Issue 33

T. Itoh et alii, Frattura ed Integrità Strutturale, 33 (2015) 289-301; DOI: 10.3221/IGF-ESIS.33.33 291 The “Max” denotes taking the larger value from the three in the bracket. The maximum value of S I ( t ) during a cycle is defined as the maximum principal value, S Imax , at t = t 0 as follows, I 1 0 2 0 3 0 Imax 0 ( ) Max ( ) , ( ) , ( )       S S S S S t t t t (2) Figure 2 : Definition of principal stress and strain directions in XYZ coordinates. Definition of Principal Stress and Strain Directions Fig. 2 illustrates two angles,  ( t )/2 and  ( t ), to express the rotation or direction change of the maximum principal vector, S I ( t ), in the new coordinate system of XYZ , where XYZ -coordinates are the material coordinates taking X -axis in the direction of S I ( t 0 ) with the other two axes in arbitrary directions. The two angles of  ( t )/2 and  ( t ) are given by i i i i ( ) 1 0 0 ( ) ( ) ( ) cos ( 0 ) 2 ( ) ( ) 2 2 t t t t t t                 S S S S (3) i i 1 ( ) ( ) tan ( 0 ( ) 2 ) ( ) Z Y t t t t                S e S e (4) where dots in Eqs (3) and (4) denote the inner product and e Y and e Z are unit vectors in Y and Z directions, respectively. S i ( t ) are the principal vectors of stress or strain used in Eq. (1) and the subscript i takes 1 or 3. The rotation angle of  ( t )/2 expresses the angle between the S I ( t 0 ) and S I ( t ) directions and the deviation angle of  ( t ) is the angle of S I ( t ) direction from the Y -axis in the X -plane. Definitions of Stress and Strain in Polar Figure Fig. 3 shows the trajectory of S I ( t ) in 3D polar figure for a cycle where the radius is taken as the value of S I ( t ), and the angles of  ( t ) and  ( t ) are the angles shown in the figure. A new coordinate system is used in Fig. 3 with the three axes of S I 1 , S I 2 and S I 3 , where S I 1 -axis directs to the direction of S I ( t 0 ). The rotation angle of  ( t ) has double magnitude compared with that in the specimen shown in Fig. 2 considering the consistency of the angle between the polar figure and the physical plane presentation. The principal range,  S I , is determined as the maximum projection length of S I ( t ) on the S I 1 - axis. The mean value, S Imean , is given as the center of the range.  S I and S Imean are equated as,   I I I I I Δ Max ( ) max max min cos ( ) S S t S t S S      (5)   Imean I I max min 1 2 S S S   (6) S Imin is the S I ( t ) to maximize the value of the bracket in Eq. (5). The sign of S Imin in the figure is set to be positive if it does not cross the S I 2 -S I 3 plane and the sign negative if it crosses the plane. The advantage of the definitions of the maximum principal range and mean value above mentioned is that the two are determinable without human judgments for any loading case in 3D stress and strain space. The range and mean value are consistent used in simple loading cases which are discussed in the case studies in the followings. S I ( t ) can be replaced by equivalent values of stress or strains, such as the von Mises and the Tresca, in case of necessity from user’s requirement.