Picture-frame testing of woven prepreg fabric: An investigation of sample geometry and shear angle acquisition

This paper examines different concepts in relation to the picture-frame test for shear characterization of a woven prepreg fabric. The influence of the sample arms is investigated by means of cut slits as well as removed transverse tows. Shear angles are obtained using Digital Image Correlation (DIC) and also from images taken during the test which are processed for fiber angles directly from the weave texture. The image processing relies on the Hough transform in MATLAB. The concept of constant shear strain rate is discussed and implemented in the test software by a multi-linear crosshead velocity profile. Finally, bias-extension data are obtained and used for comparison. It is found that the sample arm modifications have a pronounced effect on the measured shear load whereas the uniformness of the shear strain field in the samples is not improved considerably.