Fatigue Crack Growth Rate Data Research Articles

마찰교반용접된 7075-T651 알루미늄 합금 용접부의 피로균열전파율의 공간적 변동성에 미치는 균열 방향의 영향

Abstract: In this investigation, the effect of crack orientation on spatial randomness of fatigue crack growth rate (FCGR) in friction stir welded (FSWed) 7075-T651 aluminum alloy joints has been statistically analyzed by Weibull distribution. The fatigue crack growth tests are conducted under three different constant stress intensity factor range (SIFR) control at room temperature with R = 0.1 and frequency 10Hz on compact tension (CT) specimen machined at base metal (BM) and weld metal (WM). The experimental fatigue crack growth rate data were obtained for two types of specimens having LT and TL orientations. LT specimens both base metal and weld metal showed higher fatigue crack growth rate as compared to TL specimens. In the lower SIFR region, FCGR were found to be almost 3 times higher in higher SIFR region. The shape parameter of Weibull both LT and TL orientation for FCGR was increased with increasing SIFR, the scale parameter was also increased with increasing SIFR. The smallest value of the shape parameter was shown in weld metal specimens having LT orientation at lower SIFR region.

Improved test method and analytical modelling for fatigue crack growth in coarse‐grain titanium alloy with rough fatigue surfaces

ABSTRACTFatigue crack growth rate properties are typically determined by experimental methods in accordance with ASTM Standard E647. These traditional methods use standard notched specimens that are precracked under cyclic tensile loads before the main test. The data that are produced using this approach have been demonstrated elsewhere to be potentially adversely affected by the test method, particularly in the threshold region where load reduction (LR) methods are also required. Coarse‐grained materials that exhibit rough and tortuous fatigue surfaces have been observed to be strongly affected by the tensile precracking and LR, in part because the anomalies caused by crack closure and roughness‐induced closure become more important. The focus of the work reported in this paper was to further develop methods to determine more accurate fatigue crack growth rate properties from threshold through to fracture for coarse‐grained, β‐annealed, titanium alloy Ti‐6Al‐4V extra low interstitial thick plate material. A particular emphasis was put upon the threshold and near threshold region, which is of strong importance in the overall fatigue life of components. New approaches that differ from the ASTM Standard included compression precracking, LR starting from a lower load level and continuing the test beyond rates where crack growth would otherwise be considered below threshold. For the threshold regime, two LR methods were also investigated: the ASTM method and a method where the load is reduced with crack growth such that the crack mouth opening displacement is held constant, in an attempt to avoid remote closure. Constant amplitude fatigue crack growth rate data were produced from threshold to fracture for the titanium alloy at a variety of stress ratios. Spike overload tests were also conducted These data were then used to develop an improved analytical model to predict crack growth under spectrum loading and the predictions were found to correlate well with test results.

Fatigue Crack Growth Law Covering Threshold and Fracture Toughness for Railway LZ50 Axle Steel

Long fatigue crack growth rate (FCGR) model covering the range from threshold to fracture toughness is investigated through a series of experiments on the fatigue crack growth behaviour during nearby the threshold district of stage I, steady growth stage II, and the toughness of railway LZ50 axle steel. It reveals that test data in stage II can be reasonably described by employing a statistical distribution for the FCGR data at a given stress intensity factor range level. While the test data nearby the threshold district can be appropriately characterized using a statistical distribution for the factor range data at a given FCGR level and the case is similarly for the test toughness data. Therefore, the model is constructed using the test data in stage II under a control of the statistical parameters of threshold and toughness data on a principle of concurrent probabilities. Availability is indicated by the modeled result for the present material.

