PSI - Issue 68

Reza Afsharnia et al. / Procedia Structural Integrity 68 (2025) 1153–1158 Reza Afsharnia et al. / Structural Integrity Procedia 00 (2025) 000–000

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1. Introduction The lifetime of a component subjected to cyclic loading can be divided into two distinct phases. In the first phase, known as crack initiation, traditional S-N curves can be employed for lifetime estimation. However, once a crack has formed in the component, S-N curves are no longer applicable for lifetime prediction; instead, fracture mechanics analysis should be utilized (Sander 2008). In short fiber reinforced polymers, the primary function of the fibers is to enhance the material properties. However, each fiber also presents a potential defect site. By considering these fibers as possible defect locations, lifetime estimation can be conducted using only fracture mechanics data. This approach can significantly reduce testing time

and costs. 2. Theory

Figure 1 illustrates the correlation between the S-N curves and the crack kinetic curves derived from fracture mechanics. A lower slope for both curves indicates a higher stress amplitude for the same number of cycles, as well as a lower crack growth rate for the same stress intensity. Furthermore, if both curves shift to the right, this indicates improved material performance. These trends suggest a potential correlation between fracture mechanics and fatigue analysis. In Figure 1, two models are employed to correlate the S-N curve with the crack kinetic curve: Model I addresses the very high cycle fatigue regime, while Model II focuses on the high cycle fatigue regime. (Haibach 2006; Janzen und Ehrenstein 1991; Mösenbacher 2009; Zahnt 2003).

Model II

Model I

Figure 1. Correlation between fatigue analysis and fracture mechanics.

Model I calculate the maximum stress amplitude for very high cycle fatigue regime based on the defect shape, threshold stress intensity from the crack kinetic curves and defect size. For a triangular defect the maximum stress amplitude for very high cycle fatigue ( !" ) could be determined using to Eq.1 (Zahnt 2003; Mösenbacher 2009; Haibach 2006). Eq. 1 Threshold stress intensity ( ∆ #$ ) could be achieved from the crack kinetic curves, geometry factor ( Y ) in this case would be considered 1 (Zahnt 2003; Balika et al. 2006). The initial crack length ( a i ) is half of the defect length. The number of cycles to failure for a component ( N b ) could be written in the form of Eq.2. In Eq.2, N i is the number of cycles to crack initiation and N a is the number of cycles to failure while the crack is growing. Eq. 2 !" " !" #$ % & D # ! " # = ! " # $ $ $ = +