Shorter Fatigue Life Research Articles

The Effect of Weld Reinforcement and Post-Welding Cooling Cycles on Fatigue Strength of Butt-Welded Joints under Cyclic Tensile Loading

This research deals with the fatigue behavior of butt-welded joints, by considering the geometry and post-welding cooling cycles, as a result of cooling in quiet air and immersed in water. ASTM A-36 HR structural steel was used as the base metal for the shielded metal arc welding (SMAW) process with welding electrode E6013. The welding reinforcement was 1 mm and 3 mm, respectively; axial fatigue tests were carried out to determine the life and behavior in cracks propagation of the tested welded joints, mechanical characterization tests of properties in welded joints such as microhardness, Charpy impact test and metallographic analysis were carried out. The latter were used as input for the analysis by finite elements which influence the initiation and propagation of cracks and the evaluation of stress intensity factors (SIF). The latter led to obtaining the crack propagation rate and the geometric factor. The tested specimens were analyzed, by taking photographs of the cracks at its beginning in order to make a count of the marks at the origin of the crack. From the results obtained and the marks count, the fatigue crack growth rate and the influence of the cooling media on the life of the welded joint are validated, according to the experimental results. It can be concluded that the welded joints with a higher weld reinforcement have a shorter fatigue life. This is due to the stress concentration that occurs in the vicinity of the weld toe.

Study on the Induction Quenching Process and Fatigue Testing for Cr-Mo Steel Ball Screw

Due to the deficiencies such as shallow hardened layer and short fatigue life of the ball screws treated by the traditional induction quenching process, a new process has been developed. The new induction quenching process has four-turn induction coil. The power of the this process is 150-160kw, feeding speed 270mm/min and the spraying cooling water distance is 35mm. It has been successfully used on the Cr-Mo steel ball screw with a diameter of 80 mm. The microstructure and hardness distribution of the induction hardened layers were investigated by a metallographic microscope and a micro-hardness apparatus. The fatigue life of the ball screw was measured through a constant stress accelerated fatigue test. The degree of fatigue wear on the raceway of the ball screw was analyzed by a field emission scanning electron microscopy (FE-SEM). The experimental results showed that the microstructure and hardness distribution of the hardened layer of the Cr-Mo steel ball screw could satisfy the design requirements of contact fatigue resistance. The fatigue life was longer than the theoretical service life and there was no obvious fatigue wear on the raceway after the fatigue testing. It shows that the safety and reliability of the Cr-Mo steel ball screw fabricated through the new induction quenching process could be guaranteed in practical application.

Influence of the kissing bond on the mechanical properties and fracture behaviour of AA5083-H112 friction stir welds

The kissing bond phenomenon in AA5083-H112 friction stir butt welds was investigated in joints welded using a matrix of welding parameters, with tool rotation speeds of 800, 1000, and 1200 rpm and feed speeds 100, 200, and 300 mm/min. The length of the kissing bond along the cross-section normal to the welding direction was measured to quantify its influence on the mechanical properties. A combination of optical microscopy, scanning electron microscopy, transmission electron microscopy, tensile and fatigue testing were used to elucidate the impact of the kissing bond on the microstructural and mechanical properties development. The fracture type, location, and morphology were studied for the various conditions. The results showed that the welding parameters had a substantial effect on the length of the kissing bond, which was found to decrease with the increase in the welding heat input, as estimated based on the rotation and feed speeds. Moreover, the length and morphology of the kissing bond had a significant influence on the tensile and fatigue fracture type. A shear fracture type was characteristic for welds showing high tensile properties and long fatigue life, whereas fracture along the kissing bond was characteristic for poor tensile properties and short fatigue life.