The influence of negative and positive stress ratios on crack growth rate in AM60B magnesium alloy

Abstract In this paper, the effect of stress ratio on fatigue crack growth rate (FCGR) of AM60B magnesium alloy plates at room temperature is studied. The plates are loaded under a wide range of stress ratios (−1, −0.8, −0.6, −0.3, 0, 0.25, 0.5 and 0.75) and the behavior of the center crack is investigated. Most FCGR data available in the literature have been presented using various FCGR models that are mainly based on a positive stress intensity range as well as the entire stress intensity range. In this paper, a modified model, developed based on the Walker model, is proposed. The model condenses and presents the data obtained under the wide stress ratio range into a single line on a logarithmic scale. Finally, the variances in the FCGR data that were presented with different models from the best-fitted line to the zero stress ratio were calculated and compared to each other to clarify the efficiency of the model.

Simulation and Experimental Analysis of Fatigue Crack Growth under Cyclic Loading

The problem of crack growth is a major issue in the prediction and maintenance of engineering structural elements. Prediction of expected life of a structural element due to constant (static) and alternating loading (fatigue) is of major concern to the designers. Prediction of remaining life of the structural elements influences the decisions of maintenance engineers (checking intervals, corrections, and replacements).The fatigue crack growth rate, which determine the fatigue life of the components after crack initiation need to be experimentally and theoretically investigated. In this study, fatigue crack growth tests were conducted under constant amplitude loading at a stress ratio of 0.1, using three-point bend (TPB) specimens of ASTM A533 steel material. For the simulation part of this study, three fatigue crack growth models, i.e. the Paris, modified Forman and Austen were examined. None of the models has a fit for the fatigue crack growth rate data in a similar behaviour compared to others. The modified Forman model provided a closer fit than the Paris model with respect to the experimental results. However, the Austen model provided the best fit to the fatigue crack growth rate data as compared with the other two models. Therefore, this model is suggested for use in critical applications.

Crack Closure Behavior under High-Load Ratio for AISI 4340 Steel

In this paper, fatigue crack growth rate (FCGR) analyses were conducted on compact specimens of an AISI 4340 alloy to study the behavior over a range in load ratios (0.1 ≤ R ≤ 0.95) and constant maximum stress intensity factor (Kmax) condition. Previous study had indicated that high R > 0.7 and constant Kmax test conditions near threshold conditions were suspected to be free of crack-closure and that any differences were caused by Kmax effects, from threshold to near fracture conditions. Cracks in high-cycle fatigue (HCF) components spend a large portion of their fatigue life near threshold conditions. In order to characterize the evolution of damage and crack propagation during these conditions, fatigue crack growth rate (FCGR) data at threshold and near-threshold conditions are essential in predicting service life and in determining the proper inspection intervals. Fatigue crack growth model, namely the Forman model were examined, this model implicit the effect of R ratio and ease of curve fitting to measured data. The Forman model may be suggested for use in critical applications in studying fatigue crack growth for different load ratios.

Technical Basis for Acceptance/Rejection Criteria for Flaws in High Pressure Gas Cylinder

The objective of this paper is to present the technical basis used for developing acceptance/rejection limits for seamless, high pressure gas cylinders that can be used at the time of retesting the cylinders. The development of acceptance/rejection limits for cylinders is done in three steps. First, the “critical flaw sizes” (e.g., depth and length or area) for selected types of flaws are established by an analysis procedure that has been verified by experimental tests. Next the “allowable flaw sizes” are calculated by modifying (reducing) the size of the critical flaw sizes for each cylinder by adjusting for fatigue crack growth that may occur during the use of the cylinder. Finally, the “acceptance/rejection criteria” is established to take into account other factors, such as all the expected operating conditions that the cylinders may see in service, and the reliability and detectability of the specific inspection equipment to be used and to adjust the allowable flaw sizes to provide an additional margin of safety. This acceptance/rejection limits have been incorporated in a recently published ISO Technical Report No. TR 22694:2008 (2007, “Gas Cylinders—Methods for Establishing Acceptance/Rejection Criteria for Flaws in Seamless Steel and Aluminum Alloy Cylinders at Time of Periodic Inspection and Requalification,” The International Standards Organization, Geneva, Switzerland, Technical Report No. 22694). In this work, the API 579 “Recommended Practice for Fitness-for-Service” (2000, API 579: Recommended Practice for Fitness-for-Service, 1st ed., American Petroleum Institute, Washington, DC) was used to calculate the critical flaw sizes for a range of cylinder sizes and strength levels. For this study, the critical flaw size is defined as the size of the flaw that will cause the cylinders to fail at the test pressure of the cylinder. The results of flawed-cylinder burst tests were used to experimentally verify the calculated critical flaw sizes. The allowable flaw sizes were then calculated by using well established fatigue crack growth rate data for steel and aluminum alloys to allow for the expected amount of fatigue crack growth that may occur during the specified retesting intervals. A limited number of tests was conducted to verify the allowable flaw size calculations. Further adjustments are made to the allowable flaw sizes to define the acceptance/rejection criteria to be used during cylinder retesting.

