PSI - Issue 2_A

Hide-aki Nishikawa et al. / Procedia Structural Integrity 2 (2016) 3002–3009 Author name / Structural Integrity Procedia 00 (2016) 000–000

3003

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in welding affects its performance and it is difficult to evaluate each factor quantitatively. For example, since fatigue crack is initiated from weld toe, not only stress concentration but also effect of variable microstructure in the heat affected zone (HAZ) is necessary to be considered. Furthermore residual stress formed around HAZ which affect fatigue crack growth behavior is also have to be considered. By the way, it is well known that fatigue life is mainly occupied by small fatigue crack growth life. However, small fatigue crack growth behavior in the welding is not enough investigated. To clarify the small fatigue crack growth behavior in the welding is possibly helpful to considering complex effective parameter such as residual stress and stress concentration quantitatively. This study focusing on the effect of HAZ microstructure and residual stress (mean stress) on small fatigue crack growth behavior. By the way, measurement of crack closure effect which is important to mean stress dependency of fatigue crack growth rate is difficult especially for small crack. Since fatigue life is mainly occupied by crack growth life, conventional fatigue parameter for fatigue fracture life of smooth specimen is possibly useful also for small crack growth evaluation. For example, Smith et al. (1970) proposed stress-strain function which is able to fix mean stress effect on fatigue life of smooth specimen. In this study, to clarify the effect of these factor on the small fatigue crack growth behavior, fatigue testing with successive small fatigue crack growth observation were carried out for low carbon steel with several simulated HAZ heat treatment under some tensile mean stress conditions. Furthermore, crack opening-closing behavior which is important for mean stress dependency was investigated with Digital Image Correlation (DIC) technique.

Nomenclature σ a

stress amplitude

maximum stress during fatigue cycle Crack opening stress during fatigue cycle maximum strain during fatigue cycle crack opening strain during fatigue cycle strain amplitude

σ max

σ op

ε a

ε max

ε op ε eq

equivalent strain calculated from Smith-Watson-Topper (SWT) equation

l

surface fatigue crack length

a

fatigue crack depth

∆ K stress intensity factor range √ area projected area of the fatigue crack c materials constant for SWT equation C

materials constant for small fatigue crack growth law materials constant for small fatigue crack growth law

n

2. Material and testing procedure

Materials used in this study were three types of low carbon steel. Table 1 shows chemical compositions. Three types of simulated HAZ heat treatments were conducted with controlling cooling rate after holding 5 second at 1400 ℃ for each materials. Table 2 shows heat treatment conditions for each materials. Finally, three different simulated HAZ microstructures were obtained. Fig. 1 shows microstructure morphologies. Microstructures were acicular ferrite, perlite and grain boundary ferrite for material A, ferrite and bainite for material B and martensite for material C respectively. Vickers hardness were HV148, HV182 and HV350 respectively. Fatigue tests were conducted with load controlled uniaxial loading by servo hydraulic type fatigue testing equipment. Fig. 2 shows specimen configurations. Specimen surface were etched by 3 % nital after mirror polished to crack observation. Small fatigue crack growth behavior is observed using digital microscope. In addition, strain amplitude was measured by strain gage on the back surface of fatigue specimen.