Fatigue Crack Propagation Rate Research Articles

Effect of dynamic embrittlement on high temperature fatigue crack propagation in IN718 – experimental characterisation and mechanism-based modelling

At elevated temperatures, the nickel-base superalloy IN718 is prone to the failure type ‘dynamic embrittlement’. Dynamic embrittlement is assumed to be driven by tensile stress-controlled oxygen grain boundary diffusion. Oxygen embrittles the grain boundaries in front of a crack and results in a fast brittle intercrystalline crack propagation. In order to reveal the mechanism of dynamic embrittlement, high temperature fatigue crack propagation tests were carried out at 650 °C in vacuum and air applying various dwell times and testing frequencies. The crack growth was monitored by the ACPD technique and a far-field microscope. The observations show that at low stress intensity factor ranges, crack propagation mainly occurs in the unloading and loading parts of the cycle and only minor crack propagation takes place during the dwell time. With increasing dwell time, the contribution of the crack propagation at constant stress increases. A mechanism-based model was developed on the basis of these findings which allows for a quantitative description of the effect of dynamic embrittlement on fatigue crack propagation rate. The results of respective simulations correspond very satisfactorily to the experimental data. Hence, the model is suitable for lifetime assessment.

On the role of texture in governing fatigue crack propagation behavior of 2524 aluminum alloy

The texture evolution of AA2524 cold-rolled sheet during annealing process and texture effect on fatigue crack propagation (FCP) behavior were investigated. Results showed the sample annealed at 400°C for 45min and solid solution-treated at 490°C for 20min had the strongest Goss and P textures, and possessed the lowest FCP rates. EBSD examination indicated Goss, P, Q, Cube, R and S grains induced great crack deflection to retard crack growth, when having large twist and tilt angle component boundary or pure twist boundary with neighboring grains. In the case of small angle boundary or relatively large angle tilt boundary with adjacent grains, Cube, Copper and R grains induced little crack deflection. Additionally, SEM results showed, in the presence of large twist angle component boundary or pure twist boundary, more severely plastic deformation presenting numerous slip bands occurred in Cube, R and S grains than that in Goss, P and Q grains during crack propagation. In contrast, no evident slip bands formed in Cube, Copper and R grains when having small angle boundaries or relatively large tilt angle boundary.

On the fatigue performance of laser hybrid welded high Zn 7000 alloys for next generation railway components

The fatigue behavior of laser hybrid welded 7000 series aluminum alloys with high Zn was investigated by electron backscattered diffraction, transmission electron microscopy, energy-dispersive spectroscopy, basic mechanical tests, fatigue cracking rate, synchrotron radiation X-ray computed micro-tomography, high-cycle fatigue and residual stress-based fatigue crack growth simulation. The results show that hybrid laser welding process leads to essential changes of microstructure and micro-hardness. It has also been found that the fatigue performance of grinded laser shock treated joints is significantly improved compared with that of as-received welded joints. Besides, hybrid laser welded joints present slightly higher fatigue crack propagation rate than base metal under load ratios of R=0.1 and 0.3, which can be intrinsically attributed to the higher crackability of weld and inner porosity in terms of grain disorientation and three-dimensional gas pores. Based on finite element modeling, a new fatigue crack propagation life model is established by incorporating weld residual stress, and the predictions are in good agreement with experimental results under different welding parameters.

