Determination of Compressive Properties of Fibre-Reinforced Polymers in the In-Plane Direction According to ISO 14126. Part 2: A Critical Investigation of Failure Behaviour

In Part 1 of this paper, we have compared the compression modulus and compressive strength obtained during a round robin test with the so-called Celanese-type equipment in four laboratories. The scatter of the measured values was considerable, with a variation coefficient of about 10% for the modulus and about 15% for the compressive strength. The main source responsible for this disappointing result is supposed to lie in the stress/strain concentrations induced by the tabs that support the specimen and transfer the load from the machine to the specimen. Detailed optical examinations have shown that cracks are initiated in this region or at least run into it after being initiated away from the tabs. Additionally, the local deformations were determined during testing with a grating technique. The local strains in different directions agree qualitatively with the results of finite-element modelling of the Celanese-type testing. The strain concentrations near the free surface of the tabs, which are responsible for the crack initiation, can be reduced if the tabs do not have a sharp 45° edge but a certain curvature.