Fatigue Crack Propagation Data Research Articles

A Critique on Evaluating Fatigue Crack Propagation Properties Using Miniature Specimens

There are situations where baseline fatigue crack propagation (FCP) data are needed but the available material for its evaluation is far from enough to fabricate full sized standard specimens. Miniature specimen testing techniques have been proposed but published data seems to be slightly but consistently slower under the same stress intensity than that from standard specimen results. In this work, an aluminum alloy and a steel, which have vastly different mechanical behaviors, are used to generate different stress states and premature crack closure to allow the validity of miniature specimen FCP data to be critically examined.

Parametric (Non)-Variance of the Mid-Regime Fatigue Crack Propagation in an Aluminium Alloy AA6056-T6

Abstract Extent of experiment-related variability in fatigue crack propagation is essential to be known while comparing the performance with other materials, for assessment of welds or damage tolerance analysis. Since such a variability can be dependent on material or parameter investigated, and may originate from the size effect, thin sheets of the relatively new airframe alloy AA6056-T6 have been investigated for baseline data using C(T) and M(T) specimens. As a screening step, various parameters such as surface cladding, crack plane orientation, specimen width and thickness, and initial notch length have been varied on C(T) specimens. The material also stems from different heats. The maximum crack opening displacement (CODmax), obtained from a displacement gauge, is calibrated against the optical crack length for the indirect crack length measurement and its precision is increased by acquiring an individual calibration curve for each specimen. The variability in the mid-regime fatigue crack propagation range is found to be relatively low (± 35%). With this range as the tolerance, the alloy is found to exhibit mostly a parametric non-variance, since, except for the specimen type, other parameters investigated are found to be non-influential. M(T) specimens are found to yield conservative fatigue crack propagation data.

Fatigue crack propagation rates in PMMA bone cement cannot be reduced to a single power law

Cement mantles around metallic implants have pre-existing flaws (shrinkage induced cracks, laminations, and endosteal surface features) and their fatigue failure is related to the fatigue crack propagation (FCP) rate of bone cement. We estimated the relevant in vivo range of cyclic stress intensity factor (DeltaK) around a generic femoral stem (0-1 MPa square root(m)) and determined that previous FCP data did not adequately cover this range of DeltaK. Vacuum-mixed standard bone cement was machined into ASTM E647 standard compact notched tension specimens. These were subject to sinusoidal loading (R = 0.1) at 5 Hz in 37 degrees C DI water, covering a DeltaK range of 0.25-1.5 MPa square root(m) (including a decreasing DeltaK protocol). FCP-rate data is normally reduced to a power-law fit relating crack growth rate (da/dn) to DeltaK. However, a substantial discontinuity was observed in our data at around DeltaK = 1, so two power-law fits were used. Over the physiologically plausible range of DeltaK, cracks grew at a rate of 2.9 E -8 x DeltaK(2.6) m/cycle. Our data indicated that FCP-rates for 0.5 > DeltaK > 0.3 MPa square root(m) are between 10 E -8 and 10 E -8 m/cycle, 1 or 2 orders of magnitude greater than predicted by extrapolating from previous models based on higher DeltaK data.

Fuzzy Method to Determine Probability Distribution Type of Fatigue Crack Data

Since the number of fatigue crack propagation data is related to cost and time and the crack propagation data are generally finite, adopting median rank or average rank as the probability value of a test datum will result in significant error. On the basis of analyzing disadvantages of traditional methods to identify distribution functions, which begin with overall fitting effect, this paper proposed a method by applying numerical simulation and fuzzy linear regression. It can be used to determine the distribution function of crack propagation data. Through fuzzy linear regression, the result that crack propagation size under certain load cycles are in logarithmic normal distribution can be acquired. In the same way, the number of load cycles is also in logarithmic normal distribution when the crack propagates to a certain size.

