Alternative holder design for thick specimens intended for compressive strength testing of UD composites

Piotr Czarnocki

Pre-print pages 1-15

DOI: https://doi.org/10.62753/ctp.2026.01.1.1

keywords: composites, strength, tests

abstract The standard specimens used to determine the compressive strength of unidirectional (UD) composites are of a relatively small cross-section. The compressive strength of such materials depends on the volume of the tested material and decreases with its increase. Therefore, the results of tests carried out using such specimens may have limited applicability to the design of actual structures, and larger ones should replace standard-sized specimens. The performance of a hybrid holder designed for non-standard specimens of relatively large cross-sections, intended to determine the compressive strength of UD composites, was investigated numerically. Selected numerical results were compared against preexisting experimental data.  The holder consisted of a metal cup filled with resin surrounding a specimen. The results showed that (i) concentrations of longitudinal stress occurred in two locations: at the tip of the specimen touching the bottom of the cup and in the resin meniscus plane, (ii) the proportion between them could be controlled by (a) the presence or absence of adhesion between the resin and specimen, and the resin and cup wall, (b) by friction at the mentioned interfaces, and (c) by the specimen immersion depth. Eliminating adhesion and friction at the specimen/resin/cup interfaces reduced the differences in the values of the stress concentration factor present at the end of the specimen and in the meniscus plane, indicating the possibility of optimizing the holder design.