Fatigue Tests Of Specimens Research Articles

Miniature CoCr laser welds under cyclic shear: Fatigue evolution and crack growth

Miniature laser welds with the root depth in the range of 50–300 μm represent air-tight joints between the components in medical devices, such as those in implants, growth rods, stents and various prostheses. The current work focuses on the development of a fatigue test specimen and procedure to determine fatigue lives of shear-loaded laser welds. A cobalt-chromium (CoCr) alloy is used as a benchmark case. S–N graphs, damage process, and fracture surfaces are studied by applying x-ray analysis, atomic force microscopy, and scanning electron microscopy both before and after the crack onset. A non-linear material model is fitted for the CoCr alloy to run finite element simulations of the damage and deformation. As a result, two tensile-loaded specimen designs are established and the performance is compared to that of a traditional torque-loaded specimen. The new generation specimens show less variation in the determined fatigue lives due to well-defined crack onset point and, therefore, precise weld seam load during the experiments. The fatigue damage concentrates to the welded material and the entire weld experiences fatigue prior to the final, fracture-governed failure phase. For the studied weld seams of hardened CoCr, a regression fatigue limit of 10.8–11.8 MPa, where the stress refers to the arithmetic average shear stress computed along the region dominated by shear loading, is determined.

Tensile-Fatigue Behavior of Sintered Copper Die-Attach Material

The tensile-fatigue behavior of sintered-copper die-attach materials was investigated by focusing on sintering-pressure dependence. Specimens for fatigue tests were prepared by sintering copper oxide paste made from copper oxide particles with a mean diameter of 200 nm in hydrogen atmosphere under low (4 MPa) and high (13 MPa) pressures at 623 K for 15 min. The tensile-fatigue test was conducted using micro-load testing equipment in air at room temperature. The fatigue tests were conducted under pulsating load conditions. The fatigue strength of the high-pressure specimens was higher than that of the low-pressure specimens when comparing them within the nominal stress range $\Delta \sigma $ . The fracture at the narrow neck part and the dimpled fracture at the peripheral part of the voids were observed in both types of specimens from fractographic studies. The $\Delta \sigma $ was converted to the total strain range $\Delta \varepsilon _{t}$ by using finite element analysis (FEA). It was confirmed that the fatigue strength of the high-pressure specimens was higher than that of the low-pressure specimens even when comparing them within $\Delta \varepsilon _{t}$ . The strain distributions at the neck part and peripheral voids were calculated through FEA using a microporous structure model to clarify the cause of the fatigue lifetime difference between the low-pressure and high-pressure specimens. The calculated strain in the low-pressure specimens was locally concentrated at some of the narrow necks and could be linked to perpendicular cracks with a high strain level, whereas that in the high-pressure specimens was broadly distributed between the voids with a low strain level. This difference in strain distribution seemed to be the cause of the difference in the fatigue lifetime between the low-pressure and high-pressure specimens.

Role of microstructure on the actuation fatigue performance of Ni-Rich NiTiHf high temperature shape memory alloys

The focus of the present study was to systematically investigate the influence of microstructure on the actuation fatigue performance of a Ni-rich NiTiHf high temperature shape memory alloy (HTSMA). Different aging heat treatments led to the formation of H-phase nano-precipitates with different sizes and morphology within the matrix, enhancing thermo-mechanical stability and enabling control of transformation temperatures. The actuation fatigue testing of specimens was performed until failure through thermally-induced reversible martensitic transformation under a constant stress (300 MPa) with two distinct upper cycle temperatures (UCT) of 300 °C and 350 °C. Consequently, the specimens heated to 700 °C and furnace cooled to 100 °C in 48 h, with relatively large precipitates, failed at average fatigue life of 15,500 cycles and exhibited 1.0% average actuation strain in the 300 °C UCT experiments, while those aged at 550 °C for 3 h, with the precipitate sizes less than 20 nm, attained an average fatigue life of 10,800 cycles and an actuation strain of 2.5%. Samples with intermediate precipitate sizes after aging at 600 °C for 10 h failed at the shortest average fatigue life of 8,200 cycles with an intermediate average actuation strain of 2.0%. Furthermore, increase in UCT decreased the fatigue life and resulted in larger average actuation strains for all samples. Overall, the current findings constitute the first systematic results demonstrating the microstructure dependence of the evolution of actuation strain, irrecoverable strain, and transformation temperatures during actuation fatigue, and actuation fatigue life of the Ni-rich Ni50.3Ti29.7Hf20 HTSMA, demonstrating the importance of controlling the H-phase precipitate size.

