Difference In Fatigue Strength Research Articles

Microstructural effects on the rotating bending fatigue behavior of Ti–6Al–4V produced via laser powder bed fusion with novel heat treatments

The rotating bending fatigue (RBF) behavior (fully reversed, R = −1) of additively manufactured (AM) Ti–6Al–4V alloy produced via laser powder bed fusion (PBF-L) was investigated with respect to different microstructures achieved through novel heat treatments. The investigation herein seeks to elucidate the effect of microstructure by controlling variables that can affect fatigue behavior in Ti–6Al–4V, such as chemistry, porosity, and surface roughness. In order to control these variables, different hot isostatic pressing (HIP) treatments at 800 °C, 920 °C, and 1050 °C with a 920 °C temper were applied to three sets of Ti–6Al–4V cylinders that originated from the same PBF-L build, such that there were 30 tests per condition. After HIP treatment, the specimens were machined and tested. The highest runout stress was achieved after sub-β transus HIP at 800 °C for 2 h at 200 MPa of pressure. A significant drop in fatigue strength was attributed to large prior-β grains and grain boundary α resulting from super-β transus HIP treated specimens. For the sub-β transus HIP specimens, differences in fatigue strength were attributed to α lath thickness, relative dislocation density, and dislocation boundary strengthening.

Frequency dependent fatigue behaviour of additively manufactured titanium lattices

Additively manufactured (AM) porous titanium lattices, with their ability to match the mechanical properties of bone and avoid stress shielding, are a popular candidate material for orthopaedic implants. Such implants are now emerging as treatments for conditions like osteoarthritis and fixation of bone fractures. Fatigue tests are critical due to the cyclic loading environment and must be carried out at an accelerated loading rate to simulate many years of use. Tests are typically performed with servohydraulic instruments, which limits the cyclic compression to relatively low frequencies (15 Hz). Fatigue testing at a higher frequency would accelerate research, however, may introduce phenomena such as heat accumulation and strain rate effects. In this study the fatigue behaviour of a pure titanium stochastic lattice was determined at two test frequencies, 15 Hz and 110 Hz. Testing was conducted using an electromechanical dynamic system. The fatigue strengths at 106 cycles were 5.607 ± 0.106 MPa and 5.764 ± 0.214 MPa at 15 Hz and 110 Hz respectively. A hypothesis t-test at a 95% confidence level stated that there was significant evidence that the population means were not the same, demonstrating evidence of a difference in fatigue strength with testing frequency. However, we can conclude that the 2.8% increase in fatigue strength due to test frequency effects is inconsequential relative to the time saved (16 h per test) and typical batch-to-batch variability in fatigue strength of approximately 12%. The results of this study should help accelerate research into the fatigue properties of AM porous lattices.

VHCF of the 3D-Printed Aluminum Alloy AlSi10Mg

The paper is focused on the very high cycle fatigue (VHCF) properties of aluminum alloys proceeded in two different technological procedures. The hot-rolled D16T alloy is compared with the selected laser melting (SLM) AlSi10Mg alloy. The fatigue tests were performed at a high frequency (20 kHz) in the laboratory air environment at room temperature. The experimental results showed a significant difference in fatigue strength between hot-rolled and SLM materials. The VHCF properties of AlSi10Mg were more than two times lower than those of D16T in spite of the comparable quasi-static tensile properties. The difference in fatigue properties was explained based on fractographic analysis of fracture surfaces. The morphology of the fracture pattern was qualitatively different. In the case of the hot-rolled alloy, the three clear zones of crack growth could be outlined. The main part of the pattern was covered by quasi-brittle facets that are typical for VHCF fractures in Al alloys. In the case of the SLM material, unregulated structures were found in the microstructure. In some local zones, numerous non-melted particles were observed on the fracture surface. The boundaries of certain layers also played an important role in the fracture. Large, separated surfaces were observed on the fracture pattern. It is important to note that these boundaries were not associated with the layer-by-layer building of the specimen. The distance between such features was significantly larger than the thickness of an individual layer.

