Reprint of “Multiaxial fatigue property of Ti–6Al–4V using hollow cylinder specimen under push-pull and cyclic inner pressure loading”

This paper studies a multiaxial fatigue crack mode and a fatigue life of Ti–6Al–4V. Load controlled fatigue tests at room temperature were carried out using a hollow cylinder specimen under multiaxial loading with principal stress ratio λ equal to 0, 0.4, 0.5 and 1.0 and loading ratio R kept constant and equal to 0. λ is defined as λ = σ₂/σ₁, where σ₁ and σ₂ are maximum and intermediate/minimum principal stresses, respectively. Here, the test at λ = 0 is a uniaxial loading test and that at λ = 1.0 an equi-biaxial loading test. A testing machine employed was a newly developed multiaxial fatigue testing machine which can apply push-pull and reversed torsion loading with inner pressure into the hollow cylinder specimen. Based on the obtained results in this study, multiaxial fatigue properties are examined, where the fatigue life evaluation and the crack mode are discussed. The fatigue life is reduced with an increase of λ, due to cyclic ratcheting and crack mode in multiaxial loading. The crack mode is also affected by the surface condition resulting from cut-machining.     

Multiaxial fatigue property of Ti–6Al–4V using hollow cylinder specimen under push-pull and cyclic inner pressure loading

This paper studies a multiaxial fatigue crack mode and a fatigue life of Ti–6Al–4V. Load controlled fatigue tests at room temperature were carried out using a hollow cylinder specimen under multiaxial loading with principal stress ratio λ equal to 0, 0.4, 0.5 and 1.0 and loading ratio R kept constant and equal to 0. λ is defined as λ = σ₂/σ₁, where σ₁ and σ₂ are maximum and intermediate/minimum principal stresses, respectively. Here, the test at λ = 0 is a uniaxial loading test and that at λ = 1.0 an equi-biaxial loading test. A testing machine employed was a newly developed multiaxial fatigue testing machine which can apply push-pull and reversed torsion loading with inner pressure into the hollow cylinder specimen. Based on the obtained results in this study, multiaxial fatigue properties are examined, where the fatigue life evaluation and the crack mode are discussed. The fatigue life is reduced with an increase of λ, due to cyclic ratcheting and crack mode in multiaxial loading. The crack mode is also affected by the surface condition resulting from cut-machining.     

Crack initiation in VHCF regime on forged titanium alloy under tensile and torsion loading modes

This paper is focused on the VHCF behavior of aeronautical titanium alloy under tensile and torsion fatigue loadings. Tensile tests were carried out with two different stress ratios: R = −1 and R = 0.1. Both surface and subsurface crack initiations were observed. In the case of subsurface crack initiation several fatigue life controlling mechanisms of crack initiation were found under fully-reversed loading conditions: initiation from (1) strong defects; (2) ‘macro-zone’ borders; (3) quasi-smooth facets and (4) smooth facets. Tests with stress ratio R = 0.1, have shown that initiation from the borders of ‘macro-zones’ becomes the dominant crack initiation mechanism in presence of positive mean stress. Like for the tensile results, surface and subsurface crack initiations were observed under ultrasonic torsion in spite of the maximum shear stress location on the specimen surface. But the real reason for the subsurface crack initiation under torsion was not found.     

Influence of casting defect and SDAS on the multiaxial fatigue behaviour of A356-T6 alloy including mean stress effect

The effect of mean stress on the multiaxial High Cycle Fatigue (HCF) behaviour of cast A356-T6 alloy containing natural and artificial defects with varying Secondary Dendrite Arming Spacing (SDAS) has been investigated experimentally. Tension, torsion and combined tension–torsion fatigue tests have been performed for two loading ratios: R_σ = 0 and R_σ = −1. A Scanning Electron Microscopy (SEM) was used to perform fractographic analysis of the fracture surfaces to characterise the defect causing failure. In order to gauge the effect of mean stress and defects, the results are reported with standard Kitagawa and Haigh diagrams. A surface response method has been employed to characterise the influence of defect size and SDAS on the fatigue limit. Relationships and correlations describing the observed behaviour have been incorporated in the Defect Stress Gradient (DSG) criterion with the goal of determining the influence of defects on the fatigue limit through a stress gradient approach.Results clearly show that: (i) the mean stress has a detrimental effect on the fatigue limit. This effect is a function of the loading, which is most pronounced under tension, less under combined tension–torsion, and least pronounced under torsion conditions; (ii) in the absence of defects, the SDAS controls the fatigue limit of cast A356, this effect is much more important under torsion loading; (iii) the DSG criterion is improved by the mean of a parameter describing the microstructure effect through the SDAS.     

