Fatigue Crack Propagation Rate Research Articles

Crystal Plasticity Assessment to Crystallographic Stage I Cracking in a Ni-Based Single Crystal Superalloy

Stage I fatigue crack propagation along crystallographic slip planes was experimentally and analytically investigated in a single crystal Ni-base superalloy, NKH-304. Fatigue crack propagation tests at room temperature were conducted using four types C(T) specimens with different combinations of primary and secondary orientations. It was revealed in the experiments that the fatigue cracks propagated along crystallographic slip plane in mixed mode with Mode I, II and III components. Mixture ratio and fatigue crack propagation rate was strongly influenced by the primary and secondary crystal orientations. In order to interpret the effect of crystal orientations on the Stage I cracking, a crystal plasticity finite element analysis was conducted considering the actual geometry of the crystallographic crack planes. Analytical results for slip activities on the individual octahedral slip systems and a damage parameter based on the critical plane approach provided reasonable explanations to the effect of crystal orientations on the cracking path and propagation rate of the crystallographic Stage I cracking.

A Finite Element Study of the Influence of Graphite Nodule Characteristics on a Subsurface Crack in a Ductile Cast Iron Matrix under a Contact Load

This paper describes a study of the effects of graphite nodule characteristics on a subsurface crack in austempered ductile iron (ADI). A representative specimen of ADI, subjected to sliding contact load, is modeled using finite elements aiming to obtain the shear stress intensity factor (KII). The parameters varied were (i) the nodule diameter (two different values were considered), (ii) the distance between the nodule and the tip of the crack and (iii) the position of the load relative to the tip of the crack. The results of the numerical simulations show that the smaller diameter nodule has a larger influence on KII, suggesting a higher contact fatigue crack propagation rate in the material with the smaller nodule. These results are the opposite of those observed in experimental studies and would appear to indicate that other factors should be also considered to ensure realistic estimates of the contact fatigue strength of ADI.

Analytical theory for fatigue crack propagation rates of mixed-mode I–II cracks and its application

The Manson–Coffin law for the low-cycle fatigue (LCF) of a representative volume element (RVE) and fatigue crack propagation (FCP) rate of cracked components can characterize material fatigue resistance. This paper proposes an LCF–FCP model for predicting the FCP rate of plane-stress mixed-mode I–II cracks. A semi-analytical model is also proposed for the Je integral of mixed-mode I–II compact tension shear (CTS) specimens based on energy equivalence method. The combination of these two models predicted the FCP rates of CTS specimens made from 30Cr2Ni4MoV rotor steel with mixed-mode I–II cracks and subjected to four different loading angles. The theoretical results were consistent with the experimental results.

Fatigue crack propagation in welded joints X70

Structural failure assessment approaches take into account local parameters, specimen geometry, loading and material. In the case of welded joints, in addition to these parameters, consideration must be given to theeffect of the heterogeneity of properties due to welding. The objective of our work is to study the fatigue crack propagation of welded joint in API X70 pipeline steel. This experimental study focused on welded joints in thedifferent parts, base metal, weld metal and heat affected zone. The concepts of fracture mechanics are used to analyze the harmfulness of defects in welded joints and the main part of fatigue life falls on the crackpropagation. The results obtained show that the fatigue crack propagation rate of cracks in the heat affected zone is delayed compared to the other zones.The effect of the microstructure and the quality of submerged arc welding of the studied X70 steel are significant. Tensile tests, hardness and measurement of energetically parameters complemented this work.

Energy response of S355 and 41Cr4 steel during fatigue crack growth process

In this research, a novel approach of the fatigue crack growth rate description has been proposed. Based on theoretical and experimental approach, the mean stress effect expressed by R-ratio is present in classical da/dN–Δ K diagram. According to energy approach – based on the irrevocably dissipated energy accumulated in material (hysteresis loop) during fatigue process – the mean stress effect can be minimalized. Experimental validation of the proposed model was performed using results of fatigue crack propagation data for S355 and 41Cr4 steels in terms of strain energy density parameter Δ S or cyclic J-integral range –Δ J. In contrast to the force approach based on Kmax (or Δ K), the energy parameters Δ S or Δ J represent unambiguously the fatigue crack propagation rate, without influence of mean stress effect – R-ratio. However, in near threshold range of kinetic fatigue fracture diagram, the energy parameter displays a slight dispersion of the experimental data. According to the crack closure theory and its U-Elber parameter, the dispersion of experimental data is decreased. Therefore, the crack closure effects have a high significance in energy model – similar to the ‘force approach’ based on Δ K concept.

