Fatigue lifetime and tearing resistance of AA2198 Al–Cu–Li alloy friction stir welds: Effect of defects

The fatigue strength and failure mechanisms of defect-free (“sound”) and flaw bearing friction stir butt-welds of 3.1 mm-thick AA2198-T8 Al–Li–Cu alloy have been investigated via S–N curves at R = 0.1 using cross weld specimens. The fatigue strength of sound welds is only reduced by 10–15% at the aimed lifetime of 10⁵ cycles compared to the base material. Joint Line Remnant (JLR) bearing welds have a similar fatigue strength as sound welds and the JLR is not the crack initiation site. Kissing Bond (KB) bearing welds that have undergone a weld root polishing show a reduction in fatigue strength by 17% compared to sound welds. For specimens loaded at or above yield strength of the weld nugget the crack systematically initiates from the KB during the first cycle, which is interpreted further using fracture mechanics. The strongest reduction, about 28% in fatigue strength, is found for welds with an initial gap between the parent sheets (GAP welds) along with initiation at intergranular surface microcracks. Kahn tear tests show a reduction in tearing resistance for the flaw bearing welds with a similar ranking as for the fatigue strength.     

Fatigue design of axially-loaded high frequency mechanical impact treated welds by the effective notch stress method

The effective notch stress method (ENS) as defined by the International Institute of Welding is widely used by design engineers to assess the fatigue strength of welded components. This paper provides a comprehensive evaluation of published data for welded joints improved by high frequency mechanical impact (HFMI) treatment. The goal is to verify already-known fatigue classes for the ENS with the available axially-loaded fatigue data. In total, 280 experimental test results obtained from longitudinal, cruciform and butt welds subject to stress ratio of R = 0.1 axial loading are evaluated. Notch stress concentration factors (K_n) for each joint geometry are analysed based on the finite element method. Calculated K_n and reported nominal stress values are used to determine local stresses. Fatigue strength assessment of the all available data is performed by the previously-proposed and verified correction procedure for yield strength (f_y). A formerly-defined minimum K_n values as a function of f_y is used for butt welds. The already-known fatigue classes are found to be conservative with respect to available fatigue test data.     

High cycle fatigue behaviour of impact treated welds under variable amplitude loading conditions

A comprehensive variable amplitude (VA) fatigue testing program and analysis was performed to address a number of concerns raised regarding the use of impact treatments for the fatigue enhancement of welds in the high and ultra-high cycle (up to 100 million cycles) domains. A total of 67 fatigue tests were conducted on two different welded joints representing load-carrying and non-load carrying welds in steel structures. Two different VA loading spectrums, generated using traffic data and influence lines for highway bridge girders, were used. The effects of load cycles with high stress ratios (R > 0.4) and large tensile overloads (greater than the yield strength) were studied. The test results were then used to evaluate a number of previously proposed recommendations for the fatigue design of impact treated welds. The nominal, structural, and effective notch stress approaches were considered. Finite element (FE) analysis was performed to determine the structural and effective notch stresses. A statistical analysis of the fatigue test results was conducted and characteristic S–N curves with slope, m = 5 are proposed for the fatigue design of treated welds under VA loading in the high cycle domain.     

Influence of defects on fatigue strength and failure mechanisms in the VHCF-region for quenched and tempered steel and nodular cast iron

Investigations are presented in this paper on quenched and tempered steel 42CrMoS4 from two batches, with two different tensile strengths (R_m = 1100 MPa, 1350 MPa) but with similar microstructure, and a nodular cast iron EN-GJS-900-2 (R_m = 930 MPa). Fatigue tests with smooth (K_t = 1) and notched (K_t = 1.75) specimens were performed at R = −1 and R = 0 up to the number of cycles N = 2·10⁹ in order to determine the fatigue strength behaviour and failure mechanisms, especially in the VHCF-region. Failure in smooth specimens often initiated at material defects such as oxides in the quenched and tempered steel and shrinkage holes in the nodular cast iron. Firstly, a fatigue strength analysis was performed that did not consider these defects. A possibility of analysis of experimental data including VHCF-results has been discussed. Next, a linear elastic fracture mechanics analysis was performed in order to describe the defect behaviour, assuming that the defects act like cracks. The results showed that there are lower limit or threshold values of the stress intensity factor range ΔK for crack propagation in both materials. Analysis of defects and defect distribution in run-out specimens confirmed this conclusion. From the comparison of the results with an S–N curve from the design code FKM-Guideline Analytical strength assessment of components, recommendations for design and assessment of components have been derived.     