Accumulation and evolution of elastically induced defects under cyclic loading: Quantification and subsequent properties

This article quantifies the evolution of bulk material defects in α-iron during cyclic elastic loading and investigates its subsequent effect on quasi-static and dynamic mechanical response. Fatigue test specimens were cycled at a constant stress amplitude, 70% of the yield stress, to cause microscopic defects throughout the material due to a macroscopic elastic load. Afterwards, wire-cut electrical discharge machining was used to obtain multiple sub-tensile specimens from within the gauge section of the fatigued material. X-ray computed tomography of the sub-tensile specimens was used to quantify distribution and location of voids that were induced at different stages of fatigue life. Then, tension tests were performed on the sub-tensile specimens at low (10−3 s−1) and high (103 s−1) strain rates to study changes in mechanical properties. Yield stress and ultimate tensile stress were found to change under both quasi-static and dynamic strain rates based on the amount of prior cyclic loading. Changes in mechanical behavior were correlated to the characteristics of observed defects that exhibited distinct grouping at a certain point during the fatigue life. Mechanical properties of the sub-tensile specimens after fatigue loading displayed different trends depending on the strain rate. Low strain rate deformation resulted in decreases of approximately 20% in the yield stress, ultimate tensile stress, and ductility due to increased fatigue loading. Conversely, during high strain rate deformation some samples with more fatigue loading displayed higher maximum flow stresses. Results demonstrate the significance of bulk material changes which occur at mechanical loads below the yield stress. The systematic experimental procedures developed here for investigating the accumulation, evolution, and effect on subsequent properties due to elastically induced microscopic defects could potentially be applied to a wide range of material systems.

Effect mechanism and equivalent model of surface roughness on fatigue behavior of nickel-based single crystal superalloy

The global objective of this study was to systematically research the rule and mechanism how surface roughness affects the fatigue property of nickel-based single crystal superalloy. Fatigue tests of specimen with various levels of surface roughness were conducted at 980 °C and measured surface topography models were used to perform FE analysis based on crystal plasticity theory and fatigue damage model considering the resolved shear stress and slip rate of slip systems. Eventually, an geometrical equivalent simplification model with surface roughness parameters Rz, Rsm and Ra as modeling parameters for predicting fatigue life more convenient is proposed. Results indicate that micro-defects caused by surface roughness generate stress, plastic strain and damage concentration under fatigue load, which generate fatigue crack initiation, and greater surface roughness produces more severe concentration. The error between simulation prediction life and test result is within 3 times error band.

Effect of optimal shot peening time on fatigue life for aluminum alloy 6061-T651

The compressive residual stresses generated by shot peening, is increased in a direct proportional way with shot peening time (SPT). For each metal, there is an optimum shot peening time (O.S.T) which gives the optimum fatigue life. This paper experimentally studied to optimize shot peening time of aluminium alloy 6061-T651 as well as using of and analysis of variance (ANOVA).
 Two types of fatigue test specimens’ configuration were used, one without notch (smooth) and the other with a notch radius (1,25mm), each type was shot peened at different time. The (O.S.T) was experimentally estimated to be 8 minutes reaching the surface stresses at maximum peak of -184.94 MPa.
 A response surface methodology (RSM) is presented to optimize the surface properties of fatigue life due to effect of two parameters (shot peening time and fatigue stresses). A statistical software was used to perform analysis of variance (ANOVA) to predict the optimum shot peening time. The results were 8.007 minutes without notch and 7.53 minutes with notch. Two 2nd degree polynomials were obtained for the two studied cases with confidence level of 95%. Experimentally26.67% enhancement in fatigue life was obtained, but after using (RSM), the results gave 61% improvement, compared with ref. [2].

Analysis of fatigue notch effect due to axial misalignment for ultra high-strength steel butt joints

The majority of fatigue tests of welded specimens is based on shop-made samples generally exhibiting minor misalignment. Due to the challenge of ensuring misalignment-free joints in industrial manufacturing processes, investigations focussing on the effect of misalignment on the fatigue strength are important. Therefore, this paper deals with the influence of axial misalignment on the fatigue resistance of butt-welded ultra high-strength steel specimen. In addition, the effect of high frequency mechanical impact treatment (HFMI) on the fatigue performance is researched. In the course of the experimental investigations, specimens exhibiting three different levels of axial misalignment are manufactured. Fatigue tests at a stress ratio of R = 0.1 in as-welded condition reveal a significant drop in fatigue strength with increasing axial misalignment. Fatigue assessments of the as-welded test results based on nominal, structural and effective notch stress approach are performed taking into account the sample-dependent misalignment factor. The given equations enable an improved consideration of axial misalignment regarding to fatigue strength. The HFMI treatment increases the fatigue strength compared with the as-welded state; the detrimental effect of misalignment is less pronounced. A comparison to the current IIW guideline for HFMI treatment reveals a conservative assessment if IIW-recommended FAT values for as-welded condition are applied.