Influence of the welding process on the residual welding stresses in an orthotropic steel bridge deck

An orthotropic steel bridge deck consists out of a number of longitudinal and transverse stiffeners. These stiffeners are welded to the deck plate which cause residual stresses to be present near the vicinity of this connection. In addition, the welding operation causes multiple fatigue problems across the bridge deck which lead to the formation of fatigue cracks. For an accurate fatigue design of an orthotropic bridge deck, the residual stresses should also be taken into account to determine the effect of these stresses with respect to the load-induced stresses. Tensile residual stresses should be avoided since they tend to open fatigue cracks which results in a shorter fatigue life. The welding of the longitudinal stiffener to the bridge deck plate is simulated with finite element modelling. The effect of the size of the tack welds, the welding current and the welding speed on the residual stresses is determined. These different welding sequences are modelled in order to minimize tensile residual stresses near the weld connection. The welding configuration which leads to smaller tensile residual stresses will result in an orthotropic steel bridge deck with a longer fatigue lifetime.

Fatigue Tests of Concrete Slabs Reinforced with Stainless Steel Bars

Experimental studies on fatigue behavior of reinforced concrete slab with stainless steel rebar and carbon steel rebar have shown that, at the same reinforcement ratio, the slope of the deflection‐cycle number curves of stainless steel‐reinforced concrete slab is lower than that of ordinary steel‐reinforced concrete slab. The higher the reinforcement ratio is, the smaller the maximum crack width would be. Higher stress level contributes to larger deflection and reinforcement strain in midspan and shorter fatigue life. Compared to the ordinary steel‐reinforced concrete slab, the stainless steel‐reinforced concrete slab shows narrower maximum crack under the same number of loading cycles. Less significant midspan deflection, reinforcement strain, and longer fatigue life are observed in stainless steel‐reinforced concrete slab at the same reinforcement ratio, stress level, and cycling time. With the increase of reinforcement ratio, the deflection and fatigue life extended.

Short Fatigue Crack Behavior of LZ50 Axle Steel Under Rotating-Bending Cyclic Loading

The short fatigue short crack replica tests using LZ50 axle steel hourglass-shaped specimens of were carried out under rotating-bending cyclic load. Seven sets of effective test data were obtained. The analysis demonstrated that short fatigue cracks generally originate in ferrite grains or on their boundaries, and then short cracks start propagating, which is strongly influenced by the microstructure of the material. The crack growth rate increases serratically, followed by its decrease double-fold because of the limit of ferrite grain boundaries and the banded pearlite structure. Based on the three principles of the best fit, which include the overall fitting effect, correlation with fatigue physics, and safety of residual strength prediction, the statistical evolution of the dominant effective short fatigue crack length and life share were analyzed. The statistical results show that the minimum value distribution exhibit the best fit of the dominant effective fatigue short crack length and the life share. The short fatigue crack growth model, which includes various microstructure barriers, provides a good fitting result and reflects the cyclic effect of the microstructure on the short fatigue crack behavior.

Physical vapour deposition of diamond-like carbon coatings on a 7075-T6 substrate for corrosion protection at long and short fatigue lives

