Critical analysis of crack growth data

Creep-fatigue interaction would accelerate the crack growth behaviour and change the crack growth mode, which is different from that presenting in pure creep or fatigue regimes. In addition, the constraint ahead of crack tip affects the relationship between crack growth rate and fracture mechanics and thus affects the accuracy of the life prediction for high-temperature components containing defects. In this study, to reveal the role of constraint caused by various specimen geometries in the creep-fatigue regime, five different types of cracked specimens (including C-ring in tension CST, compact tension CT, single notch tension SENT, single notch bend SENB, middle tension MT) were employed. The crack growth and damage evolution behaviours were simulated using finite element method based on a non-linear creep-fatigue interaction damage model considering creep damage, fatigue damage and interaction damage. The expression of (Ct)avg for different specimen geometries were given. Then, the variation of crack growth behaviour with various specimen geometries under creep-fatigue conditions were analysed. CT and CST showed the highest crack growth rates, which were ten times as the lowest crack growth rates in MT. This revealed that distinctions in specimen geometry influenced the in-plane constraint level ahead of crack tip. Furthermore, a load-independent constraint parameter Q* was introduced to correlate the crack growth rate. The sequence of crack growth rate at a given value of (Ct)avg was same to the reduction of Q*, which shown a linear relation in log-log curve.

In order to conduct effective and rational maintenance activity of components in nuclear power plants, it is proposed to manage fatigue degradation based on crack size corresponding to an extent of cumulative fatigue effect. This study is aimed at developing a prediction model for fatigue crack growth in simulated reactor coolant environment. In order to investigate influence of reactor coolant environment on crack initiation and crack growth, two-step replica observations were conducted for environmental fatigue test specimens (type 316 stainless steel) subjected to three kinds of strain range. Crack initiation, growth, and coalescence were observed in the experiments. It is clarified that crack coalescence is one of the dominant factors causing fatigue life reduction, and fatigue life reduction depends on crack size and distance of two coalescing cracks. Then, a model was developed for predicting statistical crack initiation and growth behavior. The relationship between dispersion of crack initiation life and strain range was approximated by the Weibull model to predict crack initiation. Then, the statistical crack growth was modeled using the relation of crack growth rate and strain intensity factor. Furthermore, the crack coalescence was taken into account to the crack growth prediction considering the distance between two cracks. Finally, the crack growth curve, which is the relationship between crack size and operation period, was derived through Monte Carlo simulation with the developed model. The crack growth behavior and residual life in the simulated reactor coolant environment can be reviewed by the crack growth curve obtained with crack initiation, and the growth model developed was compared with the fatigue test results.

Effects of pearlite morphology and specimen thickness on fatigue crack growth resistance in ferritic–pearlitic steels

Modeling of I + II mixed mode crack initiation and growth from the notch

The authors have been carrying out a series of fatigue crack growth tests under two-step varying load conditions to reveal the fundamental characteristics of fatigue crack growth behavior under varying load conditions, and have established an empirical law governing the crack growth rate during the retardation period observed after the load change from high to low load amplitudes. This report describes the results of fatigue crack growth tests carried out on Al alloy CT specimen to investigate precisely the effects of pre-straining and aging treatment on the crack growth behavior during the retardation period under two-step varying load conditions. The results indicated that the crack growth rate under constant amplitude load conditions was not affected by the pre-straining and the aging treatment, but that the crack growth rate during the retardation period was remarkably influenced by these treatments. By taking the above mentioned crack growth behaviors into account, the analytical crack growth law to estimate the crack growth rate in the retardation period, derived empirically in our previous studies, was modified to be available for a wide range of material conditions. Furthermore, a reasonable crack growth model was also proposed.

In the present work the effect of microstructure on Fatigue crack growth (FCG) behavior of AISI304L with different hydrogen content was investigated. The FCG tests were carried out at 10Hz, in air at room temperature, according to ASTM Standard. C (T) specimens with 2.5 mm thickness and 30 mm width were used for the tests. Specimens, with ASTM grain sizes of about 8, were charged with hydrogen by the electrolytic method at 50oC before testing. The electrolytic solution used was 1N H2SO4 an acid solution with 0.25g/l of As2O3 oxide. Different current densities, varying between 50 and 300 mA/cm2 were used. During the fatigue test, the length of the crack was measured with an extensometer and also optically, with a telescope and a video camera connected to a screen. The crack growth behavior as a function of the pre-charge parameters has been analyzed and various techniques (OM, DRX, SEM) have been applied in order to analyze the micro structural transformations produced and to relate them with the crack growth behavior.

The improvement of acceleration behavior of crack growth was investigated with constant stress amplitude under negative stress ratio R=-1. Then a technical method to detect the fatigue crack growth was discussed. For example, when the stress amplitude exceeds a critical value, crack growth rate of overloaded specimen became higher than that of baseline which was obtained by crack growth test without applying overload. In some experimental cases, the acceleration of crack growth was observed and that could be happened on practical cases. Stop-holes were drilled at crack tips or in the vicinity of crack tips to remove the plastic zone and the effect of that on crack growth behavior were investigated. Also, steel pins were inserted into the stop-holes and its effect was discussed. Finite element method (FEM) was used to analyze the stress concentration at the edge of stop-holes. Positions of center of the stop-holes were varied at different distance from the crack tips to investigate the effect of stop-holes on fatigue crack growth. Also, stress intensity of base and stop-holed specimen was calculated. Then, the effect of stop-hole was discussed by both the experimental and analytical results. Specially, it was discussed whether the crack growth behavior was improved or not in the case of relatively smaller width specimen.

