Issue 40

L. Zou et alii, Frattura ed Integrità Strutturale, 40 (2017) 137-148; DOI: 10.3221/IGF-ESIS.40.12 145 indicates an attribute. Considering the advantages of the nodal force based structural stress, take it as the stress factor that influence the fatigue life of the aluminum alloy welded joints in S . Thus the fatigue decision system S of the aluminum alloy welded joints is built up in this paper, where the condition attributes of S is C ={material type( C 1 ), welding method( C 2 ), thickness( C 3 , mm ), Ratio( C 4 ), load type( C 5 ), joint type( C 6 ), Eq. structural stress( C 7 ， MPa )} ， the decision attribute of S is D ={ lgN }. Part data of the decision table is shown as Tab. 4. U Condition attributes Decision attributes C 1 C 2 C 3 C 4 C 5 C 6 C 7 D 1 5083H11 MIG 10 0.1 4B TJ:p 161 4.7973 2 5083H11 MIG 10 0.5 4B TJ:p 121 5.3299 3 AlMg4MnCr GMAW 2.5 0.1 T LJ_SS:p 174 4.4950 4 AlMg4MnCr GMAW 2.5 0.1 T LJ_SS:p 135 4.7163 5 AlMgSi1 TIG 3 0 T LJ_DS:p 160 4.9341 6 AlMgSi1 TIG 3 0 T LJ_DS:p 97 5.5098 7 NP5/6 Manual Arc 4.8 0 T SJ_DS:p 116 5.2742 8 NP5/6 Manual Arc 4.8 0 T SJ_DS:p 77 6.0969 9 HP30 Manual Arc 4.8 0 T SJ_DS:p 155 5.2742 …… Table 4 : Part data of the decision table. In the experiment, ( ) ( ) / i i C STD C    , 2   , 0.01   .After attributes reduction, the reduction result of the neighborhood decision system of the aluminum alloy welded joints is obtained, namely{ C 1 (Material type), C 4 (Ratio) ， C 7 (Eq. structural stress)}. S-N Curve Modeling Based on Fatigue Characteristics Domain In Eq. SS method, one master S-N curve is obtained at last thus the uncertain problem of S-N curve choice has been overcome. Compared with the nominal stress method, dispersion of the fatigue data samples in the nodal force based structural stress method has been greatly reduced. But from the design point of view, the dispersion degree of the fatigue data samples indicated by the value of RMSE is still relatively high, which is about 0.0791 here. In this work, a novel S-N curve modeling method is put forward by using the nodal force based structural stress. In the proposed method, fatigue characteristics domains are divided on basis of the reduction result of the welding fatigue decision system obtained by using rough set granularity theory. Subsequently, S-N curves are fitted on each fatigue characteristics domain rather than on the whole domain. As a result, a series of S-N curves instead of only one master S-N curve are obtained at last. In the process of welding fatigue design, we should also design according to each fatigue characteristics domain rather than in the whole fatigue domain. The fatigue characteristics domains of the aluminum alloy welded joints are determined according to the reduction result, that is, { C 1 (Material type), C 4 (Ratio), C 7 (Eq. structural stress)} obtained by using rough set theory. All the fatigue data samples are divided into 6 series from S 1 to S 6 , where S 1 ： { X ∈ U ∣ X C 1=5083H11 and X C4 =0.1 } S2 ： { X ∈ U ∣ X C 1=5083H11 and X C4 =0.5 } S3 ： { X ∈ U ∣ X C 1 =AlMg4MnCr and X C4 =0.1 } S4 ： { X ∈ U ∣ X C 1 =AlMgSi1 and X C4 =0 } S5 ： { X ∈ U ∣ X C 1 =NP5/6 and X C4 =0 } S6 ： { X ∈ U ∣ X C 1 =HP30 and X C4 =0 }, among which, each series of fatigue test samples corresponds to a specific fatigue characteristics domain and the determine of the fatigue characteristics domains is shown as Fig. 6. Fitting the S-N curve in each fatigue characteristics domain and 6 Mean S-N curves from Mean 1 to Mean 6 are obtained as is shown in Fig. 7. As could be seen from Fig. 7, fatigue data with the same characteristics scatter in a relatively independent area. For example, the scatter of green asterisk ‘*’ which denote all the fatigue samples whose material name is 5083H11 and Ratio is 0.1 in the fatigue experiment are relatively concentrated, corresponding with characteristic domain S 1 . Accordingly, the whole fatigue test samples of aluminum alloy welded joints are divided into six fatigue characteristics domains from S 1 ~ S 6 . The dispersion degree of the fatigue samples are further reduced when S-N curves are fitted according to each series instead of the whole fatigue samples. Six mean S-N curves from Mean 1 ~Mean 6 are obtained in the proposed method at last. The coefficients of the Basquin equation of Mean and Mean 1 ~Mean 6 are shown in Tab. 5.