Short Crack Propagation Research Articles

Effect of microstructural features on short fatigue crack growth behaviour in SA508 Grade 3 Class I low alloy steel

Aim of the paper is to understand the effect of microstructural features of SA508 Grade 3 Class I low alloy steel (LAS) on short crack propagation rate under cyclic loading. The complex upper bainitic microstructure of this LAS consists of low angle bainitic ferrite lath boundaries and high angle prior austenite grain boundaries (PAGBs). Compared to bainitic ferrite lath boundaries, the PAGBs provided major hindrance to short fatigue crack propagation in the subject LAS. The high angle PAGBs strongly resist the dislocation motion ahead of the crack tip as the crack tip approaches the PAGBs compared to that of low angle bainitic ferrite lath boundaries. This restriction of dislocation motion ahead of the crack tip based on hindrance provided by PAGBs resulted in retardation in short fatigue crack propagation rate along the crack path. The short fatigue crack propagated at stress intensity factor (SIF) range ‘ΔK’ values lower than threshold SIF range ‘ΔKth’ for the long cracks. The growth rate of short fatigue cracks cannot be predicted by Paris law which is applicable for long crack growth. This is due to the fact that crack growth rate undergoes acceleration and retardation in short crack regime because of microstructural effects.

Fatigue short crack propagation behavior of selective laser melted Inconel 718 alloy by in-situ SEM study: Influence of orientation and temperature

The influence of orientation and temperature on fatigue short crack propagation behavior of Inconel 718 alloy was studied at both 25 °C and 650 °C by in-situ fatigue testing under scanning electron microscope. The fatigue crack growth rates (FCGR) showed evident dependence on orientation at both temperatures, following: XZ < X < Z. The FCGR difference of XZ and Z orientation became less pronounced at 650 °C. Fatigue cracks exhibited commonly transgranular fracture at both temperatures, with enhanced FCGR at 650 °C. Microstructurally short crack propagation showed evident fluctuations, which were closely related to the blocking effects of grain boundaries.

Modelling short crack propagation in a single crystal nickel-based superalloy using crystal plasticity and XFEM

Short cracks appearing under fatigue conditions are of major concern for safety-critical components. In this paper, a computational approach based on crystal plasticity and extended finite element method is developed to predict the slip-controlled short crack growth in a single crystal nickel-based superalloy. The onset of fracture is controlled by cumulative shear strain of individual slip system and the direction of crack growth follows the crystallographic slip plane. Simulations are carried out for [111] orientation at 24 °C and 650 °C, and the results confirm the capability of this approach in predicting the tortuous crack path and irregular propagation rate.

Microstructure-interacting short crack growth in blocky alpha Zircaloy-4

Microstructurally short fatigue crack growth in blocky alpha Zircaloy-4 is experimentally investigated in cyclic three-point bend testing. The short crack propagation is sensitive to the local microstructure with respect to grain crystallographic orientation and grain boundaries. Polycrystals with predominant c-axis texture aligned out-of-plane and normal to loading give alternating crack paths along prismatic planes. Samples with c-axis texture aligned in plane and normal to loading typically show straight paths along prismatic planes, sometimes tortuous paths, but always crystallographic. Prismatic <a>-direction crack growth rate is low compared to that for prismatic <c>-direction growth for given loading. Hence the crystallographic plane within which cracks grow is important for determining overall growth rate. For tortuous cracks, with the predominant c-axis texture in plane and normal to loading, crack growth occurs along basal, prismatic and pyramidal planes, deflecting from one slip plane to another during transgranular propagation.

