Decrease In Fatigue Life Research Articles

Chloride Diffusivity, Fatigue Life, and Service Life Analysis of RC Beams under Chloride Exposure

This study investigates the combined actions of flexural fatigue loading and chloride exposure by an experimental examination of the initial fatigue loading cycles (0.2, 0.4, 0.8, and 1.2 million), chloride exposure duration (0, 3, and 6 months), and modeling estimating service life. The results show the following: (1) at the initial fatigue loading cycles of 0.4, 0.8, and 1.2 million, (a) the chloride diffusivity of tensile concrete is similar, which are greater than the specimens initially fatigue loaded with 0.2 million cycles; (b) the chloride diffusivity of compressive concrete increases as the initial fatigue loading cycles increase; and (c) specimens present similar values of fatigue life and service life, which are shorter than the specimens initially fatigue loaded with 0.2 million cycles. (2) As the chloride exposure duration increases from 3 to 6 months, (a) chloride contents of tensile concrete increases more than that of compressive concrete, but both diffusion coefficients decrease; and (b) the fatigue life decreases by ~25%–75% and service life decreases by ~28%–55%. (3) There is an exponential correlation between the fatigue life and service life of beams.

Increasing fatigue life of gte compressor disks made of titanium alloys by discs treatment in fluidized abrasive layer

Purpose. Increasing fatigue life of GTE compressor disks made of titanium alloys by discs treatment in the fluidized abrasive bed.Investigation methods and equipment. The investigations were carried out using LPC stage I discs of ВТ3-1 alloy and HPC stages IV and V discs made of ВТ-9 titanium alloy of Д-36 aircraft engine.In the process of discs manufacture the following manufacturing methods were used: ultrasonic hardening and treatment in the fluidized abrasive bed.The ultrasonic hardening was carried out using Motor Sich serial installation and the fluidized abrasive bed treatment – using the АПС-600А installation.The discs were tested before and after the treatment on a special electrohydraulic test bench with a multi-axis loading system using a fixture reproducing radial and circumferential stresses at the base of an inter-slot projection.The testing was carried out at specimens temperature of 390 °С and 430 °С.For measuring the length of cracks and the time of their occurrence in the slots, the discs were inspected by ЛЮМ-1-ОВ method ant using the МПБ-2 microscope and КМ-8 cathetometer.Obtained results. It has been established that for LPC stage I discs made of ВТ3-1 alloy the increased fillet radius combined with subsequent treatment in the fluidized abrasive bed increases fatigue life of the discs by 6 times before crack occurrence, and increases durability of the disc with a crack by 5.5 times.Durability of a disc is within 41 % to 77.8 % of their total fatigue life .Treatment of the compressor dics made of ВТ-9 titanium alloy in the fluidized abrasive bed creates residual compression strain up to 300 MPa in the surface layer.Testing of the LPC stage 1 discs showed that frequency and shape of the discs effect both fatigue life prior to a crack occurrence and durability prior to discs destruction within the entire investigated loading frequency range of 0.01 Hz to 1.0 Hz.Testing of HPC stages IV and V discs of ВТ-9 titanium alloy showed that temperature variation within 390 °С to 430 °С did not contribute significantly to the fatigue life change. Introduction of endurance into the loading cycle has a considerable effect on the low-endurance fatigue of the compressor discs.Increase of the soak time up to 60 seconds under the same loading conditions decreased fatigue life of the full-scale specimens by 10 times.Scientific novelty. It was demonstrated that treatment of the compressor disks made of titanium alloys in the fluidized abrasive bed increases their cyclic life by 5 to 6 times in comparison with their treatment in accordance with serial technology.Practical effect. A process sheet was proposed and the modes of compressor discs treatment in the fluidized abrasive layer were developed that provide increase of the fatigue life by 5 to 6 times as compared with a serial technology.

