The main purpose of this paper is to explain the shear strain differen
ces observed between the two faces of certain Iosipescu specimens. The
different parasitic moments that can act on the specimen are investig
ated and their effects assessed. Different Iosipescu fixtures from the
literature are checked for these parasitic moments and it is shown th
at modification of the Wyoming fixture can lead to the elimination of
its parasitic in-plane and out-of-plane movements. Finally, it is show
n that the strain differences that remain are caused by Saint-Venant e
ffects due to the proximity of the loading points to the notch line bu
t that averaging the strains between the two faces eliminates this eff
ect. This is an important feature since for certain material configura
tions (thick and transversely stiff specimens), preventing twisting of
the fixture is not sufficient to enable the use of only one strain ga
uge.