Texture Effect on Fatigue Crack Propagation Behavior in Annealed Sheets of an Al-Cu-Mg Alloy

The effects of texture, including Brass, Goss + Cube and Cube, on the fatigue crack growth behavior in annealed Al-Cu–Mg alloy sheets were characterized in this investigation. Results showed that the Goss + Cube sheet possessed the greatest fatigue threshold value and the lowest fatigue crack propagation (FCP) rate, and the Brass sheet presented the smallest threshold value and the greatest FCP rate among the three textured sheets. SEM results showed more slip bands and more tortuous crack path formed in Goss + Cube sheet during cyclic loading, and EBSD results showed that numerous dramatic crack deflections occurred when fatigue crack propagated across Goss-grain boundary in Goss + Cube sheet, indicating that the grain orientations including Goss and Cube had positive effect on enhancing the FCP resistance. On the one hand, Goss and Cube grains have more high Schmid factor slip systems than Brass grain, promoting persistent slip bands formation and externally applied energy consumption, consequently giving rise to a high resistance to FCP. On the other hand, Goss grains could induce crack deflection with large twist angles, resulting in more energy consumption during the cracking process and thereby enhance the FCP resistance.

Prediction of crack tip plasticity induced due to variation in solidification rate of weld pool and its effect on fatigue crack propagation rate (FCPR)

Effect of solidification rate on crack tip plastic zone size at various crack lengths was calculated analytically and numerically by simplified Sih’s and Irwin’s models, respectively. Influence of plastic zone size is explained in terms of COD and elastic stress intensity factor within valid range of small scale yielding approximation. Up to plastic zone size range of 4–5 mm, a good agreement between numerical and analytical plastic zone size and elastic stress intensity factor for all weldments was observed. For high loads and greater crack lengths, experimentally obtained COD values were found 15–19 % more than simulation ones due to rapidly induced plasticity at high crack dimensions. Solidification rate showed a significant influence on FCPR, for solidification rates 13.75 °C/s, 6.97 °C/s and 4.32 °C/s the obtained fatigue strength was 35.29 MPa, 36.26 MPa and 41.32 MPa, respectively.

Enhanced low-cycle fatigue and crack propagation resistance of an Al-Cu-Mg-Si forging alloy by non-isothermal aging

Low-cycle fatigue (LCF) and fatigue crack propagation (FCP) behavior of an Al-Cu-Mg-Si forging alloy with different non-isothermal aging (NIA) degrees were investigated. The main question addressed was about the interactions between precipitates and dislocations and their influences on LCF and FCP resistance. Strain-controlled LCF and ΔK-controlled FCP were tested. The characterization of precipitates and fractography were performed on TEM/HRTEM and SEM, respectively. The results showed that under cyclic loading, shearable precipitates were suffered from particle shearing mechanism associated with slip localization, resulting in cyclic softening behavior and low FCP rates; non-shearable precipitates were suffered from bypassing mechanism associated with homogeneous slip, resulting in cyclic hardening behavior and high FCP rates. The mixed precipitation characteristics obtained by complete NIA treatment, consisting of shearable and non-shearable precipitates, led to cyclic stability behavior and low FCP rates, which means a simultaneous enhancement of LCF and FCP resistance.

The effect of inhomogeneous microstructures on strength and fatigue properties of an Al-Cu-Li thick plate

The inhomogeneity of the microstructure of an Al-Cu-Li thick plate in the thickness direction results in inhomogeneity in the tensile property and fatigue crack propagation (FCP) rate. The grain size gradually increases from the surface layer to the central layer. The contents of low-angle grain boundaries (LAGBs) were characterized by an electron backscattered diffraction (EBSD) technique, and the LAGBs, which act as nucleation sites, induced a greater number of T1 precipitates in the central layer. In the surface layer, the random texture resulted from the greater recrystallization degree due to the high degree of cold deformation during the rolling process. The 1/8 layer contained a rotated cube texture, which is a typical shear texture. A strong β fiber rolling texture was found in the central layer, including S, brass and copper texture components, because the adequate recovery during hot rolling decreased the recrystallization driving force. Both the yield strength (YS) and ultimate tensile strength (UTS) gradually increased from the surface layer to the central layer, i.e., UTS from 479.7 MPa to 546.0 MPa, YS from 455.9 MPa to 514.0 MPa. The inhomogeneity of the tensile properties was primarily caused by the different textures, LAGB content and T1 precipitate density. However, the surface layer had a distinctively lower FCP rate than the central layer. The disparity in the FCP rate between the surface layer and the central layer could be attributed to the roughness and plasticity, which are related to the LAGBs, the T1 precipitates and the grain size.

