Near-threshold Crack Growth Behavior Research Articles

Understanding the near-threshold crack growth behavior in an aluminum alloy by x-ray tomography

The near-threshold behavior of long cracks is evaluated using pre-cracked flat dogbone specimens of a commercial aluminum alloy in two heat treatment states. Once the threshold was known, the crack initially introduced by cyclic compression was propagated under load control with approximately constant range of the stress intensity factor at values close to the threshold values. The 3D morphology of the crack and the spatial distribution of the primary precipitates obtained by μ-CT were reconstructed with the aim of analyzing the role of the microstructure and primary precipitates on crack growth. Results pointed out that there were two major mechanism keeping the crack from continuous extension. First, the crack front was pinned by primary precipitates leading to kinking of the crack front. The second mechanism was pronounced shear-controlled crack growth of long cracks seen in terms of hilly structures on the crack faces. The first mechanism was found to be dominant at R=-1, whereas tests performed with R=0.1 showed an increased propensity towards shear controlled crack growth.

1215 普通鋳造ニッケル基超合金の下限界近傍における高温疲労き裂進展挙動の荷重波形依存性(GS-3 疲労特性)

1215 普通鋳造ニッケル基超合金の下限界近傍における高温疲労き裂進展挙動の荷重波形依存性(GS-3 疲労特性)

OS2103 普通鋳造二ッケル基超合金材の下限界近傍における高温疲労き裂進展挙動の繰返し速度依存性とその破面

OS2103 普通鋳造二ッケル基超合金材の下限界近傍における高温疲労き裂進展挙動の繰返し速度依存性とその破面

325 Ni基超合金材の大気中および真空中における下限界近傍の高温疲労き裂進展挙動の周波数依存性(GS-3 熱応力・熱弾性(2))

325 Ni基超合金材の大気中および真空中における下限界近傍の高温疲労き裂進展挙動の周波数依存性(GS-3 熱応力・熱弾性(2))

Fatigue Crack Growth Experiments of Resonating Micro-Samples

The reliability and optimal design of Micro Electro Mechanical Systems (MEMS) can be achieved only with the determination of material properties at the micro-scale. The major challenges in performing fatigue tests at the micro-scale are related to the accurate measurement of tiny deformations, to the control of very low forces and to the preparation, handling and positioning of μm-sized samples. In order to investigate the fatigue behaviour of MEMS components a new experimental setup based on the Phase Lock Loop (PLL) technique and a continuum mechanical model were developed for the characterization of micro-sized test samples. The main advantage of PLL is the achievable resolution in the crack length measurement, which increases with the decreasing of specimen size. Therefore, micro-beams with notches and without notches were prepared by electroplating Nickel in a SU8 photoresist mold (UV-LIGA). Investigations on the initiation and near-threshold crack growth behavior were performed to improve the understanding of the micro-mechanisms involved in fatigue phenomena.

Fatigue crack growth mechanisms at the microstructure scale in Al–Si–Mg cast alloys: Mechanisms in the near-threshold regime

The near-threshold crack growth behavior (Δ K th/Region I) of hypoeutectic and eutectic Al–Si–Mg cast alloys was investigated for both long and short cracks. To isolate and establish the mechanistic contributions of microstructural features (dendritic α-Al matrix/eutectic phases/Mg–Si strengthening precipitates), alloys with various Si content/morphology, grain size level, and matrix strength were studied; the effect of secondary dendrite arm spacing was also assessed. The mechanisms in Paris-Region II and Region III will be presented in a subsequent publication. The long-crack threshold behavior of the materials is attributed to combined effects of microstructure/roughness-induced and residual stress (introduced during heat treatment)-induced closure mechanisms. The threshold behavior of small cracks is explained through closure-unrelated mechanisms and barrier effects of the characteristic microstructural features of each alloy. The fatigue crack growth tests were conducted on compact tension and corner crack tension specimens under constant stress ratio, R = 0.1, in ambient conditions.

Debonding Under Fatigue Loading at Polymer/Inorganic Interfaces

AbstractThe mechanisms associated with cycle-by-cycle damage accumulation resulting in fatigue crack propagation between a highly constrained polymer layer and an adjacent elastic substrate are explored. Specifically, cyclic fatigue-induced crack growth between a bisphenol F model epoxy system and a passivated silicon substrate under Mode I loading is reported. Preliminary findings regarding the effects of fatigue load ratio on interfacial crack growth rates are presented. While intermediate crack growth rates were significantly accelerated under cyclic loading, the near-threshold crack growth behavior under cyclic and monotonic loading was surprisingly similar.

206 SUS304 鋼の下限界近傍の真空中高温疲労き裂伝ぱ挙動およびその疲労破面のフラクタル特性

Fatigue crack growth behaviors at elevated temperatures were investigated for SUS304 stainless steel. Fatigue crack growth tests were conducted in the near-threshold region using single-edge-cracked specimens at 350,550,and 650(℃). Crack growth behaviors of SUS304 in a vacuum were compared with those in air. It was found that the crack growth rate da/dN in a vacuum was as low as one-tenth of that in air. Threshold value, ΔK_<th>, was higher than that in air. The crack growth restarted when the maximum stress intensity factor, K_<max>, was increased for a crack exhibiting threshold behavior. An investigation of the fractal properties of fatigue fracture surfaces in SUS304 steel tested at 550℃ was performed. It was found that measured values of fractal dimension was scarcely dependent on stress intensity factor range ΔK.

