Steady-state Crack Growth Rates Research Articles

Fatigue crack propagation behaviors in Ti-5Al-3Mo-3V-2Zr-2Cr-1Nb-1Fe alloy with STA and BASCA heat treatments

Fatigue crack propagation behaviors of a novel high strength Ti-5Al-3Mo-3V-2Zr-2Cr-1Nb-1Fe alloy with equiaxed and lamellar microstructures were systematically investigated. Fatigue crack propagation tests at both increasing and decreasing stress intensity factor range values were carried out with a frequency of 15 HZ and stress radio R of 0.1 by using compact tension samples. The sample with lamellar microstructure displayed the faster fatigue crack propagation rates in near-threshold regime compared to the sample with equiaxed microstructure. The fatigue threshold (ΔKth) was 2.7 MPa·m1/2 for lamellar microstructure, and 3.2 MPa·m1/2 for equiaxed microstructure. The steady state crack growth rate in lamellar microstructure was slightly slower than that in equiaxed microstructure in the Paris regime, while much faster in the rapid growth regime. The interactions between crack path and microstructures as well as the microscopic fracture surface morphology in both microstructures were detected and analyzed by scanning electron microscope (SEM), revealing the different crack propagation behaviors in three regimes. The results showed that the crack propagation behaviors were a result of two contributing factors: interface obstacles and crack front profile. The increased fatigue crack propagation resistance of equiaxed microstructure in near-threshold regime can be attributed to the interface obstacles which dominated over the rougher crack front profile. While the higher fatigue crack growth resistance for lamellar microstructure in Paris regime was attributed to the more positive effect of rougher crack front profile. In the rapid growth regime, the crack in lamellar microstructure was found to be extended along the grain boundary α, which was regarded as a low energy crack path and led to the faster crack propagation rate.

Experimental Study on Fatigue Crack Propagation of Glare3-3/2 with Mechanics Properties

This study is to investigate the fatigue crack growth behavior of Glare3-3/2 under various stress levels. The Glare3-3/2 specimen consists of three 2024-T3 aluminum alloy sheets and two layers of glass/epoxy composite lamina. Tensile-tensile cyclic fatigue tests were conducted on centrally notched specimen at four stress levels with various maximum values. A digital camera system was used to take photos of the propagating cracks on both sides of the specimen. Image processing software was adopted to accurately measure the length of the cracks on each photo. The test results show that 1) Compared to 2024-T3 aluminum alloy, the fatigue properties of Glare3-3/2 are much better: under the same loading condition with maximum stress level of 120MPa, the crack growth rate of Glare3-3/2 is roughly 5% of the corresponding value of 2024-T3 aluminum alloy, while the fatigue life is 4 times higher than that of 2024-T3 aluminum alloy. 2) The maximum stress level shows strong influence on fatigue crack propagation behavior of Glare3-3/2. The value of steady state crack growth rate increases linearly, while the number of load cycles decreases exponentially, with respect to the maximum stress values used in the fatigue tests.

The dependency of the crack growth rate on the loading pattern and temperature in stress corrosion cracking of strain-hardened 316L stainless steels in a simulated BWR environment

The stress corrosion cracking (SCC) growth rate of a warm-rolled (WR) 316L stainless steel contoured double cantilever (CDCB) specimen was measured in high purity water at various temperatures and under various loading patterns. An alternating current potential drop (ACPD) technique was used to monitor the crack growth kinetics throughout the tests. The fracture surface exhibited typical intergranular SCC characteristics. Depending on the test conditions, three kinds of crack growth kinetics, i.e., increasing with time then becoming steady, being constant during the whole period, decreasing with time then becoming steady, were identified and are described. The steady state crack growth rate (CGR) values are used to quantify the effects of the loading pattern and the environmental temperature. A moderate increase in the crack growth rate was encountered by employing periods of unloading and reloading to form a trapezoidal loading pattern and the enhancement factor was found to depend on the holding time and the times for unloading and reloading. It was found that the crack growth is thermally activated; however, the apparent activation energy is not constant but seems to be greater at higher temperatures. Several types of temperature-dependent crack growth kinetics are proposed based on the rate-determining step for the crack growth. The present experimental results can be rationalized by considering multiple element processes such as aqueous mass transport and solid-state mass transport in the crack growth. The cracking mode, the temperature dependence of the crack growth rate, and the transient crack growth behavior for WR 316L SS after changing the environmental temperature are quite similar to those for a cold-worked(CW) 316L SS tested in the same environment, despite their different absolute crack growth rate values. The effect of yield strength on CGR is more significant at lower temperatures and the apparent activation energy for the crack growth rate seems to be lower in the material with a higher yield strength. Time-lag crack growth behavior was found at points during several test steps on WR 316L SS, for example, just after in situ pre-cracking and after increasing or decreasing the temperature, which is quite consistent with the results obtained with CW 316L SS. The importance of in situ monitoring of the crack growth for obtaining steady state crack growth rates is emphasized, especially for those steps for which a nonlinear crack growth period occurs after changing the test condition.

Transient and steady state crack growth kinetics for stress corrosion cracking of a cold worked 316L stainless steel in oxygenated pure water at different temperatures

The stress corrosion cracking (SCC) growth kinetics for a cold worked 316L stainless steel was continuously monitored in high purity water at different temperatures and dissolved oxygen (DO) levels under a K (or K max) of 30 MPa m 0.5. The total SCC test time was more than 8000 h to make sure the steady state crack growth rate under each test condition could be reached. Crack growth rate (CGR) increases with increasing temperature in the range 110–288 °C. A typical intergranular-cracking mode is identified. Depending on the previous test condition, especially the temperature, three kinds of crack growth kinetics, i.e., increasing with testing time then becoming steady, being constant during the whole period, or decreasing with test time then becoming steady, are identified and discussed. Time-dependent and testing history-dependent crack growth modes were confirmed in two series of tests in 2 ppm DO and 7.5 ppm DO pure water. The apparent activation energies are calculated and compared with other data in different environments under different applied loading levels for understanding the cracking mechanism.

