Research on fatigue behavior and residual stress of large-scale cruciform welding joint with groove

Fatigue fracture behavior of the 30 mm thick Q460C-Z steel cruciform welded joint with groove was investigated. The fatigue test results indicated that fatigue strength of 30 mm thick Q460C-Z steel cruciform welded joint with groove can reach fatigue level of 80 MPa (FAT80). Fatigue crack source of the failure specimen initiated from weld toe. Meanwhile, the microcrack was also found in the fusion zones of the fatigue failure specimen, which was caused by weld quality and weld metal integrity resulting from the multi-pass welds. Two-dimensional map of the longitudinal residual stress of 30 mm thick Q460C-Z steel cruciform welded joint with groove was obtained by using the contour method. The stress nephogram of Two-dimensional map indicated that longitudinal residual stress in the welding center is the largest.     

Improving the fatigue life of electro-discharge-machined SDK11 tool steel via the suppression of surface cracks

The present study performs an experimental investigation to identify the EDM processing parameters which suppress the formation of surface cracks in the machined surface of SKD11 tool steel specimens. In the EDM trials, the specimens are machined using pulse currents of 4 A, 16 A or 32 A with pulse-on durations of either 4 μs or 16 μs. The various specimens are then fatigue tested at loads ranging from 1470 to 2401 N in order to determine their respective fatigue lives. A polished SKD11 specimen is also fatigue tested for comparison purposes. Finally, the fracture surfaces are examined using scanning electron microscopy to examine the crack propagation characteristics.The results show that increasing the pulse current and reducing the pulse-on duration provides an effective means of suppressing the surface cracking phenomenon. Higher values of the pulse current and pulse-on duration are found to increase the average thickness of the recast layer. Overall, the present results show that the four specimens considered in the fatigue test can be ranked in order of reducing fatigue life as follows: (1) the polished specimen, (2) the specimen with a thin recast layer and no surface cracks, (3) the specimen with a thick recast layer and no surface cracks and (4) the specimen with surface cracks.     

Effect of laser quenching on fatigue properties and fracture morphologies of boronized layer on Cr12MoV steel

A boronized layer of Cr12MoV steel was processed with LQ (laser quenching), and the fatigue limits of original samples before and laser quenched samples were calculated with Locati tension–tension fatigue test, and the fracture morphologies were observed with a SEM (scanning electronic microscope). The results show that the compressive residual stress of −382 MPa is introduced by LQ, the fatigue strength improves from 368 MPa to 422 MPa, increasing by 14.7%, and the fatigue crack is initiated at the subsurface after LQ. The compressive residual stress of the Cr12MoV by LQ is of the main mechanism of the improving of fatigue property.     

Effect of nitriding and shot-peening on the fatigue behavior of 42CrMo4 steel: Experimental analysis and predictive approach

In this work, we are interested in the bending fatigue resistance of the nitrided 42CrMo4 steel improvement by shot-peening. The micro-structure, the micro-hardness, the residual stresses distribution and the crack resistance of the hardened steel are determined. The gains, expressed in term of endurance limit, brought by these treatments are established by three-points bending fatigue tests and discussed in relation to the residual stresses evolution under the cyclic loading conditions. The fatigue fracture resistance after a combined process of surface hardening including shot peening followed by nitriding is analyzed by methods of fracture mechanisms. This reveals that the gain provided by the nitriding is about 8% against 35% for the nitriding with shot-peening. This is primarily allotted to a high level of compressive residual stresses for nitrided + shot-peened state compared to the nitrided state. The fast relaxation of these stresses in the low cycle fatigue domain is at the origin of the fatigue fracture at surface, which leads to a lower fatigue fracture resistance compared to the untreated state. Based on multiaxial HCF criterion of Sines and taking into account the different surface properties, a local predictive approach was developed.     

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.     

