Round-robin programme for longitudinal tensile testing of unidirectional composites: results, conclusions, and recommendations

The longitudinal tensile behaviour is one of the most important and basic characteristics of a unidirectional composite, yet its experimental characterisation is not straightforward. Therefore, we conducted a round-robin programme with seven test labs and six coupon designs where the panels were manufactured and cut by the same lab. Our results indicate that careful manufacturing and cutting enable consistent and reliable results for the different labs and coupon designs. High-speed videos confirmed that many coupons failed away from the grips, which is the targeted failure location. In addition to the modulus, failure strain and strength results, we present results and novel recommendations related to the coupon preparation and testing methodology. We recommend using a non-contact strain measurement method and discarding tests with significant stress drops before final failure.