Effects of metallic microstructures on fatigue fracture of Q345 steel

Effects of high-frequency cyclic loading on the banded ferrite–pearlite steel were analyzed through crack initiation and propagation. Interfaces of ferrite and pearlite colony with a small angle deviation from the loading axis were verified to be the most potential sites to fabricate the microcracks caused by the high strain gradient. The initial crack extension inside ferrite grain was driven by shear stress in model II along the direction with a 45° angle to the loading axis. Banded pearlite colony and the high-angle grain boundaries were considered as the dominant factors that promote the fatigue resistance of the material through arousing crack deflection in short crack propagation range and crack branching in long crack propagation range to reduce the crack propagation driving force in the crack tip. P–S–N curves were used to quantify the dispersion of fatigue lifetimes and evaluate the effect of elevated volume content of pearlite colony on the fatigue performance of the material.

Numerical simulation model for the competition between short crack propagation and wear in the wheel tread

A numerical model is developed to simulate the competition between RCF-initiated short crack propagation and wear in a wheel tread. The crack is assumed to initiate when the total accumulated plastic shear strain reaches the critical value. In the early crack growth simulations, the two-stage short crack growth model proposed by Hobson is applied. With regard to wear, the Archard model is adopted as the basis. The model is shown to operate properly when applied to the results of unidirectional twin disc tests. When an outward route comprising a 1000 km straight run is simulated based on the Shinkansen routes, it is found that the tread is hardly damaged. The results obtained from the rolling reversal fatigue tests show that if the rolling direction of the wheel is reversed, the delamination wear becomes dominant.

Study on Short Fatigue Crack Behaviour of LZ50 Steel Under Non-Proportional Loading.

The low cycle fatigue tests using the replica technique for LZ50 steel under non-proportional cyclic loading were carried out, and eight groups of effective test data were obtained. The evolution behaviour of short cracks was studied based on the effective short cracks criterion. The results show that short cracks generally originate in the grain or along the grain boundary. At the microstructural short crack stage, the crack propagation is influenced strongly by the microstructure of the material, and the growth rate of the short crack slows down several times according to the number of obstacles encountered. At the physical short crack stage, the crack propagation breaks through the banded structure of pearlite. Thus, the dominant effective short fatigue crack is formed, and the crack growth rate increases rapidly. Based on the modified parameters of the uniaxial short crack model, an approach is presented to calculate the growth rate of short cracks under multi-axial non-proportional loadings, and the new model can consider the non-proportional factor F. The fitting results of the multi-axial microstructural obstacles model are compared with test data. The comparison results show that this model can reflect the trend of short fatigue crack propagation rate under non-proportional loadings.

An investigation of short crack propagation in a single crystal Ni-based superalloy using crystal plasticity and the extended finite element method

Short cracks propagating under fatigue conditions are major concerns for the structural integrity of safety-critical applications. These defects tend to grow at high and irregular rates compared to long cracks under similar load, making the prediction of their evolution a challenging task. In this study, a computational approach comprising a crystal plasticity constitutive model and the Extended Finite Element Method (XFEM) is developed to investigate the slip-controlled short crack growth in a single crystal Ni-based superalloy. The onset of fracture is controlled by the cumulative shear strain of individual slip systems and crack develops along crystallographic directions. The model is calibrated from low-cycle fatigue experiments and used to evaluate short crack growth paths and rates in [111] and [001] orientations at 24 °C and 650 °C. Furthermore, the slip behaviour around cracks is investigated. The obtained results show that this modelling approach can capture the tortuous short crack paths and predict the fluctuating propagation rates.

Fatigue Crack Growth Model Incorporating Surface Waviness For Wire+Arc Additively Manufactured Components

Wire+Arc additive manufacturing, due to its high deposition rate, is particularly suited for the construction and repair of large-scale components. For the very same reason, however, an important drawback of this process is the pronounced surface waviness. This waviness introduces multiple adjacent peaks and valleys at the component surface, acting as stress concentration sites which can undermine the fatigue performance. In the present work, the authors propose a framework to model fatigue crack propagation considering the effect of waviness at the component surface. The growth model takes the stress concentration factor as input and incorporates it in a two-fold crack propagation model of a planar flaw. The fracture mechanics based growth models are developed based on El Haddad’s short crack model and the Paris equation for long crack propagation. The effects of stress concentration sites are firstly considered by modifying the short crack propagation parameters. Secondly by introducing a new crack length dependent stress intensity factor threshold, surface waviness is incorporated in the transition from short crack into long crack regime. The results show that short cracks originating from an as-built WAAM surface grow faster as compared to their counterparts originating from machined surfaces.