Cyclic torsion very high cycle fatigue of VDSiCr spring steel at different load ratios

Cyclic torsion fatigue tests with superimposed static torsion loads are performed with VDSiCr spring steel with shot-peened surface in the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) regime. Fatigue properties are investigated at load ratios R = 0.1, R = 0.35 and R = 0.5 up to limiting lifetimes of 5 × 10⁹ cycles with a newly developed ultrasonic torsion testing method. Increasing the load ratio reduces the shear stress amplitude that the material can withstand without failure. Fatigue cracks are initiated at the surface in the HCF regime. In the VHCF regime, cracks are preferentially initiated internally in the matrix, below the surface layer with compression residual stresses, and less frequently at the surface. Cyclic and mean shear stresses with 50% survival probability in the VHCF regime are presented in a Haigh diagram. Linear line approximation delivers a mean stress sensitivity of M = 0.33 for load ratios between R = −1 and R = 0.5.     

Evaluation of residual stress relaxation and its effect on fatigue strength of AISI 316L stainless steel ground surfaces: Experimental and numerical approaches

This paper is aimed at evaluating the residual stress relaxation and its effect on the fatigue strength of AISI 316L steel ground surfaces in comparison to electro-polished surfaces. An experimental evaluation was performed using 3-point and 4-point bending fatigue tests at R_σ = 0.1 on two sets of notched specimens finished by electro-polishing and grinding. The residual stress fields were measured at the notch root of specimens, before and after fatigue tests, by means of the X-ray diffraction technique. It was found a degradation of about −35% for the 4-point bending fatigue limit at 2 × 10⁶ cycles of the ground specimens in comparison to the electro-polished ones. This degradation is associated with a slight relaxation of the grinding residual stresses which remain significant tensile stresses at the stabilized state. While under the 3-point bending test, these residual stresses relax completely and provoke a noticeable increase of the fatigue limit estimated at about 50% in comparison to the 4-point bending fatigue test. The numerical evaluation of residual stress relaxation was carried out by FE analyses of the cyclic hardening behaviour of the ground layer. The isotropic and nonlinear kinematic model proposed by Chaboche was used and calibrated for the base material and the ground layer. The results show that residual stresses relax to a stabilized state characterized by elastic-shakedown response. This stabilization is occurred after the first cycle of the 4-point bending test corresponding to the higher stress concentration (K_t-4p = 1.66), while it requires many cycles under the 3-point bending test corresponding to the lower stress concentration (K_t-3p = 1.54). The incorporation of stabilized residual stress values into the Dang Van’s criterion has permitted to predict with an acceptable accuracy the fatigue limits under both bending modes.     

Threshold crack growth behavior of shear and tensile cracks

Crack-face interference-free mode I and mode II crack-growth data was combined with smooth axial (λ = ε_xy/ε_xx = 0) and torsional (λ = ∞) endurance limit data to develop unified crack growth models that incorporate both shear and tensile cracking. The crack growth models incorporated growth from a slip band (including short crack behavior) size crack until the final failure of a long crack, and the ability to switch between crack growth on shear planes to growth on tensile planes. The models successfully predicted smooth specimen crack-face interference-free fatigue lives and gave reasonable estimates of the smooth specimen endurance limits of crack-face interference free tubular tests run at intermediate strain ratios (λ = 3/4, 3/2, and 3). The series of Kitigawa–Takahashi (threshold fatigue) diagrams developed from the models help illustrate the competition between shear and tensile cracking at the fatigue limit under crack-face interference-free crack growth.     

Consideration of mean-stress effects on fatigue life of welded magnesium joints by the application of the Smith–Watson–Topper and reference radius concepts

Three types of welded joints have been assessed with regard to their fatigue strength based on the mean-stress damage parameter model according to Smith, Watson, and Topper (P_SWT) and on the reference notch radius concept. These analyses were performed with three different stress ratios, R = −1, R = 0 and R = 0.5, under axial loading. For each stress level, the corresponding Neuber-Hyperbolas, Masing-loops and their maximum stress and maximum strain values were determined in order to calculate damage parameter (P_SWT) values. For a given weld geometry, this damage parameter is able to unify the fatigue results for different R-values within at a tight scatter band and therefore to consider the mean-stress effect. The unification of the results for different weld geometries is performed by applying the reference radii r_ref = 0.05 and r_ref = 1.00 mm as suggested by the IIW-Recommendations.     