Fatigue reliability analysis of crack growth life using maximum entropy method

In this article, a new method for fatigue reliability analysis of crack growth life based on the maximum entropy theory and a long crack propagation model is proposed. A modified generalized passivation-lancet model for long fatigue crack propagation rate is presented with explicit physical meaning. Experimental results for turbine disk alloy ZSGH4169 under different strain ratios and temperatures (at 650°C and room temperature) are used to verify the applicability of the new model. Results show that predictions by the proposed model are almost identical to the experimental data. The presented model is better than the other three models to reflect the rapid propagation characteristics of the crack. In order to perform fatigue reliability estimation, the probabilities of failure are calculated using the maximum entropy theory based on the fatigue crack growth life that derived from the proposed modified crack propagation model and the above existing three models. Results have shown that maximum entropy theory is very apt for fatigue reliability analysis of turbine disk under different loading conditions with a limited number of samples because it does not need any distribution assumptions for random variables. The effectiveness and accuracy of the combination of fatigue crack propagation models and maximum entropy method for fatigue reliability analysis are demonstrated with examples.

The effect of external magnetic flux field in the QTS weldment on the change of fatigue crack propagation behaviors

This investigation discusses fatigue crack propagation behaviors on the welded joint of Hot Rolled Quench Tempered Steel (QTS) in which during welding process the fusion zone of the joint was subjected to magnetic flux field. The QTS weldability is not really excellent due to the change of microstructure into tempered martensite, and the possibility of microcrack defect on the welding area is still high. The purpose of the investigation is to know the effect of External Magnetic Flux (EMF) field during welding process on fatigue crack propagation behaviors. The external magnetic flux is applied transversely from two sides of the workpiece using a DC powered solenoid of 0, 3, 6, 9 and 15 Amperes. The effect of EMF is more sensitive to decrease the tensile strength and the fatigue crack propagation rate of the weld area. The result shows that the electromagnetic force on the weld pool increases. It causes the liquid metal circulation rate to increase and welding defects to decrease. This indicates that the liquid metal and filler metal are easily mixed, the release of gas from liquid metal to surface before solidification easily happens. The finding shows that the effect of EMF is more efficient.

Analysis of empirical relation between microstructure, texture evolution and fatigue properties of an Al-Cu-Li alloy during different pre-deformation processes

The microstructure and fatigue properties of Al-Cu-Li alloy sheets were investigated respectively under the conditions of without pre-deformation, 3% pre-stretching deformation and 8% pre-rolling deformation utilizing scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) in present work. Results show that the intensive Goss texture and relatively weaker Brass, Cube texture were formed in without pre-deformed alloy after aging treatment. The pole density level of Goss texture was decreased remarkably and the intensity of Brass, Cube texture was increased lightly after 3% pre-stretching deformation and 8% pre-rolling deformation. The magnitude order of fatigue crack propagation (FCP) rate of the alloy under different pre-deformation conditions was dadNpre−rolling > dadNpre−stretching > dadNnon−predeformation in stage II (Paris regime). The combined contribution of T1 phase with different size, amount and the intensity of Goss texture is the main factors affecting the FCP rate of Al-Cu-Li alloy.