3D laser shock peening – A new method for improving fatigue properties of selective laser melted parts

Although Selective Laser Melting (SLM) currently revolutionizes the way parts are being manufactured, certain process inherent limitations such as accumulation of residual stresses and increased porosity content can lead to a decrease in fatigue life of produced parts. 3D Laser Shock Peening is a new hybrid additive manufacturing process whereby a periodic Laser Shock Peening (LSP) treatment is added to the standard SLM process. LSP can be applied selectively in the 3D volume of metallic parts, and is shown to influence the bulk internal stress state and hardness, as well as the distribution of porosities in the material. Here, a decoupled 3D LSP process was used, i.e. LSP treatments were applied during the SLM process, by removing the sample from the SLM chamber whenever needed. We report fatigue lives of 316 L stainless steel multiplied by more than 15 times compared to standard SLM parts, and more than 57 times compared to conventional manufacturing, with a significant improvement over other state-of-the art methods. The effect of the 3D LSP treatment on SLM parts and the consequence on crack initiation and propagation is investigated by measuring the near surface residual stresses, imaging the fractured surface, measuring the porosity content and analyzing the microstructure and microhardness.

The Effects of Anodizing Process on the Corrosion rate and Fatigue Life of Aluminum Alloy 7075-T73

This research aims to study the effect of using the anodizing process on the corrosion rate, mechanical properties as well as the fatigue life for aluminum alloy (7075-T73), which is one of the most commonly used aluminum alloy in production of aircrafts, vehicles and ships structures. The anodizing process was employed through using sulfuric acid for time (20) min in a salty atmosphere. The mechanical properties and fatigue life of the AA7075-T73 were obtained before and after the anodizing process. All the results were listed in detailed tables and figures for comparison purpose. Generally, these results showed a decrease in corrosion rate by (155.06%) in comparison with untreated, an increase in hardness by (21.54%) and a slight decrease in fatigue life by (7.7%) due to anodizing for a time of 20 min at the stress level of (σa = 491.10 MPa). It was concluded that this technique could be applied on other aluminum alloys in order to know the magnitude of change in the mechanical characteristics and their fatigue life.

Experimental and numerical investigation of effect of size, position and geometry of some cutouts on fatigue life and crack growth path on AISI1045 steel plate

The main factor in determining path and rate in fatigue crack growth (FCG) problems is the stress field around the crack tip that is defined as stress intensity factor (SIF). Cutouts cause a stress concentration that depends on their size and geometry. This stress concentration can affect the stress field near the crack tip vicinity and deviates the FCG path. In this paper in order to study the effect of cutout parameters on FCG, first, a taguchi statistical analysis is used. Then, controlling and changing the FCG path and rate in specimens with different cutouts are studied. The results show that the size and position of cutouts are the most effective parameters on the crack growth path. According to the cutout’s features, an increase or decrease in fatigue life can be expected. An effective range for the distance between a cutout and a crack tip was determined. To verify the results of the extended finite element method (XFEM), the mode I SIF was obtained experimentally and compared with XFEM results. It was concluded that numerical results are in such a good agreement with experimental results.

An approach to wind-induced fatigue analysis of wind turbine tubular towers

This study discusses the wind-induced fatigue analysis of typical pitch-controlled 1.25MW wind turbine structures, with the reasonable consideration of the influence of low stress range fatigue damage, wind direction and the rotating effect of the blades. An integrated finite element model consisting of blades, nacelle, tower and foundation is established in ANSYS software. The wind speed and wind direction joint distribution function at the construction site of the wind turbine structures based on the observation datum of the meteorological station in China is put forward. The wind-induced response time history analyses of the tubular tower structures corresponding to different wind speed and wind direction cases are conducted. Finally, the wind-induced fatigue analysis theory of the tubular tower structures based on the time domain and time-frequency domain method are systematically established. The results indicate that the rotating effect of the blades should be considered in the fatigue life analysis of the tubular towers. Neglecting the influence of the wind direction and the low stress range on the fatigue damage of the tubular towers could lead to the decreased fatigue life which is conservative but not economic. Compared with the time domain method, the fatigue life of the tubular tower based on time-frequency domain method decreases, but the calculation is simpler. Adopting the equivalent stress range method considering rain flow correction to conduct the fatigue analysis of the tubular tower is recommended in this study.