Statistical modeling of fatigue crack growth rate in pre-strained 7475-T7351 aluminum alloy

In this study, 7475-T7351 aluminum strips were subjected to two tensile pre-strain levels of 3% and 5%. Using compact tension C(T) specimens, fatigue crack growth tests were conducted under constant amplitude loading at stress ratios of 0.1 and 0.5 in air and at room temperature. Three fatigue crack growth rate (FCGR) models, namely, Collipriest, Priddle, and NASGRO were examined. To handle the effect of stress ratio on FCGR, Walker equivalent stress intensity factor model was used. Consequently, generalized Collipriest (GC), generalized Priddle (GP), and generalized NASGRO (GN) models were developed and fitted to the FCGR data. It was shown that both GC and GP models fit the FCGR data in a similar fashion. However, the GP model provided a better fit than the GC model. The GN model was found to be the most appropriate model for the data. Therefore, this model may be suggested for use in critical applications, such as aeronautical structural design.

Fatigue Crack Growth Behavior of Sensitized Type 304 Stainless Steel under Boiling Water Reactor Conditions

Abstract The fatigue crack growth behavior of sensitized Type 304 (UNS S30400) stainless steel (SS) with an electrochemical potentiokinetic reactivation (EPR) value of 30 C/cm2 was investigated under boiling water reactor (BWR) conditions at 288°C. Tests were performed under either cyclic or trapezoidal loading conditions. Test environments were high-purity water either with 200 ppb dissolved oxygen (DO) under normal water chemistry (NWC) conditions or with 10 ppb DO plus 160 ppb dissolved hydrogen (DH) under hydrogen water chemistry (HWC) conditions. It was observed that whether the loading waveform was cyclic or trapezoidal, the fatigue crack growth rates (FCGR) could not be reduced by HWC conditions. None of the current FCGR data was encompassed by the reference curves of the ASME Code Section XI, Appendix C (ASME-XI, Appendix C), whereas all of the current data could be bounded by the JSME flaw evaluation code (JSME S NA1-2000). Finally, the examination of the accuracy of the superposition principles ...

Predicting fracture and fatigue crack growth properties using tensile properties

The safe-life assessment of components requires information such as the plane stress ( K c ), plane strain ( K Ic ), part-through fracture toughness ( K Ie ), and the fatigue crack growth rate properties. A proposed parametric/theoretical approach, based on an extended Griffith theory is used to derive fracture toughness properties and generate fatigue crack growth rate data for a range of alloys. The simplicity of the concept is based on the use of basic, and in most cases available, uniaxial stress–strain material properties data to derive material fracture toughness values. However since the methodology is in part based on an empirical relationship a wide ranging validation with actual data is required. This paper uses steel, aluminum and titanium based alloys from a pedigree database to quantify material properties sensitivity to the predictions for K Ic and K c and the subsequent estimation of Δ K th threshold and the Paris constants, C and n values. A sensitivity analysis using experimental scatter bounds show the range of d a/d N predictions can be achieved. It is found K Ic /Δ K th ratios designated as α has a range of 5–25 irrespective of tensile ductility, ε f , and is insensitive to it. The value of Δ K th for all the alloys considered was found to be proportional to the final elongation, ε f , and an empirical relationship describing Δ K th as a function of ε f was established. Furthermore it is suggested that, with the knowledge of appropriate tensile properties and the estimated range of K Ic /Δ K th ratios for the different alloys applying this method could be an appropriate tool that can be used to conservatively predict fracture and fatigue in similar alloy categories. Thus helping to reduce costs and optimize the number of experimental tests needed for alloy characterizations.