Analysis of the effects of compressive stresses on fatigue crack propagation rate

In this study, the effects of compressive stresses on the crack tip parameters and its implication on fatigue crack growth have been studied. Elastic–plastic finite element analysis has been used to analyse the change of crack tip parameters with the increase of the applied compressive stress level. The near crack tip opening displacements and the reverse plastic zone size around the crack tip have been obtained. The finite element analysis shows that when unloading from peak tensile applied stress to zero applied stress, the crack tip is still kept open and the crack tip opening displacement gradually decreases further with the applied compressive stress. It has been found that for a tension–compression stress cycle these crack tip parameters are determined mainly by two loading parameters, the maximum stress intensity K max in the tension part of the stress cycle and the maximum compressive stress σ maxcom in the compression part of the stress cycle. Based on the two parameters, K max, and σ maxcom, a fatigue crack propagation model for negative R ratios only has been developed to include the compressive stress effect on the fatigue crack propagation rate. Experimental fatigue crack propagation data sets were used for the verification of this model, good agreements have been obtained.

Fatigue Crack Growth Threshold of Austenitic Stainless Steels in Simulated PWR Primary Water

Many studies have revealed that fatigue crack growth (FCG) rate of austenitic stainless steels is accelerated in light water reactor environment compared to that in air at room temperature. Major driving factors in the acceleration of FCG rate are stress ratio, temperature and stress rise time. Based on this knowledge, FCG curves have been developed considering these factors as parameters. Hewever, there are few data of FCG threshold ΔKth in light water reactor environment. Hence it is necessary to clarify FCG rate under near-threshold condition for more accurate evaluation of fatigue crack growth behavior under cyclic stress with relatively low ΔK. In the present study, therefore, ΔKth was determined for austenitic stainless steels in simulated PWR primary water, and FCG behavior under near-threshold condition was revealed by collecting fatigue crack propagation data. The results are summarized as follows : No propagation of fatigue crack was found in high temperature water, and there was a definite ΔKth. Average ΔKeff, th was 4.3MPa·m0.5 at 325°C, 3.3MPa·m0.5 at 100°C, and there was no considerable reduction compared to currently known ΔKeff, th in air. Thus, it was revealed tha ambient conditions had minimal effect, on ΔKeff, th, ΔKth increases with increasing temperature and decreasing frequency. As a result of fracture surface observation, oxide-induced-crack-closure was considered to be a cause of the dependency described above. In addition, it was suggested that changes in material properties also had influence on ΔKth, since ΔKeff, th itself increased at elevated temperature.

Modeling of Stress Ratio Effect on Al Alloy SAE AMS 7475-T7351: Influence of Loading Direction

The main goal of this work was to evaluate the effectiveness of Walker’s equation in collapsing the fatigue crack propagation data of a SAE AMS 7475-T7351 aluminum alloy loaded either longitudinally (L-T) or transversely (T-L) to the rolling direction. T-L orientation testpieces presented lower ductility and fracture toughness values than L-T orientation. As a consequence, during the fatigue crack propagation tests, T-L testpieces exhibited a stronger influence of monotonic modes of fracture, resulting in higher Paris exponent values,m. Walker’s model was able to collapse fatigue crack propagation data of L-T test pieces at different applied stress ratios,R. However, for the T-L orientation, due to theR ratio dependency onm andC, simply averaging ofm values for the calculations of Walker’s exponent proved to be inefficient. A simple analytical procedure was proposed by the authors to modify Walker’s model to take into account such effect. For T-L test pieces, when Walker’s model is modified by considering both Paris’s exponent as well the coefficient as a function of theR ratio, the fatigue crack growth data collapses within a narrow band, thus allowing predictions to be made satisfactorily. The collapsed band is even narrower if the empirical relationm=a+blogC is used instead of simple polynomial equations due to a better correlation coefficient.

Evaluating fatigue crack propagation properties using a cylindrical rod specimen

In the evaluation of fatigue crack propagation properties, standard specimens such as the compact tension (CT) or center cracked specimens are commonly employed. However, these specimens are not applicable when the available material for testing is of a long axial length but small cross-sectional area. Surface cracked rod specimens may be used for this purpose. In the current work, to shed light on their applicability to the evaluation of fatigue crack propagation properties, surface cracked rod specimens were tested under axial tension. The development of the surface cracks was examined. Fatigue crack propagation data at the surface and in the deepest interior point of the surface cracks in different diameter rods were obtained. Comparison showed that rods with diameters of 8 mm and larger give fatigue crack propagation data similar to that given by compact tension specimens. For a 6 mm diameter rod specimen, growth rates were slightly higher than the standard if correlated in terms of Δ K but, fall in line with the standard, if crack closure was taken into account.

