4 Conclusions - POLYTECH - ETH Zürich

(often assumed to be equal to that of the isotropic material) are known, the shearmodulus, G lt , can be calculated from E 45 , the off-axis stiffness determined in a tensiletest at θ = 45 °, as:G ⎛ltEl E ⎞l= ⎜4 −( 1−2νlt ) − ⎟El⎝ E45Et⎠−1(2)Similarly, the anisotropic failure criterion of Tsai-Hill allows calculation of the offaxisstrength, σ θ , from the longitudinal, transverse and shear strength (respectively, σ l ,σ t , and τ lt ) as:( − )2 2 2⎛cos θ cos θ sin θ 4 2 2cos θ cos θ sin θ ⎞σθ= + +2 2 2⎜ σl σt τ ⎟⎝lt⎠Here, the shear strength,. τ lt , can be determined from the off-axis tensile strength, σ 10 ,−1(3)measured at an angle θ = 10 °, as: 60 10τlt= σ sin(10 ° )cos(10 ° )(4)The influence of the elastic constants on the anisotropic behavior of a single foil isdepicted in Figure 5. Obviously, the longitudinal and transverse modulus determine,respectively, the upper and lower limit of the foil stiffness. However, from Figure 5, itcan be seen that the off-axis stiffness of the foil strongly depends on the shearmodulus, G lt .-19-