The effects of building position on surface and fatigue of DED-arc steel components

Recently, additive manufacturing (AM) of structural metallic components is analyzed regarding its potential use by industry and research. Next to the development of manufacturing processes, the mechanical properties are under investigation today. One of the quality measures of metallic components is the surface topography. DED-arc processes (direct energy deposition) result in relatively coarse surfaces, characterized by a distinct waviness with wave amplitudes in the mm-range. This is enhanced when applying horizontal building position in comparison to vertical position. Next to increased waviness, the load-bearing net cross sections are reduced as well. The surface topography determines the fatigue life properties of metallic components. While stress raising surface effects are generally well understood and fatigue (Structures 31: 576–589, 2021) of welded metals is established well, the fatigue behaviour of additively manufactured components is less investigated yet. In order to define surface quality levels for DED-arc components, the effects of surface topography on mechanical performance need to be understood. This article presents the manufacturing of high strength steel test coupons by the DED-arc process. The process parameters were varied with regard to the building position and different levels of surface quality were generated. The surfaces of different specimens were characterized and fatigue tests were conducted. The results were used to derive the surface influence on both, the effective load-bearing wall thickness and notch effects induced by the layer-by-layer building approach. A correlation between building position, surface waviness and fatigue strength was proven. In general, higher waviness resulted in reduced effective wall thickness and lowered fatigue strength. A difference in fatigue strength at 2 million load cycles of 20 to 30% was proven when printing in different building positions. The surface effect can be captured in the design concept when applying the effective notch stress approach with an averaging length of of ρ* = 0.4 mm. The fatigue strength is describable by a design S–N curve FAT160 and a k-value of 4.

Effect of welding processes on the fatigue behaviour of ultra-high strength steel butt-welded joints

In the last decades, advances in steel manufacturing made possible the use of high-strength steel (HSS) and ultra-high strength steel (UHSS) for several applications, such as bridges, cranes, offshore structures, oil pipelines and automotive parts. Capacity of withstanding loads with reduced cross-section and minimum weight could be efficiently increased. Since most structures need to be joined, welding procedures are a major issue in mechanical design of HSS elements. Particularly in construction codes and design documents, it is normally assumed that fatigue resistance of as-welded joints is independent of strength level. Nevertheless, fatigue loaded as-welded components with high quality welds or post-weld treated joints could experience benefits from the use of HSS as the base material (BM).The purpose of the present work is to analyse fatigue behaviour of ultra-high strength steel butt-welded joints, by means of experimental testing and a fracture mechanics approach. Sheets of steel S960MC and S960QL were joined with different welding techniques: Gas Metal Arc Welding (GMAW), Laser Hybrid Welding (LHW) and Electron Beam Welding (EBW). Fatigue tests were performed with stress ratio R=0.1, under four points bending loading.All specimens exhibited fatigue crack initiation and subsequent propagation from the weld toe area, near the heat affected zone (HAZ). Different SN curves were obtained for the different welding processes. The Resistance Curve methodology was employed to assess the effect of microstructure, defect size, hardness, and joint geometry resulting from each technique. This fracture mechanics approach allowed to estimate the relative influence of the different geometrical and mechanical parameters of the weld and showed that joint geometry could not explain by itself the differences in fatigue strength. It was observed that microstructure and the size of defects played an important role in early crack growth, and they can reduce the benign effect of a high-strength base material.

Machine Learning-Assisted probabilistic fatigue evaluation of Rib-to-Deck joints in orthotropic steel decks

