Tsuneshichi Tanaka

Crystallographic Study of the Fatigue Crack Nucleation Mechanism in Pure Iron

The fundamental question posed in this study is, What crystallographic conditions are satisfied at fatigue crack nucleation sites in polycrystalline iron? To answer this question it is vital to establish an effective technique to determine the crystallographic orientation of each grain under observation. This problem was solved by developing an etch pit method of grain orientation analysis. Stereographic projection geometry was successfully applied in determining the orientation of each grain with great accuracy from the measured angles between ridge lines of sharp-edged etch pit patterns on the film. The critical experiment, then, is to examine the geometrical relationship between the nucleated fine cracks and the orientations of surrounding grains on fatigued specimen surfaces. The final step is to extract such crystallographic conditions as are commonly satisfied at the sites of crack nucleation. Electrolytically polished commercial base pure iron plate specimens were tested under completely reversed plane bending stress in the high-cycle fatigue range. Tests were interrupted at about half of the fracture life, and the crystallographic configuration of microcracks with respect to the grain orientation was thoroughly examined by making use of the newly developed technique. Cracks were observed to nucleate only along grain boundaries on the specimen surface. And it was found that there are five conditions satisfied at those cracked sites. These conditions concern the favorable configuration of slip systems, Schmid factor, grain size, and the direction of grain boundaries. Three more conditions were added from information pertaining to the crystallographic structure below the surface gained by observation after removal of the surface layer and of cross sections containing a crack. A brief discussion is also given on some statistical aspects of important parameters.

Creep Rupture Map of Engineering Fine Ceramics

The main objective of this study is to clear the strength and strain behaviors of engineering fine ceramics in long term high temperature creep. For this purpose, a series of creep tests was carried out on a silicon nitride and a silicon carbide at high temperature levels up to 1723K and during long term up to 1000 hours by using a newly developed high temperature tensile creep testing machine. The results indicate that the relationship of applied stress vs. rupture time of both ceramics at each temperature level can be approximated by a straight line on full log paper with a little scatter. Then, discussion on the creep behaviors of the silicon nitride from the viewpoint of creep strain revealed that the minimum creep strain rate well governed the rupture time, and the creep rupture maps constructed from two different viewpoints indicated that the tertiary creep stage was a coalescence stage of multi-site microcracks.

Fatigue crack growth and microscopic crack opening behaviour under impact fatigue load

Abstract The fatigue crack growth rates and crack opening behaviours were observed under impact and non-impact fatigue loads for high strength, low alloy HT-60 steel and austenitic SUS316 stainless steel, which had almost the same ultimate tensile strength of about 600 MPa. Particular attention was paid to the microscopic crack opening behaviour near the crack tip under two load conditions, evaluated using a fine grid method. The results showed no difference between the crack growth rates under impact or non-impact fatigue loads in the case of HT-60 steel, but in the case of SUS316 steel the crack growth rate under impact fatigue was much higher than that under non-impact fatigue. Such a difference between the two steels could be correlated with the microscopic crack opening behaviour under two load conditions; in the case of SUS316 steel, both the effective stress intensity range at the crack tip and the effective crack opening displacement behind the crack tip were larger for the crack formed by impact fatigue load than for the crack formed by non-impact load, whereas in the case of HT-60 steel, the difference between the values under impact and non-impact fatigue loads was significantly smaller.

Fatigue Behavior of Solder Joint under Simple Shear Stress.

In order to clarify the simple shear strength of solder joint, we carried out static strength tests, fatigue tests, fatigue crack growth tests and creep crack growth tests on Cu/60Sn·40Pb/Cu joint under simple shear stress. Major conclusions obtained in this study are summarized as follows : 1) The maximum simple shear stress increased with the increase in the simple shear strain rate. 2) The effect of the width of the solder on the fatigue strength were not observed. 3) The fatigue cyclic crack growth rates were governed by neither the stress intensity factor range nor the maximum stress intensity factor. 4) The fatigue and creep loading time crack growth rates were governed by the modified J integral. 5) The fatigue and creep crack growth behaviors depended on the process zone (small scale creep zone) at the crack tip.

Fatigue Crack Growth Behavior of Alumina Short Fiber Reinforced Aluminum Alloy at Room and Higher Temperatures.