Desarrollo de un método de simulación física de zonas térmicamente afectadas en soldaduras de acero, para estudios de propagación de grietas por fatiga

Se presenta el desarrollo de un método de simulación física, a través del sistema Gleeble, de las zonas térmicamente afectadas de uniones soldadas de aceros estructurales (A283 Gr. C y A106 Gr. B), para el estudio de la propagación de grietas por fatiga. Para ello fue necesario utilizar probetas de 90 mm de largo, 49,5 mm de ancho y 7 mm de espesor, en una configuración que no se encuentra dentro de los estándares del sistema para este tipo de simulaciones. Con este método se reprodujeron satisfactoriamente, y por separado, en diferentes probetas del metal base correspondiente, la zona de grano fino, la zona de grano grueso y una zona de múltiples pasadas presentes en uniones soldadas fabricadas por FCAW. Las zonas simuladas fueron reproducibles, homogéneas, no presentaron defectos ni tensiones residuales y tuvieron un tamaño considerablemente grande, además que las características microestructurales (como el tamaño de grano y el porcentaje de fases) fueron muy similares a su contraparte real, lo que permitió estudiar por primera vez el efecto neto de la microestructura sobre el comportamiento a la fatiga en estas zonas. De esta manera, el desarrollo realizado permitió aumentar considerablemente las aplicaciones y las ventajas que esta técnica puede tener, además de mejorar la comprensión del comportamiento a la fatiga de las soldaduras de acero.

Multistage-aging process effect on formation of GP zones and mechanical properties in Al–Zn–Mg–Cu alloy

The tensile properties and fatigue behavior of an Al–Zn–Mg–Cu alloy were investigated by performing tensile tests and fatigue crack propagation (FCP) tests. The tensile results show that lower aging temperature modified retrogression and re-aging (RRA) process enhances the elongation, but reduces the strength of the alloy, as compared to conventional RRA process which employs peak aging temperature. Both ductility and strength, however, are increased by employing a natural aging prior to re-aging based on the former modified RRA process. Fatigue test results show that both routes reduce FCP rate. Especially, the lower re-aging temperature modified RRA process obtains the lowest FCP rate. Natural aging treatment could enhance the nucleation rate of GP zones. A large amount of GP zones could be cut by dislocations, which is responsible for the highest tensile strength and elongation, as well as lower FCP rate.

Effect of laser shock peening on microstructure and fatigue crack growth rate of AZ31B magnesium alloy

The fatigue crack growth tests were introduced for studying the effect of Laser Shock Peening (LSP) on fatigue crack growth rate of AZ31B Magnesium (Mg) alloy. The surface layer structures of laser treated samples were analyzed by using transmission electron microscopy (TEM) and X-ray diffraction (XRD) method. The fracture surface morphologies of samples were characterized by scanning electron microscopy (SEM). Moreover, the residual stress and surface roughness were also examined. A modified incremental polynomial method was used to obtain the fatigue crack propagation rates of six samples each group under the same crack length. The results show that the nanometer grains (with an average size of 17.5 nm) can be generated in the surface layer by using the optimized laser parameters. Surface roughness decreased from 1.177 μm to 0.713 μm after LSP. The depth of compressive residual stress induced by LSP reached to about 0.8 mm from the top surface. Comparing with the original samples, the obviously lower fatigue crack growth rates for the LSP treated samples was observed due to a combination of grain refinement, residual compressive stress and the barrier effect of surface nanocrystalline layer.

Debonding of Carbon Fiber–Reinforced Polymer Patches from Cracked Steel Elements under Fatigue Loading

This paper summarizes the details of a numerical and experimental research program that was conducted to study the debonding of carbon fiber–reinforced polymer (CFRP) patches from cracked steel members under fatigue loading. A preliminary numerical model was developed to investigate the influence of patch debonding on the fatigue life of cracked steel elements. Results indicated that altering the shape and increasing the size of the debonded region could change the calculated crack growth rate by up to 54 times. To validate the model, six steel edge–notched tension coupons were patched with CFRP materials and tested under fatigue loading, while full-field strain distributions were monitored using a digital image correlation (DIC)–based measurement system. Based on the experimental results the numerical model was refined to incorporate the interfacial traction-separation behavior. A parametric study was conducted using the refined numerical model. The results indicate that the size and shape of the debonded region, and therefore the fatigue crack propagation rate, are not only dependent on the fatigue detail and the crack length, but also on the maximum magnitude of the applied fatigue load and the properties of the bonded interface.

