PSI - Issue 2_B

Balázs Fekete et al. / Procedia Structural Integrity 2 (2016) 2164–2172 Author name / Structural Integrity Procedia 00 (2016) 000–000

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Striations were not so prominent, as is typical for ferritic steels. The distances between striations lines are very short at the beginning of the crack but become coarser with increasing crack length (Fig. 5(c)). With increasing total strain amplitude, the distance between striations firstly rises and subsequently decreases dependent on the length of crack (Fig. 5(d)). 3.3. New energy based LCF model Based on the Taylor-Quinney parameter investigated for this steels [Fekete and Szekeres (2015)] we have developed and validated a new low cycle fatigue prediction model. The strain energy in the stabilized cycle can be calculated by the following form:

  

stab

W

d  

,

(1)

p

cycle

where W stab is the strain energy per cycle; σ is the stress; d ε p is the plastic strain. It is proposed that the fatigue toughness to start of crack propagation, W SCP , can be determined with the number of cycle to start of crack propagation (N SCP ), as follows:

SCP N

1 i W W    SCP

stab

.

(2)

The proposed new measure of LCF damage is attempted to quantify the critical amount of stored energy (W STF ) required to introduce into the material to failure. The strain energy in the stabilized cycles can be calculated by the following form:   1 STF SCP SCP W W D N       , (3) where β is the Taylor Quinney parameter, D and δ are the fatigue toughness coefficient and the fatigue toughness exponent, respectively. The best fitted results on material constants for the model are reported along with the coefficients of determination (R²) in the embedded table on each diagram as shown in Fig. 6.

Fig. 6. Cumulated strain energy (W STF ) with the cycles start of crack propagation (a) 15Ch2MFA (b) 08Ch18N10T