In order to clarify the fatigue crack growth mechanism of an alumina short fiber reinforced aluminum alloy (FRM), fatigue crack growth tests were carried out at several stress ratios on the center cracked tension (CCT) specimens at room and elevated temperatures. Aluminum alloy, A6061-T6 was also used as a reference material. Comparison of fatigue crack growth rate curves determined for A6061-T6 and FRM specimens indicated that the effect of temperature on the crack growth rate curves was much smaller for FRM than that for A6061, and the crack growth property of FRM was superior to A6061 in the region of low crack growth rate including the threshold, when the comparison was made on Kmax and ΔK. By estimating the values of ΔKeff, it was shown that the fatigue crack growth rates of A6061 and FRM at each temperature were governed by ΔKeff and KmaxαΔKeff(1-α), respectively. Moreover, the detailed evaluation indicated that the fatigue crack growth rates of A6061 and FRM at different stress ratios and temperatures could be expressed by introducing new parameters, excepting in the threshold region, as follows:da/dN=C1{UR(1-R)/UR=0.1Kmax(Eσ0.2)-1/2}m1 for A6061da/dN=C2{{UR(1-R)/UR=0.1}(1-α)Kmax(Eσ0.2)-1/2}m2 for FRMwhere R is the stress ratio, UR is the crack opening ratio at stress ratio R, UR=0.1 is the value of U at R=0.1, E and σ0.2 are Youngs modulus and 0.2% proof stress at each temperature, and C1, C2, m1 and m2 are material constants.

Transient crack growth behaviors under two-step varying loads

Transient crack growth behaviors were observed on aluminum alloy A2017-T3 under two-step varying amplitude loads. The prediction of the crack growth rates by the linear cumulative rule is valid only in limited cases. Retardation of crack growth occurs at the lower level of the two-step load patterns, due to the deceleration and the subsequent acceleration of the growth rate. The equations governing these processes are formulated as functions of the stress intensity factors at high and low load levels. The effects of strain hardening and aging treatment on the transient behaviors are discussed, and a reasonable crack growth model is proposed.

Crack growth behavior under repeated impact load conditions

Fatigue behavior of metallic materials under repeated impact load conditions differs from that under ordinary load conditions; fatigue strength in impact fatigue is lower than that in non-impact fatigue except for the case of torsional impact fatigue, and most of metallic materials show higher crack growth rate in impact fatigue, especially in high K region, compared with that in non-impact fatigue. The characteristics of impact fatigue behaviors are influenced by impact stress pattern. In order to have fundamental understanding for impact fatigue properties of metallic materials, it is necessary to evaluate fatigue data under simple impact load pattern. Impact fatigue crack growth behavior is described in detail from several points of view, and the principle of impact fatigue testing machine used in this study to obtain fatigue data under simple impact stress pattern and a brief summary of impact fatigue strength characteristics of metallic materials are also given.

Fatigue Behavior of Alumina Short Fiber Reinforced Aluminum Alloy at Room and Higher Temperatures.

As a first step in the study on the fatigue mechanism of fiber and/or particle reinforced aluminum alloys, we carried out, in this study, a series of fatigue tests and fatigue crack growth tests on an alumina short fiber reinforced aluminum alloy in push-pull load conditions. The results indicated that the fatigue failure of this fiber reinforced aluminum alloy was governed by two main mechanisms: One was a debonding of the fiber and the matrix induced by the localized plastic strain, depending on the mismatch of elastic constants, and another was the failure of the matrix induced by the accumulation of fatigue damage around the bond interface. In the higher applied stress range, and at room and relatively lower temperatures, the former mechanism is predominant, but in the lower stress range, and consequently in longer life region, the latter mechanism plays a main role and this trend becomes more dominant with each increase in temperature.

STRENGTH AND STRAIN BEHAVIORS OF ENGINEERING FINE CERAMICS IN HIGH TEMPERATURE CREEP

ABSTRACT By using a newly developed high temperature tensile creep testing machine, a series of high temperature and long-term creep tests was conducted on a sintered silicon nitride and a sintered silicon carbide. The creep rupture curves of both ceramics showed straight line relationship at respective temperature levels, and the scatter of rupture lives could be negligible when compared with that of static and cyclic fatigue lives. The sintered silicon nitride showed 3-stage creep deformation process analogous to that of metallic materials, and the minimum creep rate well governed the rupture time of this ceramic.

Impact fatigue characteristic of martensitic stainless steel and the relationship between the impactive fatigue crack growth behavior and the dynamic stress-strain response of marerial.

It has already been indicated, as a general trend in impact fatigue, that the impact fatigue strength is lower than the ordinary fatigue strength, and that the crack growth rate under impact fatigue is higher than that under non-impact fatigue, excepting a few materials with relatively high static strength. The first aim of this study is to clarify the impact fatigue behaviors of high-strength alloy steels with the ultimate tensile strength of about 1800 MPa, and the second is to evaluate the crack growth behavior under impact fatigue load in connection with the dynamic stress-strain response. For these purposes, a series of impact fatigue tests were first carried out on martensitic stainless steel, and then single impact tests were performed to obtain dynamic stress-strain relation-ships of the martensitic stainless steel and the other three steels whose crack growth behavior in impact fatigue were already obtained by the authors. The S-N property of the martensitic stainless steels is similar to the other steels but the fatigue crack growth rate under impact fatigue load is equivalent to that under ordinary fatigue load. An interesting finding is that the crack growth behavior of each material under impact fatigue load is well correlated with the dynamic stress-strain response of that material.