Application of strain-controlled fatigue testing methods to polymer matrix composites

This paper presents a short investigation of the benefits of non-contact strain measurement for monitoring and control of fatigue tests on composites. Recent developments in measurement technology offer the means to effectively measure both axial and transverse strain, instantly, throughout cyclic and highly dynamic tests. Several test scenarios are examined which demonstrate potential benefits of current state-of-the-art video extensometry, for strain controlled fatigue tests on thermoplastic composites.Live extensometry enables continuous monitoring and live calculations throughout the test and provides the option to automatically collect additional data for loading cycles with anomalous behaviour. It also allows a good control system to safely apply accurately strain-controlled loading of specimens. Digital image correlation techniques can offer complimentary information on the full-field strain behaviour of a specimen, but the data processing and storage requirements are considerably too large for live or continuous measurements during fatigue tests.Composites fatigue investigations, to date, have generally been focused on high cycle fatigue, typically under sinusoidal stress-controlled conditions. The authors propose that there is a growing need to understand composite behaviour, at well-defined strain-rates and under conditions of occasional, non-catastrophic, overload; furthermore that non-contact extensometry is an important enabling technology for such investigations.