Remaining Useful Life for Heavy-Duty Railway Cast Steel Knuckles Based on Crack Growth Behavior with Hypothetical Distributions

The current research on the integrity of critical structures of rail vehicles mainly focuses on the design stage,which needs an effective method for assessing the service state.This paper proposes a framework for predicting the remaining useful life(RUL)of in-service structures with and without visible cracks.The hypothetical distribution and delay time models were used to apply the equivalent crack growth life data of heavy-duty railway cast steel knuckles,which revealed the evolution characteristics of the crack length and life scores of the knuckle under different fracture failure modes.The results indicate that the method effectively predicts the RUL of service knuckles in different failure modes based on the cumulative failure probability curves for different locations and surface crack lengths.This study proposes an RUL prediction framework that supports the dynamic overhaul and state maintenance of knuckle fatigue cracks.

EFFECT OF NON-PROPORTIONAL OVERLOADING ON FATIGUE CRACK GROWTH

The effect of proportional and non-proportional overloading on mode l fatigue crack growth have been studied,and the influences of crack tip plastic zone,crack tip blunting as well as crack closure were discussed.Proportional(model I)overloading may cause more serious crack growth retardation than non-proportional(mixed mode)overloading.Therefore,for estimating the fatigue life of engineering structures to simplify a real overload which may of- ten be non-proportional as a proportional one is not always safe.

The Effects of Fatigue Cracks on Fastener Loads during Cyclic Loading and on the Stresses Used for Crack Growth Analysis in Classical Linear Elastic Fracture Mechanics Approaches

High strength threaded fasteners are widely used in the aircraft industry, and service experience shows that for structures where shear loading of the joints is significant, like skin splices, fuselage joints or spar caps-web attachments, more cracks are initiated and grow from the edges of the fastener holes than from features like fillets radii and corners or from large access holes. The main causes of this cracking are the stress concentrations introduced by the fastener holes and by the threaded fasteners themselves, with the most common damage site being at the edge of the fastener holes. Intuitively, it is easy to visualize that after the crack initiation, during the growth stages, some of the load transferred initially by the fastener at the cracked hole will decrease, and it will be shed to the adjacent fasteners that will carry higher loads than in uncracked condition. Using currently available computer software, the method presented in this paper provides a relatively quick and quantitatively defined solution to account for the effects of crack length on the fastener loads transfer, and on the far field and bypass loads at each fastener adjacent to the crack. At each location, these variations are determined from the 3-dimensional distribution of stresses in the joint, and accounting for secondary bending effects and fastener tilt. Two cases of a typical skins lap splice with eight fasteners in a two rows configuration loaded in tension are presented and discussed, one representative for wing or fuselage skins configurations, and the second case representative for cost effective laboratory testing. Each case presents five cracking scenarios, with the cracks growing from approx. 0.03 inch to either the free edge, next hole or both simultaneously.

Fatigue Crack Growth Rate of Ti-6Al-4V Considering the Effects of Fracture Toughness and Crack Closure

Fatigue fracture is one of the main failure modes of Ti-6A1-4V alloy,fracture toughness and crack closure have strong effects on the fatigue crack growth（FCG）rate of Ti-6A1-4V alloy.The FCG rate of Ti-6A1-4V is investigated by using experimental and analytical methods.The effects of stress ratio,crack closure and fracture toughness on the FCG rate are studied and discussed.A modified prediction model of the FCG rate is proposed,and the relationship between the fracture toughness and the stress intensity factor（SIF）range is redefined by introducing a correcting coefficient.Notched plate fatigue tests（including the fracture toughness test and the FCG rate test）are conducted to investigate the influence of affecting factors on the FCG rate.Comparisons between the predicted results of the proposed model,the Paris model,the Walker model,the Sadananda model,and the experimental data show that the proposed model gives the best agreement with the test data particularly in the near-threshold region and the Paris region,and the corresponding calculated fatigue life is also accurate in the same regions.By considering the effects of fracture toughness and crack closure,the novel FCG rate prediction model not only improves the estimating accuracy,but also extends the adaptability of the FCG rate prediction model in engineering.

