PSI - Issue 68

Soran Hassanifard et al. / Procedia Structural Integrity 68 (2025) 77–83 S. Hassanifard and K. Behdinan / Structural Integrity Procedia 00 (2025) 000–000

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Table 1 summarizes the mechanical properties and strain-based fatigue parameters of the raw materials used as filaments, including the specified GNP contents of 0.1, 0.5, and 1.0 wt.%. In this table, UTS is the ultimate tensile strength of the filaments, E is Young’s modulus, ! and ! are coefficient and exponent of Romberg-Osgood’s equation, and " ! , " ! , b , and c are fatigue parameters. Table 1. Mechanical properties and fatigue parameters of filaments. Property Filament UTS (MPa) E (MPa) ! ! " ! " ! b c Pure ABS 37.0 1643 127.3 0.31 64 0.13 -0.13 -0.39 ABS/0.1% GNP 43.8 1942 192.4 0.38 101 0.89 -0.18 -0.73 ABS/0.5% GNP 39.8 2081 128.7 0.31 107 0.51 -0.17 -0.53 ABS/1.0% GNP 37.8 2095 99.4 0.24 72 0.35 -0.15 -0.61 2.2. Fatigue life prediction models To predict fatigue life of 3D-printed samples, strain-based fatigue damage models were utilized. Under completely reversed loading condition, Coffin-Manson strain-based equation can express the relation of − in a log-log scale (1) In this equation, # is the strain amplitude, which consists of both elastic and plastic components. The parameters " ! and b represent fatigue strength coefficient and exponent, respectively, while " ! and c represent fatigue ductility coefficient and exponent. To account for the effect of mean stress, several models have been developed, including the modified Morrow and SWT models. In the modified Morrow fatigue damage model, the influence of mean stress ( ( ) is only incorporated into the elastic strain component of the Coffin-Manson equation, as shown below: # = $ !" % ,1− $ !" $ # . (2 " ) & + " ! (2 " ) ' (2) While SWT model assumes that the fatigue life for any loading condition involving non-zero mean stress depends on the product of the maximum stress and the strain amplitude: (#) # = ($ !" ) $ % (2 " ) ,& + " ! " ! (2 " ) &-' (3) To plot a complete loading-unloading hysteresis loop, it is necessary to identify two key points in the − coordinate system such as ( (#) , (#) ) and ( # , # ). This requires solving two sets of equations: Neuber’s equation and the Ramberg-Osgood equation, simultaneously. = (. !" 0) $ % (4) = % & +( ' % ( ) 1 ′ ⁄ (5) During cyclic loading with constant amplitude stress, the unloading and reloading behavior can be described by the following equations: = (#) − 2 ( $ #%& , 4$ ) (6) = (56 + 2 ( $4$ #'( , ) (7) as follows (Dowling, 2008): # = $ !" % (2 " ) & + " ! (2 " ) '