Kin-ichi Nagai

Stress-Strain Response and Fatigue Damage of Mild Steel Plate under Variable Stress Amplitude

In this study, seven types of load controlled fatigue tests were carried out under variable stress amplitude in order to clarify the stress-strain response and the fatigue strength of SS 41 and STPY 41 plate specimens. The tests were conducted in constant stress amplitude loading, step stress amplitude loading, repeated high-to-low and low-to-high block loading, two types of block-random loading and random loading with exponential distribution. In each test, stress-strain loops were recorded with X-Y plotter from the start of cycling to visual crack initiation by using the strain gauges put them on the test piece.As a result, the following conclusions were obtained.(1) Sa-epa (stress amplitude-plastic strain amplitude) curves are different with each variable loading patterns.(2) Cyclic softening for random loading is greater than that for constant stress amplitude in the conditions of Sa≥σy and Sa≤Saw, while the relation is contrary to above in the conditions of Saw<Sa<σy, where Saw is a fatigue limit and σy is a static yield stress of the material.(3) Sa-epa curve for random loading with exponential distribution can be obtained approximately by the step stress amplitude loading.(4) Fatigue life estimations by Miners rule and Modified Miners rule make over estimation compared with experimental results for block, block-random and random loadings.(5) Fatigue life estimations by Manson-Coffins law do not coincide with the experimental results for block-random loading and random loading with exponential distribution included the large repeated number of stress amplitudes below fatigue limit.(6) Fatigue life estimations by Corten-Dolans method fairly coincide with experimental results for various kind of variable stress amplitude loading.

Effects of Cycle Frequency, Mean Stress, Temperature and Cathodic Protection on Fatigue Crack Growth in 3% Salt Water

In this report, the effects of cycle frequency, mean stress, temperature of corrosive and cathodic protection on the fatigue crack growth of mild steel are examined by using the center-notched specimens under more extensive test conditions of corrosion fatigue than those in the previous report.The results obtained are summarized as follows ;(1) The correlation between the range of stress intensity factor ΔK and the crack growth rate da/dN for mild steel in 3% salt water could be given by the powers law shown in the following equationda/dN=C (ΔK) 3.5irrespective of the test conditions of corrosion fatigue such as cycle frequency f, mean stress and temperature of corrosive.(2) The coefficient C in above equation could be given by the following equationC=-A log f+B, where the coefficients A and B are material constants depending on the intensity and temperature of corrosive.(3) The crack growth rate under pulsating tension in salt water is unaffected by mean stress, and dependent on stress amplitude and cycle frequency.(4) The cathodic protection is scarcely valid for retarding the crack growth in salt water.

Static Tensile Strength and Deformation Behaviors of Wide Specimens of Composite Weldment

The aim of the application of high strength steel to mammoth structures is to maximize their allowable stress in designing and at the same time reduce their weight as a whole by the use of high strength steel in critical parts of the structures. In accordance with the existing concepts of elastic design, high strength steel has been generally used for the members subject to higher stress or inducing a stress concentration in the structures built up by welding together high strength steel and mild steel. However, the strain behaviors should be considered as one of the most important factors in the application of high strength steel to the structures, because the high strength steel is more sensitive to the strain concentration than mild steel isl). In the previous report2), the plastic deformation behaviors of the smaller size of composite weldment, in which high strength steel and mild steel are symmetrically arranged about the longitudinal axis, have been investigated under static tension parallel to the weld line. There are still problems of the similarity of the strain behaviors of the larger size of composite weldment to those of the smaller one. In this research, mechanical behaviors of the wide specimens having the heterogeneity are investigated in various combinations of the size of dissimilar steels with different strength. It is concluded as a result that ultimate tensile strength and yield strength of the wide specimens of composite weldment can be estimated by the strength of each steel and the hybrid ratio of high strength steel, and that their equivalent uniform elongation increases with the rate of mild steel to high strength steel.