Effect of creep aging forming on the fatigue crack growth of an AA2524 alloy

Effects of creep aging forming on the microstructural evolution, conventional mechanical properties and fatigue crack propagation (FCP) behavior for 2524 aluminum alloy were studied by tensile and hardness testing, the FCP behavior testing and transmission electron microscopy (TEM) observation. The results show that in the low stress intensity range, the characteristics of precipitates have a significant effect on fatigue performance of the creep-aged aluminum alloy, and the mainly precipitated S″ phases in the alloy creep-aged for 4 h is beneficial to the reversibility of coplanar slip and dislocation slip, and the alloy presents better FCP resistance and relatively lower FCP rate, while the FCP rate increases with increasing aging time. In the Paris regime, effects of microstructure on the FCP rate are no longer obvious, and the FCP rates are almost consistent. Meanwhile, the presence of creep stress significantly accelerates the aging precipitation process. In terms of yield strength σ0.2, hardness and FCP resistance, the 9 h artificial aged alloy is analogous to the 4 h creep aged alloy. The peak strength of the alloy creep-aged for 24 h is the highest, but the fatigue crack growth resistance degraded obviously. It is not comprehensive to determine the optimal process of creep aging only in accordance with the peak strength.

Very-high-cycle fatigue crack initiation and propagation behaviours of magnesium alloy ZK60

ABSTRACTThe aim of this paper is to assess the very-high-cycle fatigue (VHCF) behaviour of a magnesium alloy (ZK60). Results indicate that the fatigue crack initiates from an area consisting of many distributed facets, while the region of early crack propagation is characterised by parallel traces, based on a fractographic analysis. The significant differences in morphology around the crack initiation area result from the interaction between the deformation twinning and the plastic zone at the crack tip. In addition, the fatigue crack propagation rate around the crack initiation site is also estimated based on a modified Murakami model. It is found that the formation stage for the fatigue crack is of great importance to the fatigue failure mechanism in the VHCF regime.

Effect of magnetic field on crystallographic orientation for stainless steel 316L laser-MIG hybrid welds and its strengthening mechanism on fatigue resistance

In order to extend fatigue life of structures, traditional methods involving controlled creation of boundaries and local microstructure discontinuities are used to obstruct dislocation motion and change the crack propagation path during fatigue service. However, such strategies inevitably sacrifice ductility due to the reduced ability to accommodate dislocations. Herein, an external magnetic field is used to assist laser-MIG hybrid welding for 316L austenitic stainless steel. Fatigue crack propagation rate is decreased by 33% at f = 0.1 Hz and 12% at f = 1 Hz in air. The external magnetic field optimizes fatigue resistance of 316L welds from two aspects: diverse orientation distributions of ferrite and non-K-S orientation relationship (OR) between ferrite (δ) and austenite (γ). Diverse orientation distributions of δ induced by magnetic field activate different slip systems, promoting crack deflection in γ grain interior during cyclic deformation. Semi-coherent interface between δ-γ with non- Kurdjumov-Sachs (K-S) OR not only impedes transmission of the dislocation slip but also accommodates a considerable amount of plastic strain by continual loss of coherency. Thus, external magnetic field assisted welding of 316L extends fatigue life by increasing strength and ductility, and promoting crack deflection in grain interior.

Exploring the role of reinforcement in controlling fatigue crack propagation behavior of perfluorosulfonic-acid membranes

Reinforced composite membranes in polymer-electrolyte fuel cells (PEFCs) are attracting more and more attention due to their simultaneously superior durability and high performance. Despite its improved durability during accelerated stress testing, the underlying mechanism of reinforcement in improving mechanical durability, particularly the crack propagation resistance, has not well been investigated. In this paper, we report the fatigue crack propagation behaviors of reinforced Nafion XL membrane and unreinforced Nafion 212 membrane. It is found that the fatigue crack growth rate of Nafion 212 membrane depends on stress ratios, and that of Nafion XL membrane depends on initial crack length and membrane orientation. In addition, the microstructure changes of reinforced Nafion XL membrane are examined through microscopy studies where two distinct features are observed: crack propagation in outer ionomer layers and interfacial delamination. These two features well explain the distinct fatigue crack propagation behavior of Nafion XL membrane, such as slow and almost constant fatigue crack propagation rate. The findings reported here are not only beneficial for understanding fatigue crack propagation mechanism of reinforced composite membrane but also helpful for designing and optimizing composite ion-conductive membranes.