This study integrates the fatigue test and numerical prediction to derive a comprehensive probability-stress-life (P-S-N) curve for rib-to-deck (RD) welded joints in orthotropic steel decks. Fatigue tests of RD joints are conducted to measure fatigue strength and crack growth data. Based on the test, a probabilistic fatigue crack growth (PFCG) model is established to predict the distribution of fatigue life under various stress ranges. Two machine learning tools are adopted to assist the PFCG model-based prediction, i.e., the Gaussian process regression (GPR) and dynamic Bayesian network (DBN). The GPR is used to train a surrogate model solving stress intensity factors for the PFCG prediction, using 2,000 samples generated from finite element (FE) analyses. The trained model is then validated by a new dataset of 100 FE samples. An adapted DBN model is proposed to update the PFCG model with the fatigue crack growth data measured from ten specimens. According to the result, the application of GPR can reduce the solution cost of the PFCG prediction by approximately 1,875 times. Compared with the prior PFCG model, the updated posterior model shows an improved agreement with the test data, i.e., the maximum difference in fatigue strength between model prediction and test data decreases from 12% to 3%. Based on the posterior PFCG model, the P-S-N curve of RD joints is statistically derived using sufficient numerical samples.

Influence of high initial porosity introduced by laser powder bed fusion on the fatigue strength of Inconel 718 after post-processing with hot isostatic pressing

Laser-based additive manufacturing (AM) of metallic materials can produce AM parts of high quality. The obtained density of metal parts manufactured by laser powder bed fusion (LPBF) is mostly greater than 99% and high mechanical strengths can be achieved; however, a long processing time is typically required to reach a high quality. Possibilities to shorten the LPBF processing time involve increasing either the scanning speed or the hatch distance, but both will result in greater porosity. In general, the porosity of AM parts can be reduced by subsequent post-densification of the materials by hot isostatic pressing (HIP). In this manner, not only can the LPBF process be accelerated but the fatigue strength of the parts can be increased by reliable reduction of porosity and the simultaneous homogenization of the microstructure. Nevertheless, argon porosity introduced by the LPBF process cannot be completely eliminated by HIP due to the insolubility of argon in metallic materials. In the present study, the influence of the initial porosity on the fatigue strength of IN718 specimens after HIP and different heat treatments were investigated with a view to understanding the impact of argon porosity. The fatigue strength under a rotating bending load was determined for four different test batches and one almost fully dense as-built state as a reference. For the test batches, two different initial porosities of 3.5% and 9.9% were introduced by increasing the hatch distance. These porous samples were subjected to the same HIP treatment but aged differently: half of the samples were annealed, quenched, and aged within the HIP vessel under pressure and the other half were annealed and quenched within the HIP vessel but aged in an ambient air atmosphere. HIP post-processing significantly improved the fatigue strength of IN718 samples produced by LPBF, even in the case of a high initial porosity and residual argon content. In the case of a 3.5% initial porosity, no difference was observed in fatigue behavior with respect to aging with or without pressure. In contrast, samples with an 9.9% initial porosity showed a large difference in fatigue strength according to different aging conditions.

The Effect of the Transformation of ε-Fe2-3N into γ′-Fe4N Phase on the Fatigue Strength of Gas-Nitrided Pure Iron

The effect of the iron nitride phases, ε-Fe2-3N and γ′-Fe4N, on the fatigue strength was investigated. Pure iron was used to observe only the effect of nitride, excluding the effects of factors, such as residual stress, depending on the alloy composition and microstructural change according to working on the fatigue strength. In this work, ε and γ′ phases were respectively grown at a time on the surface of the pure iron specimens using the appropriate nitriding potential KN, the mixture rates of ammonia and hydrogen gases, at same temperature of 570 °C according to the Fe-N Lehrer diagram. Another γ′ phase was prepared by first growing the ε phase and then transformed from ε phase into γ′ phase by changing the KN at the same temperature of 570 °C in the 2-stage gas nitriding. The fatigue strengths of the iron nitride consisted of ε and γ′ phases, γ′ phase, and γ′ phase grown by the 2-stage gas nitriding were evaluated, respectively. As a result, first, it can be seen that the diffusion layer of ε phase was deeper than γ′ phase, but fatigue strength was lower. On the other hand, fatigue strength of both the γ′ phases are higher than that of the ε, and the fatigue strength of γ′ phase nitride grown by 2-stage gas nitriding was almost similar to that of γ′ phase nitride grown at a time, i.e., fatigue strength was not significantly related to diffusion depth and depended on nitride phases in this study. Secondly, we cannot clearly conclude that there was the difference in fatigue strength according to the thickness of nitride layer consisted of γ′ phase. However, it is clear that when ε phase was transformed to γ′ phase, fatigue strength had the same level as γ′ phase formed at one time.