EFFECTS OF MICROSTRVCTURE ON FATIGUE CRACK GROWTH BEHMIOR IN Ni-BASE SUPERALLOY GH586

Threshold stress values, muging from ～8 to 16MPa·m1/2 can be obtained in a Ni-base alloy GH586 by varying the microstructure through heat treatments. The threshold and low crack growth rate behaviors at room temperature, with varying groin size and γ'-distribution, have been investigated. The results indicate that grain size is an important microstructurol parameter that affects fatigue crack growth threshold and propagation behaviors, as the values of △Kth increase with increasing grain size, but the γ' -distribution also has important effect. Analyses show that the effects of groin size on threshold and low crack-growth rate behavior result from heterogeneous deformation and roughness-induced crack closure due to crystallographic slipping and cracking in coarse microstructure. The higher △Kth and lower fatigue crack growth rate with increasing amounts of fine γ' phases are closely related to less damage accumulation level in deformation zone of crack-tip resulted from heterogeneous deformation due to dislocations' shearing γ' precipitates.

A GENERAL CONSTITUTIVE RELATION FOR FATIGUE CRACK GROWTH ANALYSIS OF METAL STRUCTURES

Crack growth rate curves are the fundamental material property for metal structures under fatigue loading. Although there are many crack growth rate curves available in the literature, few of them showed the capability to explain various special phenomena observed in tests. A modified constitutive relation recently proposed by McEvily and his co-workers showed very promising capability. This modified constitutive relation is further generalized by (1) introducing an unstable fracture condition; (2) defining a virtual strength to replace the yield stress; and (3) defining an overload and underload parameter. The performances of this general constitutive relation for fatigue crack growth is extensively studied and it is found that this general constitutive relation is able to explain various phenomena observed with particular strong capability on load sequence effect.

Finite Element Method Investigation of the Effect of Cold Expansion Process on Fatigue Crack Growth in 6082 Aluminum Alloy

Cold expansion is an efficient way to improve the fatigue life of an open hole. In this paper, three finite element models have been established to crack growth from an expanded hole is simulated. Expansion and its degree influence are studied using a numerical analysis. Stress intensity factors are determined and used to evaluate the fatigue life. The residual stress field is evaluated using a nonlinear analysis and superposed with the applied stress field in order to estimate fatigue crack growth. Experimental test is conducted under constant loading. The results of this investigation indicate expansion and its degree are a benefit of fatigue life and a good agreement was observed between FEM simulations and experimental results.

Effect of Notch Location on Fatigue Life Prediction of Strength Mismatched HSLA Steel Weldments

Welding of high strength low alloy steels (HSLA) involves usage of low, even and high strength filler materials (electrodes) than the parent material depending on the application of the welded structures and the availability of the filler material. In the present investigation, the fatigue crack growth behaviour of weld metal (WM) and heat affected zone (HAZ) regions of under matched (UM), equal matched (EM) and over matched (OM) joints has been studied. The base material used in this investigation is HSLA-80 steel of weldable grade. Shielded metal arc welding (SMAW) process has been used to fabricate the butt joints. Centre cracked tension (CCT) specimen has been used to evaluate the fatigue crack growth behaviour of the welded joints. Fatigue crack growth experiments have been conducted using servo hydraulic controlled fatigue testing machine at constant amplitude loading (R=0).A method has been proposed to predict the fatigue life of HSLA steel welds using fracture mechanics approach by incorporating influences of mismatch ratio (MMR) and notch location.