Experiments under pure shear and rolling contact fatigue conditions: Competition between tensile and shear mode crack growth

In the present paper, the mechanism of shear crack growth under both pure torsion and mixed mode loadings, simulating rolling contact fatigue testing conditions, has been investigated for a bearing steel and the role of the superimposed compressive stress in subsurface RCF has been clarified both numerically and experimentally. In particular a previous data set of fatigue tests on micro-notched specimens subjected to torsion and out-of-phase loads with |σ_min|/τ_max ≅ 3.5 (LP1) has been complemented with the new tests onto micro-notched specimens loads with |σ_min|/τ_max ≅ 0.7 (LP2) and a test under pure compression. The same tests have been also simulated numerically with a non-linear FE analysis of crack advance. The numerical analyses have been conducted with the aim of demonstrating that the compressive stress fully suppresses the tendency to tensile mode growth as the crack extends.Eventually, the competition between tensile and shear mode growth during a fatigue cycle has been investigated theoretically in terms of local branch SIFs. In particular, the conditions for the branch crack growth have been examined on the basis of the effective SIFs: the crack tip shielding effects due to the crack surface interference (both the mode I contribution caused by the asperity mismatch and the shear attenuation produced by the frictional stresses) have been quantified by employing a model for crack sliding interaction under pure mode III and mixed mode I + III loadings.     

Gigacycle fatigue initiation mechanism in Armco iron

Microstructure irreversibility plays a major role in the gigacycle fatigue crack initiation. Surface Persistent Slip Bands (PSB) formation on Copper and its alloy was well studied by Mughrabi et al. as typical fatigue crack nucleation in the very high cycle fatigue regime. In the present paper, Armco iron sheet specimens (1 mm thickness) were tested under ultrasonic frequency fatigue loading in tension–compression (R = −1). The test on the thin sheets has required a new design of specimen and new attachment of specimen. After gigacycle fatigue testing, the surface appearance was observed by optical and Scanning Electron Microscope (SEM). Below about 88 MPa stress, there is no PSBs even after fatigue cycle up to 5 × 10⁹. With a sufficient stress (above 88 MPa), PSBs in the ferrite grain was observed by optic microscope after 10⁸ cycles loading. Investigation with the SEM shows that the PSB can appear in the body-centered cubic crystal in the gigacycle fatigue regime. Because of the grain boundary, however, the local PSB did not continually progress to the grain beside even after 10⁹ cycles when the stress remained at the low level.     

Effect of welding parameters on mechanical and microstructural properties of dissimilar AA6082–AA2024 joints produced by friction stir welding

The effect of processing parameters on the mechanical and microstructural properties of dissimilar AA6082–AA2024 joints produced by friction stir welding was analysed in this study. Different samples were produced by varying the advancing speeds of the tool as 80 and 115 mm/min and by varying the alloy positioned on the advancing side of the tool. In all the experiments the rotating speed is fixed at 1600 RPM. All the welds were produced perpendicularly to the rolling direction for both the alloys. Microhardness (HV) and tensile tests performed at room temperature were used to evaluate the mechanical properties of the joints. The mechanical tests were performed on the joints previously subjected to annealing at 250 °C for 1 h. For the fatigue tests, a resonant electromechanical testing machine was employed under constant loading control up to 250 Hz sine wave loading. The fatigue tests were conducted in the axial total stress–amplitude control mode, with R = σ_min/σ_max = 0.1. In order to analyse the microstructural evolution of the material, the welds’ cross-sections were observed optically and SEM observations were made of the fracture surfaces.     

Short cracks initiated in Al 6013-T6 with the focused ion beam (FIB)-technology

The fatigue crack growth behaviour of short corner cracks in the Aluminium alloy Al 6013-T6 was investigated. The aim was to determine the crack growth rates of small corner cracks at a stress ratio of R = 0.1, R = 0.7 and R = 0.8 and to find a possible way to predict these crack growth rates from fatigue crack growth curves determined for long cracks. Corner cracks were introduced into short crack specimens, similar to M(T) – specimens, at one side of a hole (Ø = 4.8 mm) by cyclic compression (R = 20). The precracks were smaller than 100 μm (notch + precrack). A completely new method was used to cut very small notches (10–50 μm) into the specimens with a focussed ion beam. The results of the fatigue crack growth tests with short corner cracks were compared with the long fatigue crack growth test data. The short cracks grew at ΔK-values below the threshold for long cracks at the same stress ratio. They also grew faster than long cracks at the same ΔK-values and the same stress ratios. A model was created on the basis of constant K_max-tests with long cracks that gives a good and conservative estimation of the short crack growth rates.     

