Fatigue Crack Growth Rate Data Research Articles

A comparison of test methods for the determination of fatigue crack growth rate threshold in titanium at elevated temperature

The near-threshold fatigue crack growth rate behavior of Ti-8A1-1Mo-1V at elevated temperature was investigated using three different test methods and two different specimen geometries. The first two methods were performed on compact tension specimens where crack closure effects were accounted for by monitoring crack opening load using automated compliance measurements. The third method was performed on a cantilever bending specimen that was designed to minimize closure effects. Crack opening load was not monitored for the bend specimens. All tests were performed at 500°F (260°C) in a laboratory air environment. It was observed that the fatigue crack growth rate data for each of the first two methods collapsed to one curve when crack closure was incorporated into the analysis, with a unique value of threshold stress intensity range, ΔK th,eff . However, the value of threshold stress intensity range, ΔK th , determined from the third method differed slightly from the values obtained from the other two procedures. This suggests that a unique value of ΔK th may not exist for this material under these test conditions even when crack closure is accounted for, and that it may be a function of the test method and the specimen geometry that are used in the investigation.

EFFECTS OF HYDROSTATIC PRESSURE ON THE RATE OF FATIGUE CRACK GROWTH

Abstract— Fatigue crack growth rate data have been obtained, for both positive and negative R stress ratios, in pressurised and unpressurised oil and in inert argon gas. Expected results show that for negative R the rate data should be correlated against the tensile stress intensity range. For tension‐compression loading constant hydrostatic pressure has a detrimental effect increasing the tensile stress intensity range. For tension‐tension loading constant hydrostatic pressure has a beneficial effect decreasing crack growth rates. A simple theoretical model has been proposed which describes these trends.

CRACK GROWTH BEHAVIOUR IN A THERMAL FATIGUE TEST. EXPERIMENTS AND CALCULATIONS

Abstract— This study is concerned with the results of experiments in which thermal cycles have been repeatedly applied through the wall thickness of axisymmetrically cracked tubular specimens. The investigated material is a Cr–Mo steel used for the moulds when fabricating centrifugally cast iron pipes. Crack growth rates have been measured by using the interrupted tests technique. A methodology is proposed to model the crack growth rates under such thermal fatigue loadings. The elastic and plastic stress‐strain fields are calculated on the uncracked specimen by means of a finite element code. Special attention was paid to reach a mechanical steady state regime. Fatigue crack growth rates data were obtained, both under isothermal and anisothermal conditions, on CT and SEN specimens. The latter specimens were tested under large‐scale yielding in order to obtain the data appropriate to the cyclic stress‐strain field calculated in the thermal fatigue specimens. An effective stress intensity factor, which takes into account both plastic strains and crack closure effect, was used to correlate the results of isothermal tests on CT and SEN specimens and to calculate the thermal fatigue crack growth rates in tubular specimens. It is shown that the use of the effective stress intensity factor gives a satisfactory agreement between the observed and the calculated crack growth rates.

The effect of stress ratio on the near-threshold fatigue crack growth behavior of Ti-8A1-1Mo-1V at elevated temperature

An investigation of the effects of stress ratio and temperature on the near-threshold fatigue crack growth characteristics of Ti-8Al-1Mo-1V was performed. Computer-controlled decreasing stress intensity range tests using compact tension specimens were conducted for various stress ratios at temperatures of 76°F (24°C) and 500°F (260°C) in laboratory air. Crack opening load was determined throughout the tests based on automated compliance measurements and was used to adjust the fatigue crack growth rate data to effective stress intensity range ( ΔK eff ). The effects of stress ratio at a constant temperature were able to be explained by the concept of crack closure and ΔK eff . However, crack closure was not able to account for the effects of temperature at a fixed stress ratio of 0.1, as higher near-threshold crack growth rates were observed at 500°F (260°C) when the data were plotted as a function of ΔK eff . This difference in crack growth rate is believed to be attributable to significant crack front branching and secondary cracking as a result of the increased oxidation at 500°F (260°C). A unique value of fatigue crack growth rate threshold, ΔK th was observed for all of the test conditions when crack closure was accounted for in the data analysis.

The significance of crack closure under high temperature fatigue crack growth with hold periods

A finite element model has been developed that simulates crack growth under time dependent loading conditions. The model accounts for the hold time and frequency effect under high temperature fatigue crack growth. Time dependent deformation due to creep modifies the crack tip stress-strain fields and crack tip displacements. As the creep strain rates increase, the crack opening displacement and effective stress range over which the crack is open increases. The effective stress range is strongly dependent on the creep constitutive relations and increases with hold period at maximum stress. The increase in effective stress range depends on the level of applied stresses relative to yield strength and is influenced by the sensitivity of creep strain rates on stress. The implications of crack closure results in describing high temperature fatigue crack growth rate data is outlined.

