Variation Of Fatigue Life Research Articles

Development of a novel fatigue damage model with AM effects for life prediction of commonly-used alloys in aerospace

In the aerospace industry, more and more alloy parts with requirements of complex geometry and light weight are fabricated by additive manufacturing (AM) process, which has significant influence on their high-cycle fatigue properties. However, so far no work has been done to predict fatigue life of AM alloy parts through the damage mechanics based method. In this paper, a novel fatigue damage model with AM effects is proposed to address the issue, in which laser power, scan speed, hatch spacing and powder layer thickness are integrated in terms of the volumetric energy density, and the material parameters are calibrated with reported experimental data for the damage-coupled elastoplastic constitutive equations. After that, a good agreement is achieved numerically between the present theoretical model and published experimental results. Then the three most commonly-used alloy (SS316L, Ti6Al4V and AlSi10Mg) parts fabricated by AM process are studied in detail to investigate their several important characteristics, including the variation of fatigue life with the volumetric energy density, the variation of damage evolution rate with fatigue life subject to different volumetric energy densities, the relations between Young's modulus and fatigue life, and so on. Finally, several recommendations are presented for selection of the commonly-used AM alloy parts in aerospace, based on the engineering requirements and economy consideration.

P-S-N Curve Description of Laser Metal Deposition Ti-6.5Al-2Zr-1Mo-1V Titanium Alloy after Duplex Annealing.

Fatigue tests were conducted on standard smooth laser metal deposition (LMD) Ti-6.5Al-2Zr-1Mo-1V titanium alloy specimens under three constant-amplitude stresses. The mixed failure behaviors and the influence of internal pores on the fatigue life were discussed in detail. The double-peak characteristics of the fatigue life distribution have been observed, and a bimodal lognormal distribution (BLG) with five variables can be used to describe the fatigue life variation. A parameter estimation method based on the maximum likelihood estimation (MLE) method was proposed to estimate the BLG distribution parameters, and the Newton–Raphson algorithm was used to solve the nonlinear equations derived from the likelihood functions. A P-S-N curve description of LMD titanium alloy was established based on the BLG and verified by fatigue life data obtained from a fatigue test.

Fatigue behavior of direct laser deposited Ti-6.5Al-2Zr-1Mo-1V titanium alloy and its life distribution model

The present work aims to investigate the fatigue behavior of Direct Laser Deposition (DLD) Ti-6.5Al-2Zr-1Mo-1V titanium alloy under constant amplitude stress. 22 pieces of DLD Ti-6.5Al-2Zr-1Mo-1V titanium alloy standard cylinder specimens were tested under a stress level of 800 MPa with a stress ratio of 0.06. Fatigue fractography and fatigue life data were obtained. Through the fracture surface analysis, the specimens were divided into two categories in accordance with the location of crack initiation and defect types. Comparison of fatigue life and behavior between two specimen types was given, which was followed by a discussion about the impact of defect type, size and position on the fatigue life of the specimen. The fatigue test results also show a large variation of fatigue life. To illustrate the statistical characteristics of the fatigue life, probabilistic analysis was performed, and a novel bimodal lognormal model was established. The model has a good fit with the experimental data and can reduce the scatter of the fatigue life significantly.

An investigation on residual strength and failure probability prediction for plain weave composite under random fatigue loading

The residual strength and the failure probability of the plain weave carbon-fiber reinforced epoxy composite subjected to random fatigue loading are investigated. According to the constant amplitude residual model and the experimental S-N curves, a new model is proposed, in which both the probability density function of stress peaks and the power spectrum density of stochastic processes are employed. The degradation of residual strength (dSr) in a time increment (dt) is calculated via an integration of the decrease of residual strength due to the stress peaks (dSrp) in the definitional domain of the peak probability density function. A linear damage accumulation of residual strength is used in this time increment. The evolutions of the residual strength with time are determined via a double integral method. The new model’s prediction results are in good agreement with the experiment results and it can give the curves of the variation of fatigue life with failure probability.

An integrated methodology for fatigue life prediction of carbon fiber-reinforced polymer matrix composites under constant and variable R-ratio loadings

The progress in the anisomorphic constant fatigue life (CFL) diagram approach for efficiently identifying the fatigue of carbon fiber-reinforced composites under constant waveform loading over the whole range of mean stress is reviewed. The asset of this approach is the concept of critical stress ratio. The anisomorphic formulation of CFL diagram is flexible enough not only to increase the number of segments to accommodate the transient change in sensitivity to mean stress, but also to take a generalized form that considers the effect of temperature as well as the effect of variability in fatigue life. The anisomorphic approach is shown to succeed in describing the asymmetric and nonlinear CFL diagrams for carbon fiber composites of different kinds. It is suggested that the classic procedure for prediction of spectrum loading fatigue life of composites using the Miner rule with the rain flow cycle counting method is powered by integrating with the anisomorphic CFL diagram approach.