Fatigue Life of Welded Joints of High-Strength Structural Steel S960QL

The article presents the results of research on low cycle fatigue strength of welded joints of structural steel S960QL. Two types of butt welds were analysed: I-joints and V-joints. The tests were performed under load controlled using the total strain amplitude εac. Fatigue life analysis was conducted based on the Manson-Coffin-Basquin equation, which made it possible to determine fatigue parameters. High concordance was found of the adopted description model with experimental results. Studies have shown differences in the fatigue life of the various joints analysed, wherein I-joints showed about 20-50% higher fatigue life. Fractographic tests of fatigue fractures in joints revealed the details of fatigue cracking and differences in the propagation rate of fatigue cracks.

Fatigue crack propagation behavior of Ni-based superalloys after overloading at elevated temperatures

The fatigue crack propagation behavior of three superalloys subjected to a single overloading at elevated temperatures was investigated. The fatigue crack propagation rate FCPR versus stress intensity factor range data da/dN—ΔK were calculated using the two-point secant method. It was found that the crack growth rates of the investigated materials were retarded after overloading with an overload ratio ROL=1.6. The size of the plastic zone in the front of the crack tip and its relation to loading level were discussed. The overload retardation effects are attributed to crack closure. The fatigue damage in the plastic zone can also be a factor to explain the overload retardation.

Properties of nanofiller-loaded poly (methyl methacrylate) bone cement composites for orthopedic applications: a review.

There is a large body of literature on new generations of poly (methyl methacrylate) bone cements that address one or more of the material's shortcomings. Among these are cements in which one of the constituents is a nanofiller, such as nano-sized barium sulfate, multiwalled carbon nanotubes, natural nanoclay, mesoporous silica nanoparticles, or oleic acid-capped silver nanoparticles. This article is a review of the literature on the properties of these nanofiller-loaded bone cements (NFLBCs). Some key characteristics of the literature are that (1) in a number of studies, clinically relevant properties were determined, examples being maximum exotherm, setting time, fatigue life, and compressive modulus; (2) in some studies, properties were not determined in accordance with approved bone cement testing specifications, an example being fatigue life; and (3) there are a number of clinically relevant properties that were not determined in any of the studies, examples being fatigue crack propagation rate and dynamic compression creep life. These observations, as well as other considerations, suggest 12 areas for future study, such as determination of dynamic creep compliance (using nanoindentation), determination of compressive fatigue life for cements to be used in vertebral compression fracture augmentation, elucidation of toughening mechanism(s) in each type of NFLBC, and conducting well-designed clinical trials. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 1260-1284, 2017.

The effect of single overloads in tension and compression on the fatigue crack propagation behaviour of short cracks

The crack propagation behaviour in cast quenched and tempered steel after one overload cycle in tension as well as in compression on short cracks is investigated in deep notched specimens. The overload cycle exhibits a significant influence on the fatigue life endurance, due to the formation of an overload plastic zones in front of the crack tip. The crack propagation after overload cycles is investigated by inspection of the fatigue threshold R-curve and fatigue crack propagation rate. Tension overload increased the long crack threshold and reduced the R-curve effect, whereas overloads in compression reduced the crack growth resistance and shifted the threshold value to larger crack extension. A simple FE simulation was also performed to investigate the variation in the contribution of plasticity induced crack closure during crack propagation after the overload. Macroscopic mechanistic and dislocation models are introduced to explain the results obtained.