Statistical Design of Fatigue Crack Growth Test Programs

Abstract An experimental design method for determining statistically significant differences in fatigue crack growth rate data due to the influence of various test conditions (factors) is presented. This method is based on a completely randomized factorial design and a three-way classification Analysis of Variance (ANOVA) procedure. Although the example presented is for fatigue crack growth rate testing, this method is generic to any materials test program where comparisons of properties or behavior are made. The procedure allows decisions regarding these comparisons to be based on statistical tests of significance.

A REVIEW OF FATIGUE CRACK GROWTH CHARACTERISATION BY LINEAR ELASTIC FRACTURE MECHANICS (LEFM). PART I—PRINCIPLES AND METHODS OF DATA GENERATION

Abstract— The application of linear elastic fracture mechanics (LEFM) to the characterisation of fatigue crack growth has been reviewed. In Part I the present understanding of the factors influencing growth rate is summarised, and the current methods of fatigue crack growth rate data generation are considered in the light of this. In Part II, documents are reviewed which provide advice on the prediction of fatigue crack growth and also those national standards in which fatigue crack growth considerations are implicit.

A 4-electrode analogue for estimating electrochemical reactions with bare metal surfaces at the crack tip

Summary A 4-electrode in situ fracture technique has been developed as an analogue for estimating, under potentiostatic control, the kinetics of electrochemical reactions with a bare metal surface which is coupled to an “oxidized” surface of the same material. The technique simulates the equilibration reactions that occur at the tip of a growing crack. The relevance and effectiveness of this technique have been demonstrated by the good correlation between independently measured charge transfer data and corrosion fatigue crack growth rate data obtained under an externally applied cathodic potential. The results suggest that the electrode potential at the crack tip does not deviate substantially from the free corrosion potential. With the aid of this technique, a consistent and quantifiable picture of the role of electrochemical reactions in corrosion fatigue crack growth is beginning to emerge. Further quantitative understanding of corrosion fatigue crack growth of steels in aqueous environments is now possible, and is being fully explored.

Fatigue crack growth and closure at high cyclic strains

The concept of a cyclic J integral ΔJ is briefly reviewed and equations are given for estimating its value. Fatigue crack growth rate data for various test specimens of an alloy steel are analyzed using ΔJ with the aid of rough crack closure measurements. Without the plasticity correction of ΔJ, there is an extreme lack of correlation for the cases of cracks growing under reversed plastic strains. Including a correction for crack closure together with ΔJ gives the best correlation of growth rates among various geometries, degrees of plasticity and R ratios of 0 and −1. Crack closure is decreased by increasing plasticity, with the crack being open during even most of the compressive portion of completely reversed loading at high cyclic strains.

The influence of waveform and long hold time on the corrosion fatigue crack growth behavior of an austenitic steel

An inexpensive test set-up for conducting fatigue crack growth tests in corrosive environments under hold times of up to 24 h is described. The set-up uses modified creep machines for conducting slow cyclic tests for cyclic times up to 24 h. Corrosion fatigue crack growth rate data were generated for a high strength austenitic steel in an environment consisting of a mixture of hydrogen and air at several relative humidity levels. It was observed that frequency, waveform and load ratio had significant influence on the corrosion fatigue crack growth behavior. The level of relative humidity appeared to have no measurable influence on the corrosion fatigue crack growth behavior.

A Stochastic Model for Fatigue Crack Growth Rate Data

This paper summarizes a new approach to the probabilistic modeling of fatigue crack growth. The material’s resistance to fatigue crack growth is modeled as a stochastic process, which varies randomly along the crack path. Model parameters are determined through time analysis of fatigue crack growth rate data. Predictions of the statistics of crack growth are excellent, especially for small cracks.

A study of the effect of mechanical variables on fatigue crack closure and propagation

In this paper an analytical crack closure model is developed, based on the Dugdale model, but modified to take into account the plastically deformed material left in the wake of an advancing crack. For specified maximum and minimum stress values expressed as fractions of the yield stress, the model predicts a crack opening stress from which an effective stress intensity factor may then be computed. Using this factor, the constant amplitude fatigue crack growth rate data for several stress ratios for each of three different aluminium alloys are shown to reduce to a single curve. From these data and the model, growth rates may be predicted without further tests for all steady-state cyclic conditions.

Time series analysis of fatigue crack growth rate data

Previous treatments of fatigue crack growth rate (FCGR) statistics have sought to determine the best testing and data analysis techniques with which to process fatigue crack growth data. The best techniques were denned as the ones that produced the least scatter in the data. Recent studies suggest that the scatter in FCGR data has physical significance, which must be understood in order to predict the growth of small cracks. This paper presents a stochastic model which treats the material's resistance to fatigue crack growth as a spatial stochastic process evolving along the path of the crack. The parameters of the model are found by a time series analysis which accounts for the statistical correlation that has been observed between adjacent FCGR measurements. The data to be analyzed is assumed to consist of an ensemble of replicate crack growth tests sampled at equal increments of crack growth. The model is shown to be useful for explaining the moderate scatter observed in mid-range ΔK tests. It is also likely to be useful for understanding the large scatter observed in small crack tests.

