PSI - Issue 13

Hugo Wärner et al. / Procedia Structural Integrity 13 (2018) 843–848

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Hugo Wärner et al./ Structural Integrity Procedia 00 (2018) 000 – 000

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(b)

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Fig. 3. Cyclic hardening/softening curves for CF tests. Stress amplitude vs. number of cycles; (a) Virgin material at different strain amplitudes and dwell times; (b) Pre-aged and virgin material at different dwell times.

The fatigue hardening coefficient ( K’ ), the fatigue hardening exponent ( n’ ), the fatigue ductility coefficient ( ε’ f ) and the fatigue ductility exponent ( c ) were derived using this procedure and can be viewed in Table 1. From the Coffin Manson graph, it is notable that both the TMF and CF conditions with different configurations are shifted to lower fatigue lives compared with pure LCF testing for the same plastic strain amplitudes. This means that it is very important to correctly identify whether LCF, TMF or CF condition apply for the real application. The CSSCs graph shows, in accordance with the cyclic hardening/softening curves in Fig. 2 and Fig. 3, that TMF without dwell time and lower temperature range has a more extensive hardening behavior and has a higher cyclic hardening coefficient than the other conditions. The exception is the aged CF condition, but there are too few data points in order to establish a reliable trend.

(b)

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Fig. 4. Fatigue evaluation graphs for all the investigated conditions; (a) Cyclic stress-strain curves; (b) Coffin-Manson fatigue life curves.