In this study, fatigue crack growth (FCG) behavior of cemented carbide under the repeated thermal shock (RTS) was experimentally evaluated by using the thermal-shock experiment method developed by the authors. Tests were carried out using cemented carbide having two different WC crystal grain sizes. In addition, FCG behavior under rotating bending fatigue (RBF) test was investigated using the same cemented carbides. Then the FCG results obtained by the RTS test and the results of the RBF test obtained at stress ratio, R = -1, were compared with each other. Here, the stress ratio R is defined as, R = σmin/σmax; σmin and σmax are the minimum and the maximum stresses, respectively. From this comparison, it was found that the relation between the rate of fatigue crack growth (FCG) and the maximum stress intensity factor in the RTS tests was equivalent to the one obtained under the RBF tests at stress ratio of -1. From a practical point of view, this result is important as it indicates that it is not necessary to purposely perform RTS experiments. In this research, the effect of WC grain size on the short surface FCG behavior of the cemented carbide was also studied and discussed.

The crack growth behavior in a highly filled composite propellant with strain gradients was studied through the use of centrally cracked, wedge-shaped sheet specimen. The specimens were tested under a constant strain rate condition at room temperature. Two crack lengths were considered. The experimental data were analyzed to calculate the instantaneous stress intensity factor K} and the associated instantaneous crack growth rate da/dl. In the data analysis, four data processing methods—the secant method, the modified secant method, the spline fitting method, and the total polynomial method—were considered. The effect of the data processing method and the time interval for crack length measurement on the accuracy of the calculated crack growth rate was investigated. In addition, the effect of the initial crack length, the nonuniform gross strain field, and the data processing method on the crack growth behavior was also investigated, and the functional relationship between the stress intensity factor and the crack growth rate was determined.

The creep crack growth (CCG) and the fatigue crack growth (FCG) behavior of a commercial pipe grade PVC material was studied based on a linear elastic fracture mechanics (LEFM) methodology. The FCG tests were performed under sinusoidal load control at a frequency of 5 Hz and at R-ratios (F min /F max ) of 0.1, 0.3 and 0.5; the test temperatures were 23 °C and 60 °C. The creep crack growth behavior (corresponding to R = 1) was studied at a test temperature of 60 °C. The results of the FCG tests revealed that fatigue crack propagation is primarily controlled by the cyclic component of the crack tip stress field rather than by the mean stress level. Comparing FCG and CCG data in terms of K Imax and K I , respectively, also confirmed the deteriorating effect of the fatigue loading on the crack growth resistance. Fracture surface investigations for both fatigue and static loading were performed to gain insight into the micromechanisms of crack advance.

The effects of water vapor in air and hydrogen gas on the tensile and fatigue crack growth behavior of Fe{sub 3}Al alloys have been studied at room temperature. Fe-28a% Al-5a% Cr alloys to which either Zr alone or Zr and C have been added and tested in controlled humidity air environments as well as in 1.3 atm hydrogen or oxygen gas and in vacuum. As with other Fe{sub 3}Al alloys, oxygen produces the lowest crack growth rates as well as the highest critical stress intensities and tensile ductility in each of the alloys tested. However, while Zr lowers crack growth rates in the Paris regime, there is no apparent beneficial effect on crack growth thresholds. Hydrogen gas also produces unusual results. While crack growth rates are very high in hydrogen in the Paris regime for all alloys, hydrogen only lowers the crack growth threshold relative to air in ternary Fe-28Al-5Cr; it does not lower the threshold in the Zr-containing alloys. It was found that decreased test frequency leads to increased crack growth rates in a Zr-containing alloy which points to a moisture-induced embrittlement mechanism responsible for the higher crack growth rates in air. Fracture path tends to be insensitive to environment for each alloy.

Effect of stress ratio on fatigue lifetime and crack growth behavior of WC–Co cemented carbide

Characterizing crack growth behavior and damage evolution in P92 steel under creep-fatigue conditions

Fatigue failure tests were firstly carried out. The material constant derived from S-N curve was evaluated and compared with previous study. The compliance method was subsequently used to evaluate the fracture toughness of MgO–C brick and the relationship between the stress factor and crack growth rate. In order to use this method, the effects of brick carbon content on the crack growth rate and crack growth rate in high temperature were investigated.As a result, it was found that the crack growth rate increased when the carbon content in brick decreased. This result was confirmed by X-ray CT scans, which revealed large cracks in bricks with a lower carbon content, even in the middle of fatigue failure tests.Furthermore, the material constants obtained from fatigue failure test and K-V diagram were compared. A material constant was derived by evaluating the relationship between the crack growth rate and stress intensity factor, and the result was found to agree with the value derived from fatigue failure tests. This result confirms that that material constant derived from fatigue failure tests is the inherent property of the material and corresponds to the variation of crack growth rate with changes in stress factor. The effects of carbon content in MgO–C brick on the crack growth behaviour and fatigue failure mechanism were also investigated.

Embrittlement of laser surface-annealed 17-4 PH stainless steel