The effects of fatigue and residual strain on the mechanical behavior of poly(ethylene terephthalate) unreinforced and nanocomposite fibers

Poly(ethylene terephthalate) (PET) control fibers (nominal diameter ～24 ± 3 μm) and PET fibers with embedded vapor-grown carbon nanofibers (PET-VGCNF) (nominal diameter ～25 ± 2 μm) were exposed to cyclic loading and monotonic tensile tests. The control fibers were processed through a typical melt-blending technique and the PET-VGCNF samples were processed with approximately 5 wt.% carbon nanofibers present in the sample. Under uniaxial fatigue conditions, the fibers were subjected to a maximum stress that was approximately 60% of the fracture stress of the sample at an elongation rate of 10 mm/min in uniaxial tension. The fibers were subjected to a frequency of 5 Hz. Subsequent to non-fracture fatigue conditions, the fibers were tested under uniaxial stress conditions for observation of the change in mechanical properties to assess the effects of fatigue loading. The elastic modulus, hardening modulus, fracture strength, work done, and yield strain of both PET control and PET-VGCNF samples in uniaxial tension subsequent to fatigue were shown to be dependent on the residual fatigue strains. Relative mechanical properties were used to quantify the difference in PET and PET-VGCNF samples as a function of residual strain. In most cases, the results indicated a strengthening mechanism (strain hardening effect) in the low residual strain limit for fatigued PET samples and not for fatigued PET-VGCNF samples. In comparison with the unreinforced PET sample, the PET-VGCNF fibers showed greater degradation of mechanical properties as a function of residual strain due to fatigue when cycled at 60% of the fracture stress. The effects of the fatigue process on the change in mechanical properties have been quantified and supported through existing qualitative, quantitative, and scanning electron microscopy (SEM) techniques.     

Low cycle fatigue behavior of Cr–Mo–V low alloy steel used for railway brake discs

The cyclic stress–strain response and the low cycle fatigue (LCF) behavior of Cr–Mo–V low alloy steel which was used for forged railway brake discs was studied. Tensile strength and LCF properties were examined over a range from room temperature (RT) to 600 °C using specimens cut from circumferential direction of a forged disk. The fully reversed strain-controlled LCF tests were conducted at a constant total strain rate with different axial strain amplitude levels. The cyclic strain–stress relationships and the strain–life relationships were obtained through the test results, and related LCF parameters of the steel were calculated. The studied steel exhibits cyclic softening behavior and behaves Masing type, especially at higher strain amplitudes. At higher than 600 °C, carbide particles aggregated and a decarburized layer developed near the specimen surface. Micro voids distribute within the depth of 50 μm from the specimen surface could coalesce with fatigue cracks. Multiple crack initiation sites were observed on the fracture surface. The oxide film that generated at 600 °C covered the fatigue striations and accelerated the crack propagation. Final fracture area with bigger and deeper dimples showed better ductility at higher temperature. The investigated LCF behavior can provide reference for brake disc life assessment and fracture mechanisms analysis.     

The effects of texture and grain morphology on the fracture toughness and fatigue crack growth resistance of nanocrystalline platinum films

The fracture toughness and fatigue crack growth resistance of nanocrystalline materials are significantly affected by the thickness of the specimen. In this work we relate the mechanical properties of nanocrystalline platinum films to their texture and grain morphology. Tensile, creep and fatigue testing of annealed, ∼1 μm films resulted in mechanical properties similar to the as-received films (yield strength of ∼1.2 GPa, fracture toughness ∼17.8 MPa √m, and a fatigue crack growth power law exponent of ∼4.2). However, the breakdown of the initially columnar grain morphology had a marked effect on the transition point from an intergranular to transgranular fatigue cracking mode. Finite element modeling suggests that cyclic (fatigue) grain coarsening and the transition from inter- to transgranular cracking modes are a result of the relative importance of dislocation slip accommodation on in-plane and through-thickness oriented slip directions.     

