Evaluation of bias-extension and picture-frame test methods for the measurement of intraply shear properties of PP/glass commingled fabrics

Efforts are currently devoted to the development of numerical tools for predicting the forming process of continuous fiber-reinforced thermoplastic composites. To ensure good predictions, reliable experimental measurements of the laminate properties must be performed at the processing temperature. This paper presents experimental results of the intraply shear properties measured on a polypropylene/glass fabric using two different methods known as the bias-extension and the picture-frame testing methods. Based on kinematic equations, both methods of measurement are discussed in regards to their respective weaknesses, mainly related to the difficulty to measure precise shear angles in the bias-extension method and the undesired fiber tension invalidating the load–displacement curve in the picture-frame test. To remove these difficulties, a modified version of the picture-frame test is proposed. This new method avoids fiber tensioning during testing and is considered more appropriate to capture the influence of the fabric architecture on the measured properties. Good correlations between the load–displacement curves and viscosities measured with the modified picture-frame and bias-extension methods were obtained compared to the standard picture-frame method.