The main purpose of this paper is to explain the shear strain differences observed between the two faces of certain Iosipescu specimens. The different parasitic moments that can act on the specimen are investigated and their effects assessed. Different Iosipescu fixtures from the literature are checked for these parasitic moments and it is shown that modification of the Wyoming fixture can lead to the elimination of its parasitic in-plane and out-of-plane movements. Finally, it is shown that the strain differences that remain are caused by Saint-Venant effects due to the proximity of the loading points to the notch line but that averaging the strains between the two faces eliminates this effect. This is an important feature since for certain material configurations (thick and transversely stiff specimens), preventing twisting of the fixture is not sufficient to enable the use of only one strain gauge.
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