Static and Fatigue Tests of Steel-Precast Composite Beam Specimens with Through-Bolt Shear Connectors

Static and Fatigue Tests of Steel-Precast Composite Beam Specimens with Through-Bolt Shear Connectors

In-plane anisotropy in deformation micro-mechanism of commercially pure titanium during monotonic tension and cyclic loading

In the present investigation in-plane anisotropy in tensile and ratcheting behavior of cold rolled and annealed commercially pure titanium plate has been studied. Flat tensile and fatigue test specimen oriented at 0, 45, and 90 degree to the rolling direction in the rolling direction–transverse direction (RD–TD) plane of the plate has been machined out. Specimens with loading axis at 0, 45 and 90 degree to RD have been designated as 0T, 45T and 90T for tensile and 0R, 45R and 90R for fatigue. Owing to initial TD split basal texture of as received plate, 0T sample has crystallographic direction   aligned with loading axis. It shows lowest yield strength but highest ductility in monotonic tension. Although ultimate tensile strength (UTS) and strain to failure of samples 45T and 90T are similar, the former has significantly lower yield strength than the latter, indicating different strain-hardening behavior due to different slip/twin activity. On the other hand, 0R sample exhibits longer ratcheting life while 90R sample accumulates highest ratcheting strain. This is attributed to higher propensity for twinning with the formation of intersecting multiple variant twins during cyclic deformation of 0R sample. Viscoplastic self-consistent (VPSC) simulations of one-cycle tension-compression-reload tension indicate that there is alternate activity of pyramidal <c+a> slip and extension twinning during cyclic loading. This induces cross slip activity and detwinning of extension twins in 90R which causes rapid accumulation of strain leading to early failure.

Study on Initiation and Growth of Fatigue Crack of Pre-corroded 7B04 Aluminium Alloy Specimen

The purpose is to study the fatigue crack initiation and propagation behavior of aluminum alloy under pre corrosive damage. The high strength aluminum alloy 7B04-T74 with main bearing structure of aircraft was used to prepare single edge notched specimens for pre corrosion and fatigue tests, and the effects of corrosion damage on fatigue crack initiation and propagation are analyzed. It is found that the fatigue crack sprout at the bottom of a single corrosion pit at a low corrosion period, and the length of the crack is equivalent to the width of the corrosion pit, about 0.047mm, and the crack propagates in a straight line. At high corrosion stage, cracks occur at multiple corrosion pits, and the crack propagation paths become tortuous, resulting in glyph cracks.

The Effect of Plasma Nitriding on the Fatigue Behavior of the Ti-6Al-4V Alloy

The Ti-6Al-4V alloy is widely used in the manufacture of components that must have low density and high corrosion resistance and fatigue strength. The fatigue strength can be improved by surface modification. The aim of this study was to determine the influence of plasma nitriding on the fatigue behavior of a Ti-6Al-4V alloy with a lamellar microstructure (Widmanstätten type). Nitriding was executed at 720 °C for 4 h in an atmosphere with N2, Ar, and H2. Microstructure characterization of the samples was carried out by X-ray diffraction analysis, optical microscopy, and scanning electron microscopy. The average roughness of the specimens was determined, and fatigue tests were executed in a bending–rotating machine with reverse tension cycles (R = −1). X-ray diffraction analysis of the nitrided alloy revealed the following matrix phases: α, β, ε-Ti2N, and δ-TiN. A nitrogen diffusion layer was formed between the substrate and the titanium nitrides. Plasma nitriding resulted in an increase in low-cycle fatigue strength, whereas at high cycles of 200 MPa, both conditions exhibited similar behaviors. The fracture surface of the fatigue-tested specimens clearly revealed the lamellar microstructure. The fracture mechanism in the non-nitrided specimens appears to be due to cracking at the interface of the α and β phases of the lamellar microstructure.

Large versus Small Grain Sizes on Fatigue Life of Aluminum Aircraft Wheels

The high cycle fatigue behavior of an aluminum alloy of small and large grains was investigated. Samples of small and large grains were provided as rotating bending fatigue test specimens. The samples were tested at eight different stress levels from 103 MPa (15 ksi) to 345 MPa (50 ksi) with a 34 MPa (5 ksi) increase at each level. As-received samples were evaluated for optical microstructural analysis. Scanning electron microscopy (SEM) images were used to evaluate the fractured surface of several samples. High cycle fatigue S-N curves were generated. The fatigue test results showed that grain size has a strong influence on the fatigue life. The high cycle fatigue S-N curves of small grains showed better fatigue life as compared to large grains in the lower stress levels. At higher stresses, the situation was reversed.

