Terutoshi Yakushiji

Fatigue dependence of residual magnetization in austenitic stainless steel plates

In order to estimate the amount of bending fatigue damage on austenitic stainless steel plates, we have investigated the relationship between plane bending stress and residual magnetization caused by martensitic structure in austenitic stainless steel plates. Magnetic flux density in the Z component caused by residual magnetization at 1 mm above a specimen is measured by using a high sensitivity fluxgate magnetic sensor. It can detect a very small magnetic flux density. From our experiments, the magnetic flux density at 1 mm above a specimen caused by residual magnetization has clear dependences on the increase of bending stress and the number of stress cycles. In this paper, the relationship between bending stress and residual magnetization in austenitic stainless steel plates is shown and the bending fatigue dependence of residual magnetization on them is discussed.

Fatigue Evaluation for a Ferritic Stainless Steel (SUS430) by the Eddy Current Method Using the Pancake-Type Coil

To prevent accidents caused by the accumulation of fatigue damage, it is important to evaluate fatigue damage in stainless steel, a widely used structural material. SUS430 (ANSI430) is a ferritic stainless steel. It is well known that the electromagnetic properties of SUS430 such as permeability and resistivity, change due to fatigue damage. The eddy current method can detect change in the electromagnetic properties of SUS430. In this research, the eddy current method using the pancake-type coil was used to find fatigue damage in SUS430. The pancake-type coil was put on a specimen made of SUS430, and excited with alternating current. When a pancake-type coil and a specimen are adjacent, the change in permeability and resistivity of the specimen due to fatigue influences coil constants such as inductance L and phase angle ¿ of impedance. In our experiment, L and ¿ clearly depended on an increase in the bending stress and the number of stress cycles. It can, therefore, be said that L and ¿ are clearly influenced by fatigue damage. In this paper, the results of investigating the relationships between the amount of fatigue damage, bending stress, L, and ¿ are reported.

NON‐DESTRUCTIVE EVALUATION OF FATIGUE DAMAGE FOR SUS316 BY USING ELECTROMAGNETIC METHODS

There are some fatigue damage estimation methods for an austenitic stainless steel that uses martensitic transformation. For instance, those are the remanent magnetization method, the excitation method using the differential pick‐up coil, and so on. We are researching also those two methods in our laboratory now. In the remanent magnetization method, it is well known that the relationship between fatigue damage and the remanent magnetization is simple, clear, and reproducible. In addition, the excitation method can be easily used at the job site because the special magnetizer is unnecessary. But, these methods have some disadvantages shown as follows. For instance, the former needs a special magnetizer and the latter’s output signal is small. On the other hand, it is well known that the inductance of a pancake type coil put on the metallic specimen changes according to the electromagnetic properties of the metallic specimen. In this paper, the assessment method of fatigue of an austenitic stainless steel (SUS316) that uses the change by fatigue of the inductance of the pancake type coil measured with the LCR meter is shown. In addition, the fatigue evaluation performance of this method is described.There are some fatigue damage estimation methods for an austenitic stainless steel that uses martensitic transformation. For instance, those are the remanent magnetization method, the excitation method using the differential pick‐up coil, and so on. We are researching also those two methods in our laboratory now. In the remanent magnetization method, it is well known that the relationship between fatigue damage and the remanent magnetization is simple, clear, and reproducible. In addition, the excitation method can be easily used at the job site because the special magnetizer is unnecessary. But, these methods have some disadvantages shown as follows. For instance, the former needs a special magnetizer and the latter’s output signal is small. On the other hand, it is well known that the inductance of a pancake type coil put on the metallic specimen changes according to the electromagnetic properties of the metallic specimen. In this paper, the assessment method of fatigue of an austenitic stainless steel ...

ESTIMATION OF FATIGUE DAMAGE FOR AN AUSTENITIC STAINLESS STEEL (SUS304) USING A PANCAKE TYPE COIL

There are some fatigue damage estimation methods of an austenitic stainless steel that uses martensitic transformation. For instance, those are the remanent magnetization method, the excitation method, and so on. Those two methods are researched also in our laboratory now. In the remanent magnetization method, it is well known that the relationship between fatigue damage and the remanent magnetization is simple, clear, and reproducible. However, this method has the disadvantage to need a special magnetizer. Then, this method cannot be easily used at the job site such as the factory. On the other hand, as the special magnetizer is unnecessary, the excitation method can be easily used at the job site. But, this method has some disadvantages shown as follows. For instance, the output signal of this method is small. And the surface state of the specimen strongly influences the noise component of the output signal. It is well known that the inductance of a pancake type coil put on the metallic specimen changes...