Purpose7075-T6 is the most widespread structural aluminium alloy due to its high mechanical strength. However, use of this alloy in critical aeronautic, maritime, and automotive sectors is limited by the susceptibility of T6 treatment to cracking and pitting corrosion. To improve fatigue behaviour in aggressive environments, several authors have proposed the use of different coatings to protect the substrate. Studies have investigated the application of thin hard coatings on light alloys by physical vapour deposition (PVD). Different contributions of residual stresses, thermal modification of the substrate, and mechanical interaction between the coating and aluminium substrate were investigated. The purpose of this paper is to investigate the rotating bending fatigue behaviour (R=−1) of 7075-T6 PVD diamond-like carbon (DLC)-coated specimens in air and in a corrosive environment. Tests were conducted at different applied stresses. Scanning electron micrographs of the fracture surface are provided to investigate the influences of mechanical and environmental driving forces on the failure mechanism.Design/methodology/approachThe paper conducted an experimental study of the fatigue resistance of DLC coatings on a 7075-T6 substrate for corrosion protection at long and short fatigue lives, which includes rotating bending fatigue tests, step-loading fatigue test procedure, tests in aggressive environment (methanol), tests at high and low fatigue lives, analysis of the fracture surface, and analysis of the driving forces.FindingsTests performed in air showed that the coating anticipates crack nucleation for high applied loads, whereas for lower loads, the difference among fatigue curves decreases. This result is very interesting from an industrial standpoint because the obtained material shows improved corrosion and wear resistance, without the fatigue resistance loss generally associated with hard coatings. The methanol environment accelerates crack nucleation and propagation, resulting in a sensible deterioration of the fatigue behaviour. A minimum soaking time seems to be necessary before the damaging effect of the environment begins. The coating has a certain protective effect against the environment, but this protection is insufficient for the specimen to achieve fatigue limits beyond those of the uncoated specimens. This deficiency can be related to small pores or defects in the coating, which allow contact between the substrate and the environment. Further tests are necessary to verify whether there exists a load under which the fatigue behaviour of the coated specimens is better than that of the uncoated specimens. Crack nucleation due to fatigue occurs close to the outer surface for all observed samples. For coated samples tested at the lowest stress level, crack nucleation seems to be located below the surface. This observation means that premature coating cracking, which characterises the nucleation mechanism at higher loads, did not occur at lower stress levels. The fracture surface of uncoated samples was clearly damaged by the aggressive solution, justifying the poor fatigue resistance.Research limitations/implicationsThe obtained data do not represent actual S-N curves, which would necessitate a larger number of tests with proper statistics. Nevertheless, some indications of the DLC effects on 7075-T6 specimens in air and methanol environments can be deduced. The step-loading technique seems to be critical for tests in corrosive environments, probably because the total soaking time in the corrosive environment is generally higher than it is for the single-run test.Originality/valueThe originality of the paper lies in the application of the step-loading test procedure to quickly detect the mechanical and chemical driving forces that control the damage and structural integrity of light alloys components in very aggressive environments.

Experimental clarification of the cyclic deformation mechanisms of β-type Ti–Nb–Ta–Zr-alloy single crystals developed for the single-crystalline implant

Quaternary Ti–Nb–Ta–Zr (TNTZ) type β-Ti alloys with a body-centered cubic (bcc) structure have been of great interest as new implant biomaterials for bone replacement. Recently, we have proposed a “single crystalline β-Ti implant” as a novel hard-tissue replacement to suppress the stress shielding to bone after healing. To develop this, the fatigue properties and its controlling mechanisms of the TNTZ single crystals must be clarified. In this study, we focused on three alloys: Ti–25Nb–10Ta–5Zr(25Nb), Ti–29Nb–13Ta–4.6Zr(29Nb), and Ti–35Nb–10Ta–5Zr(35Nb) (mass%), and the cyclic deformation behaviors were examined using their single crystals. The shapes of the hysteresis loops were significantly different for these alloys, which was ascribed to the operation of different deformation modes: stress-induced α”-martensite (α”-SIM), {332}<113> twinning, and {101}<111> slip in the 25Nb, 29Nb, and 35Nb alloys, respectively. The hysteresis loops during cyclic deformation exhibited very irregular shapes for 25Nb and 29Nb, accompanied by a flow-stress asymmetry in tension and compression. This was caused by the polarized feature on the formation of them in tension and compression. In addition, the apparent yield stress in a hysteresis loop greatly decreased after the second cycle, owing to the reversible motion of the interfaces of the SIM and {332} twins by altering the direction of the applied stress. Although the cyclic deformation behavior was controlled by a similar mechanism in them, 29Nb was found to exhibit a much shorter fatigue life than 25Nb. This is because of the difference in the decreasing tendency of the reversibility for the SIM and {332} twins as cyclic deformation proceeded. The obtained results demonstrate that the controls of their asymmetric features and the reversible motion are the important factors to develop these alloys as a practical “single crystalline β-Ti implant”.