Physically short and long-crack growth behavior of MIG welded Al-5.8%Mg alloy

In this study, we investigated the physically short and long-crack growth behavior of Al-5.8%Mg alloy, a special hardened alloy used for welding of Al-5083 plates. Microstructural analysis on both longitudinal and cross-sectional plane of welded plates showed no crystallographic texture with nearly equi-axed grains of size 85 μm and gas porosities. Fatigue tests were done at 50 Hz using CT-specimens at different R-ratios (0.1,0.5 and 0.8) to characterize the long-crack growth behavior. Compliance method was used for crack-closure measurements. An increase in crack growth rates and reduction in threshold ΔK was observed with increasing R-ratio. This R-ratio effect could be rationalized after compensating for crack-closure. Special specimens for short-crack growth fatigue tests were cut from pre-cracked CT-specimens. Constant load-range fatigue tests at 50 Hz were conducted at three different R-ratios (-1,0.1 and 0.5) to characterize the effect of mean-stress on short-crack growth behavior. It appeared that the physically short-cracks initially propagated in closure-free environment and then deviated towards their long-crack growth rates. The microstructural effects on short-cracks were more pronounced at lower mean-stresses or R-ratios. The physically short-crack growth effect could not be removed completely using the crack-closure concept alone, suggesting the role of other growth-retarding mechanisms. A modified empirical model including the effect of other possible growth-retarding mechanisms was proposed to capture the behavior of physically short-cracks in this material.

Microstructure and fatigue performance of SLM-fabricated Ti6Al4V alloy after different stress-relief heat treatments

Abstract The main interest in Additive Manufacturing (AM) technology relates to its ability to produce complex components with relatively reduced weight that are difficult to produce or cannot be produced by other conventional technologies. Selective laser melting (SLM) is extensively used, as one of the AM technologies to fabricate metallic parts. This advanced method allows to produce various parts with complex geometries with high three-dimensional (3D) accuracy from fusion powders in a layer-by-layer style. Ti6Al4V alloy is a widely used material for structural applications in aerospace and biomedical due to high specific fatigue strength. SLM processing makes this alloy attractive when weight reduction is a design objective. The SLM Ti6Al4V microstructure is influenced by process parameters and build orientation. The localized high energy input during very short interaction times leads to the formation of very fine structures and to the generation of internal stresses. Therefore, the SLM parts are heat treated to decrease or completely remove residual stresses. The present study aims at evaluating the effect of stress-relief heat treatments on the microstructure, the mechanical properties and the fatigue performance of SLM Ti6Al4V alloy. Ti6Al4V alloy specimens were manufactured according to the SLM process with an EOS M290 system. Post fabrications heat treatments at different temperatures (i.e. 740˚C vs. 900˚C) resulted in different structure and mechanical properties that were identified and measured. Fatigue testing of specimens with as-built surfaces was performed at room temperature on modified Schenk-type fatigue testing machine applying a pulsating plane bending (load cycle ratio R = 0) to the specimens at a frequency f = 15 Hz.

Fatigue behaviour of carbon/epoxy Non-Crimp Fabric composites for automotive applications

Facing the stringent requirements for fuel economy in automotive industry, composite materials are seen to be utilized for structural lightweighting. This paper focuses on the fatigue test development and fatigue behavior of a biaxial [0/90] carbon/epoxy non-crimp fabric composites for automotive applications. A new fatigue test specimen was successfully developed for fatigue testing under fully reversed loading, i.e. stress ratio R=-1, at room temperature and 130 ℃. The developed test specimen prevents buckling during testing without use of an anti-buckling fixture. 3D Digital image correlation strain mapping also revealed a uniform strain distribution on the surface of this specimen geometry. Fatigue test results indicate that the fatigue behavior at both room temperature and 130 ℃ is matrix dominated. The fatigue data is a good fit to the Basquin equation σa=σf(2N)b.