Predicting Fatigue Life for Finite Line Contact under Starved Elastohydrodynamic Lubrication Condition

Surface contact fatigue is the main failure mode in many mechanical components, such as gears, bearings, and cam-followers. A fatigue life prediction model is proposed for finite line contact under starved thermal elastohydrodynamic lubrication (TEHL) condition in this paper. Then, the effects of inlet oil-supply thickness, slide-to-roll ratio (SRR), and operating conditions on the lubrication performance and fatigue life are investigated. The results show that the lubrication characteristics and fatigue life of finite line contact are obviously different from those of fully flooded situation by introducing the starved lubrication condition. For example, the severe starved conditions lead to a significant increase in friction coefficient and decreased fatigue life. The variation of SRR has an important influence on the fatigue life. With the increase of SRR, the fatigue life decreases firstly and then increases. The stress concentration occurs near the surface when speed is low. In addition, under the low-speed situation, rotation speed variation has little effect on the fatigue life.

Performance investigation and design optimization of micro scale compressed air axial turbine for domestic solar powered Brayton cycle

This research investigation aims to characterize the aerodynamic and structural performance of a micro scale axial turbine operated with the Brayton cycle, at various boundary conditions, by using the numerical integration finite element method and 3D computational fluid dynamics; the stresses on the rotor blade, in particular, were investigated. Firstly, the turbine was designed with a power output of 0.5–1 kW and efficiency of 81.3%. Then, together the turbine’s shaft and its blades were structurally investigated under a variety of loading conditions, with the purpose of visualising the effect of different geometrical and operational factors on the stress values, distributions and displacements over the rotor’s blades. After evaluating the structural stresses, rotor blade design changes to decrease these stresses will be proposed, to attain the best turbine performance, using multidisciplinary optimization; these will be reported in the next research publication from this study. The results showed that the maximum Von Mises and maximum principle stresses are highly influenced by the rotor stagger and trailing edge wedge angles, the turbine’s rotational speed, the inlet pressure and the working fluid inlet temperature. Additionally, the maximum allowable deformation was highly influenced by the rotational speed, around 16.5%. Moreover, the fatigue life was also determined and both the rotor stagger and trailing edge wedge angles significantly affected its value. The simulated increases in fluid temperature lead to a decrease in the rotor fatigue life of 38%, at an inlet pressure of 5 bar. Such a result can open the door for more research studies and investigations to develop this (domestic) system for ground based (i.e. sea level) uses.

Effect of freeze–thaw on fatigue damage characteristics of cement emulsified bitumen mastic

Cement emulsified bitumen mastic (simplified as CEBM) is a type of composite binder composed of cement, emulsified bitumen, water and various additives. Freeze–thaw cycles have an important influence on the fatigue properties of CEBM. In this paper, freeze–thaw test and fatigue test were carried out. Damage variable characterizing freeze–thaw effect and total damage variable characterizing freeze–thaw and fatigue coupling effect were established. They were based on the initial complex modulus and fatigue properties parameters of fatigue test. Effects of freeze–thaw cycles on fatigue properties and damage evolution process were analysed. This study demonstrates that, as freeze–thaw cycles increasing, fatigue life decreases, and the sensitivity of fatigue life with stress level is reduced. With the increase of freeze–thaw cycles, both freeze–thaw damage and total fatigue damage increase, and the proportion of freeze–thaw damage in total damage increases. Effect of freeze–thaw cycles on the fatigue properties of CEBM is enhanced. With the increase of bitumen content, freeze–thaw damage reduces and total damage increases. The increase of bitumen content can improve the frost resistance of CEBM and improve the sensitivity of fatigue properties to freeze–thaw cycles.

