PostprintThis is the accepted version of a paper published in Journal of composite materials. This paper has been peer-reviewed but does not include the final publisher proof-corrections or journal pagination. AbstractThe work presented herein aims to investigate the in-plane properties of cross-plied unidirectional prepreg using the bias extension method. The study is focusing on different generations of carbon fibre/epoxy material systems and testing is performed at elevated temperatures to enhance formability. Using digital image correlation (DIC), the specimen deformation and fibre rotation is captured during tests. The study shows that most cross-plied unidirectional material deforms at different characteristic length scales as deformation continues: first seemingly continuous and later in form of bands.Further, the different type of prepreg materials investigated behaves differently; for one type the pin-jointed net theory fit well, enabling simple estimation of resulting fibre angle, while for the another two not. Different loading speed and temperatures are investigated in order to pin-point its influence on the deformability.