An integrated methodology for separating closure and residual stress effects from fatigue crack growth rate data

ABSTRACTTo properly interpret the results of standard fatigue crack growth tests it is often necessary to incorporate corrective techniques to the ΔK applied data. This is especially true in the near‐threshold regime where long crack data need to be closure corrected to predict small crack behaviour. It is also an issue in the presence of residual stress. A methodology to separate the influence of sample size, geometry, crack length and residual stress from the standard crack growth test data to obtain a true material response is presented. Stress ratio and residual stress contributions from known combinations of assumed crack size, applied stress and residual stress are also addressed and incorporated in the fatigue crack growth behaviour.

A methodology for predicting creep/fatigue crack growth rates in Ti 6246

A methodology has been developed which is capable of predicting creep/fatigue crack growth rates at ambient and elevated temperatures in Ti 6246. Predictions are based on finite element analysis and strain-control testing of plain specimens. The prediction of fatigue crack growth rates for a given crack configuration and cyclic plastic zone size is assumed to be consistent with the processes leading to crack initiation in plain specimens. Such an assumption leads to the conclusion that a similar stress–strain profile will lead to similar lives in both the plain specimens and in the cyclic plastic zone ahead of a crack in a notched specimen. Therefore, fatigue crack growth results from the accumulation of damage in the cyclic plastic zone ahead of the crack tip. Once the damage accumulated in this element of material becomes critical, the crack propagates through the damaged region into a new region of virgin material where the process of damage accumulation begins again. The creep/fatigue model is described and assessed with reference to measured fatigue crack growth rate data for Ti 6246 at 20 °C and 500 °C.

A study of the stress ratio effects on fatigue crack growth using the unified two-parameter fatigue crack growth driving force

A unified two-parameter fatigue crack growth driving force model was developed to account for the residual stress and subsequently the stress ratio effect on fatigue crack growth. It was found that the driving force should be expressed as a combination of the maximum stress intensity factor, K max, and the stress intensity range, Δ K, corrected for the presence of the residual stress. As a result, the effects of residual stresses manifest themselves in changes of the applied maximum stress intensity factor and the applied stress intensity range. A two-parameter function of the maximum total stress intensity factor, K max,tot, and the total stress intensity range, Δ K tot, was proposed to model the fatigue crack growth rate data obtained at various R-ratios. Based on the analysis, the unified two-parameter driving force, Δ κ = K max,tot p Δ K tot ( 1 - p ) , was derived accounting for the mean stress or the stress ratio effect on fatigue crack propagation. It was shown that the two-parameter driving force, Δ κ = K max,tot p Δ K tot 0.5 , was capable of correlating fatigue crack growth data obtained under a wide range of load ratios and fatigue crack growth rates spanning from the near threshold to the high growth rate regime. The model was successfully verified using a wide range of fatigue crack growth data obtained for Al 2024-T351 aluminium alloy, St-4340 steel alloy and Ti–6Al–4V titanium alloy with load ratios, R, ranging from −1 to 0.7.

Fatigue crack propagation in the wing to fuselage connection of the new trainer aircraft M346

