Comparison of strain measurement techiques for thermoplastic composite material characterization

Abstract

Characterizing the in-plane shear response of thermoplastic composite materials is receiving considerably attention in order to provide important design parameters for industrial manufacturers. In this report, the in-plane shear behaviour of a ±45º laminate will be quantified based on the test method ASTM D3518 while comparing different strain measurements techniques. In addition to the standard methods, the strain will also be measured by digital image correlation (DIC) and laser speckle extensometers (LSE) and the results and applicability of these methods will be theoretically and experimentally compared. These two non-contacting techniques allow to measure in both longitudinal and transversal direction simultaneously, and extend their applicability to complex and in real-service parts. In addition, the effect of laminates misalignment is experimentally evaluated. The material used is Cetex#®, a PPS thermoplastic matrix reinforced with 5-Harness Satin carbon fabric supplied by TenCate Advanced Composites. The experimental tests revealed that the effect of laminates misalignment in ±45º laminates is not very notable up to 10º. Comparing the three strain measurements techniques, DIC appears to be the most suitable, applicable and advantageous method while measuring at laboratory conditions. This technique also gives a very good post-processing of the inelastic zone. Laser speckle extensometer presents more limitations when measuring longitudinal and transversal strain simultaneously. Finally, failure analysis demonstrates that final failure occurs along the fibres, which try to reorient to the loading direction. Final failure is due to fibre-matrix debonding and scissoring as shown in optical microscopy and SEM analysis.