Effect of shot peening on short crack propagation in 300M steel

The short crack growth behaviour of high strength martensitic 300M steel was studied for as-machined, polished and 2 peening conditions subjected to axial fatigue at 2 stress amplitudes. It was observed that prior austenite grain boundaries acted as barriers to short crack propagation in peened specimens within the peening induced compressive residual stress layer, causing crack decelerations upto 6 grains. Using the da/dN-ΔK trend of the short cracks, fatigue lives of the peened specimens were predicted at both the stress amplitudes. These analytical predictions (which reflect the effect of microstructure and compressive residual stresses) were within 15% of the experimental fatigue lives.

Investigation of the stress corrosion cracking behavior in annealed 5083 aluminum alloy sheets with different texture types

Al–Mg alloys are commonly employed for anti-corrosion parts in marine and rail vehicles. However, the corrosion resistance of these alloys is affected by the alloy microstructure. In this study, 5083 aluminum alloy sheets with different recrystallized textures (a low-intensity texture, a cube-based texture, and a brass-based texture) were obtained by cold rolling and annealing. The stress corrosion cracking and short crack propagation behavior were investigated based on the microstructural characterization, intergranular corrosion, electrochemical corrosion, and slow strain rate testing. The results showed that the tested samples exhibited large differences in stress corrosion properties, although they had a similar resistance to intergranular corrosion. The sample dominated by the brass texture exhibited the best resistance to stress corrosion cracking (SCC) with an ISSRT of 11.7%, while the sample with the low-intensity texture exhibited the worst resistance to SCC with an ISSRT of 29.7%. Moreover, the stress corrosion crack in the sample dominated by the brass texture propagated along a more tortuous path. The crack was deflected at brass texture grains, which was attributed to the combined effects of the relatively large twist angle between the grain boundaries of brass grains and neighboring grains and the low grain boundary energy configuration of {011} orientation grains.

Detection and analysis of high temperature fatigue fracture surface oxygen diffusion of GH4133B superalloy used in turbine disk of aero-engine

The chemical contents on the fatigue fracture surface of GH4133B superalloy piece suffering tension-tension fatigue and operated at high temperature 600 ºC is analyzed using an energy dispersive spectrometer, and the element content percentages at the position of carbide inclusion and that of second-phase particle in the fatigue source region are measured. It is shown that under the conditions of high temperature and air environment, the carbon element in the carbide inclusion can be easily oxidized, which results in accelerating fatigue crack nucleation and microcrack generation, reducing the fatigue crack initiation lifetime. The oxygen concentration is detected on the fatigue fracture surface along the crack propagation direction, the oxygen diffusion model is constructed, and the oxygen concentration gradient equation is derived. It is found that the oxygen content decreases gradually along the crack propagation direction, suggesting that at the crack initiation and short crack propagation stage, the alloy element reacts sufficiently with oxygen for a long time, and the oxidation accelerates the crack initiation, while at the long crack growth stage, the crack propagation rate is faster, so the alloying elements do not have adequate time for reacting with oxygen, resulting in the low oxygen content, and the oxidation has a small influence on the long crack propagation rate.

