All R-curves are linear with ∆a in the range studied, but their slopes vary between the compositions. AP and AP-Zm have a slope ranging from 12.10 3 to 15.10 3 MPa.m -0.5 , AP-Zt, AP ng -Zm and AP ng -Zt around 20.10 3 MPa.m -0.5 and finally AP ng around 30.10 3 MPa.m -0.5 .