The fatigue behavior of irradiated Reactor Pressure Vessel steel

Fatigue specimens of A508-3 steel were irradiated in the swimming-pool test reactor in China Institute of Atomic Energy, the fluence was 3 × 10¹⁹ n/cm² at 300 °C, then low-cycle fatigue tests were carried out at ambient temperature, with the fatigue strain range is 0.32–1.8%. The results indicate that, irradiated A508-3 specimens exhibit cyclic softening and instability behavior during the test, and the cyclic softening rate increased with strain range increased; fatigue life decreased from 1.7 × 10⁵ to about 5 × 10², as the strain range increased from 0.32% to 1.8%, the fatigue life of A508-3 steel increased after the neutron irradiation; fatigue fracture initiated at the surface of specimen, and more individual cracks formed on the specimens of higher strain range compared with the specimens of lower strain range.     

Effects of dynamic mechanical properties on the ballistic performance of a new near-β titanium alloy Ti684

A comprehensive study of the newly developed near-β titanium alloy Ti684 has been carried out to determine the influence of the dynamic strength, dynamic hardness and critical failure strain on the ballistic impact properties. Two heat treatments of Ti684, namely β solution-treatment and α + β solution-treatment followed by aging, were carried out and the results were compared with Ti–6Al–4V. Ballistic impact tests were conducted on 7 mm thick front plates with a 20 mm thick A3 steel backing plate, using 7.62 mm armor piercing projectiles. The ballistic performance was evaluated by measuring the residual depth of penetration (DOP) in the A3 steel backing plates. It was found that the DOP values did not show obvious corresponding relation with both dynamic strength and dynamic hardness. The 800 °C solution +550 °C aged Ti684, which had the maximal dynamic strength, presented the worst ballistic performance, with a maximum DOP of 12.5 mm. In addition, the Ti–6Al–4V plate in the study with highest dynamic hardness did not show the best ballistic performance, having a DOP of 11.86 mm. However, as the critical failure strain increased, the DOP of the A3 steel backings were observed to decrease. This relationship was revealed from post ballistic microstructural observations.     

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.     

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 and fracture behavior of laser clad repair of AerMet® 100 ultra-high strength steel

The effect of laser cladding on the fatigue and fracture behavior under variable amplitude loading is a major consideration for the development of laser cladding process to repair high value complex fatigue critical aerospace military components, that otherwise would be replaced. The selected material, AerMet®100, is a widely used ultra-high strength steel in current and next generation aerospace components, such as landing gears. Laser cladding was performed using AerMet® 100 powder on AerMet® 100 fatigue substrate specimens. No micro-cracking and very little porosity were observed in the clad layer. The fatigue tests were performed under variable amplitude loading with a maximum stress of 1000 MPa. Residual stress, microstructure, and hardness, was also evaluated. Both the as-clad and post-heat treated (PHT) samples were compared to a baseline sample with an artificial notch to simulate damaged condition. Results show that laser cladding significantly improves fatigue life, as compared to the baseline sample with a notch. However, the fatigue life of the as-clad sample is lower as compared to a baseline sample without a notch. A compressive residual stress of 300–500 MPa was observed in the clad region and HAZ. The fracture modes in the as-clad specimen consisted mainly of tearing topology surface and some regions of decohesive rupture through the columnar austenite grains. The PHT condition however was not effective in improving the fatigue life. The fracture modes showed mainly decohesive rupture, and as a consequence, reduced the fatigue life.     

