Hiroshi Misawa

X-Ray Stress Measurement Method for Single Crystal with Unknown Stress-Free Lattice Parameter

In this study, a new principle, which can determine the stress states in a single crystal even if the lattice spacing of d0 in the stress-free condition is unknown, was proposed. In this principle, the stress states in the plane stress condition can be determined by considering not only the stress components but also the stress-free diffraction angle of 2θ0 as constant values in multiple regression analysis. The stress states calculated by this principle is not affected by the slight change of the predicted diffraction angle 2θ0 which occurs due to the setting error of the specimen. Therefore, this principle can determine the plane stress condition without the effect of the reliability of d0. High reliability of this principle was verified by carrying out the applied stress measurements of silicon single crystal.

Research of Strength and Fracture Micromechanism for Cold Rolled Steel Foil of Semi-austenitic Precipitation Hardening Stainless Steel SUS631. Fatigue Behaviour and It's Fractography on Fracture Surface.

In order to clarify the fatigue behavior and fracture micromechanism for cold rolled steel foil of semi-austenitic precipitation hardening stainless steel SUS631 so as to evaluate its fatigue characteristics, fatigue tests and hardness measurements before and after the fatigue tests were carried out on the stainless steel foil of 0.05mm in thickness for as-received material and two kinds of ageing treated materials. The fracture surface of these foil specimens was observed by Scanning Electron Microscopy (SEM). The main results obtained are summarized as follows. (1) The fatigue strength of as-received material was improved by ageing hardening treatment, the fatigue limit of the material ageing treated at 753 K for 1 hour was larger than that at 703 K for 1 hour. (2) After fatigue testing, the hardness of as-received material was increased, but the hardness of ageing hardening treated materials was decreased. The variations in the hardness before and after fatigue tests for these materials indicated that as-received material showed cyclic hardening behavior, and ageing hardening treated materials showed cyclic softening behavior. (3) From these observed results of the fracture surface and the cross section of fatigue cracks, there were distinctions in the crack growth path, failure pattern and fracture micromechanism on these materials, furthermore, the fatigue behavior of the foil material was very different from that of a bulk material.

Quantitative Analysis by X-Ray Fractography of Fatigue Fractured Surface under Variable Amplitude Loading.

X-ray fractography is a method of analysing the causes of accidental fracture of machine components or structures. Almost all of the previous research on this problem has been carried out using constant amplitude fatigue tests. However, the actual loads on components and structures are usually of variable amplitudes.In this study, X-ray fractography was applied to fatigue fractured surfaces produced by variable amplitude loading. Fatigue tests were carried out on Ni-Cr-Mo steel CT specimens under the conditions of repeated, two-step and multiple-step loading. Residual stresses were measured on the fatigue fractured surface by an X-ray diffraction method. The relationships between residual stress and stress intensity factor or crack propagation rate were studied. They were discussed in terms of the quantitative expressions under constant amplitude loading, proposed by the authors in previous papers.The main results obtained were as follows:(1) It was possible to estimate the crack propagation rate of the fatigue fractured surface under variable amplitude loading by using the relationship between residual stress and stress intensity factor under constant amplitude loading.(2) The compressive residual stress components on the fatigue fractured surface correspond with cyclic softening of the material rather than with compressive plastic deformation at the crack tip.

Effect of Stress Level of Gust Cycles on Fatigue Crack Propagation Behavior. Acceleration and Retardation of Crack Propagation Under Simplified Flight Simulation Loading.

Fatigue tests were carried out on 2024-T3 aluminum alloy sheet material under 11 kinds of flight simulation loadings and its crack propagation behavior was observed. In this experiment, flight simulation loadings were made to simplify a complex stress history so that they consisted of the ground to air cycle of large stress range and gust cycles of relatively small stress range.The results obtained were as follows:(1) When the maximum stress of the ground to air cycle was equal to the maximum stress of gust cycles, the crack propagation rate under flight simulation loading was faster than that estimated by the linear damage law on the basis of crack propagation of constant stress amplitudes.(2) When the maximum stress of gust cycles was 40MPa or 80MPa lower than that of the ground to air cycle, the crack propagation rate in a short crack length was almost the same as the estimated value. With increasing crack length, the crack propagation clearly showed the retardation behavior. As the crack length increased further and approached to the critical value of final fracture, the crack propagation rate gradually recovered to the estimated values.(3) When the maximum stress of gust cycles was 120MPa or 160MPa lower than that of the ground to air cycle, the crack propagation rate under the flight simulation was nearly equal to the estimated value and the retardation behavior could not be observed in the whole range of crack propagation.(4) The retardation behavior of crack propagation was not observed just after changing the stress of the ground to air cycle to the gust stress, but the crack propagation clearly showed the retardation behavior with increasing the number of gust cycles in the same flight.