A two-parameter model to predict fatigue life of high-strength steels in a very high cycle fatigue regime

In this paper, ultrasonic （20 kHz） fatigue tests were performed on specimens of a high-strength steel in very high cycle fatigue （VHCF） regime. Experimental results showed that for most tested specimens failed in a VHCF regime, a fatigue crack originated from the interior of specimen with a fish-eye pattern, which contained a fine granular area （FGA） centered by an inclusion as the crack origin. Then, a two-parameter model is proposed to predict the fatigue life of high-strength steels with fish-eye mode failure in a VHCF regime, which takes into account the inclusion size and the FGA size. The model was verified by the data of present experiments and those in the literature. Furthermore, an analytic formula was obtained for estimating the equivalent crack growth rate within the FGA. The results also indicated that the stress intensity factor range at the front of the FGA varies within a small range, which is irrespective of stress amplitude and fatigue life.

Study on Fatigue Lifetimes and Their Variation of Mg Alloy AZ61 at Various Stress Ratios

In this study, fatigue tests under different R ratios were conducted on the AZ61 Mg alloy to investigate its fatigue lifetimes and fatigue crack growth (FCG) behavior. The fracture surface of the failed specimens was investigated using a scanning electron microscope to study the size of the intermetallic compounds from which the pioneer fatigue crack initiated and led to the final failure of the specimen. To determine the maximum size of the intermetallic compounds existing within the cross section of the specimen at higher risk, Gumbel’s extreme-value statistics were utilized. In the present study, the intermetallic compounds contained within the specimen were assumed to be the initial cracks existing in the material before the fatigue tests. A modified linear elastic fracture-mechanics parameter, M, proposed by McEvily et al., was used to analyze the short FCG behavior under different stress ratios, R. The relation between the rate of FCG and M parameter was found to be useful and appropriate for predicting the fatigue lifetimes under different R ratios. Moreover, the probabilistic stress-fatigue life (P-S-N) curve of the material under different R ratios could be predicted with this method, which utilizes both the FCG law and a statistical distribution of sizes of the most dangerous intermetallic compounds. The evaluated results were in good agreement with the experimental ones. This correspondence indicates that the estimation method proposed in the present study is effective for evaluation of the probabilistic stress-fatigue life (P-S-N) curve of the material under different R ratios.

Numerical simulation of fatigue crack propagation in heterogeneous geomaterials under varied loads using displacement discontinuity method

Heterogeneous brittle geomaterials are highly susceptible to cyclic loads.They contain inherent flaws and cracks that grow under fatigue loads and lead to failure.This study presents a numerical model for analyzing fatigue in these materials based on the two-dimensional(2D)boundary element method and linear elastic fracture mechanics.The process is formulated by coupling the displacement discontinuity method with the incorporation technique of dissimilar regions and the governing equations of fatigue.The heterogeneous media are assumed to consist of materials with different properties,and the interfaces are assumed to be completely bonded.In addition,the domains include multiple cracks exposed to constant and variable amplitude cyclic loads.The stress intensity factor is a crucial parameter in fatigue analysis,which is determined using the displacement field around crack tips.An incremental crack growth scheme is applied to calculating the fatigue life.The growth rate values are employed to estimate the length of crack extension when there are multiple cracks.The interaction between cracks is considered,which also includes the coalescence phenomenon.Finally,various structures under different cyclic loads are examined to evaluate the accuracy of this method.The results demonstrate the efficiency of the proposed approach in modeling fatigue crack growth and life estimation.The behavior of life curves for the heterogeneous domain was as expected.These curves illustrate the breakpoints caused by utilizing discrete incremental life equations.At these points,the trend of the curves changed with the material properties and fatigue characteristics of the new material around the crack tips.

Fatigue crack growth behavior of a 170 mm diameter stainless steel straight pipe subjected to combined torsion and bending load

In a nuclear powerplant,the rotary equipment,such as a pump directly fitted with hanger in the piping system,experiences torsional and bending loads.Higher crack growth rate occurs because of this torsional load in addition to the bending load.Hence,it is necessary to study the fatigue behavior of piping components under the influence of combined torsional and bending load.In this study,experimental fatigue life evaluation was conducted on a notched stainless steel SA312 Type 304LN straight pipe having an outer diameter of 170 mm.The experimental crack depth was measured using alternating current potential drop technique.The fatigue life of the stainless steel straight pipe was predicted using experiments,Delale and Erdogan method,and area-averaged root mean square-stress intensity factor approach at the deepest and surface points of the notch.Afterward,the fatigue crack growth and crack pattern were discussed.As a result,fatigue crack growth predicted using analytical methods are in good agreement with experimental results.

