Issue 30

V. Chaves et alii, Frattura ed Integrità Strutturale, 30 (2014) 273-281; DOI: 10.3221/IGF-ESIS.30.34 276 B IAXIAL FATIGUE CURVE he fatigue limits obtained from biaxial loading tests on cylindrical specimens can be used to construct σ–τ plots. As can be seen from Fig. 5, our experimental values fitted an elliptical quadrant better than an elliptical arc. Based on experimental results of Gough et al. [3], fragile materials in fatigue are optimally approximated by an arc and ductile materials by a quadrant. AISI 304L steel has a near-unity σ FL /τ FL ratio (288/316 = 0.91); therefore, it behaves as a fragile material in fatigue. Then, the present experimental results contradict experimental results of the previous authors for materials exhibiting a fragile behaviour under fatigue. But, certainly, the results of Gough et al. corresponded to cast irons alone. 0 50 100 150 200 250 300 350 0 50 100 150 200 250 300 350  (MPa)  (MPa) Experimental Ellipse quadrant Ellipse arc Figure 5 : Experimental fatigue limits for the biaxial tests. C RACK DIRECTION DURING STAGE I his section deals with crack direction across the first few grains (Stage I) as determined experimentally or predicted with various models. The X axis ran across the specimen and the Y axis along it (see Fig. 6). Determinations involved examining the crack at the specimen outer surface and measuring the angle, called α, from the X axis. A second angle, named θ , which is the angle that forms the crack with the direction that is normal to the first principal stress (σ 1 direction) is defined for the comparison with the models. x y  1 direction Crack direction F F T T   Figure 6 : Axes and angles for the study of the crack direction. T T