Hydrostatic Retesting of Existing Pipelines

An analytical model is presented for predicting hydrostatic retest intervals in liquid pipelines which are subjected to frequent large pressure cycles. The model utilizes pressure cycle history, hydrostatic test history, and fatigue crack growth rate data for the pipe material to calculate time to failure for the largest possible defect which could have survived a previous hydrostatic test. An example problem is described which shows the value of maximizing the margin between test pressure and operating pressure in order to achieve long time intervals between tests.

Cryogenic temperature near-threshold fatigue crack growth rate data for JBK-75 stainless steel

An important structural component of the Westinghouse Large Coil Programme superconducting magnet is the JBK-75 (modified A-286) stainless steel conductor sheath. Because the presence of pre-existing cracks or flaws in the conductor sheath is a potential possibility, the structural reliability of the conductor sheath would be enhanced if a threshold level of stress intensity range ( ΔK th ) was established below which fatigue crack growth would not occur. Consequently, near-threshold fatigue growth rate data were generated at two load ratios on JBK-75 stainless steel at room and cryogenic temperatures. No load ratio effect on near-threshold fatigue crack growth rate was observed at cryogenic temperatures.

Fatigue crack propagation behaviour of rotor and wheel materials used in steam turbines

The fatigue crack propagation characteristics of several rotor and wheel materials that are commonly used in rotating components of steam turbines were investigated. Particular emphasis was placed on the behaviour at near-threshold growth rates, ie below 10 −5 mm/cycle, approaching the fatigue-crack propagation threshold, ΔK th . The lifetimes of the cracks of interest lie mostly in this region, and it is also the region where few data are available. The effects of load ratio on the fatigue crack growth rates were examined, as well as the tensile, Charpy V-notch and fracture toughness properties of the rotor and wheel materials. The relationship between fatigue crack propagation behaviour and fractographic features was examined. Fatigue crack growth rate data, da/dN vs stress intesity range ΔK, were fitted with a four parameter Weibull survivorship function. This curve fitting can be used for life estimation and establishment of ΔK th . The results show that load ratio and microstructure play a role in determining the fatigue crack threshold and fatigue crack growth behaviour.

SEGMENTED FITTING OF FATIGUE CRACK GROWTH DATA

Abstract— Segmented fitting of fatigue crack growth rate data is shown to give a more accurate prediction of fatigue life with less statistical scatter than by a single fitting using the straight line of a Paris‐type plot. Changes in the slope of the log (da/dN) vs log ΔK data are seen to correlate well with changes in the mode and mechanism of fracture.

Statistical modeling of fatigue-crack growth in a nickel-base superalloy

Two fracture mechanics-based statistical models for the fatigue crack propagation of engine materials are developed. The models are applied to fatigue crack growth rate data for IN 100, a nickel-based superalloy used in F100 engine disks, at various elevated temperatures, loading frequencies and stress ratios. The lognormal and the randomized parameter models allow the incorporation of the statistical variability associated with crack growth data into the life prediction process. The statistical distributions of (i) the crack growth rate, (ii) the propagation life to reach a given crack size (iii) the crack size at any given service life have been derived. A correlation analysis is performed to compare the results of the statistical models with test data. The correlation is demonstrated to be very good.

Computer controlled decreasing [formula omitted] fatigue threshold test

An automated fatigue threshold test was developed which decreases testing time, eliminates need for constant operator attention, and produces a plot of fatigue crack growth rate (FCGR) data as the test proceeds. A minicomputer interfaced to a servo-hydraulic test stand adjusts loads during the FCGR test to achieve a specified stress intensity range (AA). The value of ΔK is decreased exponentially with crack growth to get the fastest load shedding without crack arrest. The procedure is as follows: apply load levels to obtain desired ΔK for the crack length, a; using the specimen compliance, compute a new value for a; adjust loads to obtain corresponding ΔK for that a; record a, ΔK, and cycle count ( N); repeat. Plots of a vs N, and da/dN vs ΔK are made by a separate program. The optimized load shedding and 24hr/day operation result in a considerable reduction in test time. The program was written in BASIC on a MTS Automated Test System. Visual crack growth measurements showed good agreement with computer acquired data for a test of ETP Cu.

Near-threshold fatigue crack growth behavior in copper

Near-threshold fatigue crack growth rate data were developed in annealed, quarter-hard, and full-hard copper at various load ratios, (R = σmin/σmax). Increasing theR value decreases the resistance to threshold crack growth. At a fixed value ofR, annealed copper has the slowest near-threshold crack propagation rate while full-hard copper has the fastest crack growth rate. Waveform (sine and triangle) and specimen geometry (WOL, CT, and CCT) do not appear to affect the rates of near-threshold crack propagation. The influences of load ratio and material strength on threshold crack growth behavior can be rationalized by crack closure.