Fatigue crack propagation of 444 stainless steel welded joints in air and in 3%NaCl aqueous solution

Fatigue crack propagation (FCP) was studied on ferritic stainless steel welded joints in air and in 3%NaCl aqueous solution. In air, when the crack propagated normal to the weld line, FCP rates decreased temporarily as it reached the heat affected zone (HAZ) and then increased monotonously. When the crack propagated within HAZ and the weld metal, FCP rates were lower than those of the base metal. After allowing for crack closure, the FCP data of all the welded specimens became similar and coincided with those of the base metal. In 3%NaCl aqueous solution, all the welded specimens still showed enhanced FCP rates at high effective stress intensity factor range region, where extensive intergranular and quasi-cleavage fractures were seen.

Fatigue crack propagation testing using subsized rotating bending specimens

In many industrial applications, mechanical properties characterization is needed yet sufficient amount of material for standardsized specimens is not available. Miniaturized specimen testing technique has to be employed. Currently there are a lot of effort in developing subsized specimen technique for impact, fracture toughness and tensile properties. Work on fatigue properties testing is limited and largely confined to stress/strain life tests. In the current work, evaluation of fatigue crack propagation behaviour using surface crack growth in a rotating bending rod has been attempted. Nine different rod geometries have been tested. The resulting fatigue crack propagation data is more sensitive to rod diameter than to rod length. Difference in crack growth behaviour can largely be understood when crack closure is taken into account. All in all, the crack propagation data obtained from these small sized rods agree well with those obtained from standard testing employing compact tension specimens. A more precise picture about the crack growth behaviour can be obtained if crack closure is considered. If crack closure is not monitored, rods with longer length and smaller diameter are more likely to give the conservative upper bound fatigue crack propagation behaviour.

ガスタービン動翼後縁冷却孔部のき裂進展評価(FP3 設備診断・運用技術2)

A small crack has infrequently been detected at cooling hole machined at trailing edge of a gas turbine blade. This paper describes crack propagation evaluation at the cooling hole, which includes an estimation of temperature distribution near the cooling hole, a calculation of thermal stress distribution and a evaluation of stress intensity factor K. These results are utilized to predict crack propagation behavior at cooling hole together with fatigue crack propagation data of blade material that has been obtained in other studies.

Fatigue Fracture Resistance Analysis of Polymer Composites Based on the Energy Expended on Damage Formation

Analysis of the fatigue crack propagation behavior of filled polytetrafluoroethylene (PTFE) composites using the energy expended on damage formation was performed. A 25% chopped glass fiber filled PTFE with a commercial name of Rulon LR and a 25% polymer particle filled PTFE (Rulon J) were chosen as candidate materials for the study. Fatigue crack propagation (FCP) tests were performed using both unnotched and single edge notched (SEN) specimens under tension-tension load control condition. The notched specimens were prepared with the same notch length to sample width ratio (a/w) of 0.09. The applied frequency was 3 Hz. The maximum stress was 6 MPa, and the ratio of minimum stress to maximum stress was kept at 0.10. FCP data such as the crack length and the number of cycles were used to calculate the crack speed. The hysteresis energies for both notched and unnotched specimens during cyclic loading were obtained. The energy release rate was determined as the first derivative of the relationship between the potential energy and the crack length. The potential energy was calculated as the area above the unloading curve from the hysteresis loops at various crack lengths. The cyclic rate of energy dissipation on damage formation was calculated based on the hysteresis energies for both notched and unnotched specimens. It was found that the fatigue kinetics are related to the cyclic rate of energy dissipation into the active zone evolution. The specific energy of damage (γ) a material parameter characteristic of the fatigue fracture resistance was evaluated using the Modified Crack Layer (MCL) theory. It was found that the value of γ for the particle filled PTFE is 3200 kJ/m3, which is about three times higher than that for the fiber filled PTFE (750 kJ/m3). This indicates that the particle filled PTFE has higher fracture resistance than the fiber filled PTFE. The fracture surface of the second region for both materials was examined using scanning electron microscopy (SEM). Interfacial debonding, drawn ligaments and large voids were associated with the fatigue fracture of the fiber filled PTFE. The particle filled PTFE displayed more fibrillation than that of the fiber filled PTFE, which indicates a higher energy consuming process related to the fatigue crack growth in the particle filled PTFE.