Effect of broken wire on bending fatigue characteristics of wire ropes

The life of damaged wire rope is difficult to judge and the damaged wire rope is prematurely replaced resulting in an enormous economic waste. This paper studies the effect of different distributions of pre-broken wires on bending fatigue behavior of wire ropes using a self-made bending fatigue test facility. The fatigue lives of wire ropes with different distributions of pre-broken wires were studied using both visual inspection and commercial non-destructive inspection technology, combined with electron microscopy. It is found that broken wires on the surface reduce the bending fatigue life of wire ropes. The broken wires increase the stress in the inner wire strands as well as the contact force between the wires. This leads to a concentration of severe wear, which accelerates the density of broken wires locally, leading to short fatigue lives especially in the case with the most concentrated number of pre-broken wires (1×4). After numerous wires fail by wear and fatigue, others fail by tensile overload, as evidenced microscopically by necking and ductile failure. The damage in the rope at failure defined by counting broken wires is nearly 50% smaller than the damage value given by the commercial non-destructive damage value, based on magnetic measurements.

Corrosion-fatigue testing of Ni-based superalloy RR1000

Operation of gas turbines at high temperatures can lead to corrosion of turbine materials. A programme was designed to understand the corrosion-fatigue behaviour of two grain size variants of RR1000 under cyclic loading at elevated.Tests were conducted on cylindrical specimens under a range of loading conditions. Results indicated that salted specimens tested in air-SOx had shorter fatigue lives than unsalted specimens tested in air, and this deficit was dependent on stress and/or test duration. Examination of specimens showed that the extent of corrosion was dependent on exposure time and stress. Additional salt loading resulted in faster initial pit growth which led to lower fatigue lives at higher stresses.This paper is part of a thematic issue on the 9th International Charles Parsons Turbine and Generator Conference. All papers have been revised and extended before publication in Materials Science and Technology.

A comparison of low-cycle fatigue behavior between the solutionized and aged Mg-3Nd-0.2Zn-0.5Zr alloys

The present paper compares the low cycle fatigue characteristics between the solution-treated (T4) and peak-aged (T6) Mg-3Nd-0.2Zn-0.5Zr magnesium alloys produced by semi-continuous casting. The cyclic stress amplitudes of the T6-treated counterpart are higher than those of the T4-treated alloy, which is due to precipitate strengthening. At the same stress amplitude, the T4-treated alloy with higher hysteresis energies undergoes more fatigue damage, resulting in shorter fatigue lives. For the T4-treated alloy, the cyclic stress amplitude is found to increase with increasing the cycle number, which is attributed to its soft matrix and the increased volume fraction and number density of twins under continuous loading. Cyclic hardening followed by the decrease of the stress is observed in the T6-treated alloy. It is shown that the cyclic deformation behavior and failure mechanism of the T4-treated alloy are dependent on the dislocations-slip plus twinning, while those of the T6-treated counterpart are dependent on the dislocations-slip. Based on discussion, the Coffin-Manson law and Basquin equation or the energy-based concepts can be well applied to the prediction of the LCF lives of the NZ30K alloys.

Experimental Study on Extremely Low-Cycle Material Fatigue Resistance for Two Types of Welded Structural Details

Extremely low-cycle fatigue and fracture is one of the most critical damage types of the welded beam-to-column connections when the steel frame subjected to the strong earthquake excitation. The investigation on extremely low cycle fatigue behavior of welded structural details is useful for the fatigue design of welded structure. In the present study, extremely low cycle material fatigue tests were carried out to examine the fatigue failure mechanism of two types of welded structural details, which were the plate butt weld (PB) specimen and cruciform load-carrying groove weld (CLG) specimen. Fatigue tests were performed on 12 welded specimens under strain-controlled constant loading in the plastic range. The fitting S-N curves of the two types were presented. The fatigue S-N curves of the similar welded structural details in IIW Fatigue Recommendation was adopted to correlate the fatigue resistance of the two types of welded structural details. The results demonstrated that the failure mode of PB specimens was not the same as that of CLG specimens. The extremely low-cycle material fatigue strength of the investigated welded structural details was lower than expected high cycle fatigue S-N curves provided by IIW, showing shorter fatigue lives, which indicated that the cumulative damage of welded details involved ductile damage besides cyclic fatigue damage in extremely low-cycle fatigue situation.