An anisotropic damage model for tensile fatigue

AbstractFatigue damage in materials is considered to be the effect of material degradation, and the dispersion in fatigue life is attributed to variability in microstructure. This paper presents a numerical model to simulate fatigue damage evolution using continuum damage mechanics to characterize material degradation. An explicit microstructure topology representation is achieved using Voronoi tessellations. Unlike conventional models which use a scalar approximation for damage, this model treats the damage variable as an anisotropic tensor. The model is used to simulate tensile fatigue failure in thin steel specimen. The fatigue life estimations from the model compares well with published experimental results. The results predict a high variability in fatigue life that is characteristic of metals and alloys, as compared with the existing isotropic damage models available in the literature. The model was also used to study the influence of material inhomogeneity on fatigue life dispersion.

Effects of hydrogen on fatigue behavior of near-alpha titanium alloys

The influence of hydrogen content on fatigue response was examined for a near-α titanium alloy, Ti-6Al-2Sn-4Zr-2Mo-0.1Si, in bimodal microstructural condition with ~70 vol% primary α. The hydrogen content was varied in the range 7–127 ppm (by weight). The fracture mechanism at crack-initiation sites changed from ductile tearing with localized plasticity in low (<50 ppm) hydrogen alloys to a more brittle fracture with facets in high (≥100 ppm) hydrogen alloys. The variation in fatigue life (by a factor of ≤1.8) was partially associated with the differences in hydrogen level and partially with the statistical scatter in life.

Effect of pre hot corrosion on high cycle fatigue behavior of the superalloy IN718 at 600 °C

High cycle fatigue behavior of the superalloy IN718 was studied in the air at 600 °C in fully reversed stress controlled mode (R = −1), in the as heat treated condition as well as pre hot corroded in the salt mixture of 75Na2SO4 + 15NaCl + 10V2O5 (wt.%) at 600 °C for 100 h. Fatigue life of the pre hot corroded specimens was drastically reduced at all the stress amplitudes studied. The variation of fatigue life with stress amplitude followed the Basquin relationship. The endurance limit of pre hot corroded specimens was reduced by 100 MPa in respect of that of the as heat treated sample. The reduction in fatigue life was found to be associated with pits formed on the surface due to evaporation of chlorides, from the prior hot corrosion in the above salt mixture, which led to early fatigue crack initiation and rapid crack propagation.

Fatigue life prediction for parallel-wire stay cables considering corrosion effects

Corrosion in stay cables of long-span cable-stayed bridges is a severe problem. This paper studies the quantitative influences of corrosion on fatigue life for parallel-wire stay cables. First, accelerated corrosion tests were conducted using acetic acid salt spray (AASS) for exposure periods from 1 d to 60 d; subsequently, a multi-parameter Weibull model for corrosion-stress-life (C-S-N) is employed to describe the influence of corrosion on fatigue life of such steel wires. Fatigue damage evolution models of multi-step loading are then used to establish recursion formulas for the probabilistic character of the fatigue life of parallel-wire stay cables. Monte Carlo simulation (MCS) is conducted to simulate the fatigue life of the stay cable, assuming load redistribution after the failure of each wire. Results indicate that the fraction of wires with short fatigue lives dominates the fatigue life of stay cable. Corrosion effects on fatigue life for both steel wire and stay cables are nearly the same. Although the variance of fatigue life of stay cable is much smaller than that of the steel wire, the fatigue performance of the stay cable becomes stable. The fatigue life of stay cable decreases significantly, as the corrosion degree of the steel wire increases, especially, in the lower stress ranges. Therefore, to prevent premature fatigue failures of parallel-wire stay cables, avoiding corrosion in the steel wires during the in-service period is crucial.

Statistical analysis of the influence of defects on fatigue life using a Gumbel distribution

Location and scale parameters, λ and δ, of a Gumbel distribution are used to quantify defects in metallic materials. To determine the impact of these parameters on fatigue life performance and variability, fatigue crack growth simulations of 7075-T651 aluminum alloy were performed using a modified strip-yield model to produce S-N data. It was observed that defects representing initial crack lengths following a Gumbel distribution yield a fatigue life that follows a log-normal distribution. It was determined that location and scale parameters impact the fatigue life mean and standard deviation, but have little to no effect on the coefficient of variation.