Fatigue properties of X80 pipeline steels with ferrite/bainite dual-phase microstructure

Fatigue properties are important parameters for the safety design and security evaluation of gas transmission pipelines. In this work, the fatigue life at different stresses of full-thickness X80 pipeline steel plates with a ferrite/bainite dual-phase microstructure was investigated using a MTS servo-hydraulic universal testing machine; the fatigue crack propagation rate was examined with CT specimens by using an INSTRON 8874 testing machine. Results indicate that fatigue life increases as maximum stress decreases; as the maximum stress decreases to the maximum operating stress (440MPa), the fatigue life is approximately 4.2×105 cycles. The fatigue crack of the full-thickness fatigue life specimens is generated at the surface of rolled steel plates and then the crack propagates and grows inward until a fracture is formed. During fatigue crack growth, a transitional turning point appears in the curve of da/dN with ΔK in the Paris region. The transitional turning point that divides the Paris region to two stages is approximately ΔK≅30MPam1/2. The change in the growth rate (da/dN) is related to the variation of the crack path and in the fracture mode because of the possible microstructural sensitivity of fatigue crack propagation behavior. This study also discussed the effect of duple phase ferrite/bainite microstructure on fatigue crack initiation and propagation.

In situ delta ferrite estimation and their effects on FCPR at different orientations of multipass shielded metal arc welded SS304L

Solidification and re-solidification with subsequent phase transformation in different welding zones of multipass weldment play a vital role to define mechanical properties of joint structures. To avoid from penetration defects and inclusion contamination in molten metal pool, multipass shielded metal arc welding (SMAW) of SS304L is performed in this research using E308L filler metal. The objective of this research is to investigate out the multiple welding passes effects on microstructural evolution, in situ delta ferrite percentage and different mechanical properties. A metallurgical microscope is used to examine the delta ferrite phase in austenite. Instead of depending on general ferrite number (FN) obtained through rigorous physical attempts, a MATLAB® code for image processing is developed to estimate the local percentage of delta ferrite, within the austenitic phase. To reveal delta ferrite effects, its local percentage is discussed in relation to various properties. With the help of this technique, a good estimation is also made to detect the localized delta ferrite morphologies. The fusion zone (FZ) solidified in ferritic-austenitic mode (FA mode) comprises two basic delta ferrite morphologies; lacy and vermicular. Grain growth behavior and ferritic percentage of base metal (BM), heat affected zone (HAZ) and fusion zone (FZ) are discussed in single, double and triple pass weldments. Effects of delta ferrite percentage and morphology on fatigue crack initiation as well propagation are also investigated in different zones of each pass weldment. Fatigue crack propagation rate (FCPR) in different orientation with respect to welding zones is checked and it is noticed that propagation rate is much lower in perpendicular direction as compared to parallel one. Due to specific microstructural pattern, HAZ has greater fatigue crack propagation rate (FCPR) in comparison to BM and FZ. Fatigue crack propagation found highest resistant in triple pass weldment due to high concentration of lacy ferrites. However, in some cases it is found that crack mechanics is actually responsible instead of microstructural effects.

Crack Closure Effects on Fatigue Crack Propagation Rates: Application of a Proposed Theoretical Model

Structural design taking into account fatigue damage requires a thorough knowledge of the behaviour of materials. In addition to the monotonic behaviour of the materials, it is also important to assess their cyclic response and fatigue crack propagation behaviour under constant and variable amplitude loading. Materials whenever subjected to fatigue cracking may exhibit mean stress effects as well as crack closure effects. In this paper, a theoretical model based on the same initial assumptions of the analytical models proposed by Hudak and Davidson and Ellyin is proposed to estimate the influence of the crack closure effects. This proposal based further on Walker’s propagation law was applied to the P355NL1 steel using an inverse analysis (back-extrapolation) of experimental fatigue crack propagation results. Based on this proposed model it is possible to estimate the crack opening stress intensity factor,Kop, the relationship betweenU=ΔKeff/ΔKquantity and the stress intensity factor, the crack length, and the stress ratio. This allows the evaluation of the influence of the crack closure effects for different stress ratio levels, in the fatigue crack propagation rates. Finally, a good agreement is found between the proposed theoretical model and the analytical models presented in the literature.