327 SUS304鋼真空中およびニッケル基超合金の下限界近傍における高温疲労き裂伝ぱ挙動(OS1-5 環境・高温および熱疲労)(OS1 高強度材・表面改質材の疲労と破壊)

327 SUS304鋼真空中およびニッケル基超合金の下限界近傍における高温疲労き裂伝ぱ挙動(OS1-5 環境・高温および熱疲労)(OS1 高強度材・表面改質材の疲労と破壊)

703 下限界近傍の高温疲労き裂伝ぱ挙動に関する研究(GS-4 機械材料・材料加工(1))

703 下限界近傍の高温疲労き裂伝ぱ挙動に関する研究(GS-4 機械材料・材料加工(1))

Fatigue crack growth behavior of 4340 steels

Fatigue crack growth behavior of 4340 steels was investigated in four gaseous environments; laboratory air, wet hydrogen, dry hydrogen and dry helium. Specimen orientation does not affect crack propagation rate results. The effects of R-ratio (load ratio) and environment on crack growth rate properties are interrelated. Increasing R -ratio increases the rates of near-threshold crack propagation. Nevertheless, the effect of R-ratio on crack growth rates in air is much more significant than that in the two dry environments. Interestingly, the R-ratio effect in wet hydrogen is comparable to that in dry environments. At an R-ratio of 0.1, the rates of crack propagation in air are slower than those in dry environments while crack growth rates are essentially identical in wet hydrogen and dry environments. Increasing R -ratio was found to decrease the environmental effect. Furthermore, increasing yield strength from 700 to 1040 MPa does not affect crack propagation behavior. While surface roughness-induced crack closure is thought to be minimal in affecting gaseous-environment near-threshold crack growth behavior of 4340 steels, oxide-induced crack closure governs crack propagation kinetics. It is suggested that in moisture-containing environments, thick oxide deposits measured on fracture surfaces may not result in high crack closure levels. Nevertheless, oxide-induced crack closure rationalized the effects of R-ratio and environment on near-threshold crack growth rate properties. Furthermore, hydrogen embrittlement is believed not to play an important role in influencing wet-hydrogen environment near-threshold crack propagation behavior. At higher ΔK levels (⩾ 12 MPa √m), an “intrinsic” dry hydrogen effect seems to be present, and crack closure, however, cannot account for the environmental effect.

The influence of vacuum on near-threshold fatigue crack growth behavior in 60-40 brass.

The influence of vacuum environments on near-threshold crack growth behavior in an annealed 60-40 brass have been studied by means of fracture mechanics and fractography. In vacuum, the crack growth rate was smaller than that in air and decreased with decreasing vacuum pressure. When the R ratio and ΔK level increased, the crack growth rate in vacuum approached that in air. The near-threshold crack growth behavior in vacuum was markedly influenced by a fracture roughness, which resulted from a crystalline fracture mode, in addition to a plastic induced crack closure.

クリープ条件下の疲労き裂伝ぱ下限界挙動とき裂開閉口

The fatigue crack growth behavior of SUS 304 stainless steel at 650°C was investigated under low stress ratios R on the basis of the non-linear fracture mechanics. Emphasis was placed upon the near-threshold fatigue crack growth behavior and crack closure under creep conditions. It was found that in the very low da/dN region and the high da/dN region the relation between the cycle-dependent crack growth rate da/dN and stress intensity factor range ΔK obtained for -1≤R<0.2 deviated from the same correlation obtained under 0.5≤R. The values of fatigue threshold ΔKth for low R were much greater than those obtained for high R. Such a strong R dependence of the crack growth behavior found in the low R region was eliminated by the use of J-integral range ΔJf, eff, in which the effect of crack closure as well as that of material non-linearity were taken into account. The near-threshold crack growth behavior was found to be very sensitive to the loading history including frequency. This was rationalized in terms of the effect of oxide-induced crack closure enhanced by fretting as well as that of time-dependent creep deformation.

The Influence of Load Ratio and Temperature on the Near-Threshold Fatigue Crack Growth Rate Properties of Pressure Vessel Steels

Near-threshold fatigue crack propagation behavior was examined in four pressure vessel steels (SA508 Cl 2a, SA533 Gr A Cl 2, SA508 Cl 3a, and SA533 Gr B Cl 2) and two submerged arc weldments (SA508 Cl 2a and SA533 Gr A Cl 2). At a given temperature and load ratio, near-threshold crack growth rates in the four base materials and two submerged arc weldments were essentially identical. Increasing the load ratio from 0.20 to 0.50 increased the rate of near-threshold crack propagation at a given temperature. At a fixed load ratio, increasing the temperature from 24 to 288°C (75 to 500°F) increased the rate of near-threshold crack growth. The effects of load ratio and temperature on the near-threshold crack growth behavior of pressure vessel steels can be explained based on a crack closure concept.

Influence of oxide-induced crack closure on near-threshold fatigue crack growth behavior

The characteristics of oxide formation during air environment near-threshold fatigue crack growth were extensively investigated in annealed, quarter-hard and full-hard copper. Previous oxide measurements by Ritchie et al. in steels were discussed. Results show that the effect of R-ratio ( σ min / σ max ) on the threshold stress intensity range, ΔK th , can be correlated with the development of oxide thickness at the crack tip during near-threshold testing. Oxide-induced crack closure provides a possible rationale in explaining the interrelated effects of yield strength. R-ratio and environment on near-threshold crack growth behavior.