Issue 30

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

geometry, in addition to a small amount of delta ferrite bands. The mean size of the austenite grains was 80 μm (see Fig. 1).

Figure 1 : Microstructure of the stainless steel AISI 304L.

The mechanical properties of the steel as determined from 5 tensile tests were as follows: tensile strength (σ UTS ) = 654 MPa, yield stress 0.2% (σ y ) = 467 MPa and elongation = 56%. The material was subjected to various biaxial fatigue laboratory tests including tensile, torsional and in-phase tensile– torsional, all at R = –1. Tests were conducted on specimens of cylindrical cross-section having a central diameter of 12.5 mm (see Fig. 2). All specimens were carefully polished to an average surface roughness ( R a ) not exceeding 0.1 μm. Also, all tests were performed on a biaxial fatigue hydraulic machine under controlled loading conditions, using a sine wave and a frequency of 6–8 Hz. Each test was finished when the crack grew several millimetres long (in some cases the specimen broke completely) or a number of 3.5×10 6 cycles (run-outs) was reached.

F

T

60 57 60

R 110

Ø12,5

Ø20

T

F

Figure 2 : Geometry of the cylindrical specimens tested in biaxial fatigue (in mm).

S-N CURVES

he results of the above-described tests were used to construct S – N curves in accordance with ASTM E 739-91 (2004) [1]. Based on this standard, the variables stress and number of cycles to failure can be approximated by a linear logarithmic relationship, excluding run-outs. The fatigue limits were calculated using the maximum T

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