炭素短繊維強化PPSの射出成形平板における疲労き裂進展に及ぼす試験温度の影響

The temperature effect on the fatigue crack propagation behavior was studied with center-notched specimens which were cut from injection-molded plate of short fiber reinforced plastics, PPS, at three angles of the loading axis relative to the molding flow direction. The crack propagation behavior was investigated at four temperatures : room temperature (RT=298 K), 343K, 373 K and 403 K. In the relation between crack propagation rate, da/dN, and the stress intensity factor range, ΔK, the propagation rate of fatigue cracks was slowest for the zero angle, MD, and increased with increasing fiber angle at all temperatures. For each orientation, da/dN was nearly the same at RT and 345K, and increased greatly at temperatures of 373 K and 403K above Tg (=363 K). When da/dN was correlated to the J-integral range, ΔJ, the relations for different orientations became closer, and also the influence of temperature on da/dN was decreased. The inelastic deformation and the decrease of elastic constants are responsible for crack acceleration seen in da/dN vs ΔK relation at high temperatures.

Fatigue crack growth behaviour of the 6082-T6 aluminium using CT specimens with distinct notches

This paper presents a study aiming at characterizing the fatigue crack growth behaviour of the 6082-T6 aluminium alloy using standard CT specimens. The fatigue crack growth threshold is also characterized according to the ASTM E 647 standard. Besides the standard CT specimens with sharp notches, specimens with initial circular notches were also fatigue tested. The threshold stress intensity factor ranges, ΔKth, are compared for the distinct notches under study. The fatigue crack propagation rate in the regime II are obtained using the Paris law. A comparison is made between the CT specimens geometries with distinct notches under consideration. The ΔKth values for the geometry of standard CT specimens present lower values when compared with the geometry of the CT specimens with blunted notches.

Influence of variable stress ratio during train operation on residual fatigue lifetime of railway axles

Railway axles are subjected to cyclic loading, which could lead to fatigue failure. Therefore, it is desired to know residual fatigue lifetime of railway axles to ensure safe operation of trains. Because of detection of relatively small fatigue cracks is not guaranteed an estimation of residual fatigue lifetime is based on damage tolerance approach. The acting stress ratio is variable due to variable amplitude loading and load caused by existence of press-fitted wheel in the vicinity of assumed crack. The contribution is focused on influence of variable stress ratio in EA4T steel on residual fatigue lifetime of railway axles. The influence of stress intensity factor on fatigue crack propagation rate was experimentally evaluated for three different stress ratios, which correspond to operation conditions. Two different expressions of fatigue crack propagation rate were used and mutually compared to show influence of the stress ratio on residual fatigue lifetime of structure made of EA4T steel. The first expression considers stress intensity factor range (respecting stress ratio R) and the second one uses maximal value of the stress intensity factor. The paper shows ability of both expressions to describe experimental data obtained under different stress ratios and their influence on estimated residual fatigue lifetime values. The results obtained contribute to the better estimation of residual fatigue lifetime of railway axles and generally to the safer rail transportation.

Fatigue crack propagation properties of submicron-thick freestanding copper films in vacuum environment

Fatigue crack propagation experiments were conducted in approximately 500 nm thick freestanding copper (Cu) films in both air and vacuum environments to clarify the effects of vacuum environment on fatigue crack propagation properties. First, we newly developed an experimental setup for fatigue crack propagation experiments of the freestanding Cu films inside a vacuum chamber of a field-emission scanning electron microscope (FESEM). Fatigue crack propagation experiments were conducted in ambient air and vacuum environment of the FESEM chamber (˜10-4 Pa) under load-control conditions with constant maximum stress and at a stress ratio R of 0.1. In situ FESEM observations of fatigue crack propagation confirmed that preceding intrusions/extrusions were formed ahead of the fatigue crack tip, and the fatigue crack then propagated preferentially through these intrusions/extrusions in the lower stress intensity factor range (ΔK). In the higher ΔK, the fatigue crack propagated in tensile fracture mode. These mechanisms of fatigue crack propagation were similar to those in air. The relationships between fatigue crack propagation rate (da/dN) and stress intensity factor range (ΔK) in both environments were roughly within a narrow band in the region of ΔK ≳ 4—5 MPam1/2. On the other hand, da/dN in vacuum became smaller than that in air in the region of ΔK ≲ 4—5 MPam1/2. FESEM observations confirmed that the fracture surfaces morphologies depended on the environments in ΔK ≲ 4—5 MPam1/2: flat fracture surface were mainly observed in air, whereas, in vacuum environment, blunt fracture surface with fine roughness were mainly observed. This suggests that reversible cyclic slip deformation and rewelding occurred in vacuum environments, resulting in smaller da/dN in vacuum than air.

