presents the failure analysis concentrated on finding evidence of failure mechanisms and plausible root-cause(s) of the fractured spar stiffener. Chemical analysis, stereo and scanning electron microscopy, as well as finite element analysis employed as the main analytical tools for material characterization and failure investigation. Findings The overall evaluation of the findings suggest that the failure caused by a synergy of two mechanisms; a crack initiated in the longitudinal, extrusion direction by an environmentally assisted corrosion attack, then propagated by the superimposed transverse stress field, branched/deflected due to a low crack driving force and extended in a transverse path through a high cycle fatigue process. Finally, the complete fracture occurred as fast fracture, resulted by a ductile overload. Originality/value This paper deals with an industrial damage case study, providing analysis and modeling from structural engineering standpoint. The aforementioned findings concerning the fractured aircraft component allow gaining a deeper knowledge about the mechanisms of crack initiation and propagation which, in turn, can produce a valuable feedback to design, inspection and maintenance procedures. This includes a modified heat treatment from T6 to T73 temper for the redesigned component.

In-situ SEM study of crack initiation, propagation and interfacial debonding of Ni-P coating during tensile tests: Heat treatment effect

This study focuses on the effect of annealing temperature onto the mechanisms of crack initiation, propagation and interfacial debonding of the electroless nickel/phosphorus (Ni-P) coatings deposited on steel. An in-situ tensile test has been carried out in a SEM chamber for coated samples annealed at 400 and 600 °C, and at a strain rate of 0.1μm/s. It is observed that when an as deposited Ni-P on steel system is subjected to a uniaxial tensile test, a bended crack is formed on the surface of coating and leads to a debonding of coating at a small strain level. Incontrast, in case of an annealed sample, the N-iP coating shows a severe multiple parallel crackings due to the localized plastic deformation of the substrate. The crack density firstly increases with the rising tensile strain, and then reaches saturation. The spacing between the resulting saturated parallel cracks in the coating surface is defined in the shear lag model to quantify the interfacial shear strength. Despite the large plastic deformation of the substrate, there is no spallation observed at the interface between the two materials. The hardness and the young's modulus of the coating before and after heat treatment are determined by nano-indentation tests. The stiffness of the coatings increases owing to the Ni3P phase precipitation during heat teatment.

Experimental study of the low-cycle fatigue life under multiaxial loading of aluminum alloy EN AW-2024-T3 at elevated temperatures

This paper presents the results of an experimental study conducted on technical aluminum alloy EN AW-2024-T3 specimens under multiaxial low-cycle-fatigue loading at elevated temperatures. An original experimental setup, consisting of a biaxial servo-hydraulic MTS 858 MiniBionix material testing system, Maytec resistance furnace, high-temperature MTS extensometer and hydraulic grips of original design, was used in tests. Tubular specimens were subjected to multiaxial fatigue loading, proportional and non-proportional (with phase shift), under isothermal conditions of the air atmosphere. Multiaxial tests were performed under combined axial-torsional loading. Fractography analysis were conducted on the tested samples, and crack initiation mechanism under conditions of multiaxial loads at elevated temperatures were identified. Obtained fatigue life values were analyzed, particularly from the perspective of the possibility of approximating them by means of simple relations.

High cycle fatigue behavior of Ti–5Al–5Mo–5V–3Cr–1Zr titanium alloy with bimodal microstructure

In this study, high–cycle fatigue (HCF) damage behavior of Ti–55531 alloy with bimodal microstructure (BM) was studied at room temperature. Fatigue crack initiation and propagation mechanisms of the alloy were thoroughly investigated by studying fracture morphology, crack front profiles, polished microstructure and dislocation structures beneath fatigue main–crack initiation sites of HCF samples. The results indicate that this alloy presents an excellent HCF strength (107cycles, R = −1) as equal to 656 MPa. Dislocation analyses exhibit that typical dislocation structures include straight prismatic slip lines, curved dislocation lines, dislocation tangles and {1¯011}α type twins in fatigued specimens. Primary αp particles and secondary αs lamellae accommodate more cyclic deformation than retained βr laths. Furthermore, the dislocation free zone can be observed in the αp/βtrans (β transformed microstructure) boundary. As a result, microcracks primarily nucleate at the αp/βtrans interface or at αp particles. Moreover, a few microcracks initiate at the αs/βr interface or at αs plates of βtrans microstructure. These fatigue crack initiation behaviors promote the fracture of Ti–55531 alloy.