Chloride concentration distributions in fatigue damaged RC beams revealed by energy-dispersive X-ray spectroscopy

In this laboratory study, the chloride concentrations at the steel-concrete interfaces of fatigue damaged RC beams were investigated by energy-dispersive X-ray spectroscopy. The beam specimens were subjected to fatigue loadings followed by 3.5 wt% NaCl solution wet-dry cycles. For the two bars in one beam, the fatigue-fractured steel exhibited severe corrosion (30%–40% corrosion area) and high chloride concentrations (0.5 wt%–0.9 wt%) in the pure flexural segment. When the load level (ratio of maximum fatigue load to ultimate static strength) reached 0.45 or after 1.2 million cycles of fatigue loading, the fatigue damage affected the chloride concentrations of compressive concrete. The increase of load level from 0.4 to 0.45 caused at least 30% increase in the chloride concentrations of tensile concrete and 6.24 times decrease in the residual fatigue life, but the initial fatigue loading cycles barely affected either of them. Results of the chloride concentrations were consistent with those of the residual fatigue life test.

The effect of environment and fatigue loading on the behaviour of TEPs-modified adhesives

ABSTRACTIn this work, the effect of environment and fatigue loading on the mechanical behaviour of a structural adhesive modified with thermally expandable particles (TEPs) was investigated. Tensile tests were performed to get the tensile properties of the TEPs-modified adhesive as a function of moisture. It was found that the presence of moisture affects the mechanical properties of TEPs-modified adhesive. Large increases in ductility and reductions in tensile strength and Young´s modulus were measured for the water-saturated specimens, but after drying the tensile properties are recovered. Further, fatigue tests were performed on single lap TEPs-modified adhesive joints (0, 10 and 20 wt% TEPs) and a decrease in fatigue life was found for TEPs-modified adhesive joints compared to unmodified joints. Finally, a scanning electron microscope (SEM) analysis was performed in order to examine the fracture surfaces of the tensile specimens tested. SEM analysis showed that the absorbed moisture changes the fracture mechanisms and the morphology of the specimens.

Hysteresis energy based low cycle fatigue properties analysis in extruded Al-7Zn-2Mg-1.5Cu-0.2Sc-0.1Zr alloy at low temperature

The influences of low-temperature on low cycle fatigue (LCF) behaviors and fracture mechanisms of extruded Al-7Zn-2Mg-1.5Cu-0.2Sc-0.1Zr alloy with T6 state were investigated by strain-controlled LCF tests. The results show that the tensile strength increases, elongation and LCF life decreases with decreasing the temperature. The cyclic stress response behaviors of alloy are stable at various temperatures of room temperature (RT), 0 °C, −20 °C and −40 °C. And cyclic hysteresis energy densities are also stable after a short period of fluctuations at different conditions. In consideration of the different temperatures, the fatigue life evaluation model based on hysteresis energy is adopted. And the relationship between temperature and LCF life is derived from the parameters (Wo and β) of hysteresis energy model. The increase of hysteretic energy will lead to greater plastic damage, which is the main reason for the decrease of fatigue life, and the corresponding value of Wo significantly decreases. In addition, the fatigue crack initiates from the free surface of specimens and there exist a mass of fatigue striations in crack propagation region. The secondary cracks appear at low temperature, and the corresponding value of β slightly increases.