Experimental study on the fatigue failure mechanism of QP-16 type ball-eye under asymmetric load

The ball eye (BE) is a key connecting component between the insulator and transmission tower, whose fatigue characteristics concern the safety of transmission lines. To understand the fatigue mechanism and characteristics of it, the fatigue test was conducted based on the following data: r = 0.25, S = 500 MPa,then plotting of SN and Δε_axis − N, to analyze the fatigue failure of the test specimen from the macro and micro point of views. The research results show that: the life of BE significantly reduces with the increase of the stress amplitude, but the relative reduction in life is not the same; softening and strain amplitude of the specimen change differently before and after the stress amplitude of 300 MPa; when S ≤ 300 MPa, the fracture is more smooth, the fatigue crack propagation is slow; when S > 300 MPa, the rate of fatigue crack growth is faster, and the fatigue crack growth zones are not obvious. The cracks are easily detectable appear at the joint of the BE and insulator cap, and the cracks along the fracture cross section are constantly expanding, showing multiple fatigue sources and fatigue steps. The number of fatigue steps increases as the magnitude of the tensile stress increases. When S = 500 MPa, the yield strength decreases during the lifetime, the decrease rate of the tensile strength and microstructure strength in each stage are different. Axial lengthening and section shrinkage ratio decrease with the development of fatigue, fatigue evolution process is accompanied by phenomenon of crystalline slip, deformation, dislocation, at the same time, dissipation and decomposition of pearlite occur, and carbide precipitates from the matrix, growing and moving to the grain boundaries, the specific phenomenon of grain growth appears.     

Fatigue properties and fracture analysis of a SiC fiber-reinforced titanium matrix composite

Tension–tension fatigue properties of SiC fiber reinforced Ti–6Al–4V matrix composite (SiC_f/Ti–6Al–4V) at room temperature were investigated. Fatigue tests were conducted under a load-controlled mode with a stress ratio 0.1 and a frequency 10 Hz under a maximum applied stress ranging from 600 to 1200 MPa. The relationship between the applied stress and fatigue life was determined and fracture surfaces were examined to study the fatigue damage and fracture failure mechanisms using SEM. The results show that, the fatigue life of the SiC_f/Ti–6Al–4V composite decreases substantially in proportion to the increase in maximum applied stress. Moreover, in the medium and high life range, the relationship between the maximum applied stress and cycles to failure in the semi-logarithmic system could be fitted as a linear equation: S_max/μ = 1.381 − 0.152 × lgN_f. Fractographic analysis revealed that fatigue fracture surfaces consist of a fatigued region and a fast fracture region. The fraction of the fatigued region with respect to the total fracture surface decreases with the increase of the applied maximum stresses.     

Very high cycle fatigue of nitrided 18Ni maraging steel sheet

Thin sheets of nitrided 18Ni maraging steel are tested under cyclic tension (load ratio R = 0.1) in the very high cycle fatigue (VHCF) regime. The ultrasonic fatigue testing method with a cycling frequency of about 20 kHz has been further developed for these experiments. Sheet specimens with 0.35 mm thickness are mounted on a carrier specimen, they are pre-stressed and are forced to vibrate jointly. Between 10⁷ and 10⁹ cycles, fatigue cracks are initiated exclusively at internal TiN inclusions. The areas of the crack initiating inclusions projected perpendicular to the applied tensile stress are evaluated. The square root of inclusion areas, (area_INC)^1/2 lies between 2.5 μm and 5.3 μm. Considering inclusions as cracks, their stress intensity range is between ΔK_INC = 1.3 MPa m^1/2 and 2.4 MPa m^1/2. The sizes of crack initiating inclusions influence fatigue lifetimes. This is considered in a crack propagation model and by presenting lifetimes versus the stress amplitudes multiplied by (area_INC)^1/12. A mean lifetime of 10⁹ cycles is found at a stress amplitude of 22% of the tensile strength, which is comparable to other high strength steels tested under cyclic tension.     

J-integral-based approach to fatigue assessment of laser stake-welded T-joints

The aim of this paper is to investigate the influence of the plate thickness on the fatigue strength of laser stake-welded T-joints under the tension loading condition. Fatigue tests were conducted on specimens with plate thicknesses below 5 mm subjected to tension loading with the load ratio R = 0. The statistical analysis of the weld geometry showed a normal distribution of the each parameter that was measured. In addition, the parameters had similar proportions in comparison to the specimens with plate thicknesses above 5 mm. FE analysis was performed with the aim of determining the stress state in the joint along with the J-integral. If the square root of the J-integral, √ΔJ, is used as the fatigue strength assessment parameter, the fatigue strength obtained at five million cycles is similar as in the case of other steel welded joint types. The investigation concluded that the stress state changes with the reduction of the plate thicknesses and the contribution of fracture mode II becomes significant. However, using √ΔJ as a fatigue strength assessment parameter ensures that the complex state of the mixed fracture mode loading is accurately accounted for. This fact further enables the fatigue strength of laser stake-welded T-joints of any plate thicknesses to be described by means of a narrower scatterband than the one obtained by the nominal stress approach.     