Effect of crystallographic orientation and geometrical compatibility on fatigue crack initiation and propagation in rolled Ti-6Al-4V alloy

Fatigue crack initiation and short crack propagation was systematically investigated in a rolled α/β Ti-6Al-4V alloy through a combination of in situ optical microscopy observations during fully-reversed uniaxial fatigue experiments and post-mortem electron backscattering diffraction analyses. The effect of microstructural attributes (phase, crystal orientation, grain boundary) on fatigue crack formation and early growth was statistically quantified based on the analysis of a large dataset of grains. Crack initiation was mainly observed on prismatic slip systems which was related to the effect of the rolling texture. Crack propagation was found to be mostly transgranular and crystallographic on both basal and prismatic slip systems. An analysis of the effect of the β phase was proposed to explain the preferential slip systems for propagation in lamellar colonies. Crack deflection was quantified for a large set of grain boundaries. A combination of elastic, plastic and geometric compatibility criteria was identified to consistently predict the preferential slip systems for crack propagation across grain boundary. Finally, a discussion on the transition from microstructure-sensitive to microstructure-insensitive crack propagation was proposed.

RESEARCH ON PROPAGATION CHARACTERISTICS AND REINFORCEMENT METHOD OF PENETRATING CRACK AT RIB-TO-DIAPHRAGM WELDED JOINTS IN STEEL BRIDGE DECK

The rib-to-diaphragm welded joints in the deck of an orthotropic steel bridge is most prone to fatigue cracking. A FE model was established and fracture mechanics was used to study the reinforcement effect of angle steel reinforcement methods on the penetrating crack at fatigue vulnerable details. Based on the finite element model corresponding to a full-foot segment fatigue test model, a penetrating fatigue model was established for the cracks in rib-to-diaphragm welded joints, and the reinforcement effect was evaluated on two reinforcement techniques:bolted angle steel and the bolted steel plate beside a longitudinal rib. The results indicate that:the fatigue cracks at the welded joint in the rib-to-diaphragm of a steel bridge deck expand into a certain length and will develop into penetrating cracks, the deformation of the crack surface under stress conditions is complicated, the fatigue crack propagation characteristics of the inner and outer side of longitudinal ribs are not the same, and as the crack propagation progresses, the cracking mode of the crack tip will be dominated by composite cracking. The existing research results show that:the bolted angle steel reinforcement method can well inhibit the longitudinal rib and diaphragm connection details of the vulnerable part with the fatigue crack type I cracking. Therefore, the short crack propagation can be well suppressed. However, the reinforcement effect on penetrating fatigue cracks extended in a composite type in this detail is not as good as the short crack that have not penetrated the web; the reinforcement method of the half U rib steel plates bolted to the outside of longitudinal ribs can effectively reduce the equivalent stress intensity factor of penetrating fatigue cracks, and stay below the crack propagation threshold after reinforcement. It shows that the reinforcement method has a good reinforcement effect on penetrating fatigue cracks.

The effect of wear on short crack propagation under fretting conditions

The effect of wear on the crack propagation behavior for short cracks under fretting conditions is investigated by using numerical method. The numerical simulation effectively combines the remeshing technology with the adaptive meshing technology, and the evolution of contact force and crack length caused by wear can be considered in this study. The critical distortional elastic energy criterion is used to determine the threshold condition of short crack propagation, and the Archard equation is employed for wear simulation. The results show that wear reduces the short crack propagation rate. Furthermore, the effect of wear on the short crack propagation rate is more obvious for the shorter initial crack length.

Role of prior austenite grain boundaries in short fatigue crack growth in hydrogen charged RPV steel

Short crack propagation under cyclic loading is compared experimentally in hydrogen charged and un-charged SA508 Gr. 3 Cl. I low alloy steel (LAS). This LAS is used in manufacturing of pressure vessels installed in nuclear power plants and is susceptible to hydrogen embrittlement (HE) during operation. Single edge notch tension (SENT) specimens with an initial notch of 85 μm–90 μm are used for this short fatigue crack propagation study. The short crack growth from the notch of SENT specimen subjected to cyclic loading is measured using moving digital microscope fitted on the servo-hydraulic fatigue testing machine. The short fatigue crack growth rate in hydrogen charged SA508 Gr. 3 Cl. I LAS is found to be one order higher as compared to the un-charged subject reactor pressure vessel (RPV) steel. Both trans-granular and inter-granular crack propagation is observed in un-charged and hydrogen charged specimens. In un-charged specimens, inter-granular crack propagation along the prior austenite grain boundaries (PAGBs) occurred only when the short fatigue crack encounters the martensite/austenite (M/A) island present along the PAGB. Whereas, in hydrogen charged specimens, inter-granular crack propagation occurred along the PAGBs of large size prior austenite grains and trans-granular crack propagation through the small size prior austenite grains. Strong resistance to short fatigue crack propagation is provided by PAGBs in un-charged specimens whereas, these PAGBs offered negligible resistance to short fatigue crack growth in hydrogen charged subject RPV steel. A new perspective of studying HE in materials by conducting short fatigue crack growth experiments is demonstrated.