Correlating Scatter in Fatigue Life with Fracture Mechanisms in Forged Ti-6242Si Alloy

Unlike the quasi-static mechanical properties, such as strength and ductility, fatigue life can vary significantly (by an order of magnitude or more) for nominally identical material and test conditions in many materials, including Ti-alloys. This makes life prediction and management more challenging for components that are subjected to cyclic loading in service. The differences in fracture mechanisms can cause the scatter in fatigue life. In this study, the fatigue fracture mechanisms were investigated in a forged near-α titanium alloy, Ti-6Al-2Sn-4Zr-2Mo-0.1Si, which had been tested under a condition that resulted in life variations by more than an order of magnitude. The crack-initiation and small crack growth processes, including their contributions to fatigue life variability, were elucidated via quantitative characterization of fatigue fracture surfaces. Combining the results from quantitative tilt fractography and electron backscatter diffraction, crystallography of crack-initiating and neighboring facets on the fracture surface was determined. Cracks initiated on the surface for both the shortest and the longest life specimens. The facet plane in the crack-initiating grain was aligned with the basal plane of a primary α grain for both the specimens. The facet planes in grains neighboring the crack-initiating grain were also closely aligned with the basal plane for the shortest life specimen, whereas the facet planes in the neighboring grains were significantly misoriented from the basal plane for the longest life specimen. The difference in the extent of cracking along the basal plane can explain the difference in fatigue life of specimens at the opposite ends of scatter band.

Continuum damage mechanics based approach to the fatigue life prediction of cast aluminium alloy with considering the effect of porosity

A damage mechanics based approach is applied for the study of fatigue behaviour of high pressure die cast ADC12 aluminium alloy. A damage coupled elastoplastic constitutive model is presented according to the concept of effective stress and the hypothesis of strain equivalence. An elastic fatigue damage model taking into account the pore-induced stress concentration is developed to investigate fatigue damage evolution of the specimens subjected to cyclic loading. The predicted lives for the specimens with different sizes of pores are consistent with the experimental data. The pore-induced fatigue damage and the variation of fatigue life along with the size of pores are also investigated.

Uniaxial ratcheting behaviour of 304L stainless steel and ER308L weld joints

In this study, a series of uniaxial strain cycling and ratcheting tests were conducted on 304L stainless steel and weld metal made with ER308L stainless steel welding wire. The ratcheting strain and fatigue life of the materials were measured at different loading levels and their cyclic behaviours are discussed. Microstructural evolution during cyclic loading was investigated by electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM). The experimental results show that the cyclic behaviours of the base metal and the weld metal are different. The cyclic behaviour of the base metal includes primary hardening, slight softening, and significant secondary hardening, while the weld metal exhibits a short hardening stage for several early cycles and later exhibits cyclic softening until failure. The evolution of dislocations during strain cycling and ratcheting deformation is discussed and the cyclic and ratcheting behaviours of the two materials are explained qualitatively. The final ratcheting strain of the materials depends greatly on the mean stress and stress amplitude. However, the fatigue life variation in the two materials is different. The base metal exhibited minimum fatigue life when a mean stress of 30MPa was loaded; at constant mean stress, the fatigue life decreases when the stress amplitude is increased. In the case of the weld metal, the fatigue life decreases with increasing mean stress and stress amplitude.

Fluid volume at 12 hours is associated with outcomes in acute pancreatitis: A retrospective study

In this study, a series of uniaxial strain cycling and ratcheting tests were conducted on 304L stainless steel and weld metal made with ER308L stainless steel welding wire. The ratcheting strain and fatigue life of the materials were measured at different loading levels and their cyclic behaviours are discussed. Microstructural evolution during cyclic loading was investigated by electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM). The experimental results show that the cyclic behaviours of the base metal and the weld metal are different. The cyclic behaviour of the base metal includes primary hardening, slight softening, and significant secondary hardening, while the weld metal exhibits a short hardening stage for several early cycles and later exhibits cyclic softening until failure. The evolution of dislocations during strain cycling and ratcheting deformation is discussed and the cyclic and ratcheting behaviours of the two materials are explained qualitatively. The final ratcheting strain of the materials depends greatly on the mean stress and stress amplitude. However, the fatigue life variation in the two materials is different. The base metal exhibited minimum fatigue life when a mean stress of 30 MPa was loaded; at constant mean stress, the fatigue life decreases when the stress amplitude is increased. In the case of the weld metal, the fatigue life decreases with increasing mean stress and stress amplitude.