The series version of the M346 military trainer aircraft is currently under construction at Aermacchi (Venegono Superiore, Italy). The design target life of the aircraft, which will be certified for Damage Tolerance, is 12 000 flight hours (FH), with the possible extension to 16 000 FH after specific inspections. Fatigue tests were performed on critical elements at the Department of Aerospace Engineering at University of Pisa in order to verify crack propagation calculations. The wing to fuselage connection is one of the most interesting elements from the fatigue point of view. Spars and frames, both integrally machined, are connected by two lug-fork joints; the base material is aluminium alloy 7050-T7451 for both the elements. High interference bushings, ForceMate®, produced by FTI (Fatigue Technology Inc., Seattle, WA) were used in the lug/fork connections. Experimental activity was carried out on two different specimens. The first, a Compact Tension specimen, was tested under constant amplitude loading to verify the fatigue crack growth rate data contained in NASGRO 4, the software used for Damage Tolerance evaluations. Experimental results were fully comparable with the NASGRO 4 material database. Additional variable amplitude loading tests were carried out in order to calibrate crack growth prediction models used in the analyses. The second specimen was a lug-fork joint designed as the actual joints present on the aircraft. Both constant and variable amplitude loading fatigue tests were carried out in this case too. Results obtained clearly indicated the beneficial effect of ForceMate bushings, compared to shrink fit bushings.

Improved modeling of the effect of R-ratio on crack growth rate

Most engineering structures with crack-like defects experience varying crack-tip loading during their service. This variation may result from combinations of varying applied loads and displacements and/or varying body force including residual stress. It is known that crack growth rates differ according to the R-ratios when expressed via a single parameter, stress intensity factor range, Δ K. Many methods have been proposed to incorporate the effect of the R-ratio, including effective stress intensity factor range models based on crack closure and the two-parameter driving force model etc. However there are few clear statements about the choice of material constants for the calculation of crack growth lives under varying amplitude loading. This is a significant omission. In this paper, a simple crack growth rate equation has been proposed, which can condense the crack growth data under different R-ratios to the curve corresponding to R = 0. Fatigue crack growth rate data for several materials taken from the literature, were analyzed using a new formulation. These data then cluster around the R = 0 crack growth rate curve. The result implies that crack growth rates for fatigue life calculation under different R-ratios may be obtained directly from the constants corresponding to R = 0. Thus the most commonly tested crack growth rate constants corresponding to R = 0–0.1 are sufficient for fatigue crack growth calculation under different loading conditions.

Statistical modeling of fatigue crack growth rate in Inconel alloy 600

Inconel alloy 600 is widely used in heat-treating industry, in chemical and food processing, in aeronautical industry, and in nuclear engineering. In this work, fatigue crack growth rate (FCGR) was evaluated in air and at room temperature under constant amplitude loading at a stress ratio of 0.1, using compact tension specimens. Collipriest and Priddle FCGR models were proposed to model the data. In addition, these models were modified to obtain a better fit to the data, especially in the near-threshold region. Akaike information criterion was used to select the candidate model that best approximates the real process given the data. The results showed that both Collipriest and Priddle models fit the FCGR data in a similar fashion. However, the Priddle model provided better fit than the Collipriest model. The modified Priddle model was found to be the most appropriate model for the data.

Development of a Circumferentially Throughwall Cracked Tube Specimen for Fatigue Crack Growth Rate Tests

Abstract This paper describes the development of a nonstandard specimen geometry for fatigue crack growth rate studies. Most fatigue crack growth rate data, available in open literature, for Alloy 690 and Alloy 600 were developed using standard, thick section C(T) specimens. Recently a need arose to study a circumferentially throughwall cracked tube specimen. The goal of the development was to use this tube specimen to determine any effects of section thickness or grain size on corrosion enhancement of fatigue crack growth rates in a simulated pressurized water reactor (PWR) primary side environment. Along with the final specimen geometry, the stress intensity equation and crack length monitoring equation are discussed. The stress intensity equation was converted to a standard polynomial equation in order to use a conventional automated fatigue crack growth rate software package. Symmetry arguments were made to justify the use of direct current potential drop (DCPD) and the use of the standard Johnson's equations for measuring crack lengths. Also, some discussions of the validity requirements for this geometry are presented such as stress intensity limits to remain with linear elastic fracture mechanics (LEFM) bounds and crack length limits to avoid excess bending stresses. A baseline test program was conducted on Alloy 690 tube specimens in ambient air and these results were compared with literature data. Also, a sensitivity investigation was preformed to determine the error between the polynomial equations developed and the actual equations in literature. The results of this study confirmed that a circumferentially cracked tube specimen could produce valid fatigue crack growth rate data.