Three-dimensional stress and strain around real shape Si particles in cast aluminum alloy under cyclic loading

The crack initiation mechanism of cast Al-Si-Mg alloy under low-cycle fatigue was addressed by using the synchrotron X-ray computed tomography (CT) and the image-based finite element analysis. The fatigue test and its in situ CT observation were conducted to visualize the crack initiation behavior. In the low-cycle fatigue, the cracking generally started with the voiding by the fracture of silicon particles, and the coalescence of these voids formed the crack. To elucidate the mechanism of silicon particle fracture, the finite element elastic-plastic analyses were performed with regard to twelve silicon particles including the fractured and intact particles detected by the chronological CT observation. By using the image-based modeling technique, the interested particle was embedded in the finite element model along with the surrounding particles as they were in the specimen. The material properties of silicon phase and aluminum matrix were identified by the nanoindentation tests. Ten cycles of loading by the uniform stress which was equivalent to the load in the fatigue test was applied to the finite element model, and the stress, strain and their cyclic response around the silicon particles were simulated. The morphology analysis was also carried out for the interested particles, and the geometrical parameters affecting the particle fracture were examined. By comparing the results of fractured and intact particles, we found that there were some geometrical conditions for the fracture of silicon particles, and a certain magnitude of hydrostatic stress was required to break the particles.

Crack initiation sensitivity of wrought direct aged alloy 718 in the very high cycle fatigue regime: the role of non-metallic inclusions

Fatigue crack initiation in the direct aged version of the nickel-based superalloy Inconel 718 has been investigated at room temperature in the low stress/very high cycle regime via ultrasonic fatigue testing. Three different microstructures have been examined at the same strain amplitude in order to understand the influence of non-metallic inclusions (NMIs), i.e. carbides, carbonitrides and nitrides, and Σ3 twin boundary density on lifetime and failure mode. A slight refinement in grain structure and a higher Σ3 twin boundary density is associated with substantial reductions in lifetime. Decreasing Σ3 twin boundary density for fine grain microstructures results in a change in crack initiation mechanism from strain localization within grains at the high end of the grain size distribution to cracking of NMIs. To study the early stages of crack initiation and growth, specimens with pre-cracked NMIs were also tested in order to examine the role of the surrounding grain structure. Pre-cracked NMIs mainly result in macroscopic failure initiation at NMIs independent of the wrought microstructure. However, pre-loading specimens within the plastic domain highlighted the competition in crack initiation mode between cracked-NMIs and favorably oriented twin boundaries. Crack arrest from most of the pre-cracked NMIs demonstrates that surrounding grain structure (grain orientation, local plasticity and roughness in the vicinity of crack tip due to pre-straining) play a key role in the fatigue life of components stressed in the nominal elastic regime.

Effect of Stress Ratio and Loading Frequency on the Corrosion Fatigue Behavior of Smooth Steel Wire in Different Solutions.

In this work, the effects of loading condition and corrosion solution on the corrosion fatigue behavior of smooth steel wire were discussed. The results of polarization curves and weight loss curves showed that the corrosion of steel wire in acid solution was more severe than that in neutral and alkaline solutions. With the extension of immersion time in acid solution, the cathodic reaction of steel wire gradually changed from the reduction of hydrogen ion to the reduction of oxygen, but was always the reduction of hydrogen ion in neutral and alkaline solutions. The corrosion kinetic parameters and equivalent circuits of steel wires were also obtained by simulating the Nyquist diagrams. In corrosion fatigue test, the effect of stress ratio and loading frequency on the crack initiation mechanism was emphasized. The strong corrosivity of acid solution could accelerate the nucleation of crack tip. The initiation mechanism of crack under different conditions was summarized according to the side and fracture surface morphologies. For the crack initiation mechanism of anodic dissolution, the stronger the corrosivity of solution was, the more easily the fatigue crack source formed, while, for the crack initiation mechanism of deformation activation, the lower stress ratio and higher frequency would accelerate the generation of corrosion fatigue crack source.