Fatigue Life Re-Assessment of FSO Spread Mooring System

Re-assessment fatigue life on spread moored system of Floating Storage and Offloading (FSO) vessel moored in irregular wave has been carried out based on hybrid time-frequency domain approach. Previous assessment work result obtained the fatigue life very conservative about thousands of year, whilst existing mooring system was designed for the operational life of 10 years. Different method from previous work was used in fatigue damage calculation approach for undertaking a fatigue mooring analysis in present work. Present works aim to show that the simple summation method used for previous assessment that cause the large variation in fatigue life obtained by comparing different methods of combining fatigue damage. Comparison between the result of fatigue life both previous assessment and present work (re-assessment) of fatigue life are made and discussed.

Competing fatigue failure behaviors of Ni-based superalloy FGH96 at elevated temperature

Fatigue experiments were performed on a polycrystalline P/M processed nickel-based superalloy, FGH96 at 600°C to investigate competing fatigue failure behaviors of the alloy. The experiments were performed at four levels of stress (from high cycle fatigue to low cycle fatigue) at stress ratio of 0.05. There was large variability in fatigue life at both high and low stresses. Scanning electron microscopy (SEM) was used to analyze the failure surfaces. Three types of competing failure modes were observed (surface, sub-surface and internal initiated failures). Crack initiation sites were gradually changed from the surface to the interior with the decreasing of stress level. Roles of microstructures in competing failure mechanism were analyzed. There were six kinds of fatigue crack initiation modes: (1) surface inclusion initiated; (2) surface facet initiated; (3) sub-surface inclusion initiated; (4) sub-surface facet initiated; (5) internal inclusion initiated; (6) internal facet initiated. Inclusions at surface were the life-limiting microstructures at higher stress levels. The probability of occurrence of inclusions initiated is gradually reduced with decreasing of stress level, simultaneously the probability of occurrence of facets initiated is increasing. The existence of the inclusions resulted in large life variability at higher stress levels, while heterogeneity of material caused by random combinations of grains was the main cause of fatigue variability at lower stress levels.

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.

Assessment of the mechanical behaviour of glass fibre composites with a tough polydicyclopentadiene (PDCPD) matrix

The use of a tough thermoset polydicyclopentadiene (PDCPD) as a matrix material for composites was explored. A PDCPD–glass fibre composite and an equivalent epoxy composite were compared. Fibre–matrix adhesion quality was assessed by transverse bending tests. The materials were subjected to compression tests, impact tests, static tensile tests and tensile fatigue tests. The results indicate that the tough behaviour of the PDCPD matrix markedly influences the composite damage resistance. The size of the impact damage in the PDCPD composite was half of that in the epoxy composite. The tensile tests indicated no significant difference in tensile strength, but the damage before failure was found to be much more severe in the epoxy samples. The fatigue results showed a much lower variation in fatigue life for the PDCPD material than for the epoxy material, as well as clear differences in damage development for the two materials.

Fatigue life estimation of aero engine mount structure using Monte Carlo simulation

The estimation of fatigue life of mount structures in aero engine has become crucial for safe fly-home period in the event of a fan blade off situation since it has many random factors which will be influencing the durability of these structures either direct or indirect way. The imbalanced rotation during windmilling post fan blade off (FBO) creates vibratory loads of such low frequency but in high amplitude that leads to have the shorter fatigue life of these mount structures. Regression analysis during the evaluation of fatigue life under this circumstance provides the frequency of the front rotor shaft as most influencing parameter affecting the durability of the mount structures even though there are many other factors like, tribological conditions, material properties, impact fraction, environmental and thermal effects. Since it has low probability of occurrences but with high impact, the degree of conservatism in the estimated life is still hidden. Monte Carlo simulation of randomly selected lives from a predicted distribution of all dependent factors brings the variations in fatigue life of the mount structure. This can be estimated as a function of populated size. The mean and standard deviation of the simulated fatigue life converges with more number of randomly varied samples.

G0301104 スポット溶接継手の疲労寿命に及ぼすナゲット径と打点位置変動の影響

In this paper, the influence of simultaneous variation of nugget diameter and spot welding location on fatigue life of the spot-welded joints was investigated under two kinds of loading, tensile shear and bending. The relationship between spot welding position and the nugget diameter and the nominal structural stress, which is used as fatigue life evaluation parameters, were examined. The variation of fatigue life due to varying the influence factors was quantitatively assessed by the sensitivity analysis. The fatigue test was conducted to verify the results of the sensitivity analysis. As a result, it was shown that the influence of the factors of the variation to the fatigue life was able to be accurately evaluated using the sensitivity.