Study on a Parameter for Correspondence Between Notched and Unnotched Specimen Fatigue Data (III) : Conception and Formulation of Criteria on Fatigue Strength and Fatigue Crack Propagation Rates in Notched Specimens Based on the Equivalent Stress Ratio

In the present study, a creative consideration is developed for the problem of the correspondence between notched and unnotched specimen’s fatigue data. The proposed concept is essentially different from that of volumetric damage, and it has the great advantage of being avoidable to encounter the fatally invalid point of the volumetric approach, such as determination of damage-volume sizes. Moreover, the fatigue criteria derived are very easy to be applied to practical notch problems. The approach is developed as follows; (1) A hypothesis of plastic adaptation in a surface layer is proposed and mechanical models reflecting the hypothesis are constructed. (2) From these models, equivalent stress ratios REQ are derived as the new parameter for the correspondence between the cyclic stress conditions of the notched and unnotched specimens. REQ is generalized for proportional multiaxial cyclic loading through the consideration for the case of torsional cyclic loading. Moreover, REQ is derived from the mechanical model for the stress field of the mode I fatigue crack tip. (3) Experimental data on fatigue strength and fatigue crack propagation rates are rearranged on the basis of REQ and are formulated. (4) Practical criteria are proposed for the fatigue strength and the fatigue crack propagation rates.

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Fatigue Strength Assessment of a Structure Considering Corrosion Wastage and Corrosion Fatigue

Volume 3: Structures, Safety, and Reliability ◽

10.1115/omae2018-78188 ◽

2018 ◽

Author(s):

Norio Yamamoto ◽

Tomohiro Sugimoto ◽

Kinya Ishibashi

Keyword(s):

Fatigue Crack ◽

Fatigue Strength ◽

Crack Propagation ◽

Fatigue Crack Propagation ◽

Corrosion Fatigue ◽

Life Assessment ◽

Actual State ◽

Corrosive Environment ◽

Corrosion Fatigue Crack ◽

Long Life

It is known that the fatigue strength decreases in corrosive environment and many experiments were carried out to comprehend the decrease in fatigue strength in corrosive environment. In order to comprehend the actual state, a cycle speed of fatigue test loads should correspond to a wave frequency. Therefore, an experiment in the long life region is practically difficult, then the corrosion fatigue data available for the life assessment of the structure is quite limited. In this study, the fatigue strength of the welded joints in long life service was evaluated according to the calculations of corrosion fatigue crack propagation subjected to the random loadings which followed an exponential distribution. In the crack propagation calculations, the progress of corrosion wastage from the plate surface and the resultant stress increase were considered simultaneously. In the high stress and the short life region, the decrease in fatigue strength due to the accelerated crack propagation in corrosive environment was dominant because the progress of corrosion wastage was little. On the other hand, in the low stress and the long life region, the decrease in fatigue strength became dull as longer the fatigue life because the corrosion fatigue crack propagation was suppressed by the corrosion wastage, but after that the fatigue strength showed the precipitous decrease due to the increase in stress resulted by the progress of corrosion wastage.

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