Three Stages of Fatigue Crack Growth in GFRP Composite Laminates

Multiple fatigue crack growth behavior has been studied in model transparent GFRP laminates. Detailed experimental observations have been made on the growth of individual fatigue cracks and on the evolution of cracks in off-axis layers in 0/90/±45S and ±45/90S laminates. Three stages of fatigue crack growth in the laminates have been identified: initiation, steady-state crack growth (SSCG), crack interaction and saturation. The results show that SSCG rate is essentially constant under constant load, independent of crack length and crack spacing. Finite element models have been developed and used to calculate the strain energy release rates associated with the off-axis matrix cracking. A correlation has been achieved between fatigue crack growth rates in off-axis layers and the total strain energy release rates.

Creep rupture in ceramic matrix composites with creeping fibers

A model has been developed for crack growth in an elastic matrix with creeping fibers. Specific solutions have been presented for the steady-state crack growth rate. These solutions provide straightforward connections to the constituent properties through well-known parameters that characterize matrix cracking with elastic fibers. An expression has also been given for the time taken to extend matrix cracks across the plies, as a preliminary estimate of the creep rupture life. A significant finding is the strong stress dependence of the life, even when the fibers creep linearly with stress.

Slow Crack Growth in Single-Crystal Silicon

Time-dependent crack growth has been measured on a precracked, single-crystal silicon cantilever beam 75 micrometers long that was excited at resonance. Growth of the precrack changes the resonant frequency of the beam, which is correlated to crack length. The measured steady-state crack growth rate was as slow as 2.9 x 10(-13) meter per second, although the apparatus can measure crack growth rates as low as 10(-15) meter per second. It is postulated that static fatigue of the native surface silica layer is the mechanism for crack growth. These experiments demonstrate the possibility of rate-dependent failure of silicon devices and the applicability of linear elastic fracture mechanics to small-scale micromechanical devices. The results indicate that slow crack growth must therefore be considered when evaluating the reliability of thin-film silicon structures.

Steady-state creep crack growth by continually nucleating cavities

F or creep crack growths by the nucleation and growth of grain boundary cavities, steady-state crack growth rates ( ȧ) are derived when the cavity nucleation is assumed to be continual and strain controlled. For the crack growth under HRR fields, a is linearly proportional to C∗ as reported in other models. However, when the crack growth occurs under elastic fields, a ̇ K 1 2 + 3n 5 (n > 8 3 ) for diffusional cavity growth and a ̇ ∼ K n 1 (n > 2) for cavity growth by power law creep. Predictions of the model are applied to a Ni-Cr steel and a Nimonic 80A, and compared with other models through analyses on the dependence of a on macroscopic load parameters and the activation energy of a.

Macroscopic creep crack growth in type 316 stainless steel. I. Considerations of notch ductility

Notched bar rupture and creep crack growth experiments have been performed in air on three different casts of Type 316 stainless steel at 625°C and 550°C. Although notch weakening are observed in one cast at 550°C, the notched bar strengthening observed in all three casts is shown to be a consequence of increased initiation times before steady state crack growth took place. Crack initiation times were found to be dependent on the particular cast in question but steady state crack growth rates were comparatively unaffected by cast-to-cast variations.

A Study of Transient Crack Growth in AISI 4340 Steel in NaCl Solution

Abstract Subcritical crack growth in high strength AISI 4340 steel in 3.5% NaCl aqueous solution at 25 C in response to sudden changes in applied stress intensity was studied. The material-environment system was potentiostatically controlled at cathodic ( −1.0 VSCE). open circuit (−0.6 VSCE), and anodic (−0.55 VSCE) potentials in order to ascertain the effect of potential on the crack growth transient behavior. Two loading schemes were used. In loading scheme I, the initial crack was stationary, whereas with loading scheme II, the sudden change in crack tip stress intensity, KI, was applied to a crack growing at a steady-state rate. The results obtained from loading scheme I show that the steady-state crack growth rate is achieved only after an incubation period of no crack extension and a transient period of time-dependent crack growth rate. The time taken to achieve the steady-state crack growth rate was found to decrease with increasing applied stress intensity. The results obtained from loading scheme...

Creep crack growth behavior in 316 stainless steel at 594�C (1100�F)

The creep crack growth behavior of a type 316 stainless steel was characterized at 594°C (1100°F) using precracked single edge notch specimens loaded in displacement rate control. The steady-state crack growth rate, da/dt, correlated with J-integral and did not correlate with C*. The creep crack growth behavior in this material and temperature is compared with our previous creep crack growth rate data on a Cr-Mo-V steel at 538°C (1000°F) and on type 304 stainless steel at 594°C in which da/dt correlated with C*. A detailed discussion is included on why in some materials creep crack growth rate correlates with J integral and in others it correlates with C*.

Stress history effect on incubation time for stress corrosion crack growth in AISI 4340 steel

The effect of stress history on stress corrosion cracking of AISI 4340 steel in an aqueous environment has been studied with the use of double-cantilever beam specimens. The stress history effect was found to influence the incubation time period with changes in the stress intensity. When the stress intensity was decreased, the incubation time period was dependent on the △K and finalK f during stress corrosion testing. When the stress intensity was increased, the incubation time period was independent of the applied stress intensity. However, the stress history effect did not influence the steady-state crack growth rates. In this report, the stress history effect is explained by using the hydrogen embrittlement mechanism.