Lecture - Simple test, still a challenge: Accurate tensile testing of carbon fibre reinforced unidirectional composites

As some of you may know, material testing is crucial in engineering practice to provide design allowables for basic strength calculations and input for numerical modelling of more complicated components and structures. This is the so-called coupon level, where usually a small and simple representative piece of a flat plate is loaded until failure, to obtain mechanical properties. Higher level (i.e. component or structural) testing is required to validate the numerical models and to comply with safety regulations in critical fields. Although it sounds simple to break a rectangular piece of composite plate reinforced by fibres parallel to the longer edge, even this most basic test has several pitfalls. My lecture concentrates on the issues around the gripping areas where the clamps of the test machine grab the specimens and introduce additional load compared to that encountered by the free section of the specimen. These so-called stress-concentrations can result in premature failure during the tests and significant drop in the measured mechanical properties, which may lead to overdesigned components and limited exploitation of composite materials’ potential. I review the state of the art solutions to reduce the detrimental stress-concentrations and highlight their potential to obtain the most accurate material properties possible. I put special emphasis on our recently developed hybrid composite specimens, where continuous high elongation glass fibre reinforced layers protect the lower elongation carbon/epoxy material.

If you are interested in the hybrid test specimens, please check out our paper: https://www.sciencedirect.com/...

If you have any questions regarding my lecture please find me at  czel@pt.bme.hu

Author: Gergely Czél