Takayoshi Nosho

Characteristics of surface roughening during plane strain compression of polycrystalline iron

Roughening behavior of the free surface of polycrystalline iron during plane strain compression is investigated experimentally. The changes in the shape of the free surface, which is roughened during plastic deformation, are observed in the three-dimensions. It is found that the mountains and the valleys of the roughened shape tend to elongate in the constrained direction for the specimen with isotropic grain shape. The shapes of the roughness curves in the loading direction and in the constrained direction are compared. The normalized height distribution of the roughness curve in the constrained direction is symmetric, while that of the roughness curve in the loading direction is asymmetric and positively skew during plane strain compression. Based on a simple simulation of the roughness curves by a random midpoint displacement method, this difference is supposed to be caused by the constraint of the material flow under plane strain condition.

Roughening of Contact Surface During Plane Strain Compressive Plastic Deformation of Polycrystalline Copper.

Changes in surface roughness and three-dimensional microscopic shapes of contact surface of polycrystalline copper during plane strain compressive plastic deformation are studied. The roughening of contact surface is measured with a stylus instrument. The microscopic change in surface shape is also observed with a scanning electron microscope. The roughness of contact surface measured in the free direction is larger than that measured in the constrained direction for the specimen having transverse contact surface. Meanwhile, the roughness of contact surface measured in the loading direction of the specimen having longitudinal contact surface is a little larger than that measured in the free direction. From the observation by a scanning electron microscope, it is seen that the mountains and the valleys which appear on the free surface are sharp, while those on the contact surface are relatively rounded.

Technology of Plasticity. Microscopic Plastic Deformation Behavior on Free Surface of Copper during Plane Strain Compression.

The free surface of polycrystalline metal becomes roughened in metal forming process due to its microscopic inhomogeneity. In the present study, two-dimensional surface roughness and three-dimensional microscopic shapes of the roughened surface of pure copper caused by compressive plastic deformation under a plane strain condition were investigated, and the characteristics of the microscopic deformation behavior during plane strain compression were clarified in comparison with those during uniaxial compression. It was found that the surface roughness increased almost in proportion to the applied strain, but the roughness measured in the compressive direction was larger than that in the transverse direction. This is because the mountains which appear on the free surface during plane strain compression tend to elongate in the transverse direction. The primary wavelength of the surface shape was also obtained by means of the maximum entropy method. The primary wavelength in the transverse direction was almost twenty times of the average grain size, and it decreased in correspondence with the applied strain. This implies that the inhomogeneity of deformation behavior of copper appears under the scale of twenty grains in the transverse direction.

Acceleration of Fatigue Crack Growth under Intermittent Overstressing. Effect of Mean Stress and Crack Closure.

Fatigue crack growth behaviour under intermittent overstresses of different mean stress levels was investigated in dry air with a low carbon steel. A very small number of cycles of overstress applied intermittently between a very large number of cycling of understress below threshold caused significant acceleration in crack growth rate amounting to about one hundred times as compared to the case of steady cyclic stress for all cases of mean stress level. The acceleration in the cases with tensile mean stress was slightly smaller than that in the cases without it, and it was related to a little higher crack closure level in the former case. However, correlation in the acceleration versus understress diagram was not improved appreciably by using effective stress intensity taking account of the crack closure. Stress history in precracking had some effect on the crack closure and, consequently, on the acceleration. The correlation in this case was appreciably improved by using the effective stress intensity. Difference was not recognized in fracture surface morphology among the cases with different mean stress levels, and so the micromechanism of crack growth acceleration would be the same throughout all cases of different mean stress. The acceleration under multi-level intermittent overstresses with mean stress could be predicted from the two-level intermittent overstress test data by linear summation in the same way as the case without mean stress.

Effects of environmemt on the acceleration of fatigue crack growth under periodic overstressing.

The acceleration of fatigue crack growth under periodic overstressing was studied with centernotched plate specimens of low-carbon steel under push-pull loading. Tests were carried out in moist air, dry air, nitrogen, and vacuum. A very small number of cycles of overstress applied intermittently during a very large number of cyclings of undersress below the theshold stress intensity range ΔKth caused a significant acceleration of crack growth in the range of relatively large understress ΔK1(ΔKth/ΔK1>0.5) in moist air, dry air, and nitrogen. In the region of relatively small understress, however, acceleration of carck growth in moist air was lesss than that in dry air and nitrogen. It was found by means of Auger spectroscopy that the maximum thickness of excess oxide within the cracks enhanced crack closure and reduced the effective stress intensity range of understress, (ΔK1)eff, resulting in less acceleration of crack growth in moist air. Accelaration of crack growth in vacuum, on the other hand, was lesss than that in other environments in all understress levels. Fractographic study suggested that rewelding in vacuum resulted in the less acceleration. There was no sequential effect of overload on the acceleration under two-level variable stress. The acceleration under multi-level variable stresses was predicted by a method using a linear cumulative concept on the basis of test data under simpler two-level variable stresses.

Surface roughening of Nylon 6 during plastic deformation.

Uniaxial tension tests were carried out using Nylon 6 which is a typical thermoplastic resin. The surface roughening during plastic deformation was investigated with a stylus instrument and the change in the microscopic surface profile was observed by a scanning electron microscope. The surface roughness increased in proportion to the applied plastic strain until the beginning of the necking of the specimen. After that the rate of increase of the surface roughness with increasing strain gradually slowed down. It was found that the degree of crystallinity was an important factor for the surface roughening during plastic deformation. The initial surface roughness of specimen also affected the surface roughness. The specimen thickness from 2mm to 6mm had little effect on surface roughening. The surface roughness in the loading direction was smaller and had larger wavelength than that in the transverse direction. Macro-fibrils, that is, fibrous structures were observed on the plastically deformed surface. The measured surface roughness was considered to consist of the irregularity between neighbouring macro-fibrils and the irregularity caused by the motion of group of macro-fibrils.

Acceleration of crack growth under intermittent overloading at elevated temperature.

Acceleration of crack growth under intermittent overloading was studied at 650°C by using two types of center notched plate specimens of different thickness made of a SUS 304 stainless steel. When the hold time of overload was very short (20 seconds), the crack growth rate was significantly accelerated to be about 20-50 times as large as that of static creep cracks, and the fracture surface morphology was different from that under static loading, indicating extremely ductile transgranular fracture by glide plane decohesion or microvoid coalescence. The acceleration in growth rate could be ascribed to recovery of the material during the low stress period. In the thinner plate specimens, the crack growth rate under intermittent loading was correlated well with modified J-integral J and agreed with the growth rate of static creep cracks in J-da/dt diagram. In the thicker plate specimens, however, the crack growth rate under intermittent loading did not agree with that of the thinner plate specimens in J-da/dt diagram, being about 1/5 times. Transgranular fatigue type crack growth appeared in the low growth rate region, and the growth rate was different from that of creep cracks.