PSI - Issue 22

C.L. Niu et al. / Procedia Structural Integrity 22 (2019) 361–368 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

363

3

（ 1 ）

The stress variation amplitude of the evaluation point is obtained by using the calculation formula (2). （ 2 ） 4 ） The minimum principal stress and the range of principal stress are calculated by formulas (1) and (2). The stress factor of the welded joint is evaluated by the ratio of the calculated stress variation amplitude to the allowable stress variation amplitude corresponding to the FAT grade of the welded joint, and then the stress state grade of the welded joint can be determined. Based on the above process, the evaluation process based on IIW-2008 standard is programmed by APDL language. The calculation results are saved with the output of TXT file. 2.2 Stress State Grade Assessment Based on ASME Structural Stress Method The structural stress in ASME reflects the stress on the weld which is related to stress concentration, while the equivalent structural stress is deduced by using the principle of fracture mechanics, considering the factors affecting fatigue assessment comprehensively, and its value is related to fatigue life. The master S-N curve is obtained by studying the fatigue characteristics and fracture mechanics of thousands of weld joints. The influence of various factors of fatigue fracture on the fatigue characteristics of weld seams is considered in the curve. The steps for predicting fatigue life of welded structures by structural stress method are as follows. 1 ） The number of stress-strain cycles at different locations is determined by cyclic counting method. The total number of cyclic stress cycles is defined as M . 2 ） According to the elastic membrane stress and bending stress in the normal direction of the crack surface at the beginning and end of the k th cycle obtained in the first step ( m t for the beginning and n t for the end of a weld), the following parameters are calculated: The variation range of elastic membrane stress at the starting point e k m ,   and elastic bending stress e k b ,   , the maximum variation range k max ,  , the minimum variation range k max ,  and the average variation range k min ,  in the cycle are calculated as follows.  e k m n e k m m e k m e k b n e k b m e k b , , , , , ,             ；              e b k e n m k e n b k e m m k m k e b k e n m k e n b k e m m k m k , , , , max , , , , , min , , max , min                 ； min, max, , k k mean k      ； 3 ） The stress variation range of elastic structure and equivalent structural stress variation range of the k th cycle are calculated. e b k e m k e k , ,         min max       2



k 

ess k ,   S

（ 3 ）

2     2

m m m ss 1    ss

t

I f ss

M k ,

ess

k   and equivalent

In formula 3 ， the unit of parameters, thickness t , crack length a, stress variation range

, ess k S  are mm, mm, MPa and

(2 )/2 / ( ) ss ss m m MPa mm  , respectively. The calculation

structural stress variation range

methods of formula ss m , ess t and M k f , , I (r) are shown in ASME standard. 4 ） According to the fatigue curve of welded joints and the variation range of equivalent structural stress obtained in step 3, the number of cycles N k and the parameters C d and h in the master S-N curve are calculated. See ASME standard.