On the mechanism of dynamic embrittlement and its effect on fatigue crack propagation in IN718 at 650°C

IN718 is a commonly used nickel-base alloy for high temperature applications, e.g., in gas and steam turbines. At elevated temperatures, this and other superalloys are prone to the failure mechanism "dynamic embrittlement". Dynamic embrittlement can be considered as a kind of stress corrosion cracking, driven by tensile-stress-controlled oxygen grain boundary diffusion. Oxygen embrittles the grain boundaries by weakening the grain boundary cohesion resulting in fast and brittle intercrystalline crack propagation. In order to reveal the mechanism of dynamic embrittlement, high-temperature fatigue crack propagation tests were carried out at 650°C applying various dwell times and testing frequencies. Most of the tests were performed in laboratory air, but some experiments were run in vacuum as well, in order to eliminate environmental effects and, hence, to define the reference fatigue crack propagation behaviour. The observations show that at low stress intensity factor ranges ΔKI, continuous crack growth occurs. At intermediate values of ΔKI, no crack propagation takes place during the dwell part of the cycle. Rather, the crack extends during unloading and reloading between subsequent hold times. The time necessary to grow the crack under sustained load during the dwell time was found to decrease with increasing stress intensity factor. Therefore, at high values of ΔKI, there is a contribution of the crack propagation at constant stress, since the incubation time is shorter than the dwell time. A mechanism-based model was developed for the range of test parameters, where intergranular and transgranular areas exist side by side in the fracture surface. The total crack growth per cycle is calculated by a linear combination of the intergranular and the transgranular contribution using the corresponding area fractions as weighting factors. It is shown that simulation calculations based on this model approach correspond very reasonably to the experimental observations. Hence, the model provides a quantitative mechanismen-related description of the effect of dynamic embrittlement on fatigue crack propagation rate.

Evaluation of fatigue crack propagation in dissimilar Al/steel friction stir welds

The fatigue crack propagation (FCP) behavior was investigated in the interfacial boundary of the dissimilar joint between 6061-T6 aluminum (Al) alloy and type 304 stainless steel fabricated by a friction stir welding (FSW) technique. The tensile tests were conducted using the FSW Al/steel joints fabricated with different welding conditions and the tensile strength of 194 MPa was obtained in the condition of the rotating speed of 800 rpm and the tool offset of 0.2 mm. Near the welded boundary, the hardness decreased in the Al side due to the resolution of precipitates during the FSW process while the hardness increased in the steel side due to the work hardening. The compact tension (CT) specimen was sampled from the FSW joint so that the initial notch aligned with the interface of the joint and side-grooves were machined to propagate a crack along the interface. Prior to the FCP test, re-T6 heat treatment after the FSW was conducted on the CT specimen to even the hardness in the Al side. In the FCP test using the re-heat-treated CT specimen with a side-groove, the crack propagated straight along the welded interfacial boundary and the FCP rate in the interfacial boundary was measured. The energy release rate was calculated by a finite element method (FEM) and used to evaluate the interfacial crack tip stress intensity instead of the conventional stress intensity factors. The FCP rate for the same energy release rate range was comparable or slightly faster in the interfacial boundary of the Al/steel FSW joint than that of the Al base metal.