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.     

Fatigue of 2024-T351 aluminium alloy at different load ratios up to 1010 cycles

Fatigue properties of 2024-T351 aluminium alloy are investigated in the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) regime. Endurance tests are performed with ultrasonic equipment at 20 kHz cycling frequency at load ratios of R = −1, R = 0.1 and R = 0.5 up to 10¹⁰ cycles. Additional servo-hydraulic tests between 8 and 10 Hz at R = 0.1 show no frequency influence on fatigue lifetimes. Linear lines in double logarithmic S–N plots are used to approximate data. Slope exponents of approximation lines increase with increasing numbers of cycles for all load ratios. Failures above 5 × 10⁹ cycles (R = −1 and R = 0.1) or 10¹⁰ cycles (R = 0.5) occur, and no fatigue limit is found. Fatigue cracks leading to failures above 10⁹ cycles are initiated at the surface or slightly below at broken constituent particles or at agglomerations of fractured particles, which are probably Al₇Cu₂(Fe, Mn). Specimens stressed with more than 10¹⁰ cycles at R = −1 without failure show several cracks starting at constituent particles. Maximum crack lengths are 30 μm, which is considerably below grain size.     

Effect of stress ratio on VHCF behavior for a compressor blade titanium alloy

Effect of stress ratio on fatigue properties of a titanium alloy (TC-17) in the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) were investigated by electromagnetic and ultrasonic fatigue testing. The S–N curves at R = −1, 0.1, 0.5 and 0.7 at 110 Hz and 20 kHz were obtained and discussed. The effects of frequency on fatigue strength was also investigated. It was concluded that the fatigue strength with 50% fatigue failure probability at R = 0.1, 0.5 and 0.7 is lower to the Goodman line and shows a bilinear decreasing trend. Cleavage fracture of primary grains in the surface and interior initiation zone were observed. The formation of the facets induced by the basal or prismatic slips of the H.C.P grains decreased the fatigue strength with variation in mean stress.     

Fatigue strength of sharp V-notched specimens made of ductile cast iron

In the present, the high cycle fatigue strength of notched ductile cast iron is investigated. Experimental tests under axial loading, has been carried out on sharp V-notched specimens taken from heavy section casting considering nominal load ratios (R = 0). All specimens, taken from a heavy section casting, are characterized by a notch tip radius less than 0.1 mm, a notch depth of 10 mm and a notch opening angle = 90°. In order to evaluate the influence of chunky graphite morphology on fatigue life, fatigue tests were carried out also on a second set of specimens without that microstructural defect.Metallurgical analyses were performed on all the samples and some important microstructural parameters (nodule count and nodularity rating, among others) were measured and compared. It was found that a mean content of 40% of chunky graphite in the microstructure (with respect to total graphite content) does not influence significantly the fatigue strength properties of the analysed cast iron.     

Effects of inclusion size and stress ratio on fatigue strength for high-strength steels with fish-eye mode failure

Experimental results indicate that the fatigue life reduces by about two orders of magnitude when inclusion size doubles. Then, a model is proposed for predicting the fatigue strength of high-strength steels with fish-eye mode failure based on the experimental results for the effect of inclusion size and stress ratio. In the model, the effect of inclusion size a₀ and stress ratio R on fatigue strength σ_a is expressed as σ_a ∝ a₀^m[(1 − R)/2]^α, where m and α are material parameters. The predicted results are in good agreement with our experimental results and the ones reported in literature.     

