Accelerated Fatigue Crack Growth Research Articles

The Relationship Between the Bending Fatigue Strength of Laminated Inhomogeneous Plates and the Fatigue Crack Growth Law

An expression for the evaluation of fatigue life is proposed on the basis of the fatigue crack growth low, confirming the fact that tensile mean stress and tensile residual stress do not influence the fatigue crack initiation life but accelerate fatigue crack growth. By using this expression, bending S-N curves for base metals and laminated inhomogeneous plates (clad plates) are discussed. As a result, for the surface microcracks of base metal subjected to bending fatigue, the constant m of the fatigue crack growth law in which the growth rate is proportional to the mth power of stress level has been evaluated. The ratios of crack initiation to fatigue lives have also been estimated. In addition, especially in the laminated inhomogeneous plates, tensile mean stress has remarkably decreased fatigue lives, because fatigue crack growth constitutes a significant portion of fatigue lifetime.

Electrochemical study of the corrosion fatigue of copper and alpha brass in ammoniacal copper sulphate solutions

The corrosion fatigue of copper and alpha brass was investigated in ammoniacal copper sulphate solutions within the pH range 3.7–9.2 using a rotating bend test machine. The influence of the amplitude and frequency of the cyclic stress, pH and electrode potential on both fatigue life and nature of cracks was systematically analysed. Data obtained from fatigue tests and fractograhic examination on the fracture surfaces were correlated to the change in the mechanisms of environmentally accelerated fatigue crack growth with the solution pH which is the primary factor controlling the formation of oxide layers and solution composition in chemically reactive microcracks.

Fatigue crack growth along planar slip bands

Fatigue crack propagation along planar slip bands under a mixed mode loading is analyzed based on the micromechanical theory of fatigue crack nucleation proposed previously by the authors. Fatigue damage is systematically accumulated within the slip bands with increasing dislocation dipoles and the resulting strain energy density to be a critical value yields crack growth. The propagation rate is given as the crack growth distance in each step divided by the number of cycles to the formation of a new crack ahead of the old crack. The range of cyclic shear stress in a mixed mode acting on the slip plane directly controls the rate of damage accumulation, and the normal stress component provides an additional strain energy release to accelerate fatigue crack growth. The material parameters involved in the theory are the grain size, the dislocation friction stress, and the specific fracture energy. The theory is applied to Stage I fatigue crack growth observed in high-strength alloys, and it is further extended to the near-threshold growth of a long fatigue crack.

A New Parameter for Characterizing Corrosion Fatigue Crack Growth

The role of mechanical factors, such as ΔK, R, and K˙ (loading rate), and its significance on corrosion fatigue crack growth acceleration were discussed in terms of crack tip strain rate and/or nucleation rate of fresh metal surface. A new parameter for characterizing corrosion fatigue crack growth was proposed, paying attention to rates of crack tip mechanochemical reactions, i.e., oxide film rupture rate, passivation rate, and solution renewal rate, which are influenced by the crack tip mechanical condition, microstructure of material, and environment. Hence a new parameter da/dt]air, the time base pure fatigue crack growth rate which was related closely to crack tip deformation rate, was introduced as a measure of actual crack tip strain rate. In various combinations of materials and environments, it was shown that the value of da/dt]air determines a crack growth rate in the environment, irrespective of mechanical factors such as ΔK, Kmax, R, and K˙, or frequency.

Martensitic transformation at a propagating crack

Localized transformation in the inhomogeneous stress field at a crack tip is found in alloys that are capable of strain-induced martensitic transformation (austenitic steels). Acceleration or deceleration of fatigue crack growth can occur, depending on stress intensity K and the difference of test temperature T and martensite temperature M s . It is proposed in the present paper that internal compressive stresses as induced by a positive local volume change can retard crack growth. Measurements of fatigue crack growth rates of specially designed austenitic alloys with M s little below room temperature were used to test this hypothesis.

超高真空中におけるＡｌ合金の疲労き裂の挙動

The crack growth rate of type 5056 aluminum alloy in reversed bending is much reduced in vacuum from that at atmospheric pressure, being independent of pressure in the range 10-4 to 10-6mmHg. On the contrary, the crack rate increases in high vacuum of 10-7mmHg under lower stress amplitude due to vacuum outgassing effect by prolonged exposure to high vacuum. The enhanced crack growth due to the vacuum outgassing appears more clearly in the ultra-high vacuum of 10-9mmHg irrespective of applied stresses. The plastic size generated at crack tips increases with the decrease of vacuum pressure, extraordinal plastic deformation being observed at 10-9mmHg. The acceleration of fatigue crack growth due to high vacuum outgassing can be remarkably observed on the surface crack extension of thin sheet specimens, which grows greater in the case of more prolonged exposure to higher vacuum.

The Effect of Load Condition and Specimen Configuration on Fatigue Crack Propagation

A proposal has been made for the law of fatigue crack propagation rate and may be expressed asda/dN=ΔδT/ΔδiC0 (ΔK) m0where da/dN; rate of fatigue crack propagationΔK; range of stress intensity factorC0, m0; material constantΔδT; variation of crack tip opening displacement from minimum stress to maximum stress in test specimenΔδi; crack tip opening displacement at Kin=ΔK where Kin is stress intensity factor in an infinitely wide plate with a through crack under uniform tension.This equation is in very good agreement with results of fatigue crack propagation test under various stress ratios and with various test configurations. This suggests that C and m in Paris's equation da/dN=C (ΔK) m are affected by load conditions and specimen configurations. It also suggests that a region of accelerated fatigue crack growth leading to rapid terminal failure at high ΔK values, so called “region 3”, might not be appeared in an infinitely wide plate.

Subcritical crack growth and its effect upon the fatigue characteristics of structural alloys

The subcritical crack growth behavior noted during the fracture toughness testing of several structural alloys was investigated and compared with the crack propagation characteristics observed under fatigue loading conditions. The stress intensity factor, which describes the stress conditions necessary to initiate monotonie loading subcritical crack growth, was found to correspond to that associated with an accelerated fatigue crack growth rate. From these observations, it appears that irregularities in the fatigue growth characteristics at higher K levels may be the direct result of subcritical crack growth occurring in conjunction with fatigue damage.