Crack growth in discontinuous glass fibre reinforced polypropylene under dynamic and static loading conditions

Crack propagation in single edge notched tensile specimens of isotactic polypropylene reinforced with short E-glass fibres has been investigated under both fatigue and creep loading conditions. Fatigue crack propagation (FCP) experiments have been performed at three different frequencies (0.1, 1, 10 Hz) and at a mean applied tensile load of 1200 N. Isothermal creep crack propagation (CCP) tests have been conducted under a constant tensile applied load of 1200 N at various temperatures in the range from 32 to 60 °C. Analysis of FCP data allowed an estimation of the pure fatigue and pure creep components of the crack velocity under the adopted cyclic loading conditions. Crack growth at low frequencies (0.1 and 1 Hz) is mainly associated with a non-isothermal creep process. At higher frequency (10 Hz), the pure fatigue contribution appeared more pronounced. Finally, the comparison of FCP and CCP as a function of the mean applied stress intensity factor confirmed the major contribution of creep crack growth during FCP process at low frequencies.

High temperature fatigue crack growth in two metals under constant and variable amplitude loading

Results are presented for fatigue crack growth at elevated temperatures during both constant amplitude and variable amplitude loading. A careful experimental investigation is presented to show that the concept of the effective stress-intensity factor range Δ K I eff can be used to eliminate the load ratio effect on fatigue data and produce one single set of crack growth data. The fatigue crack propagation data corrected for crack closure is then used for comparison of mean crack propagation rates for variable amplitude loads with simple time histories. It is concluded that measured crack closure can not fully explain the discrepancies between measured and predicted propagation rates. A reduction factor fitted to the experimental data could be used to successfully correlate the results.

802 ステンレス鍛鋼品の疲労き裂進展特性について

LBB evaluation guide in Nuclear Power Plants (NPP) was developed based on material properties of austenitic stainless steel piping in Japan. On the other hand, we use stainless steel forging in piping systems of NPPs. On the LBB evaluation, both the estimation of fatigue crack propagation and evaluation of crack instability play important roles. So we prepared stainless steel forging to obtain material properties such as strength, fracture toughness and fatigue crack propagation data in the simulated primary coolant of PWR. As for the strength and fracture toughness, stainless steel forging is as same as stainless steel pipes. On the fatigue crack propagation, experimental data of stainless steel forging is slower than the estimation formula in LBB evaluation guide. Taking these data of stainless steel forging account, we can treat stainless steel forging same as stainless steel pipes on LBB evaluation

918 ガスタービン動翼の熱疲労下における複数き裂進展挙動評価

Frequent start-stop operation of a gas turbine sometimes causes coating cracks on the surface of the first stage blades. Since the coating cracks tend to propagate into the substrate, accurate prediction of crack propagation behavior is important to make a reasonable judgment for the repair or replacement of blades. This paper describes the development of a crack propagation prediction methodology for first stage blades, which includes evaluation of the stress intensity factor K, and estimation of temperature distribution and stress distribution between the outer surface and the inner cooling hole of blades. Since the coating cracks of blades reveal themselves as multiple parallel cracks with narrow intervals, a primary concern was directed to establish evaluation method of K values of multiple parallel cracks. For this purpose, a series of finite element analyses were performed for multiple edge cracked plate with finite width subjected to a crack face pressure expressed as σ=σn (x/W)n , where x is a distance from the mouth of the crack and σn is the stress at plate surface (x=W). The result was used to make approximate equations for the influence coefficient Gn , defined as K=Gn σn (a/W)n (πa)1/2 , where a is a crack length. Temperature distribution was evaluated by observation of microstructure (gamma prime phase) change of a first stage blade used in a power plant for more than 20,000 hrs. Thermal stress distribution was calculated by finite element analysis using the evaluated temperature distribution. These results and the approximate equation of Gn were used to evaluate the crack propagation behavior of the blade surface, together with the fatigue crack propagation data of blade material that had been obtained in other studies.Copyright © 2004 by ASME

Fatigue crack propagation in austeno-ferritic duplex stainless steel 22 Cr 5 Ni

Fatigue crack propagation in Duplex Stainless Steels (DSSs) is strongly affected by microstructure and therefore by the choice of the steel grade or by the heat treatment conditions. In the present work fatigue behaviour of 22 Cr 5 Ni DSS has been investigated both after solution annealing and after different embrittling thermal cycles. Fatigue tests have been done in air considering the influence of both the stress ratio and the loading direction. Roughness analysis has been performed to point out a relationship between fracture morphology and fatigue crack propagation data and it shows that fracture surface roughness influence on fatigue crack advancing depends on loading conditions. A scanning electron microscope (SEM) fracture surface analysis has been performed to investigate the fatigue crack propagation micro mechanisms in 22 Cr 5 Ni duplex stainless steel.