The effects of fiber ply orientation, laminate layer and PLA content on mechanical properties of carbon fiber reinforced bioplastic composites

In this study, two kinds of bioplastic materials, where the first consists of 10% PLA, corn starch of 80% and CaCO3 10%, and the second consists of 45% PLA content, corn starch of 45% and CaCO3 10%, were used. The composites were also reinforced by the carbon fibers, which were prepared with one and two layers of carbon fiber and then ply orientations of [0[Formula: see text]] and [45[Formula: see text]]. The maximum tensile strength was observed for PLA 45% with a [0[Formula: see text]] ply orientation of two layers of carbon fiber. For composite with two layers of carbon fiber, the tensile strength showed higher for the [0[Formula: see text]] ply orientation than for the [45[Formula: see text]] ply orientation. The fatigue strength strongly depends on the orientation of carbon fiber, but in the long fatigue life range, the difference of fatigue strength between the fiber ply orientations reduces.

Bonding Strength and Grain Size Control of Dissimilar Joints of Al-Mg Alloys with Forge Welding

High-productivity dissimilar bonding between A2024 aluminum (Al) alloy and AZ80 magnesium (Mg) alloy with a pure titanium (Ti) interlayer was achieved using forge welding with an alternating current (AC) servo press. This process involved diffusion bonding with plastic deformation in air. The tensile and fatigue strengths were compared between specimens prepared in the conventional way, which is considered the standard for bonding quality, and under the new rough condition. The tensile strengths were almost identical, but the fatigue strength with increasing number of cycles of specimens prepared under the rough condition was significantly lower than that of specimens prepared under the standard condition. Elemental analyses of cross-sections and the bonding interface revealed intermetallic compound (IMC) formation due to rupture of the Ti sheet. The difference in fatigue strength was attributed to a nonlinear effect on fatigue crack growth rate. Furthermore, in view of controlling grain size for practical use, the dynamic recrystallization behavior of the Mg alloy during this bonding process was investigated. The relationship between the grain size and Vickers hardness was in good agreement with the Hall–Petch relationship, and the Zener–Holloman parameter (Z parameter) explained the dynamic recrystallization process.

Microstructure-dependent fatigue behavior of aged Cu-6Ni-1.5Si alloy with discontinuous/cellular precipitates

The effect of microstructure on the fatigue strength of age-hardened Cu–6Ni–1.5Si alloy was investigated in this study. The aging was conducted at 500 °C for 0.5 h for the precipitation of disk-shaped δ-Ni2Si precipitates (continuous precipitates) and 3 h for the discontinuous precipitation of fiber-shaped, stable δ-Ni2Si precipitates to strengthen the Cu matrix. The tensile strength of 0.5 h-aged specimens was about 1.3 times greater than that of the 3 h-aged counterparts, but with inferior electrical conductivity. Despite the considerable difference in tensile strength, no notable difference in fatigue strength was observed between 0.5 h- and 3 h-aged specimens. The 3 h-aged specimens had superior electrical conductivity without sacrificing fatigue strength of the 0.5 h-aged specimens. The physical background of high-cycle fatigue strength in Cu–6Ni–1.5Si alloys with continuous precipitates and discontinuous precipitates is discussed considering the microstructure-sensitive behavior of fatigue cracks.

Fatigue Bending Strength of Jones Fracture Specific Screw Fixation.

