I have few basic questions about elements selection/usage in FEM for fracture mechanics problems,

1, In abaqus why near the contour regions ONLY quadrilateral or Hexahedron elements should be used ? any specific importance.

2, But with other LEFM programs triangular/tetrahedron elements are used, with 2nd order elements(elements with mid-nodes) and quarter point method, why any specific importance, some comments about quad/hexa Vs tria/tetra elements and 1st order Vs 2nd order and quarter point method?

3, In abaqus why the mid-nodes are selected in the contour regions, if 2nd order elements are used ?