Fatigue crack growth in fiber-metal laminates

Fiber-metal laminates(FMLs)consist of three layers of aluminum alloy 2024-T3 and two layers of glass/epoxy prepreg,and it(it means FMLs)is laminated by Al alloy and fiber alternatively.Fatigue crack growth rates in notched fiber-metal laminates under constant amplitude fatigue loading were studied experimentally and numerically and were compared with them in monolithic 2024-T3 Al alloy plates.It is shown that the fatigue life of FMLs is about 17 times longer than monolithic 2024-T3 Al alloy plate;and crack growth rates in FMLs panels remain constant mostly even when the crack is long,unlike in the monolithic 2024-T3 Al alloy plates.The formula to calculate bridge stress profiles of FMLs was derived based on the fracture theory.A program by Matlab was developed to calculate the distribution of bridge stress in FMLs,and then fatigue growth lives were obtained.Finite element models of FMLs were built and meshed finely to analyze the stress distributions.Both results were compared with the experimental results.They agree well with each other.

Fatigue Life Evaluation Method for Foundry Crane Metal Structure Considering Load Dynamic Response and Crack Closure Effect

To compensate for the shortcomings of quasi-static law in anti-fatigue analysis of foundry crane metal structures,the fatigue life evaluation method of foundry crane metal structure considering load dynamic response and crack closure effect is proposed.In line with the theory of mechanical vibration,a dynamic model of crane structure during the working cycle is constructed,and dynamic coefficients under diverse actions are analysed.Calculation models of the internal force dynamic change process of dangerous cross-sections and a simulation model of first principal stress-time history are established by using the steel structure design criteria,which is utilised to extract the change of first principal stress of danger points over time.Then,the double-parameter stress spectrum is obtained by the rain flow counting method.The fatigue life calculation formula is corrected by introducing a crack closure parameter that can be calculated by the stress ratio and the effective stress ratio.Under the finite element model imported into Msc.Patran,crack propagation analysis is performed by the growth method in the fatigue integration module Msc.Fatigue.Taking the metal structure of a 100/40t-28.5m foundry crane with track offset as an example,the accuracy of calculation results and the feasibility and applicability of the proposed method are verified by theoretical calculation and finite element simulation,which provide a theoretical basis for improvement of the fatigue resistance design of foundry cranes.

Influence of Low Load Truncation Level on Crack Growth for Al 2324-T39 and Al 7050-T7451

Tests with middle-crack tension （M（T）） specimens made of Al 2324-T39 and Al 7050-T7451 are conducted to investigate the influence of low load truncation level on fatigue crack growth. The six different truncated spectra are obtained by removing the small cycles of which amplitudes are less than the specified percentages of the maximum amplitude in the basic flight-by-flight loading spectrum and the remainder of the spectrum is untouched. The tests indicate that the mean level of fatigue crack growth life （FCGL） increases as the load truncation level is enhanced. Considering both the time saving and the influence on FCGL, there is an applicable choice （i.e. spectrum S2 or spectrum S3 in this investigation） for full scale fatigue test. The scatter of FCGL becomes much larger than that under the basic spectrum when the load truncation level is increased to a specified high level, mainly due to the occurrence of crack slanting and branching under the high level truncated loading spectra.

BLOCK SPECTRUM CHARACTERISTICS AND FATIGUE CRACK RANDOM GROWTH

Two parameters,block spectrum intensity Seq and spectrum shape factor a,which describe the characteristics of the loading block spectrum are defined,and the relationship between the parameters and fatigue crack propagation behaviours is investigated.It is shown that the spectrum intensity is an 'average drive force' of fatigue crack propagation,and the variance of fatigue crack size at a given fatigue life is closely related to the spectrum shape factor α.