Intramedullary screw fixation is a common method of treating proximal metadiaphyseal fifth metatarsal (ie, Jones) fractures. Fatigue failure is a complication of this fixation. There are many screw designs available, including Jones fracture specific fixation, but the optimal choice of screw design is unknown. The purpose of this study was to compare the fatigue strength of Jones fracture specific screw designs as well as other commonly used screw designs. Our hypothesis was that there would be no difference in fatigue strength for Jones fracture specific screw designs at similar screw diameters. A study was performed to determine the fatigue bending strength of 5 different screw designs including Jones fracture specific screw designs at 3 different screw diameters. Six screws of each size and design underwent cyclic fatigue testing, and a median fatigue limit (MFL) was determined for each screw design and size. The Stryker Asnis JFX solid 4.0-mm, 5.0-mm, and 6.0-mm screws had a higher MFL than all other screws with similar diameter tested (all P < .0001). Both Jones fracture specific screw designs (Stryker Asnis JFX solid screws and Charlotte Carolina Jones screws) had higher MFLs than the other screw designs tested. This study provides comparative fatigue strength data on larger screw diameters, which have not been previously reported. There was a statistically significant difference in screw fatigue properties at the screw diameters tested. The clinical significance of this study is that it provides surgeons with fatigue strength data to aid in screw selection for Jones fracture fixation.

Experimental study of transverse attachment joints with 40 and 60 mm thick main plates, improved by high-frequency mechanical impact treatment (HFMI)

In recent years, high-frequency mechanical impact (HFMI) treatment has grown in popularity due to its efficiency in improving the fatigue strength of welded joints. The fatigue performance of HFMI-treated welded steel joints has, however, not been thoroughly studied for plate thicknesses above 30 mm. In this study, 40 and 60 mm thick main plates with non-load-carrying transverse attachments have been fatigue tested under constant amplitude four-point bending, both in as-welded and HFMI-treated condition to investigate the fatigue performance for large plate thicknesses, typical for weldments in bridges. Axial fatigue strengths were estimated by a modification of the experimental results with fracture mechanics calculations. The main conclusions are that HFMI treatment can result in significant fatigue strength improvement even for large main plate thicknesses and that the difference in fatigue strength between bending and axial loading is negligible for the specimen geometries used in this study.

The effects of heat treatment on very high cycle fatigue behavior in hot-rolled WE43 magnesium

The role of crack initiation and short crack growth on fatigue life in the very high cycle fatigue regime (VHCF) is investigated for three heat treatments of the wrought magnesium alloy WE43. As-received (T5) WE43 with a relatively fine grain size was solution treated and aged to produce precipitation strengthened coarse-grained microstructures in the underaged and peak-aged (T6) conditions. Ultrasonic axial fatigue tests with a cyclic frequency of 20kHz were conducted using smooth specimens. Heat treatment was shown to have a strong effect on fatigue strength, with the fine-grained, strain-hardened T5 condition exhibiting much higher values than the coarse-grained conditions. No significant difference in fatigue strength was observed between the underaged and peak-aged microstructures of equivalent grain size. Crack initiation and short crack growth behaviors in each condition were investigated to determine if one of these behaviors dominated the VHCF lives. It was found that average short crack growth rates for the three conditions were similar and had no clear dependence on microstructural condition. Crack initiation was shown to occur through cyclic slip deformation in particularly large and favorably oriented grains in each condition. Subsurface crack initiation was observed at low stresses and high lifetimes in the coarse-grained conditions, but not in the fine grained T5 condition. Crack growth rates in vacuum were investigated using a unique combination of ultrasonic fatigue instrumentation and scanning electron microscopy (UFSEM) in order to simulate subsurface crack propagation. Environment was shown to have a significant effect on crack growth rate, with rates in vacuum nearly two orders of magnitude lower than in laboratory air.

The multiaxial weld root fatigue of butt welded joints subjected to uniaxial loading

AbstractIn this study, the fatigue strength of inclined butt welds subjected to a proportional multiaxial stress state generated by uniaxial loading is studied in nominal and local stress concepts. The local methodologies studied included principal stress hypothesis, von Mises stress hypothesis and modified Wöhler curve method. Nominal methodologies included modified Gough–Pollard interaction equation, the design equation in Eurocode3 and the interaction equation in DNV standard. Results are evaluated along with data published in relevant literature. It is observed that both local and nominal stress assessment methods are able to estimate multiaxial fatigue strength. No obvious difference in fatigue strength is observed in the nominal stress concept, but the notch stress concept is able to capture a decrease in fatigue strength in shear‐dominated joints. It is concluded that modified Wöhler curve method is a suitable tool for the evaluation of fatigue strength in joints dominated by both normal and shear stresses.

