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.     

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.     

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.     

A neutron-diffraction study of the low-cycle fatigue behavior of HASTELLOY® C-22HSTM alloy

The HASTELLOY^® C-22HS^TM alloy is a face-centered cubic (fcc), nickel-based, corrosion-resistant superalloy. In the present study, the low-cycle-fatigue behaviors of the alloy were examined by in situ neutron diffraction at room temperature. The fatigue parameters included a total strain range of Δε = 2% and a strain ratio of R = −1 (R = ε_min/ε_max, where ε_min and ε_max are the applied minimum and maximum strains, respectively). The effect of cyclic deformation on the lattice strains was studied as a function of cyclic straining. The cyclic hardening and softening behaviors during fatigue is discussed in light of the relationship between the peak widths and lattice strains.     

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.     

The fatigue crack growth rate and crack opening displacement in 18G2A-steel under tension

In the paper, the results of crack tip opening displacement (CTOD) and crack opening displacement (COD) in place of crack initiation as well as the fatigue crack growth rate in higher strength steel are presented. The investigation were carried out on flat specimens with central notch under constant amplitude tensile fatigue loading at stress ratio R = 0.2 and different value of the stress σ_max. The test results showed that with growth of crack length l grew values of the CTOD and COD. In the work, it was proposed calculation of the CTOD value on basis various dependence of plastic zone radius on crack tip.     

Crack closure and fatigue crack growth near threshold of a metastable austenitic stainless steel

In this paper R-ratio effects on fatigue crack growth near threshold region of a metastable austenitic stainless steel (MASS) in two different conditions, i.e. annealed and cold rolled, is investigated. The authors present two approaches to correlate FCGR data for R = 0.1, 0.3, 0.5, 0.7 and K_max = 23 MPa√m using a two-parameters approach (ΔK, K_max and α in Kujawski’s model) and crack closure model (using Elber’s K_op and in Donald’s ACRn2 approaches). The K_op and ACRn2 were experimentally measured on a single edge tension specimens. The K_op measurements were performed using a modified method and based on ASTM standards. While the two driving force approaches correlate data well in the Paris region, they fail to correlate them in the threshold region. However, this correlation can be improved in the threshold region when a different α value from the Paris region is used. The authors indicated that two different mechanisms operate; one in the Paris region and another in the near threshold. Hence, they proposed to combine the two-parameter and crack closure approaches where ΔK is replaced by ΔK_eff (estimated by a new method proposed in this paper), which is shown to correlate the FCGR data for different stress ratios for annealed steel. The correlation for cold rolled condition shows improvement with the new approach but is not as good as for the annealed one. The author further suggests to modify K_max in the two-parameter approach.     

Numerical simulation of fatigue crack propagation in friction stir welded joint made of Al 2024-T351 alloy

In this work, fatigue crack propagation in thin-walled aluminium alloy structure with two friction stir welded T joints has been simulated numerically. Crack propagation in stiffened part of the structure between two friction stir welded T joints is analysed by using the eXtended Finite Element Method (XFEM), including software ABAQUS, as well as MORFEO, for modelling and results display. Tensile fatigue loading is applied, with stress ratio R = 0, and maximum stress σ_max = 10 MPa. Material properties (Al 2024-T351, as used in aeronautical industry) in different welded joints zones are adopted from available literature data. Following results are obtained by numerical analysis: stress–strain and displacement state in the structure, position of the crack tip and value of stress intensity factor for every crack propagation step, as well as the structural life estimation, i.e. number of load cycles, N, also for each crack propagation step. Using these results the number of cycles at which the crack starts to propagate in an unstable manner is predicted.     

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.     

Effect of welding residual stresses on fatigue crack growth thresholds

Fatigue crack growth thresholds ΔK_th were determined for friction stir welded butt joints made from aluminium alloys AA2024 and AA6013. Plotting the thresholds as a function of load ratio R showed distinctly higher amounts for welded joints as compared to those for parent material at small load ratios, but differences became smaller with increasing load ratio, until thresholds became finally identical for the highest R values. Applying Döker’s concept of two controlling parameters, namely ΔK and K_max [1], and plotting ΔK_th versus K_max, however, revealed that the effective threshold ΔK_th,eff determined at very high R ratios was nearly independent on the alloy and, simultaneously, was identical for parent material and respective welded joints. Thus, differences in threshold behaviour were only caused by the second threshold K_max,th, which was significantly higher for welded joints as compared to parent material. Differences in K_max,th coincided with compressive residual stresses determined by cut-compliance measurements in terms of stress intensity factors K_rs acting at the crack tip. Based on the analytical approach described by Döker [1], only one characteristic K_rs value was needed to calculate all thresholds of welded joints for 0 ⩽ R ⩽ 1 provided a base material master curve is available.     