Experimental Study on the Coaxing Effect of Multi-Level Stresses with Different Sequences

The coaxing effect in fatigue refers to an increase in the fatigue strength of material imduced by prior cycling at a stress level below the fatigue limit. In this study, the coaxing effect of multi-level stresses in the fatigue process in the automotive drive shaft material 40Cr was investigated by applying three levels of stress amplitudes below the fatigue limit in different sequences prior to conducting the conventional constant amplitude fatigue test with standard round bar specimens. The existence of coaxing effect under multi-level stress with a load sequence effect was observed and confirmed by the results. The longest fatigue life is achieved through three levels of coaxing load with a step increase sequence, while a decrease sequence will result in the shortest fatigue life. The mechanisms of the coaxing effect which increased the fatigue life at the crack initiation and propagation stages were observed, through fracture morphology analysis, as the regulation of micropores, decreasing of pore density and the narrowed striation spacing. This finding is instrumental for extending the service life of mechanical components.

An investigation on crack growth rate of fatigue and induction heating thermo-mechanical fatigue (TMF) in Hastelloy X superalloy via LEFM, EPFM and integration models

In this research, fatigue and induction heating thermo-mechanical fatigue (TMF) were performed on Hastelloy X superalloy in the small and large scale yielding in plane stresses mode. The crack growth rates were measured and formulated by fracture mechanics parameters. Furthermore, the fatigue life was predicted by employing resistance curves technique. The TMF behavior of this superalloy was investigated. The results demonstrated that in-phase loading TMF conditions lead to short fatigue life (more crack growth rate) at high strain amplitudes and temperatures up to 600°C. For higher temperatures, the predominant damage was due to creep. A model based on damage contributions due to pure fatigue and cyclic creep has been presented for predicting TMF crack growth rates. Fracture mechanic method was used to suggest a model for fatigue part of TMF crack growth rate, while the temperature effects during TMF crack growth rate was considered to be due to cyclic creep. In addition the TMF crack healing or crack closure occur during application of induced eddy currents were investigated explicitly, as environmental effects of induction heating. The results show the higher current density in the crack tip area produced more heat and resulted in a significant rise in temperature. So it was concluded that the compressive thermal stress due to change of thermal expansion causes crack healing.

Fatigue Damage of V-Lock Chain Ring Under Random Load

Abstract To study the influence of the random load on the V-lock chain ring for mining, the numerical simulation technology is used. The dynamic tension is obtained by using the dynamic model of the plough. The life and damage nephograms are obtained by using ANSYS Workbench. The analysis results show that the short fatigue life region of the V-lock chain ring for mining is mainly concentrated on the transition region between the medial straight edge and arc, and the fatigue damage of the link chain on the side of the motion direction of the plow head is larger than that on the other side. This link chain has strong anti-fatigue performance.

Low cycle fatigue behavior of the extruded AZ80 magnesium alloy under different strain amplitudes and strain rates

Abstract Low cycle fatigue behavior of extruded AZ80 magnesium alloy was investigated under uniaxial tension-compression at different strain amplitudes and strain rates. The results show that the extruded AZ80 magnesium alloy exhibits cyclic hardening at strain amplitudes ranging from 0.4% to 1.0%, the asymmetry of hysteresis loops becomes increasingly obvious when the strain amplitude increases. Higher strain rates correspond to higher stress amplitudes, high mean stresses and short fatigue life. {10–12} extension twins play a role in the cyclic deformation under higher strain amplitudes (0.8%, 1.0%). The relationship between total strain energy density and fatigue life can be described by the modified Morrow model. The effect of strain rate on the fatigue life can also be predicted by the model.