Fatigue Resistance of C30 Structural Steel

In this paper the authors present and discuss results of both fatigue life tests and fatigue crack growth rate tests performed for C30 steel which is used for railway axle manufacturing. The obtained results demonstrate the differences in fatigue strength and fatigue crack growth resistance in two different parts of one railway axle which are subjected to different levels of plastic deformation, namely the axle bolster body and axle body.

G0301505 冷間工具鋼の回転曲げ疲労強度に及ぼす一次炭化物の影響

SKD11 contains many coarse carbides to improve wear resistance and hardness. However fatigue strength of SKD11 decreased due to coarse carbides. Therefore new SKD11 was developed for improving adhesion to surface coating and less primary carbide than SKD11. Rotary bending fatigue test in air at room temperature using hourglass shaped specimen is conducted to clarify S-N property and effect of the carbide on fatigue strength of cold work tool steels. From the experimental results of fatigue test, it was found that the fatigue strength of improved showed high values in the range from 10^4 to 10^7 cycles than that of JIS SKD11. Fatigue strength was increased 220〜240 MPa in low cycle and 250〜260 MPa in high cycle, significant difference in fatigue strength was observed in high cycle. From the results of fracture surface observation by SEM and EDX, it was found that fracture origin of JIS SKD11 was internal coarse carbide. Fracture origin of new SKD11 was surface.

Corrosion Fatigue Behaviour of a Common AZ91D Magnesium Alloy in Modified Simulated Body Fluid

Use of Magnesium alloys as body implants are breaking into a new paradigm of biomedical engineering as they are biocompatible, biodegradable and have mechanical properties close to that of bone. Even though corrosion fatigue (CF) and stress corrosion cracking (SCC) failures are among the most common concerns for metallic implants, CF behaviour of magnesium alloys in physiological environments has received little attention. This article reports the CF results of a common cast magnesium alloy (AZ91D) in modified simulated body fluid (m-SBF). Results showed that there was a remarkable difference in fatigue strength of Mg alloys when tests were performed in m-SBF.

Reconfirmation and new discussions on frequency effect of fatigue property of materials based on numerous published data

Ultrasonic fatigue testing is a convenient method to examine the S-N property in long life region, within a definite period. However, a distinct gap of the loading stressing frequency between conventional and ultrasonic testing methods (100–1,000 times larger) urges us to study a potential effect of the loading frequency. The present paper deals with a comparison of results obtained from conventional and ultrasonic tests, in the high cycle fatigue region, from two viewpoints. Firstly, a short direct comparison of fatigue data from conventional and ultrasonic tests, with similar conditions, has been undertaken. Secondly, overview analyses based on a far larger amount of data have been carried out for structural steels and aluminium alloys. Results obtained from this work give us some idea of the extent of differences between fatigue test results obtained at usual and ultrasonic frequencies in the fatigue regime of N f < 10 7 cycles. Particularly, the second study gives a potential 18% difference of fatigue strength for steels, whereas it reveals only a slight difference of fatigue behaviours for aluminium alloys.

Fatigue Strength of Al Cast Alloy in Bio-Diesel Fuel

In order to investigate the effect of bio-diesel fuel (BDF) on fatigue strength of a squeeze cast Al alloy, AC4CH-T6, rotating bending fatigue tests were carried out using plain specimens in BDF made by rapeseed oil. The results were compared with those in turbine oil and in ambient air. Fatigue life was longer in turbine oil and shorter in BDF than that in ambient air. Fatigue life in each environment tested was mainly occupied by the growth life of a crack smaller than 1mm, meaning that the difference in fatigue strength affected by environment was mainly yielded in the growth process of a small crack. The crack growth rates were suppressed in turbine oil and accelerated in BDF in comparison with those in air. BDF contained much water dissolved from air in the early stage of fatigue process. The suppression of crack growth was caused by wedging effect of oil and the acceleration was caused by corrosion due to dissolved water in BDF, which was larger than the suppression due to the wedging effect.