This paper gives the general expressions for the compliance
s'(ijkl), Young's modulus E(hkl) and Poisson's ratio v(hkl, theta) along arbitrary loading
direction [hkl] for tetragonal crystals. The representation surface for which the length of the
radius vector in the [hkl] direction equals E(hkl) and representation curve for which the length
of the radius vector with angle theta deviated from the reference directions [00 (1) over bar],
[100], [00 (1) over bar], [10 (1) over bar] and [11 (2) over bar] equals v(100, theta), v(001,
theta), v(110, theta), v(101, theta) and v(111, theta) respectively, are constructed for nine
tetragonal crystals (ammonium dihydrogen arsenate, ammonium dihydrogen phosphate, barium
titanate, indium, nickel sulfate, potassium dihydrogen arsenate, potassium dihydrogen phosphate,
tin and zircon). The characteristics of them are analysed in detail.