Properties of solid wood and laminated wood lumber manufactured by cold pressing and heat treatment

Physical, mechanical, and morphological properties of solid wood lumbers which were cold pressed in a press and then heat treated in a kiln. Two different kinds of domestic thinning small-diameter softwood (Ginko biloba L.) and hardwood (Tilia amurensis Rupr.) were used in this study. First 50 mm thick lumbers were cold pressed until 35 mm (30% of control lumber) using a stopper for 5 min. Then the cold pressed lumbers were heat treated in an electric kiln at 180 °C for 6, 12, 24, or 48 h. To increase the utilizability of woods, the LVLs were produced from 4 mm thick veneers prepared from the heat treated lumbers using a veneer saw. Each LVL sample consisted of 5 layers which were subsequently 48 h-, 24 h-, 12 h-, and 6 h-treated veneers and untreated veneer (from top layer to bottom layer). The shrinkage rates of softwood and hardwood were considerably decreased with increasing temperature. The mechanical properties of heat treated samples were better than those of unpressed control samples. The bending strength and modulus of elasticity of the LVLs manufactured from cold pressed and then heat treated lumbers were slightly lower than those of untreated woods. The colour values obtained from the heat treated wood samples showed a clear effect of the temperature on the colour changes.     

The role of delamination in strength,failure mechanism and hole size effect in open hole tensile tests on quasi-isotropic laminates

Results of several different series of open hole tension tests on quasi-isotropic IM7/8552 carbon fibre/epoxy laminates with the same stacking sequence but different ply block thicknesses and numbers of sublaminates are summarised. Specimens with single 0.125 mm thick plies failed by fibre fracture, with the strength decreasing with increasing hole size. Ones with 0.5 mm thick blocks of plies all delaminated, with the failure stress increasing with increasing hole diameter, the opposite to the usual hole size effect. Specimens with 0.25 mm thick ply blocks showed intermediate response, with small ones failing by delamination, and large ones by fibre failure, and constant strength over a range of hole sizes from 1.6 to 12.7 mm diameter. The crucial role of delamination in the strength and failure mechanism in open hole tension is examined in order to explain these results and show why conventional hole size corrections may not always be applicable.     

Fatigue behavior and failure mechanism of basalt FRP composites under long-term cyclic loads

This paper studies the fatigue behavior of basalt fiber reinforced epoxy polymer (BFRP) composites and reveals the degradation mechanism of BFRP under different stress levels of cyclic loadings. The BFRP composites were tested under tension–tension fatigue load with different stress levels by an advanced fatigue loading equipment combined with in-situ scanning electron microscopy (SEM). The specimens were under long-term cyclic loads up to 1 × 10⁷ cycles. The stiffness degradation, S–N curves and the residual strength of run-out specimens were recorded during the test. The fatigue strength was predicted with the testing results using reliability methods. Meanwhile, the damage propagation and fracture surface of all specimens were observed and tracked during fatigue loading by an in-situ SEM, based on which damage mechanism under different stress levels was studied. The results show the prediction of fatigue strength by fitting S–N data up to 2 × 10⁶ cycles is lower than that of the data by 1 × 10⁷ cycles. It reveals the fatigue strength perdition is highly associated with the long-term run-out cycles and traditional two million run-out cycles cannot accurately predict fatigue behavior. The SEM images reveal that under high level of stress, the critical fiber breaking failure is the dominant damage, while the matrix cracking and interfacial debonding are main damage patterns at the low and middle fatigue stress level for BFRP. Based on the above fatigue behavior and damage pattern, a three stage fracture mechanism model under fatigue loading is developed.     

Residual stress and strain in MIG butt welds in 5083-H321 aluminium: As-welded and fatigue cycled

This paper presents single-line residual stress profiles for 8 mm 5083-H321 aluminium plates joined by gas metal arc (MIG) welding. The data were obtained by synchrotron diffraction strain scanning. Weld metal stresses (up to ～7 mm either side of the centreline) are quite scattered and unreliable because of the large epitaxial grain size in the fusion zone. Peak magnitude of the transverse stresses varies between +50 MPa (19% of parent plate proof strength) at the HAZ boundary to ?150 MPa (57% of PP proof strength) at the weld centreline. Equivalent values for longitudinal stresses are +90 MPa (34% of PP proof strength) some 22 mm from the weld centreline to ?120 MPa (45% of PP proof strength) at the weld centreline. Plate-to-plate variation in the as-welded transverse and longitudinal residual stress values across the weld toe region is around 40 MPa. The effect on residual stress and strain values of a sequence of applied fatigue loads was also considered and reported.     