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.     

Fatigue behavior and modeling of short fiber reinforced polymer composites including anisotropy and temperature effects

Effects of anisotropy and temperature on cyclic deformation and fatigue behavior of two short glass fiber reinforced polymer composites were investigated. Fatigue tests were conducted under fully-reversed (R = −1) and positive stress ratios (R = 0.1 and 0.3) with specimens of different thicknesses, different fiber orientations, and at temperatures of −40 °C, 23 °C, and 125 °C. In samples with 90° fiber orientation angle, considerable effect of thickness on fatigue strength was observed. Effect of mold flow direction was significant at all temperatures and stress ratios and the Tsai–Hill criterion was used to predict off-axis fatigue strengths. Temperature also greatly influenced fatigue strength and a shift factor of Arrhenius type was developed to correlate fatigue data at various temperatures, independent of the mold flow direction and stress ratio. Micromechanisms of fatigue failure at different temperatures were also investigated. Good correlations between fatigue strength and tensile strength were obtained and a method for obtaining strain–life curves from load-controlled fatigue test data is presented. A fatigue life estimation model is also presented which correlates data for different temperatures, fiber orientations, and stress ratios.     

Effect of frequency on very high cycle fatigue behavior of a low strength Cr–Ni–Mo–V steel welded joint

Axially fully-reversed fatigue test of a low strength Cr–Ni–Mo–V steel welded joint was conducted up to the very high cycle fatigue regime under the frequency of 110 Hz and 20 kHz. The S–N curve shows a duplex shape at low frequency while decreases continuously at high frequency. Sites of crack initiation and fracture of the welds depend on stress level and loading frequency, hence leading to changed fatigue strength. In addition, frequency effect varies among different parts of the welded joint and fatigue lifetime.     

Zr–Si biomaterials with high strength and low elastic modulus

In order to develop new biomaterials for hard tissue replacements (HTR), the Zr–8.8Si–xNb (x = 0.0, 0.3, 0.6 and 0.9) alloys with required properties were designed and prepared for the first time. Experimental results show that these alloys can provide excellent mechanical compatibility for the special demands for substitution of human bones. The highest compression strength of the alloys is 1189.30 MPa, while the highest yield strength of alloys is 850.25 MPa. The elastic energy is determined to be 5.001–12.01 MJ/m³, and the Young’s modulus is in the range of 25.08–29.63 GPa. The composition of high strength and low elastic modulus of Zr–8.8Si–xNb alloys offer potential advantages for biomedical applications.     

Effect of residual stresses on fatigue strength of high strength steel sheets with punched holes

The effect of residual stresses on the reverse bending fatigue strength of steel sheets with punched holes was studied for steels with tensile strength grades of 540 MPa and 780 MPa. Tensile and compressive residual stresses were induced around the punched holes. Heat treatment of the specimens with punched holes at 873 K for 1 h decreased the residual stresses around the holes and improved the fatigue strength of the sheets. This result means that the tensile residual stresses induced in the sidewalls of the holes and near the hole edges by punching reduced fatigue strength. The effect of the residual stresses on the fatigue limits of the edges was estimated by the modified Goodman relation using the residual stresses after cyclic loading and the ultimate tensile strength at the fatigue crack initiation sites.     

The relative effects of residual stresses and weld toe geometry on fatigue life of weldments

The weld toe is one of the most probable fatigue crack initiation sites in welded components. In this paper, the relative influences of residual stresses and weld toe geometry on the fatigue life of cruciform welds was studied. Fatigue strength of cruciform welds produced using Low Transformation Temperature (LTT) filler material has been compared to that of welds produced with a conventional filler material. LTT welds had higher fatigue strength than conventional welds. A moderate decrease in residual stress of about 15% at the 300 MPa stress level had the same effect on fatigue strength as increasing the weld toe radius by approximately 85% from 1.4 mm to 2.6 mm. It was concluded that residual stress had a relatively larger influence than the weld toe geometry on fatigue strength.     