Effect of freeze-thaw cycling on fatigue behaviour in concrete

Freeze-thaw damage is a deterioration mechanism caused when saturated concrete is exposed to temperature variations that cycle above and below its freezing point. The expansion of water in the concrete pore space due to freezing can lead to internal pressures that induce cracking, thus expediting the deterioration process through freeze-thaw cycles. The purpose of this investigation was is to evaluate the effect of freeze-thaw deterioration on the tensile fatigue life of air-entrained concrete at early stages of deterioration. Concrete cylinders were prepared with 0.45 and 0.65 water to cement ratios (W/C), then cured for 28 days before being subjected to the freeze-thaw cycling. After 0, 25 and 50 freeze-thaw cycles, the pressure tension (PT) test was used to induce tensile fatigue loading cycles until failure. The PT was capable of determining the decrease in fatigue life, represented as the number of cycles to failure, of the concretes when subjected to pure cyclical tensile loading after having first been exposed to freezing and thawing cycles. The results indicated that even though the ultimate static tensile strength of the specimens did not vary significantly due to the freeze-thaw cycles, the residual fatigue properties were degraded. UPV monitoring was able to determine the increase in internal damage, as the UPV of the specimens decreased with continued freeze-thaw cycling. It was shown that freeze-thaw deterioration reduces the ability of concrete to withstand cyclic loading, even at early stages prior to a decrease in static tensile strength.

Ductility and fatigue properties of low nickel content type 316L austenitic stainless steel after gaseous thermal pre-charging with hydrogen

The susceptibility of low nickel content type 316L austenitic stainless steel to hydrogen was quantified using low strain rate tensile tests and strain-controlled low-cycle fatigue life measurements. Both tests were performed under air condition after charging with high-pressure 10-MPa hydrogen gas at 300 °C for eight days. No significant influence of hydrogen was recognized in 0.2% proof stress, but the strain at fracture and reduction area was decreased significantly in both hydrogen pre-charged and in gaseous hydrogen conditions compared to companion tests conducted in air. The decrease of fatigue life in the high strain amplitude region was related to a significant decrease in the plastic component while the effect of hydrogen on the elastic component was negligible. Highly localized deformation and a pronounced martensite transformation occurred near the site of the fracture surface in the high strain amplitude regime, resulting in the early formation of abundant micro-surface cracks in this regime of the hydrogen pre-charged samples.

Three-Point Bending Fatigue Test of TiAl6V4 Titanium Alloy at Room Temperature

A polycrystalline alpha-beta TiAl6V4 alloy in the annealed condition was used for the three-point bending fatigue test at frequencyf∼100 Hz. The static preloadFstat. = −15 kN and variable dynamic forceFdyn. = −7 kN to −13.5 kN were set as fatigue test loading parameters. The fatigue life S-N curve presented the stress amplitudeσaas a function of a number of cycles to fractureNf. A limiting number of cycles to run out of 2.0 × 107cycles were chosen for the 3-point fatigue tests of rectangular specimens. In addition, the Smith diagram was used to predict the fatigue life. The alpha lamellae width has a significant influence on fatigue life. It is assumed that the increasing width of alpha lamellae decreases fatigue life. A comparison of fatigue results with given alpha lamellae width in our material to the results of other researchers was performed. The SEM fractography was performed with an accent to reveal the initiation sites of crack at low and high load stresses and mechanism of crack propagation for the fatigue part of fracture.

Low cycle fatigue properties and microstructure of P92 ferritic-martensitic steel at room temperature and 873 K

Low cycle fatigue tests for P92 heat-resistant steels (P92 HRS) were conducted at room temperature (RT) and elevated temperature (873 K). The fatigue test results showed that P92 steel showed softening characteristics during fatigue at RT and 873 K, and the increase of temperature and strain amplitude was the main reason for the decrease of uniaxial fatigue life. The specimens before and after fatigue test at two temperatures were observed and analyzed by transmission electron microscope (TEM) coupled with energy-dispersive X-ray spectroscopy (EDS) and selected area electron diffraction (SAED). Microstructure observations revealed that the cyclic softening was caused by the annihilation of dislocations, as well as the fragmentation and polygonization of lath structure. High temperature promoted the formation of subgrains by accelerating the motion of dislocations and migration of low-angle grain boundaries (LABs). With the continuously decreasing of dislocation density, the growth of equiaxed subgrains and the weakening of the precipitation strengthening, the softening rate of the material was further increased until the final fracture occurs. Therefore, the evolution of microstructure of 9Cr martensitic steel during the low cycle fatigue process was deduced and described.

