Shotaro Kodama

Crack propagation behaviour in fretting fatigue

Abstract Fretting fatigue and normal, or unfretting, fatigue tests of a stainless steel SUS304L and an aluminium alloy A2024-T3 were carried out to investigate the effects of the contact pressure and the stress ratio on the crack propagation behaviour. The crack propagation behaviour was represented by the crack propagation rate da/dNversus the crack length a or the stress intensity factors ΔKeff and Kmax In fretting fatigue, crack propagation was divided into two stages, namely SI and SII. The value of da/dN in the SI stage was very high, even under a stress intensity factor less than the threshold for normal fatigue, and decreased gradually with crack growth because of crack closure and the decreasing fretting effect. The decrease in da/dN was marked in the case of high contact pressure and low stress ratio such as when R = −0.33, where R denotes the minimum stress divided by the maximum stress. During fretting fatigue crack closure occurred at an oblique short crack in the early stages of crack propagation in both the SUS304L steel and the A2024-T3 alloy; it also occurred at the oblique cracked surface of the shear lips formed in the A2024-T3 alloy during crack growth. However, in the SII stage, which followed the SI stage, da/dN increased with crack growth as for normal fatigue.

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.

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.

Fretting-fatigue crack propagation behavior of A2024-T3 aluminum alloy.

The crack propagation behavior in fretting-fatigue of A2024-T3 strip-typed specimens was investigated under the repeated stress of 60MPa with three different stress ratios, namely R=0.33, 0, and -0.33, and the contact pressure of 50MPa. The crack length a, the effective stress intensity factor range ΔKeff proposed by J. Schijve, and the maximum stress intensity factor Kmax were plotted as a function of crack propagation rate da/dN. It was seen that the crack propagation behavior in fretting-fatigue was affected by the stress ratio, and was able to be divided into two stages. In ‘SI-stage’ where the crack length is up to about 0.5mm, the da/dN in fretting-fatigue was very high compared with that in unfretting-fatigue, but with crack growth the da/dN in fretting-fatigue decreased markedly in the order of decreasing R. In ‘SII-stage’ the da/dN in fretting-fatigue increased with crack growth, and was higher than that in unfretting-fatigue for R=0.33, lower for R=-0.33, and almost the same for R=0. This behavior was interpreted by the reduction of fretting effect with crack growth and by the crack surface contact due to slantwise crack characterized in fretting-fatigue and shear lips characterized in A2024-T3.

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.