Fatigue failure analysis of V–4Ti–4Cr alloy

In the present work, the fatigue fracture and failure behavior of a V–4Ti–4Cr has been studied. Static tests were conducted to study the overloading behavior and to select the magnitude of the stress level for the fatigue studies. Fatigue tests were performed using single edge notched (SEN) specimens under tension–tension load control conditions. Fatigue crack propagation (FCP) data such as the crack length, number of cycles, and hysteresis loops were recorded to calculate the crack speed, the energy release rate, and the change in work expended on damage formation and dissipative processes within the material. Parameters characterizing the fatigue fracture resistance of V–4Ti–4Cr alloy, namely the specific energy of damage ( γ ′), and the dissipative coefficient ( β ′), were determined from the fatigue data using the modified crack layer (MCL) theory. Fracture surface examination using scanning electron microscopy (SEM) revealed ductile failure mechanisms under tensile overloading conditions. The fatigue fracture surface of the V–4Ti–4Cr consists of three distinct regions, corresponding to the threshold, stable and unstable crack propagation stages.

Evaluation of the fatigue fracture resistance of unfilled and filled polytetrafluoroethylene materials

Polytetrafluoroethylenes (PTFEs) and their composites are a special class of fluorocarbons with very high chemical resistance and wide service temperature. This makes them good candidate materials for load-bearing components exposed to harsh environments, including some space applications. In the present work, fatigue crack propagation (FCP) behavior of four materials from the fluorocarbon family, including PTFE without filler (virgin PTFE), PTFE with 15% glass fiber, PTFE with 15% graphite particles, and PTFE with 25% glass fiber, were studied. Tension/tension FCP experiments were carried out using single-edge notch (SEN) specimens under load control. The maximum stress was kept constant at 8 MPa for each material at a frequency of 3 Hz. The minimum to maximum stress ratio was 0.27. FCP data such as the number of cycles, crack length, and hysteresis loops were recorded in order to establish the crack speed, the energy release rate, J*, and the change in work Wɛi. Parameters that characterize the resistance of PTFEs to FCP have been successfully determined by the modified crack layer (MCL) model. These parameters are γ′, the specific energy of damage, which reflects the FCP resistance of the PTFE materials, and the dissipative characteristic of the materials, β′. It has been found that the MCL model describes the behavior of the PTFEs over the entire range of the energy release rate and discriminates the subtle effects introduced by changing the filler type and dosage as well as the processing conditions. The values of the specific energy of damage γ′ have been found to decrease by increasing the dosage of the fiberglass fillers. Graphite particulate filler also reduced the value of γ′ more than fiberglass filler for the same dosage. Microscopic analysis of the fracture surface in the stable crack propagation region of each material revealed that there exists a strong correlation between the value of γ′ and the amount of damage energy manifested by different mechanisms and species during the fatigue process.

Malfunctions of a steam turbine mechanical control system

This paper is aimed at elucidating the cause of a series of malfunctions involving the bending or breaking of main steam turbine throttle valve spindles which occurred at service times ranging from hundreds to several thousand hours in a number of 270 MW steam raising units. It was clearly established, by two distinct approaches (one engineering, one micromechanistic) that the stresses which produced these malfunctions were bending in nature and were the result of out-of-alignment deflections. In the case of the bent spindles the stresses were very high and approached flow strength levels of around 8000 MPa while the broken spindles were the results of fatigue initiation and subsequent growth from a thread root (stress concentration) location on the spindle. Using relevant fatigue crack propagation data for the valve spindle material at 300°C it was demonstrated that fatigue failures occurred at spindle deflections of between 0.9 and 1.6 mm. Finally, it was demonstrated that the fatigue breakage problem could be significantly reduced, especially at the lower end of the valve spindle deflection range, by a combination of re-profiling the thread root and shot peening.