Recombination zone in white organic light emitting diodes with blue and orange emitting layers

White fluorescent OLED devices with a 10 nm thick blue-emitting layer and a 31 nm thick orange-emitting layer have been fabricated, where the blue-emitting layer is stacked on a hole transport layer. An interlayer was inserted between the two emitting layers. The thickness of the interlayer was changed among 0.3, 0.4, and 1.0 nm. White emission with CIE coordinates close to (0.33, 0.33) was observed from all the OLEDs. OLED with 0.3 nm thick interlayer gives the highest maximum luminous efficiency (11 cd/A), power efficiency (9 lm/W), and external quantum efficiency (5.02%). The external quantum efficiency becomes low with increasing the interlayer thickness from 0 nm to 1.0 nm. When the location of the blue- and orange-emitting layers is reversed, white emission was not obtained because of too weak blue emission. It is suggested that the electron–hole recombination zone decreases nearly exponentially with a distance from the hole transport layer.     

TIG–MIG hybrid welding of ferritic stainless steels and magnesium alloys with Cu interlayer of different thickness

The joining of ferritic stainless steels and magnesium alloys is light and economic for weight reduction of automobiles. Unlike previous conventional welding method, a novel TIG–MIG hybrid welding is applied for the joint successfully in this study. The melted Mg weld metal wets the ferritic stainless steels surface to form a brazed Mg–Cu to steel connection when the interlayer thickness is 0.02 mm. When the interlayer thickness is 0.1 mm, the intermetallic compounds transition layer determined the tensile-shear strength of joints. Intermetallic compounds transition layer has been found in the 0.1 mm thick interlayer joints and no particle has been found in the 0.02 mm thick interlayer joints. Based on the analysis of microstructure and properties, joining and strengthen mechanisms of the joint were discovered. As the thickness of the Cu interlayer increases, the joining mechanism changed. The joining and strengthen mechanisms are mainly determined by the thickness of the interlayer. The tensile-shear strength of 0.1 mm thickness Cu interlayer joints is improved by 47% compared to 0.02 mm Cu.     

Fatigue fracture of friction-stir processed Al–Al3Ti–MgO hybrid nanocomposites

This paper presents experimental results on the fatigue properties of Al-matrix nanocomposites prepared by the friction stir processing (FSP) technique. An Al–Mg alloy (AA5052) with different amounts (∼2 and 3.5 vol%) of pre-placed TiO₂ nanoparticles were FSPed up to 6 passes to attain homogenous dispersion of nano-metric inclusions. Microstructural studies by electron microscopic and electron back scattering diffraction (EBSD) techniques showed that nano-metric Al₃Ti (50 nm), TiO₂ (30 nm), and MgO (50 nm) particles were distributed throughout a fine-grained Al matrix (<2 μm). Consequently, a significant improvement in the tensile strength and hardness was attained. Uniaxial stress-controlled tension–tension fatigue testing (R = 0.1) were utilized to evaluate the fatigue behavior of the prepared nanocomposites. The results were compared with the un-processed (annealed) and FSPed alloy without pre-placing TiO₂ particles. It was found that FSP of the aluminum alloy increased the fatigue strength (at 10⁷ cycles) for about 28% and 32% compared with the annealed specimen when the concentration of the reinforcing particles was 2 and 3.5 vol%, respectively. Fractographic analysis determined a ductile fracture behavior with deep-equiaxed dimples for the annealed and FSPed alloy. The facture surface of the nanocomposites revealed a combined ductile–brittle fracture mode with finer dimples. The mechanism of the fatigue fracture and the role of nano-metric inclusions were elaborated.