Bug: Wrong angle for dynamic beam rotation

[victorkemp]

The tip of the arm should rotate by 0.01 radian but it actually rotates by about 0.055 radian. It approaches the correct value with mesh refinement. The node with the rotation constraint rotates correctly but there's a twist in the shaft that seems to come from spurious rotational stiffness at the connection between the two elements.

It works OK in static.

*NODE
1,0,0,0
2,2,0,0
3,0,1,0
*ELEMENT,TYPE=B31,ELSET=Eall
1,3,1
2,1,2
*MATERIAL,NAME=Material
*ELASTIC,TYPE=ISOTROPIC
200000000000,0
*DENSITY
100
*BEAM SECTION,ELSET=Eall,MATERIAL=Material,SECTION=RECT
0.01,0.01
0,0,1
*BOUNDARY
1,1,3,0
2,2,3,0
*AMPLITUDE,NAME=A_1
0,0
0.5,0.01
*STEP
*DYNAMIC,DIRECT
0.01,1,0,0
*BOUNDARY,AMPLITUDE=A_1
2,4,,1
*NODE FILE
U
*END STEP