The effects of pre-cyclic stress on fracture properties and fatigue crack propagation life of 7N01 aluminum alloy

Material properties are of great significance for structural safety. In this paper, the mechanical property tests are described and fatigue crack propagation (FCP) behavior for material that had experienced a certain number (10e7) of pre-cyclic stress (PCS) cycles are analyzed. The results illustrate that the material properties increase with the increase of PCS level for relatively lower PCS, and decrease with further increase of PCS. The FCP experiment shows that the FCP rates of the specimens subjected to PCS become lower than that of the specimen without PCS. ‘Turning’ points are clearly shown in the FCP rate curves, indicating that the FCP rate in the range of small ΔK is more sensitive to PCS. This paper presents a bi-linear approximation to describe the FCP behavior and predict the FCP life. It is found that the PCS has ‘coaxing’ effect on material FCP behavior. The fracture morphologies of the material with and without PCS are analyzed to discuss the ‘coaxing’ effect on microstructures. The PCS decreases the void number and void size in fatigue crack initiation zone, decreases the striation width in FCP zone, and increases the dimple size in final rupture zone.

Improving the Fatigue Crack Propagation Resistance and Damage Tolerance of 2524-T3 Alloy with Amorphous Electroless Ni-P Coating

The surface microhardness, as well as the fatigue crack propagation (FCP) resistance of 2524-T3 alloy, is improved by producing a 20-μm-thick amorphous electroless Ni-12% P coating on its surface. Compared to the substrate, this deposited EN coating possesses higher strength properties and exhibits a greater ability of accommodating the plastic deformation at the fatigue crack tip, thereby remarkably improving the FCP resistance in near-threshold and early Paris regimes. Regardless of the similar FCP rates in Paris regime (ΔK ≥ 16.2 MPa m0.5), the coated sample exhibits extended Paris regime and enhanced damage tolerance.

Over-aging influenced matrix precipitate characteristics improve fatigue crack propagation in a high Zn-containing Al-Zn-Mg-Cu alloy

A high Zn-containing Al-Zn-Mg-Cu alloy was researched due to excellent overall performances. The hardness, electrical conductivity and mechanical properties were investigated and detailed aging parameters subjected to T6, T79, T74 and T73 were proposed. The matrix precipitates of various aging tempers were investigated by transmission electron microscope (TEM) and high-resolution transmission electron microscope (HREM) techniques and quantitative information of matrix precipitates was extracted from the bright-field TEM images projected along 〈110〉Al orientation with the aid of an imaging analysis. The fatigue crack propagation (FCP) behaviors subjected to different aging tempers were investigated, the related fracture morphology of the stable expanding regions was analyzed and corresponding fatigue striations were measured to verify the FCP rates. The results showed that with the deepening of aging degree, the matrix precipitates coarsened with an expanding of precipitate size distribution and an enlargement of average precipitate size while the precipitates evolved from GP zones and η' phase to η' phase and η phase. The FCP resistance was improved with the aging degree deepens and the evolution of related tearing ridge, tearing dimple and fatigue striation also proved it. Due to a smaller cyclic plastic zone of the alloy with various tempers compared with the average grain size, the FCP rate was significantly influenced by matrix precipitate characteristics and a theoretical model which directly correlated FCP rate with matrix precipitate characteristics was proposed. From T6 state to T73 state, the enlargement of cuttable GP zones and η' phase, its evolution to η phase and the nucleation, growth and coarsening of η phase were in favor of enhancing the FCP resistance.

Effect of Tensile or Compressive Overload on the Fatigue Crack Growth of Friction Stir Welded 19501 Aluminum Alloy

Abstract In this paper, crack growth tests on 19501 aluminum alloy friction stir welded butt joints after a single overload cycle in tension or in compression on compact tension (CT) specimens, along with constant amplitude tests, are investigated. The effect of the tensile overload or compressive overload applied just after 20,000 load cycles on the crack growth rate, crack growth path, and growth profile are investigated. The overload cycle exhibits a significant influence on fatigue life endurance. Increases of 39.14 % and 131.58 % in fatigue life with delayed cycles of 13,874 and 46,646 are seen because of the application of 1.78 times tension or compression overload. The crack propagation after overload cycles is investigated by inspection of the fatigue crack propagation rate. Results are discussed with reference to the retardation characteristic parameters, such as retardation ratio, plastic zone size, crack deviation, etc. Compressive overload is found to be more effective in retarding the growth rates compared to tensile overload of the same magnitude in friction stir welded joints. Secondary crack formation is seen after the tensile overload, whereas no such secondary crack formation is observed with compressive overload. The possible reasons for the crack growth behavior due to tensile or compressive overload are further analyzed in terms of scanning electron micrographs and plastic zone size.