Dynamic response and fatigue life of Vacuum cast Polyurethane polymer material

The objective of this paper is to measure the dynamic response transmissibility and fatigue life of vacuum cast Polyurethane (PU) polymer material used in the construction of automotive lamps. Polymers used in automotive lamps, if mass produced, are injection moulded. For ultra-low volume production, however, the cost of tooling for these traditional manufacturing methods is often very high. Hence, alternative approaches such as Silicone Tool – Vacuum (STV) casting are used. The parts produced using alternative approaches may have inferior dynamic performance. The specimens for dynamic response and fatigue testing were cut-out from a vacuum cast polyurethane plate. An instrumented beam was mounted on a shaker table and using sine sweep base excitation the dynamic response was measured. Using measured acceleration, response transmissibility was calculated. The bending fatigue properties were measured on a vibration shaker using a 4-point bending based resonance test apparatus. A series of tests were carried out to obtain average performance parameters. The dynamic response shows nonlinearity, as expected of polymers, and a large variation in dynamic properties is observed. The obtained fatigue life curve shows significant scatter at 40% fatigue load level. The assessed fracture surface micrograph of the fatigued specimens at three load levels shows surface tear at 80% and 60% load levels. Overall, the Vacuum cast polyurethane shows less fatigue performance than the conventional polymers of automotive lamps.

Fatigue Resistance of Laminated and Non-laminated TRIP-maraging Steels: Crack Roughness vs Tensile Strength

Pre-cold-rolling before annealing in transformation-induced plasticity (TRIP) maraging steels changes the microstructural morphology from lamellar to granular, increasing the tensile strength. However, fatigue tests of plain specimens at ambient temperature and the stress ratio − 1 show that increased strength does not improve the fatigue strength compared to laminated TRIP-maraging steel, irrespective of stress amplitude. The disappearance of fatigue crack roughness suppresses the improvement in fatigue resistance with increasing strength.

Influence of the gas hydrates on the corrosion rate of gas gathering pipelines

The regularities of corrosion degradation of the low carbon ferrite pearlite St 20 (0.2C) steel of gas gathering pipeline under the mutual effect of corrosion environment, mechanical stresses and hydrate formation were determined. A change in corrosion mechanism in chloride media at increasing chloride concentration from 2.5 mol/L to higher concentration was found. The revealed more significant influence of mechanical stress on localized corrosion rate than on uniform corrosion one was associated with the formation of local galvanic elements on the metal surface and intensification of the metal dissolution in the tensile zone due to the weakening of interatomic interaction caused by increase the distance between grains. Fatigue tests of the pipe steel specimens after exposure to gas hydrate showed a significant decrease in durability (by 15 – 25% depending on test environment), the reason of which was obviously associated with increasing of corrosion damaging of the specimen surface as a result of the aggressive action of gas hydrates.

The Influence of Temperature and Stress Cycle Asymmetry at Fatigue Cracks Initiation and Propagation in Cylindrical Specimens of Ni-based Alloy EP741NP

Using the scanning electron microscope, information on the initiation and kinetics of the fatigue cracks development in the powder Ni-based superalloy EP741NP was obtained. This alloy is used for the manufacture of GTE disks. The low-cycle fatigue test of cylindrical specimens was carried out in the wide range of temperatures from 20 °C to 750 °C under strain control (fixed strain range) and different asymmetry ratios. The fractographic investigation of broken specimens made it possible to determine the kinetics of the fatigue crack growth and durability, which also made it possible to determine the period of crack initiation. After the fractographic analysis, the mathematical modeling was carried out using the ANSYS software, which made it possible to determine the Paris parameters which are responsible for the growth of fatigue crack in this material. The comparison of obtained parameters with the Paris factors derived with standard procedure during the tests of CT specimens revealed the mismatch at high temperatures. The obtained information will enable to predict the durability of parts manufactured of this material.

Fatigue life assessment of a tack welded high-strength wire mesh for reinforcement of precast concrete bridge girders

The research provides experimental data and assesses the effect of tack welding on the fatigue life of a high-strength electro-welded wire reinforcement (WWR) mesh used as reinforcement of precast concrete girders for bridge construction. The experiments were performed on samples obtained from one girder dismantled when ready for service. Tack welding is a controversial construction technique that entails structural risks for the WWR meshes consisting of substantial loss of strength, local embrittlement and fatigue life shortening. Hence, fatigue tests of specimens containing two types of cruciform joints, respectively obtained by electro-welding and by tack welding, were carried out and interrupted in case of surpassing the endurance limit. The experiments were followed by tensile tests of the survival specimens in order to detect the damage effects from the tensile failure of the main mesh reinforcement. Micro fractographic analysis revealed the physical mechanisms of fatigue damage induced by tack welding. Fatigue testing and fractography showed that adequate designed and executed tack welds can bear fatigue loads higher than the fatigue resistance of WWR specified by technical codes, since the type of damage that might be generated does not affect the fatigue life nor the tensile loading capacity of the WWR.