PSI - Issue 42

Robert Basan et al. / Procedia Structural Integrity 42 (2022) 655–662 R. Basan et al. / Structural Integrity Procedia 00 (2019) 000–000

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Regarding the preparation of data and datasets required for the analysis, methodology established in previous study by Basan et al (2015) performed on set of 32 differently heat treated low-alloy steel 42CrMo4 was used in this work as well. Experimental and estimated numbers of load reversals to crack initiation 2 N f,exp and 2 N f,estUML , 2 N f,estH and 2 N f,estM were calculated for all considered materials for 8 different values of total strain amplitudes Δ ε /2: 0,25 %, 0,3 %, 0,35 %, 0,4 %, 0,45 %, 0,5 %, 0,9 % and 1,5 %, using original Basquin-Coffin-Manson equation (8) with experimentally obtained fatigue parameters and expressions (5), (6) and (7), respectively. Fatigue lives pairs with unrealistically high values of load reversals 2 N f,exp (higher than 5·10 6 ) excluded from the analysis. Materials were divided into low strength (LS) and high strength (HS) subgroups according to value of their ultimate tensile strength. This division and separate consideration was additionally motivated by the fact that high strength steels, as a distinct subgroup of steels, are of particular interest and relevance in terms of application since they are utilized to manufacture heavily stressed (and often vital) parts of machines and other equipment and devices. Even though this may seem like a simple division, different values for ultimate tensile strength are provided in different literature sources as a measure by which steel is judged to be of high strength. The classification now includes new terms/subgroups such as very high strength steels and ultra high strength steels since over time new techniques and steel materials with higher ultimate strengths are continuously developed. Thus, it can be anticipated that using older estimation methods, which were created on a significantly different set of material data, without those representative of today’s high strength steels, might introduce significant extrapolation and introduce an additional risk of getting unreliable estimation results. In addition to the value of ultimate tensile strength, number of datasets which would fall into each subgroup was also considered in order to avoid too much imbalance. Material datasets have been subdivided in two strength subgroups in the following manner: • unalloyed steels: 71 materials with R m < 750 MPa (LSUA) ; 57 materials with R m > 750 MPa (HSUA) • low-alloy steels: 29 materials with R m < 1000 MPa (LSLA) ; 35 materials with R m > 1000 MPa (HSLA) • high-alloy steels: 43 materials with R m < 750 MPa (LSHA) ; 32 materials with R m > 750 MPa (HSHA) The complete range of fatigue lives was also divided into two groups, where "LCF" group (low–cycle fatigue) implies fatigue lives data pairs (2 N f,exp ; 2 N f,est ) with 2 N f,exp ≤ 20000 and "HCF" subgroup (high–cycle fatigue) implies data pairs (2 N f,exp ; 2 N f,est ) with 2 N f,exp > 20000. In order to determine how the proposed detailed evaluation methodology compares to common evaluation procedure, a control groups containing fatigue lives data pairs (2 N f,exp ; 2 N f,est ) calculated for all materials and for the complete range of fatigue lives (2 N f,exp ≤ 5·10 6 ) were formed and designated as "ALL". 4. Results Based on calculated values of experimental and estimated numbers of load reversals 2 N f,exp , 2 N f,estH , 2 N f,estUML and 2 N f,estM , values of evaluation criteria defined in section 2 have been determined for all groups and subgroups as defined in subsection 3.2. Results i.e. values of evaluation criteria are presented separately, those for unalloyed steels in Table 1, for low-alloy steels in Table 2 and for high-alloy steels in Table 3. The higher the value of individual criterion, the better, with value of 1 for “ideal” estimation. It must be noted that since Uniform Material Law was originally developed only for unalloyed and low–alloy steels, for this method values of evaluation criteria were not calculated for high-alloy steels group. ( ) N f 2 ( ) c N f 2 b E f ε + ′ f σ ε ′ p e ε 2 2 2 Δ Δ = Δ + ε = (8)