Impact of Electrocautery on Fatigue Life of Spinal Fusion Constructs-An In Vitro Biomechanical Study.

Instrumentation failure in the context of spine surgery is attributed to cyclic loading leading to formation of fatigue cracks, which later propagate and result in rod fracture. A biomechanical analysis of the potential impact of electrocautery on the fatigue life of spinal implants has not been previously performed. The aim of this study was to assess the fatigue life of titanium (Ti) and cobalt-chrome (CoCr) rod-screw constructs after being treated with electrocautery. Twelve spinal constructs with CoCr and Ti rods were examined. Specimens were divided into four groups by rod material (Ti and CoCr) and application of monopolar electrocautery on the rods’ surface (control-group and electrocautery-group). Electrocautery was applied on each rod at three locations, then constructs were cyclically tested. Outcome measures were load-to-failure, total number of cycles-to-failure, and location of rod failure. Ti-rods treated with electrocautery demonstrated a significantly decreased fatigue life compared to non-treated Ti-rods. Intergroup comparison of cycles-to-failure revealed a significant mean decrease of almost 9 × 105 cycles (p = 0.03). No CoCr-rods failed in this experiment. Electrocautery application on the surface of Ti-rods significantly reduces their fatigue life. Surgeons should exercise caution when using electrocautery in the vicinity of Ti-rods to mitigate the risk of rod failure.

Effect of stress ratio and mean stress on high cycle fatigue behavior of the superalloy IN718 at elevated temperatures

The present work shows the effect of mean stress on high cycle fatigue behavior of the superalloy IN718 at 400, 500 and 600 °C, at stress ratios (R) −1, 0.5, and 0.7. Fatigue life is decreased with increase in tensile mean stress. At high mean stress, decrease in fatigue life is associated with multiple crack initiation sites at the specimen surface. Fatigue limit is highly affected by the tensile mean stress and stress ratio, R = 0.7, since the maximum stress approaches near yield stress and causes cyclic ratcheting.

Laboratory investigation on the properties of asphalt concrete containing reclaimed asphalt pavement and waste cooking oil as recycling agent

ABSTRACT In this study different dosages of waste cooking oil (WCO), namely, 5%, 10% and 15% (by the weight of total asphalt) have been added into an asphalt concrete containing 25%, 50% and 75% of reclaimed asphalt pavement (RAP) and the effects on volumetric and Marshall properties, indirect tensile (IDT) strength, creep and fatigue behaviour have been investigated. Results show that air voids and optimum asphalt content of the mixtures decrease with the increase of WCO and RAP content, respectively. The Marshall stability, flow and IDT grow with the increase of RAP content and drop with increasing WCO dosage. The rutting resistance was found to increase with adding RAP, with lower increase for adding 50% RAP than 25% and 75%, and decrease with adding WCO, with lower decrease for adding 10% than adding 5% and 15%. Stress-controlled fatigue tests showed that the fatigue resistance of non-rejuvenated mixtures grows with increasing RAP content, and, in general, the increase of WCO dosage results in the decrease of fatigue life. According to the results of this research, inclusion of, approximately, 4%, 7% and 10% of WCO into the mixtures containing 25%, 50% and 75% of RAP, respectively, results in a mixture with almost similar properties of the control mixture.

Deformation mechanism and fatigue life of an Al-12Si alloy at different temperatures and strain rates

The low-cycle fatigue (LCF) deformation mechanism and fatigue life of an Al-12Si alloy were investigated at different temperatures and strain rates. With increasing temperature, the fatigue life increases at first and then reduces, which is derived from the deformation mechanism. At the lower temperatures, the primary Si cracking induced by piling-up of dislocations dominates fatigue damage behaviors. Both of increasing temperature and reducing the strain-rate can restrain the phase cracking, thus increase fatigue life. However, with further increasing temperature, the micro-scale void caused by dislocation annihilation dominates the phase debonding behavior, leading to the decrease of fatigue life.