Issue 30

V. Chaves et alii, Frattura ed Integrità Strutturale, 30 (2014) 273-281; DOI: 10.3221/IGF-ESIS.30.34

likelihood method proposed by Betinelli [2]. This procedure, which uses both the failures and the run-outs, is an effective alternative to classical choices such as the staircase method and requires less experimental testing. The two straight lines were plotted in a single graph in order to obtain predictions throughout the entire life. Fig. 3 and 4 show in a semi-logarithmic graph the experimental S – N curves for the monoaxial and biaxial tests, respectively. As can be seen, the curves were extremely horizontal. In fact, the difference between withstanding a few thousands of cycles only and not breaking was a few megapascals. This material exhibits a dual behaviour: either it exhibits a very short lifetime or does not break at all. It was therefore impossible to detect breaking points after around a million cycles since any specimens reaching those lifetimes failed to break beyond that point. Also, having such a horizontal S – N curve had the advantage that fatigue limit calculations were subject to very little uncertainty. The tensile–compressive fatigue limit ( R = –1) for the material as determined in cylindrical specimens and expressed in stress amplitude was σ FL = 316 MPa and its torsional counterpart τ FL = 288 MPa.

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Stress  , MPa

Stress  , MPa

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200

S-N curve:  =369.6N -0.0193 Fatigue limit:  =288 MPa

S-N curve:  =366.7N -0.0126 Fatigue limit:  =316 MPa

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100

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Number of cycles, N

Number of cycles, N

a) b) Figure 3 : S-N curves for the monoaxial tests (R=-1): a) push-pull, b) torsion.

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Stress  , MPa

Stress  , MPa

200

200

S-N curve:  =346.5N -0.0172 Fatigue limit:  =283 MPa

S-N curve:  =400.3N -0.0494 Fatigue limit:  =215 MPa

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Number of cycles, N

Number of cycles, N

a)

b)

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Stress  , MPa

Stress  , MPa

200

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S-N curve:  =517.3N -0.0557 Fatigue limit:  =250 MPa

S-N curve:  =906.9N -0.113 Fatigue limit:  =283 MPa

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100

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10 6

Number of cycles, N

Number of cycles, N

c) d) Figure 4 : S-N curves for the biaxial tests (R=-1): a) 0.5 σ = τ, b ) σ = τ, c ) 2 σ = τ, d ) 4 σ = τ .

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