ON THE SHARP BEND IN THE S-N CURVE OF THE AZ31 EXTRUDED MAGNESIUM ALLOY

Abstract Rotating bending fatigue test was carried out using AZ31 extruded magnesium alloy in order to clarify the S-N curve, crack initiation and propagation behaviour of the alloy. The present material is divided roughly into 2 kinds of banded texture, the phase A and B. The phase B further consists of the crystal grains (phase C). The fatigue crack generates at the upper and lower edges of the grain boundary (phase C) in the early stage of the fatigue life. At stress amplitude of 122.5 MPa which is slightly larger than the fatigue limit, the crack develops to the next B-phase without being blocked by the grain boundary (phase B), however at 120 MPa which is slightly less than the fatigue limit, the crack advance was blocked by the grain boundary. The sharp curved S-N curve is attributable to the crack arrest, which is caused by the grain boundary of the phase B

Distribution of Corrosion Fatigue Crack Lengths in Carbon Steel : 1st Report, The Cracks Which Grow Individually

It has been known that very small distributed cracks can be observed on the surface of a smooth specimen subjected to corrosion fatigue. The fracture process can be characterized by the interaction and coalescence of these small distributed cracks. In order to analize this corrosion fatigue fracture process, high cycle fatigue tests were performed on carbon steel sheet specimens under completely reversed plane bending stresses in salt water (3.0%NaCl). Initiation and growth of cracks on the surface of smooth specimen were observed in detail during corrosion fatigue process. The distribution of crack lengths at a certain stress cycles was able to be explained by a statistical calculation which took into account both the variation of number of cracks during stress cycling and the scatter of crack growth rates

Distribution of Corrosion Fatigue Crack Lengths in Carbon Steel : 2nd Report, The Distributed Cracks which Interact and Coalesce

It has been known that very small distributed cracks can be observed on the surface of smooth specimen subjected to corrosion fatigue, and the fatigue crack growth rate is accelerated by the interaction and coalescence of them. In this report, following the previous report of the authors, the interaction and coalescence behaviour of the distributed cracks on the specimen surface were observed in detail. Based on the experimental results, distribution of corrosion fatigue crack lengths after a certain cycles can be estimated theoretically. These estimated distributions of crack lengths coincided with those obtained experimentally

Effect of Specimen Thickness and Stress Intensity Factor Range on Plasticity-Induced Fatigue Crack Closure in A7075-T6 Alloy

Fatigue crack growth experiments are performed using A7075-T6 compact tension (CT) specimens with various thicknesses t (1–21 mm). The stress intensity factor at the crack opening level Kop is measured, and the effects of t and the stress intensity factor range ΔK on Kop are investigated. In addition, the change in Kop value due to specimen surface removal is investigated. Furthermore, we clarify that the radius of curvature of the leading edge of the fatigue crack decreases as t becomes thinner. Using the three-dimensional elastoplastic finite element method, the amount of plastic lateral contraction (depression depth d) at the crack tip after fatigue loading is calculated quantitatively. The following main experimental results are obtained: In the region where ΔK is 5 MPam1/2 or higher, the rate of fatigue crack growth da/dN at a constant ΔK value increases as t increases from 1 to 11 mm. The da/dN between t = 11 and 21 mm is the same. Meanwhile, in the region where ΔK is less than 5 MPam1/2, the effect of t on da/dN is not observed. The effects of t and ΔK on the da/dN–ΔK relationship are considered physically and quantitatively based on d

Effect of Specimen Thickness and Stress Intensity Factor Range on Plasticity-Induced Fatigue Crack Closure in A7075-T6 Alloy

Fatigue crack growth experiments are performed using A7075-T6 compact tension (CT) specimens with various thicknesses t (1–21 mm). The stress intensity factor at the crack opening level Kop is measured, and the effects of t and the stress intensity factor range ΔK on Kop are investigated. In addition, the change in Kop value due to specimen surface removal is investigated. Furthermore, we clarify that the radius of curvature of the leading edge of the fatigue crack decreases as t becomes thinner. Using the three-dimensional elastoplastic finite element method, the amount of plastic lateral contraction (depression depth d) at the crack tip after fatigue loading is calculated quantitatively. The following main experimental results are obtained: In the region where ΔK is 5 MPam1/2 or higher, the rate of fatigue crack growth da/dN at a constant ΔK value increases as t increases from 1 to 11 mm. The da/dN between t = 11 and 21 mm is the same. Meanwhile, in the region where ΔK is less than 5 MPam1/2, the effect of t on da/dN is not observed. The effects of t and ΔK on the da/dN–ΔK relationship are considered physically and quantitatively based on d

Effects of Fluid Flow Rate and Stress Amplitude on the Initiation and Growth Behavior of Corrosion Pits on an Annealed Carbon Steel

In order to clarify the effects of the fluid flow rate and stress amplitude on the initiation and growth behavior of corrosion pits, round, smooth specimens were rotated at various speeds in sodium chloride aqueous solution. The results are summarized as follows: (1) The pit growth behavior was adequately predicted by Eq. (1), regardless of the fluid flow rated and stress amplitudes. (2) The pit initiation time decreases with an increase in the fluid flow rate and stress amplitudes. (3) For small pits whose sizes are about 0.03mm, the pit growth rates increase with an increase in the fluid flow rate, while the growth rates for pits above 0.08mm are independent of the fluid flow rate and smaller than those of 0.03 mm. The pit growth rates in the case of 100 MPa are 5～10 times larger than those of the nonstressing condition