Heat treatment,surface roughness and corrosion effects on the damage mechanism of mechanical components in the very high cycle fatigue regime

Many engineering components are subjected to combined torsion and axial loading in their working conditions, and the cyclic combined loading can result in fatigue fracture after a very long life fatigue regime. The present investigation extends over a wider range of test conditions involving surface treatment and manufacturing effects such as machining, so as to understand the fatigue properties and damage mechanisms of the material beyond 10⁹ cycles.This work reviews the effect of surface conditions on the fatigue behaviour of mechanical components in the gigacycle regime. Evidently, surface conditions can be variable and are due to very different reasons such as manufacturing effects like machining or final surface processes on the parts, heat treatment before and after manufacturing or environmental conditions like corrosion. In fact, this is a detailed comparative study based on the results of experiments carried out by our research team working in this domain. For this reason, it reveals a continuous decrease of the fatigue strength in the VHCF domain for the investigated materials under different surface conditions as important information for design engineers.Experimental investigation on the test specimens was performed at a frequency of 20 kHz with different stress ratios varying between R = ?1 and R = 0.7 at room temperature. All of the fatigue tests were carried out up to 10¹⁰ cycles. The damage mechanism was evaluated by Scanning Electron Microscopy (SEM).     

22 Cr 5 Ni duplex and 25 Cr 7 Ni superduplex stainless steel: Hydrogen influence on fatigue crack propagation resistance

Duplex stainless steels (DSS) fatigue crack propagation resistance is strongly affected by both microstructure and environment. In this work, environment influence on the fatigue crack propagation in a 22 Cr 5 Ni duplex and in a 25 Cr 7 Ni superduplex stainless steels is investigated considering three different stress ratios (R = K_min/K_max = 0.1, 0.5, 0.75). Tests are performed according to ASTM E 647 standard, both in air and under hydrogen charging conditions (0.1 M H₂SO₄ + 0.01 M KSCN aqueous solution, ?0.9 V/SCE). Crack fracture surfaces are extensively analysed by means of a scanning electron microscope. Furthermore, crack paths are investigated by means of a crack profile analysis performed through a light optical microscope. Nickel coated fracture surface sections obtained for constant ΔK values are considered in order to analyse the loading (R values) and environment influence on fatigue crack paths.     

Effect of anisotropy on fatigue properties of 2198 Al–Li plates joined by friction stir welding

Al–Li alloys are characterized by a strong anisotropy in mechanical and microstructural properties with respect to the rolling direction. In the present paper, 4 mm sheets of 2198 Al–Li alloy were joined via friction stir welding (FSW) by employing a rotating speed of 1000 mm/min and a welding speed of 80 mm/min in parallel and orthogonal direction with respect to the rolling one. The joints mechanical properties were evaluated by means of tensile tests at room temperature. In addition, fatigue tests were performed by using a resonant electro-mechanical testing machine under constant amplitude control up to 250 Hz sinusoidal loading. The fatigue tests were conducted in axial control mode with R = σ_min/σ_max = 0.33, for all the welding and rotating speeds used in the present study.     

Compression fatigue performance of a fire resistant syntactic foam

Compression–compression fatigue test study of a fire resistant Eco-Core was conducted at two values of stress ratios (R = 10 and 5). Tests were conducted at S_min/S_o values of 0.9–0.6 for R = 10 and 0.95–0.8 for R = 5. Here S_min is the maximum compression stress and S_o is the compression strength. The study showed that Eco-Core has well defined failure modes and associated fatigue lives. The failure modes are: damage on-set; damage progression, and final failure. The damage on-set, propagation and final failure were characterized by 2%, 5% and 7% changes in compliance. The three failure modes were found to be same for both static and fatigue loadings. The endurance limit was found to be 0.72S_o, 0.75S_o and 0.76S_o, respectively for three failure modes for R = 10 and 0.81S_o, 0.82S_o and 0.82S_o, respectively for R = 5. The fatigue life is defined by a power law equation, S_min/S_o = A_oN^α. Constants of the equation were established for all three modes of failures and the two stress ratios. Finally, fatigue life was found to be less sensitive to R ratio when expressed in terms of stress range versus number of load cycles, which is similar to that of metallic materials.