Effect of Stress Ratio on Fatigue Life of A356 Aluminium Casting Alloys

• Published in: Journal of materials engineering and performance Vol. 31; no. 2; pp. 1066 - 1076
• Main Author: Lee, ChoongDo
• Format: Journal Article
• Language: English
• Published: New York Springer US 01-02-2022
• Subjects: Characterization and Evaluation of Materials; Chemistry and Materials Science; Corrosion and Coatings; Engineering Design; Materials Science; Quality Control; Reliability; Safety and Risk; Tribology; stress ratio; defect susceptibility; fatigue property; porosity; A356 aluminium alloy
• ISSN: 1059-9495; 1544-1024
• DOI: 10.1007/s11665-021-06297-9

The present study aimed to investigate the dependence of fatigue properties of A356 casting alloy on the variation of stress ratio (
R
-value) in terms of the mutual contribution of strain components and porosity to fatigue life. The fatigue test was carried out in axial high-cycle fatigue mode through the variation of
R
-value by the addition of compressive load to a fixed tensile stress amplitude (
R
= 0.0, − 0.5, − 1.0) and by the variation of tensile and compressive loads in a fixed total stress range (
R
= − 0.58 ~ − 1.25), respectively. The decrease in
R
-value by the addition of a compressive load at a fixed tensile stress amplitude wholly reduces the slope of the S-N curve (fatigue strength exponent), because the nominal level of fatigue life becomes increasingly sensitive with the porosity variation as
R
-value decreases; fundamentally, the increase in total stress range induces a remarkable increase in the plastic strain as well as the elastic strain. Meanwhile, the decrease of
R
-value by the variation of tensile/compressive load to a fixed total stress range induces a different aspect on the dependence of fatigue life on the porosity variation: the nominal value of defect susceptibility coefficient remarkably increases with a wide range and the decrease of maximum fatigue life is restricted to a small range, by a relatively small variation of both strain components. However, as the
R
-value decreases in both loading conditions, the propagation path of a fatigue crack gradually transitions from a cracking mode, in which the cracking damage of eutectic Si particles dominates, to a mixed mode of matrix penetration by local plastic deformation and cracking damage of eutectic Si particles, which mainly accumulate by compressive loading conditions.