Issue34

T. Itoh et alii, Frattura ed Integrità Strutturale, 34 (2015) 487-497; DOI: 10.3221/IGF-ESIS.34.54 493       2 2 2 1 2 eq r r            (9) The inner and outer surfaces at the gauge part of the specimen were ground to an averaged surface roughness 1.6  m by honing machine. After the honing, some specimen were also received polishing and buffing to 10  m Al 2 O 3 denoted by ‘Polish’ in Tab. 1. Stress Time t Axial stress σ z Hoop stress σ θ Stress Time t Axial stress σ z Hoop stress σ θ Stress Time t Axial stress σ z Hoop stress σ θ Axial load, Inner pressure Time t Axial load F Inner pressure P Axial load, Inner pressure Time t Axial load F Inner pressure P Axial load, Inner pressure Time t Axial load F Inner pressure P Load & Pressure Axial & Hoop stresses  =0  =0.4, 0.5  =1 Figure 9 : Wave forms of axial load, inner pressure, axial and hoop stresses. Stress path Maximum stress (MPa) Polish/ Non-polish Uniaxial/Biaxial λ Axial stress σ z max Hoop stress σ  max Mises stress σ eq max Uniaxial Fatigue 0 800 ― 800 Non-polish Polish Biaxial Fatigue 0.4 985 400 858 Non-polish Polish 0.5 924 462 800 Polish 1.0 800 800 800 Polish Table 1 : List of fatigue test condition. E XPERIMENTAL RESULTS AND DISCUSSION Failure life ig. 10 shows a comparison of failure lives. In this figure, failure lives are normalized by the mean value of failure life in the uniaxial fatigue tests (λ=0 tests) tested using polished and non-polished specimens. In comparing the failure lives tested using the polished specimen, failure lives ( N f ) tend to decrease with increase of the principal stress ratio (λ) although N f at λ=0.4 is slightly shorter than N f at λ=0.5. The shorter life in the λ=0.4 test may come from that the maximum Mises’ equivalent stress in the test is slightly larger than those in the other λ’s tests as shown in Tab. 1. The specimen polishing effect can be seen in the λ=0 and λ=0.4 tests. In the λ=0 test, N f with non-polished specimen have longer life by about 20 % than that with polished specimen. In the λ=0.4 test, conversely, N f with non-polished specimen is shorter than that with the polished specimen and the reduction ratio is down to approximately 50 % depending on each specimen. The effects of λ and polishing on the failure life will be discussed in following sections. F