Fatigue Crack Propagation in Steel A131 Under Ice Loading of Crushing,Bending and Buckling

Three types of ice loading, which are most commonly present when ice acts on structures, are chosen and simulated for use of fatigue crack propagation tests on offshore structural steel A131. The three types of ice categorized in accordance with the failure modes when acting on structures called Crushing ice, bending ice, and buckling ice, respectively. This paper presents an experimental investigation on the fatigue crack propagation behavior of widely used high strength steel A131 for offshore jackets in the loading environment of ice crushing, bending, and buckling. The test results of fatigue crack propagation in steel A131 tinder these simulated ice loading at temperature 292K are presented and analyzed in detail in this paper, The amplitude root mean square stress intensity factor is optimized to be the fundamental parameter of fatigue crack propagation for all types of ice loading histories. The results are also compared with constant amplitude fatigue crack propagation conclusions as in wave load mode, and a joint investigation on the results from ice forces, ice-induced vibrations, and ice-induced fatigue crack propagation is conducted. Conclusions are drawn for reference in structural design and material selection for offshore structures in ice environments.

CALCULATIONS TO ITS FATIGUE DAMAGE FRACTURE AND TOTAL LIFE UNDER MANY－STAGE LOADING FOR A CRANK SHAFT

Suggests some calculating formulas and methods with respect to the damage evolvingrate da / dN|i and the fatigue life and in varied history from uncrack to microcrackinitiation until fracture for a crankshaft, which are suitable to stress concentration positionsabout its journal fillets and oil holes on a crankshaft, that it is undergone to bending, twistingand shearing loading and subjected to unsymmetric cyclic many-stage loading. Last the total lifein whole process is estimated by

STUDY ON THE SURFACE CRACK GROWTH BEHAVIOR IN 14MnNbq BRIDGE STEEL

Three-dimensional crack closure correction methods are investigated in this paper.The fatigue crack growth tests of surface cracks in 14MnNbq steel for bridge plate subjected to tensile and bending loadings are systematically conducted.The experimentally measured fatigue crack growth rates of surface cracks are compared with those of through-thickness cracks in detail.It is found that the crack growth rates of surface cracks are lower than those of through-thickness cracks.In order to correct their differences in fatigue crack growth rates, a dimensionless crack closure correction model is proposed.Although this correction model is determined only by the experimental data of surface cracks under tensile loading with a constant ratio R=0.05, it can correlate the surface crack growth rates with reasonable accuracy under tensile and bending loadings with various stress ratios ranging from 0 to 0.5.Furthermore, predictions of fatigue life and crack aspect ratio for surface cracks are discussed, and the predicted results are also compared with those obtained from other prediction approaches.Comparison results show that the proposed crack closure correction model gives better prediction of fatigue life than other models.

Study on the Overload and Dwell-Fatigue Property of Titanium Alloy in Manned Deep Submersible

With the rapid development of ocean technology, the deep-sea manned submersible is regarded as a high-tech equipment for the exploration and exploitation of ocean resources. The safety of manned cabin has a decisive effect on the whole system. Ti-6 Al-4 V with the superior strength-to-weight ratio and corrosion resistance has been used for the manned cabin. The manned cabin experiences loading spectrum with different maximum stresses and different dwell time during their service life. The load sequence effects on dwell fatigue crack growth behavior of Ti-6 Al-4 V under different dwell time are investigated experimentally in this paper. The experimental results show that the crack tip plastic zone is enlarged by the dwell time and the overload retardation zone increases with dwell time under the same overload rate. A dwell fatigue crack growth model is proposed by modifying the crack tip plastic zone under the loading history with combinations of the single overload and dwell time factors are included in the modified model. Based on the experimental data, the overload retardation zone and the crack growth rates of Ti-6 Al-4 V are predicted by the modified model. A reasonable model for the load sequence effect on the dwell fatigue crack growth rates of Ti-6 Al-4 V is verified.