A correlation of ΔKth-value with the exponent, m, in the equation of fatigue crack growth for various steels

Literature (mainly in Japan) relating to fatigue-crack-growth-data at R=0 in an air environment for a wide range of steels is reviewed with particular attention to the threshold stress intensity, ΔKth. The collected data are analyzed in terms of the exponent, m(the slope of the linear portion of the log(da/dN)-logΔD relationship) by taking account of microstructure, material strength, fracture toughness and specimen thickness. The mean rate of fatigue crack growth for ductile steels in the range from the intermediate growth rate to threshold level and the relevant threshold values at R=0, ΔKth0, can be represented asda/dN = 1.700 × 10-4(ΔK103.6)m - 10-6andΔKth0 = 103.6(5.88 × 10-3)1/m,where (da/dN) and ΔK are measured by the units of mm/cycle and kgf/mm3/2, respectively. Contrary to this, in the case of extremely brittle steels with KIC-value below 200 kgf/mm3/2 (the fracture occurs by the intergranular separation), the relationships are given byda/dN = 2.893 × 10-5(ΔK/49.94)m - 2.5 × 10-7andΔKth0 = 49.94(8.64 × 10-3)1/m.RésuméLa littérature, principalement japonaise, relative aux données sur la propagation des fissures de fatigue pour R=0 dans un environnement d'air et pour une large catégorie d'aciers fait l'objet d'une revue avec une attention particulière pour l'intensité de contrainte de seuil, Kth. Les données collectées sont analysées en terme de l'éxposant m (qui représente la pente de la portion linéaire de la relation log(da/dN)-log ΔK, en tenant compte de la microstructure, de la résistance du matériau, de la ténacité à la rupture et de l'épaisseur de l'éprouvette. La vitesse moyenne de propagation d'une fissure de fatigue dans le cas des aciers ductiles dans la fourchette entre la vitesse intermédiaire de fissuration et le niveau critique, et les valeurs correspondantes de seuil à R=0, ΔKth0 peuvent être représentées par la relation:da/dN = 1.700 × 10-4(ΔK103.6)m - 10-6etΔKth0 = 103.6(5.88 × 10-3)1/m,où (da/dN) et ΔK sont mesurés en unités de mm/cycle et en kgf/mm3/2 respectivement. En contraste, dans le cas d'aciers extrèmement fragiles avec des valeurs KIC en dessous de 200 kgf/mm3/2 (la rupture se produit par une séparation intergranulaire), ces relations sont données par:da/dN = 2.893 × 10-5(ΔK/49.94)m - 2.5 × 10-7etΔKth0 = 49.94(8.64 × 10-3)1/m.

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