On unified crack propagation laws

The anomalous propagation of short cracks shows generally exponential fatigue crack growth but the dependence on stress range at high stress levels is not compatible with Paris’ law with exponent m=2. Indeed, some authors have shown that the standard uncracked SN curve is obtained mostly from short crack propagation, assuming that the crack size a increases with the number of cycles N as dadN=HΔσha where h is close to the exponent of the Basquin’s power law SN curve. We therefore propose a general equation for crack growth which for short cracks has the latter form, and for long cracks returns to the Paris’ law. We show generalized SN curves, generalized Kitagawa–Takahashi diagrams, and discuss the application to some experimental data. The problem of short cracks remains however controversial, as we discuss with reference to some examples.

Effect of hydrogen on short crack propagation in SA508 Grade 3 Class I low alloy steel under cyclic loading

The effect of hydrogen on short crack propagation under cyclic loading in SA508 Grade 3 Class I low alloy steel is investigated. This low alloy steel is used in manufacturing of pressure vessel installed in Indian nuclear power plants. During operation these pressure vessels are subjected to continuous supply of pressurized hot water at 600 K and hence are susceptible to hydrogen embrittlement. In past, research has been conducted on the effect of hydrogen embrittlement on long fatigue crack propagation in this material but, the mechanistic understanding and correlation of hydrogen embrittlement with microstructural features in the material can be understood well by studying the effect of hydrogen embrittlement on short fatigue crack propagation. Short fatigue cracks are of the order of 10 µm to 1 mm and unlike long cracks these short cracks strongly interact with the microstructural features in the material such as grain/phase boundaries. The effect of hydrogen embrittlement on short crack propagation is studied by artificial hydrogen charging of the material through electrochemical process. The single edge notch tension (SENT) specimens with an initial notch of the order of 85 to 90 µm are used to study the short crack propagation. The short cracks in hydrogen charged samples initiated from the notch at lower number of loading cycles as compared to the uncharged notched samples for the same value of applied stress range (Δσ). After initiation, the short fatigue crack in hydrogen charged samples propagated at higher rate as compared to uncharged samples. This dissimilarity in crack propagation behavior is due to the difference in the interaction of short fatigue crack with the microstructural features for a hydrogen charged and uncharged samples.

Effect of temperature condition on short crack propagation in a single crystal Ni-base superalloy under thermomechanical fatigue

This study investigated the behaviors of short cracks in a single crystal Ni-base superalloy under the thermo-mechanical loading. The effect of temperature condition on the propagation behavior of short crack was discussed taking into account of crack closure. The experimental results indicated that the short crack growth rates under the higher temperature condition are remarkably higher than those under the lower temperature condition even if the crack closure and the temperature dependence on the deformation resistance take into consideration. Based on the crack propagation mechanism, the summation law of the crack propagation rates controlled with the mechanical and environmental factors was proposed to predict the fatigue short crack propagation rate. It was found that the experimental results can be predicted with good accuracy by the proposed method.

Influence of microstructures on microstructurally short fatigue crack initiation and propagation in the forged and rolled Ti-6Al-4V bars

Influence of microstructures on microstructurally short fatigue crack initiation and propagation in the forged and rolled Ti-6Al-4V bars