Kiyoshi Taniuchi

An optical interferometric band as an indicator of plastic deformation front

The Luders’ front and a previously discovered optical interferometric band structure were observed simultaneously in steel specimens under tensile loading. The observed Luders’ front and optical band structure show the same propagation characteristics, confirming our previous interpretation that the optical band structure represents the plastic deformation front. Analysis shows that the stress at which the optical band structure begins to appear is approximately 10% lower than the corresponding Luders’ front, indicating that the optical band structure reveals the plastic deformation front with higher sensitivity than the Luders’ front.

Detection of Crack Initiation by Observations of Free Surface Condition

A simple method to detect crack initiation is presented. By polishing the surface of carbon steel structure properly and using the Luders bands and orange peel as indicators for damage, it is possible to predict crack initiation with the naked eye. The initiation of the Luders bands is the yellow light of the crack initiation, for instance, in the traffic signal, and orange peel is the red light. The relationship between the observed surface condition and the cyclic stress-strain characteristics is discussed.

Yield Strength in Relation to Cyclic Loading

The yielding phenomenon under cyclic loading has been investigated for low-carbon steel. Under various loading ratios, the surface state of the specimen was observed with the naked eye, using the Luders band as an indicator for the yield point elongation. The stress-strain curve was measured simultaneously. The results show that both the surface states and the cyclic stress-strain characteristics vary depending on the load ratio. This makes it questionable to use the yield strength evaluated under monotonic loading to describe the mechanical properties of the specimen material for fatigue tests, though it is a prevailing practice recommended by ASTM. The result of this study also indicates that the Luders band can be used as a smart sensor to investigate the yield phenomenon.

Observation of Optical Interferometric Band Structure Representing Plastic Deformation Front under Cyclic Loading

An interferometric band structure previously observed under monotonic tensile loading has been observed under cyclic loading as well. Like the previous monotonic loading case, the band structure is found to represent the plastic deformation front, indicating that it can be used for visualization of stress concentration. Observed correlation between the motions of the plastic deformation front and the test machines crosshead indicates that the plastic deformation front moves in proportion to the strength of the displacement field induced by the test machine. This is consistent with a previous report that the propagation speed of the plastic deformation front varies in proportion to the monotonic loading speed.

Prediction of crack generation

The following propounds a simple way to predict the appearance of cracks. When plane carbon steel reaches the yield point, Lueders line appear. When stress exceeds yield stress the Lueders lines change into an orange peel texture. These phenomena are in all cases visible. Therefore it is possible to use the visible change in the appearance of the Lueders lines that accompanies the increase in loading as a simple device to predict the appearance of cracks. Since the visible surface conditions are used as the predictor, it is unnecessary to use any special instrument. All one has to do is to make the surface that is to be observed flat and smooth. It is an extremely simple technique. Observation was made of the change of state on the flat, smooth surface of a basic material undergoing cyclic loading. Investigation was made of the relation between Stretcher Strains, or the orange peel texture, and the S-N diagram. It was possible to apprehend the process by which the state of the flat, smooth surface changed from the moment that the Stretcher Strains developed until the appearance of cracks. The utilization of the results of this experiment render a concrete example of the possibility of predicting the appearance of cracks by visual observation of stretcher strains. One may consider the use of Lueders lines or the orange peel texture to be a reliable method for experimental stress analysis.

Detection of fatigue damage zone by visualization

Concerning the appearance of fine ripples on the surface of members and structures in which fatigue damage is produced, investigations from a variety of points of view have been made. Each of these investigations were made with the aid of various types of measurement apparatus. In this study, surface damage of the members and structures in the fatigue process was examined using direct naked-eye observation as the primary investigate measure. This type of examination has enable d the authors to better understand the state of the surface corresponding to the change of metallographic structure in the materials fatigue process. Thus, the authors have successfully gathered data that will be useful in predicting fatigue failure from a nondestructive evaluation.