Compressive Residual Stress on Fatigue Fractured Surface

X-ray fractography is a technique for analysing the cause and mechanism of fracture from the information obtained by X-ray irradiation on the fractured surface. It has been shown that a good correlation exists between the residual stress or the half value breadth of diffraction profile and the stress intensity factor that had caused the fracture. X-ray fractography has been successfully applied for the in-service failure of many types of fracture. However, in some cases the residual stresses on the fatigue fractured surface in service are compressive, which have not been found in the laboratory experiments so far. In the present study, fatigue experiments were carried out on 0.5% carbon steel to investigate the stress condition that produces compressive residual stress on the fractured surface. The specimen was a centre notched rectangular plate of 8mm thick, and a wide range of stress ratio R= o min/ o max was applied from tensile to compressive, namely R=0.50, 0.25, 0.20, 0.00, -1.67,-2.33, -2.40, and -3.Q0. From the results of experiments, it was found that, when the stress ratio was -3.00 and the minimum stress was -150MPa, the residual stress on the fractured surface became compressive. Since the minimum stress was far smaller than the compressive yield stress, the cause of the compressive residual stress was considered to be the result of crack closure. In this case, the crack opening ratio U=(σmax-σop)/Λσ, where σ op is the crack opening stress, was about 0.3 and almost constant.

A .DELTA.Kth testing method based on the plastic zone size at the fatigue crack tip.

き裂進展下限界条件値ΔKthを迅速に求めるために,き裂先端塑性域寸法に着目したΔK漸減法を提案した.SNCM 439,HT 55 およびA 5083-O を用いたこの方法によるda/dN-ΔK関係とΔKth値はASTM基準による結果とよく一致した.荷重が緩やかに減少すれば,き裂開口点Kopは前履歴の影響をあまり受けないことがこの理由である.荷重減少率に注意すれば,き裂増分は前荷重の塑性域寸法程度でよく,この方法により迅速なΔKthが得られる.

Fatigue crack propagation behavior under stress changing in SCM 435 steel.

The constant and variable stress amplitude fatigue tests were carried out on smooth and pinholed specimens of SCM 435 steel under rotating bending conditions.In the constant stress amplitude tests, the fatigue crack propagation rate was not affected by the stress level and it was represented well by ΔK.The crack initiated by the over-stress σa1 cycles was retarded by the under-stress σa2 cycles, when the difference between σa1 and σa2 was large. In the case of non-propagating cracks, their length was shorter than that in plain carbon steels. The condition of crack retardation under σa1 repetition following σa2 application was elucidated.The crack propagation behavior of pin-holed specimens in two step multi-fold tests was similar to that in two step two-or three-fold tests. The discussion was made on the shift from unity in cumulative cycle ratio for smooth specimens, based on the results of crack propagation behavior of pin-holed specimens.

Effects of the Stress Amplitudes Below the Fatigue Limit on Fatigue Lives in the Cylindrical Specimens Having a V-shape Groove

The effects of the stress amplitudes below the fatigue limit on the fatigue lives were investigated by the two steps multi-fold program fatigue tests in which the higher stress was above the fatigue limit and the lower stress was below the fatigue limit. Cylindrical specimens having V-shape groove of the structural carbon steel JIS S35C were used, and the stress concentration factors α of the specimens were selected as 2, 4 and 6. The test results revealed that the cumulative cycle ratios of the program fatigue tests were from 0.2 to 0.6 when α was 2, from 0.8 to 1.2 when α was 4, and from 0.9 to 1.4 when α was 6. The following behavior was also clarified : the fatigue crack was propagated even by the stress amplitude below the fatigue limit and in most cases the crack propagation was stopped as the number of cycles increased. Some discussions were made on the experimental results.