Crystal growth,structure, infrared spectroscopy,and luminescent properties of rare-earth gallium borates RGa3(BO3)4, R = Nd,Sm–Er,Y

Crystals of the rare-earth gallium borates RGa₃(BO₃)₄, where R = Nd, Sm–Er, or Y, were grown by the flux method. The crystal structures of RGa₃(BO₃)₄ (R = Eu, Ho) were studied on the basis of single-crystal X-ray diffraction measurements. The hexagonal unit-cell parameters are a = 9.4657(1) Å, c = 7.4667(1) Å and a = 9.4394(2) Å, c = 7.4322(1) Å for EuGa₃(BO₃)₄ and HoGa₃(BO₃)₄, respectively, space group R32. Structure model was determined by “charge flipping” method and refined to R = 1.93% [EuGa₃(BO₃)₄] and R = 1.89% [HoGa₃(BO₃)₄] in anisotropic approximation. All grown gallium borates were investigated by infrared (IR) spectroscopy technique in a middle and far IR region. IR spectra of rare-earth gallium borates correspond to a pure rhombohedral (R32) polytype structure. Small inclusions of a monoclinic phase were detected only in Eu and Nd compounds. Luminescence of Eu and Ho gallium borates was studied at room temperature. The measured decay times for the most intensive emission lines of EuGa₃(BO₃)₄ (∼614 nm) and HoGa₃(BO₃)₄ (434 nm) are 940 μs and 140 μs, respectively. The scheme of crystal-field energy levels of Eu³⁺ in EuGa₃(BO₃)₄ was built on the basis of the temperature-dependent optical transmission measurements combined with the luminescence data. The measured UV absorption edge for RGa₃(BO₃)₄ is at about 300 nm.     

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.     

An experimental investigation of fatigue behavior and deformation of double spot friction welded joints

Fatigue behavior of double spot friction welded joints in aluminum alloy 7075-T6 plates is investigated by conducting monotonic tensile and fatigue tests. The spot friction welding procedures are carried out by a milling machine with a designed fixture at the best preliminary welding parameter set. The fatigue tests are performed in a constant amplitude load control servo-hydraulic fatigue testing machine with a load ratio of (R = P_min/P_max) 0.1 at room temperature. It is observed that the failure mode in cyclic loading (low-cycle and high-cycle) resembles that of the quasi-static loading conditions i.e. pure shearing. Primary fatigue crack is initiated in the vicinity of the original notch tip and then propagated along the circumference of the weld’s nugget.     

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.     

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 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.     

Deposition and characterization of ultra-high barrier coatings for flexible electronic applications

A barrier structure consisting of SiO_x and SiN_x films was deposited on the polymer substrate at 80 °C via plasma-enhanced chemical vapor deposition (PECVD). However, the low radius of curvature (R_c) of the barrier-coated substrate may cause the inconvenience of the following fabrication processes. By depositing a 150 nm-SiN_x film, the R_c of the barrier-coated polycarbonate (PC) substrate can increase from 80 to 115 mm without inducing any cracks in the barrier structure. Furthermore, the thermal stress of the barrier structure can be adjusted via extending the PECVD process duration in the chamber and replacing PC by the polyethersulone (PES) substrate. The R_c can increase to ∼356 mm by depositing the 150 nm-SiN_x film on the other side of the PES substrate. Finally, the calcium test result of the barrier films/PES/SiN_x sample was calculated to be around 3.05 × 10⁻⁶ g/m²/day, representing that the barrier structure did not fail after modification.     

Fatigue assessment of root failures in HSLA steel welded joints: A comparison among local approaches

Fatigue tests were performed on welded joints made of high-strength, low-alloy steel (S690). Different welding processes were tested, resulting in welds with different defects essentially consisting in lack of penetration. Fatigue tests were run with both constant and variable amplitude loading. The experimental results were compared to predictions obtained by applying local approaches (local stress and local strain) and the concepts of fracture mechanics. The local stress approach allowed the fatigue strength of joints in constant amplitude loading (for fatigue above 2 × 10⁶) to be predicted, but the assumption of a constant value of the slope k = 3 for all S–N curves led to non-conservative predictions of shorter lives. The local strain approach allowed the fatigue strength of the joints under constant amplitude to be predicted. Although, these predictions matched the experimental data well for both small and large defects in the entire cycle number range, they failed to predict the behaviour of joints under variable amplitude loading. Conversely, the fracture mechanics approach proved to be more efficient in predicting the fatigue behaviour of welded joint under variable amplitude loading.     

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.     

Determining fracture toughness from cutting tests on polymers

A cutting test method has been developed for polymers and analysis schemes derived for the determination of the fracture toughness, G_c from the cutting data. The experimental scheme requires the measurement of forces for a cut of width b, in both the cutting direction, F_c/b and the transverse direction, F_t/b. Depths of cut were varied from 0.025 mm to 0.25 mm and the tool rake angle was varied from ?20° to 30°. Cutting was performed at a speed of 10 mm s^?1 on three polymers (HIPS, PA 4/6 and LLDPE). In addition, values of fracture toughness and yield strength were determined for the polymers using standard tests for comparison with the values obtained from cutting. Three analysis methods were derived to analyse the cutting data with the most favoured scheme based on an energy balance and using Merchant’s force minimisation criterion to determine the shear plane angle. This avoids the need to measure the cut chip thickness. Results for HIPS and PA 4/6 gave values of G_c in good agreement with the values determined via LEFM. However, the cutting method is intended for materials such as LLDPE which has a low yield stress and moderately high toughness, i.e. materials which cannot be tested using standard LEFM fracture mechanics tests. The cutting analysis appeared to give valid values of G_c for LLDPE in that they were independent of rake angle. There were some complications when analysing this polymer due to visco-elastic recovery effects in the chip and these have been considered. Finally, the cutting analyses always determined high values of yield stress which would appear to indicate work hardening.