Lüders-like band front motion and fatigue life of pseudoelastic polycrystalline NiTi shape memory alloy

We conducted tensile fatigue tests on polycrystalline NiTi strips with in-situ optical observation on the evolution of the localized deformation (Lüders-like band) in order to trace the deformation history of the fatigue-failure material point in the strips under various loadings. It is found that the band front motion triggering localized cyclic phase transformation leads to a short fatigue life while the “elastic” deformation (without the band front motion) results in a much longer life whose failure crack nucleates away from the immobile band front. The results provide hints to derive a material fatigue criterion from the structure fatigue tests.

Effects of Lüders-like bands on NiTi fatigue behaviors

Although the formation of Lüders bands in NiTi polycrystalline structures (strips/tubes) has been well reported and is believed to have considerable influence on the material's fatigue behaviors, there is no direct experimental evidence showing the relation between the band formation and the material's fatigue failure. For each NiTi polycrystalline strip in this study, a strain-controlled isothermal test was firstly conducted to check the pseudoelastic properties and then a stress-controlled fatigue test was performed with synchronized optical observation on the evolution (cyclic nucleation/annihilation) of Lüders bands in the strip. It is found that the fatigue failure crack always occurs within the band or at the band front, because the band formation is an indicator of the weak zones of the samples and the cyclic band nucleation/annihilation accelerates microstructure degradation in those zones. Under the same loading scheme of the fatigue tests, the samples with bands have much shorter fatigue life than the samples without bands. The experiments provide the bases for future theoretical study on the interaction between the martensitic phase transformation and the plasticity/defects and give some hints for developing a proper fatigue criterion for the softening material in engineering applications.

Microstructural Features Controlling the Variability in Low-Cycle Fatigue Properties of Alloy Inconel 718DA at Intermediate Temperature

The low-cycle fatigue behavior of two direct-aged versions of the nickel-based superalloy Inconel 718 (IN718DA) was examined in the low-strain amplitude regime at intermediate temperature. High variability in fatigue life was observed, and abnormally short lifetimes were systematically observed to be due to crack initiation at (sub)-surface non-metallic inclusions. However, crack initiation within (sub)-surface non-metallic inclusions did not necessarily lead to short fatigue life. The macro- to micro-mechanical mechanisms of deformation and damage have been examined by means of detailed microstructural characterization, tensile and fatigue mechanical tests, and in situ tensile testing. The initial stages of crack micro-propagation from cracked non-metallic particles into the surrounding metallic matrix occupies a large fraction of the fatigue life and requires extensive local plastic straining in the matrix adjacent to the cracked inclusions. Differences in microstructure that influence local plastic straining, i.e., the δ-phase content and the grain size, coupled with the presence of non-metallic inclusions at the high end of the size distribution contribute strongly to the fatigue life variability.

3D造形コバルトクロム合金の疲労特性に及ぼす熱処理の影響

The Co-Cr-Mo alloy is used for a prosthetic slide part to be superior in wear resistance. The custom-made medical application is examined for the Quality Of Life (QOL) improvement of the person with skeletal system medical care disease. Therefore, the Electron Beam Melting (EBM) that a little and free shape can manufacture is effective for the production of custom-made devices. However, crystal structures are different for the molding position. The end samples (No.5) had short fatigue life. On the other hand, the center samples had long fatigue life. This depends on crystal structure. This is because the central samples has γ phase and the end samples has ε phase. Thus, γ phase influences in improvement of the fatigue life. Therefore I thought that crystal structure is changed γ phase by the heat-treatment. EBM sample was performed heart-treatment (change the ε phase) and reverse transformation heart-treatment (change the γ phase).I performed a hardness examination afterwards. As a result, all specimens became the hardness of the equal degree. The hardness decreased in comparison with before heat-treatment. Thus, I was able to become a γ phase.