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.     

Use of electronic speckle pattern interferometry in the detection of fatigue failure in high strength steels

The fatigue behaviour of next generation high strength steels (σ_UTS = 950–1000 MPa) has been studied. Specifically, this study is focused on the initiation stage of fatigue microcracks. With this purpose, high cycle fatigue tests under uniaxial loading have been performed. During these tests, the deformation history of the specimen has been tracked by means of speckle interferometry. This technique allows monitoring the evolution of the displacement field and its derivatives on the specimen surface, so that it can be used as a tool for detecting microcracks in the first stages of crack initiation. The observation of the fracture surfaces provides complementary information about the localization of the initiation of failure thus, a correlation between the observations made by interferometry and the actual location of the fatigue nucleus and the evolution of the crack during its propagation can be established. Results appoint speckle interferometry as a promising technique for the detection of fatigue failures.     

High yield strength bulk Ti based bimodal ultrafine eutectic composites with enhanced plasticity

Ti-based bulk metallic glass (BMGs) and their bimodal composites are linked with the pronounced strain hardening after yielding but with much low value of strength. Therefore, developing Ti-based alloys with high yield strength and high plasticity is the current challenge. Here, we report the synthesis of ultra-fine grained bulk (UFG) (Ti_0.705Fe_0.295)_100−xGa_x (0 ⩽ x ⩽ 2) bimodal eutectic composites with not only high strength and larger plasticity but also with high yield strength which is one of the important mechanical property for structural application. Reasonably high strength, high yield strength, strain to failure ratio, and enhanced plasticity of ∼7 ± 0.8% was observed in (Ti_70.5Fe_29.5)₉₈Ga₂ composite which is superior than Ti-based BMGs and bimodal composites. Modification of degree of eutectic structure refinement and volume fraction of constituent phases with the addition of Ga are the crucial factors in enhancing the mechanical properties of Ti–Fi–(Ga) composites.     

Fatigue strength analysis of laser-hybrid welds in thin plate considering weld geometry in microscale

Utilization of thin plates together with laser-based welding processes allows manufacturing of large weight efficient steel structures. However, the fatigue strength of welds in thin-plate structures with plate thicknesses below 5 mm is observed to have large variation, which brings challenges to fatigue strength assessment. One possible reason for this variation is the increased influence of actual weld geometry that is neglected in common fatigue strength assessment approaches utilizing geometry idealization. To reveal this influence the fatigue strength of 3 mm thick laser-hybrid welded butt joints were studied using the measured microscale weld geometry and the notch stress approach. Notch stresses were defined using Neuber’s stress averaging approach which allows the determination of the fatigue-effective stress without fictitious geometric modifications. For the studied specimens the large scatter of fatigue strength in the high-cycle region could be explained using this approach with high-resolution weld profile measurements combined with thorough finite-element analysis. It was observed that axial misalignment in narrow laser-hybrid welds causes a significant notch stress increase on the root side reducing the fatigue strength dramatically in terms of structural and nominal stress. In order to capture the increased notch stress it is crucial to use a significantly smaller stress averaging length than commonly assumed for welded joints.     

Effects of strength level and loading frequency on very-high-cycle fatigue behavior for a bearing steel

Rotating bending (52.5 Hz) and ultrasonic (20 kHz) fatigue tests were performed on the specimens of a bearing steel, which were quenched and tempered at 150 °C, 300 °C, 450 °C and 600 °C, respectively, to investigate the influence of strength level and loading frequency on the fatigue behavior in very-high-cycle regime. Influences on fatigue resistance of materials, characteristics of S–N curves and transition of crack initiation site were discussed. The specimens with higher strength showed interior fracture mode in very-high-cycle regime and with slight frequency effect, otherwise cracks all initiate from the surface and the fatigue strength was much higher under ultrasonic cycling.