Anomalous Fatigue Crack Growth Data Generated Using the ASTM Standards

Abstract The ASTM standard for fatigue crack growth, E 647, was co-developed by industry and government in the late 1970s to define a standard procedure for generating material crack growth behavior under a variety of loading conditions. The standard specifies tolerances on laboratory procedures, specimen configurations to generate material characterization information of a high integrity. Recent research developing fatigue crack growth rate data has uncovered significant issues with the standard test method and specimen configurations. The data generated has been largely dependent on specimen configuration, i.e., M(T) and C(T) specimens produce very different crack growth rate curves; C(T) specimens of different sizes do not generate the same material response; and finally, the development of steady-state, as defined using constant ΔK testing, does not agree with the data generated with the standard ASTM test method. Therefore, in this paper the authors present data that does not lend confidence in the integrity of the standard C(T) specimen tested using the ASTM standard method and investigate causes for the observed anomalous crack growth rate behavior.

The response of alloy 690 tubing in a pressurized water reactor environment

In this paper, the corrosion-fatigue data on Alloy 690 plate documented in the open literature is compared with test data obtained using steam generator tubing. The results on plate material documented in the open literature is not representative of actual behavior of steam generator tubing due to the mutually interactive influences of microstructural features, section thickness and processing variables. This paper also examines if response of steam generator tubing in a pressurized water reactor environment can be modeled using test data obtained for the material on plate stock. The fatigue crack growth rate data for alloy IN 690 used in actual steam generator tubing in low dissolved oxygen PWR environment was generated using a circumferentially through-wall cracked tube specimen. The laboratory test data is compared with the data published in the open literature using a modified corrosion-fatigue model. Test results reveal the tube material to have near similar fatigue crack growth rate behavior when compared to the plate material. This provides conclusion that the standard specimen geometry, such as compact tension specimen, can be used to characterize the response of the tube in adverse environments.

Use of ACR Method to Estimate Closure and Residual Stress Free Small Crack Growth Data

Abstract Life predictions for small cracks emanating from material inhomogeneities or small flaws caused by manufacturing or corrosion damage are becoming increasingly important for several reasons, including: concerns over widespread fatigue damage; a shift in philosophy for fatigue initiation design from stress-life (S-N) to the equivalent initial flaw size (EIFS) approach, and the greater usage of unitized structure. For these reasons, development of accurate fatigue crack growth rate (FCGR) data in the near threshold region is needed, which is not influenced by closure or residual stress. Threshold data ARE often generated from long crack specimens by load shedding as outlined in ASTM E 647. However, this data can be non-conservative and variable due to load history effects caused by crack closure. Generating closure-free short crack FCGR data requires much more sophisticated instrumentation and testing techniques than long crack testing, making it prohibitively expensive to perform on a routine basis. Small crack tests also sample only a small volume of material and typically exhibit large scatter, so multiple tests must be performed in order to determine the typical or average behavior of the material. As a result of these disadvantages, use of long crack tests to estimate short crack behavior is the preferred approach. The objective of this study was to evaluate the adjusted compliance ratio (ACR) method for obtaining residual stress and closure-free data from long crack tests and to compare the ACR method against the current ASTM opening load method. The ACR method was developed by Donald for estimating the closure-free ΔKeff FCGR curve from standard long crack tests. To evaluate the method's ability to estimate closure-free data, standard ASTM long crack tests and short corner crack tests were conducted on 7075-T651 and 2324-T39 alloys. The ΔKeff FCGR data resulting from the application of the ACR method to the ASTM long crack test results were then compared with the closure-free short corner crack data. Additionally, to assess the method's ability to account for residual stresses, long crack tests using ASTM standard middle crack and compact tension specimens, prepared from alloy 7055-T74511, were conducted, and the resulting ΔKapplied and ΔKeff FCGR data were compared.