FATIGUE DAMAGE OF ULTRAFINE GRAINED OXYGEN-FREE COPPER

In the present study, using an oxygen-free copper with 300 nm grain size processed by ECAP, rotating bending fatigue tests of smooth specimens were carried out. Successive observations of surface were carried out to clarify the fatigue mechanism. Experimental results showed that the fatigue life at a high stress amplitude was dramatically increased, however a negligible increase in the fatigue limit stress at 107 cycles was observed. This phenomenon was related to the large drop in surface hardness in the long-life field in excess of 107 cycles. There were good correlation between the hardness drop trend and the formation of surface damage. The physical basis of drop in surface hardness was discussed.

The Evaluation of Fatigue Caused by Plane-Bending Stress on Stainless Steel Using the Stacked-Coil Type Magnetic Sensor

To prevent an accident due to the metal degradation of stainless steels, we have previously proposed fatigue evaluation methods (such as the remnant magnetization method using a thin-film flux-gate magnetic sensor [1] and the inductance method using a pan-cake type coil [2]). These two fatigue evaluation methods demonstrated a good correlation between the magnetic sensor output signal and the amount of plane-bending fatigue damage in stainless steels. We developed a stacked-coil type magnetic sensor shown in Fig. 1(a) in order to achieve a magnetic sensor for an accurate fatigue evaluation. This magnetic sensor was composed of two detection coils that are connected differentially, an excitation coil, and a ferrite core. Fig. 1(b) shows the connection of the excitation coil and the two detection coils. Fig. 2 shows the detection result of fatigue and crack using this magnetic sensor. The material used for this specimen was an austenitic stainless steel (SUS304), and plane-bending stress was applied. From Fig. 2, it can be seen that this magnetic sensor detected defects well. The evaluation results of plane-bending fatigue damage distribution will be shown in in detail the complete paper.

Relationship between Grain Growth and Formation of Fracture Surface of Ultrafine Grained Cu in High-Cycle Fatigue

Fatigue tests were conducted on specimens of ultrafine grained copper produced by equal channel angular pressing. The growth behavior of a fatal crack was monitored successively. The morphological features of the crack growth paths and fracture surfaces were influenced by variation of grain size and damaged areas of the crack tip, depending on the stage of progress of the fatigue damage. The change in fracture surface was discussed by considering the interrelation between the reversible plastic zone size at the crack tip and the microstructure evolved during cyclic stressing.

Fatigue-induced Grain Growth And Formation Of Slip Bands In Cu Processed By Equal Channel Angular Pressing

In order to elucidate the effect of the microstructure on fatigue behavior of ultrafine grained copper, fatigue tests were conducted using copper processed by equal channel angular pressing through 4, 8 and 12 pass numbers. The evolution of microstructure and surface damage during cyclic stressing was monitored. For samples processed by 8 and 12 passes, dynamically recrystallized grains were formed and these grew with subsequent stressing. Slip bands were initiated inside such grains. For samples processed by 4 passes, the formation of slip bands was likely to occur inside dynamically recovered coarse grains. The physical basis on the phenomena leading to the initiation of fatigue cracks was discussed from the viewpoint of microstructure and surface damage evolution.

High-Cycle Fatigue Damage of Oxygen-Free and Deoxidized-Phosphorous Copper Processed by Equal Channel Angular Pressing

Two Cu samples: oxygen-free copper (99.99 wt% Cu) and deoxidized low-phosphorous copper (99.9 wt% Cu) were processed by equal channel angular pressing (ECAP). After the ECAP processing using 4 passes, equiaxed grains (~300 nm grain size) and elongated grains were formed for both samples. Fatigue strength of ultrafine grained Cu was enhanced by the addition of trace impurities. The formation behavior of surface damage and the change in surface hardness during stressing were monitored. A close relationship was observed between the change in hardness and the formation behavior of damage. The effect of trace impurities on the fatigue damage was discussed from the viewpoints of the grain coarsening and the crack initiation/growth behaviors.

FATIGUE DAMAGE FORMATION PROCESS OF ULTRAFINE GRAINED OXYGEN-FREE AND DEOXIDIZED LOW-PHOSPHOROUS COPPER

In order to study the fatigue damage formation process of ultrafine grained copper, fatigue tests were carried out for two samples: oxygen-free copper and deoxidized low-phosphorous copper. After the processing by equal channel angular pressing (ECAP) using eight passes, equiaxed grains with an average size of 300 nm were formed for both the samples. The formation behavior of surface damage was monitored. The effects of trace impurities on surface damage formation behavior were discussed from the viewpoints of change in surface morphological feature during stressing.